丁壩局部沖刷與掛淤保護機制之研究

Abstract

丁壩為河川常見之橫向水工建造物。藉由丁壩傳遞適宜之人為擾動，可改變水流沖刷勢能在河床上之分布，進而達到保護岸壁邊界之目的。不過目前相關丁壩研究，仍多著墨於單座丁壩壩頭之局部沖刷及束縮沖刷，鮮少從丁壩群角度切入，探討河道主流泥砂進入丁壩壩田落淤之運移路徑、及分析不同水流條件下丁壩壩田泥砂流失機制等課題。因此，本研究以臺灣濁水溪流域為研究對象，透過現場調查取得實際丁壩局部深度變化、透過數值模式分析現場丁壩壩田可能之掛淤深度、透過實驗水槽分析丁壩壩田泥砂運移路徑等，隨後整合相關分析內容，釐清現場丁壩壩頭局部沖刷與壩田泥砂掛淤機制之運行規則。根據研究成果，相同入流條件下，丁壩在未浸沒時保持壩田完整之能力優於浸沒時；而浸沒時各壩田泥砂之流失情形，可透過首座丁壩壩頭局部沖刷深度、往下游距離及壩高-水深比等參數，加以估算。此外，利用現場觀測資料提出局部沖刷公式，用以預測研究範圍內丁壩局部沖刷變化，並提出相關丁壩設計改善建議供參。

Spur dikes are hydraulic structures that extend from a river bank into the river channel, and in on-site applications, they are present in groups. In river engineering, spur dikes have diverse functions. When applied to erodible banks, spur dikes can deflect near-bank currents to the river center. In addition, the recirculation zones behind spur dikes have a slow flow velocity, which increases sedimentation at the riverbank footing and reduces riverbank erosion. Although spur dikes serve multiple river management functions, their inherent structure hampers water flow from upstream to downstream. Thus, when water flows approach a group of spur dikes, the potential energy of the flows is increased to overcome the resistance engendered by the spur dikes. This increase in potential energy indicates that water flows passing through spur dikes demonstrate increasing turbulence intensity of flow, resulting in complex bed change near spur dikes. The local scour issue of a single spur dike has been studied by many researches, but fewer studies analyzed local scour of spur dikes. In addition, less study discussed the relationship between local scour mechanism and sediment exchange around spur dike field. Based on the field investigation, numerical simulation and flume experiment, this study explained the mechanism of spur dike scour and sediment loss in the spur dike fields. In the submerged case, the sediment loss in the spur dike fields caused by upward vortex and the scour zone development of first spur dike. The thickness of sediment loss in the spur dike fields can be calculated by local scour depth of first spur dike, the distances from upstream and overtopping rate. In the emerged case, the sediment loss in spur dike field is less than the submerged case. Moreover, an empirical formula was developed to estimate the maximum scour depth around the on-site spur dike group.