Need of Two-Dimensional Consideration for Modelling Urban Drainage

Abstract

The computation of unsteady free surface flow is essential in urban drainage systems to estimate values of various flow parameters like depth and velocity of flow in the different parts of a drain at different times of flow. Generally, 1-D governing equations of unsteady free surface flow (Saint-Venant equations) are solved for simulating storm water flow in drains. This gives acceptable result for turbulent flow in a channel of small width. However, for relatively larger channel width, flow parameters may vary significantly within a channel cross section and therefore, for proper analysis, 2-D simulation becomes essential. Need of using 2-D simulation for analyzing flow characteristic in a typical city drain is studied and presented in this paper. Comparison of 1-D and 2-D results has revealed that though both the models give same surface elevations along the drain at different times, flow velocity varies significantly across the channel in 2-D simulation. 2-D result shows higher velocity at middle of the channel and lower flow velocity near the bank, whereas, because of inherent limitation, the variation of flow velocity across the channel cross section never gets reflected in the result computed by 1-D analysis. Thus, the fact that a channel having low velocity near the bank may experience sediment deposition, might go unnoticed if it is analyzed by a 1-D model.