ML20126J651
| ML20126J651 | |
| Person / Time | |
|---|---|
| Issue date: | 05/17/1985 |
| From: | Higginbotham L NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Themelis J ENERGY, DEPT. OF |
| References | |
| REF-WM-58 NUDOCS 8506100665 | |
| Download: ML20126J651 (10) | |
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DE Martin NMSS s/f MR Knapp Mr. John G. Theiifelis, Project Manager J0 Bunting JT Greeves Uranium Mil! Tailings Project Office RE Browning RD Smith, URF0 U.S. Department of Energy MJ Bell GN Gnugnoli Post Office Box 5400 Albuquerque, New Mexico 87115
Dear Mr. Themelis:
We have completed a review of the Shiprock construction documents including design changes and construction specifications transmitted to the NRC by Morrison-Knudsen (MK) letters dated February 11 and March 26, 1985. 0ur review of these documents was indicated in Item 3 (Rock Sizing) and Item 5 (Construction Specifications) of the summary of open issues transmitted by letter to you dated April 19, 1985. Comments pertinent to these reviews are attached.- Drafts of these comments have already been given to Mr. T. R. Wathen of.MK during the NRC'.s recent visit to the Shiprock site on May 9,1985.
Should you have any questions on these issues, please contact Mr. Daniel Gillen of my staff at (FTS) 427-4160.
Sincerely, orisinal gema by Leo B,11ta'd 5 " "
Leo B. Higginbotham, Chief Low-Level Waste and Uranium Recovery Projects Branch Division of Waste Management
Enclosures:
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. SHIPROCK RAP SURFACE WATER HYDROLOGY QUESTIONS AND COMMENTS
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1.
Our review of the design of the erosion protection to be provided for the onsite drainage ditches indicates that adequate protection has not been provided. Specifically, we conclude that improper assumptions and/or calculation techniques were applied in the following areas:
A)
Use of average, rather than localized, shear stresses B)
Use of inappropriate Manning's 'n' values for estimating velocities and shear stresses C)
Inadequate riprap design at channel bends and channel junctions D)
Inappropriate reduction of PMP rainfall intensity E)
Inappropriate calculation of time of concentration.
A.
Use of Average Shear Stress for Riprap Design The size of the riprap for protection of the drainage ditches was determined by estimating the average shear stress over the entire ditch cross-section.
Because the adopted design cross-section is basically a rather inefficient one (the hydraulic radius is small relative to the area of the flow, particularly in the outer flow areas away from the center of the channel), the shear stress and resulting riprap size will also be small if the shear forces are averaged across the entire section.
In actuality, most of the flow will be concentrated in the center of the ditch, with the outer edges of the ditch carrying proportionately very little flow.
This will prodsua much higher velocities and shear forces in the center of the ditch. The rock protection for the ditches should be designed for the velocities and the localized shear forcc: produced at the most critical locations in the ditch, which for this design, is the center of the ditch.
The riprap size for the drainage ditches should be re-analyzed in accordance with methods which account for uneven flow distribution over the channel cross-section width and for localized, rather than average, shear forces.
Acceptable methods for estimating the variation of velocities, discharges, and shear forces may be found in Corps of Engineers EM 1110-2-1601, " Hydraulic J
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Design of Flood Control Channels" and ETL~ 1110-2-120, " Additional Guidance for Riprap Channel Protection."
B.
Use of Inappropriate Manning's 'n' Values The flow velocities, water surface profiles, and shear forces for riprap design for the drainage ditches were estimated using a Mannings 'n' value (a dimensionless measure of the roughness and frictional effects on channel flow) of 0.038. Our review indicates that this estimate may be too high, resulting in flow velocities and riprap sizes which are too small.
