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_ ~. <br /> <br />Sheet flow <br />Sheet flow is flow over plane surfaces. It usually <br />occurs in the headwater of streams. With sheet flow, <br />the friction value (Meaning's n) is an effective <br />' roughness coefficient that includes the effect of <br />raindrop impact; drag over the plane surface; <br />obstacles such as litter, crop ridges, and rocks; and <br />erosion and transportation of sediment. These n <br />values are for very shallow flow depths of about 0.1 <br />foot or so. Table 31 gives Meaning's n values for <br />sheet flow for various surface conditions.' <br /> <br />1 <br />i <br /> <br /> <br /> <br />1 <br /> <br />CJ <br /> <br /> <br />For sheet flow of less than 300 feet, use Mannatg's <br />kinematic solution (Overton and Meadows 1976) to <br />compute Tt: <br />where <br />Tt =travel tune (hr), <br />n = Meaning's roughness coefficient (table 3-1), <br />L =flow length (ft), <br />PZ = 2-year, 24-hour rainfall (in), and <br />s =slope of hydraulic grade tine (land slope, <br />ft/ft). <br />This simplified form of the Meaning's kinematic <br />solution is based on the following: (1) shallox• steady <br />uniform flow, (2) constant intensity of rainfall excess <br />(that part of a rain available for runoff), (31 r-.irtfall <br />duration of 24 hours, and (4) minor effect of <br />infiltration on travel time. Rainfall depth can be <br />obtained from appendix B. <br />_ 0.007 (nLp-s [Eq. 33] <br />Tt = (peps so.a Shallow ocncentrated flow <br />Table 3-l.-Roughness ccefticients (Meaning's n) for <br />sheet flow <br />Surface description n' <br />Smooth surfaces (concrete, asphalt, gavel, or <br />bare soil) ................................... 0.011 <br />Fallow (no rot;idue) .......................... 0.0.5 <br />Cultivated sods: <br />Residue cover ~ZOrc ...................... 0.06 <br />Residue cover >Z09c ...................... 0.17 <br />Grass: <br />Short grass prairie ........................ 0.15 <br />Dense gtasses3 ............................ 02a <br />Bermudagrass ............................. 0.41 <br />Range (natural) ............................. 0.13 <br />R'oodsP <br />Light underbrush .......................... 0.40 <br />Dense underbrush ......................... 080 <br />s7'he n values are a composite of intcrmation mmp0ed by Engman <br />(1986). <br />alneludrs apeties such as weeping lovegnss, bluegrass, Duttalo <br />grass, blue grams gran, and rutive gnu mixtwrs. <br />!When selecting n, wnstder rover to a height of about 0.1 n This <br />is the only part of the pLnt ewer :hat w~71 obetrun sheet sow. <br />After a ma~amum of 300 feet, sheet flow ustuilly <br />becomes shallox• concentrated flow. The average <br />velocity for this flow can be determined from figure <br />31, in which avenge velocity is a function of <br />watzrcoutse slope and type of channel, For slopes <br />less than 0.005 ftJft, use equations given in appendix <br />F for figure 31. Tillage can affect the direction of <br />shallox• concentrated flow. Flow may not always be <br />directly dorm the watershed slope it tillage runs <br />across the slope. <br />After determining average velocity in figure 3-1, use <br />equation 31 to estimate travel time for the sha]!ow <br />concentrated flow segment. <br />Open channels <br />Open channels are assumed to begin where surveyed <br />cross section information has been obtained, where <br />channels are visible on serial photographs, or where <br />blue lines (indicating s;a~eamsJ appear on United <br />States Geological Surrey (USG3) quadrangle sheets. <br />Meaning's equation or water stuiace profile <br />information can be used to estimate average flow <br />velocity. Average flox velocity is usually determined <br />for bank-full elevation. <br />(210-VI-TR-55, Second Ed., June 1986) 33 <br />