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4 Operation of Glen Canyon Dam INTRODUCTION Glen Canyon Dam and Lake Powell represent a major source of water, energy, and Rood protection in the southwestern United States. It is therefore not surprising that objections from groups that use water and energy appeared quickly when significant changes in the dam's operating rules were proposed for environmental reasons. While objections related to increases in the cost of energy did have a valid basis, concerns about changes in water supply, and flood protection did not. The nature of the changes resulting from the effort to protect the environment belowthe dam had no effect on the ability of upper-basin states to deliver lower-basin water (the Law of the River) or the degree of flood protection provided by the dam. The reason is be- cause the monthly release targets are totally independent of any constraint related to either interim release rules or preferred alternative criteria of the environmental impact statement (EIS). These changes, which were intro- duced for purposes of environmental protection, speak only to variations during a day, not the average volume released during a day (and therefore the month or year). This conclusion is supported by the simulation model results included in the Bureau of Reclamation's (BOR) EIS. The median annual release is the same (8.5 million acre feet (math) for all of the op- erational rule alternatives considered (BOR, t994b, Table 11-7~. This chapter compares the hydrology of the Colorado River before and after construction of the Glen Canyon Dam, describes operating rules for the dam and their evolution, and provides an overview of water supply above the dam. These three topics define the scope within which the dam's operation can be adapted to environmental objectives. 50
Operation of Glen Canyon Dam HYDROLOGY THROUGH THE GRAND CANYON Hydrology Prior to Construction of Glen Canyon Dam 51 Priorto construction of Glen Canyon Dam, the flows at Lee's Ferry varied seasonally (Figure 4.1~. The average annual discharge of 16,800 cubic feet per second (cfs) included periods of high flows sometimes exceeding 100,000 cfs (byline) and flows as low as 2,500 cfs pall and winter). Monthly average (flung) flows were much more constant following construction of the dam. This is not surprising given that a principal justification of the reservoir was to reduce spring floods and to store this water, thereby providing subsequent increases in low-flow seasons. The daily pre~dam fluctuations were much smaller than seasonal var- ~ations, but occasional daily fluctuations were very significant. For example, variations in stage height of 5 to 10 feet for 1 to about 5 days are shown in Figure 4.2. These variations occurred during all seasons because of pre- cipitation in tributarywatersheds or temperature variations during the snow- melt season. Figure 4.2 also shows that minor daily fluctuations of about 1 to 3 inches were very common. Hydrology Following Closure of the Dam Following the initial filling of Lake Powell in 1980, and until interim flows began in 1991, the average annual flows were unchanged except in response to short-term droughts or floods, but daily fluctuations in discharge were large, as shown by Figure 4.3. Daily operating rules reflected variations in peak demand for hydropower. Maximum controlled daily peaks approached the 31,500-cfs capacity of hydropower turbines, and daily minima were as low as 1,000 cfs in winter and 3,000 cfs in summer. The peaks exceeded 24,000 cfs 10 percent of the time and were below 5,000 cfs 10 percent of the time. Tributary Inflows Below the Dam Flows through the Grand Canyon are normally dominated by the water released from Glen Canyon Dam; tributaries belowthe dam such asthe Paria River, Little Colorado River, and Kanab Creek (Figure 1 .1 ) contribute less than
