石門水庫上游白石溪集水區蝕溝產砂量推估之研究

Abstract

蝕溝係土壤沖蝕作用初期在坡面上切割所形成之深溝，且為坡面土壤強烈沖蝕作用之表徵與地形夷平過程之濫觴。蝕溝形成後即會因下切、橫向與溯源沖蝕而擴張，造成大量的地表沖刷。本研究使用全站儀，進行石門水庫集水區內白石溪流域內四條蝕溝變化地形測量，並結合航空攝影圖估算研究區域內的蝕溝數量及其規模，再分析白石溪流域蝕溝沖蝕產砂的總量。蝕溝測量的結果顯示蝕溝產砂狀態依照其發育程度的不同，可分為產砂型蝕溝以及穩定型蝕溝，產砂型蝕溝發育期較為年輕，其平均單位面積產砂量為0.2758m^3/m^2；穩定型蝕溝發展成熟，產砂量較少，平均單位面積產砂量為0.0108m^3/m^2。由航照圖判釋集水區共120條蝕溝，其中，面型蝕溝41條與線型蝕溝79條。因此，2006年7月至12月，集水區蝕溝產砂總量推估為27,744m^3。假設產砂量與研究期間內累積降雨量為一線性正比例關係，則集水區內蝕溝沖蝕產砂量為每年176,280噸，相較於崩塌地產砂量每年1,023,480噸，以及土壤沖蝕量每年30,960噸，蝕溝產砂量佔集水區總產砂比14%，為一不可忽略之產砂來源。 Gully is a landform with narrow channel cutting by runoff on hillslope. Gully landform indicates rapid soil erosion and initiation of denudation process on hillsope. After gullies are formed, a large quantity of sediment is produced by the incision, lateral erosion and head-cutting. This study aims to estimate the sediment yield due to gully erosion in the Baishi watershed of Shihmen Reservoir. We measured the morphology change of gullies by using Pulse Laser Total Stations, and interpreted the gullies from aerial photographs. Then, sediment yield from gully erosion was estimated in the watershed.The results show that gullies can be categorized into two types, active gully and stable gully, based on its sediment yield. Active gully, which is in the initial stage of gully evolution, has an average sediment yield of 0.2758 m^3/m^2. In contrast, stable gully, which is highly-developed, has with a smaller sediment yield of 0.0108 m^3/m^2 relatively. There are 120 gullies interpreted from aerial photographs, which 41 are polygon shapes and 79 are line shapes. The total sediment yield from gully erosion in the Baishi Watershed is 27,744 m^3 during July to December in 2006. The annual sediment yield from gully erosion is 176,280 t yr^(-1) based on the assumption of linear ratio of sediment yield with accumulated rainfall. The gully erosion contributes 14% of annual sediment in the watershed, comparing with the landslide erosion of 1,023,480 t yr^-1 and soil erosion of 30,960 t yr^(-1). Gully erosion is a significant source of sediment yield in the watershed.