利用動態水壓分佈評估岩石沖刷坑形狀與深度

Abstract

當水流沖刷外力大於地質材料的抵抗能力時，沖刷便會產生，水利建造物的下游沖刷，可能造成基礎的損壞，進而造成結構的不穩定。因此，如何預測水利建造物的下游沖刷，一直是水利工程在規劃設計時的重要議題。傳統上對沖刷問題著重於砂、礫石等無凝聚性材料，利用室內試驗或現場調查資料而發展出其經驗公式，在這些方法中，顆粒尺寸大小是地質材料的主要評估因子。而岩盤沖刷特性相較則複雜許多，但因沖刷機制及評估方法尚未完全成熟，仍須深入進行探討研究。為評估水庫溢洪道下方之沖刷及防治，近年來已有部分研究涉及岩盤沖刷之課題，惟因岩石特性差異，主要仍以岩塊抽離為評估方法。且目前評估方式僅著眼於沖刷深度，而完整瞭解沖刷範圍及形狀有助於評估消能設施是否合宜，提高水利建造物整體安全性。 本研究結合地質評分法(Annandale, 2006),及縱向動態水壓分佈(Castillo et al, 2014)觀念，蒐集包括加拿大的Revelstoke Dam、Peace Canyon Dam、W.A.C. Bennett Dam及國內義興壩與曾文水庫等5個實際案例進行驗證，流況則包含有自由跌水及Ski jump 2種型式。並依據實際流況對以自由跌水流況所得現代室內試驗資料進行修正，使其可應用各種流況條件。同時考量在入射角的影響，在流功作用面積及消能水深進行修正。驗證案例，分析結果顯示，自由跌水流況下與實際變化相當吻合，但在溢洪道條件則有低估其擴散程度的情形。但相較目前沖刷坑之評估方法，本研究成果已有相當程度的改善。

Scour would occur when the erosive capacity of water exceeds the erodability resistance of geo-materials. The scour on the doundation of a hydraulic structure could cause foundation damage and endanger the structural stability. To determine the local scour holes downstream of a hydraulic structure plays an important role in the planning and design stages. In general, geo-materials include granular soil, cohesive soil and rock. Most of current researches deal with granular materials using laboratory or field data to develop their empirical or semi-empirical formulae. In those researches, the grain size is the sole material parameter characterizing the erodibility property of geo-materials. The erosion behavior of rock materials is much more complex than that of granular materials. Therefore, it deserves more study to understand and to determine the local scour holes on rocks. Several approaches were developed for the plucking mechanism in order to estimate the ultimate depth of scour holes in rocks. However, not many approaches have dealed with the range and shape of scour holes. This study proposes an approach for estimating the shape and the depth of the scour hole on weak rocks. This approach integrates the concepts for the stream power of water jet (Annandale, 2006), the dissipation energy and the longitude pressures distribution (Castillo et al, 2014). A new method with applicable procedures for estimating the shape and depth of the scour hole was developed. This study compared the simulated data and in-situ data surveyed at the stilling basin downstream the spillway of several dams to examine the applicability of the proposed approach. The comparison reveals that the proposed approach models reasonablly well for the freefall condition, but may underestimate the extent and shape of sour holes for the ski-jump condition. Nevertheless, it appears the new method is able to improve the estimation of scourhole depth, shpae and extention, comparatively to other existing methods.