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Memorandum Job No. <br /> Forward,Inc.Compost Facility Stormwater Improvements July 2014 <br /> RAINFALL <br /> Rainfall and intensity duration frequency data was obtained from NOAA Atlas 14. The 25 year 24 <br /> hour rainfall depth is 2.83 inches and the 25 year 10 minute intensity is 2.18 inches per hour. <br /> RUNOFF <br /> Soil ground cover is designed to be low permeability.Runoff using the SCS method was computed <br /> using a Curve Number CN=88.Runoff for the Rational Method used a Coefficient of C=0.8. <br /> TIME OF CONCENTRATION <br /> Time of concentration was computed using the TR-55 method. <br /> DESIGN FLOWS <br /> Design flows were computed for three conditions using the(1)SCS TR-55 Method,(2)Rational <br /> Method,and(3)Rational with fixed 10-Minute time of concentration method.Method(3)gave the <br /> higher peak design flows and was used for design. <br /> CHANNEL DESIGN <br /> Channels were designed for open channel flow condition using Manning's Equation(Q=1.49 A W' <br /> 51/2/N)for a uniform steady state flow condition and 0.5 foot minimum free board plus additional <br /> freeboard for surface waves and super-elevation. Channels use the peak flow at the downstream end <br /> of the channel and flow decreases proceeding upstream.Channel bed shear is low and earth channel <br /> is stable. If erosion control mat is to be used, it raises the channel friction, increases the water depth, <br /> and decreases the freeboard depth. It is recommended that if erosion control blanket is used, that it be <br /> for a short duration of time. <br /> CULVERT DESIGN <br /> Culverts were designed using Extension for AutoCAD(Autodesk,Inc. (BEE), 2013). Hydraflow <br /> Express Extension uses sophisticated energy-based methods to compute the hydraulic grade line <br /> (HGL). It can handle inlet control and outlet control in any flow regime from partial depth, full depth, <br /> surcharged,roadway overtopping,as well as supercritical flow profiles with hydraulic jump.Methods <br /> used are those generally described in HDS-5 (Federal Highway Administration HDS-5, 2001). <br /> Design is based on HDPE pipe having an N-Value of 0.10, 18"minimum pipe cover, and H-20 <br /> vehicle loading. <br /> REFERENCES <br /> Autodesk, Inc. (HEE). (2013). Hydraflow Express Extension for AutoCAD Civil 3D 2013, Version 10. <br /> Autodesk, Inc. (HSSE). (n.d.). Hydraflow Storm Sewers Extension for AutoCAD Civil 3D 2013, <br /> Version 10. <br /> Federal Highway Administration HDS-5. (2001). Hydraulic Design of Highway Culverts. FHW4- <br /> NHI-01-020. <br /> Regional Water Quality Control Board, Order No. R5-2014-00006. <br /> 2 <br />