7
FLOOD CONTROL AND ENGINEERING CONSIDERATIONS

Issue 5 The Potential for Flood and Erosion Control Devices to Fail

THE WILSHIRE GROUP POSITION

The Wilshire group has expressed concern about the long-term stability of engineered flood and erosion control facilities. In their first memorandum (Wilshire et al., 1993a), referencing the draft EIR/S, they referred to possible failure of ''flood control devices'' without specifying which devices they were addressing. In their expanded report (Wilshire et al., 1993b), they referred to "site evaluation documents that incorrectly claim" that the upslope diversion berms will protect the primary flood control berm against erosion. In addition, they predicted channelization of surface water at the ends of the diversion berms and consequent threats to the integrity of the trench cover. Thus their two main concerns are: (1) the proposed flow diversion or breakup berms upslope to the west of the 70-acre LLRW site and (2) the proposed rip-rapped flood-protection barrier around the LLRW site. The Wilshire group questioned the purpose or function of the breakup berths and postulated that channelization at the end of the failed berms would force concentrated runoff toward the western edge of the main rip-rapped flood-protection barrier. They claimed that water would pond in front of the barrier, giving rise to potential leakage into the trenches. They raise an additional concern that channelized runoff from the breached breakup berms would undercut the rip-rapped flood-protection barrier and cause slope failure (Wilshire et al., 1993b; Wilshire et al., 1994).

THE DHS/U.S. ECOLOGY POSITION

DHS has indicated that the design of the durable flood-protection structures at the disposal site is based on conservative U.S. NRC guidance to protect LLRW facilities. This guidance is based on Corps of Engineers design procedures for erosion protection during large storms on alluvial fans, and on U.S. NRC guidance for erosion protection at uranium mill tailings facilities. DHS reputes that this guidance "represents the state of the art in hydraulic design and erosion protection" (Brandt, 1994).



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology 7 FLOOD CONTROL AND ENGINEERING CONSIDERATIONS Issue 5 The Potential for Flood and Erosion Control Devices to Fail THE WILSHIRE GROUP POSITION The Wilshire group has expressed concern about the long-term stability of engineered flood and erosion control facilities. In their first memorandum (Wilshire et al., 1993a), referencing the draft EIR/S, they referred to possible failure of ''flood control devices'' without specifying which devices they were addressing. In their expanded report (Wilshire et al., 1993b), they referred to "site evaluation documents that incorrectly claim" that the upslope diversion berms will protect the primary flood control berm against erosion. In addition, they predicted channelization of surface water at the ends of the diversion berms and consequent threats to the integrity of the trench cover. Thus their two main concerns are: (1) the proposed flow diversion or breakup berms upslope to the west of the 70-acre LLRW site and (2) the proposed rip-rapped flood-protection barrier around the LLRW site. The Wilshire group questioned the purpose or function of the breakup berths and postulated that channelization at the end of the failed berms would force concentrated runoff toward the western edge of the main rip-rapped flood-protection barrier. They claimed that water would pond in front of the barrier, giving rise to potential leakage into the trenches. They raise an additional concern that channelized runoff from the breached breakup berms would undercut the rip-rapped flood-protection barrier and cause slope failure (Wilshire et al., 1993b; Wilshire et al., 1994). THE DHS/U.S. ECOLOGY POSITION DHS has indicated that the design of the durable flood-protection structures at the disposal site is based on conservative U.S. NRC guidance to protect LLRW facilities. This guidance is based on Corps of Engineers design procedures for erosion protection during large storms on alluvial fans, and on U.S. NRC guidance for erosion protection at uranium mill tailings facilities. DHS reputes that this guidance "represents the state of the art in hydraulic design and erosion protection" (Brandt, 1994).

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology THE COMMITTEE'S APPROACH The following discussion about the erosion-control and flood-control facility features is based on the committee's examination and analyses of (1) available design documentation, including appropriate license application materials; (2) summary reports and materials provided by both opponents and proponents of the project; (3) written information provided during the two NAS/NRC committee meetings in Needles in July and August/September, 1994; (4) field visits to the facility site and to nearby I-40 drainage facilities in July and August, 1994; and (5) other relevant scientific references. DESCRIPTION OF PROPOSED FACILITIES Location and general description of the Ward Valley low-level radioactive waste (LLRW) disposal facility has been presented in Chapter 2. A detailed description of the major elements of the proposed LLRW facility which involve engineered flood and erosion control facilities follows. Radiological Control Area Facilities General Trench Area Design The proposed Radiological Control Area consists of a 5321 m by 532 m (approximately 28 hectares) area surrounded by a 0.9 m to 1.5 m high flood protection berm and an electrified security fence, within which near surface LLRW disposal operations will take place in a series of five progressively developed trenches. Four unlined Class A waste trenches and one unlined Class B/C trench are planned and are illustrated in Figures 7.1, 7.2, and 7.3 (U.S. Ecology, 1990). A planned buffer zone will extend 122 m around the perimeter of the fenced control area for carrying out environmental monitoring activities, maneuvering construction equipment, and allowing corrective actions to be implemented. The base of the Class A trenches will be 18 m below the natural surface, and the Class B/C trench will be 13 m deep to eliminate any potential lateral flow from the bottoms of the Class A trenches into the B/C trench. All waste will be buffed up to 6.1 m below the original ground surface and beneath the deepest projected scouting action predicted by a Probable Maximum Flood (PMF). The four Class A trenches are about 470 m long by 88 m wide at final grade and 34 m wide at the bottom. The interior side slopes of these trenches are a ratio of 1.5 horizontal (H) to 1 vertical (V) (1.5H:1V). A 12 m wide space at the surface separates each completed 1   All conversions from English system (feet/pounds) to metric (meters/grams) have been rounded to two significant figures. See conversion table in Appendix C.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Figure 7.1 LLRW control site, perimeter flood-protection berm, part of the upslope breakup berms, typical excavations for both trench classes, and other site features superimposed on an existing topographic contour map for the area. (James L. Grant & Associates, 1989)

