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<br />I. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />i. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />, <br />~ . <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />. <br />i. <br />. <br />. <br />. <br />. <br />. <br /> <br />Wash <br /> <br />Master Plan <br /> <br />Phase I: Watershed Floodplain Delineation <br /> <br />4.1.2 Time of Concentration (Tc) Determination <br /> <br />For the lower portions of the watershed, below 1-70, computations for time of concentration were <br />developed using a procedure that considered both overland (sheet) flow and shallow <br />concentrated flow, as outlined in Technical Release 55 (TR-55, SCS, 1986), The equation used <br />for calculation of overland flow time is: <br /> <br />O,007(nL)O,8 <br />Ttl = 05 04 ,where <br />(Pz)'s' <br /> <br />TtI = travel time as overland flow (hr) <br />n = Manning's roughness coefficient <br />L = flow length as overland flow (ft) <br />Pz = 2'year, 24-hour rainfall (in) <br />s = land slope (ft/ft) <br /> <br />The calculation of overland flow assumed the longest non-channelized route in a particular <br /> <br />subbasin based on contours, not to exceed 300 feet. Anything in exceedance of 300 feet was <br /> <br /> <br />assumed to be shallow concentrated flow, A Manning's 'n' value of 0,17 was used in evaluation <br /> <br />of sheet flow; this coefficient is based on an assumption of cultivated soils with greater than 20% <br /> <br />residue cover as described in Table 3-1 of the Drainage Criteria Manual. developed by Urban <br /> <br />Drainage and Flood Control District (UDFCD, 1969), The 2-year, 24-hour rainfall depth was <br /> <br />estimated to be 1.1 inches, based on values from the NOAA Atlas, The land slope is estimated <br /> <br /> <br />from mapping provided by the County GIS/Mapping Department. <br /> <br />Travel time as shallow concentrated flow is: <br /> <br />L <br />T,z = 3600*V ' where <br /> <br />T,z = travel time as shallow concentrated flow (hr) <br /> <br />L = flow length as shallow concentrated flow (ft) <br /> <br />v = average velocity (ftls) <br /> <br />_.~~~~~:!~\k;<f',,~ <br />hi" <br />~ <br /> <br />lil"1 <br /> <br />c~~1i1~ <br /> <br /> <br />:M" <br />i,.,~,,", .~"0r':;~~ <br /> <br />. <br /> <br />Page 9 <br />