Priorities of dredging sites for the river sections of debris disasters caused by Typhoon Morakot in the Chishan streams

Taiwan is located on the Eurasian plate coupled with the fragile geological conditions and the phenomenon of earthquake occurred frequently. The abundant but uneven distribution of annual rainfall is concentrating in the rainy and typhoon season from May to September, which is easily to cause the disasters of floods, landslides and debris flows, and show a great threat to the human lives and properties. Typhoon Morakot hit southern mountains seriously in Taiwan caused several collapses and heavy sedimentation in the watersheds. It is necessary to set the priorities of dredging, promote the dredging efficiency and control the flow chart of dredging in the channels due to a wide range of debris deposition. This study focused on the assessment of sediment yields for the river sections of main channel and/or tributaries located at the watershed of Chishan streams, which suffered severe damages during the hits of typhoon Morakot. The results show that the amounts of landslides is about 7.624 x107 m3 in the watershed, and the outputs of total sediment yields is some 1.262 x107 m3 from the outlet, there is still 6.362 x107m3 remains in the watershed. Seventeen control points are placed at the junctions of main stream and tributaries to analyze the sediment yields of each channel section.While the section of A11-12 and A14-A15 depicts tremendous deposition because of several landslides resulted in sedimentation. The section of A14-A15 is screened as the first priority of dredging because of the amount of sediment sedimentation and the protected targets along the river banks. The treatments and quantities of debris deposition can be effectively controlled and the dredging efficiency can then be further promoted through the establishment of mechanism for the dredging management. This study can be applied as the references of dredging related authorities.台灣位處歐亞板塊與菲律賓海板塊之交界，地質條件脆弱，加上地震及高強度、長延時降雨頻繁，且雨量多集中於五月至九月的梅雨與颱風季節，易造成洪水、地滑、土石流等災害，對民眾生命財產威脅極大。八八風災重創南部山區，集水區多處崩塌，使河川水系堆積土砂料源範圍廣大且河道淤積嚴重，為有效選出河道優先清疏區位之順序，及提升河道清疏效率、砂石流向管控，本研究針對莫拉克颱風受創嚴重之旗山溪集水區為樣區，推估風災後旗山溪主、支流各河段之泥砂產量，考量各河段未輸出土砂量、兩岸保全對象之安危作為河段清疏區位優選之參考。結果顯示莫拉克風災旗山溪集水區之崩塌量約7,624 萬立方公尺，河道總輸出土砂量1,262 萬立方公尺，未輸出土砂量約為6,362 萬立方公尺。針對旗山溪主流處標示17 處控制點，進行控制點間河段之土砂量分析，各控制點以A11-A12 及A14-A15 河段堆積嚴重。考量各河段未輸出土砂量及兩岸保全對象之安危，以A14-A15 河段為最優先清疏區位；經由制定清疏之管控機制，有效的管控砂石流向及數量，進而提升清疏效率，研究結果可提供相關單位清疏之參考