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Figure 7.2 LLRW disposal site showing finished grade contours and cross section locations through trench (James L. Grant & Associates, 1989)

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Figure 7.3 Typical south-north (BB') and west-east (CC') cross sections through trench and other typical sections (James L. Grant & Associates, 1989) (Not converted to metric scale; see appendix C for conversion factors).

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology trench. The Class B/C trench is about 470 m long by 69 m wide at final grade and 31 m wide at the bottom. The inside side slopes of this trench are also 1.5H:1V. All five trenches will be excavated parallel to the slope of the ground surface and the approximate dip of sediments comprising the alluvial fans. Both Class A and B/C trench bottoms will slope about 2 percent west to east away from the direction of disposed wastes. The trench covers for all five disposal trenches will consist of a 2.4-m thick silty sand vegetative-support layer overlying 5.2 m of reworked sediments that were removed from the trenches. This means that the total trench cover thickness of 7.6 m includes the 6.1 m cover of fill over both classes of waste plus a permanent 1.5-m high flood-protection cap/berm. The waste disposal trench cover is designed to shed surface flow resulting from the Probable Maximum Precipitation (PMP) storm event, and the resulting surface runoff will be directed laterally into four shallow 0.3 m deep swales which slope about 2 percent toward the east. Drainage from the swales and off the cover will be directed across a system of rip-rapped chutes and outlet controls along the edge of the eastern berm toward Homer Wash. No design details are apparent in the drawings or license application for the chutes and outlet controls. Onsite storm water that falls outside the trench, but inside the perimeter flood-protection berm during active operation, will be prevented from entering the trenches by a planned combination of temporary berms and ditch system to divert water from the excavated trench areas over the natural ground. No specific management plan is indicated for dealing with rain water that falls directly into the open trenches and collects during active waste disposal operations, other than through natural evaporation, removal if necessary, or "proper" handling (LA Sections 3100.1.1 and 3200.4). The California DHS noted in its Summary Report, however, that "standing water or moist sediments will be removed from the trenches and tested for radioactivity" (Brandt, 1994). Flood Protection Berm The design plans for the site indicate that offsite storm water will be prevented from entering the trench area during operation and after closure by a permanent flood-protection berm surrounding the disposal site. The berm, which is to be constructed when site construction begins and incorporated into the final site cover at closure, is designed to withstand the Probable Maximum Flood (PMF). The berm will rise 1.5 m above the original ground surface on the upslope (western) side to 0.9 m on the downslope (eastern) side and is designed to prevent overtopping by wind and wave action during the PMF event. The purpose of this berm is to contain surface-water flow during a PMF event and, from the approximately 10 km2 drainage area upslope from the facility, for the same flood, to divert flow around the north and south sides of the facility during operations and after closure. The permanent berm has 12H:1V outer exposed side slopes and 1.5H:1V interior side slopes. Embankment armoring consisting of a 0.9 m thick layer of 0.6 m average size stone rip rap and a 45.7 cm thick filter base of 7.6 cm maximum size gravel is proposed to stabilize the

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology surface against wind and water erosion. The outer embankment armoring system is to be extended at a 2H:1V slope to a depth of 1.5 m into the subsurface to provide some scouting protection from adjacent surface water flow and to interrupt any lateral flow of infiltrating precipitation through the shallow calcrete layers. The outer stone flood protection berm represents about 3 ha (7 acres) of exposed surface area, or about 10 percent of the approximately 28 ha (70-acre) radiological control area. This highly permeable surface will encourage directly-falling precipitation, or non-flood water, to infiltrate into the berm slope fill and possibly add recharge water to the edges of the trench zone over several decades of exposure. Because the upstream toe of the facility flood berm is nearly parallel with the topographic contours, little gradient exists for water to flow away from that side, and there could be a tendency for water to pond in the middle of the western side. No floodwater, however, is expected to encroach on the disposal site from Homer Wash, which is located about 760 m to the east, because the nearest natural surface elevation at the site is reported to be over 13 m above the estimated 100-year flood level and 13 m above the estimated PMF elevation. Interpretation of these data indicates that the bottom of the Class B/C trench would be about 0.6 m higher than the Homer Wash PMF peak and 1.5 m higher than the 100-year flood, but the Class A trenches would be about 4 m and 5 m lower than these floods, respectively. Breakup Berms Offsite storm water flows onto the LLRW site from the west as sheet flow or in small rills. A series of shallow-flow breakup berms will be placed in a staggered, offset chevron pattern upslope and west of the disposal facility, (Figure 7.4), primarily (1) to increase sheet flow roughness for the purpose of reducing the sheet flow velocity and inducing more tranquil subcritical hydraulic conditions near the permanent primary flood control berm to reduce scour potential. and (2) to divert storm runoff to the north and south of the facility (LA Section 3310.2). Each berm that forms part of the chevron pattern would be 122 m long, 3 m wide and 0.3 m high. These shallow flow breakup berms, however, are intended to be temporary features that will be constructed during the initial trench excavation with materials removed from the trenches and maintained during the operations and institutional control periods. A more detailed analysis of the breakup berms is discussed later in this chapter. HYDROLOGICAL SETTING The general Ward Valley Watershed geology, hydrology and climatology were described in Chapter 2. Details of the hydrological characteristics of the 127 km2 Homer Wash Watershed upstream from the LLRW site and the 9.8 km2 local site subbasin hydrology