Recognizing that estimation of Manning's 'n' values can sometimes be very subjective, there are however, several methods available to directly calculate the 'n' value if the hydraulic radius and average rock size are known. A specific check of the computations (by tne NRC staff) of the 'n' value in ditch segment D-7, using Corps of Engineers EM 1110-2-1601 fonnulae (Plate 4),
indicates that the 'n' value in this segment is approximately 0.027. Use of this value would produce much higher velocities-in the ditch, resulting in the i
need for larger rock to be used for erosion protection.
(We note that this method was previously used - See Sheet 12/24 Calc. No. 04-11-RO-02).
- The riprap sizes for. each of the diversion ditch segments should be re-evaluated using estimates of Manning's 'n' value which correspond to the depth of flow and rock size in a particular ditch segment. Acceptable methods for estimating these values may'be found in Corps of Engineers EM 1110-2-1601,
" Hydraulic Design of Flood Control Channels."
It should be noted that several trial-and-error calculations may be necessary to arrive at the final 'n' values and riprap size.
It should also be pointed out that Manning's 'n' may be larger than 0.038 in those ditch segments when the depth of flow is low, relative to the' rock size.
L C.
Inadeouate Riprap Design at Channel Bends and Junctions The size of riprap to be placed in curved sections of the channel was concluded to be the same as for straight sections of the ditches. However, there is an apparent incorrect conclusion drawn from the results of calculations presented on Sheet 18/24 - Calculation No. 04-11-R0-02, dated 9/20/84. We conclude that the forces produced in the curved channel portions will be considerably larger than the forces produced in the straight portions.
The riprap size in these areas should be increased, accordingly.
We note from a review of Calculation 04-11-R0-02-Sheet 6/24 that there is an
-intent to design channel junctions with a selected radius-of-curvature to width (r/w) ratio.
If the intent is to provide a r/w ratio of at least 2, the selected factor of 1.84 for increasing shear stresses is acceptable.
. Otherwise, for different ratios, EM 1110-2-1601 provides acceptable guidance for determining appropriate factors for increasing shear stresses.
In addition, our review of the details of the channel junctions that were provided on Drawing SHP-PS-10-0016, REV A, indicate that the channel junctions, as designed, will be subjected to excessive shear forces-that'have not been properly accounted for. These junctions are apparently not designed using the proposed r/w ratio of 2 and do not represent adequate transition zones for high velocity flow. We co.:clude that the channel junctions should be redesigned.
EM 1110-2-1601 (pp. 57-62) provides acceptable general guidance for designing channel confluences.
It also appears that the roadway crossings through the diversion ditches will develop very undesirable flow conditions.
In any redesign of the channel junctions, consideration should be given to redesigning the roadway crossings so that undesirable flow currents are not created.
D.
PMP Rainfall Distribution and Reduction of Rainfall Intensity The peak flow in the drainage ditches was computed based on a reduction of rainfall intensity (see sheet 7/24 - Computation 04-11-R0-02) based on Soil Conservation Service (SCS) methods. The NRC staff concludes that this reduction is not appropriate for rainfall as severe as the PMP.
In most accepted computations of the runoff from a PMP, including other accepted SCS methods, it is assumed that the peak burst of rainfall (with a duration corresponding to the time of concentration) occurs at a time when previous rainfall has sufficiently saturated the ground so that nearly total runoff occurs during the most critical period. We conclude that any reductions in intensity are already accounted for by the computation of the time of concentration. Since it appears that peak flow rates in the ditches could be increased by about as much as 1/3 if nearly 100% runoff is assumed, we conclude that the calculations should be revised to reflect more severe rainfall intensities. (See also Comment 2)
E.
Time of Concentration The curve number (lag) method of computing the time of concentration for the drainage ditches is not considered conservative for rainfall as severe as the PMP. We conclude that the times of concentration will be significantly smaller if other methods are used.
For example, in the design of Ditch D-1, the time of concentration for flow over and through the rock layer on the 560-foot long, 4% slope is computed to be 11.4 minutes; in the calculations presented for design of the embankment rock layer, the time of concentration for the same 4% slope (595 feet long) is calculated to be 3.69 minutes.