52 55,000- . 50,000 03 - a) co ._ o a) < ~20,000 a) 45,000 40,000 35,000 30,000 25,000 1 5,000 1 0,000- 5,000 O River Resource Management in the Grand Canyon Before1 963 1963 to 1990 ,\ - - - 0 2 4 6 8 Month (October through September) 10 12 FIGURE 4.1 Comparison of monthly average discharge at Lee's Ferry before and after closure of Glen Canyon Dam in 1963 SOURCE: Bureau of Reclamation (1994~. 2 percent of the average flow, as shown in Table 4.1. Small tributaries such as Havasu Creek and Bright Angel Creek contribute an insignificant amount of water. In a particular tributary drainage, however, thunderstorms can for a short period produce a major increase in discharge and sedimenttransport. For example, the 1 20,000-cfs extreme event shown in Table 4.1 for the Little Colorado River is an order of magnitude higher than the average release from the dam. In January 1993 such an unusual event moved a large amount of sediment from the Little Colorado River into the main river. Hydrological Regimes During the GCES Research Periods During much of the data collection phase for Phase I of the Glen Canyon Environmental Studies (GCES), releases from Lake Powell were unusually high (1983 and 1984~. During GCES Phase 11, hydrological conditions included years of both normal and low runoff (each of which resulted in the prescribed annual releases of 8.23 maf). Thus, the entire GCES study interval
Operation of Glen Canyon Dam 30.0 25.0 20.0 a) a) - LL~ 1 5.0 u, 10.0 5.0 0.0 53 1 :~- .~ I \: 0.0 50.0 100.0 150.0 200.0 250.0 300.0 350.0 DAYS (beginning 10^01-28) FIGURE 4.2 Example of fluctuations in stage height of the Colorado River prior to construction of Glen Canyon Dam (October 1928 through September 1929 at Colorado River near Grand Canyon gauge). SOURCE: D. Wegner, Bureau of Reclamation. spanned a wide range of conditions for dam operation. Prediction of future releases using data from the period of record since dam closure is difficult because much of the postdam historical record is biased by the large number of years during the filling period when releases were more restricted than they are now. During the initial filling of Lake Powell (1963 to 1980), water releases to the lower basin were reduced by 27 mat of reservoir storage plus 10 mat to 16 mat of bank storage. This was followed by an unusually wet period in 1983 and 1984 that resulted in flows approaching (and for a few weeks exceeding) turbine capacity for almost 2 years. Travel Time Through Grand Canyon During 1991, a dye study coinciding with experimental flows provided measurements of water velocities through the canyon at both steady and un
54 1100 000 900 ~ 800 a, 7co 600 ~ son Q ~ coo 300 100 o River Resource Management in the Grand Canyon 0 so too 150 200 2s0 3~ 3so coo also son see 600 6s0 70c 7so Hourly Data FIGURE 4.3 Example of Glen Canyon Dam operational fluctuations prior to interim flows, July 1982. SOURCE: WAPA (1988). TABLE 4.1 Discharge of Tributaries in the Vicinity of Grand Canyon . Annual Average River (cfs) Highest Annual Lowest Annual Extreme Average (cfs) Average (cfs) Event (cfs) l Colorado, 17,850 29,000 3,200 30O, 000 Lee's Ferry Paria River 29 65 11 16,100 Uttle Colorado 248 1,127 27 120,000 Kanab Creek 14 28 8 3.030 From streamflow record 1922 to 1993. SOURCE: USGS (1993~. steady flows (Graf, 1993~. Wave velocities following rapid increases in flow rate were also documented and were used to verify calibration of an unsteady flow simulation model (Smith and Wiele, 1993~. The flow rates shown in Tables 4.2 and 4.3 are from both the dye study and the flow simulation model. The average water flow rates were the same for steady flows (1 5,000 cfs) and unsteady flows with the same average rate (3,000 to 26,000 cfs). As shown in Table 4.2, the wave caused by a rapid increase of water depth at the dam travels about two to two and three-tenths times the speed of the associated water mass. Wave celerity increases as a function of water
Operation of Glen Canyon Dam 55 TABLE 4.2 Travel Time and Velocities Through Grand Canyon with Daily Flows Varying from 3,000 to 26,000 cfs and Averaging 15,000 cfs Water Travel Time Wave Travel Time Days mph Days mph Location River Miles Lee's Ferry 0 0 0 Uttle Colorado 62 1.4 2.2 0.6 4.3 Phantom Ranch 88 2.0 2.2 0.8 4.6 National Canyon 166 3.7 2.2 1.3 5.3 Diamond Creek 225 4.4 2.2 1.8 5.2 SOURCE: Graf (1993). TABLE 4.3 Variations in Travel Time with Variations in Average Discharge (Lee's Ferry to Diamond Creek) Average Discharge (cfs) Time (days) Velocity (mph) 5,000 10.0 1.0 15,000 4.4 2.2 30,000 2.9 3.4 SOURCE: Smith and Wlele (1993). depth and change in depth. Water velocity increases with average discharge but less than linearly. For example, as shown in Table 4.3, as discharge increases by a multiple of 6 (from 5,000 to 30,000 cfs), mean velocity in- creases by a multiple of 3.4. The travel times shown in Table 4.2 are for an average discharge (15,000 cfs) that is 36 percent greaterthan the annual average discharge (1 1,400 cfs). Thus, average travel times are somewhat longer than those shown in the table.