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Figure 7.4 Schematic plan view of temporary breakup berms. which drains toward the LLRW from the west are described in Box 7.1 and Box 7.2, respectively. These summarize the information from which the committee has drawn certain conclusions about the ability of the design to divert water from the radiological control area and about the stability of the flood-control berm. The hydrologic and hydraulic information were extracted from the administrative record license application (sections 2410, 3200, 3200.2, 3440, and 5110) and from the license application interrogatories/responses (0449A3440.2 through 0453A3440.2, 0454A3440.4 thorough 0454A3440.4, and 0449B3440.2) and from the DHS summary report (Brandt, 1994). Homer Wash Watershed A complete hydrologic description and analysis of Homer Wash Watershed above the LLRW site was needed to define the proximity of the site to the estimated 100-year flood plain and the Probable Maximum Flood (PMF) (Box 7.1); to classify the FEMA flood zone for the site; and to determine the short- and long-term flood protection requirements, if necessary, against the PMF. Because no historical flow data were available for Homer Wash

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Box 7.1  Design Storm Parameters for the Homer Wash Watershed 24-hour, 100-year storm depth 3.5 in (8.9 cm)1 1 sq. mile, 1 hr. Prob. Max. Precip. (PMP) depth 10.9 in (27.7 cm) 1 sq. mile, 6 hr. PMP depth 14.7 in (37.3 cm) 9.2 sq. mile, 6 hr. PMP depth for Homer Wash 10.7 in (27.2 cm) HEC-1 Computer Program Flood Peak Results for Homer Wash: 24-hr, 100-year flood flow peak 1,461 cfs (41.4 m3/s) 6-hour PMF 19,230 cfs (544.6 m3/s) Estimated LLRW Site Disposal Area Elevation Above Calculated Peak Flood Level, using HEC-2 Computer Program Water-Surface Profile Analysis: 100-year Flood 47 ft (14.3 m) hour PMF 44 ft (13.4 m) 1 Conventionally, English units are used for this type of measurement and its tributaries, appropriate hydrologic data were developed for conducting computer simulation modeling for various assumed floods. The following data summarize the Homer Wash Watershed characteristics upstream from the LLRW site: Drainage Area 127.4 km2 Soil Conservation Service (SCS) Hydrologic Soil Group (described in Chapter 2): Zone 1 (HSG "D") 32% of Drainage Area Zone 2 (HSG "C") 0% Zone 3 (HSG "B 13% Zone 4 (HSG "A 55%

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The Homer Wash Watershed was subdivided into 17 sub-basins. Standard SCS procedures were used to determine parameters such as SCS weighted Curve Number, which combines the effects of soil type, surface cover and land use, and assumed antecedent soil moisture condition (AMC) plus time of concentration and lag time (USDA/SCS, 1986; USDA/SCS, 1972). These parameters are summarized in Table 7.1 for each of the sub-basins (LA Section 2410.3.1). The hydraulic lengths for the 17 sub-basins range from 1,676 m to 12,800 m; the average sub-basin watershed land slopes range from 1.5 to 14 percent; the SCS Curve Numbers vary from 52 to 80, and the times of concentration from 1.03 to 4.18 hours. The studies indicate, based on the above hydrologic data and computer results, that the site is located well above the Homer Wash floodplain for floods through the 100-year flood and up to the Probable Maximum Flood (PMF), which is based on Probable Maximum Precipitation (PMP). The PMF is the flood that may be expected from the most severe combination of critical meteorologic and hydrologic conditions that are reasonably possible in the region. While the PMF is considered a rare event that is not generally associated with a fixed return period or exceedance probability, it is typically assigned a return period of 1,000 to 1 million years (Moser, 1985). A 100-year flood is an event that has a 1 percent chance of occurring or being exceeded in a given year (or once, on average, every 100 years). From the principles of probability, however, the chance of a 100-year flood occurring during any given 100-year interval is about 63 percent. To put the current LLRW site situation into perspective, the probability of a 100-year flood occurring during the 30 years of active facility operation is 26 percent. For the expected 500-year project design period, the chance of a 100-year flood happening is 99.34 percent. These probability calculations do not consider any future effects of global warming, land-use changes, or other long-term temporal changes that would affect precipitation or runoff processes. The committee agrees with the license applicant that I-40, which acts like a low dam across Homer Wash, does not represent a realistic threat to the LLRW site. The likelihood is small that the road will fail and threaten the LLRW site. Dam failure analysis is, therefore, not necessary in the Homer Wash flood analysis. Local Site Surface Hydrology The local site watershed is defined as the area upgradient to the west of the site that could contribute storm-water runoff over the site. This area is given as sub-basin 65 (Table 7.1). The upslope local drainage rises to the west to a maximum elevation of about 1086 m above mean sea level (AMSL) in the Piute Mountains. This compares to an elevation of about 645 m in the middle of the facility. The estimated PMF and its associated Probable Maximum Precipitation (PMP) (Box 7.2) were chosen as the bases for the design of flood protection features for the facility during operations, as well as for the site closure and stabilization period (LA Section 3440.2). While the use of the PMF is clearly acceptable for the relatively short-term operational design of low-level waste facilities, its use is not required, and a less conservative design basis such as a

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Table 7.1 Homer Wash sub-basin hydrologic summary (Note: Sub-basin 65 represents the local site drainage hydrology) (LA Section 2410) (Original data not converted to metric units; see Appendix C for conversion factors).  

