ML20148F940
| ML20148F940 | |
| Person / Time | |
|---|---|
| Issue date: | 02/19/1988 |
| From: | Starmer R NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Fliegel M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-58 NUDOCS 8803280332 | |
| Download: ML20148F940 (7) | |
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hln)- 3f FEB 19198g MEMORANDUM FOR: Myron Fliegel, Section Leader Operations Branch Division of Low-Level Waste Management and Deconmissioning, NMSS FROM: R. John Stamer, Section Leader Technical Branch Division of Low-level Waste Management and Decennissionir.g, NMSS
SUBJECT:
SHIFROCK TECHNICAL EVALUATION MEMORANDLM in accordance with your recent request, we have revised the surface water hydrology and erosion protection portion of the Shiprock Technical Evaluation Memorandum (TEM). These revisions were made to the original draft TEM which was transmitted to you on September 11, 1987. These revisions also reflect your suggested changes to the TEM which you provided to us in the form of a rarked-up copy.
The enclosed TEM was prepared by Ted Johnson. If you have any questions, he may be reached at X23440.
g.ig Mf*1 gignea W R. Jchn Starmer, Section Leader Technical Branch Division of Low-level Waste Management and Deconrnissioning, NMSS
Enclosure:
Shiprock Technical Evaluation Memorandum DISTRIBUTION:
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DATE :02//9/88 :02/['l/88 : : :
OFFICIAL RECORD COPY l 8803280332 880219 I PDR WASTE j WM-58 PDR .Q
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. . ENCLOSURE TECPNICAL EVALUATION MEMCPANDUM SURFACE WATER HYDROLOGY AND EROSION PROTECTION EVALUATION OF THE SHIPROCK, NM REMEDIAL ACTION SITE
- 1. Hydrologic Description and Site Conceptual Design The Shiprock site is located near Shipreck, New Mexico and is situated on a steep escarpment on the left bank of the San Juan River. The San Juan River is located inwediately r.crtheast of the site and flows in a northwesterly direction.
In order to comply with EPA standercs, which require stability of the tailings for a 1,000-year (or minimum 200-year) period, COE proposes to stabilize the tailings and contarainated materials in an engineered embankment to protect them from flooding and erosion. The design basis events for protection of the embankrrent slopes included the Probable Maximum Precipitation (PMP) and the Probable Maximum Flood (FMF) events, both of which are considered to have very low probabilities cf occurrence during the 1,000-year stabilization period.
The tailings will be consolidated into a single pile, which will be protected by soil and rock covers. The covers will have maximum slopes of 2-4% on the top and 20% on the sides. The irregularly-shaped pile will be surrounded by drainage channels which will convey flood runoff away from the tailings. The channels will n.erge into one single channel which will terminate at an energy dissipation area, designed to prevent erosion where the outlet of the diversion channel meets the existing ground. Portions of the escarpment on which the pile is located will be stabilized with riprap'crosion protection, and the pile will be set back approximately 300 feet from the edge of the escarpment.
l I 2. Flooding Deteminations i
In order to determine site impacts from flooding, DOE analyzed peak flows and velocities and evaluated the need for erosion protection features. DOE esti-l mated the PMF peaks resulting from an occurrence of the PMP over the various small drainage areas. These design events meet the criteria outlined in the Standard Review Plan (Reference 15) and are, therefore, acceptable.
The details of DOE's flood cerrputations were analyzed by the NRC staff as follows:
A. Probable Maximum Precipitation (PMP) l A PMP rainfall depth of approximately 9.1 inches in one bcur was used by DOE to compute the PMr for the small drainage areas at the site. This rainfall esti-mate was developed by DOE using Hydrometeorological Report (HMR) 49 (Refer-ence 14). Based on a check of the rainfall computations, we conclude that the PMP was acceptably derived for this site.
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B. Infiltration losses In computing the peak flow rate for the design of the rock crosion protection, DOE used the rational formula - Q = C i A. In this formula, the runoff l coefficient (C) was assumed by DOE to be unity; that is, DOE assumed that no infiltration losses would occur. Based on a review of the computations, we conclude that this is a very conservative assumption when using the rational formula and is therefore acceptable.
C. Time of Concentration The time of concentration is the amount of time required for runoff to reach the cutlet of a crainage basin from the mest remote point in that basin. The peak runoff for a given drainage basin is inversely prcportional to the time of concentration for that basin. If the time of concentration is conservatively computed to be small, the peak discharge will therefore be conservatively large.
Various times of concentration (tc) for the ditches and embankments were esti-mated by DOE using procedures discussed in Reference 3. Based on our review, we conclude that the procedures used for ccmputing tc are representative of the small steep drainage areas present at the site. For very small drainage areas with very short times of concentration, DOE utilized tc's as 1cw as 5 minutes, which is generally considered to be conservative.