56 River Resource Management in the Grand Canyon OPERATING RULES FOR GLEN CANYON DAM Seasonal and Annual Operations Formal Description of Operating Rules The dam's daily operating rules are defined in terms of maximum and minimum releases and rates of change. Monthly releases are defined in a more complex way. The complexity derives from the fact that monthly releases involve conditional probability. They are recalculated monthlyburing the spring and early summer. The outcome of the calculations is a function of both snowpack and current storage level in Lake Powell. The monthly releases therefore differ every year and can be described only in statistical terms. Monthly and Annual Releases The annual release target for Glen Canyon Dam has always been 8.23 million acre-feet. This amount satisfies the Law of the River (Chapter 3) while maximizing the storage remaining in Lake Powell for use during future droughts. During wet years, when it appears from snowpack measurements that storage capacity will require more than this minimum release, the objective is to schedule monthly targets so that increased releases are distributed over several spring and summer months. This strategy reduces the likelihood of flood damage to the dam and, second, avoicis bypass of hydropower turbines. Typical monthly release targets vary from about 0.5 to 1 mat. The smaller releases are in spring and fall; the largest releases are in summer and winter. Summer peaks accommodate demands for both hydropower and recreation, while winter peaks meet energy demands for heating. The only change in monthly operating rules since GOES Phase I relates to the management of potential spills (bypass of turbines). During the unusually high runoff of 1983, a major spill resulted from the operating criteria, which then required that a minimum of 2.4 mat of storage be available on January 1. Releases were then estimated as necessary to fill the reservoir by July. The BOR's Colorado River Simulation Model (CRSM) estimated that this rule would result in a spill in 1 of every 4 years on average (BOR, 1986~. Following the 1983-1984 floods, however, the criteria were revised. While the storage
Operation of Glen Canyon Dam 57 target on January 1 remains the same, the monthly release targets during years of high snowpack are now higher, particularly during spring months. Also, the estimates are made with a July storage target that is 0.5 mat lower than actual capacity. The new criteria have yet to be tested because the reservoir is now refilling after several years of drought, but the BOR's sim- ulation model estimates that the frequency of spill with the new criteria will be 1 year in 20 on average. The adjustments in monthly targets should cause no reduction in the probability that water can be delivered downstream as specified by the Law of the River. The annual target of 8.23 mat has the unusual characteristic of being both the probable minimum and the median annual release. It will be the minimum unless a long-term drought occurs that is much more serious than any in the 90 years of record. It is the median because during at least 50 percent of years the release has been, and will likely continue to be, no greater than 8.23 mat. Daily and Hourly Operations Daily Operations Short-term operating rules prior to 1991 were designed almost entirely for maximizing the value of hydropower, except for a small accommodation to environmental resources in terms of minimum flow criteria. The releases were characterized by large daily fluctuations with peaks at turbine capacity of 31,500 cfs and minima of 1,000 cfs in winter and 