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology stable for at least 25 years, even though they have been exposed to direct wash flow conditions since I-40 had been constructed. No geotechnical stability analysis was performed on the 12H:1V external slope of the 1.5 m high flood protection berm for the site because of the low height and the relatively flat slope (LA Section 3100.3). The license application does not appear to present or address any plan for monitoring differential settlement in the 70-acre trench area. Temporary Breakup Berms In addition to the 1.5 m high flood protection berm, fifteen 0.3 m high flow "breakup berms" arranged in a chevron pattern, as described previously in this chapter, are proposed to be constructed west of and upgradient from the western-facing flood protection berm, to "slow and divert storm runoff to the north and south of the (LLRW) facility, and to ensure that flood flows remains subcritical near the permanent primary berm." The berms would be constructed during initial trench excavation from materials removed from the trenches, but not tip rapped (LA Sections 3310.2; 3200.5; 3440.1). Earlier, it was pointed out that no clear hydrologic flood criteria or basis for designing the upslope temporary breakup berms was apparent in any reviewed LLRW documents. At the end of a series of interrogations and responses on the breakup berms, U.S. Ecology's response to an interrogatory asking them to expand on the details of the breakup berms was: The small-flow breakup berms are constructed from the native material and are designed to withstand and divert the 100-year flood events. Once the primary berm is constructed and trench cover is complete, there will be no need for maintenance of the breakup berms (California DHS, 1990, Interrogatory No. 0216A Section 3100.1.1, 1990). Therefore, although it is not apparent that the design of the temporary breakup berms was based on a consistent hydrologic criterion, i.e., 100-yr or PMF, the main purpose of the berms is to provide hydraulic roughness for slowing down the water velocity as it approaches the western flood protection berm at the LLRW site. The long-term integrity of the temporary breakup berms does not appear to be critical. Site opponents, however, have raised the possibility of secondary effects resulting from postulated breaching of the berms. The Wilshire group postulated that "channelization of runoff by breaching of the berms, and integration of individual small channels into larger ones by capture induced by the berms" (Wilshire et al., 1994) could have one of two consequences: (1) runoff trapped by the N-S main flood control berm, ponding and potential infiltration and leakage into the trenches, and (2) undercutting of the rip-rap cover at the upslope corners of the main flood-control berm leading to its deterioration. The Wilshire group cite, as anecdotal evidence for their concern about the stability of these breakup berms with respect to erosion and breaching, nearby graded road berms, an old

