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61 HOUSTON engineering, inc. MODELING GUIDELINES The following set of modeling guidelines has been prepared for reference for partners of the District. The guidelines provide a description of some of the methodologies used to develop inputs for the maintained DWMP products. Conforming to the following guidelines will allow RCWD to incorporate relevant third -party changes into the DWMP. VERTICAL DATUM All elevations within the models are in reference to North American Vertical Datum of 1988 (NAVD88). CATCHMENT BOUNDARIES The third -party users shall use the RCWD subcatchments shapefile as an initial starting point. Any subdivision of catchments will edge match to the RCWD subcatchment shapefile at the outer boundary of the study area. Per request, the District can provide the following shapefiles: • Subcatchment boundaries HYDROLOGY The SWMM models were developed using SCS hydrology method further defined below. Time of Concentration Time of Concentrations were estimated using methods described in Chapter 15, Part 630 Hydrology, National Engineering Handbook (NRCS, 2010). Curve Numbers Land use data and soil data were used to develop pervious Curve Number grids, impervious percentage grids, and a composite Curve Number grid. Area -weighted calculations using GIS tools can be performed on the Composite CN grid to develop CN values for each catchment. Per request, the District can provide the following gridded data sets: • Pervious Curve Number • Impervious Percentage • Composite Curve Number RAINFALL DATA NOAA Atlas 14 (Volume 8, NWS; 2013) rainfall depths are used to simulate the 2-year, 10-year, 100-year, and 500-year, 24-hour rainfall events. The rainfall events are simulated with a temporal distribution known as MSE3. In addition to the Atlas 14 rainfall events, a 10-day event is simulated to represent a runoff condition. The 100- year, 10-day runoff event is generated using an SCS Type I distribution with a total runoff depth of 7.2". Models provided to the District shall be run for the 2-, 10-, 100-, and 500-year rainfall events. Per request, RCWD can provide • Atlas 14 Rainfall depths • MSE3 Temporal distribution MODEL GEOMETRY Culverts, channels, and other structures added to the model shall be based on as -built survey data. Points of overflow for 500-year and lower rainfalls shall be discreetly modeled and may be based on LiDAR elevation data. STARTING WATER ELEVATION Starting water elevation in ponds and basins shall be set at the lowest pond outlet elevation, except: • For locations of HSG A or B soil, starting water elevation may be at the bottom of the basin. • Historic water level data may be used to establish the starting water level. PAGE20F2