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, Our review indicates that the latter method of computation is more appropriate for PMP rainfalls and should be adopted for design of the ditches.
2.
The methodology for determining rainfall distribution and intensities, as given in NRC Staff Technical Position Paper WM-8201, has been superceded by that given in the recently published Hydrometeorological Report No. 55 (March,1984). The NRC staff no longer endorses the methodology presented in WM-8201.
WM-8201 was developed for use at active uranium mill sites, most of which are located in Wyoming, east of the Continental Divide. At the time of the development of WM-8201, reasonable guidance for rainfall distributions in that area was unavailable and/or questionable. WM-8201 was formulated to provide that type of general guidance, based on Corps of Engineers rainfall distributions. The recent publication of Hydrometeorological Report No. 55 has indicated that certain areas in Wyoming will be subject to rainfall intensities (especially of short duration) much greater than those given in WM-8201. As a result, the NRC staff intends to make appropriate modifications to WM-8201 to reflect the new data.
The modifications to WM-8201 will include recommendations to use the rainfall distribution guidance that is developed in the Hydrometeorological Report that is appropriate for a given region. These modifications will be applicable to UMTRAP sites in general.
For the Shiprock site, in particular, the rainfall distributions given in Hydrometeorological Report No. 49 should be used, since this represents the most current estimates of rainfall potential for this area of the United States.
Extrapolation of the data for time intervals less than 15 minutes will be necessary.'
3.
Our review of the erosion protection to be provided for the filled-in arroyos indicates thct the protection may not be adequate to meet EPA long-term stability criteria.
We conclude that the 1 Vertical (V) on 2 Horizontal (H) fill slopes have not been adequately protected and that the erosion protection for the toe of slopes may not be adequate to resist flood velocities in the San Juan River. We disagree with your conclusions that only minor erosion of the slopes will occur during major rainstorms and that insignificant gully erosion on the slopes should be expected. We conclude that the filled-in arroyos, as designed, will experience significant erosion due to (1) erosion of the toe of the slope during high river stages, and (2) erosion of the slope due to gullying and sheetwash.
We further conclude that the arroyos could be severely eroded due to the occurrence of relatively minor flood and precipitation events.
, Since it appears that the setback distance, coupled with rerouting of the drainage, is adequate to meet EPA standards in those areas where the existing escarpment is being cut back and re-shaped, additional protection should be needed only in the areas where the arroyos are being filled.
This additional protection is needed to assure that the arroyos do not re-form and eventually, through headcutting, extend into~the tailings stabilization area. We conclude that adequate protection has not been provided to prevent these phenomena.
Accordingly, the erosion protection design for the filled-in arroyos should be modified to prevent the re-development and growth of the filled-in arroyos. The IV on 2H slopes should be designed using similar design methods to the remediated pile slopes, and the toe of the slopes should be designed to resist velocities due to large floods in the San Juan River.
4 The larger rock that will be placed on the 1 Vertical (V) on 5 Horizontal (H) side slopes of the remediated pile should be extended for a short distance (say 30') up onto the flatter 4% slope. Since this transition area represents a very critical area where a significant amount of flow emerges from the rock layer, it is considered to be a prudent measure to provide this extra degree of flood protection.
5.
Additional information and design changes should be provided regarding the erosion protection that will be provided at the outlet of Ditch D-7.
This area could become unstable, since a large amount of runoff which formerly was discharged elsewhere is now being directed at this area of the escarpment.
Provide the details of, and bases for, the erosion protection and transition design in this area.
. GE0 TECHNICAL ENGINEERING REVIEW OF SPECIFICATIONS FOR PHASE II CONSTRUCTION SHIPROCK RAP i
The specifications for the Phase II construction of the proposed Remedial Action Plan (RAP) at the Shiprock site has been reviewed.
It is noticed that there are changes in the design between the RAP submitted in December 1984, and the remedial action as per the drawings attached to the contract documents.