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology I-40 borrow pit area located at the northwest edge of the LLRW, and General Patton's base camp at the south end of Ward Valley (Wilshire et al., 1994). However, Wilshire later acknowledged during his August 30, 1994, presentation before the committee that these noted features may not have been engineered and constructed for the purpose of withstanding flooding or erosion forces. The Wilshire group is correct in assuming that the small breakup berms will subsequently be eroded and breached over time. The small volume of sediment that may be eroded from the breakup berm will either wash past the flood protection berm in suspension (in the sheet wash) or deposit against the western upslope face of the rip-rapped flood protection berm. It is the committee's view that the additional material will not adversely affect the stability or performance of the flood protection berm. In the committee's judgment, the erosion and breaching of the breakup berms over time are not a critical problem because the berms, in some form, will remain and offer some resistance to sheet flow, thus achieving their main objective although they were not designed as permanent structures. As has been stated, their purpose is mainly redundant for ensuring subcritical flow at the downslope flood protection berm. If breaching of these berms occurs and induces channelization and concentrates flow, however, the rip-rapped flood protection berm design is adequate, in the opinion of the committee, because there is still at least a 46 m safety margin distance between scouting at this location and the proposed trenches. In the committee's judgement, the flood protection berm and rip rap/filter system were effectively engineered to maximize the protection of the LLRW site from flooding and erosion, without apparently giving consideration to possible ponding or infiltration along the upstream edge of the flood protection berm toe. It is the committee's opinion that ponding along the relatively level upstream edge of the flood protection berm and potential infiltration and leakage into the adjacent trench zone are possible because of the highly permeable stone and gravel rip rap/filter layers along the exposed and subsurface parts of the flood protection berm slope. The possibility of slow vertical movement of ponded water into the underlying vadose zone or the possibility of perched saturated lenses developing in the unsaturated zone may enhance the potential for lateral movement toward the waste trench. Similarly, the Committee considers that the approximately three ha (seven acres) of exposed rip rap around all four sides of the flood protection berm will encourage directly-failing precipitation to infiltrate into the berm slope fill and possibly contribute recharge water to the trench zone over decades of exposure. In summary, it is not clear what hydrologic design criterion was used for the small breakup berms, but this does not seem critical in terms of their intended function and lifespan. While the Wilshire group raises legitimate questions about possible channelization effects, such as scouring at the upstream corners of the downstream flood-protection berm, and possible ponding along the western edge of the flood-protection berm with seepage into the trench area, both of these concerns appear to be adequately addressed through effective flood-protection berm considerations.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The PMF Design The use of Hydrometeorological (Hydromet) Report 49 data and procedures for estimating the Probable Maximum Precipitation (PMP) in the Colorado River and Great Basin Drainages (NOAA, 1977) is appropriate and reasonable as a basis for determining the PMF design flood. The U.S. Army Corps of Engineers HEC-1 Flood Hydrograph Computer Package (1990); Soil Conservation Service (SCS) National Engineering Handbook-Section 4 (NEH-4) (1972); SCS TR-55 Urban Hydrology for Small Watersheds (U.S. SCS, 1986) for hydrologic modeling; and the Corps of Engineers HEC-2 Water Surface Profile Computer Package (U.S. Army, 1990) for hydraulic modeling are standard and reasonable methods for analyzing the assumed flood flows for flood protection and cover drainage facilities design and for predicting the potential flood impact of Homer Wash at the site. Finally, the hydrologic analysis for both Homer Wash and local site PMF and 100-year flood peak computations apparently used SCS Weighted Curve Numbers, which were based on the assumption of average Antecedent Moisture Condition, or AMC II. While this design criterion assumption is reasonably conservative and meets the minimum regulations for the design of the perimeter flood protection berm, the use of the more conservative saturated soilmoisture condition, or AMC III, would have resulted in even higher peak flows for each of the assumed flood events. Application of AMC III for analyzing the Homer Wash PMP and 100-year floods, however, would not have any significant effect on the floodplain analysis since the site is already well outside and above the 100-year and PMF floodplains. Historical Flood Experience Because of limited development in Ward Valley, no surface-water flow has been monitored and no historical flow data are available for Homer Wash (LA Sections 2410.3.2, 2410.6). Regional streamflow records are, however, available for estimating the 100-year flood and a Regional Maximum Flood, for comparing with the computed 100-year flood and Probable Maximum Flood, respectively, for Homer Wash and at the local site. Hydrologists at the U. S. Geological Survey have analyzed historical flood peaks for each of the six hydrologic regions of California, including the 129,500 km2 South Lahontan-Colorado Desert (SL-CD) Region of Southeastern California, which contains the Ward Valley and Homer Wash (Waananen and Bue, 1977). Magnitude and frequency of floods were developed from regression analysis of flood peaks located at 43 stream gage stations in the SL-CD Region. For the SL-CD Region, the 100-year flood peak is defined as where Q100 = 100-year peak flood flow (cubic feet per second [cfs]); A = Drainage area in square miles.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The 100-year flood regression equation is limited to a maximum watershed size of 65 km2. The regression equation yields a peak flow of about 2790 cfs (79 m3/s) for the 9.8 km2 drainage area above the LLRW site (Figure 2.2). This represents about an order of magnitude higher peak flow than the 233 cfs (6.3 m3/s) produced by HEC-1 computer simulation. No 100-year flood comparison was made for Homer Wash, since this 127 km2 watershed exceeded the watershed size limit for the regression equation. Maximum observed peak flood flows from 883 sites in the conterminous United States, for drainage areas of less than 25,900 km2, were analyzed by geographical regions by Crippen and Bue (1977). Envelope curves were computed that yield "reasonable limits" for estimating extreme flood potential for each region. Ward Valley and Homer Wash lie within Region 16 and the Regional Maximum Flood from the envelope curve produces about 29,000 cfs (820 m3/s) and 190,000 cfs (5,400 m3/s) for the local site drainage area (10 km2) and Homer Wash (127 km2), respectively. The local site Regional Maximum Flood peak is about three times the PMF peak of 10,270 cfs (290 m3/s) from HEC-1 computer analysis and the 190,000 cfs (5,400 m3/s) Homer Wash Regional peak is about an order of magnitude higher than the calculated PMF peak. No frequency or probability values were assigned to these Regional Maximum Floods. The above comparisons of computed flood peaks, which were based on average soil moisture conditions and used in the LLRW facility design, to significantly higher regional flood peak potential, suggest that the conservative flood peak estimate used for the flood protection berm design may be exceeded and, therefore, should not be regarded as the outer limit of flood potential at the site. GEOMORPHIC EVIDENCE OF EROSIONAL STABILITY The Wilshire group (Wilshire et al., 1994) referred to older or late Pleistocene surfaces that: . . . are in the process of erosional degradation as seen by the degree of dissection and the disturbed pavements on remnants of the surfaces in medial and distal parts of the alluvial aprons. In contrast U.S. Ecology, (LA Section 2310, p. 26), claims that: [t]he presence of surface and near surface relict and buffed paleosols at the site indicates that the area is geomorphologically stable and has low sedimentation rates. These statements seem to be important contrasts in understanding the stability of the surface and surficial deposits at the Ward Valley site. The geology of the alluvial fans and depositional processes provide a longer history pertaining to the surface stability and flooding potential for the site.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Drainage Incision and Alluvial Surfaces The alluvial fan deposits along Ward Valley in the area of the proposed site have commonly coalesced into relatively uniformly sloping deposits or bajadas. The drainage channels, especially in the vicinity of the site, are very shallow and are interwoven into a braided network (Figure 2.3). The network of rills and shallow channels indicates little incising or downcutting within the channels. Connections in this network may change locally with tune. All flow in these channels down the alluvial fan slope is ephemeral. A single geomorphic surface covers much of the site area, and within the administrative record the thin alluvial deposits on this surface are designated Qf1. As described by Shlemon (Appendix 2310.A, Addendum C), Qf1 deposits show no soil development and are inferred to be geologically young and pedologically immature. The network of shallow channels is developed on this surface in the vicinity of the site. Local areas are slightly higher than the surface of Qf1, and these areas are underlain by moderately developed relict soils or paleosols on which a surface designated Qf2 developed. From the trench descriptions, these Qf2 paleosols underlie much of the area, though they may be covered by very thin Qf1 deposits. Qf2 paleosols are developed similar to other paleosols in the region; from this Shlemon infers an age of 35,000 to 40,000 yr for the Qf2 paleosols. A deeper paleosol (Qf3) exposed in some trenching at the site is also compared to other regionally developed paleosols. The age of Qf3 is estimated by Shlemon to be about 100,000 yr by comparison. Evidence For Fan Deposition The relationships between soil units and surficial deposits in the vicinity of the site provides evidence that there has been some slight regrading of the alluvial fan. This added a thin veneer of more recent sediment over much of the area. The smaller areas that are slightly higher and have older Qf2 at the surface are more vulnerable to erosion. They would be expected to be regraded if no baselevel changes or tilting occur. In general, however, the thin veneer of deposits indicates a relatively low net deposition at the site area. The committee finds that surface deposits, paleosols, and a network of very shallow channels or rills are consistent with a surface in dynamic equilibrium for the past thousands to tens of thousands of years. Engineering analysis provides estimates of substantial potential channel erosion based on hypothesized large and infrequent rainfall and runoff events. Nonetheless, in the committee's judgement, the surficial to near-surface deposits do not indicate such events under natural conditions for thousands of years in the past; from a geological perspective, the engineering estimates of channel erosion appear conservatively large.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Homer Wash Erosion Potential Homer Wash is an ephemeral stream draining Ward Valley from north to south. It has a low-sinuosity channel occupying approximately a mid-valley position. With a relatively high sediment flux from the alluvial fans and low runoff, Homer Wash has little potential to develop sinuosity or extensive floodplain deposits. Larger vegetation persists along the channel banks because of ephemeral runoff. Homer Wash and its equivalents in the past have apparently maintained much the same position, assuming the resistivity measurements partly reflect finer-grained deposits. Alluvial fan systems between these ranges will tend to maintain this position as well, as long as no uneven tilting occurs across the valley. Conclusions From Geomorphic Evidence The geomorphology of the Ward Valley site and immediate surroundings indicates dynamic equilibrium over tens of thousands of years. This reflects a certain balance between the climate and tectonics of the area. Changes in climate or relative base level would have to be significant and in the vicinity of the site to have an appreciable effect within a timeframe of less than thousands of years. A significant baselevel change at the distance of Danby Dry Lake would require a long period of time before stream gradients would adjust and cause either erosion (because of a baselevel drop) or deposition (because of a baselevel rise) at the site. If a significant fault scarp were to develop downslope of the site, it could cause considerable erosion and regrading across the site. The probability of such an event in this specific position appears to be low. A significant increase in future precipitation could eventually change the gradient in Homer Wash and could result in some regrading across the site. However, as discussed above, the paleosol and other geomorphic evidence for long term stability over tens of thousands of years despite intervening major climate changes, as well as the stabilized, vegetated banks of the wash, suggest that any increase in precipitation would take some time to cause geomorphically significant changes to Homer Wash. The site and immediate area show no discernible evidence of having undergone a change in their basic geomorphic stability. CONCLUSIONS: ADEQUACY OF PROPOSED FLOOD PROTECTION SYSTEM DESIGN Based on the foregoing discussion, observations, and analysis, the committee concludes the following concerning the potential for flooding and the engineered barriers and flood controls.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology The hydrologic and hydraulic criteria, procedures and documentation used for analyzing the Homer Wash floodplain appear adequate. Because both the 100-year and Probable Maximum flood peak elevations on neighboring Homer Wash were estimated to be between 14.3 and 13.4 m below the LLRW facility site, potential flooding from Homer Wash is not considered to be a safety concern at the site. The hydrologic and hydraulic criteria, procedures, and documentation used for designing the 1.5 m high flood protection berm appear to meet the minimum requirements for this type of facility. The proposed 12H:1V sloped flood protection berm and engineered rip-rap armoring system appear adequate to protect the berm from the long-term, desert surface runoff or sheet flow from a PMF event, which is postulated to occur during assumed average soil-moisture conditions. While this design criterion assumption is reasonably conservative, the design flood still could be exceeded, either assuming saturated conditions or considering regional historical flood data. Erosion and breaching of the breakup berms will likely occur over a period of a few decades, but this is not critical to their main function and purpose in providing resistance to flow. Furthermore, any postulated channelization toward the LLRW site and any resulting scouring around the upstream corners of the flood protection berm appear to be adequately addressed by the above- and below-grade rip-rap design, especially because (a) at least a 46-m safety margin lies between scouring at this corner location and the LLRW material in the trenches and (b) waste is to be buried deeper than the estimated maximum scour depth during the design flood event. The flood protection berm appears to have been effectively engineered to maximize the protection of the LLRW site from flooding, without giving consideration to possible ponding or infiltration along the upstream edge of the flood protection berm toe. Ponding along the relatively level upstream edge of the flood protection berm and resulting potential infiltration and leakage into the adjacent trench zone are possible because of the highly permeable stone and gravel rip rap/filter layers along the exposed and subsurface parts of the flood protection berm slope. The possibility of slow vertical movement of ponded flood water into the underlying unsaturated zone or the possibility of perched saturated lenses developing in the unsaturated zone may enhance the potential for lateral movement toward the waste trench. The alluvial-fan geomorphology and shallow paleosols indicate dynamic equilibrium and little natural erosional potential. Ephemeral Homer Wash shows little evidence of floodplain development, consistent with engineering analyses placing the site well above the level of a hypothetical PMF or 100-year flood. The above conclusions and opinions were reached, following a review of the methods, proposed design, and stated assumptions, and are considered to be within a degree of reasonable engineering and scientific certainty. No one can predict exactly whether or when the assumed design flood will occur or be exceeded by a catastrophic storm event and exactly how the facility will perform under either circumstance. Construction quality and commitment to long-term maintenance and performance

