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Flood Risk Management and the American River Basin: An Evaluation (1995)

Chapter: 2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES

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Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
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Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
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Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 34
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 35
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 36
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 37
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 38
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 39
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 40
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 41
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 42
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 43
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 44
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 45
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 46
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 47
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 48
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 49
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 50
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 51
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 52
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 53
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 54
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 55
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 56
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 57
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 58
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 59
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 60
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 61
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 62
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 63
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 64
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 65
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 66
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 67
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 68
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 69
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 70
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 71
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 72
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 73
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 74
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 75
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 76
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 77
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 78
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 79
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 80
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 81
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 82
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 83
Suggested Citation:"2 IDENTIFICATION AND EVALUATION OF ALTERNATIVES." National Research Council. 1995. Flood Risk Management and the American River Basin: An Evaluation. Washington, DC: The National Academies Press. doi: 10.17226/4969.
×
Page 84

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Identification and Evaluation of Alternatives In the 1991 American River Watershed Investigation (ARWI), the Sacra- mento District presented various alternative plans to provide flood control to Sacramento, including supporting analysis (USAGE, Sacramento District, 1991~. For each alternative plan, the 1991 ARWI provided estimates of the cost, ex- pected benefits, and net benefits; the level of protection; and the environmental impacts and proposed environmental mitigation. Formal decisionmaking on the alternative plans was then based on these estimates. In the USAGE's planning process, the benefit-cost ratio is calculated to screen out inefficient alternative plans, as plans with negative net benefits are not eligible for federal funding. The alternative plan with the highest expected net benefits, consistent with applicable environmental laws and regulations, is desig- nated the National Economic Development plan (NED) and is generally the plan recommended by the federal government. In the American River case, the NED plan included construction of a dam and 894,000-acre-foot reservoir at a site near Auburn. However local interests, as represented by the Sacramento Area Flood Control Agency (SAFCA), preferred a plan featuring a smaller dam and after consultation a plan including a smaller structure, offering, a 200-year rather than 400-year level of protection, became the selected plan. During review of the 1991 ARWI by federal and state agencies and by public interest groups, concern about a number of technical issues emerged. These issues played some role in the rejection of the selected plan by Congress in 1992 and ultimately led to the creation of this committee. In a more recent document, the 1994 Alternatives Report (USAGE, Sacramento District, 1994a) the Sacra- mento District presented a revised set of alternative plans, including estimates of 32

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 33 costs and benefits. Unfortunately, the analysis supporting those new estimates is not scheduled for release until July 1995. In preparing the 1994 Alternatives Report, the Sacramento District had the opportunity to benefit from the technical debate that was generated by the 1991 ARWI and from interactions with this committee and many other parties. In addition, the 1994 Alternatives Report previewed the first application of USAGE's new approach to evaluating flood control projects, an approach based on risk and uncertainty analysis. This chapter discusses the development of alternative plans and the technical analysis used to estimate costs, benefits, and levels of protection. Subsequent chapters consider the analysis of environmental impacts and the new USACE approach to risk and uncertainty analysis. The committee's consideration of these issues was based largely on written and oral information provided by USACE, SAFCA and its consultants, and various critics of the 1991 ARWI. The committee was able to make firm recommendations on a number of technical issues, but many issues remain unresolved owing to lack of data and to the fact that the supporting technical analysis is not yet available. This latter fact has proven particularly problematic. Information related to that future document, received informally during briefings, indicates that the analysis supporting the 1994 Alternative Report is significantly different in many crucial respects from that which supported the 1991 ARWI. But the committee did not have formal written documentation of the analysis, and in most cases was uncomfortable about commenting on oral presentations and the few supporting documents that were available. SELECTION OF PROJECT ALTERNATIVES Perhaps the most critical step in the development of a flood control project is the selection of alternatives that will receive detailed analysis. Regardless of the potential effectiveness of a particular alternative, if it is not identified, it will not be selected. Furthermore, if popular alternatives are not selected for detailed analysis, it may be difficult to win support for the selected alternative, regardless of the potential effectiveness of the popular choices. Thus, this section looks specifically at the selection of alternatives in the American River planning pro- cess. (Additional discussion of the selection of alternatives and project planning in general is found in Chapter 6.) Flood Control Measures In developing project alternatives, USACE begins by identifying flood con- trol measures that can be used alone or in combination. In the 1991 ARWI, the Sacramento District identified 23 flood hazard reduction measures, 13 pertaining to the main stem of the American River and 10 pertaining to Natomas. Of the 13 main stem measures, 4 were retained for further consideration and incorporated

34 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN into flood protection alternative plans: (1) structural modifications to Folsom Dam to increase outlet efficiency; (2) increased downstream channel capacity to allow greater flood releases (so-called "objective releases") from Folsom Reser- voir; (3) increased allocation of storage space in Folsom Reservoir to flood control; and (4) construction of a dam upstream of Folsom Reservoir (at Auburn). In the 1994 Alternatives Report, which excluded consideration of the Natomas Basin, the Sacramento District presented 17 measures, 8 of which were retained for further consideration. The latter included 4 measures for increasing the outlet efficiency of Folsom Dam, in addition to measures for increasing downstream channel capacity, increased flood control storage space in Folsom Reservoir, construction of a dam at Auburn, and raising of Folsom Dam and its spillway. The 1991 and 1994 flood control measures are summarized in Table 2.1. Flood Control Alternative Plans In the 1991 ARWI, the 4 surviving flood control measures were bundled into 6 alternative plans. Two alternatives were based on construction of a flood control dam at Auburn. Two other alternatives combined increasing flood con- trol storage and outlet efficiency at Folsom with increasing downstream flow capacity. The fifth alternative was based solely on increasing the downstream channel capacity. The final alternative was based solely on increasing the pro- portion of flood control storage in Folsom Reservoir. Seven alternative plans were presented in the 1994 Alternatives Report. Three of these were based on construction of a flood control dam at Auburn. Three other alternatives combined increasing flood control storage and outlet efficiency at Folsom with increasing downstream flow capacity. The final alter- native combined increasing flood control storage and outlet efficiency at Folsom, without increasing the downstream flow capacity. The alternative plans presented in the 1991 ARWI and 1994 Alternatives Report are summarized in Table 2.2, along with the estimated levels of protection and ratios of the net benefits to the net benefits of the NED plan. Note that the methods that the Sacramento District used to estimate the levels of protection in 1991 differed from those used in 1994; hence the estimates are not strictly com- parable. Criticisms of the 1991 Measures and Alternatives The measures and alternatives presented in the 1991 ARWI were criticized on a number of grounds. Many of these criticisms focused on the evaluations of the alternatives; these are addressed in subsequent sections. However, some of the criticisms had to do with the perceived failure of the Sacramento District to consider and evaluate potentially effective alternatives. The most serious criti- cisms focused on Folsom Reservoir. In particular, critics argued that the district

IDENTIFICATION AND EVALUATION OF ALTERNATIVES TABLE 2.1 American River Flood Control Measures (Excluding Natomas) 35 Measure 1991 Reporta 1994 Preprojectb 1994 ReportC Listed/Retained Condition Listed/Retained Increased Outlet Efficiency of Folsom Dam and Reservoir Normalized use of auxiliary spillway No No Yes/No Structural modifications Lower main spillway Yes/Yes No Yes/Yes Enlarged river outlets No No Yes/Yes New river outlets No No Yes/Yes New tunnel outlets No No Yes/No Conjunctive use of river outlets and main spillway (without modifying outlets) No No Yes/No Use of existing diversion tunnel No No Yes/No Improved flood forecasting and reservoir operation Yes/No No Yes/No Increased Flood Releases from Folsom Reservoir Levee/channel modifications Yes/Yes Yes Yes/Yes Setback levees Yes/No No Yes/No Flood control bypass south of Sacramento (Deer Creek) Yes/No No Yes/No Increased Flood Storage in the American River Basin Flood detention at Auburn Yes/Yes No Yes/Yes Existing upstream reservoirs Yes/No No Yes/No Multiple small-detention reservoirs Yes/No No Yes/No Offstream storage near Folsom Yes/No No No Out-of-basin storage on Deer Creek Yes/No No Yes/No Increased flood space in Folsom Yes/Yes Yes Yes/Yes Raised Folsom Dam and spillway Yes/No No Yes/Nod Other Measures Divert flood flows into Sacramento River deep water ship channel Yes/No No No Miscellaneous nonstructural Yes/No No No aMeasures listed for consideration in the 1991 American River Watershed Investigation, Sacra- mento District, U.S. Army Corps of Engineers. bMeasures from the 1991 ARWI that were treated as part of the pre-project condition (i.e., mea- sures already or planned to be implemented) in the 1994 Alternatives Report, Sacramento District, U.S. Army Corps of Engineers. CMeasures listed for consideration in the 1994 Alternatives Report, Sacramento District, U.S. Army Corps of Engineers. dMeasures that may be reconsidered before final recommendations are made.

36 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN TABLE 2.2 American River Flood Control Alternative Plans Alternative Level of Protectiona (years) Net Benefits/ NED Net Benefitsb 1991 ARWI Auburn Dam 894,000 acre-feet Auburn Dam 545,000 acre-feet Folsom Modification and Reoperation (1) Increase maximum Folsom flood control storage to 650,000 acre-feet Lower Folsom spillway Increase objective release to 130,000 cfs Folsom Modification and Reoperation (2) Increase maximum Folsom flood control storage to 470,000 acre-feet Lower Folsom spillway Increase objective release to 130,000 cfs Levee Modification Increase objective release to 145,000 cfs Increased Folsom Flood Storage Maximum flood control storage-590,000 acre-feet 1994 Alternatives Report Auburn Dam 894,000 acre-feet Auburn Dam 545,000 acre-feet Auburn Dam 380,000 acre-feet Folsom Modification and Reoperation (3) Modify Folsom outlet works Increase objective release to 180,000 cfs Folsom Modification and Reoperation (4) Variable Folsom flood control storage 450/670,000 acre-feet Modify Folsom outlet works Increase objective release to 145,000 cfs Folsom Modification and Reoperation (5) Variable Folsom flood control storage 475/670,000 acre-feet Modify Folsom outlet works Increase objective release to 130,000 cfs Folsom Modification and Reoperation (6) Variable Folsom flood control storage 495/670,000 acre-feet Modify Folsom outlet works Maintain objective release at 115,000 cfs 400 1.0 200 0.80 0.56 100 0~30 00 100 455 270 200 244 217 0.30 0.34 1.0 0.70 0.30 0.24 0.21 185 0.19 152 0.32 aLevel of protection was computed differently in 1991 and 1994. bFor the 1991 ARWI, the divisor is the net expected benefit for the 1991 NED plan; for the 1994 Alternatives Report, the divisor is the net expected benefit for the 1994 NED plan.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 37 failed to adequately consider modification of the operation of Folsom Dam, which, coupled with improvements in the dam's outlet capacity, might signifi- cantly increase the effectiveness of the existing storage. Some of these criticism were addressed in the 1994 Alternatives Report. Most notable is a reoperation plan for Folsom Reservoir that will increase the winter flood control space based on the availability of storage space in the three largest reservoirs in the upper American River basin. This plan is expected to be implemented independently of the ongoing planning process and hence is considered an existing condition in the 1994 Alternatives Report. Issues of Importance in the 1991 and 1994 Alternative Plans In considering the alternative flood control plans in both the 1991 and 1994 reports, the committee elected to focus on four specific elements: use of Folsom Reservoir, the question of gates in the Auburn Dam alternatives, the Deer Creek alternative, and nonstructural measures. Folsom Reservoir As noted above, the 1991 ARWI was criticized for failing to give sufficient consideration to ways to maximize the flood mitigation potential of Folsom Reservoir, including the use of flood forecasts. How valid is that criticism? Before addressing this question, consider how the operation of Folsom Reservoir determines its effectiveness at reducing flood risk in Sacramento. Folsom Reservoir provides the primary means of reducing flood flow in the lower American River. The flood reduction potential of the reservoir depends on the amount of water that can be stored as compared to the difference between the amount that enters the reservoir during major flood events and the amount that can be safely released. At full pool, Folsom Reservoir has a storage capacity of about one million acre-feet. But Folsom is a multipurpose reservoir; in addition to flood control, its purposes are water supply, hydropower, and recreation. Un- fortunately, there are conflicts among these objectives. If the reservoir were to be operated for an assured water supply alone, the optimal strategy would be to keep the reservoir as full as possible. If the reservoir were to be operated for flood control alone, the optimal strategy would be to keep the reservoir as empty as possible. Clearly, the reservoir cannot be operated to maximize both of these objectives simultaneously. One solution to this dilemma is to allocate storage amounts separately to flood control and water supply. Nominally, the top 400,000 acre-feet of storage space in Folsom Reservoir is allocated to flood control; the remainder is allocated for water supply. This allocation is not rigid, however, owing to the timing of flood events in the watershed. Potentially damaging floods occur only during the winter storm season, which lasts from the beginning of November through the

