Investigation of Watershed Sediment Estimation Before and After 921 Chi-Chi Earthquake -Peikang Stream a Tributary of Wuchi River as an Example

Abstract

There caused a large landslide after the CHI-CHI earthquake in Wu-Chi watershed, the central area of Taiwan, at 1999 year. After that the sediment yield has been increasing obviously for being washed off by the peak flow caused by Typhoon and Storm. How to estimate the quantity of sediment yield reasonably is becoming to be a very important work for watershed management. In this study we selected the Peikang creek watershed as a sample for researching. We estimated the sediment yield by using the Universal Soil Loss Equation(USLE), Watershed Information System WinGrid and Soil Erosion Index Model(SEIM) to estimate the sediment yield of the watershed and then to compare with the observed data. The sediment yield was defined to equal “slopeland Soil Erosion” and “sediment transport of stream bed in mountain area”. After we compared the result with the observed suspended yield at the hydrological station of the downstream, we found it that the transported sediment from landslide is about 76% of the total estimated quantity, and to be the main quantity, and it will take 33 years to transport it up. On the other hand, to compare the estimated yield with the suspensing yield in the bower stream hydrologic station, it was a reasonable approach. It can be used as important reference for a watershed manager to make program for controlling and preventing flood disaster.

1999年9月21日之集集大地震引起台灣中部地區上游集水區大規模崩塌，其後經過颱風豪雨之沖洗，其泥砂生產量明顯增加，如何合理評估其泥砂之生產量對集水區之經營管理相當重要，此亦為集水區治理或河道治理工作者極需掌握的工作重點。 本研究旨在利用『通用土壤流失公式（USLE）』、『集水區資訊系統 （WinGrid）』及『土壤沖蝕指標模式（SEIM）』等三種泥砂推估模式，推估集水區泥沙之產量，並將推估之結果與實測值進行探討與比較。經選定烏溪流域內面積最大，於桃芝颱風時崩塌最嚴重的北港溪集水區為研究範圍，就九二一地震前後對該集水區之泥砂生產評量採用『坡地土壤沖蝕量』及『山區河道土砂輸送量』兩項相加。結果顯示，崩塌土方輸移泥砂量佔總推估量之76 ﹪，為集水區泥砂主要來源，子集水區崩塌地泥砂需約33年始可輸移完畢。另外，再将推估之結果與下游水文流量站之懸浮載輸砂量實測值進行比較，結果推估量與實測值相當。應可作為地震前後北港溪集水區出口泥砂生產量估算作業之參考及擬訂集水區水土防災治理計畫時之空間決策的重要參考資料。