3,000 cfs in summer. In 1991 the daily operating rules were revised. These interim flow targets, which remained in effect as of September 1995, are: · daily maximum releases not more than 20,000 cfs; · minimum flows not lower than 5,000 cfs at night and 8,000 cfs during the day; · change in release rate not to exceed 5,000, 6,000 or 8,000 cfs per day as monthly release targets vary from <0.6, 0.6 to 0.8, and >0.8 mat, respectively; and ~ hourly changes in release rate not to exceed 2,500 cfs when increasing and 1,500 cfs when decreasing. These interim flow rules are intended to reduce the adverse effects of dam
58 River Resource Management in the Grand Canyon operations on the environmental resources in Grand Canyon. The preferred alternative (modified low flow) that was selected in the operations EIS (BOR, 1 994 b) is very similar to the interim flow rules, but some modifications were made as a result of experience with operations during 1993 and 1994, as described below. Experience with Interim Flows and Exception Criteria The changes in daily operating rules have had no effect on the ability of the BOR to meet monthly or annual release targets because monthly release targets are totally independent of interim flow rules. It is, however, now much more difficult for the Western Area Power Administration (\/VA PA) to respond to hourly changes in energy load because of the constraints on both daily and hourly ramping rates (the rate of change in dam releases). Absolute en- forcement of the revised operating rules would have decreased WAPA's ability to "meet system regulation needs, maintain transmission reliability, maintain operating reserve requirements, and serve firm load requirements" tWAPA, 1994~. Therefore, in October 1991, WAPA and the BOR signed an interagency agreement. This exception criteria agreement allows WAPA to violate flow restriction rules not more than 3 percent of the time in any 30-day period (WAPA, 1994~. The range between the 20,000-cfs maximum and the 5,000-cfs minimum release limits appears to still allow substantial flexibility for response to chan- ges in demand for hydropower. Because of the daily limits on ramping rates, however, the 1 5,000-cfs nominal range is reduced to 5,000 cfs in months with low-release targets and to 8,000 cfs in high-release months, as indicated in the following EIS preferred alternative section. A recent example (one week in April 1995) of diurnal variations in both release rates and rates of change in release rates (ramping rates) are given in Figures 4.4 and 4.5. Clearly, the limiting parameter is ramping rate, not the 5,000- to 20,000-cfs bounds on release rate. EIS Preferred Alternative Operating criteria for the modified low-fluctuating-flow alternative, which was identified in the final EIS as the preferred alternative, are shown in Table 4.4.
Operation of Glen Canyon Dam 35,000 30,000 2~;,000 - ~ 20,000 C1 1 5,000 1 0,000 5,000 59 , . . . . . . . : : : : : 20,000 cl, a'';~ea . ..................... ~ .......... . N ~ \ I ~ ; U: ~ ·~ h ~ N 12 APR 95 13 APR 95 14 APR 95 15 APR 95 16 APR 95 D" and Time 17 APP' 95 18 APR 95 19 APR 95 FIGURE 4.4 Hourly releases from Glen Canyon Dam for 1 week in April 1995. SOURCE: D. Wegner, Bureau of Reclamation. 4,000 3,500 3,000 2,500 2,000 1,500 , 1,000 :~ 500 ~ O He -500 c) -1 ,000 -1, 500 -2,000 -2,500 -3, 000 -3,500 0~ 12 APR 95 13 APR 95 14 APR 95 . . .. .. _ . . _ _ _ _ . _ 15 APR 95 16 APR 95 17 APR 95 18 APR 95 19 APR 95 Dam and lime FIGURE 4.5 Ramping rates for release of water from Glen Canyon Dam for 1 week in April 1995. SOURCE: D. Wegner, Bureau of Reclamation.
