固床工下游拋石保護區間之水力特性研究

Hydraulic Characteristics in the Vicinity of Riprap Protection at the Downstream of Groundsill

吳雅筠(Ya-Yun Wu)

指導教授 ： 吳嘉俊

國立屏東科技大學/工學院/水土保持系所/碩士(2016年)

摘要

造成固床工趾部局部淘刷的主要原因為射流之投潭作用，過去曾有學者研究固床工趾部保護工末端的局部淘刷問題，並提出相對紊流強度與局部淘刷係數的觀念，但卻鮮少見到保護工的長度與糙度對相對紊流強度與局部淘刷係數影響的研究。因此，本研究之目的，為探討固床工下游配置拋石保護段後所產生的相對紊流強度、局部淘刷係數與水力特性。研究方法利用1：50縮尺之水工模型試驗，以量測當拋石保護段下游淘刷坑達到動態平衡階段時的淘刷坑特徵參數、水力相關參數及流速剖面。
研究結果發現，Breusers相對紊流強度、Hoffmans-Booij相對紊流強度及局部淘刷係數與拋石保護段末端的單寬淘刷體積相關性不佳。相同拋石粒徑下，短拋石保護段所產生的單寬淘刷體積顯著大於長拋石保護段，因此，本研究建議拋石保護段的長度應至少超過固床工間距的1/4。本研究亦提出不同條件之拋石保護段末端的曼寧糙度建議值；其中，當渠床坡降為0.030、拋石保護段長度與固床工間距比為1/2、拋石粒徑與床砂平均粒徑比為14.1時，其所對應的曼寧糙度範圍為0.025 ~ 0.028；當渠床坡降為0.085、拋石保護段長度與固床工間距比為1/2、拋石粒徑與床砂平均粒徑比為33.9時，其所對應的曼寧糙度範圍為0.071 ~ 0.085。

關鍵字

拋石保護 ； 相對紊流強度 ； 局部淘刷係數 ； 固床工 ； 曼寧糙度

並列摘要

Local scour at the toe of a hydraulic structure is mainly caused by the plunging action of the turbulent jet. The concept of relative turbulence intensity and local scour coefficient that related to local scour at the interface of channel bed protection and channel bed has been documented in the past. Nevertheless, limited researches have been carried to investigate the effect of length and roughness of bed protection on related turbulence intensity and local scour coefficient. Therefore, the objective of this study is to experimentally explore relative turbulence intensity, local scour coefficient, and hydraulic characteristics in the vicinity of riprap boundary at the downstream of groundsill. An array of 1:50 scaled groundsill models was installed in a semi-circulation flume. Relevant parameters that characterized the geometry of scour as well as velocity profiles at selected sections were measured after scour reached equilibrium.
Results from this study indicate that the scour volume per unit channel width at the tail end of riprap protection is poorly correlated with Breusers relative turbulence intensity, Hoffmans-Booij relative turbulence intensity, and scour coefficient. Under the same riprap size, the scour volume per unit channel width at the tail end of short riprap length protection is much significant than that under long riprap length protection. Hence this study suggests that the length of riprap length should excess 1/4 of the groundsill interval. In addition, this study provides Manning roughness coefficient at the tail end of riprap protection. When riprap length is one-half the groundsill interval and riprap size is 14.1 times the channel bed material, Manning roughness coefficient at the tail end of riprap protection recommended value is 0.025 ~ 0.028. Under the same riprap length to groundsill interval ratio but higher riprap size to channel bed material, say 33.9, Manning roughness coefficient at the tail end of riprap protection recommended value is 0.071 ~ 0.085.

並列關鍵字

Riprap bed protection ； Relative turbulence intensity ； Local scour coefficient ； Groundsill ； Manning roughness coefficient

參考文獻

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