D. PMP Rainfall Distributions DOE derived rainfall distributions and intensities from HMR 49 (Reference 14),
which is acceptable. In the determination of peak flood flows in ditches, rainfall intensities for durations as short as 5 minutes were used. Based on our review, the staff concludes that the peak rainfall intensity of about 55 inches / hour (corresponding to a tc of 5 minutes) is conservative, and therefore, acceptable.
E. Corputation of PMF DOE utilized the rational fomula (Reference 3) to compute the peak sheet flows down the slopes and PMF ficws in the ditches, given the input parameters dis-cussed above. Based on our review of the calculations presented, we conclude .
that this method of computation has been conservatively applied.
The PMF for the San Juan River was estimated by COE using data from other government das projects and adjusting for drainage area. The peak outflow from a postulated failure of Navajo Dam was added to this discharge, resulting in a peak PMF discharge of approximately 844,000 cfs. In order to verify its adequacy, the staff compared the estimate with the estimates given in Reference 2. This regulatoiy guide was developed to estimate PMF flood peaks for the siting of nuclear power olants. The estimates given in this reference represent conservative ficod estnates and computational techniques of other gr.ernment agencies including the Corps of Engineers. The FMF estimates given for this region are truch less thtn the PMF estimate computed by DOE on a discharge per square mile basis. In addition, the PMF estimate is conservative when compared to the regional maximum values given in Reference 11. Thus, the 2
staff concludes that DOE has conservatively estimated the PHF for the San Juan River at this location.
- 3. Water Surface Profiles and Channel Velecities, Water surface profiles and velocities in the drainage channels were computed using Manning's formula. The NRC staff checked the water level and channel velocity computations in accordance with slope-area procedures given in Refer-ence 6 to determine their accuracy. Based on this check, we conclude that the estimates are appropriate for the given channel configuration.
Water surface profiles fcr the San Juan River were developed by DOE for the FMF using the slope-area methoo (Reference 6). CCE estimated that the peak PMF ficw of 844,000 cfs resulted in a water level which would be 44 feet below the base of the tailings embankment. Our review of the data provided by DOE indi-cates that the water surface profiles have been acceptably computed.
Because the velocities are very high (14 feet per second for the PMF), erosion of the escarpment can be expected to occur during major floods. To prevent long "u stream meander and migration of the channel bank, DOE proposes to install erosion protection along the escarpment. (See 4, below for staff anal-ysis of the adequacy of the erosion protection.)
- 4. Erosion Protecticn A. San Juan River The riprap along the escarpment was designed to reduce the rate of slope erosion and stream meandering toward the pile. Based on our expc ience with flood erosion and meandering streams, the NRC staff concludes that the riprap
. will be helpful in reducing meandering provided that careful monitoring of slope erosion is performed along the stream. Our experience indicates that progressive meandering will occur on actively-eroding channels in this area, if adequate measures are not taken to assure that significant erosion and damage is repaired innediately after the occurrence of any large flood. However, geomorphic evidence indicates that the erosion will not reach the tailing in a t 1,000-year period. The erosion protection provided thus only serves as a backup preventive measure to slow down the rate of erosion in the existing arroyos.
In order to minimize erosion, DOE will also assure that the existing edge of the escarpment remains a considerable distance from the contaminated material.
00E proposes that this can be accomplished through a monitoring and maintenance program where the exact lateral erosion distance can be observed and the eroded material can be replaced to its original configuration. NRC staff review of the Remedial Action Inspection Plan (RAIP) indicates that the proposed surveil-lance and maintenance program for this area is acceptable. It should be emphasized that this program is not required to assure stability of the i
tailings for a 1000-year period, but serves to enhance the performance of the l tailings protection system.
B. Orainace Ditches 3
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l Drainage ditches are proposed along the perimeter of the pile at the toe of the l stabilized embankment slopes; these perimeter ditches merge and form one collector ditch which will convey flows to a ditch cutlet which is designed to reduce flow velocities to non-erosive levels. DOE proposes that the erosion protection in the perimeter drainage ditches will be designed for an occurrence of a local PMP. This design basis meets the criteria outlined in the SRP and is, therefore, acceptable.
Additionally, we conclude that the computational models used by DOE are conser-vative ano that the proposed riprap design meets the criteria cutlined in References 7 and 8. At those locations where the proposed civersion ditches transition into natural grcund, the riprap protection will be designed so that erosive velocities at the end of the protected areas do not affect the tailings. Energy dissipation areas (EDA's) will be proviced at those locat ons l where these transitions occur. The riprap in these areas will be designed for the PMF, which is acceptable. J C. Top and Sides of Pile _s The rock covers, which will be used to protect the soil cover from wind and water erosion, are designed for an occurrence of the local PHP. For the top of the pile (maximum 4% slopes), DOE proposes to provide a 12-inch layer of rock with a minimum D of about 1.15 inches. For the sides of the pile (20%
slepes)DOEprop$esa12-inchlayerofrockwithaminimumD GO of about 4 inches. Each of the rock layers will be placed on bedding Tayers. The Safety Factors Method (Reference 12) was used to determine required rock sizes for the top slopes of the pile. The Stephenson Method (Reference 16) was used for the steeper side slopes.