60 River Resource Management in the Grand Canyon TABLE 4.4 Operating Criteria for Modified Low Fluctuating Flow Minimum Maximum Daily Fluctuations Ramp Rates (cfs/h) Releases (cfs) Releases (cfs) (cfs/24h) 8,000/day 25,000/day 5,000/night to 8,000 4,000 up 1,500 down Orate of change in release from the reservoir. bSee Table 4.5. The two significant changes from the interim flow criteria are the increase in maximum release rate from 20,000 to 25,000 cfs and the increase in ramp- ing rate up from 2,500 to 4,000 cfs/h. The maximum flow rate occurs very infrequently, however, owing to the constraints on ramping rates. The down ramping rate (which is unchanged) was found to be much more important in terms of environmental impact than the up rate. The range of variations in monthly release targets results in the app- roximate ranges in allowable daily fluctuation shown in Table 4.5, which are much less than the theoretically allowable variation between maximum and minimum daily rates. The preferred alternative also includes beach/ habitat- building flows as discussed below. Beach/Habitat-Building Flows Releases exceeding 25,000 cfs are included in the preferred alternative for most years (except when storage in Lake Powell is greaterthan 19 mat on January 1~. These releases would occur during March at a steady rate of 33,200 cfs (power plant capacity) for 1 to 2 weeks. The purpose of these flows is to maintain physical habitat. Beach-building flows at rates higherthan power plant capacity (45,000 to 52,000 cfs) are also part of the preferred alternative. They would occur with less frequency: 1 in 5 years, except when high runoff requires greater frequency. The final EIS combines beach- and habitat-building flows. The combined flows are to occur either in May-June (high runoff) or in late summer during years when summer thunderstorms have added large amounts of sand from tributaries of the Colorado River below the dam. The flows are to be at least 35,000 cfs and are to last 1 to 2 weeks at a frequency of 1 in 5 flood years
Operation of Glen Canyon Dam TABLE 4.5 Fluctuation Range Experience Under Interim Flow Criteria 61 Monthly Release Volume (acre-feet) Minimum-Flow day (cfs) Minimum-Flow Night (cfs) Allowable Daily Fluctuation (cfs) <600, 0~)0 8,000 5,000 5,000 6001000 to 800,000 8,000 5,000 6,000 >800,000 8,000 5,000 8,000 SOURCE: Bureau of Reclamation EIS. except when high runoff requires that they be more frequent. The actual size of these flows is to be determined from the results of an experimental flood exceeding 35,000 cfs, which has yet to occur. Operational Constraints on Experimental Floods Beach-building flows would exceed turbine capacity (33,200 cfs). A release rate of 45,000 cfs can be achieved by using the river outlets, which have a capacity of about 15,000 cfs (actual capacity depends on reservoir storage level). Releases above 50,000 cfs would also require some flow through the spillway tunnels, which are controlled by radial gates (50 feet high). This can be done only during years when the lake level is above the bottom of the radial gates (elevation 3,648 feet above sea level). This elevation corresponds to 17 mat of storage, which the lake level is expected to exceed except following extended droughts. BOR considers use of the spillway undesirable on a routine basis, because it is considered to have the shortest useful life of any of the components of the dam, even after being modified to prevent cavitation following the 1983 flood. WATER SUPPLY ABOVE THE DAM Water Balance in Lake Powell All predictions of future probabilities for release or spill of water from Glen Canyon Dam are based on assumptions that are inherent in the mass balance equations of the CRSM. This model uses gauge records of inflow to Lake
62 River Resource Management in the Grand Canyon Powell, releases through the turbines, and gauge data for the Colorado River at Lees Ferry. Computation of mass balance also requires estimates of evaporation and change in bank storage (these variables are not measured). The mass balance equation used is S,+' USE+ Qi- Q°-E-.O8(a SJ, where So+' is ending storage, So is beginning storage, Qi is flow into the reservoir, Q° is flow released, E is evaporation, and AS is change in bank storage. The CRSM estimate of evaporation appears to be much lower than predicted by various researchers (Dawdy, 1991; Hughes, 1974~. Also, change in bank storage is estimated as a constant 8 percent of the annual change in storage, regardless of reservoir level, even though bank storage would be expected to vary in relation to reservoir level. The recent cirought (1987-1992) lowered the reservoir to an unprecedented extent. Data on water balance over this interval should provide a way to improve the bank storage estimate. Mass balance estimation could be improved through an analysis of the data on flow and reservoir stage, including the recent drought period, combined with a corrected estimate of evaporation over the same period. There is also an apparent discrepancy between reservoir release rate as measured by the energy generated at Glen Canyon Dam and the flow rate measured at Lee's Ferry. Part of this difference is due to seepage from the reservoir, but an analysis of