FLOOD03686 - Laserfiche WebLink

Home Browse Search

so that flow information needed for highway design might be computed. The guide presents step-by-step procedures that enable determination of Manning's n values for natural channels and flood plains. This guide builds on the report by Aldridge and Garrett (1973), who attempted to systematize the selection of roughness coefficients for Arizona streams. An attempt is made to broaden the scope of that work, in particular to describe procedures for the selection of roughness coefficients in densely vegetated flood plains. The n values for channels are determined by evaluating the effects of certain roughness factors in the channels. 'IWo methods are presented to determine the roughness coefficients of flood plains. One method, similar to that for channel roughness, involves the evaluation of the effects of certain roughness factors in the flood plain. The other method involves the evaluation of the vegetation density of the flood plain to determine the n value. This second method is particularly suited to handle roughness for densely wooded flood plains. There is a tendency to regard the selection of roughness coefficients as either an arbitrary or an intuitive process. This design guide presents specific procedures to determine the values for roughness coefficients in channels and flood plains. Photographs of flood plains with known roughness coefficients are also presented for comparison. APPROACH Values of the roughness coefficient, n, may be assigned for condi- tions that exist at the time of a specific flow event, for average conditions over a range in stage, or for anticipated conditions at the time of a future event. The procedures described in this report are limited to the selection of roughness coefficients for application to one-aimensional, open-channel flow. Further, in most instances, the values are intended for use in the energy equation as applied to one-dimensional, open-channel flo~-such as in a slope-area or step-backwater procedure for determining flow. The roughness coefficients apply to a longitudinal reach of channel and (or) flood plain. The cross section within the reach may be of regular geometric shape (such as triangular, trapezoidal, or semicircular) or of an irregular shape typical of many natural channels. The flow may be confined to one or more channels; and, especially during floods, the flow may occur both in the channel and in the flood plain. Such cross sections may be termed compound channels, consisting of channel and flood plain subsections. Cross sections are typically divided into subsections at points where major roughness or geometric changes occur. Fbr example, such changes may be at the juncture of dense woods and a pasture or a flood plain and main channel. However, subsections should reflect representative conditions in the reach rather than only at the cross section. Roughness coefficients are determined for each subsection, and the procedures described herein apply to the selection of roughness coefficients for each subsection. 2