ENFORCE21130 - Laserfiche WebLink

Home Browse Search

Memo to Dan Hernandez - 3 - October 27, 1989 b. The 54-inch receiving culvert downstream is designed to accommodate this additional 30 cfs of flow on top of its design flow. It should be noted that the culvert constructed to drain i:he area near the guard station and the hydraulics of the reduction in the 54-inch culvert down to 30 inches (as noted on Plate 2 of 2), should be considered in this analysis. If the demonstration of sufficient flow capacity and stability of the channels can be made, then the proposal can be approved as submitted. If not, then the Division would recommend a 36-inch culvert be installed at HR-6. This would imply a replacement of the existing 24-inch culvert on Cedar Heights Drive East with a 36-inch size or that a new 3E-inch culvert be placed at the location specified in the Drainage Master Plan and leaving the existing 24-inch culvert in place. Local drainage regulations may need to be consulted as to who is responsible for insuring properly-sized culverts are installed on downstream properties before this option can be approved. Water Quality A silt retention facility is proposed as part of the corrective action plan to protect water quality. As mentioned earlier, this facility would replace the function served by the ponded area upstream of the Area 1 slide which will be filled (reference page 11 ). The silt retention facility would control sediment from the drainage area encompassing the visual berm, processing area, and area east of the access road. This corresponds to the area drained by Culvert No. HR-1. The inclusion of the silt retention facility in the plan to replace the existing sediment trap appears to be appropriate. Detailed plans for this proposed facility need to be developed as stated on page 12. A careful survey of the contributing watershed area should be completed as is also mentioned. The design parameters and methodologies to be used in the analysis, i.e. USLE derived sediment volume, 40 micron particle size setting velocity, 10-year, 24-hour storm runoff volume and 100-year design storm emergency spillway discharge are appropriate. The preliminary sizing of the facility as noted on page 13 were reviewed. My calculations verify the pond dimensions would provide 1.55 acre-feet of volume which is larger than the 1.15 acre-feet of runoff expected from the 11 acre basin area. The final designs should specify the dimensions of all structures and the corresponding hydrologic and hydraulic parameter values used in the design methodology. All supporting documentation including maps, figures, calculations and references should be submitted for Division review. Response to Other Parties Comment In the following sections I am providing you with my response to comments each of the parties submitted which are relative to hydrology. I have numbered them the same as they are presented in the respective letters.