ML20197D809
| ML20197D809 | |
| Person / Time | |
|---|---|
| Issue date: | 05/01/1986 |
| From: | Fliegel M NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| To: | Hawkins E NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV) |
| References | |
| REF-WM-48 NUDOCS 8605150058 | |
| Download: ML20197D809 (4) | |
Text
,,
s a
DISTRIBUTION
'Wi s/f" WiGT r/f Wek d GAY 01 W MISS r/f REBrow6ing
-3000/OC/04/30/E-MBell J0 Bunting PSJustus TJohnson & r/f legel MEMORANDUM FOR:
Ed Hawkins, URF0 FROM:
Myron H. Fliegel, Section Leader g
Hydrology Section, WMGT MHaisfield, WMLU
SUBJECT:
REVIEW 0F RAC FINAL DESIGN - DURANGO (B0D0 CANYON) SITE In accordance with your recent requests, Ted Johnson has reviewed surface water hydrology and erosion protection aspects of final design documents for the Durango (Bodo Canyon) site. These documents were submitted by the remedial action contractor (RAC) and are dated April, 1986. Our questions and comments are enclosed.
In general, we conclufe that the design, as proposed, is deficient in several areas and will not meet EPA standards. These deficiencies are detailed in the enclosure.
If you have any questions, please contact Ted Johnson at 427_4490.
S Myron H. Fliegel, Section Leader Hydrology Section, WMGT
Enclosure:
As Stated WM Record File WM Project Dacket fb. _ _
POR V_ _
Distribution:
~ ~ ~ ~
8605150058 860501
~ ~ ~ - ~ ~
PDR WASTE
~ ~ ~ - - -
[~ itusi_~io_l'/_11_'6_2fS5_)___ ___
R
~~~
)FC
- WMGT
- WMGT I
_l.:....... ____:._________.: ___________:________...:. ____ ___
4AME :TJ h mt :MFliegel y_.__:____________:____________:____________:____________:____________:.........__ :___________
) ATE :86/05/t
- 86/05/\\
Durango/Bodo Canyon Final RAP Surface Water Hydrology Questions and Comments 1.
Our review of the embankment design indicates that the toe of the embankment in the southwest area of the pile will be located in a steep arroyo, with arroyo flows parallel to the embankment tot. This design, as proposed, is not considered to be acceptable for assurind long-term stability of the remediated pile.
This conclusion is based on the following:
1.
The arroyo appears to be very steer (longitudinal slope of greater than 10%), and extremely high velocities and shear forces will be produced by even floods of small magnitude.
Because the erosion protection is not keyed into competent rock, these high velocities can produce lateral erosion of the channel banks, such that undercutting of the erosion protection is likely.
2.
The embankment erosion protection does not appear to be placed above the level of flood flows in the arroyo.
3.
The arroyo has some curvature and several natural bends, which will tend to increase shear forces.
4.
There are several other gullies and arroyos opposite the embankment (flowing northeast) where flood flows could impinge directly on the embankment.
In order to resolve the above concerns, we suggest that one or more of the following design changes be made:
1.
Erosion protection capable of resisting PMF velocities should be provided in the arroyo. This alternative, however, is likely to require extremely large rock and may not be practical to implement.
2.
The toe of the embankment should be located above the arroyo PMF level and should be keyed into competent rock, such that the bottom of the key trench is at an elevation above the PMF.
In providing a revised design, further information will be needed regarding the elevations and competency of the bedrock. Additionally, arroyo cross-sections and details of the embankment toe design will be needed to evaluate water surface profiles and velocities.
2.
Examination of the proposed design indicates that there are several areas of the embankment where the peak runoff rates on the embankment will be greater than normal sheet flow rates. This is due to outside drainage
7 2
areas contributing additional runoff to the embankments. Affected areas of the embankment include one at the extreme southwest corner of the site, another at the southeast corner (See also Question 3), and another at the extreme western corner.
The erosion protection should be checked, and redesigned if necessary, to assure that the erosion protection is adequate to resist these additional flows.
3.
We note that the rock which will be provided for the small embankment, which partially fills an existing arroyo in the southeast portion of the site, has been designed for sheet, rather than concentrated, flows. Due to the configuration of the embankment, it is evident that a significant amount of flow concentration can be expected, particularly in the toe area.
In addition, it also appears that the embankment will receive a significant amount of runoff from drainage areas outside the embankment area (See also Question 2). Such runoff will likely be concentrated into the downstream toe area. Based on these considerations, we conclude that the proposed erosion protection for this embankment area is unacceptable.
Unless the embankment configuration is changed, the erosion protection on the embankment should be redesigned to account for runoff from the total
. contributing drainage area.
Additionally, it should be emphasized that since this area will act much like a channel, special measures will be needed to assure that either (1) the exit velocity at the downstream end is non-erosive or (2) the toe of the channel is keyed into competent bedrock, similar to the termination of Ditch #1. The design should be revised, as necessary.
4.
We note that the erosion protection to be placed at the outlet of Ditch #1 is not designed to resist flow velocities occurring in the channel to the north.
Based on a review of the layout of the ditch termination (relative to the north channel), it appears that the erosion protection could be affected by both lateral erosion and headcutting in the north channel.
The riprap in the outlet area should be checked, and redesigned if necessary, to assure that the rock can resist the flow velocities produced by a PMF in the north channel. An acceptable assumption for such computations would be to assume that an eroded cross-section is formed and that a PMF occurs at this eroded section (for example, if the north channel must widen 94 feet to erode the outlet erosion protection, an additional channel width of 94 feet may be assumed in the computation of flow velocities).
4 Additionally, the contributing drainage area for the PMF should include the area of the pile which drains to the north channel; this area was not previously considered in estimating the PMF on the north channel.
(
e '.,'
i~
3 5.
Our review of the computations associated with flushing of sediment from the drainage ditches indicates that the analyses presented may not adequately reflect conditions that will actually exist. In general, the assumptions used to compute the flow velocities associated with relatively frequent flood events should not be the same conservative assumptions that were used to compute the magnitude and velocity of the PMF. The calculations should be revised, using the following:
(1) A runoff coefficient of less than 1 should be used in the rational formula.
(2) The rainfall intcnsity and discharge should be based on the time of concentration and drainage area at various points along Ditch 1 and Ditch 2.
(3) Mannings's 'n' for lesser floods should be assumed to be greater than the 'n' value assumed for the PMF, due principally to decreased depths of flow for lesser floods.
l 1
i 9
n.