(a) Models. All computations of runoff volumes and peak flow rates used in the development of stormwater management plans in accordance with this ordinance shall be based on United States Department of Agriculture – Natural Resources Conservation Service (NRCS) methodology. Models such as Hydraflow Hydrographs Extension, HydroCAD, PondPack or other P&D approved models may be used to evaluate the efficiency of the design required to meet this ordinance. Models such as SLAMM, RECARGA or other P&D approved models may be used to evaluate the efficiency of the design in meeting the infiltration requirements of this ordinance. Models distributed and supported by the Wisconsin Department of Natural Resources may be used to determine compliance with calculating soil loss on construction sites.

(b) Rainfall Depths. To determine compliance with this ordinance, the following design storm rainfall depths shall be used, which are derived from National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Precipitation-Frequency Atlas of the United States, Volume 8, Version 2.0: Midwestern States, published in 2013 and consistent with the NRCS National Engineering Handbook, Wisconsin Supplement Part 650:

(c) Runoff curve numbers. All computations of pre-development conditions as specified in this ordinance shall use those NRCS runoff curve numbers assigned for a “good” hydrologic condition for each land cover type. For lands where the pre-development land use was woodland, grassland/meadow, or cropland, the following NRCS curve number values shall be used as maximums:

Note: NRCS SSURGO soil data shall be used to determine hydrologic soil groups Soil hydrologic groups are available from P&D and can be found on the Kenosha County’s Interactive Mapping System website.

(d) Average annual rainfalls. All modeling involving average annual rainfall or runoff volumes shall use rainfall data from the Milwaukee area between March 28 and December 6, 1969 as the typical annual rainfall pattern for Kenosha County, unless otherwise prescribed in BMP design standards.

(e) Rainfall distribution. All peak flow calculations shall use MSE3 rainfall distribution patterns, as defined in NRCS methodologies. Pre- and post-development conditions shall be analyzed with recurrence intervals of 1-, 2-, 10-, and 100-year design storms. Any hydrologic and hydraulic study required as part of this ordinance involving a floodplain analysis shall use the SEWRPC recommended rainfall distribution and rainfall depths instead of the Type II distribution. It is necessary to perform a critical duration analysis using various storm durations to determine the largest peak flow using the SEWRPC distribution.

(f) Other methods. All velocity and peak flow computations for open channels and storm sewer pipe flows shall be based on Manning’s Formula. Flow routing, culvert design, weir and orifice flow and other related hydraulic computations used to design stormwater management facilities shall be based on standard applicable engineering formulas. Any design data or methodology proposed to be used for hydrologic or hydraulic computations other than those prescribed in this ordinance shall be approved by P&D. Revisions or updates to the rainfall depths and distribution prescribed above may be allowed upon approval by the applicable regulatory agencies and P&D. [Code § 17.11-1.]