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology monitoring are key factors in predicting how effectively a reasonably well-designed facility will meet its intended purpose. Recommendations The above analysis of flood protection facility design information and the breakup berms leads to the following recommendations for consideration if the site is developed for a LLRW facility. While erosion and breaching of the breakup berms will likely occur over a period of a few decades, the berms will probably continue to provide flow resistance for several additional decades. If greater stability or longer-term performance is desired for these breakup berms, a more substantive design to minimize breach failure and possible channelization may be considered. One possible solution would be to use low, permeable rock dikes instead of local trench material. Such rock would provide desired roughness and be more stable over the lifetime of the facility, thus reducing the possibility of negative secondary effects such as the formation of small flow channels. To address the possibility of ponding and infiltration, the committee recommends consideration of the use of an engineered channel (sloped to eliminate the possibility of ponding along the upstream edge of the flood protection berm and lined to reduce the possibility of infiltration into the underlying unsaturated zone), for conveying flood water around the west, north, and south sides and corners of the flood protection berm. For additional infiltration protection, the committee recommends the use of an impermeable geosynthetic or alternative barrier under the flood protection berm's gravel filter layer to reduce both flood and non-flood rainfall event infiltration into the berm zone and lateral seepage toward the trench area. The committee recommends testing the integrity of the proposed flood-protection berm for an assumed saturated antecedent soil-moisture condition (AMC III) when the PMP event is simulated. The designed flood-protection berm should be able to withstand this event, for at least the stillwater condition, with zero freeboard. In light of the relatively high Regional Maximum Flood peak potential compared to the computed design PMF peak, the committee recommends that the peer-review panel of hydrologic experts also assist DHS in reviewing extreme hydrologic event potential at the site and in recommending if any additional engineering design response is needed to defend the flood protection berm against such a rare flood event. The committee recommends that more design and construction detail be provided for the rip-rapped chutes and outlets, which are proposed for conveying concentrated surface water drainage from the four cover swales while minimizing long-term scouring and infiltration.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology In the committee's view, stability analysis should be conducted of 1.5-m high flood protection embankment slope to confirm stability under assumed seismic and toe-water conditions. The committee recommends that a long-term monitoring plan be developed for detecting significant differential settlement of the trench-cover area and a response program for mitigating its potential negative effect(s) on surface drainage and floods. This plan also should include a comprehensive, operational and long-term flood and erosion facility monitoring and response program for identifying, repairing, or mitigating any stability, scouring, or sediment deposition problems which develop. The committee reemphasizes the Chapter 6 recommendation for an independent scientific oversight advisory panel of experts to review all monitoring data and the proposed response plan. The committee recommends the development of a management plan for removing and properly disposing standing water from the open trenches during construction. REFERENCES Ad Hoc Interagency Committee on Dam Safety/Federal Coord. Council for Science, Engineering and Technology. Jun 1979. Federal Guidelines for Dam Safety. U. S. Govt. Printing Office, 041-001-00187-5. Brandt, E. C. October 7, 1994. Summary submittal of the California Department of Health Services to National Academy of Sciences Committee to review specific scientific and technical issues related to the Ward Valley, California LLRW site. pp. 8, 32, 43. California Department of Health Services (DHS), submitted by Roy F. Weston, Inc., Consultants. May 10, 1990. Interrogatories of concerns answered by U.S. Ecology, Inc. for License Application Sections Related to Flood Protection: Sections 3100, 3200 (No.s 0288A3200.2.1 and 0306A3200.5), and 3440 (No. 0457B3440.4). California Department of Health Services (DHS). Aug 1993. Summary of comments from June-August 1991; comment period on final environmental impact report/statement (FEIR/S) and Department responses to State of California Indemnity Selection and Low-Level Radioactive Waste Facility. Sections 5.8 (Weather and Flash Flooding) and 5.11 (Erosion). Chow, V. T. 1959. Open-channel hydraulics. New York: McGraw-Hill. Chapters 1-8. Crippen, J. R. and C. D. Bue. 1977. Maximum flood flows in the conterminous United States. U.S. Geological Survey, Water Supply Paper 1 887:15, 52. Federal Emergency Management Agency (FEMA). Sept 1985. Flood insurance study guidelines and specifications for study contractors. FEMA Report 37. Franzini, D. L. Freyberg, and G. Tchobanoglous. 1992. Water-resources engineering. Ch. 7, 8. License Application. 1989. U.S. Ecology, Inc. Administrative Record, Ward Valley Low-Level Radioactive Waste Disposal Facility, Sections 2310 (Geologic Site

