ML20214E412
| ML20214E412 | |
| Person / Time | |
|---|---|
| Issue date: | 03/10/1986 |
| 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-64 NUDOCS 8603260171 | |
| Download: ML20214E412 (4) | |
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}Qg3ff' WMGT r/f' NMSS r/f MAR 101986 RBrowning
-ME"0/i3l/TJ/EG/03/ u/
u ting MKnapp TJohnson & r/f MFliegel MEFJRANDu.it FOR:
Edward Hawkins, URF0 losure in DCC FROM:
Myron H. Fliegel, Section Leader URF0
SUBJECT:
REVIEW 0F REVISED LAKEVIEW DESIGN In accordance with your requests, we have reviewed the most recent revisions to the Lakeview remedial action design. These revisions were transmitted to you from Morrison-Knudsen by letter dated February 24, 1986. Our comments and questions are enclosed.
Based on our review, we conclude that additional revisions will be needed, particularly in those areas where the erosion protection terminates and meets existing topography. These revisions will need to be made before we can conclude that the design will meet EPA long-term stability criteria.
If you have any questions, please contact Te y sc 42,7 4490.
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Myron H. Fliegel, Section Leader WMGT
Enclosure:
As Stated WM Record file WM Projxt
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- MFliegel JATE :86/03//c
- 86/03/l0
MEM0/EH/TJ/86/03/07/ ENCLOSURE Lakeview Design Revisions Surface Water Hydrology and Erosion Protection Questions and Comments 1.
Our review indicates that the rock layers for the rock apron at the toe of the embankment side slope and for the energy dissipation area (EDA) at the downstream end of the diversion ditch have both been designed assuming that normal depth and velocities will occur.
In the upstream portions of the apron and the EDA, this assumption may not be valid, because normal flow depths will likely be established only at some point downstream (after the flow has had the opportunity to stabilize). A considerable amount of turbulence and energy dissipation can be expected to occur in the areas.
Because the main purpose of the apron and EDA is to dissipate the energy produced on the steeper upstream slopes, these areas should be designed under the basic premise that they will act as stilling basins.
It may be necessary to increase the average rock sizes and layer thickness in these areas, particularly on the upstream portions of the apron and EDA where most of the energy will be dissipated.
While we recognize that specific guidance for designing erosion protection in energy dissipation areas may not be available, a redesign of the apron and EDA should be performed. Use of the following will provide designs which are acceptable to the NRC staff:
(a) EM 1110-2-1603, " Hydraulic Design of Spillways" (page 45),
indicates that erosion protection in turbulent areas should be designed assuming that the stability of the rock layer is dependent upon the stability of an individual average rock in the layer. Use of the isolated cube curve and the averaqe velocity produced on the steeper upstream slope (as shown on Plate 22) is acceptable for designing rock on the apron and EDA.
(b) ETL-1110-2_120 " Additional Guidance for Riprap Channel Protection" (page 4) indicates that where uniform channel flow does not occur, local shear forces will be greater than indicated by normal computational methods, and that shear forces should be increased by a factor of about 1.5 to account for localized velocities and pressure pulsations. Use of this factor, based on the velocity on the steeper upstream slope, is l
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MEM0/EH/TJ/86/03/07/ ENCLOSURE acceptable for designing the erosion protection for the apron and the EDA.
(See also Question 2, below).
2.
We note that credit has been taken for flow through the rock layer in the diversion ditch.
In the steeper ditch segments, this can amount to a significant percentage of the total flood flow.
For example, in ditch segment D-5, the flow through the rock layer was computed to be about 120 cfs out of a total of about 300 cfs. As a result, the riprap in this ditch segment has been designed to withstand a flow of about 180 cfs.
However, it must be realized that at some point at the downstream end of this ditch segment (where it merges with the EDA), the entire flow will emerge onto the face of the rock layer. At this point, 300 cfs (not 180 cfs) becomes the design flow.
Where the slopes change from a steep to a mild grade, such as the apron and EDA, the calculations should be revised to reflect the larger flow rates occurring over. the top of the rock layer.
It should be recognized that the exact location of the occurrence of emergent flow may be very difficult to determine. General guidance may be found in Rockfill in Hydraulic Enoineering (D. Stephenson, 1979).
Lacking specific or reliable estimates, the rock should be designed for the larger flow rate for a distance of at least about 20 feet upstream of the slope breaks at the apron and the EDA.
Downstream of the slope breaks for about 20 feet, the increased flow rate and revised velocity should also be used to design the erosion protection.
(See also Question 1, above).
3.
The key trenches at the downstream end of the apron and EDA are proposed to be approximately three feet deep and have a cross-section that is approximately square. Since the trenches do not key into rock, the NRC staff does not consider this design to be acceptable in meeting EPA long-term stability standards.
First, the depth of the key trench should be greater than three feet.
Examination of an existing gully (which the proposed pile will cover) in the site area indicates that this gully is more than three feet deep.
Since it is likely that a gully will form and headcut gradually toward the downstream end of the apron and EDA, it would seem prudent to place the key trench at a depth at least as deep as the maximum depth of the existing c 'ly (If the drainage area of the assumed new gully will be significantiy different from the drainage area of the existing gully, adjustments will need to be made, accordingly).
Second, the square trench cross-section proposed may not be adequate to provide an adequate quantity of rock to make the future gully
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MEM0/EH/TJ/86/03/07/ ENCLOSURE 3-
"self-healing". The cross-sections should be revised to reflect standard practice, such as EM 1110-7-1601 (Plate 38) which provides acceptable guidance for design of riprap end protection.
(Method C provides one acceptable method). Alternate designs, if used, should be thoroughly documented.
The design of the key trenches should be revised, as necessary.
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