Laserfiche WebLink
SECTION 3 – STORMWATER <br />23 <br />ii.Runoff Hydrographs and Flood Routing Analysis <br />a.) This method shall be required when any of the following apply: <br />•If the total tributary drainage area is greater than 5 acres <br />•If there is existing depressional storage <br />•If offsite flows are detained in storage facilities <br />•If proposed flows travel through multiple storage facilities <br />b.) The USDA Natural Resource Conservation Service (NRCS) method (also known <br />as the SCS method) shall be used for calculating runoff and generating <br />hydrographs. Refer to the USDA NRCS National Engineering Handbook (NEH-4) <br />publication for additional information. <br />•The use of proprietary software is discouraged. <br />•Allowable open source software recommended for use include: TR-20, HEC- <br />HMS and EPA-SWMM. <br />3.) Rainfall Depths shown in Table 3 shall be utilized to determine total rainfall depths for South <br />Bend. These values are derived from NOAA Atlas 14, published by the U.S. Department of <br />Commerce, National Oceanic and Atmospheric Administration, and National Weather Service. <br />Table 3: South Bend 24-Hour Rainfall Depth <br />Frequency Rainfall <br />(years) (inches) <br />2 2.81 <br />10 4.09 <br />25 4.90 <br />100 6.27 <br />4.) The rainfall distribution used for the Design Storm should be the median first quartile 24-hour <br />rainfall distribution published in the latest publication of NOAA Atlas 14. The Rainfall <br />Distribution shown in Table 4 shall be utilized in the hydrologic analysis to Design Storm <br />characteristics and the calculation of release rates from the Design Storm.