possible errors in both approaches should be made so that seepage can be determined more accurately. Because of the recent large drawdown, it should now be possible to improve estimates of seepage as a function of storage in the reservoir. Upper-Basin Depletions During the 1 980s, projections of future release frequencies by the CRSM model assumed that consumptive use in the upper basin would progressively increase, which would reduce releases to the lower basin over the next 60 years (Figure 4.6~. A large number of projects for the upper basin have been authorized by Congress, and the CRSM model has in the past incorporated the assumption that these states will eventually use their entire share of the river (6.0 mat). It is now clear, however, that most of these projects will not
Operation of Glen Canyon Dam 6 5.8 5.6 5.4 5.2 co o .~ a) a, 4.8- .= 4.6- ~ 4.4- Q a) ~ 4- a) 5 4.2 2 O- 63 . / / , - - Previous Assumption EIS Assumption 1 990 2000 2010 2020 2030 2040 2050 2060 Year FIGURE 4.6 Comparison of previous and current projected depletions from Colorado River above Glen Canyon Dam. SOURCE: Bureau of Reclamation EIS. be built because of their adverse environmental effects and new criteria for allocating the costs of irrigation projects. Therefore, the CRSM estimate of upper basin depletions used for the EIS model runs was set at 4.5 mat per year on average overthe next 50 years (Figure 4.6~. This makes a significant difference (a decrease) in the number of years during which the annual release from the dam will be at the minimum level. Evaporation Loss Since the completion of Glen Canyon Dam, the Colorado River has reached the sea only during 2 years of unusually high runoff years (1983
64 River Resource Management in the Grand Canyon 1984~. This implies that, on average, the 12 mat of water produced by the watershed leaves the system in one of three principal ways: (1) as ev- apotranspiration from irrigated lands (by far the largest quantity), (2) as effluent from urban sewer treatment plants (mostly in Southern California), or (3) as evaporation in transit (mainly from reservoirs). The BOR operating plan for 1994 estimated the total evaporation loss from the entire storage system as 1.6 mat, or 13 percent of the total resource. For Lake Powell the estimate is 507,000 acre-feet (BOR, 1 994a). This is from an average summer-season surface area of 139,000 acres with an estimated annual evaporation of 3.65 feet. This may well be an accurate estimate of evaporation loss in calculating upper-basin depletions for the purpose of evaluating water rights. It is not, however, the total evaporation from the reservoir. It is calculated as the difference between the net evaporation (corrected for precipitation) from the open water surface of Lake Powell and the evapotranspiration from the land surface that was inundated by the reservoir (mostly from phreatophytes). This net loss is calculated as the dif- ference between stream flow gauges (corrected for minor ungauged streams) and the estimated flow at Lee's Ferry without the dam, as shown by gauge data at Lee's Ferry prior to 1963. This continues to be projected as the loss from Lake Powell. While this may provide a correct estimate of depletion caused by dam construction, it is much less than the evaporation that would be used in mass balance calculations for the river simulation model. Bawdy (1991) and Hughes (1974) have estimated the correct net evaporation (corrected for rainfall) as about 5.3 feet for Lake Powell. The significance of the conceptual error in estimating of evaporation can be demonstrated as follows. Since 1964 when the lake began filling, the average storage has been about 15 mat. The surface area at this elevation is 1 15,000 acres, which suggests an average annual net evaporation during the past 30 years of 609,500 acre-feet at a rate of 5.3 feet per year. The BOR estimate at this same lake level is 419,750 acre-feet. This is a difference of 189,750 per year, or 5.7 mat in 30 years. The BOR staff recently calculated all of the monthly corrections to the mass balance equation used by the simulation model during the past 30 years. The total was 6.5 mat of loss unaccounted for by the model. A correct evaporation estimate would account for 88 percent of this error. BOR, however, chose to make the correction as bank storage and added a one- time correction of 6.5 mat to the bank storage estimate in October 1993. It is correct to show the open-water minus the predam evapotranspiration as the "loss" charged to Glen Canyon Dam for upper-basin water depletion
