Ward Valley An Examination of Seven Issues in Earth Sciences and Ecology (1995) / Chapter Skim
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7 FLOOD CONTROL AND ENGINEERING CONSIDERATIONS
7 FLOOD CONTROL AND ENGINEERING CONSIDERATIONS
Pages 169-194
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From page 169... ...
the proposed flow diversion or breakup berms upsiope to the west of the 70-acre LLRW site and (2) the proposed rip-rapped flood-protection bamer around the LLRW site.
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From page 170... ...
~ m below the original ground surface and beneath the deepest projected scouring action predicted by a Probable Maximum Flood (PMF)
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From page 174... ...
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" 03randt, 1994~. Floo~Protection Berm The design plans for the site indicate that offside 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.
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From page 175... ...
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 I.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.
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From page 176... ...
and from the DHS sun~nary report pBrandt, 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 lOO-year flood plain and the Probable Maximum Flood (P~)
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From page 177... ...
FLOOD CONTROLLED ENGINEERING CONSIDERATIONS ~ Rowe Homer Was Wart 177 ~-~3-:; - Jo 27.2 amp 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 Dom the LLRW site: Drainage Area 127.4 km2 Soil Conservation Service (SCS)
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From page 178... ...
I S 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 lOO-year flood and up to the Probable Maximum Flood (P~)
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From page 180... ...
180 WARD VALLEY- SEVEN ISSUES ~~-~-~.~:~ ~-~ i. .~ ~ ~~ i.,.:, : ~~ ~ ~~ ~~ ~~-~ ~~ ~ :~ ~-:R~-~7~ ~ ~~: ~ : ;~ H13C-1: Computer Pro~ram~llood::~Peak~sults :: ~-.~o>= ..i~we=-~.-flow-.~ -- : - -- :- ~:-.~ ~ ~.~ ~~ .
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From page 181... ...
As was the case for Homer Wash, no historical flow data were available for the local site sub-basin, and appropriate hydrologic data were developed for conducting computer simulation modeling for various assumed floods. The following data summarize the reported local site watershed characteristics upstream from the LLRW site: Drainage Area: SCS Hydrologic Soil Group (described in Chapter 2~: Zone ~ (HSG "D")
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From page 182... ...
Permanent Flood Protection Berm The PMF (including its PMP storm derivatives was chosen as the basis for designing the height of and the erosion control for the permanent flood protection berm which surrounds the LLRW site. While the governing regulations (U.S.
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From page 183... ...
Members of the committee inspected two engineered training dikes for controlling flood water upstream and downstream from nearby I-40 in the Homer Wash Watershed in September 1994. The inspected embankments range Tom I.8 to 2.4 m high and have slopes from about 2H:IV to nearly SHIV, or between 6 to 12 times steeper than what is proposed for the site flood protection berm.
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From page 184... ...
The license application does not appear to present or address any plan for motoring differential settlement in the 70-acre trench area. Temporary Breakup Berms In addition to the I.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 Tom the westem-facing flood protection berm, to "slow and divert storm runoff to the north and south of the ALLOW)
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From page 185... ...
If breaching of these berms occurs and induces channel~zation and concentrates flow, however, the riprapped 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 scouring at this location and the proposed trenches. In the committee's judgement, the flood protection berm and rip rap/fiIter 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.
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From page 186... ...
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 coverage Antecedent Moisture Condition, or AMC H 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 Ill, would have resulted in even higher peak flows for each of the assumed flood events.
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From page 187... ...
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.
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From page 188... ...
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 QfI.
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From page 189... ...
With a relatively high sediment flux Tom 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.
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From page 190... ...
Pending 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.
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From page 191... ...
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.
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From page 192... ...
Administrative Record, Ward Valley LowLeve! Radioactive Waste Disposal Facility, Sections 23 10 (Geologic Site
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From page 193... ...
Sept 1977. Probable maximum precipitation estimates, Colorado River and Great River Basins.
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From page 194... ...
1994. Ward Valley, Proposed Low-Level Radioactive Waste Site.
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