ML20138E068
| ML20138E068 | |
| Person / Time | |
|---|---|
| Issue date: | 08/23/1985 |
| From: | Knapp M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Higginbotham L, Danni Smith NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV), NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-48 NUDOCS 8510240562 | |
| Download: ML20138E068 (5) | |
Text
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_1 MFliegel MKnapp JBunting MBell MEMORANDUM FOR: Leo B. Higginbotham, Chief RBrowning Low-Level Waste and Uranium Recovery Projects Branch W M h ard rite WM P@ct Division of Waste Management Dach! Ma _ __
n' Dale Smith, Director l# - - -
Uranium Recovery Field Office Dis!ribuh FROM:
Malcolm R. Knapp, Chief U
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Geotechnical Branch Division of Waste Management giurn to WM,623 S5)
SUBJECT:
TAR.85059 - DURANGO RAP - SURFACE WATER HYDROLOGY QUESTIONS AND COP 9fENTS As requested in TAR # 85059, we have completed our initial review of surface water hydrology and erosion protection aspects of the Durango Remedial Action L
Plan (RAP). Our questions and comments are enclosed.
In general, we conclude that the erosion protection proposed for the drainage u
ditches and for the prevention of gully intrusion has not been adequately designed,duetoimproperap(plicationofshearstressmodels(SeeComment1) i and rainfall distributions See Comment 5). Based on this initial review, we conclude that rock sizes and quantities for the ditches and for gully erosion L
protection will likely be much larger than the sizes and quantities proposed.
in addition, recent meetings with the National Weather Service have indicated that short duration rainfall intensities need to be dramatically increased above the intensities used in DOE's analyses for the erosion protection for the top and sides of the pile. These increases will affect the rock size for the top and sides of the pile and will also further affect the rock size in the perimeter ditches and gullies.
i Because of the potential difficulties in locating and providing rock of the size and thickness required, and the costs associated with the volumes required, the viability of the Bodo Canyon site (preferred alternative) may be t
i questionable. However, it should be emphasized that little information is available at this time to determine if large rock is economically available.
t Therefore, we suggest that upon re-design of the erosion protection.
i investigations be undertaken to locate rock of the required size.
If such rock cannot be found to be economically available, it may be necessary to locate l
additional alternative sites where topographic features and site conditions do not require the use of such large rock.
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WM-48/TJ/85/08/01 A draft copy of these questions was provided to DOE at the NRC/ DOE meeting which was held on August 14, 1985 in Denver.
This review was performed by Ted Johnson.
If you have any questions or comments, he may be reached at x-74490.
Malcolm R. Knapp, Chief Geotechnical Branch Division of Waste Management
Enclosure:
As stated i>
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SURFACE WATER HYDROLOGY AND I
EROSION PROTECTION QUESTIONS AND COMMENTS DURANGO RAP 1.
Use of Average Shear Stresses for Channel Design Our review of the information presented in the RAP and in the backup calculations indicates that average shear stresses were used to design the riprap to be placed in the perimeter ditches surrounding the remediated pile.
The NRC staff does not consider this to be an acceptable method for designing erosion protection for drainage channels similar to those proposed for this site. The proposed V-shaped channels are relatively inefficient and tend to produce shear forces which are very low on the sides and very high in the center of the channel, where the channel is the most efficient.
One acceptable method for resolving these concerns is to assume that the hydraulic radius (R) in the design equation is equal to the depth of flow in the center of the channel. This will assure that the rock is designed for local velocities, as recommended in most standard hydraulic design methods. Another acceptable method is to provide a channel design where shear stresses are distributed more uniformly across the channel section.
(Becausetheaverage'R'isabout1/2iftheR'thatshouldbeusedfor design purposes in V-shaped channels, the NRC staff expects that the D50 rock sizes needed will be approximately twice as great as the sizes proposed. Also, see comment 5, below).
2.
Gully Erosion Protection The bases for providing erosion protection for the existing on-site gullies need to be more clearly defined. As presented in the RAP and supporting calculations, it is not clear how the erosion protection will prevent further gully erosion and headcutting. Our review indicates that the following additional considerations need to be factored into the design and/or addressed in the revised RAP:
A)
Provide detailed plan views and cross-sections in the areas where gully erosion protection is provided and in those areas where the perimeter ditches merge and transition into the gully erosion protection.
8)
Because of the high velocities and supercritical flow in the ditches, consi ditch,deration must be given to further increasing rock sizes in the
-to-gully transition areas, in the ditch junctions, and in those areas where hydraulic jumps will occur.
These increases will be needed in addition to the increases noted in Comment 1, above, and comment 5, below. Additional analyses should be perfomed to detemine the increase in shear forces and the need for additional erosion protection at these locations.
2 C)
Based on the information presented, it appears that the design of the gully erosion protection should be based on the velocities which will occur in the perimeter ditches or in the existing gullies. There is no basis for the assumption that uniform sheet flow will occur over the gully erosion protection blanket (as assumed in the backup calculations).
It appears that the rock provided for gully protection should be at least as large as the rock in the perimeter ditches. The slopes of the blankets appear excessively steep and the basis for providing such blankets should be further explained.
It is difficult to determine the effectiveness of such a design when the existing erosion features in the area indicate that gully protection should extend for some distance downstream and stabilize the gully cross-sections as they now exist to prevent further erosion both laterally and headward.
D)
Based on the information presented, it is not clear what drainage areas were used to compute peak runoff rates to the ditches and gullies.
For example, the north gully erosion protection is designed for the runoff from a drainage area of 31 acres. However, it appears that flow from the north ditch, with a drainage area of about 25 acres should be combined with the flow from the existing north gully, which appears to have a drainage area much greater than 6 acres.
Further analyses and possible re-evaluation of the drainage areas should be provided. A map should be provided which outlines the drainage areas for each ditch segment and gully.
3.
Based on the configuration of the proposed pile, it appears that the concave shape (Figure B.1.12) will be conducive to the concentration of runoff at the toe of the pile. This phenomenon should be factored into the design of the erosion protection for the sides of the pile, or the pile configuration should be modified such that flow concentration is prevented.
The basis for computations should be provided along with revised analyses which document the degree to which flow concentration will occur for the final design selected.
4.
Provide a discussion of the rock that is available at the potential borrow areas cited in the RAP.
Information should be provided regarding the amounts and types of rock that are available in the sizes needed.
In addition, the feasibility and practicability of crushing rock from various sources to meet the required gradation should be discussed.
5.
The rainfall distributions used to compute peak rates of runoff in the ditches and gullies is not considered acceptable. As stated in previous comments at other sites, the rainfall distributions given in Hydrometeorological Report No. 49 should be extrapolated and used for design purposes.
3 The NRC staff recently met with the National Weather Service (NWS) staff.
Based on NWS recammendations, the following rainfall distribution should be used for the Bodo Canyon site and for any future site in the region covered by HMR 49:
Duration (minutes)
Percent of 1-hr PMP 5
45 10 61 15 74 30 89 60 100 Based upon these distributions, it will be necessary to re-compute peak rates of runoff used to design the riprap erosion protection for the top and sides of the pile, and for the perimeter ditches and gullies.
6.
In order to document that flow concentration will not occur, discussions
.__and/or analyses should_be_provided_to_show.that_ differential ~ settlement-will not be a problem. As discussed in recent meetings between NRC and TAC, a commitment should be provided to take necessary measures (such as surcharge loading, compaction of slimes, etc) that will ensure that differential settlement will be minimized over the 1000-year stability period. Otherwise, consideration must be given to flow concentration for sizing the rock for the tops and sides of the pile.