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PRELIMINARY DRAINAGE STUDY HIGHLANDS ADDITION TOWN OF COTTAGE GROVE, DANE COUNTY, WI This report analyzes the drainage impacts of the proposed Highlands Addition Subdivision. The proposed subdivision site is located in the SW 1/4 of the SE 1/4 of Section 3, and in a part of the NW 1/4 of the NE 1/4 of Section 10, T7N, R11E, Town of Cottage Grove, Dane County, WI, at the NE corner of Kennedy Road and C.T.H. "BB". The development is 73.6 acres consisting of 75 single family lots. Drainage calculations were done to analyze the impact of the proposed Residential Development using the Soil Conservation Service TR-55 Method. Ten-year, 25-year and 100-year storm events were used to compare peak flow rates for existing conditions and proposed conditions. Presently, the site is a farm field, but to take a more conservative approach to analyzing the existing conditions a meadowland condition was assumed for the selection of a SCS CN value (meadowland - Type B Soils N=58). Soil types on the site are as follows: St. Charles silt loam, Plano silt, McHenry silt, Ringwood silt, and Dodge silt. The proposed drainage pattern will continue to follow the present direction of flow as follows: Area A, 40.6 acres, drains to the southwest corner of the site to the C.T.H. "BB" ditch; Area B, 29.1 acres, drains to the northwest corner of the site to the West Ridge Addition to American Heritage Subdivision; and Area C, 3.9 acres of the site drains to the west to the American Heritage Subdivision. A CN value of 68 was used for the proposed condition analysis. (Ave. 1 acre residential lots, Type B soils). The following is the summary of the peak flow calculations: Area A: 10-Yr. 25-Yr. 100-Yr. Existing Conditions 16 cfs 26 cfs 52 cfs Proposed Developed Conditions 35 cfs 49 cfs 83 cfs Area B: 10-Yr. 25-Yr. 100-Yr. Existing Conditions 11 cfs 19 cfs 37 cfs Proposed Developed Conditions 25 cfs 35 cfs 60 cfs 0/1 �`I