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I i i Sheet flow where Sheet flow is flow over plane surfaces. It usually Tt = travel time (hr), occurs in the headwater of streams. With sheet flow, n = Manning's roughness coefficient (table 3-1), the friction value (Manning's n) is an effective L = flow length (ft), roughness coefficient that includes the effect of P2 = 2-year, 24-hour rainfall (in), and raindrop impact; drag over the plane surface; s = slope of hydraulic grade line (land slope, obstacles such as litter, crop ridges, and rocks; and ft/ft). erosion and transportation of sediment. These n values are for very shallow flow depths of about 0.1 This simplified form of the Manning's kinematic foot or so. Table 3-1 gives Manning's n values for solution is based on the following: (1) shallow steady sheet flow for various surface conditions. uniform flow, (2) constant intensity of rainfall excess (that part of a rain available for runoff), (3) rainfall For sheet flow of less than 300 feet, use Manning's duration of 24 hours, and (4) minor effect of kinematic solution (Overton and Meadows 1976) to infiltration on travel time. Rainfall depth can be compute Tt: obtained from appendix B. 1 0.007 (nL)0-8 [Eq. 3-31 Tt = (p2)p,5 s0.q Shallow concentrated flow After a maximum of 300 feet, sheet flow usually becomes shallow concentrated flow. The average velocity for this flow can be determined from figure Table 3-1.—Roughness coefficients (Manning's n) for 3-1, in which average velocity is a function of sheet flow watercourse slope and type of channel. For slopes ' less than 0.005 ft/ft, use equations given in appendix- Surface ppendixSurface description na F for figure 3-1. Tillage can affect the direction of shallow concentrated flow. Flow may not always be directly down the watershed slope if tillage runs Smooth surfaces(concrete. asphalt, gravel, or across the slope. baresoil) ................................... 0.011 After determining average velocity in figure 3-1, use Fallow:(no residue).......................... 0.05 equation 3-1 to estimate travel time for the shallow concentrated flow segment. Cultivated soils: Residue cover 00% ...................... 0.06 Residue cover >20% ...................... 0.17 Open channels 1 Grass: Short grass prairie ........................ 0.15 Open channels are assumed to begin where surveyed Dense grassesa............................ 0.24 cross section information has been obtained, where Bermudagrass............................. 0.41 channels are visible on aerial photographs, or where blue lines (indicating streams) appear on United Range(natural) ............................. 0.13 States Geological Survey (USGS) quadrangle sheets. Manning's equation or water surface profile Woods:a information can be used to estimate average flow Light underbrush.......................... 0.40 velocity. Average flow velocity is usually determined Dense underbrush ......................... 0.80 for bank-full elevation. , 'The n values are a composite of information compiled by Engman (1986). alneludes species such as weeping lovegress,bluegrass,buffalo grass,blue grama grass,and native grass mixtures. 3when selecting n.consider cover to a height of about 0.1 ft.This is the only part of the plant e(ver that will obstruct sheet flow. (210-VI-TR-55, Second Ed., June 1986) 3-3