攔河堰下游軟弱岩石沖刷坑形狀與深度分析

Abstract

西部麓山帶為台灣主要發展區域，在民生需求下興建攔河堰，但攔河堰的興建使河道坡度改變，上游的礫石無法穿越攔河堰至下游形成護甲層，使得下游岩盤出露，岩盤直接暴露在水流的衝擊下，迅速的被沖刷。大部分的攔河堰並沒有施做深基礎，在沖刷坑持續發展的情況下，容易造成堰體基礎的淘刷，進而造成堰體的不穩定；另外還會造成下游河道的深槽化，並危及下游橋樑及取水工，影響下游人民生命與財產的安全。 本研究藉由了解軟岩在承受衝擊後的發展，掌握攔河堰下游軟岩的沖刷機制及建立評估沖刷坑深度的方式，以供現有攔河堰估算最大沖刷坑深度、形狀與未來設計攔河堰時參考之用。 本論文旨在將Annandale (2006)所提出計算水面流功的方式，再加上Bollaert(2003)所提出計算尾水消能的方式，以及Melo(2006)所提出的動態水壓衰減曲線，加以整合之後，提出評估攔河堰下游軟弱岩石沖刷坑形狀與深度的分析方法，並以實際案例進行試算，結果顯示本方法可有效計算出已知流量下的沖刷坑形狀及極限沖刷坑深度；接著由Kh與ke之間的轉換結果帶入Liu(2005)所提出的公式中計算沖刷坑深度，並與上述三位學者整合之方式所計算出的沖刷坑深度結果進行比較，比較結果顯示兩者的差異不大，皆可用以估計極限沖刷坑深度。

The western foothill is the mainly development area in Taiwan. Weirs were often built across rivers to elevate river water level for providing public water supply in Taiwan. Due to low rock strength and the head difference caused by the weirs, rapid scour downstream of weirs is often observed. There is no deep foundation in most weirs, so this type of scour may induce weir instability and accelerate river-bed scour. Moreover, channel would become trench by this type of scour.

This research aims to propose an approach for estimating the shape and depth of the scour hole downstream weirs built on weak rocks.

Intergarting the formula for calculating the stream power of water jet (Annandale, 2006), for estimating the dissipation energy in tailwater (Bollaert, 2003), and for determining the dissipation hydrodynamic pressures (Melo, 2006), the new method for estimating the shape and depth of the scour hole is presented. Then, the ultimate scour hole and shape for several real cases is calculated using the proposed method. Since the constant ke depending on rock properties in the formulation for determining the scour depth proposed by Liu(2005) is not easy to determine from rock putcrop, the erodibility index(Kh) is proposed to replace it. The relationship of Kh and ke is proposed based on the regression of the data of Liu’s cases. Using the modified Liu formulation, the ultimate scour hole is also calculated. The calculated scour depths from both methods are similar.