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology Characterization), pp. 2310-26; 2410.3.1, 2410.3.2, 2410.6, 2410.7 (Surface Water Hydrology), pp. 2410-15, 2410-16, 2410-19, 2410-22, 2410-28, 2410-29, 2410-30; 3100.1.1, 3100.3 (Principal Design Features), pp. 3100-2, 3100-4, 3100-12; 3200.5 (Design Considerations for Normal and Abnormal Accident Conditions), pp. 3200-10; 3310.2 (Construction Methods and Features), pp. 3310-2, 3310-3; 3440.1, 3440.2 (Erosion and Flood Control System), pp. 3440-1, 3440-2, 3440-4; and 6310 (Surface Drainage and Erosion Protection). Moser, D. A., 1993. An Application of Risk Analysis to the Economics Dam Safety, Engineering Foundation Conference Proceedings, Risk-Based Decision Making in Water Resources, eds. Y. Haimes and E. Stakhiv, sponsored by the National Science Foundation, Universities Council on Water Resources, U.S. Army Corps of Engineers, and American Society of Civil Engineers Task Committee on Risk-Based Decision Making. Santa Barbara, Calif. December, p.179. National Research Council. 1983. Safety of Existing Dams: Evaluation and Improvement. Washington, D.C.: National Academy Press. National Research Council. 1985. Safety of Dams: Flood and Earthquake Criteria. Washington, D.C.: National Academy Press. Nuclear Regulatory Commission. Licensing requirements for land disposal of radioactive waste, 10 CFR 61, Subpart D, § 61.50 & 61.52. Jan 1, 1991. San Bernardino County (SBC), California. 1986. Hydrology Manual. August. Schreiber, D. L. Sept. 16, 1994. Detailed calculations and notes on the design for protection against flooding and erosion. Transmitted under cover letter to B.A. Tschantz. U.S. Army Corps of Engineers. 1991. Hydraulic design of flood control channels. EM-1110-2-1601. U.S. Army Corps of Engineers. Sept 1990. HEC-1, Flood hydrograph package and computer program 723-X6-L2010, CPD-1A, Version 4.0. U.S. Army Corps of Engineers. Sept. 1990. HEC-2, Water Surface Profiles Manual and Computer Program, CPD-2A. U.S. Army Corps of Engineers. 1976. Wave runup and wind setup on reservoir embankments. ETL-1110-2-221. U.S. Army Corps of Engineers. 1971. Earth and rock-fill dams-general design and construction considerations. EM-1110-2-2300. U.S. Department of Agriculture, Soil Conservation Service (SCS). June 1986. Technical Release 55. Urban hydrology for small watersheds (including TR-55 Interactive Micro-Computer Watershed Characterization Package, Version 1.11). U.S. Department of Agriculture, Soil Conservation Service (SCS). 1972. SCS National Engineering Handbook, Section 4, Hydrology. U.S. Department of Commerce, NOAA. Sept 1977. Probable maximum precipitation estimates, Colorado River and Great River Basins. Hydrometeorological Report No. 49. U.S. Department of Commerce, NOAA. 1973. Precipitation - frequency atlas of the western United States. Vol. XI-California, NOAA Atlas 2.