Operation of Glen Canyon Dam 65 calculations. It is incorrect, however, to continue to use this same quantity in the reservoir operating plan mass balance calculations. This introduces an annual error that is greater than the entire proposed Central Utah Project diversion from the river. Storage in Lake Powell There is about 2 mat of dead storage in Lake Powell. Figure 4.7 displays active (not total) storage above the elevation of the river outlet pipe centerlines (elevation 3,374~. The curve in the figure displays the probability that the lake level will be above any given storage volume or elevation (BOR EIS draft, Appendix B. p. B-154~. For example, the figure indicates that 70 percent of the time water is expected to be above the bottom of the radial gates-which is the minimum elevation for controlled releases exceeding the turbine capacity (as necessary for an experimental flood or 50,000 cfs). The elevation of the eight turbine intake pipe centerlines is 3,470 feet above sea level. The elevation below which the turbine operation would cease is 3,490 feet above sea level. This 20 feet of head above the turbine intakes is necessaryto prevent airfrom being sucked intothe turbines. Below an elevation of 3,490 feet, the only way to release water would be from the river outlets, which are at 3,374 feet. Figure 4.7 suggests the probability is zero that water level will fall below the elevation at which the turbines would have to be stopped. This frequency analysis is based upon the Bureau's simulation model CRSS, which as previously stated, underestimates evap- oration from the lake. If corrections for the more accurate evaporation losses were made, the exceedance line in Figure 4.7 would be lowered (except at the left end). Flood Control Current Rules Related to Floods Currently, 2.4 mat of storage space is reserved forfloods on January 1 of each year. This flood storage space is gradually reduced to 500,000 acre-feet in June, when the runoff peak has begun to decline. In addition, the expected quantity of peak season runoff is increased by a safety factor that is highest in January (4.98 marl and declines as uncertainty declines to 2.13 mat on June
66 26 24 22 20 18 16 - o, 1 4 ° 12 co ~ 10 . _ 8 6 4 o River Resource Management in the Grand Canyon - - 3648- Bottom of - ~Radial Gates 3490 = Minimum level for turbine Operation 0 0.2 0.4 0.6 0.8 1 Probability of exceeding 3698 ~ IL o 3656 >m 3640 Led 3603 c,' 3581 <~, 3556 3490 3374 FIGURE 4.7 Frequency analysis of reservoir water levels at beginning of water year (October 1), as predicted by the CRSM. 1. These practices are estimated to prevent floods exceeding turbine ca- pacity of 33,000 cfs in 19 of 20 years on the average. Flood Control Changes Recommended in EIS Additional flood control measures have been recommended in the operations EIS. These measures would reduce the frequency of floods exceeding 45,000 cfs to 1 in 100 years. There are two possible ways to ach- ieve this reduction in flood frequency, as described in the EIS: (1 J raise the top of the spillway radial gates by 4 5 feet, which would provide 0.75 mat of additional flood storage, and (2) change the releases to target a maximum reservoir content of 23.3 mat during spring months until the runoff peak has passed. This is 1 mat less than the previous target level. Reregulating Dam The conventional approach to mitigating the environmental effects of daily fluctuations below hydropower dams is to construct a small dam below the
Operation of Glen Canyon Dam 67 main dam. The purpose is to equalize, or at least reduce, the daily variations in releases from the dam. The amount of storage behind a secondary dam need only be a fraction of the volume of daily releases. In the case of Glen Canyon Dam, a reregulation dam could be located about 17 miles below the main dam, or 1/2 mile above Lee's Ferry in the Glen Canyon recreation area, but outside the national park. Such a dam would raise the water level by about 20 feet at the reregulation dam (BOR, 1994b, p. 49~. This would convert the reach between the two dams from a river (with a prime trout fishery) into an impoundment within which fishing would likely not be allowed because of the rapid changes in stage. The principal benefit of such an arrangement is to allow river releases at approximately the optimum level for environmental purposes while allowing the main dam to follow electrical demand and thus generate maximum hydropower revenues. The economics of the reregulation concept are interesting in that the reduction in the annual value of hydropower estimated by the EIS for the preferred alternative ($30 million annually, estimate from Bureau of Rec- lamation) would pay the capital cost of constructing the reregulating dam in a very few years. A thorough analysis should be done if a reregulation dam below Glen Canyon Dam is to be considered. A reregulation dam below Glen Canyon Dam would allow both maximization of hydropower value and optimal releases forenvironmental objectives-thereby reducing many ofthe concerns centered around operation of the dam. The disadvantage of build- ing a reregulation dam is that it would inundate another portion of the can- yon. Summary The changes in operating rules for Glen Canyon Dam resulting from the Grand Canyon Protection Act and the EIS preferred alternative have no effect on the dam's long-term operation (monthly and yearly releases). These changes affect only the way in which daily average releases are distributed hourly. While the range between revised maximum and minimum allowable release rates is still quite large (5,000 to 25,000 cfs), the possible daily fluc- tuations resulting from the preferred alternative rules are much less-5,000 to 8,000 cfs depending on the monthly release volume target. This smaller range is due to ramping rate limitations that now dominate the allowable short-term variations in dam releases.