38 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN end of March. Hence the full flood storage pool need be available only during this period. The manner in which the flood storage space is managed is specified by a flood control diagram that was originally formulated in 1956 and modified in 1977 and 1987. Under the 1987 diagram (USAGE, 1987), the flood control storage space must be increased from zero on October 1 to a maximum of 400,000 acre-feet on November 17, at which level it must be maintained until February 8. Between February 8 and May 31 the flood control space is to be varied according to the accumulated seasonal precipitation, which is closely related to the depth of snowpack in the upper American River watershed. This currently used approach to managing the flood control space in Folsom Reservoir could be modified to improve flood control effectiveness (as is being considered with the Folsom reoperation, discussed below). Such improvements may or may not come at the expense of water supply or other water resources purposes (see Chapter 6 for additional discussion). The seasonal allocation of flood storage determines the amount of storage available for flood control prior to a flood. The effectiveness of the available storage depends on how it is used during a flood event. Obviously, it is desirable to release water as rapidly as possible without causing downstream damage dur- ing a flood, since that frees up storage space in the reservoir. But there are constraints on how rapidly water can and should be released. First, there are physical limitations on the maximum discharge rate from the reservoir. Folsom Reservoir is severely limited in this regard. For example, the primary flood- release structures, the five main spillway bays, cannot discharge water at the objective release rate of 115,000 cfs until the flood control storage has been filled to about half of total capacity. (The objective release rate is the design discharge capacity of the channel and levee system downstream of the reservoir; sustained flows in excess of this rate could cause levee failure.) Second, there are admin- istrative and legal limitations on releases. The 1987 Water Control Manualfor Folsom Reservoir (USAGE, 1987) provides that as an operating guide, "releases from Folsom Dam shall not be increased more than 15,000 cfs or decreased more than 10,000 cfs during any 2 hour period. . ." This limit on the rate of increase of discharge rates (the so-called "ramping rate") is intended to minimize bank sloughing and caving downstream and to allow time to prevent downstream loss of life and damage to property. The 1987 Water Control Manual also limits the maximum controlled release to 115,000 cfs, up until the time at which the storage level of the reservoir reaches full pool. At full pool the release policy is governed by an emergency spillway release diagram that is designed to protect the reser- voir from failure due to overtopping. There is one additional constraint that is applied to the operation of the reservoir during floods: while inflows are rising, the controlled discharge from the reservoir cannot exceed the inflow rate. This requirement ensures that in no flood event will the peak discharge below the reservoir exceed the peak discharge into the reservoir. Note that this is a de facto policy that is not explicitly specified in the 1987 Water Control Manual.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 39 All of the above constraints on the operation of Folsom Reservoir can be modified to some extent. Changing the physical constraints, of course, requires structural modifications to the reservoir and levees. The remaining constraints are administrative and legal and could be changed by appropriate agreements. As noted above, the 1991 ARWI considered a number of measures for im- proving the flood control effectiveness of Folsom Reservoir, including lowering the main spillway, using flood forecasting to draw down Folsom Reservoir in advance of a potentially severe storm, increasing the objective release, increasing the allocated flood space in Folsom, use of storage in upstream reservoirs, and raising Folsom Dam. Of these, the use of flood forecasting, use of storage in upstream reservoirs, and raising Folsom Dam were not incorporated into any of the proposed alternatives. In the 1994 Alternatives Report, the original 1991 measures were reconsidered, although increasing the Folsom flood space in accordance with the amount of water stored in upstream reservoirs (Folsom reoperation) was considered to be a without- project condition. New measures in 1994 included construction of new outlet works, as well as altered use of the existing outlet works. As in 1991, measures involving flood forecasting and the raising of Folsom Dam were not incorporated into alternatives, although appar- ently the latter measure is still being considered. It is clear that the Sacramento District considered a number of strategies for increasing the flood control effectiveness of Folsom Reservoir. The most notable of these is the Folsom reoperation, which is considered a without-project condi- tion in the 1994 Alternatives report. Also relevant is the decision by the Sacra- mento District to reject use of flood forecasts, as well as some other approaches to Folsom operation. Folsom Reoperation One measure considered in the 1991 ARWI was increasing the Folsom flood control storage allocation to 650,000 acre-feet. This measure was included with lowering the Folsom spillway and increasing the objective releases in an alterna- tive that provided an estimated 150-year level of protection. The lost water supply resulting from the increased flood control allocation was computed to cost about $10 million per year, or about 20 percent of the total annual cost of the alternative. Subsequently it was realized that if the Folsom pool were lowered in accordance with the water stored in the largest upstream reservoirs, the expansion of the flood pool would not necessarily represent a loss to water supply. On the basis of this realization, several potential operating rules were considered; of these, the so-called "670 plan" became a without-project condition in the 1994 Alternatives Report. Under this plan, the flood control space in Folsom Reser- voir would vary between 400,000 and 670,000 acre-feet, based on the day of the year and the reservoir storage space available in the French Meadows, Hell Hole, and Union Valley reservoirs. Between December 1 and March 1, the Folsom

40 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN TABLE 2.3 Estimated Volume of Water That Must Be Stored in Order to Control the Flood of the Given Recurrence Interval to the Given Objective Release Required Volume (1,000 acre-feet) Recurrence Interval (years) Objective Release of 1 15,000 cfs Objective Release of 180,000 cfs 100 498 232 200 770 452 400 1,115 748 NOTE: The volume estimates are based on the USACE flood quartile estimates for the 3- and 5-day floods, without the expected probability correction, and on the design hydrograph used in the 1991 ARWI, with- out any adjustments for upstream storage. flood control space would be maintained at 400,000 acre-feet if the empty space in the three upstream reservoirs totaled at least 200,000 acre-feet. Any incremen- tal reduction in the upstream space would require a corresponding incremental increase in Folsom's flood space. When all of the empty space in the upstream reservoirs was filled, the flood-storage space at Folsom would be maintained at 670,000 acre-feet (SAFCA, 1994a). Although Folsom reoperation was consid- ered a without-project condition in the 1994 Alternatives Report, it still must be approved prior to its adoption. This proposed modification of the operation of Folsom Reservoir represents a significant increase in the flood control effectiveness of the reservoir. An idea of the relative magnitude of this increase can be obtained from Table 2.3, which gives for different levels of protection the volume of water that must be con- trolled if the corresponding flood peak is to be kept from exceeding an objective release of either 115,000 or 180,000 cfs. The table was developed by computing the area enclosed above the objective release and below the design hydrograph for the given recurrence interval. It is based on the design hydrographs used in the 1991 ARWI, without the expected probability correction. From Table 2.3 it can be seen that the maximum additional storage of 270,000 acre-feet provided by the proposed modification represents about 35 percent of the volume required to control the 200-year event to 115,000 cfs. For the 400-year events, the amount is 24 percent. Flood Forecasting and Flood Control Effectiveness In both the 1991 ARWI and the 1994 Alternatives Report, the Sacramento District considered and then rejected a measure involving the use of weather forecasts to draw down Folsom Reservoir in advance of a storm. This decision

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 41 was based on the conclusion that weather forecasting was not sufficiently accu- rate. The committee also doubts the efficacy of early releases, given the current limitations of precipitation and runoff forecasting, physical and administrative limits on pre-flood-peak release rates from Folsom, and the fact that Folsom reoperation will enable use of about 70 percent of the available storage space in the reservoir. The committee thinks, however, that forecasting may be of value in devising strategies for regulating floods that exceed the Folsom flood pool capac- ity so as to minimize the amount by which the actual Folsom outflows exceed the objective release. In addition, dam operation decisions that clearly take available forecast information into account are more likely to be acceptable to both the dam operators and the public than decisions that do not make use of all available information. The committee recommends, therefore, that the Sacramento Dis- trict, the Bureau of Reclamation, and the state of California keep abreast of developments in precipitation forecasting and develop the capability to exploit major improvements in forecasting accuracy. Folsom Operation During Flood Events As previously discussed, maximum flood-reduction effectiveness requires rapid discharge of water during a flood event. In this regard, Folsom Reservoir presents three issues: limitations in the outlet structures at Folsom; appropriate- ness of the rules governing the release of water from Folsom during floods; and actual operation of the reservoirs during past floods. During a flood event, Folsom releases water over the main spillway, through river outlets in the spillway, and through the power penstocks. The main spill- way has eight gated bays. Five of these bays discharge down the spillway into a stilling basin at the base of the dam; they constitute the main release mechanism. The river outlets were designed to operate concurrently with the five main spill- way bays. The remaining three spillway bays, called the auxiliary spillway bays, discharge to a flip-bucket energy dissipator. These bays were designed to help pass water during extreme floods to protect the dam against overtopping. Unfortunately, the existing outlet facilities are inadequate and limit the flood control effectiveness of Folsom Reservoir. When the pool is at the bottom of the current flood space (40O,OOO acre-feet of storage), the five main spillway bays can pass only 6,500 cfs. At a flood storage space of 500,000 acre-feet, the main bays cannot pass any water. The original operation of Folsom Reservoir de- pended on the concurrent use of the river outlets and the five main spillway gates. Shortly after the dam became operational, however, it was discovered that con- current use caused cavitation damage to the spillway. Even with subsequent modifications to the river gates, concurrent operation of the river and spillway gates has been avoided. These limitations on flow releases severely constrain the current operation of Folsom and would be especially constraining under the proposed reoperation.

42 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN For this reason, several of the proposed measures involve construction of new outlet structures. In addition, Countryman (1993) made a number of recommen- dations for improving the efficiency of Folsom Reservoir with the existing struc- tures. These include concurrent operation of the river outlets and five main spillway gates and use of the three auxiliary spillway gates during normal flood operations. Countryman calculated that use of his "maximum outlet plan" would increase the releases during the FEMA 100-year flood by over 60,000 acre-feet. This represents about 8 percent of the volume required to control the 200-year flood to 115,000 cfs (Table 2.3~. Although this is not a large percentage, given the low level of protection currently provided Sacramento, the recommendations of Countryman (1993) should be considered seriously. The committee was told that the main spillway gates and the river outlets are assumed to operate concur- rently in the analysis supporting the 1994 Alternatives Report. The committee did not attempt to evaluate in detail the appropriateness of the ramping rates or of the de facto requirement that outflows be less than inflows during the period of increasing inflow. The committee was told that in the analysis supporting the 1994 Alternatives Report the ramping rates were in- creased by 33 percent for flow up to 25,000 cfs and increased by 100 percent for flows above 25,000 cfs. Operating with these new rates would improve the flood-reduction effectiveness of the reservoir. The committee conducted its own analysis of the increases in water levels and velocities associated with the ramp- ing rates. The results of this analysis show no reason why ramping rates must be held at 15,000 cfs per 2 hours. The committee recommends that the Bureau of Reclamation and the Sacramento District consider the impacts of operating Folsom with higher ramping rates. The more critical issue is the way the reservoir is actually operated in prac- tice. Up to the present, the operator has had to compute reservoir inflows on the basis of observed increases in water levels. This problem alone results in a 4- hour delay in releases. It is the committee's understanding that the flow measure- ment issue is being remedied by the installation of telemetering equipment at flow monitoring stations in the three main upstream tributaries. The committee strongly supports the development of real-time capacity for monitoring inflows to Folsom Reservoir and of a means for accurately gaging outflows from Folsom and Nimbus reservoirs. Another important operational problem is the failure of operators to follow the rules. In its discussion of the 1986 operation of Folsom Dam, the Bureau of Reclamation stated that prescribed rule curve operation should be viewed as "hypothetical." The agency goes on to say (Bureau of Reclamation, 1986) operators are reluctant to rapidly increase the volume of outflow and conse quently affect the floodplain unless such increases are clearly warranted. It is estimated that actual operating efficiencies, when compared to hypothetical op eration, are about 80 percent.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 43 If this statement is true, then either a change is needed in the constraints placed on operators with clearer specification and formulation of release rules, or else plan- ning assumptions should be revised. This issue is explored further in Box 2.1. In addition, the operation of Folsom during the 1986 flood, discussed later in Box 2.2, illustrates these issues. Special training and use of forecasts are two approaches that could be used to improve the performance of reservoir operators during flood events. Forecast- based rules, if well thought out and tested in advance, can to lead to better decisions than can be made on an ad-hoc basis under emergency conditions. Special training in use of the operating rules could make effective use of simula- tion exercises, in which operators develop experience in decisionmaking under both historical and hypothetical extreme events. Simulation exercises can pre- pare operators to take those actions early in a storm that are required to reserve flood storage to control very large events. Recommendations on Folsom Operations Folsom Reservoir is the critical component in the flood control system for Sacramento. Consequently, it is essential that it be operated as efficiently as possible during floods. Based on the 1986 flood experience, it is clear that there were problems with how Folsom was operated: the ramping rates were exces- sively conservative, needed gages were not installed, operators were not careful about retaining flood control storage, and the dam did not go on alert when the rest of the state did. (See Box 2.2 and Figure 2.1.) Since 1986, several changes have been made or proposed, including new operating rules in 1987 and a pro- posed reoperation plan. But, in spite of these changes, the committee was uncer- tain about the current and future operating efficiency of Folsom Reservoir. The reasons for this uncertainty include the following: · The Folsom Flood Management Plan, referred to in the 1994 Alternatives report, was not completed in time for committee inspection. This plan is intended to "maximize the flood control capability within the existing 400,000 acre-foot flood control reservation of Folsom and improve the stream-gage network and flood-forecast system for the American River basin upstream from the reservoir" (USAGE, Sacramento District, 1994a). The committee was not provided any details on the recommendations in this report and hence unable to evaluate their potential effectiveness. · The proposed Folsom reoperation was not final at the time of this report, and represents a major change in reservoir operation. It is notable that the Folsom Flood Management Plan apparently does not consider Folsom reoperat~on. · The current ramping rates may be unduly conservative, as recognized in the 1994 Alternatives Report.

