NONRIGID, NONSUBMERGED, VEGETATIVE ROUGHNESS ON FLOODPLAINS

Individual pine and cedar tree saplings and branches were used to mode l the resistance to flow in a water flume for nonsubmerged and nonrigi d vegetation to determine the amount that streamlining decreases the d rag coefficient and reduces the momentum absorbing area. Currently, ve getation on floodplains is commonly assumed to behave as rigid roughne ss that can lead to large errors in the relationships between velocity and drag force. This presents a basic fluid mechanics problem. An ext reme variation of roughness with depth of flow can result due to a lar ge increase in the momentum absorbing area in nonsubmerged vegetation as depth is increased. This deems all the available roughness equation s (which generally are based on relative roughness approach) useless f or this application. In this paper a dimensional analysis, supported b y experimental results, is developed to obtain a relationship between roughness conditions (i.e., density and flexural rigidity) and flow co nditions (i.e., velocity and depth) for floodplains and vegetative zon es of natural waterways.