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Stantec <br /> CITY OF CENTERVILLE -2018 LOCAL WATER PLAN <br /> HYDROLOGIC CALCULATIONS/MODELING PARAMETERS <br /> 4.0 HYDROLOGIC CALCULATIONS/MODELING PARAMETERS <br /> 4.1 WATER QUANTITY <br /> As stated in Chapter 6, rate control must be provided within each development that maintains <br /> existing runoff conditions for the 2-year, 10-year, and 100-year storm events. For each <br /> proposed development, supporting calculations or modeling must be submitted. The <br /> calculations or modeling shall be based on NRCS methods. The parameters for the calculations <br /> or modeling shall be as follows: <br /> NRCS 24-HOUR Type II design storms based on NOAA Atlas 14 Precipitation Estimates for <br /> Centerville, MN: <br /> 1-year storm = 2.4" <br /> 10-year storm = 4.2" <br /> 100-year storm =7.2" <br /> Previously, Technical Paper 40 estimates were: <br /> 1-year storm = 2.4" <br /> 10-year storm = 4.2" <br /> 100-year storm = 5.9" <br /> As discussed in Section 3.4 (Climate), the state of Minnesota and watershed districts now <br /> require use of Atlas 14-Point Precipitation Frequency estimates. Previously, Technical Paper <br /> No. 40 (TP-40) estimates were used. NOAA Atlas 14 data uses more precipitation stations in its <br /> statistical analysis, includes additional precipitation records, and uses a more robust statistical <br /> analysis to determine rainfall estimates. Both TP-40 and Atlas 14 datasets are based on <br /> historical rainfall events and do not extrapolate to predict future events. <br /> In addition to analyzing the 2-year, 10-year and 100-year storm events, the 10-day snowmelt <br /> (Curve Number 100) must be used. According to RCWD requirements, analyzing the 10-day <br /> snowmelt event is not required for stormwater management detention basins with a defined <br /> outlet elevation at or below the 100-year return period, 24-hour rainfall event elevation. <br /> To analyze stormwater runoff for the design of discharge and water levels within and from the <br /> project site, a hydrograph method, or program based on NRCS Technical Release #20, must be <br /> used. Pervious and impervious areas in the model must be modeled separately. <br /> 4.13 <br /> 51 <br />