44 FLOOD RISK MANAGEMENTAND THE AMERICAN RIVER BASIN .: i ~.~..,~ ~ ~ ~ ~ ~ ~ ... ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ' ~ ' ~ ~ ~ ~ ~ ~ ~ ' ~ ~ ~ ~ ~ ' ~ ~ ' ~ ~ ' ~ ' ~ ' ' ~ ~ . ' ~ ' ~ ~ ' ~ ~ ~ ~ . ~ ~ ~ ~.~ ~,~, ~.~,~,~,~,~,~. i. i. ~ ~.~ ~ .. ~ ~ ~.~ ~ B0.X~.~2.w1-~ 'P~4'~1~G' ASSU'M'PT1.~NS'A'ND" OPERATIONAL EF~FECTIV~EN..ES~S~ There is attires a `~isconn~ect''~betwe,en~,the operating assum.phon.,s~made~ Ins ~ planning anal.. what actually hap~g'the~ ~= At. ~a' fjord ~,':~facilit~. If: ~ Sometimes such diderences~are caus~ed~by how mechanical:systerns~ perform ~ an: If deficiencies in: system, ope~ti~on a~re~.identif~'e'd.~..~ing '"' li '' ,~I.~'..~v~sed~ to .r.-.efl~,~.actu;a' I conditions ~ or,~the~,', :deficiencies~ sl~ou.ld~ '6e~.~corrected.~- ~.~. ~....... ~ . , ~., ~.~.~., ~. ~ ~ ~ .~ .... ~ , ~ . i. ... . ~.~ .~ ~ i, , ~ ,., ~..~99:~ A~.~rx~'nstance ~.~accepted:~.~many operating ~£onstral:nts~ ale :given$ ~ ~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ hi; ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ He Sacramento Distn~>s~upoom~x~'~Supplemental Norma port is~said'to be .'.~.ore~ aggr~sive~. inks oe~i.on'ng th.e,~.e re~.rai.nts end. ''.~.,,~m.,~ing.to fin ~ ays't 'it timinateproblems.. 'One~example~e i -I ' ist ' lyric nceo f ~ ~.~ ~ ~ . ~ ~ ~.~ ~ Hi. . ~ ~ ~ ~ . ~ ~ ... . -it.,., ., ~ ~ ~ ~.~.~.~ ~,~ ... ~ ~ ~,~ . ~ .. i.. ~ ~.~ ... ~ a.. ~.~ ~ ~ ~i~lil~operators~to.alIow free flow wer.the~ Folsom spiti~,"ay b"~.it.~.i.s,.~di.fficult to if' 'ti 'mate~th~e .result~t x lease ~from. lake levels ~.~this.~chnical problems co . ~ :solved ~'. i~by~inst filing ".stmam .g 'a'ges~ ~or~o.ther. ' w ~ as ' ring Levi es'.'~.'~' ~'~:~""" '. tiom~lmxe$"~ suc~n'ne.~.~$~0~.by tale nature at The assu:m~i:ors.. 0on~sewative.~assumptions ~ Tn6~icating sew ex.pectat~ f op rati ,,,.,, ~.~ ., .~ ~.~.~,,-~. ~.,~.~.~ ~ I.. :., ~.~.~..~.~.~.~.~:~.~ . i, 7, -.. . ~ .~ . ~ : If ~It~WOUl~ take ,.o~,`~!ors~x,, Aim ~ nforma ~ ~ ~ ~ ma. ~ ~ 0~.~ ~ ~.~ ~ ~.~ ~ ~,~.~ ~ ~ ~ ~.~.~ ~.~ ~. ~,. ~ ,.~ ., , ,, ..,~. ~ ~ ~,~ . ~ ., . Hi. ~.~.~ . i,, i. i .. , ~,~,~ ,.~. ~ .~ ~ , ~ . ~ ~ t+On~Tect~ava~If~ of ~ real~-tlme ~forecasts~that mlg, ~ ~ - oper~ to . ~ ~ ~ ~ ~ ~ . . ~ ~ . ~ . ~ ~ ~ ~ ~ . . . . ~ ~ ~. ~ . ~ ~ . ~ ~ .~ ~ i. ~ ~ . ~ i. . ~ ~ ~.,. Csp^.~.~.~st s~l~'x.~.~.~,~,,,,0~,~,,~,,, is s ~ t .~ ~ ~. . ~.,., , - ~i .. ~ .~ . - ..~ ~ ..~ ., . . ~ ~ ~ ~' ~ ~ a. . ~ ~ ~ ~ . - ~ ~ ~ : Aim, ,~ stir cons,'derv,1n, at,,,n,~,,majo,r,,[~o,~s,':,~,~=in and .streamf.ow...~ga.ges ca.n.'f' il .~.c' ' 'unica- ~.~ ........ .. .. ~ i .~ . ~ ...... .. ~ ~ , ~ ttons,~,~s~,,,~,0 break, genemr~ =~us~.~n ~,m,=~,< ~e, A"" ~ ~.~ ~ ~ ~ .:.~...~ ~ . . ~ . ~ ~ : i.:: -A ..::.: ..~ . ~ . i::.. i: . .. ..:::..., i ~ a-~...ti7~.~ornm`un~cat~on of Dens to operand Inns . and. Latest or other ~ sh~tu:~s In,' . .f x`~..~.o ¢~;~ i. ~ """""'"''''' ~''"''~""''''"''"'""~'''~' '~x'~"""''' "''"' '"""""""'""""""'"~""' ~.~ ~ ,. ~ ~ ~ . ~ ~ - .......... .~ ~ i ... ~.~ .. ...pmblems w~.~ms~n of spillways affecting. operands '.ncIod~e 1983 ffood~at~ ~ ~ ~ ~ ~ ~ ~ . ~ ~ i. ~ ~ ~ . ~ ~ . ~ . ~ . ~.. ~ . ~ i: ~ Din a. ~ - n fin - all.. ~ ~ .~ ~.~ ~ ~ ~ ~ it. ~ .~.~.~ ~ ~.~ ~ ~ ~ i. .,.. .. .. .~.~. ,~,~,,,x Hi. .. ~ .~ ~ .~..~ i:: :::: : :~. .. . ~d .. ; i' 1 \ . .3 ..,...~.x, · ~ .~ ~ ~ .x '': 'x .~ .t ~ ~ ~ levy issue Is. whether c~ow ~,9 ~.~es Based on the .plann~ng as-. ~ ~ . ~ i , . ~ . i. ~ ~ . ~ , . ~ - ,~ i ., - -, ~ Hi. . ~ . - i: . i. sum,phons. For instances. there is sIgOitl=~ Concern BS to ~S5 ''~tOm`'''2''''ill.i. '-p'lement..~rasti"""" 'lease~policies hi might caus o hefl :. ~ ~ Hi. ~ i. ~ , i.:., ~. ~ ~ ~ ~.~. i : .~ . ~.. ~ . ~ . ~ ~ "'''"'x''"'- ''I ~ ~ ~ , ~;~ .~w,,,~.~.o,,.r, e ~P.y: l!~: ~ Ok All tn ~ flood .As ~an,, ~xample serious p - lems,. .... . we'' 'n ~.'opemI'cn.~'.'~ainted.'Rock O 'am on the' G'i~ta'~River~in Arizo' .' .~ , .~.,.~ ~ ~.~ ~ . i. ~ ~ ~ ~ . . i ~ i . ~ . . .. i ~ ~ ~ ~ r,x .'~(R~ezac,~.'~.~1993)0..~.~',On.~Jan~ fair ...2Q~.of 1'9~,~"he~y, ins.in.the..reni~.or'result''' d~"'in~ ........... , ..~ ...... : i. ~ · The potential addition of new outlet works will require fresh consider- ation of the operating policy at Folsom. · The potential construction of a dam at Auburn will require a new operat- ing policy for Folsom, both to maximize efficiency of the combined system and . . . . . . to minimize env~ronmenta Impacts. It Is important to stress that while the committee was uncertain about the current and future operating efficiency of Folsom Reservoir, it did not believe that these uncertainties were sufficiently large to compromise the validity of the 1994 Alternatives Report. Hence the committee does not suggest that these uncertainties must be resolved before a flood control alternative is selected for Sacramento. In any case, whatever the decision regarding flood management for Sacra

=~#I-~ i3~ = all ~ ~= art ha= a! A -ad miki!ng Large ~lle~e\~beciuse.!ll~lllfe~ Manly ~g6~i~l!a~l~ll~l; ~tl~il~-sl d6!Ang~ lnighll~ll~houk (s~l~u~<~he~la~m~iol ~ si bitt ~ saga ~ ~ ~ I.:~ll=~:~: C~:~# 7~ Bias ~ : i incest of ~ Gel find aver Lion i~ ~1rom~ oh; ~6ritlcal ~ /{co6~ wh({l~ilm- (remends co/Id ~l~fi3~!~>likl~! USACE hds Baaed (USAC~E~i fag) 11~=!#1~ ! !I ELI Sips is nonnational E Flo~dll~ntrcl~!~ope~ing~is~um~pti6~>lUi!ed ==i =;= BIBS ems f <cul)~er~4I~ll(#~ spend years Paging and icons/{ing wet to end! ant hyd~p~?wdF,~w<~r~su@~!y>~i~d~!nv!~-enl#~i~ne~edsl~<!~!~l~!~l~!~!~ ~1~1~ ~ ~!~1 ~ seas saws Is sssS s s s sss s s s s s sSsssss~s ~ ~ ~ s ssss s s sss s s sss ss s s s s ssssssssssss sss sss~sss s s s s s sss sss s s s ~:~ ss ssssssssss~ssssssssss ~s~ 1 S :S S:SSS:S:: ss s s sssssssssssssssssssss s~ sssssssssssss ssssssssss sss S~S~S~S~S~S~S~S~SS sssssssssssssss ssssssss SSSsSsSsSsSsSsS~SsSsSsS S~S~S~SSS~S~S~SSS~S~S s s s ~s~s s~ssssssssssssssssssss s ~! S~S~S~S~S~S~S~S~S~S~ s~s~s~i ssssssssssssss ~i~i~'~ ~s~ 1 men10, 1here needs lo be serious cvaluabonof1bc operation of Folsom Rcservoir Tbis evaluation should be ~ coopcradvc e~r~ involving 1he Bureau of Rcclama hon, USACE, [bc s1atc of CaLfornia, and the U.S. Wcather Scrvice and should . . nc. uc e: e consideration of technological capabiL1ies in precipi1~hon and mno~ ~rc- casting, remoic sensing of rainf~lt rcal-Ome moni10ring of upstream reservoir s10rages, soil moisture, snowpack, and stccamOow~ and rain~ll-runo~ simula- 1ion; ~ consideration of operaUng rules 1ba1 c~ploi1 cu~en1 1cchnologica1 capa- bihties, including ~les goveming reservoir operadons ~hcn 1bc Cood capacity is c~cceded; - quan1~dvc asscssmen1 of v~ious operaUng ~les;

46 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 47 . . ~ ~ ~ ~ ~ . ~ ~ ~ .~ . ~ ~ . ~ ~ i. i Hi ~ ~ ~ ,~ ~ .,, ., ~ ~ i. i ~.,~ ~ i. ~. .. =~.~.jl~ ,~, ~ my. i . ~ i, ..,~, , i ....... . i. ,.~,.,,.~. i i ,. ~ . ~. ~.~.~.~. ~.~.~, .,,, Hi,,,, ,,~ ,.,,,,, ~,.,~,~,,~,~, ~.~,,,,,, ~,,,,~,.,,,.,~,,.,~,,~, ... .,.,,,,, ,, ,. ~,. ...~..~..~...... ..,..,.,... ..~ ~ · justification of constraints on release rates; and · operator training and other means of improving operator performance, including use of continuous interactive simulation of storm events. The evaluation of reservoir operating rules should be an ongoing process, so as to reflect changes in technology and in the physical system. Furthermore, there is a national need for assessment and monitoring of the effectiveness of operating rules at all major reservoirs with flood control obligations. This need

48 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN ~ 000 '_ 900 au a) 800 g 700 Go - ~ 600 O 500 oh ._ a' In a) ~ =~ nave ~ ^< A ^2,~^ . I. Bihourly Pack Inflow - 552,892 cfs driver rvv~ ',u~v,u~v~ At. `~ Response to Auburn Cofferdam Failure ~ · _~ . ~p ~- · 1~./\ ,2s, 2 1~ ~'~1 ~ x, . .. ......... 11 ~ ~ STORAGE _: l, `, ~ fir ~ OUTFLOW \ ' ~ ' cite 140 <' 0 0 120 ~ a) 100 ~ a) a) 80 ~ Cd 60 ~ o 40 20 o 12 13 14 15 16 17 18 19 20 21 22 23 24 25 February 1 986 FIGURE 2.1 Flood operation of Folsom Lake during the February 1986 flood. Note that at the start of the flood, Folsom Lake was encroached within the 400,000 acre-feet nomi- nally reserved for winter flood regulation. SOURCE: Bureau of Reclamation, 1986. arises from a number of factors, including potential changes in flood regime due to changes in climate and watershed conditions and to changes in political and economic demands on reservoir storage space. Gated Auburn Dam In the 1991 ARWI, the Sacramento District was criticized because it in- cluded gated outlet structures in its preliminary designs of the proposed flood control dams at Auburn. The argument against gates was that a gated flood control reservoir could be converted in the future into a multipurpose reservoir. In the 1994 Alternatives Report, the District held firm on its inclusion of gates, and even increased the number. The committee fully concurs with this decision on the grounds that gates are essential to safety and flood control efficiency and, as discussed later, because gates allow operational flexibility that can be used to . . . . . minimize environmental Impacts. The proposed 425-foot dam at Auburn (the 1991 "recommended" alterna- tive) would be well over twice the height of any ungated dams constructed by USACE. As with any dam of this height, there would be uncertainties regarding potential cavitation damage to the main spillways and scour at the downstream toe. If such damage occurred during a major flood, gates would make it possible

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 49 to reduce flows to less damaging levels, greatly reducing the possibility of dam failure. For this reason alone, it would be extremely unwise to construct a 425- foot-high dam without gates. The ability to control flows during a flood event would also facilitate emergency actions in the American River downstream of the dam and at Folsom Reservoir. Finally, gated outlet works make it possible to operate the dam conjunctively with Folsom Dam, potentially providing both improved flood control efficiency and reduced environmental damage in the canyon. Offstream Flood Control Storage on Deer Creek In the 1991 ARWI, the Sacramento District considered a measure involving diversion of American River flood flows to a detention basin in the Deer Creek watershed. On the basis of preliminary calculations, this measure was deter- mined to be very costly compared with other measures and hence was dropped from further consideration. Subsequently, the Sacramento District developed a conceptual design for a Deer Creek project that alone would be able to provide Sacramento with a 200-year level of protection (USAGE, Sacramento District, 1994b). The project provides for diversion of American River flood flows from Folsom Reservoir via a connecting channel to a detention basin in the Deer Creek watershed about 10 miles south of Folsom Reservoir. Design of the Deer Creek project assumes that the seasonal flood control storage in Folsom Reservoir will remain at 400,000 acre-feet. Releases would be made from Folsom Reservoir to the Deer Creek detention basin only after it had been determined that the Ameri- can River had achieved the objective release of 115,000 cfs from Folsom Reser- voir. During nonflood periods, no water would be stored in the Deer Creek detention basin. (Apparently, no water supply objective for the Deer Creek reservoir was proposed or included in the Sacramento District's investigation.) Estimated capital costs for the project range from approximately $1.2 billion to $1.8 billion depending on project design. This represents approximately $2,500 per acre foot of storage. This can be compared to the cost of $67 per acre foot of storage for the 1991 NED Auburn Dam alternative. The August 1994 draft analysis of the proposed project (USAGE, Sacra- mento District, 1994b) concluded that the project is technically feasible, but expressed serious doubt about the social feasibility. The 1994 Alternatives Re- port repeated similar conclusions. The report indicated that there are major concerns about potential environmental impacts, specifically with respect to rare and endangered species, that would require expensive coordination and consulta- tion with the U.S. Fish and Wildlife Service. In addition, the report indicated that there would be significant land use conflicts and high land costs, which would range from $50,000 to $200,000 per acre in some areas. The report also noted that construction of a 300-foot-wide channel connecting Folsom Reservoir to the proposed detention site on Deer Creek would pass through the middle of a num

so FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN her of developments and would have a severe effect on the overall development plan for the area. The report concluded that because of the lack of "social feasibility" of the Deer Creek project further study of offstream flood control storage on Deer Creek should be discontinued. The Sacramento District's analysis apparently only investigated the potential for a 600,000-acre-foot storage reservoir that would not receive any water from Folsom Reservoir until the objective release of 115,000 cfs from Folsom Reser- voir had been reached. The study did not report on the possibility of including a smaller Deer Creek reservoir together with a combination of other measures in order to produce an overall package of flood control measures to provide 200- year protection to Sacramento. It may very well be that such a package would not be competitive with the alternatives that were retained in the 1994 Alternatives Report, and that USACE analysts were able to reach that conclusion on the basis of their analysis of the full Deer Creek project. If so, discussion of this conclu- sion in the 1994 Alternatives Report would have forestalled potential criticism. Nonstructural Measures The flood protection alternatives considered in the 1991 ARWI and the 1994 Alternatives Report consist largely of structural measures (e.g., reservoir storage, levee improvement, increased channel conveyance). Nonstructural measures, including floodplain zoning, relocation, flood warning, floodproofing, minimum elevation building design, mandatory insurance, and evacuation capabilities re- ceived little consideration. This omission of nonstructural measures is discussed in more detail in Chapter 5. The committee believes that nonstructural measures can make a significant contribution to flood damage reduction, especially to flood damage reduction in currently undeveloped areas such as Natomas. There- fore, the committee recommends that nonstructural flood damage reduction mea- sures be evaluated together with the structural measures for implementation in the American River watershed. FLOOD RISK REDUCTION FROM ALTERNATIVE PLANS Once alternative plans have been developed, they must be evaluated care- fully. The most critical performance criterion of a given plan is its expected net benefits, the difference between the expected benefits of the plan and its costs. For a flood control project, the expected benefits consist mainly of the difference between the expected value of flood damages with and without the project. At any location, the expected value of flood damages is the integral over all possible flood stages of the product of the flood damage that would occur at a given stage and the probability of that stage. To evaluate a given project, it is necessary to estimate and total the expected value of flood damages at all locations subject to flood damage, with and without the project. This requires a complicated set of