68 River Resource Management in the Grand Canyon The predam flows through the Grand Canyon frequently experienced very substantial daily fluctuations due to local storms in the tributaries-a flow regime quite different than constant daily flows. The BOR's Colorado River simulation model has a conceptual error in the way in which evaporation losses from Glen Canyon Dam are calculated. As a result, the losses are significantly understated. The method of simulating both evaporation volumes and bank storage volumes should be improved. A reregulating dam below Glen Canyon Dam may allow both maximi- zation of hydropower value and optimal releases for environmental ob- jectives-thereby reducing many of the concerns that have driven the controversy over the dam's release patterns. RECOMMENDATIONS 1. An analysis of data (including the recent drought period) on reservoir inflow, outflow, evaporation, and reservoir stage is needed to improve the estimate of bank storage in Lake Powell. 2. An analysis of the comparison between releases from the dam (based on energy generated) and the flow measured at Lee's Ferry should be made so that seepage can be determined more accurately. 3. For its mass balance simulation model, the BOR should use actual open-water evaporation rather than depletion (the difference between the open-water evaporation and the predam evapotranspiration). 4. If the option to mitigate environmental effects of daily fluctuations below Glen Canyon Dam includes building a reregulation dam, a thorough analysis of its costs and possible environmental impacts should first be completed. REFERENCES Bureau of Reclamation. 1986. Special Report, Colorado River Alternative Operating Strategies for Distributing Surplus Water and Avoiding Spills. BOR, Denver, Cola. Bureau of Reclamation. 1988. Glen Canyon Environmental Studies. Final Report, U.S. Department of the Interior, Washington, D.C. Bureau of Reclamation. 1994a. Annual Operating Plan for Colorado River Reservoirs, 1994. U.S. Department of the Interior, Washington, D.C.
Operation of Glen Canyon Dam 69 Bureau of Reclamation. 1994b. Operation of Glen Canyon Dam. Environ- mental Impact Statement, U.S. Department of the Interior, Washington, D.C. Dawdy, D.R. 1991. Hydrology of Glen Canyon and the Grand Canyon. In Colorado River Ecology and Dam Management. Washington, D.C.. National Academy Press. Graf, J. 1993. GOES Travel Time and Dispersion. Draft Report. Tucson: U.S. Geological Survey. Hughes, T. C. 1974. Water Salvage Potentials in Utah, Vol. 1. Utah Water Research Laboratory, PRWA22-1. Logan: Utah State University. Smith, J., and S. Wiele. 1993. GOES SedimentTransport Study Draft Report. Boulder: U.S. Geological Survey Water Resources Division. U.S. Geological Survey. 1993. Water Supply Records, Arizona. Western Area Power Administration. 1994. Salt Lake Area Integrated Projects Firm Power Proposed Rate Adjustment Brochure, Salt Lake City. Western Area Power Administration. 1988. Salt Lake City Area, Analysis of Alternative Release Rates at Glen Canyon Dam. .
