Method to estimate effects of flow-induced vegetation changes on channel conveyances of streams in central Arizona

Abstract

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