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 51 interrelated calculations, based on statistical, hydrologic, hydraulic, geotechnical, and economic models. The analysis framework that the Sacramento District used for these calculations is the "design event" method. A design event is a hypothetical flood that produces a unique set of flows and stages, and hence damages, throughout the project area. The design floods are characterized by a single measure of event magnitude (such as peak flow or maximum 3-day volume) and a corresponding exceedance probability. It is assumed that at all locations in the project area subject to flood damage, the exceedance probability of the flow, stage, and corresponding damage is equal to that of the design event magnitude. Hence in the case of a 100-year design flood, the damage produced at all locations in the project area is assumed to have an annual exceedance probability equal to 0.01. The application of the design event method to the American River is some- what complicated. The starting point in the analysis is Folsom Reservoir. On the basis of a long-term streamflow record from the gaging station just downstream of the reservoir and the record of storage changes in Folsom Reservoir, the Sacramento District estimated the inflows to Folsom Reservoir and the probabil- ity distribution of rain-flood inflow volumes for various durations. For each exceedance probability considered, the District then constructed a "balanced" design hydrograph based on the corresponding flood inflow volumes. For an alternative without upstream storage, each inflow design hydrograph was routed through Folsom Reservoir. For an alternative with an upstream reservoir, the Folsom inflow design hydrograph was separated into two components reflecting the inflows to the upstream site and the inflows from the remainder of the Folsom drainage basin. The upstream inflows were routed through the upstream reser- voir and the resultant upstream outflow hydrograph was recombined with the inflows from the remainder of the Folsom drainage basin. The recombined inflow hydrograph was then routed through Folsom Reservoir. The discharge from Folsom Reservoir was then augmented to account for the additional drainage area between Folsom Dam and downstream locations. The additional discharge was determined from a rainfall runoff model of the contrib- uting drainage areas, based on a design storm with the corresponding exceedance probability. Hydraulic analysis was then used to determine stage hydrographs for the design event. For the lowest part of the river, the hydraulic analysis was particularly complex because of the complicating effects of water levels in the Sacramento River and in the bypass system. Next, the Sacramento District esti- mated the damage associated with each design event. This was based largely on the stage and flow hydrographs at critical locations where it was expected that levee failure would first occur. Finally, the Sacramento District estimated the expected value of flood damages for each alternative, including a without-project alternative, by integrating the product of damages and the corresponding exceedance probabilities. The design event concept has been used for well over SO years to design

52 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN engineering works involving peak flows. The classical application of this con- cept is the design of storm drainage systems in urban areas. Typically, the area of interest is small, the hydrology can be characterized by a single parameter such as maximum rainfall depth accumulated over the time of concentration over the watershed, and there is a one-to-one correspondence (such as the rational for- mula) between hydrologic inputs and outputs. In more general applications of the design event concept to a particular flow system, it is assumed that the maximum flows at all points of interest in the system can be expressed adequately in terms of one-to-one correspondences between the flow and a single numerical param- eter called the design event magnitude. Under this assumption, the peak flows at all points of interest have the same exceedance probability as the corresponding design event magnitude. Over the years, however, design event methods have been applied to increas- ingly more complicated design problems, in which many factors affect the flows within the system. For example, in reservoir storage systems, the design event magnitude may be characterized by the maximum 3-day inflow volume, but other factors relating to the time distribution and shape of the inflow hydrograph may have important effects on the reservoir outflows and on flows at critical points downstream. Under these conditions, the downstream flows are not determined solely by the design event magnitude, and the assumption that downstream flow exceedance probabilities are equal is no longer valid. Nonetheless, to avoid complexity in the risk and expected damage computa- tions, designers have tended to retain the use of the event-magnitude exceedance probability and to adopt conservative fixed values of the secondary factors. For example, the so-called "balanced" design hydrograph used in the 1991 ARWI studies is synthesized by assuming that the maximum 1-, 3- and 4-day volumes under the hydrograph all have the same specified exceedance probability as de- termined from the flood volume frequency curves; other hydrograph shapes and combinations of hydrograph volumes and probabilities are ignored for simplicity. The so-called "operational contingencies" used by USACE are other examples of such assumptions. These assumptions are made to protect the public at risk by providing some additional margin of safety. But simplifying assumptions, if overly conservative, can lead to upwardly biased flood risk estimates, and in turn to inefficient projects. In the case of contentious projects, such as flood control for Sacramento, the conservative assumptions also can be lightning rods for . . . criticism. Consider the design situation on the American River. The design events are a set of "balanced" inflow hydrographs to Folsom Reservoir, each with an as- sumed exceedance probability. But the probability distribution of peak flood discharges at downstream locations depends on a number of factors, including the time distribution and shape of the inflow hydrograph (in addition to its mag- nitude), initial encroachment of Folsom Reservoir, actual reservoir operating decisions, contributions of downstream tributaries, concurrent flows and levels in

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 53 the Sacramento River and bypass system, and the flows and water levels at which levees fail. In applying a design event method to the American River, the Sacra- mento District made a number of assumptions about these factors based on the engineering judgment of its analysts. In order to evaluate the appropriateness of any of these assumptions, it is necessary to determine whether other reasonable assumptions would have resulted in significantly different answers. In the 1991 ARWI, the results of sensitivity analyses were presented for several of the operational contingency assumptions. The new methods that USACE is developing to evaluate risk and uncertainty may reduce the need to make conservative assumptions about some of the critical factors, such as the flows and levels at which levees fail. Appar- ently some of the "contingency assumptions" made in the 1991 ARWI were handled through uncertainty analysis in the 1994 Alternatives Report. Unfortu- nately, the committee was not able to review the details of this analysis. Instead Chapter 4 gives a general evaluation of the new methods. In considering the methods and assumptions used by the Sacramento District to estimate flood damages, the committee attempted to evaluate the significance of the assumptions, as well as comment on their reasonableness. Correctness, per se, was rarely the issue. The committee was not able, however, to do formal sensitivity analysis and hence was not always able to reach firm conclusions. In such cases the committee merely indicated that the particular assumptions war- ranted further investigation. The committee's evaluation focused on the most critical components of the flood damage estimation: (1) development of design hydrographs at Folsom for unregulated conditions (estimation of probabilities); (2) development of design hydrographs below Folsom (accounting for the effects of Folsom and Auburn reservoirs); (3) computation of stage hydrographs at criti- cal damage locations; and (4) estimation of damages at critical damage locations (determining the probability at which levees fail). Generally speaking, these components correspond to hydrologic' hydraulic, and geotechnical modeling. Each of these modeling components is discussed below, with an emphasis on the methods and assumptions that have generated the most controversy and that the committee judged to be most critical. Most of the discussion focuses on the 1991 ARWI, since the 1994 Alternatives Report does not provide supporting technical documentation. Several critical issues that emerged during the committee's re- view of the USACE analysis are also discussed. Development of Inflow Design Hydrographs for Unregulated Conditions Development of the unregulated design hydrographs for use as inflow hydro- graphs for Folsom Reservoir is a key component of the design process, because it is here that probabilities are introduced into the process. There are two basic steps: estimation of the probability distribution of unregulated flood volumes and construction of unregulated hydrographs.

54 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN Estimation of the Probabilities of Unregulated Flood Volumes Estimation of the probability distribution of unregulated flood volumes was based on analysis of stream gage data collected at Fair Oaks since 1905. These data were adjusted to remove the effects of storage in Folsom Reservoir and in five upstream reservoirs. Series of adjusted annual maximum flows, representing unregulated flows to Folsom Reservoir, were developed for durations of 1, 3, 5, 7, 10, 15, and 30 days, for both rainfall and spring snowmelt floods. Probability distributions for these series were estimated using the program REGFQ: Re- gional Frequency Computation (USAGE, 1982), which was written at the Hydro- logic Engineering Center based on a method described by Beard (1962~. Only the estimated distribution for rainfall floods was used in subsequent analyses. (In cases where flooding is due to distinct climatic mechanisms, such as rainfall and snowmelt, it is prudent to separately analyze annual flood series from each mecha- nism. Because large floods on the American River never result from spring snowmelt events, the estimated probability distributions of the spring snowmelt volumes are not needed in the subsequent analysis.) The program REGFQ estimates the parameters of the Pearson Type III distri- bution for the logarithms of flow using the method of moments. On the basis of examination of the REGFQ user's manual (USAGE, 1982), it appears that the program is consistent with guidelines in Bulletin 15, published in 1967 by the Water Resources Council (WRC, 1967) to provide federal agencies with a uni- form technique for estimating flood flow probabilities. REGFQ does not incor- porate subsequent modifications to the recommended techniques, presented in Bulletin 17B (IACWD, 1982~. The most significant of these modifications in- volve adjustment for historical floods, estimation of generalized skew, and test- ing and accounting for outliers. REGFQ also includes one feature that is not included in Bulletin FIB: adjustment of the log-space moments to ensure that for all probabilities of interest the corresponding d-day average flow is always a decreasing function of duration, d. This is done by developing smoothed rela- tionships between log-standard deviation and log-mean and between log-skew and log-mean for the various durations considered. Once probability distributions have been estimated for the various durations, REGFQ applies an expected probability adjustment to the estimated flow quan- tiles. This adjustment was developed by Beard (1960) to ensure that nationwide failure rate experience for statistically designed structures would be consistent with the failure probabilities adopted for the design. However, as discussed in Chapter 4, use of the expected probability adjustment does not yield unbiased estimates of the risk of flooding or expected damages. To evaluate the significance of the various idiosyncrasies of the estimation procedure used by USACE, probability distributions of the Fair Oaks rain and flood data from 1907 through 1986 were estimated in a manner consistent with Bulletin FIB. In making the calculations, the lowest rain-flood data point was

IDENTIFICATION AND EVALUATION OF ALTERNATIVES TABLE 2.4 Comparison of Quantile Estimates for 1-Day and 3-Day Mean Flows of Annual Rain Floods 55 1-Day Quantile (1,000 cfs) 3-Day Quantile (1,000 cfs) USACE USACE Recurrence (without (without Interval Bulletin expected Bulletin expected (years) ~ 7B USACE probability) 17B USACE probability) 25 151 165 158 112 115 110 50 198 220 210 149 150 145 100 253 285 271 192 195 185 200 316 360 341 243 245 231 500 413 485 453 323 325 304 NOTE: The upper and lower 95-percent confidence limits for Bulletin 17B 100-year 1-day flows are 190,000 and 363,000 cfs, and for 100-year 3-day flows are 138,000 and 267,000 cfs. found to be a low outlier; it was removed and the conditional probability adjust- ment was used. The resulting quartile estimates for the 1- and 3-day mean flow are shown in Table 2.4. For the 1-day flows, USACE estimates are higher than those based on Bulletin 17-B by 9 to 17 percent. For the 3-day flows, USACE estimates are virtually identical to those based on Bulletin 17B. Also shown in Table 2.4 are USACE estimates without the expected prob- ability correction. For the 1-day flows, about half of the difference between USAGE's quartile estimates and those based on Bulletin 17B is due to the use of the expected probability correction. Most of the remaining difference is due to the application of the Bulletin 17B correction for low outliers. In the case of the 3-day flows, for which there were no outliers, USACE quartile estimates without the expected probability adjustment are 1 to 6 percent lower than those based on Bulletin 17B. It should be noted that the observed differences between the USACE and the Bulletin-17B estimated quartiles are much less than the uncer- tainties in the estimates. Although the effect of the expected probability adjustment is small in rela- tion to the uncertainty in the quartile estimates, the committee concluded that the Sacramento District should not have applied the expected probability adjustment. The purpose for which the adjustment was developed is not relevant in the Sacra- mento situation, and, as explained in Chapter 4, the adjustment yields biased estimates of level of protection and expected damages. It appears that the Sacra- mento District did not use the correction in its analysis supporting the 1994 Alternatives Report; however, as discussed in Chapter 4, the committee disagrees with the procedure the District did use in the 1994 Alternatives Report to estimate level of protection.

