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Mem orandum Job No. <br />Forward, Inc. Compost Facility Stormwater Improvements July 2014 <br />2 <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 R2/3 <br />S1/2 / 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. (HEE), 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. FHWA- <br />NHI-01-020. <br />Regional Water Quality Control Board, Order No. R5-2014-00006.