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<br />Chow (1959) presents a table glVWg minimum, normal, and maximum <br />values of n for flood plains made up of pasture and cultivated crops. <br />'Ihese values are helpful as a comparison for roughness values of flood <br />plains having similar vegetation. <br /> <br />Vegetation Density Methods <br /> <br />In cases where a flood plain is wooded, the vegetation-density <br />method can be used as an alternative to the previous method for deter- <br />mining n values for flood plains. In a wooded flood plain, where the <br />tree diameters can be measured, the vegetation density of the flood plain <br />can be determined. <br /> <br />Determining the vegetation density is an effective way of relating <br />plant height and density characteristics, as a function of depth of flow, <br />to the flow resistance of vegetation. Application of the flow-resistance <br />model presented below requires an estimate of the vegetation density as a <br />function of depth of flow. The procedure requires a direct or indirect <br />determination of vegetation density at a given depth. If the change in n <br />value through a range in depth is required, then an estimation of vegeta- <br />tion density through that range is necessary. <br /> <br />Techniques of Determining Vegetation Density <br /> <br />A method of analysis of the "vegetation density" to determine the <br />roughness coefficient for a densely vegetated flood plain was developed <br />by Petryk and Bosrnajian (1975). By summing the forces in the longitudinal <br />direction of a reach and substituting in the Manning's formula, the <br />following equation was developed. <br /> <br />(C*EA)(1.49)2 4/3 <br /> <br />1+2gAL ~ R <br />o <br /> <br />where: no = Manning's boundary roughness coefficient, excluding the <br /> <br />effect of the vegetation (a base n); <br /> <br />n = n <br />o <br /> <br />(7) <br /> <br />C* = the effective drag coefficient for the vegetation in the <br /> direction of flow; <br />g = the gravitational constant, in feet per second squared; <br />A = the cross-sectional area of flow, in square feet; <br />R = the hydraulic radius, in feet; <br /> <br />l'Ai = the total frontal area of vegetation blocking the flow in <br />the reach, in square feet; <br /> <br />and L = the length of channel reach being considered, in feet. <br /> <br />17 <br />