56 225,000- 200,000- 1 75,000- - C' C' I 125 000- ~' 1 50,000- 1 00,000- 75,000- 50,000- 25,000- FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN o ) 0 ~- _ ~ 1~1 ~ ) o- ~ 1 1 1 1 1 1900 1910 1920 1930 1940 1950 Year I . I 1960 1970 1980 1990 FIGURE 2.2 Annual maximum daily flow, American River at Fair Oaks, 1906-1986, adjusted for affects of regulation. A critical issue in the estimation of probabilities of unregulated flood vol- umes is the apparent increase in the frequency of large floods in the Fair Oaks record since 1950 (Figure 2.21. The six largest annual maximum 1-day flood volumes in the Fair Oaks record (adjusted for Folsom effects) occur in or after 1950. This apparent increase in flood magnitudes has led to historically increas- ing estimates of the vulnerability of Sacramento to catastrophic flooding. It is clearly a very important issue, and yet the committee was unable to discover any scientific studies that explain the apparent increase in flood magnitudes. Later in the chapter the discussion returns to this issue. Construction of Inflow Design Hydrographs-Unregulated Conditions For each exceedance probability evaluated, an inflow design hydrograph was constructed that preserved the appropriate volumes. In the 1991 ARWI the duration of the design hydrograph is 4 days. Hence, for a given exceedance probability, the maximum 1- and 3-day volumes of the associated design hydro- graph equal the 1- and 3-day volume quartiles with the same exceedance prob- ability. Similarly, the total volume of the design hydrograph equals the 4-day

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 57 volume, which was estimated by averaging the flow rate associated with the 3- and 5-day volumes. The general shape of the hydrograph is based on the prob- able maximum flood hydrograph developed in a 1980 study evaluating the ad- equacy of the Folsom spillway. In the 1991 ARWI, the 100-year flood inflow hydrograph was reduced by 47,000 acre-feet to account for storage in upstream reservoirs. The reduction was made at a linear rate for the first day of the design hydrograph. No adjustment was made for floods greater than the 100-year flood, based on the assumption that no storage would be available for these extreme events. The Sacramento District's correction for upstream storage has been criti- cized as unduly conservative (Williams and Gallon, 1987; Swanson and Associ- ates, 1992~. On the basis of a compilation of data on the storage available in the largest upstream reservoirs 15 days prior to the annual maximum flood, Swanson and Associates (1992) estimated that the value of 47,000 acre-feet used by USACE has an exceedance probability of about 98 percent. (That is, in 98 percent of the years, the available storage exceeded 47,000 acre-feet.) The mean available storage was estimated to be 289,000 acre-feet, an amount equal to about 70 percent of the available flood storage at Folsom. Williams and Gallon (1987) also criticized the fact that the Sacramento District adjusted only the first day of flow on the design hydrograph, arguing that because of the travel time involved the adjustment should have been made on the second, and most critical, day of the hydrograph. The Sacramento District's response to these criticisms is that the upstream reservoirs are not operated for flood control and cannot be counted on for storage in large floods. Furthermore, the reservoirs are located in the upper part of the watershed, capturing runoff from only 14 percent of the total water- shed area. The Sacramento District clearly made conservative assumptions in account- ing for upstream storage. The most critical of these is that upstream storage would provide no benefits during events as rare as or rarer than the 1 percent event. What is the potential impact of this assumption, and was it too conserva- tive? Consider again Table 2.3, which gives for various recurrence intervals the volume of storage required to control the design hydrograph to 115,000 or 180,000 cfs. The mean available upstream storage of 289,000 acre-feet is about 38 percent of the volume required to control the 200-year event, for an objective release of 115,000 cfs. Hence it is a significant amount of storage. However, the effective storage potential in these reservoirs is much less because of their up- stream location. The reservoirs were designed to be drawn down to low levels in the fall and re-filled by spring snowmelt, not by winter rainfall floods. Although the available flow records are not easy to interpret, the reservoir storage records show only relatively minor storage increases, consistent with the 14-percent wa- tershed area figure, for the major Folsom floods. In all cases the stored water was retained in the upstream reservoirs for at least several weeks after the flood. This is consistent with the use of the reservoirs for hydropower generation. It appears,

58 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN therefore, that the upstream reservoirs have limited flood control value not be- cause their available capacity is not effectively utilized but because they are located upstream from most of the significant rain-flood runoff-producing area. Furthermore, the operators of the upstream reservoirs have no responsibility for providing flood control for Sacramento. How can flood control credit be allo- cated to that storage when it is not managed for flood control? Fortunately, the Sacramento District has found an effective way to exploit the potential benefits of the upstream storage (USAGE, 19931. This can be done by adjusting the Folsom flood control space in concert with the available storage in the upstream reser- voirs, as specified in the Folsom reoperation plan discussed previously in this chapter. Development of Design Hydrographs Below Folsom Accounting for Effects of Folsom Dam For events with an exceedance probability of greater than about 2 percent, the effects of Folsom storage were accounted for by directly using 32 years of flows measured at Fair Oaks. For rarer events, design hydrographs were routed through Folsom, based on several assumptions: · initial flood control storage encroachment of 80,000 acre-feet; · initial release of 20,000 cfs; · outflow lags inflow by 4 hours; · releases increased by a maximum of 7,500 cfs/hr and decreased by a maximum of 5,000 cfs/hr; · for storage at or below main spillway, release based on full capacity of river outlets; · for storage above main spillway, release based on main spillway, but not river outlets; and maximum surcharge storage of 50,000 acre-feet, as prescribed by the emergency spillway release diagram. These assumptions were subject to considerable criticism. The assumed initial flood encroachment, which accounts for flood storage that is used by a lesser flood event preceding the design event, was criticized as being too large. The operating assumptions and the assumption of a maximum surcharge storage of 50,000 acre-feet were criticized as being too conservative. The initial flood control storage encroachment accounts for the occurrence of a storm event in advance of the design event. It is another example of an uncertain factor about which ad hoc assumptions have to be made when using the design storm method. It should be noted from Table 2.3 that the assumed en- croachment of 80,000 acre-feet is about 10 percent of the volume required to

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 59 control the 200-year event to 115,000 cfs and about 7 percent of the volume required to control the 400-year event to the same flow. In the 1994 Alternatives Report, the Sacramento District accounted for the occurrence of an antecedent storm by using a two-wave design hydrograph. The committee found no evi- dence that would lead to the conclusion that either of these approaches was inappropriate. In considering the assumptions in the 1991 ARWI having to do with routing of water through Folsom, the committee held to the belief that the assumptions used in the analysis of a reservoir should accurately reflect the operating rules of the reservoir. The actual operation of Folsom Reservoir during the 1986 flood event was not as efficient as the assumptions used in the 1991 ARWI. (For details, see Box 2.2 and Figure 2.1, earlier in this chapter.) After the 1986 event, modest improvements were made in the operation of Folsom Reservoir. More recently, the Bureau of Reclamation and USACE have been working on a man- agement plan that would further improve the operational efficiencies of Folsom Reservoir. Given the state of flux in the operations of Folsom, the committee did not find the 1991 operational assumptions to be unreasonable. Because the committee did not have documentation supporting the 1994 Alternatives Report, it was not able to examine in detail the methods used to evaluate the operational assumptions regarding Folsom Reservoir. It is the committee's understanding, however, that the assumptions do not reflect the improvements in operational efficiency that would be possible with telemetered discharge information and other operational changes, particularly with respect to the alternatives involving new outlet works. For example, the committee was told that the 1994 analysis assumed a 10-hour delay in initiating releases from Folsom prior to the second flood wave, if the flood reservation is evacuated after the first flood wave. A 10-hour delay seems to be too long, especially if tele- metered flow data are available from upstream gages. Ten extra hours of flow at 115,000 cfs represents about 12 percent of the total volume required to control the 200-year flood to 115,000 cfs (Table 2.3~. Hence this amount is large enough to warrant further consideration of the reasonableness of the assumption. The committee was unable to evaluate the way in which USACE incorpo- rated the Folsom reoperation plan in the analysis supporting the 1994 Alterna- tives Report. As previously discussed, this plan requires that the available flood storage in Folsom be adjusted in accordance with the status of storage in the French Meadows, Hell Hole, and Union Valley reservoirs. This dependence of Folsom operation on storage in upstream reservoirs is an example of the second- ary factors that complicate the use of the design event concept. Apparently, USACE attempted to consider this particular factor in its new risk and uncer- tainty procedures, although the committee was not able to evaluate the way in which this was done.

60 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN Accounting for Effects of Auburn Alternatives Design inflow hydrographs for Auburn alternatives were constructed by scal- ing the Folsom design inflow hydrographs by 67 percent, to account for the additional drainage area contributing to Folsom. The actual percentage used was based on drainage area ratios, analysis of normal annual precipitation, and his- toric flood flow data. The design hydrographs were routed through the Auburn alternatives; the resulting outflow hydrographs were then used as design inflow hydrographs to Folsom Reservoir. The committee was not aware of any criti- cisms of the District's approach to accounting for Auburn storage and did not investigate the issue in depth. Accounting for Downstream Inflows For each design event, discharge from Folsom Reservoir was augmented to account for the additional drainage area between Folsom Dam and downstream locations. The additional discharge was determined from a rainfall runoff model of the contributing drainage areas, based on a design storm with the correspond- ing exceedance probability. The committee was not aware of any criticisms of the Sacramento District's approach to this portion of the analysis and thus did not investigate the issue in depth. Hydraulic Modeling The next step in the analysis was to estimate stage hydrographs for critical locations on the lower American River. In the 1991 ARWI, this was done by using a one-dimensional gradually varied flow analysis. For the lowermost por- tion of the American River, which is affected by water levels in the Sacramento River, this analysis required assumptions of concurrent water surface elevations in that river. This section considers the appropriateness of those assumptions as well as the adequacy of the methods used in the hydraulic modeling. Assumptions About Confluence of American and Sacramento Rivers In modeling water levels in the portion of the American River affected by water levels in the Sacramento River, assumptions must be made about the mag- nitude of the flood hydrograph on the Sacramento River and its timing relative to that of the American River. With regard to timing, the Sacramento District used the same relative timing as occurred in the 1986 flood event. This meant that the peak of the American River hydrograph was assumed to occur about one-half day ahead of that of the Sacramento River. With respect to flood magnitude, the District assumed the occurrence of a 100-year event on the Sacramento River when modeling the 100-, 200-, and 400-year events on the American River.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 61 The use of the 1986 flood event as a model for relative timing was criticized because the sluggish operation of Folsom Reservoir during that event delayed the arrival of the American River hydrograph (Williams and Gallon, 1987~. If Folsom had been operated so that outflows equaled inflows, the American River peak would have arrived at the junction about one and one-half days earlier. Swanson and Associates (1992) estimated that had this occurred the peak flow at the confluence would have been reduced by about 30,000 cfs (Swanson and Associ- ates, 1992), a reduction of about 6 percent. Assumptions about the level of the Sacramento River during a flood event on the American River are required by the event focus of the design flood method. The District clearly made conservative assumptions, but in the absence of a very elaborate analysis of the joint occurrences of floods on the two rivers (such as discussed by Dyhouse (1985) for case of the Missouri River), the assumptions are reasonable. As the Sacramento District demonstrated in the 1991 ARWI, flood peaks on the two rivers can occur within 1 or 2 days of each other. Furthermore, the hydrographs of both rivers are typically broad, so that a 1- or 2-day lag in peaks does not significantly affect the peak of the combined flows. Hydraulic Models In the analysis supporting the 1991 ARWI, the model HEC-2 was used to compute water levels at various locations on the lower American River. HEC-2 is a USACE model for computing water-surface profiles of one-dimensional, steady-state, gradually varied flows. In recognition of the unsteady nature of flood flows, USACE subsequently has developed a one-dimensional unsteady flow network model of the lower American River, based on the USACE model UNET. The committee supports this change in modeling approach. For levee overtopping or failure, the UNET model does not consider the momentum con- servation between the river flow and the flow in the floodplain and thus the velocity and direction of flow in the floodplain are not properly calculated. In order to estimate the flood residual risk behind the levees, a two-dimensional unsteady flow model is needed to calculate the force and momentum of the flows to assess the possible damage, warning systems, floodproofing and evacuation. This two-dimensional model is also needed to calculate the flow behavior at the junction between two or more rivers such as the occurrence of flow separations downstream of the junction. These flow separations may cause the formation of eddies to cause excessive bank erosion. A one-dimensional model cannot predict flow separation. During the 1986 flood, sediment accumulated upstream from the Fremont Weir up to about 1 foot above the weir crest, although some of this material probably was deposited over time in earlier years. This sediment deposit blocked the flow over the weir. Thus, this modeling effort should also consider the

62 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN potential role of sediment accumulation at the various control structures, espe- cially at the Fremont Weir. Hydraulic modeling of the potential affects of a dam break due to earthquake should be investigated if construction of a new dam at the Auburn site is pursued because such a dam would be very close to an earthquake fault. Geotechnical Analysis Once flood peak flows and water levels were computed throughout the lower American River, the District estimated the damages that would occur for each of the design events considered and for all alternatives, including the without-project conditions. For alternatives involving increased objective releases, the process was more complicated. Before evaluating the damages associated with these alternatives, it was first necessary to design and cost measures for improving the channel and levee system to handle the increased objective releases. This is an extremely critical part of the analysis, since project costs are obviously an impor- tant factor in the decision process. The objective releases investigated by the Sacramento District in the 1991 American River Watershed Investigation are 115,000, 130,000, 145,00O, and 180,000 cfs. The feasibility of conveying these objective releases in the Ameri- can River channel downstream from Folsom Dam is determined in part by the adequacy of the downstream levees and revetments to contain flows within the channel without failure. Where the investigation showed that levees became unstable for flows above 130,000 cfs, the Sacramento District proposed to stabilize the levees with slurry cutoff walls. Flood releases over 165,000 cfs would overtop existing levees and require raising longer reaches of levee. With the higher objective releases, some areas upstream from the project levees would require new levees or floodwalls. Setback levees were also considered by the Sacramento District in its alterna- tives. These levees were considered for the lower American River in order to confine it to a narrow corridor. A component of the 1991 ARWI and the 1994 Alternatives Report focused on determining what additional work would be necessary to the levees and chan- nel revetments in order to allow conveyance of the objective releases with a reasonable degree of certainty. To answer this question, the Sacramento District investigated the possibility of levee failure by breaching, overtopping, seepage under the levee, and other causes. The following sections contain an analysis of the methodologies and data used by the District in their levee investigations, together with some suggestions for improving the analysis. Further analysis of the geomorphology conditions in the American River basin that are relevant to the levee investigation are discussed in a later section.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES The 1991 ARWI Analysis 63 As indicated in the 1991 ARWI, levees along the lower American River have been constructed and modified over many years. Near downtown Sacramento, the levees were originally designed to accommodate a peak flow of 180,000 cfs. Today, with the existence of Folsom Reservoir, flood flows can be attenuated for longer duration, but the levees cannot safely pass a sustained flow of 180,000 cfs. After the February 1986 flood, extensive geotechnical evaluation of the levees was conducted (USAGE, Sacramento District, 1991, Appendix M). The ARWI concluded that there are reaches of levees that will exhibit structural deficiencies with sustained flows as low as 130,000 cfs. The ARWI concluded that levees along the lower river are believed to be able to safely accommodate a sustained flow of only 115,000 cfs. The 1991 ARWI evaluation of levee failure on the lower American River concentrated on failure caused by encroachment on freeboard. The elevations at which the levees might fail were determined based on a projection of the impacts of various water levels on the physical system. Failure projections were based on varying degrees of encroachment, knowledge of levee conditions, exposure to high velocities or wave run-up and overtopping, and levee performance during the February 1986 flood. The analysis of levee failure was based on several factors, including: · The assumption that the levee improvements described in the Sacramento River Urban Flood Control System Evaluation, Phase 1 (Sacramento Urban Area) would be complete (i.e., Sacramento area levees are stable up to their design flow). · The observed condition of the levees in relation to geotechnical evalua- tion of the function of the system during the February 1986 high flows. · Hydrologic observations and forecasts developed in the Hydrology Ap- pendix to the 1991 ARWI. On the basis of the these parameters and procedures, the 1991 ARWI devel- oped an estimate of potential levee failures (Table 2.5) that specified the maxi- mum flow or stage that could occur on a specific reach of levee before failure. This analysis was deterministic and was based primarily, if not entirely, on re- maining freeboard. The 1991 ARWI also qualified this estimate of potential levee failure by indicating that these estimates are "for flood damage estimates only. Actual levee failures may occur at higher or lower flows and stages" (Table 2.5~. On the basis of this deterministic evaluation of levee reaches that would fail at varying flows and/or stages, the 1991 ARWI detailed a list of levee and chan- nel modification projects (including necessary revetment) to increase channel capacity of the lower American River in order to safely pass the objective re- leases of 130,000, 145,000, and 180,000 cfs (Table 2.6~. Necessary channel and

