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a ntec CITY OF CENTERVILLE -2018 LOCAL WATER PLAN 4.0 HYDROLOGIC CALCULATIONS/MODELING PARAMETERS 4.1 WATER QUANTITY As stated in Chapter 6, rate control must be provided within each development that maintains existing runoff conditions for the 2-year, 10-year, and 100-year storm events. For each proposed development, supporting calculations or modeling must be submitted. The calculations or modeling shall be based on NRCS methods. The parameters for the calculations or modeling shall be as follows: NRCS 24-HOUR Type II design storms based on NOAA Atlas 14 Precipitation Estimates for Centerville, MN: 1-year storm = 2.4" 10-year storm = 4.2" 100-year storm =7.2" Previously, Technical Paper 40 estimates were: 1-year storm = 2.4" 10-year storm = 4.2" 100-year storm = 5.9" As discussed in Section 3.4 (Climate), the state of Minnesota and watershed districts now require use of Atlas 14-Point Precipitation Frequency estimates. Previously, Technical Paper No. 40 (TP-40) estimates were used. NOAA Atlas 14 data uses more precipitation stations in its statistical analysis, includes additional precipitation records, and uses a more robust statistical analysis to determine rainfall estimates. Both TP-40 and Atlas 14 datasets are based on historical rainfall events and do not extrapolate to predict future events. In addition to analyzing the 2-year, 10-year and 100-year storm events, the 10-day snowmelt (Curve Number 100) must be used. According to RCWD requirements, analyzing the 10-day snowmelt event is not required for stormwater management detention basins with a defined outlet elevation at or below the 100-year return period, 24-hour rainfall event elevation. To analyze stormwater runoff for the design of discharge and water levels within and from the project site, a hydrograph method, or program based on NRCS Technical Release#20, must be used. Pervious and impervious areas in the model must be modeled separately. 13 108