The Influence of Flood on the Bed Evolution and Habitat in Alluvial River

• Main Authors: Ching-Nuo Chen, 陳金諾
• Other Authors: Chang-Tai Tsai
• Format: Others
• Language: zh-TW
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/94150195514184580178

博士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 94 === Most sediment was transported from upstream watershed that generated from large-scale collapse and soil erosion into the river and resulted the riverbed evolution during the process of flooding. The river habitat could be seriously destroyed by the riverbed evolution and high concentration of suspended sediment. The purpose of this study is to develop the river habitat transition model applicable for the simulation of bed evolution, habitat transition, and estimation the influence of bed evolution and flow condition on river habitat during the flood. The river habitat transition model can be used for stream restoration and river habitat improvement. The river habitat transition model was developed combining the physiographic soil erosion-deposition model (PSED Model), alluvial river-movable bed-two dimensional model (ARMB-2D Model) with habitat model. GIS is applied to the physiographical soil erosion-deposition model to simulate the hydrographs of runoff and the concentration of suspended sediment. Finally, the distribution of river habitat can be estimated using the habitat model. The physiographic soil erosion-deposition model utilizes GIS, in which, the hydrological and physiographical factors are processed instantaneously but not necessarily simplified. Any changes in these factors are incorporated on a timely basis. The runoff hydrograph, suspended sediment concentration hydrograph, soil erosion and deposition in watershed, and sediment yield could be simulated by PSED model for single small river basin, large-scale watershed with many sub-watersheds of tributaries (such as the Choshui river basin), or reservoir watershed. In order to verify the PSED model, the simulation results of discharge hydrograph, suspended sediment concentration hydrograph, and sediment yield were compared to the observed data at hydrological station. The upstream boundary conditions including runoff hydrograph and suspended sediment concentration hydrograph was first obtained and then used by the alluvial river-movable bed-two dimensional model for the unsteady flow and bed evolution calculations. The PSED model is applicable to estimate the soil erosion and sediment yield occurring in a river basin and is helpful for the watershed management. The bed evolution of the Tali river and Chi-Chi weir of Choshui river basin were simulated by alluvial river-movable bed-two dimensional model. The result indicated that suspended load would be able to affect the riverbed evolution that included the location of silting and shape of river cross-section. The channel bed evolution simulated with the inclusion of suspended load from upstream watershed was better conformed to the present silting appearance in Chi-Chi weir of Choshui river basin. Therefore, the boundary condition of alluvial river-movable bed-two dimensional model with the consideration of suspended sediment concentration hydrograph obtained from physiographic soil erosion-deposition model was necessary during the period of flooding for the simulation purpose. The influence of the riverbed evolution on flow condition can be transferred to the habitat in alluvial river. The alluvial river-movable bed-two dimensional model and habitat model can be combined to calculate the combined suitability factor, weighted usable area (WUA), and percent usable area (PUA) by habitat suitability curve of depth, velocity, and substrate for the target species. The influence of flood of different return periods and typhoon events on bed evolution and habitat could be estimated by PSED model, alluvial river-movable bed-two dimensional model and river habitat model. The sediment concentration hydrograph from upstream river basin caused by precipitation can be simulated and then the riverbed evolution can be reasonably estimated. Finally, the value of combined suitability factor and distribution of the habitat during the processes of flood can be obtained which becomes helpful to stream restoration and river habitat improvement. The river habitat transition model developed in this study may be useful for estimating the river habitat during and after the processes of flood as well as ordinary flows.