64 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN TABLE 2.5 Potential Levee Failure Levee Reach Remaining Freeboard at Failurea (feet) Stage or Return Period Flowsb (years) Reclamation District 1000 Sacramento River (left bank) NCC to NEMDC NCC (north and south levees) NEMDC (west levee) American River Levee System North (right) bank, Sacramento River to river mile 5.2 North (right) bank, upstream from river mile 5.2 South (left) bank, Sacramento River to river mile 5.2 South (left) bank, river mile 5.2 to river mile ~7.8 South (left) bank, upstream from river mile 7.8 Dry and Arcade creeks, and east levee of the NEMDC Sacramento River east (left) bank from the American River to Freeport Sacramento River west (right) bank from the Sacramento Bypass to Riverview Yolo Bypass and Tributary levees Sacramento River west (right) bank from the NCC to the Sacramento Bypass 2d 1 .5d 4 4 3 3 3 40.0 feet 35.4 feet 180,000+ cfs 140,000 cfs 200 71 85+ 71 140,000 cfs 71 145,000 cfs 73 200,000 cfse 94 d d _d NOTE: For flood damage estimates only. Actual levee failures may occur at higher or lower flows and stages. aAssumptions: (a) levee rehabilitation as part of the Sacramento River Flood Control and Sacra- mento River Bank Protection Projects in Sacramento area has been completed, and (b) the remaining sediment in Fremont Weir has been removed. bUnless otherwise noted, flows are at Fair Oaks gage. CNot applicable due to failure at other locations reducing threat. dFreeboard encroached condition chosen based on February 1986 flood conditions. eNondamaging flow is approximately 145,000 cfs. Levee failure is not the cause of flood damage on Dry Creek. gFor evaluation of flood damages, zero remaining freeboard was selected to be consistent with FEMA's approach to establishing failures. SOURCE: USACE, Sacramento District, 1991.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES TABLE 2.6 Summary of Levee and Channel Modifications to Increase Channel Capacity of Lower American River 65 Objective Release 130,000 cfs 145,000 cfs 180,000 cfs Lower American River (miles) Slurry wall 0.7 0.9 4.1 Toe drain 0.6 2.7 7.8 New levee 0.9 1.0 1.0 Levee raising 0.0 2.7 11.4 Riprap on bank 1.5 1.5 1.5 Riprap on levee 5.3 5.3 5.3 Riprap on bank and levee 3.2 3.2 3.2 Yolo Bypass Sacramento Weir Other Extensive levee raising on both sides south of Sacramento Bypass Lengthen 500 feet Lengthen 1,400 feet Raise Union Pacific Railroad Relocate American River Parkway Access Road Replace Main Replace Main Avenue Bridge Avenue Bridge and Norwood Avenue Bridge Lengthen 3,600 feet Raise H Street bridge; replace E1 Camino, Howe Avenue, Main Avenue, and Norwood Avenue budges; replace American River bike trail; replace fencing SOURCE: USACE, Sacramento District, 1991. levee modifications included slurry walls, toe drains, new levees, levee raising, bank riprap, levee riprap, and various combinations of these projects. Subsequent Investigations The 1991 ARWI generally concluded that the levee system was stable for the original design flow (i.e., objective flow) of 115,000 cfs but needed significant remedial work if flows were to be increased to 130,000 cfs or higher. A subsequent report prepared by WRC-Environmental and Mitchell Swanson and Associates (1992) reviewed the 1991 ARWI. A major conclusion from this review was that there was very little difference between the hydraulic character- istics of 115,000 cfs and 130,000 cfs and, therefore, that the system was not safe for the design flow of 115,000 cfs.

66 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN In addition, a later report by Resource Consultants and Engineers, Inc. (1993) was prepared for the Sacramento District. The scope of this effort involved a technical review of the stability and seepage analysis in the 1991 ARWI. The review and subsequent analysis were accomplished using the existing published data; no additional field investigations, borings, or soil testings were performed for this third report. The Resource Consultants and Engineers, Inc. (1993) report concluded that, In general, the results of the Army Corps of Engineers analysis are reasonable given the assumptions listed above. They show that seepage pressures and the potential for piping failures will go up significantly as the flows are increased above the 1 15,000 cfs level. However, the analysis lacks adequate detail and site-specific data to conclu- sively evaluate the relative stability of the entire levee reach at the flow level of 115,000 cfs. Exit gradients at the landward side can be much higher when a thin confined layer of pervious materials exists either within the levee or the foundation. The stratigraphy of the section can be as important as the value of permeability selected. The permeability test data were based on only two tests of remolded samples. The Resource Consultants and Engineers, Inc. (1993) report also concluded that evaluation of the levee stability analysis in the 1991 ARWI indicated that substantially more information and data are required to evaluate levee stability. The report concluded, Because layering in the levee foundations is important in the assessment of levee stability, it is recommended that foundation investigation borings be con- ducted. Additional levee and foundation configuration should be analyzed for potential seepage and piping problems. Further biaxial shear of soil materials should be carried out to better define the range of conditions within the levees. Stability analysis should be revised after the foundation conditions and the soil strength parameters have been verified. In general, the Resource Consultants and Engineers, Inc. (1993) investiga- tion emphasized the need for significantly more information and data for pur- poses of evaluating levee stability. Recent Work A new risk and uncertainty methodology is under development by USACE, and that methodology was extended by the Sacramento District for this study. The committee was provided with a series of working papers and calculations concerning the risk and uncertainty analysis being developed by the Sacramento District for evaluating the various flood control alternatives, including levees, under consideration for the American River. The 1994 Alternatives Report

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 67 (USAGE, Sacramento District, 1994a) presents the results of the flood control alternatives evaluation using the risk and uncertainty analysis procedures devel- oped by USACE. The 1994 Alternatives Report, however, does not provide additional information concerning the actual calculation procedures employed in the risk and uncertainty analysis. With respect to the analysis of channel capacity to convey the objective releases, USACE will no longer treat stage-discharge and stage-damage func- tions in a deterministic fashion, but will regard these as stochastic functions and will estimate probability distributions for these functions. Risk and uncertainty are incorporated into the analysis by estimating the probable nonfailure point (PNP) and the probable failure point (PFP). The PNP is defined as the water surface elevation below which it is highly unlikely (probability zero) that the levee would fail; the failure probability jumps to 15 percent when the water surface elevation rises above the PNP. The PFP is defined as the water surface elevation above which it is highly likely (probability one) that the levee would fail. For a water surface elevation just below the PFP, the levee would have an 85 percent chance of failure. The failure probability is assumed to vary linearly from 15 to 85 percent for water-surface elevations between the PNP and the PFP. Representative PNP and PFP elevations for the levees were identified at each index location for the without-project conditions. For each alternative the PNP and PFP would be modified to represent the levee modifications proposed for that alternative (USAGE, Sacramento District, 1994~. The percentage chance of levee failure is then calculated based on the PNP and PFP elevations. Therefore, the selection of the PNP and PFP is an important step in the risk and uncertainty analysis. It appears that no additional data were available for estimating the PNP and the PFP elevations and that the recommendations of the Resource Consult- ants and Engineers, Inc. report (1993) concerning insufficient data for dete~in- ing levee failure are still valid. These efforts by USACE to incorporate risk analysis procedures into deci- sion-making concerning the adequacy of the levees are to be commended, but it is apparent that existing data is insufficient to permit effective application of risk analysis to this decisionmaking process, especially with respect to the levees and the estimation of the PNP and PFP elevations. More data are required to com- plete the evaluation of levee stability analysis. Consequently, unless the addi- tional data detailed in the Resource Consultants and Engineers, Inc. (1993) report are developed, it would appear that the PFP and PNP elevations and probabilities are no more reliable than the qualitative engineering judgments in the 1991 ARWI and that use of risk and uncertainty in evaluation of alternatives for the American River flood control project will not necessarily increase the quality of the decisionmaking data base. Given that application of risk analysis procedures will require additional data in order to quantify the parameters of the probability distributions, it appears that a first step in applying this risk analysis procedure should be to acquire the

68 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN additional data detailed in the Resource Consultants and Engineers, Inc. (1993) recommendations. Because the various reports available express uncertainty concerning levee stability, the committee has concerns about the Sacramento District's proposed alternatives for repairing and enlarging the levees to permit conveyance of "ob- jective releases" from Folsom Reservoir larger than 1 15,000 cfs. Before alterna- tives involving raising and enlarging the levees to permit conveyance of 130,000, 145,000, or 180,000 cfs are used in the flood damage reduction project, sufficient data concerning levee stability must be available to provide assurance that the repaired or raised levees can contain these higher flows. USAGE's use of risk and reliability analysis does not eliminate the need for additional levee stability data. A recent report by the Sacramento Area Flood Control Agency's Lower American River Task Force also supports the need for additional geotechnical evaluation of federal and nonfederal levees with respect to seepage and stability at objective releases greater than 115,000 cfs (Lower American River Task Force, 1994). OTHER TECHNICAL ISSUES: FLOOD RECORD AND GEOMORPHOLOGY in reviewing the analysis performed by the Sacramento District in support of the 1991 ARWI and the criticism of this analysis, the committee identified two critical issues that had not been given adequate consideration: the apparent nonrandomness of the American River flood series and geomorphic issues that affect the long-term stability of the lower American River channel. American River Flood Record A critical issue in the management of Sacramento's flood risk is the fact that a high percentage of the largest flows in the unregulated American River flood series at Fair Oaks occurred after the design of Folsom Dam in 1945 (Figure 2.2~. For example, in the series of unregulated maximum daily flows extending from 1907 through 1986, the top six flows occurred after 1950. This has meant that the apparent magnitude of the American River flood threat is now substantially greater than what was apparent when Folsom Dam was designed. Figure 2.3 illustrates the effect of the apparent increase in flood magnitudes on the estimated quartiles for the 3-day rain floods. Shown are the Bulletin 17B frequency curves for the periods from 1905 to 1949, 1950 to 1986, and 1905 to 1986. Also shown are the one-sided upper 95 percent confidence interval for the period from 1905 to 1949 and the one-sided lower 95 percent confidence limit for the period from 1950 to 1986. Note that the estimated 200-year 3-day flood

IDENTIFICATION AND EVALUATION OFALTERNAT~ES 69 500 400 as 0 300 - ' 200 tar c:, 10~) 90 80 70 ~ _ 1 950-1 986 Lower 95% C. I. for 1950-1 986 1 905-1 986 Upper 95% C. 1. for 1 905-1 949 go 1905-1 949 25 50 100 200 500 RECURRENCE INTERVAL (yrs) FIGURE 2.3 Split-record quartile estimates for 3-day deregulated rain floods, American River at Fair Oaks, estimated by Bulletin 17B. Confidence intervals shown are one-sided. volume for the period from 1950 to 1986 is greater than the 500-year 3-day flood volume for the full period. How should we interpret this apparent increase in flood magnitudes? There are several potential interpretations, with differing implications. These include ordinary random variability, nonrandom climatic variability, changes in the wa- tershed, and nonrandomness due to systematic measurement errors. It is possible that the concentration of high flows in the unregulated Fair Oaks data is simply the result of random variability. For example, there is about a 3 percent chance that the top six values in a random series of 80 occur in the first or last 40 values. Such an occurrence has a low probability, but it is certainly possible. Although it is common practice to assume that annual flood series are ran- dom, it is widely understood that climate itself is nonrandom, even at time scales of a few years. (Nonrandomness is defined here to include nonstationarity and correlation.) For example, it has been demonstrated that the quasi-periodic oc- currence of the E1 Nino/Southern Oscillation in the North Pacific Ocean corre- lates significantly with precipitation in the western United States (Swetnam and Betancourt, 1990; Redmond and Kock, 1991; Woolhiser et al., 1993~. However, similar relationships have not been found for floods. And although a few inves- tigators have criticized the operational assumption that floods are random (Knox, 1983; Hirschboeck, 1988), little has been offered in the way of alternatives to