It is understood that a revised RAP is under preparation which will document all the changes. The present review is based on the information presented in the December 1984 RAP and the contract documents (References 1 and 2). The comments listed below address the geotechnical engineering aspects of the project.
1.
Disposallof Demolished Materials (Section 02050, Demolition, Part 3-Execution, Section 3.3, Page 02050-8)~
Item 3.3.A(2) requires that non-radioactively contaminated materials and low-level radioactively contaminated materials shall be placed in areas outside the tailings embankment. The low-level radioactively contaminated materials shall be covered with a minimum thickness of I foot of earth cover. The contract documents do not indicate the proposed location for such disposal nor do they identify specifications for the type and engineering index properties for earth cover material. These topics should be addressed in the specification / contract documents.
2.
Radon Barrier Materials (Section02200, Earthwork,Part2-Products,Section2.2.B.Page 02200-3)
Item 2.2.B specifies that the Radon Barrier material shall be non-radioactively contaminated sandy SILT available from the designated borrow area, with a gradation of 50-100 percent passing a
- 200 sieve. This gradation requirement may not be sufficiently
a restrictive. The specified gradation should be compatible with the gradation used in the radon cover thickness computation, the gradation used in the filter criteria computation in sizing the bedding material beneath rock cover, and the gradation of the material-used in developing the compaction density-moisture criteria used in the design. The gradation of the material also impacts on compaction characteristic of the material and on the long-term moisture content of the material. These parameters are important in determining the effectiveness of the material as a radon barrier.
The above concerns should be considered in finalizing the specification for the radon barrier material.
3..
Relocation and Compaction of Slimes (Section 02200, Earthwork, Part 3-Execution, Section 3.2, Page 02200-7)
Items 3.2.A.7 & 8 specifies that pockets of slime located in tailings designated to be relocated or otherwise disturbed shall be mixed with sands and placed at the lower part of the tailings embankment. Also, slimes which are not relocated or disturbed during construction shall be mixed with or covered by sands to support the construction equipment. The specification for these items are not sufficiently restrictive. Additional guidance on the minimum or acceptable ratio of the sand-slime mixture and whether slimes and sands have to be premixed prior to placing or placed in alternate layers should be provided. This is an important aspect of the tailings stabilization plan and specifications should be very precise on the work to be performed.
4.
Compaction of Radon Barrier Materials
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(Section 02200, Earthwork, Part 3-Execution, Section 3.2, Page 02200-3)
Item 3.2.C specifies that the Radon Barrier material shall be compacted to at least 95 percent of the maximum dry density (ASTM D 698) on the wet side of the optimum moisture content. Your design calculations assume a moisture content 3 % in excess of the optimum moisture content determined from' laboratory tests. The specification should state the acceptable range of moisture content in excess of the optimum moisture content with due consideration of the swelling I
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5.
Gradation of Select Rock Material and Select Bedding Material (Section 02270, Erosion Protection, Part 2-Products, Section 2.1, Page02270-3)
Item 2.1.C.4A specifies the gradation for the Type B rock material cover proposed for the tailings embankment side slopes, drainage ditches, and intercept ditches. -There is a 6-inch thick beddirg material between this rockfill cover and the radon barrier earth cover over the tailings pile. The proposed gradation of the Type B rock material and the select bedding ma rial do not comply with the filter criteria gradation requirement (
f;5. This is a design requirement and the specification of al he materials proposed for covering the tailings embankment should comply with the filter gradation requirement.
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REFERENCES:
1.;
Letter dated February 11, 1985, from R.E. Hopkins of Morrison-Knudsen Company, Inc., to L. Higginbotham of NRC;
Subject:
Shiprock Site-Shiprock, New Mexico, Phase II Construction - Preliminary Contract Documents, Revision B and Revisions to Design Calculations.
2.
Letter dated March 26, 1985, from R.E. Hopkins to Morrison-Knudsen Company, Inc., to L.-Higginbotham of NRC;
Subject:
Shiprock Site - Shiprock, New Mexico, Phase II Construction Documents.
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