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<br />Method to Estimate Effects of Flow-Induced <br />Vegetation Changes on Channel Conveyances <br />of Streams in Central Arizona <br /> <br />By Jeff V. Phillips, Dawn McDaniel, Joseph P. Capesius, andWilliam H. Asquith <br /> <br />Abstract <br /> <br />Proper estimation of Manning's roughness coefficient, n, in open channels is necessary to <br />reliably estimate channel conveyance--an important element of an open-channel hydraulic study. <br />Proper estimation of n values, however, can be difficult in the arid to semiarid southwestern <br />United States because floods may dramatically alter the roughness characteristics of the channel <br />by (1) flattening or laying over vegetation, which acts to increase conveyance; and (2) removing <br />vegetation in response to degradation of the substrate, which also acts to increase conveyance. <br />Data collected during this investigation were used to develop a semiempirical relation to assist in <br />n-value estimation for sites where flood induced changes in vegetation are considerable. <br /> <br />To investigate the potential for the flattening or laying over of vegetation in response to a <br />flood, a site-specific vegetation-susceptibility index was developed. This index is a function of the <br />type, density, and distribution of vegetation as well as the relation between depth of flow and <br />vegetation height. In this study, it was determined that the flexural stiffness of vegetation is the <br />primary control of the potential for the flattening or laying over of vegetation. The degree to <br />which vegetation is affected by flow can be evaluated using the relation between stream power, <br />which is a measure of energy transfer, and the vegetation-susceptibility index. To investigate the <br />potential for vegetation removal, evaluation of substrate degradation and exposure and weakening <br />of vegetation root systems is required. The potential for substrate degradation is related to <br />boundary shear stress. <br /> <br />Channel-conveyance calculations made for preflow- and postflow-channel conditions indicate <br />that incorrect assessment of vegetation conditions during the peak flow can result in water-surface <br />elevation differences of as much as 2.63 feet when considerable changes in vegetation were <br />observed. This scenario can lead to erroneous delineation of flood-prone areas and structure <br />capacities. An example case is presented that illustrates the application of the relation that was <br />developed in this study to assess the effects of flow-induced vegetation changes on computed <br />channel conveyances and water-surface elevations of streams in central Arizona. <br /> <br />Abstract 1 <br />