Two-Dimensional Numerical Model for Urban Drainage System

Abstract

Modeling of urban drainage system is carried out for understanding and predicting the flow behavior in the drain, so that an adequate design of the drainage system can be achieved. However, many a time urban drainage models are developed without considering some important aspects like erosion and deposition processes in the drain. Though the erosion process can be neglected in the lined canal, deposition process has a major role in the urban drainage system. Therefore, study of sediment deposition in the urban drainage system is an essential aspect for minimization of artificial flood in the urban area. Sediment deposition in the drain depends upon various factors like particle size, settling velocity, critical shear stress, and bed shear stress. Therefore, in this paper an attempt has been made to develop a numerical model, which can identify the critical section of the urban drain where sediment starts to deposit. Two-dimensional continuity and momentum equations of unsteady free surface flow are solved by second order finite difference implicit scheme. To find the settling velocity of the particle Newton’s law and Stock’s law are used. After calculating the settling velocity of the particle, the position where the particle starts to settle down is determined. Critical sections of the drain from sediment deposition point of view are identified by comparing bed shear stress and critical shear stress computed by the Shields’ equation for incipient motion. No deposition occurs if bed shear stress is greater than critical shear stress. Again, result obtained from the sensitivity analysis of average particles size has shown that with the increases of sediment size, problem of sediment deposition aggravates.