The rock to be placed in the energy dissipation areas was designed using the Safety Factors Method. Velocities of flow in these areas will be reduced so that, upon leaving the protected area, flow will be non-erosive. Based on our review, we find these designs acceptable, since they have been conservatively '
developed in accordance crith documented, referenced methods.
C. Rock Durability For the rock to be placed in the ditches and on the pile, gradation and rock durability criteria were presented. Based on a comparison of the data with the -
criteria provided in Reference 17, we conclude that the rock durability crite-ria proposed are adequate to assure that rock of acceptable quality has been provided.
- 5. Upstream Dam Failures DOE identified one impoundment (Navajo Dam) on the San Juan River whose failure could potentially affect the site. A worst-case scenario was projected by DOE for possible failure. DOE assumed that the dam failed during a PMF, resulting in a peak discharge at the site of 844,000 cfs.
l The method of computation and assumptions used by DOE in the dam failure anal-l ysis were compared to the criteria outlined in NRC Standcrd Review Plan 2.4.4 (Reference 15) and Regulatory Guide 1.59 (Reference 2). These criteria were developed for nuclear power plants and provide conservative design 4
d considerations for use in considering dam-failure floods. Overall, our review of the calculations and staff experience regarding attenuation of flood peaks indicate that dam failures pose no threat to the integrity of the encapsulated area.
- 6. Conclusions Based on our review of the information submitted by DOE, we conclude that the site design will meet EPA requirements as stated in 40 CFR 192 with regard to flood design measures and erosion protection. We conclude that an adequate hydraulic design has been provided to reasonably assure stability of the con-taminated material at Shiprock for a period of up to 1,000 years.
References and Bibliography
- 1. U.S. Army Corps of Engineers Hydrologic Engineering Center," Flood Hydrograph Package, HEC-1, continuously updated and revised.
- 2. U.S. Nuclear Regulatory Comission, Regulatory Guide 1.59, "Design Basis Floods for Nuclear Power Plants," January 1983.
- 3. U.S. Bureau of Reclamation, U.S. Department of the Interior, Design of Small Dams, 1973.
- 4. Staff Technical Position WM-8201, "Hydrologic Design Criteria for Tailings Retention Systems," January 1983.
- 5. U.S. Army Corps of Engineers, Hydrologic Engineering Center, "Water Surface Profiles, HEC-2," continuously updated ar.d revised.
- 6. Chow, V. T., "Open Channel Hydraulics," McGraw-Hill Eook Company, New York, 1959.
- 7. U.S. Army Corps of Engineers, "Hydraulic Design of Flood Control '
Channels," EM 1110-2-1601, 1970.
( 8. U.S. Anny Corps of Engineers, "Additional Guidance for Riprap Channel Protection," EM 1110-2-1601, 1970.
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- 9. U.S. Department of Comerce, U.S. Army Corps of Engineers, Hydrometeoro-logical Report No. 43, "Probable Maximum Precipitation, Northwest States,"
1966.
- 10. U.S. Army Corps of Engineers, "Engineering and Design - Standard Project Flood Determinations," EM 1110-2-1411, 1965.
- 11. Crippen, J. R. and Bue, C. O., "Maximum Floodflows in the Conterminous United States," USGS Water Supply Paper 1887(1977).
- 12. Simons, D. B., and Senturk, F., Sediment Transport Technology, Fort Collins, Colorado, 1976.
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- 13. Codell, R. B., "Design of Rock Armor for Uranium Mill Tailings Embankments," U.S. Nuclear Regulatory Commission, Unpublished Draft Report, February 1985.
- 14. U.S. Department of Commerce, U.S. Army Corps of Engineers, Hydremeteoro-logical Report No. 49, "Probable Maximum Precipitation Estimates, Colorado
- River and Great Basin Drainages," 1977.
- 15. Standard Review Plan for UMTPCA U.S. Nuclear Title Regulatory I Mill Tailings Remedial Commission,'on Acti Plans. October 1985,
- 16. Stephenson, D., Rockfill Hydraulic Engineering Develo:ments in
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Geotechnical Engineering 727, Elsevier Scientific Pu)lishing Company, 1979. ;
- 17. Nelson, J. D. et al. , "Methodologies for Evaluating Long-Term Stabilization Designs of Uranium Mill Tailings Impoundments,"
NUREG/CR-4620, June 1986.
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