民國98年8月5至10日莫拉克颱風侵襲臺灣，在中南部造成嚴重的災害。期間曾文水庫集水區降下的總雨量達2,600毫米。強度高，延時長的降雨作用下，產生大量的崩塌，颱風過後集水區的崩塌面積達1,467公頃。崩塌的土砂流出，不僅造成水庫淤積，並且在河道上淤積，危及水利及跨河構造物的安全。成功大學防災研究中心於民國98年執行水利署南區水資源局委辦計畫期間，於莫拉克颱風後進行現地調查、斷面測量，了解及水區地形地貌的變化，並以莫拉克颱風的降雨歷線輸入數值模式，計算集水區土砂生產及沖淤量。綜合測量及數值模式計算成果，集水區上游的支流為主要的土砂來源，河道上的淤積厚度達5公尺以上。因此，往後集水區的降雨事件後，仍需持續觀察集水區土砂的沖淤變化，採取清淤防砂的措施，以確保水庫的庫容，並達到保育的目標。

Typhoon Morakot struck Taiwan from 5th to 10th August, 2009 and caused serious damage in the middle and southern part of Taiwan. Totally, more than 2, 600 mm rain fell on the watershed of Tsengwen Reservoir during that period. Under the effect of a rainfall type of high intensity with long duration, a large amount of landslides occurred. The landslide area account to 1, 467 ha totally. The sediment produced by landslide not only increases the amount of sediment deposition in the reservoir but also lift the elevation of channel bed. It is even a risk for hydraulic structures. The Disaster Prevention Research Center of National Cheng-Kung University made field investigations, survey of channel sections to realize the topographic and geographic changes after Typhoon Morakot. The work belongs to a part of a project given by Water Resource Agency Bureau South. Those results were used to calibrate a numerical model. The hydrograph was then set as the initial condition to calculate the sediment budget by using the numerical model. It suggests that the arbitraries at the upper part of the watershed are the main sediment yield sources, and the deposit depth within the channel is about to 5 m. It is then necessary to make further observation of the deposited sediment. In order to keep the storage capacity and maintain the conservation of the watershed, dredge measures are needed to be taken in the next time.