The effect of building arrangement on the flow characteristics in meandering compound channels

Abstract

Due to the fact that most of the cities and villages are built on the banks of the rivers, the hydraulic characteristics of water flow always undergo changes during floods, so it is necessary to study the interaction of the flood flows of meandering compound channel in these areas. In this paper, the effect of building arrangement on the flow characteristics in meandering compound channels was investigated by using laboratory models. For this purpose, a smooth floodplain (without building) and three different types of building arrangement on the floodplain including structural obstacles perpendicular and parallel to the floodplain flow and checkered structural obstacles have been used. The tests have been conducted at three relative depths of 0.29, 0.39 and 0.49 and the corresponding discharges. All experiments were performed under a quasi-uniform and fully developed flow condition. The measurement data were recorded at different sections along the meandering main channel. Stage-discharge curves, Manning's n, velocity distribution, boundary shear stress and Darcy-Weisbach friction factor are determined over 12 test runs. The results of this research show that the arrangement of buildings has a great effect on the longitudinal velocity of the main channel. At a relative depth of 0.49, in a meandering compound channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the dimensionless longitudinal velocity of the main channel compared to the smooth floodplain increases by 113%, but with the arrangement of structural obstacles parallel to the floodplain flow, the dimensionless longitudinal velocity increases by 27%. Also, the building arrangement is effective in reducing the discharge capacity of the meandering compound channel, so that in a building density of 18.5%, structural obstacles perpendicular to the floodplain flow reduce the discharge capacity of the channel by 50% compared to the smooth floodplain. In the channel with structural obstacles arranged perpendicular to the floodplain flow, Manning's roughness coefficient increases sharply with increasing the relative depth, but in the channel with the structural obstacles parallel to the floodplain flow, Manning's roughness coefficient decreases with increasing the relative depth. In the channel with the arrangement of structural obstacles perpendicular to the floodplain flow, the bed shear stress of the main channel is higher than the other building arrangements, so that at the relative depth of 0.49, the average maximum bed shear stress in all sections increases by 42% compared to the smooth floodplain.