OCR for page 169
Ward Valley: An Examination of Seven Issues in Earth Sciences and Ecology U.S. Department of Commerce, U.S. Weather Bureau, prepared by D. M. Hershfield. Reprinted Jan., 1963. Technical Paper No. 40 (TP-40): Rainfall frequency atlas of the U.S. U.S. Department of the Interior, Bureau of Reclamation. 1987. Design of small dams. Denver: U.S. Govt. Priming Office. 260 pp. U.S. Ecology, Inc. 1990. Low-Level Radioactive Waste Management Facility Drawings CA-121-001,002,003,004,006,007,008,009, & 010. U.S. Geological Survey. June 1977. Magnitude and frequency of floods in California. Water Resources Investigations 77-21. U.S. Nuclear Regulatory Commission (NRC). Jan 1988. NUREG-1200 (Standard Review Plan 6.3.3, Sect. 4.3.2), Low-Level Waste Disposal Licensing Program. U.S. Nuclear Regulatory Commission. Jan 1988. NUREG-1200, Standard Review Plan, Low-Level Waste Disposal Licensing Program, Rev. 1. Waananen, A. O. and J. R. Crippen. June 1977. Magnitude and frequency of floods in California. U.S. Geological Survey Water-Resources Investigations 77(21):6, 96. Wilshire, H. G., K. A. Howard and D. M. Miller. Dec 1993. Description of earth-science concerns regarding the Ward Valley Low-Level Radioactive Waste Site Plan and evaluation. Wilshire, H. G., H. G. Miller, and K. Howard. 1994. Ward Valley, Proposed Low-Level Radioactive Waste Site. A Report to the National Academy of Sciences.