Variation of Roughness in a Non-Prismatic Converging Compound Channel

Abstract

For the management of rivers and floodplains, it is important to understand the behavior of flows within compound channels. Flooding situation in rivers is a complex phenomenon which affects the livelihood and economic condition of the region. The modeling of such flow is of primary importance for river engineers and scientists to work in this field. As a result of topographic changes along the open channels, designing the converging compound channel is important. Fluvial flows are strongly influenced by geometric complexity and large overall uncertainty on every single measurable property, such as roughness or velocity distribution on different sectional parameters like width ratio, aspect ratio and hydraulic parameter such as relative depth. Flow structure in non-prismatic compound channel is more complex than straight channels due to 3-dimentional nature of flow. The usual practice in one dimensional analysis is to select a value of n depending on the channel surface roughness and take it as uniform for the entire surface for all depths of flow. The influences of all the parameters are assumed to be lumped into a single value of n. The roughness of the main channel was determined by measuring the velocity of water flowing along the main channel. Patra (1999), Patra and Kar (2000), and Pang (1998) have shown that Manning’s n coefficient not only denotes the roughness characteristics of a channel but also the energy loss in the flow. The coefficients for Chezy’s and Darcy-Weisbach friction factors from in bank flow to over bank flow are found to be in line with the behavior of Manning’s n. The larger the value of n, the higher is the loss of energy within the flow. Although much research has been done on Mannings n, for straight channels, very little has been done concerning the roughness values for non-prismatic compound channels. In this paper, an experimental investigation of a non-prismatic compound channel having converging flood plains is investigated. Five sections with different cross-sections along the longitudinal direction of the non-prismatic channel are analysed to scrutinize the change in the value of Manning’s n along the path of the channel.