Effect of Encroachment on Flow Characteristics in a Compound Meandering Channel : An Experimental Study

Abstract

The behavior of flow in a compound meandering channel, consisting of a deep main channel bounded by shallow floodplains, is very complex and influenced by several kinds of forces and hydraulic parameters. Encroachment of floodplains, i.e. various human settlements and related activities modifies such overbank flow processes due to the presence of different types of interventions. Hence, in addition to the classical turbulent interaction at the interface between the main channel and floodplain, the presence of an obstacle strongly disturbs the flow dynamics of the encroached compound channels. The location of encroachment, i.e., whether encroachment occurs in the bends or straight cross-over portion of a meandering channel, also affects the velocity and shear stress distribution at and near the encroached area. Moreover, the presence of encroachment in single overbank or both overbanks is likely to have a considerable effect on the variation of these hydraulic parameters. To investigate these effects, a systematic experimental study was carried out in the physical modeling facility of the Department of Water Resources Engineering, BUET. The experiment was conducted in a compound meandering channel with five different set-ups of various encroachment conditions for depth ratio of 0.285 and 0.375. A large volume of data was collected using Acoustic Doppler Velocity-meter (ADV) from ten experimental runs. Analyses of the study showed that the distribution of flow in a compound meandering channel is affected by the encroachment condition. When encroachment occurs in bend, the floodplain flow reduces significantly compared to encroachment in crossover. The non-uniform velocity distribution near the encroached area increases the value of both the energy and momentum coefficients, specially when encroachment occurs on both floodplains of a bend. For this condition the main channel velocity was found maximum. There has been no rapid change of shear stress at main channel at any conditions but shear stress increases at the location when encroachment was placed on both side of floodplain at bend and crossover. Finally, dye tracer technique was utilized to visualize the complex flow pattern for various encroachment conditions and the observations were found consistent with the experimental measurements and analyses.