70 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN conventional analysis. One exception is Booy and Lye (1989), which discusses a simple method for accounting for the increased uncertainty in flood risk assess- ment due to apparent nonrandom climatic variability. The committee was not able to find any literature relating the apparent in- crease in large floods on the American River to nonrandom climatic variability. However, there have been relevant studies on seasonal streamflow in the Ameri- can River and nearby rivers. Roos (1987) documented a reduction in the April- July fractional flow from snowmelt in northern Sierra Nevada streams and sug- gested that the cause was slight shifts in seasonal precipitation and broad-scale temperature patterns. (The fractional flow for a given period is the ratio of the runoff for that period to the annual runoff.) Pupacko (1993) analyzed streamflow data from the North Fork of the American River and the East and West Fork of the Carson River and documented a trend of increasing and more variable winter streamflow beginning in the 1960s. He attributed this trend to small increases in temperature, which increase the rain-to-snow ratio at lower altitudes and cause the snowpack to melt earlier in the season at higher altitudes. Aguado et al. (1992) attributed much of the decline in April-July fractional flow to an increase in fractional streamflow earlier in the winter, caused by an increase in precipitation in late fall and early winter, and especially in November. The increase in November precipitation is important, since early season precipi- tation is more likely to run off immediately rather than be stored in snowpack. Aguado et al. (1992J interpreted the decline in spring-summer fractional flow to be a natural climate fluctuation, rather than a greenhouse warming effect. How does this apparent shift in seasonal flow relate to flood flows? A climate change that caused flood-producing storms to occur earlier would result in greater flood magnitudes. The most significant floods on the American River occur in the rainy season, which begins in late fall and early winter. Storms that occur later in the winter are likely to have more precipitation in the form of snow, which does not generally contribute to storm-induced flooding. However, the timing of rainfall floods on the American River does not appear to have shifted during the period of record at the Fair Oaks gage. Apparently, the increase in flood magnitudes on the American River is not directly caused by the apparent shift in seasonal flows. It is possible, however, that both are caused by the same climate change. Tree-ring analysis provides evidence that the climate in the region is subject to nonrandom variability. Using dendroclimatic methods, Earle (1991) recon- structed streamflows since A.D. 1560 for the Sacramento, Feather, Yuba, and American rivers. These reconstructions indicate that there have been a number of periods of prolonged high and low seasonal flows during the past 420 years. Furthermore, the period from 1917 to 1950 is the most extreme dry period in the reconstructed record. It is possible that the end of the dry period, the change in the timing of seasonal flows, and the increase in flood magnitudes are all caused by the same nonrandom behavior of the climate system.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 71 Nonrandomness in the unregulated Fair Oaks series could also result from changes in land use and land management practices in the watershed. The pri- mary land use in the watershed is forestry. The committee did not discover any documentation of changes in forest practices over time. While it is possible that there have been some changes in land use practices, it is unlikely that they alone would have caused the observed increase in large floods. Nevertheless, future land use may change in the American River watershed. Changes from the current land use, which is primarily forest, would most likely increase flood magnitudes. Systematic errors in flow monitoring and in the adjustment of the gaged flows for the effects of regulation by Folsom Reservoir are another potential cause of the apparent nonrandomness of the Fair Oaks unregulated series. The gage has been located at several sites and was non recording prior to 1930. As discussed in the next section, the river channel throughout the Fair Oaks area has degraded significantly through time. The stage-discharge relation used to moni- tor the flows has been re-determined frequently to reflect the changing hydraulic conditions. However, as the main channel has degraded, the flow at which over- bank flow begins has gradually increased. Because over-bank flows are subject to more measurement uncertainty than within-banks flows, it is possible that there may be more uncertainty in the measurements of very large discharges earlier in the period of record than in more recent measurements. In addition, the closure of Folsom Dam has facilitated flow measurements by reducing peak flows and providing extended periods of steady flow, and thus may have resulted in recent flood flow measurements having less uncertainty than those early in the record. Nonetheless, there is no evidence that flow measurement uncertainty has contributed to the apparent nonrandomness of the Fair Oaks flood record. It also is possible that there might be systematic error in the post-Folsom unregulated flood record as a result of the method used to adjust the gaged flows for effects of regulation by Folsom Dam. Based on comparisons of the adjusted and unad- justed flows with Folsom reservoirs contents, however, this also appears hi~hlv unlikely. If it is true that the apparent increase in the magnitude of large floods in the American River is the result of nonrandom climatic variability, what are the implications for estimating the probability of large floods? Clearly, one implica- tion would be that probability estimates are more uncertain than would be ex- pected from conventional analysis. Is the flood record since 1950 more represen- tative of the immediate future than the prior record? If so, use of the full flood record would greatly underestimate the flood risk. There also is the possibility that global climate change will increase flood peaks on the American River, by increasing either the amount of precipitation or the proportion of precipitation that falls as rain in storm events (Lettenmaier and Gan, 1990; Roos, 1994~. In summary, there is significant uncertainty about the risk of flooding in the Ameri- can River, much greater uncertainty than would normally be assumed on the basis of the long streamflow record. It would be prudent to explore the economic and l errs ---I

72 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN safety implications of estimating flood risk in the American River basin using just the second half of the American River flood record, from 1950 to the present. Geomorphology of the Lower American River The evaluation of possible flood management alternatives needs to consider pertinent aspects of the local geomorphology. Two geomorphological features are particularly important to flood hazard management in the American River basin: (1) stability of channel banks and levees and (2) potential ongoing channel enlargement and increases in conveyance. Insights about these issues can be gained by examining geomorphic stability and channel erosion in the lower American River, as evidenced by data from U.S. Geological Survey stream-flow measurements at the Fair Oaks gage. Evaluations of potential temporal changes in flood conveyance in the lower American River must consider channel stability, which in turn is dependent on channel morphology and stratigraphy. Since both morphology and stratigraphy of the lower American are largely the result of extreme and persistent channel changes induced by human activities, analyses of channel stability should begin with an understanding of the nature of historical sedimentation and subsequent channel adjustments. Historical Channel Changes The channel geomorphic history over the last 130 years is one of great change. Mining sediment, dams, and levees caused perturbations to which the fluvial system is still adjusting. Channel stability is related to these extensive but undocumented changes due to both nineteenth century aggradation and engineer- ing works. Yet an analysis of the historical record of channel changes that could reveal instabilities has not been conducted. For example, two apparent nine- teenth century channel diversions near downtown Sacramento, including a mean- der-bend cutoff near Sutter's Landing in 1862 and a northward diversion of the channel at its confluence with the Sacramento River (Bischofberger, 1975; Dillinger, 1991), are not mentioned in the geotechnical literature. These changes could represent channel shortening and steepening in critical reaches below Howe Avenue. Mining Sediment Hydraulic mining, invented in northern California in 1853, is a method of resource extraction that uses pressurized water to move large volumes of sedi- ment (Paul, 1947; James, 1994~. As hydraulic gold-mining came into widespread use in California, much sediment was delivered to main channels of northern Sierra rivers and caused channel aggradation (Mendell, 18811. The lower Ameri

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 73 can River aggraded substantially during the primary hydraulic mining period (1861 to 1884) and later degraded as sediment inflows decreased and channel deposits were transported downstream (Gilbert, 1917~. Estimates of mining sedi- ment stored along upper American River channels near the turn of the century were 20 to 25 million cubic yards in the North Fork, 10 to 15 million cubic yards in the Middle Fork, and none in the South Fork (Manson, 1882; Gilbert, 1917~. Licensed hydraulic mining continued to produce sediment from 1893 through at least the 1930s. Few of the sediment detention structures required for licensing remain, so most of this sediment was delivered downstream until the North Fork Dam was built in 1939. Little mining sediment remains in the mountains other than sediment stored behind North Fork Dam and Folsom Dam, although a low gravel terrace remains on the North Fork above Lake Clementine (behind North Fork Dam). In the lower American River, mining sediment deposits were estimated to have varied between 5 and 30 feet in depth across almost 10 square miles (Mendell, 1881; Manson, 1882), and mining sediment still dominates the active sediment. Field visits in 1994 located much historical sediment stored along the lower American River. On the basis of the mineralogy of the pebbles, it was determined that much of this sediment was produced by hydraulic mining (James, 1991b). A left-bank historical terrace 4 m high of credible unconsolidated sand and gravel at river mile 21 is representative of historical deposits in the lower American River from river mile 15 to 22 (Photos 2.1 and 2.2~. The high terrace of historical sediment on the left bank at Watt Avenue (Photo 2.3), extends laterally beneath the levees on both sides of the river and downstream below H Street through an area of critical bank erosion potential. Channel instability may arise from the morphologic changes induced by historical aggradation. Erosion of historical sediment could be relevant to con- veyance in two ways: ( 1) eroded sediment may fill channels or produce bedforms and other roughness elements during floods, thereby reducing conveyance and raising flow stages, or (2) increased channel capacities could improve flood conveyance. In addition, many levees are built on stratified mining sediment with high lateral hydraulic conductivities. Seepage beneath levees was observed in 1986 and is a substantial problem (RCE, 1993~. Geomorphic mapping is needed to identify where banks and levee founda- tions are composed of relatively erodible and permeable historic sediment. Two recent reports classified bank stratigraphy along the lower American as Pleis- tocene or Recent (Holocene) without distinguishing between prehistoric Holocene and historical sediment (WET, 1991; RCE, 1993~. No mapping of long-term historical channel changes or field descriptions of present historical deposits has been attempted in the lower river. Nor has the condition of the pre-mining channel been considered, other than base level changes.

74 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN .. , ~ I - ~ . ~.: N:~: . ~ . ~. _ . . PHOTO 2.1 Hydraulic mining was common in northern California in the late 1800s and delivered significant amounts of sediment into Sierra rivers. These historical deposits are visible, such as this terrace near river mile 21. A bike trail runs on top of the terrace. (Allen James, University of South Carolina.) PHOTO 2.2 Hydraulic mining sediment deposits often consist of erodible, unconsolidat- ed sand and gravel. At river mile 21, about six feet of historical sediment cap about 3 feet of older sediment. (Allen James, University of South Carolina.)

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 75 PHOTO 2.3 The potential for channel instability is increased in areas with hydraulic mining deposits, such as this site along the left bank of the American River near Watt Avenue. The terrace surface extends upstream and down-stream, as well as beneath the levee. (Allen James, University of South Carolina.) Bank and Lateral Stability As pointed out in the "Geotechnical Analysis" section above, the 1991 ARWI states that banks and levees were structurally stable at flows up to 115,000 cfs, but would fail due to seepage or overtopping at higher flows. The 1991 ARWI was based largely on a geotechnical perspective, neglecting geomorphic pro- cesses. Three recent reports have introduced the geomorphic perspective (WET, 1991; WRC-Environmental and Swanson, 1992; RCE, 1993~. On the basis of historical aerial photographs and field evidence, consultants for SAFCA (WRC- Swanson, 1992) concluded that bank erosion potential is high, and that sustained bank erosion since 1955 can be attributed to Folsom Dam closure and levee construction. Consultants for the District identified lateral instability and seepage failures as serious concerns, although the District does not believe that the bank erosion problem goes beyond what can be treated by standard maintenance practices (Sadoff, 1992~. Bank stability was evaluated based on stream power, which was highest in steep upper reaches below Folsom, where channels were presumed stable because of resistant strata in the bed and right banks (RCE, 19931. How- ever, extensive deposits of historical sediment on the left bank of these reaches

76 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN could be prone to erosion. In the lower reaches, stream powers were high be- tween river mile (RM) 5 and RM 6, corroborating other findings that the bends below Howe Avenue are vulnerable to bank erosion. Comparisons of aerial photographs from 1968 and 1986 indicated that channel migration rates at five critical sites (RM 12.5 to 20.1) averaged 4.8 ft/yr and ranged between 1.1 and 8.0 ft/yr (WET, 19914. Migration rates as high as 13.9 ft/yr at other sites were not deemed critical because of the channel distance from a 50-foot buffer around the toes of levee slopes. These migration rates do not include substantial channel changes from the 1965 flood, which caused an avulsion near river mile 15. Agreement on the potential for lateral channel migration is important not only to bank stability, but also to channel enlargement. Lateral planation in meandering alluvial channels can maintain a natural equilibrium system, but with the dowr~-valley sediment supply cut off by dams, eroded bank material may not be entirely replaced and erosion could result in net channel enlargement over time. Channel Lowering and Enlargement Questions relevant to channel stability and potential changes in conveyance in the lower American River include the degree and timing of aggradation and degradation, whether channels have returned to presettlement base levels, and whether channel enlargement continues. Dam closures are often associated with channel erosion downstream (Williams and Wolman, 1984), although responses to dams may be complex and may include periods of local aggradation. For example, closure of Oroville Dam in 1968 caused complex channel changes downstream on the Feather River at least through 1975 (Porterfield et al., 1978~. It has also been argued that the lower American River has been degrading in recent decades, encouraged by the closure of Folsom Dam and levee construction in the 1950s (WRC-Environmental and Swanson, 1992), although little evidence has been cited. Vertical Incision Vertical changes on the lower American River have been the subject of several investigations. Gilbert's (1917) time series of Sacramento River bed elevations just below the American River confluence showed 10 feet of bed aggradation from 1855 to 1890, and about 8 feet of degradation by 1914. These responses to hydraulic mining sediment indicate that the lower American River also must have experienced substantial channel bed aggradation and degradation. Recent studies of historical incision, based primarily on California Debris Com- mission (CDC, 1907) and subsequent topographic maps (1955 and 1987), iden- tify 10 to 20 feet of degradation in the lower river from 1906 to 1986 and conclude that thalweg incision is ongoing at some locations (WET, 1991; WRC

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 77 Environmental and Swanson, 1992; RCE, 1993~. Ten channel cross-sections, resurveyed between 1987 and 1993, showed no systematic change (RCE, 1993), but these surveys were not separated by any major flood events. At some sites the channel bed rests on resistant premining strata, and removal of historical sedi- ment from the bed is complete at these sites (RCE, 19931. Incision of resistant Pleistocene strata can result in sustained channel degradation, however, as on the nearby Bear River in response to a 1955 flood (James, 1991a). Thalweg profiles indicate that most channel degradation between RM 6 and RM 11 was complete by 1955, but that considerable incision occurred between 1955 and 1987 from RM 6 to the mouth and between RM 1 1 and RM 14 (RCE, 19934. Channel incision of about 20 feet and considerable channel enlargement had occurred in the lower American River by 1960 (Olmsted and Davis, 19614. Changes in thalweg profiles on 1957 and 1987 maps indicate an average of about 18 feet of incision between RM 2 and 3 (WET, 19914. Bed stability was modeled using USACE design 100-year hydrographs and the Parker bedload transport equation based on the median bed material size (Dso) and Shields entrainment criteria (RCE, 1993~. Most simulated channel beds experienced no scour, and maximum bed elevation change under the worst sce- nario was less than 2 feet (at RM 7~. On the basis of the model, channel beds throughout the lower American River should be stable under relatively large and infrequent events. Channel Enlargement Vertical incision is only one form of channel erosion, and vertical stability would not preclude channel enlargement by erosion of sediment stored along channel margins. It is common in aggraded systems for channels to respond initially to decreased sediment loads by incising vertically, and later to widen out; particularly when channel top widths are confined by levees or terraces. For example, it has been shown experimentally that knickpoint retreat is often fol- lowed by lateral migration and bank erosion (Schumm, 1973; Schumm et al., 1987~. Following vertical regrading of the lower American River channel profile, a period of channel enlargement by bank and berm erosion and lateral migration cannot be ruled out. In fact, due to surplus energy from decreased sediment loads and decreased channel capacities from levees and historical deposits, and due to observed channel erosion and lack of sediment replacement from above Folsom Dam, ongoing net channel erosion could be expected for the lower American River. In spite of these reasons to suspect channel enlargement and the ramifica- tions to channel conveyance and environmental concerns along the parkway, evidence of channel change in the lower American River has not been adequately studied.

