REV01387 - Laserfiche WebLink

Home Browse Search

Technical Revision No. 79 -PAR Responses Apri118, 1997 14. The sedimentation pond now includes anti-seep collars. Figure L-3 has been revised. 15. The calculations for the rip rap sizing presented in the application are enclosed. The calculations should be included in Appendix L. The method for rip rap sizing involved the following steps: Page 4 1. Use the SCS runoff curve number method to derive the peak flow for each channel. 2. Adjust channel and rip rap dimensions to accommodate peak flow with 0.5 feet freeboard and prevent excessive velocities which could result in scouring/erosion of the channel bottom. The Chery-Manning equation was used to compute the flow velocity. The modified Strickler equation was used to derive the Manning roughness coefficient from the nominal rip rap dimension and the hydraulic radius information. The rip rap had to provide adequate channel protection yet allow a velocity which could permit the required 100-yeaz, 24-hour storm peak flow to pass. Erosion control matting maybe used instead or rip rap where rip rap is specified. No rip rap or channel Gning is necessary if flow velocities are less than 5 fps because the native clay soils are cohesive enough to resist scouring. The required rip rap necessary ranged within cobble size (3 to 6 inch nominal diameter) category. 16. MCC believes blockage of the culvert under State Highway 133 is not a plausible event due to the placement of a new 7 foot diameter culvert for the RPE haul road crossing upstream from the Highway 133 culvert. Any significant blockage will occur at the 7 foot culvert, which is designed fora 100-year 24-hour storm event (this design was included in the original submittal). In addition, the culvert under State Highway 133 is a 8' X 8' box culvert providing 64 SF of flow area compared to the 38 SF of flow area for the 7' culvert, or 1.68 times the capacity of the 7' culvert. Blockage at the 7' culvert would overtop the road and flow back into Sylvester Gulch, and through the box culvert. However, in the unlikely event that the box culvert did plug, a review of the topography of the RPE versus the inlet to the box culvert shows the overflow path would be north of the RPE footprint, and would overtop Highway 133 before impacting the ItPE. Topsoil 17. MCC agrees that based on the projected salvage depths and areal extent of the identified soil series and types that a total of 23,066 cubic yards of topsoil would be salvaged from the RPE area to be disturbed. This volume would replace 9.9 inches of topsoil on the ItPE at the time of reclamation. If additional suitable topsoil is identified, it will be salvaged and replaced on the RPE during final reclamation. MCC maintains the commitment of the current topsoil replacement plan which states, "The seedbed material is replaced to an