ML20127E002
| ML20127E002 | |
| Person / Time | |
|---|---|
| Issue date: | 04/30/1985 |
| From: | Knapp M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Higginbotham L NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| REF-WM-58 NUDOCS 8505170203 | |
| Download: ML20127E002 (6) | |
Text
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WM Record File 1%! Pro M J P l'
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APR 3 01985 FDR _&
TJ/WM.58/85/04/18
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MEMORANDUM FOR: Leo B. Higginbotham, Chief, WMLU DISTRIBUTION:
Division of Waste Management
- 6 EBrowning NMSS r/f FROM:
Malcolm R. Knapp, Chief. WMGT.
WMGT r/f Division of Waste Management TJohnson & r/f MFliegel
SUBJECT:
REVIEW 0F SHIPROCK REVISED RAP DESIGNS MKnapp J0 Bunting MJBell In accordance with your request of February 26, 1985, Task No. 85006, Ted Johnson has reviewed the revised erosion protection design for the Shiprock remedial action site. Based on this review, we conclude that the design of the erosion protection to be placed in the drainage ditches is not adequate to meet EPA long-term stability criteria. Our comments and questions are enclosed.
If you have any questions, please contact Ted Johnson at x-74490.
Original Signed B7 Malcolm R. Knapp, Chief Geotechnical Branch Division of Waste Management
Enclosure:
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e WM-58/TJ/85/04/18 SHIPROCK RAP SURFACE WATER HYDROLOGY QUESTIONS AND COMMENTS 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 produce 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 forces 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 1110-1-1601, " Hydraulic I
WM-58/TJ/85/04/18 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 urface 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 the NRC staff) of the 'n' value in ditch segment 0-7, using Corps of Engineers EM 1110-2-1601 formulae (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 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-R0-02).
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.
C.
Inadequate 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-RO-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-RO-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.
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WM-58/TJ/85/04/18 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 conclude 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 givcn 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 abcut 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
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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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.m Our review indicates that the.la.tter 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 Technica1' Position Paper WM-8201, has been superceded-
- (y that given11n the recently: published Hydrometeorological Report No.-.55 Ma b
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.in WM-8201.
. WM-8201 was: developed for use at. active uranium mill sites,
.most of which are loct.ted in Wyomitig, 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 d
'was' formulated to provide that type of, general guidance, based on Corps of Engineers rainfall distributions.
The re'ent publication of c
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.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 giventin WM-8201. As a result, the NRC stafCintends to make appropriate modifications to WM-8201 to reflect the new 'ata.
d The modifications to Wi-8201 will include recommendations to use the rainfall'distributforiguidance_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.
Extrapolatidnofthedatafortimeintervals1ess'than15minuteswillbe necessary.
3.
Our' review of the erosion protection to-be provided for the filled-in arroyos indicates that the protection may not be adequate to meet EPA long-term stability criteria. We conclude that the-1 Vertical (V) on 2 Horizontal- (H) fil1 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 rainstonns 77 and that insignificant gully erosion on the slopes should be expected. We conclude that the filled-in arroyos, as designed, will experience
- r 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 a
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occurrence of relativ'ely minor flood and precipitation events.
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WM-58/TJ/85/04/18 :
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 fonnerly was discharged elsewhere is now being directed at this area of the escarpment.
Provide the details of, and bases for, tiie erosion protection and. transition design in this area.
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