Urban flood analysis for different design storm hyetographs in Xiamen Island based on TELEMAC-2D

Abstract

Urban flood is more serious and more frequent with the rapid urbanization in recent years. Numerical simulation is one of the important means to study urban flood, which is widely used in analyzing the characteristics of urban waterlogging. In this paper, a numerical model had been established to simulate the urban waterlogging in Xiamen Island based on TELEMAC-2D software. TELEMAC-2D is one of the two-dimensional hydrodynamic models, which developed by the French National Hydraulic and Environment Laboratory. The finite element method (FEM) was used to solve the Navier- Stokes equations, and Soil Conservation Service Curve Number (SCS-CN) method was used to simulate rainfall-runoff processes. The urban flooding processes under different design storm hyetographs were simulated, with return periods of 50/100 a and rain peak coefficients of 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, and 0.8, respectively. The contrastive analyses had been done for 14 simulation scenarios, which consisted the terms of the total water accumulation, inundation area and hysteresis of inundation. The results show that the total water accumulation is greatly affected by the rain peak coefficient, and the higher the rain peak coefficient is, the greater the peak value of the total water accumulation is. The inundation area is also affected by the rain peak coefficient. Under the same return period of rainfall, the inundation area increases with the increase of rain peak coefficient. The inundation area of different depth (>15 cm, >30 cm) varies with the rain peak coefficient. For the flooded area with water depth greater than 15 cm, the area increases with the increase of rain peak coefficient, while for that greater than 30 cm, the effect of rain peak coefficient is not obvious. Under the same return period of rainfall, the higher the rain peak coefficient is, the shorter the hysteresis of inundation is. These conclusions made a preliminary revelation of the characteristic of urban flooding processes in urban area, which could provide basic support for urban flood forecast and regulation. For the lack of detailed drainage pipe network, a simplified method was used in modelling the drainage effect of pipe network, which deducted the designed rainfall of drainage pipe network from the total rainfall, and then input it to the simulation model. The simplified method is basically correct during the rain events, while there are systematic errors in the receding stage after the rainfall stops. As the rainfall is zero after rain stops, there is no way to deduct the designed rainfall of drainage pipe network, so the drainage function of the pipe network can't be reflected after rain stops. That is why the simulated receding time of the simplified method is longer than the actual result. Since most of the waterlogging simulation focuses on the occurrence and expansion of inundating during rainfall, the simplified method is widely used in European and other western countries. If the process of inundation is a focus for concern, the simplified method is not applicable, under which the suggested model needs to be improved to simulate the drainage process of the pipe-network in detail.