78 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN Channel Changes at Streamflow Gages The nature of channel erosion since the closure of Folsom Dam has been examined using topographic maps and aerial photos with limited temporal and spatial resolutions (WRC-Environmental and Swanson, 1992; RCE, 1993~. To enhance the channel-change data base, the committee examined high-resolution U.S. Geological Survey cross-section measurements at the Fair Oaks gage. These analyses are based on only a few sites associated with various locations of Fair Oaks gages and soundings, so caution should be exercised before extrapolating results up- or down-stream. Channel changes are demonstrated by channel cross-sections and stage-dis- charge regression analysis. Data were derived from stream-flow measurement records (USGS archives). Cross-section plots were derived from depth sound- ings at three locations (Figure 2.4~: the old Fair Oaks Bridge (1913 to 1950), a cable about 300 feet below the bridge (1930 to 1957), and a cable 2.2 miles upstream below Hazel Street (1958 to 19941. All sections are from bridges or cables to control the longitudinal position. Numerous plots reproduced sections during stable periods indicating high accuracy of the procedure. For the sake of brevity, only five cross-sections at one site are presented here. Channel morphological changes are rarely related to changes in flood stages ~ k~ ~ ~ \ t0,,,~, i ' '. ~ ... iC'~~_b`\ 9) \,,,, ~ \ ~ ~ ,,\~ Gt / 3dc / W"-.."""222- ~\ G ~ ''' '' ".~.2...',2,.2NlV,tIr,~,,\ Ni2''..2.~.'.'2''2''''22.''. W'..,,,~,,.',,,~.,.,'..r 4'.''.2.'.~..".2.2. ~.'~ ' ' 2 '.,.2 _ e If. . ~,`4 f D a m W ~ E S fin _ Sacramento River Flood Coritrot Project Levees Em_ American River Project Levees .___ Private Levees ~_ Rae ~ .1 Fair Oaks Gage at old bridge & cable . .2. Fair Oaks Gage at Hazel Avenue cable (53 River Miles o M i I e s 1 0 Base Map: Sacramento District, 1991 FIGURE 2.4 Locations of gages and levees on the lower American River. SOURCE: A. James, University of South Carolina (adapted from USACE, 19911.

IDENTIFICATION AND EVALUATION OF ALTERNATIVES TABLE 2.7 Stage-Discharge Data 79 Location Total N Model N Model Years Bridge 528 497 Q Range R2 1905 to 1958 500 < Q < 20,000 0.85 Stage = 67.5 + 7.18 · 10 - Q - 2.00 - 10-8 Q2 + 2.30 - 10-13 Q3 Hazel Street 454 413 1958 to 1994 Q < 15,000 0.74 Stage = 76.1 + 1.52 · 103 Q- 1.66 107 Q2 7.05 10-12 Q3 in a simple manner. For example, main channel deepening may not result in lower stages of overbank floods if meander-belt flows develop greater turbulence at channel crossings (Ervine et al., 1993~. Thus, an independent analysis of stage-discharge relationships was conducted to evaluate temporal changes in stage at the two gage sites: the old Fair Oaks Bridge and Hazel Street sites. Stage integrates morphologic and hydraulic factors, providing an indicator of flow conveyance. Stage data represent gage readings at the time of discharge mea- surements (not rating curves), corrected for gage datum changes. Flow stage is strongly related to discharge, so stage was statistically re- gressed on discharge to control for these effects. A third-order polynomial pro- vided the best-fit model at both sites. Extreme discharge events were eliminated from regressions (Q-Range, Table 2.7J to emphasize changes within the inner channel rather than overbank characteristics that can be dominated by roughness elements. The regressions provide an objective estimate of the stage of a given discharge. Plots of residuals (errors in the predicted stage) against time reveal temporal changes in stages of flows up to moderate magnitude floods. These methods and some limitations to their morphologic interpretation (e.g., changes in roughness and energy gradient) are explained elsewhere (Knighton, 1974; James, 1991a). Fair Oaks Gage near Old Bridge Cross-section plots (1913 to 1950) at the Fair Oaks bridge indicate channel bed scour and fill with net thalweg erosion of about 8 feet (Figure 2.5~. Channel morphology is controlled by bridge piers and the right-bank bluff. A deep left- bank fill narrowed the channel by about 20 feet toward the end of the period suggesting that constriction by the bridge is not the dominant reason for erosion. A cable was installed about 300 feet below the bridge in 1930, where cross- section plots indicate about 5 feet of thalweg erosion from 1944 to 1952 followed by about 2 feet of fill by 1957 when the cable was moved. Deepening and narrowing of cross-sections at this site suggest that erosion at the bridge extended through the reach. Channel deepening and narrowing at this bridge site follow

80 85 80 a) 7C ID 1 A __ o is 70 > LL 65 60 55 FLOOD RISK MANAGEME~AND THE AMERICk RIVER BASIN 1 -I 1913 ~ 1917 a 1928 ~1945 ~1950 Fair Oaks Bricige r. 500 600 700 800 Station (feet) 900 1 000 FIGURE 2.5 Representative channel cross-section plots at the Fair Oaks Bridge showing about 8 feet of thalweg degradation between 1913 and 1950. Data gaps indicate bridge piers. The view is downstream. the general response observed elsewhere where channels are incised through hydraulic mining sediment (James, 1991a). Stage-discharge relationships at the bridge site indicate a systematic group- ing by period with occasional changes in flow stages (Figure 2.6~. Temporal patterns of flood stage changes are illustrated by a time series plot of regression residuals (Figure 2.7~. Flow stages at the old Fair Oaks gage rose slightly from 1905 to 1912, lowered about 2 feet by 1920, rose about 2.5 feet in the late 1930s, and dropped about 3.5 feet by 1950 to about 1.5 foot below the mean for the period. The rapid incision during the 1940s may represent a response to de- creased sediment yields following the closure of North Fork Dam in 1939. Fair Oaks Gage at Hazel Avenue In 1957 the gage and cable were moved 2.2 miles upstream to the present Hazel Street site below Nimbus Dam. From 1958 to 1994 the channel at this

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 75 of_ - o ._ Be_ a) ~70 Be_ oh 65 81 Fair Oaks at O/d Bridge 1905-1958 x ax sx ss st a~t Aim; as 2e ~ ~a,. ~ ~ ~ ~ set, a_ ~ ~ mix ~ ~ -~' .~. o "~;,~ ~ ~- . '- ~<1912 ~<1918 ~<1928<1931 s <1945 ~<1956 ° <1959Water Yr 0 1 0,000 20,000 Discharge Offs) FIGURE 2.6 Stage-discharge relationship from the Fair Oaks gage at the bridge and early cable site. Several distinct periods of high and low stages can be identified. location experienced episodes of thalweg deepening and bar deposition followed by stable periods lasting several years, and about 9 feet of net thalweg degrada- tion. The 1965 flood scoured the thalweg about 10 feet, but the channel partly refilled from 1965 to 1973 and was colonized by willows. From 1973 to 1986, the channel bed was stable, but the 1986 flood lowered the thalweg about 3 feet and widened the channel considerably. Analysis of flood stages at the Hazel Street site indicates two periods of relative stability from 1958 to 1967 (Figure 2.8~. Stage-discharge regression residuals reveal lowering of flow stages at this site, between the two stable periods (Figure 2.7~. The 1965 scour event had no effect on flow stages, presum- ably due to rapid refilling and increased vegetational roughness of the channel. From 1967 to 1970, however, flow stages rapidly lowered about 2 feet. Sus- tained incision over the period from 1958 to 1994, during which time flow stages dropped about 2 feet, suggests a long-term tendency for channel degradation and a mobile bed at this site. The close proximity of Nimbus Dam upstream severely limits replacement of eroded bed sediment, resulting in net degradation. Thalweg incision at the two gage sites was about 8 feet (1913-1950) and 9 feet (1958 to 1994), respectively. Although net stage lowering for the two periods was only about 1.75 feet and 2.5 feet, respectively, large rapid fluctua- tions characterize these changes. This evidence of rapid erosion at gages lends

82 in ~ 2 ._ En . . a) o o ._ co En a) CY -2 a a' - ~- 1 Cn -4 1 900 1 920 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN l 1 Fair Oaks Gage at Hazel Avenue : day. . .. -. 2, .. ~ ~;s,s8,,,'. ;`a!.,.,,.~.;, T -ma- -I.- ~ it; 500 ~ Q < 20~000 Fair Oaks Gage at Old Bridge Q < 15,000 , . ~ in; s. . . . I: :~~ :~ Gage Change 1 1 ! ~ 1 940 1 960 Water Year 1 980 2000 FIGURE 2.7 Stage-discharge regression residuals for the Fair Oaks gage. Left side is from Fair Oaks bridge site (see Figure 2.6) and shows two periods of low stages and two of high stages interpreted as degradation and aggravation, respectively. Right side is from Hazel Avenue site (see Figure 2.8) and shows a short period of rapid stage lowering interpreted as in response to channel degradation. Joining of the two series is approxi mate. credence to a hypothesis of continued channel deepening and enlargement in the upper reaches. If the gage sites are representative of other sections, the conclu- sion that extreme floods would cause little incision on the lower American River (RCE, 1993) could underestimate the potential for channel down-cutting. Geomorphic Conclusion Bank stability is a serious consideration when considering conveyance of high flows in the lower American River. Although the degree of hazard that bank erosion, lateral migration, or bed incision pose to lessee stability is contested, all parties appear to agree that a program of channel monitoring and maintenance is necessary. The belief that historical sediment in channels of the Sacramento Valley is now stable is based largely on evidence of elevations derived from topographic maps and numerical simulations of channel bed erosion. Thalweg

IDENTIFICATION AND EVALUATION OF ALTERNATIVES 85 o ._ - _ ~ . a' 80 LLI 1 O) 1 can Fair Oaks at Hazel A venue 1958- 1994 i. ~ ,. ~ -Or ' - . ... it, ~ G '' X A__ a ~g ~ <- WaterYear · < 1967 ~< 1969 ~ . ~1 · < 1982 ~ < 1986 ~< 1995 7 5 1 ! ~! 1 1 I ~! 1 1 1 0 5,000 10,000 Discharge (cfs) 83 FIGURE 2.8 Stage-discharge relationship from the Fair Oaks gage at the Hazel Avenue cable site. Several distinct periods of high and low stages can be identified. elevations indicate that base-level adjustments have decelerated, but ongoing vertical adjustments should not be ruled out. Nor would stabilization of long profiles necessarily indicate an end to channel bank erosion, lateral migration, enlargement, or instability. Evidence from two Fair Oaks gage sites indicates substantial local channel bed scour. From 1913 to 1958, flow stages at the Fair Oaks bridge changed considerably, showing two periods of increasing stages and two of decreasing stages, interpreted as periods of aggradation and degradation, respectively. There was a net lowering of flow stages by almost 2 feet for this period, presumably due to erosion of historical sediment. From 1958 to 1994, flow stages at Hazel Street also lowered about 2 feet. If these sites are representative of the lower river as a whole, further channel incision may be anticipated. Given historical aggradation, cessation of sediment deliveries since dam construction, and evidence of erosion, the potential for net erosional tendencies in the lower river cannot be rejected. A sediment budget deficit exists in the lower river as dams arrest sediment deliveries from upstream while erosion removes sediment, and this deficit results in net erosion. The hypothesis that channel erosion and enlargement have resulted in increased channel conveyance over the last two decades should be tested further using hydraulic models. Analysis of

84 FLOOD RISK MANAGEMENT AND THE AMERICAN RIVER BASIN stage-discharge time series provides empirical support for the hypothesis that channel stages of moderate magnitude floods have lowered by a modest amount at two locations over two different periods, but more information is needed to substantiate these results and extend them to other locations downstream. Given the critical nature of flood hazards in Sacramento and extensive nine- teenth century channel changes, the committee suggests three areas of study regarding the geomorphology of the lower American River: (1) ongoing moni- toring of channel changes, (2) historical reconstruction of channel changes, and (3) geomorphic mapping. Recent and ongoing channel changes should be docu- mented and monitored following large flood events by repeating channel cross- section surveys, and by registering aerial photos. Study of long-term historical changes should include consultation of early historical records to establish presettlement channel conditions that could estab- lish a baseline for changes to the fluvial regime that was presumably in equilib- rium with long-term flow conditions. In addition, historical changes should be documented through historical and field methods. For example, CalTrans bridge surveys could be collected and repeated, and California Debris Commission records of twentieth century hydraulic mining sediment production could be tabulated. Vast tracts of erodible historical sediment stored in the lower river should be studied and mapped. In the upper reaches they are relevant to channel enlarge- ment and sediment production, while in the lower reaches they are relevant to bank and levee stability and seepage. Mapping will reveal spatial patterns and allow more accurate interpolation between geotechnical sample points. As pointed out above, implementation of risk and uncertainty analysis in the lower American River will require appraisals of channel and levee stability (USAGE, Sacramento District, 1994a). Assignment of PNP and PEP elevation for levees should be based in part on knowledge of lower American River stratigraphy with an emphasis on the spatial pattern of historical sediment and former channels.

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This book reviews the U.S. Army Corps of Engineers' (USACE) investigations of flood control options for the American River basin and evaluates flood control feasibility studies for the watershed, with attention to the contingency assumptions, hydrologic methods, and other analyses supporting the flood control options.

This book provides detailed comments on many technical issues, including a careful review of the 1991 National Research Council report American River Watershed Investigation, and looks beyond the Sacramento case to broader questions about the nation's approach to flood risk management. It discusses how to utilize information available about flood hazard reduction alternatives for the American River basin, the potential benefits provided by various alternatives, the impacts of alternatives on environmental resources and ecosystems, and the trade-offs inherent in any choice among alternatives which does not lie in the realm of scientists and engineers, but in the arena of public decisionmaking.

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