軟岩渠槽沖蝕試驗及模擬研究

Abstract

台灣西部河川多流經西部麓山帶地質區，而此地質區多屬於極年輕之地層，然而台灣河川坡陡流急，每當豪雨挾帶充沛雨量時，部份西部軟弱岩床河段遭受劇烈沖蝕，長時間累積下的沖蝕量可是相當可觀。影響河床軟岩沖蝕的機制眾多，本文主要以水力沖蝕機制進行研究，藉由不同強度之人造軟岩試體進行水槽清水試驗，進而探討軟岩強度對於沖蝕速率及相關沖蝕因子之關係。 本研究使用鍾佩衡(2014)所建置之軟岩渠槽進行清水試驗，人造軟岩試體單壓強度定為7 MPa及11 MPa，流速設在2.5~4.47 m/s區間進行試驗，試驗結果與鍾佩衡(2014)使用試體單壓強度為0.9 MPa及5.5 MPa所量測之數據一同探討，估算各人造軟岩強度受水流的沖蝕速率及相關因子，並利用Greimann and Vandeburg(2008)所提出之經驗公式進行迴歸，可得到不同軟岩強度與水力沖蝕係數K_p及經驗指數α之關係式，發現其具有良好之關係性。 將水槽試驗所得之軟岩沖蝕經驗式植入RESED2D數值模式中應用，試以台灣西部大安溪現地資料進行模擬，並與水力沖蝕公式(USBR)及廖仲達(2013)以流功門檻概念之公式模擬結果比較，本研究以臨界剪應力門檻沖蝕機制模擬結果與流功門檻趨勢相近。

In the western Taiwan, most rivers pass through the western stratigraphic region which separates the Central Mountain Range from coastal plain. Because the sandstone (i.e., soft rock) in the western stratigraphic region is highly erodible, huge amount of sediment is yielded by the river flow during a rainstorm event. This study used flume experiment to investigate the effects of flow velocity and soft rock unconfined strength on the erosion rate. The erosion rate under various combinations of flow velocity and unconfined strength was measured. The flow velocity is varied between 2.5 ~ 4.7 m/s and the unconfined strength of soft rock is controlled at 7 and 11 MPa. The inflow is clear water. The data measured in this study and in Chung (2014) was combined to determine the values of erosion coefficients in the regression equation proposed by Greimann and Vandeburg (2008). The regression equation derived from experiment data was subsequently embedded into a 2D mobile bed model (RESED2D) to simulate the soft rock erosion in Daan River. The simulation results were compared with USBR and Liao (2013) which used the concept of stream power to calculate erosion rate. The comparison indicates that the difference in simulation results between critical shear stress and stream power approaches is not significant.