2008-09-08_REVISION - C1994082 - Laserfiche WebLink

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Technical Revision 37 Response to Adequacy Comments Page S • Response: SCC respectfully disagrees with the Division with portions of this comment. The configuration of Pond 012 has not changed. As confirmed by the May 2008 pond survey, the impoundment's configuration (e.g., the principal spillway is still a 30-inch diameter pipe, the emergency spillway is still 15 feet wide; spillway locations remain the same as before) and structure elevations (e.g., top of embankment, top of principal spillway, invert of emergency spillway) are the same as that referenced in the As-Built Report Attachment 13-12A) and shown on Exhibit 13-6. What has changed is the volume of sediment stored in the pond. The as-built configuration as certified, and approved, is correct and appropriate (Part II and Part III, RN-02 [May 2005] and As-Built Report [March, 2000], respectively). However, with the recent construction of Pond 012A, SCC acknowledges that references to "Option 1" - a SEDCAD demonstration with only Pond 012 - and "Option 2" - the two pond system - may no longer be applicable. SCC also acknowledges that including the two pond system in Attachment 13-12A (identified as Part I [the TR-37 Postmine Storage Capacity Addendum package] is no longer applicable and, consequently, Part I of the As-Built Report for Pond 012 should be removed at this time. The Division has already approved the designs for Pond 012A and Pond 012 (see Appendix 12-13.1). SCC is developing the mitigation plan to address NOV CV-2008-001 Abatement Step 2. There are several remedial scenarios that may part of the mitigation plan including removing some of the stored sediment or changing the configuration/elevations of the pond spillways/embankment. However, SCC does not anticipate significant changes from the "original design". SCC has prepared a two-pond system "design" to calculate the minimum storage requirements needed for Pond 012 to maintain compliance with effluent discharge limits. SCC also understands that the "As-Built Report" will need to be updated upon completion of the remedial plan developed to address NOV CV- 2008-001 and construction of Pond 012A. is 12. Then it an error on page 13-12A-1.3 regarding reference to (elevation deference between the calculated top of the sediment storage and the principal spillway invert) being between 0.5 and one (1) as recommended by Warner, et al., 1998. If a slow passive dewatering system it employed (such as is the case with Pond 012) then the elevation can be reduced to 0.5 to 1.0 ft. The second paragraph should be revised to read (elevation deference between the calculated top of sediment storage and the low-hole spillway/ dewatering orifice). Please clarify this discrepancy and revise page 13-12A-1.3. Response: SCC believes that the existing statement "elevation difference between the calculated top of sediment storage and the principal spillway invert" is correct. As the Division has noted in its comment, the principal spillway contains a dewatering orifice. The dewatering orifice equates to the invert elevation of the riser pipe. Consequently, the text is correct as written. Since Part I of Attachment 13-12A has been removed in response to Comment No. 11, there is no need to correct this page. However, SCC will assure that the statement is clarified in the as-built report submitted in accordance with the NOV abatement process. 13. In the SEDCAD4 User's Guide (Page 64) it is suggested that a minimum two foot elevation deference exists between the invert of the principal spillway and the top of the sediment storage area. This would apply to Pond 012A since then is no low-hole spillway. The ori ginal approved design for 012,4 (IR-36) had less than the suggested elevation deference (1.49 feet). With TR-37page 13-12,4-1- 4.13 (Pond 012 and 012A System) shows a proposed elevation between the top of sediment storage and the principal spillway of only 55 feet). A more prudent design and to avoid re-suspension of sediment would be to allow for additional space between the top of sediment and the principal spillway elevation. Please reconsider the design for Pond 012A to allow far a minimum two foot elevation deference between the invert of the princifal spillway and the top of sediment storage. Response: The elevation suggestions in the SEDCAD4 User's Guide are recommendations. It maybe appropriate to use elevation ranges that are less than suggested in the User's Guide. Scenarios where it may be appropriate to use less than the suggested elevation difference include situations of minimal sediment contribution; where the pond acts as a pre-settling basin; and, when there is not a settleable solids effluent limit on the discharge. This is the case with Pond • 012A. The SEDCAD4 modeling, utilizing July 2007 watershed conditions, indicates the sediment delivery from the 10- year, 24-hour storm event is 46.5 tons, which equates to 0.16 tons per acre. This delivery value is significantly less than