浸沒式丁壩群間局部沖刷試驗研究

Abstract

丁壩工法為國內河川治理規劃常見之工程手段，使用種類繁多，包括透水與不透水、浸沒式與未浸沒式等。不同之丁壩佈置及水流條件會對丁壩之沖刷行為造成複雜之改變。前人文獻大多注重單一丁壩之壩頭局部沖刷與束縮沖刷，鮮少進行不同浸沒程度(壩高水深比)時，丁壩群間沖刷行為之比較，因丁壩通常需形成丁壩群(兩支丁壩以上)才能發揮較明顯之河川治理效果，故本研究探討不同浸沒程度對浸沒式丁壩群間局部沖刷之影響。 本研究藉由直線渠道，於非均勻粒徑之底床砂進行丁壩群沖刷試驗，設計不同壩高水深比改變丁壩群浸沒程度，測量並分析丁壩群沖刷時壩田泥砂運移機制與丁壩群局部沖刷行為。 實驗結果發現，當丁壩群浸沒程度上升後，壩田內底床因壩頭局部沖刷而形成逆坡，反而不利泥砂落淤。降低丁壩群首支丁壩高度可使壩頭前垂向水流減弱，局部沖刷深度減少，並減少下游壩田泥砂流失量。

Spur dike, including permeable or impermeable, and submerged or emerged, is a general method of river engineering in our country. However, different arrangements and flow conditions of spur dike will cause complex change of scouring behavior. Most of early studies focused on the local scour and contraction scour of the single spur dike, rarely compare the scouring mechanism of spur-dike group during different ratio of the flow depth to the dike height. Spur dikes usually need to form as group to bring obvious effect on river engineering. Therefore, this study was to investigate the effect to local scour on submerged spur-dike group of different ratio of the flow depth to the dike height.

In order to investigate the mechanism of sediment transport around the spur dike field and the local scour of spur-dike group, this study uses long straight flume under non-uniform grain size condition, and designs cases with different ratio of the flow depth to the dike height.

Experimental results show that, when the degree of submergence ratio increases, the local scour at the spur-dike head will lead to the adverse bed slope of spur-dike field incline, which is not conducive to sediment deposition. Lowering the height of the first spur-dike in the spur-dike group can weaken the vertical vortex in front of the spur-dike head, and reduce the local scour depth and the sediment loss in the dowmstream spur-dike field.