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Rodney J. Ripley • December 13, 1994 Page 2 The contributing drainage area for this culvert is comprised of two tributaries which confluence 600-feet upstream of the culvert crossing. The drainage area is 526-acres in size, as shown in Figure 1, based on the USGS 7.5-Minute Quadrangle Topographic Map. The SCS model uses a 2-year, 24-hour storm of 2.8-inches. Based on Dane County soil maps, the soils in this drainage area are silt loams, which have a hydrologic soil group classification of type B. The TR-55 calculation is shown in Appendix 1. The 2-year peak runoff rate is 200 cubic feet per second at the culvert location. We used the Federal Highway Administration's HY8 computer model to size the culvert to convey the 2-year flow with no road overtopping. The HY8 model considers both inlet and outlet flow conditions to determine a culvert's capacity. The result of our culvert sizing analysis, as shown in Appendix 2, is that 20-feet of 60-inch corrugated metal pipe at a .012 ft./ft. slope (to match channel slope), will convey the 2-year flow. Approximately 2.5-feet of fill is needed between the top of the pipe and the bottom of the road to allow the headwater to build up to a level to force the 2-year flow through the culvert. Recommendation Based on our analysis, we recommend a 60-inch corrugated metal pipe be placed at this driveway. We also recommend riprap or other protection be installed at the upstream and downstream edge of the culvert to prevent erosion and scour. Installation of this culvert should be performed by a certified contractor. We also recommend that you contact Clark Amundson of the Wisconsin Department of Natural Resources at (608) 273-5970, and Pat Anderson of Dane County Land Regulation & Records at (608) 266-9086, to ensure that all applicable permits are identified and obtained. Zr;-906 Sincerely, MEAD & HUNT, Inc. a 01Pvi Charles E. Nahn III, P.E. Manager of Storm Water Services Enclosures R129A001/R129-94A