巴陵壩潰壩後對上游河床變遷影響之研究

Abstract

台灣山多坡陡、河川短促、地質脆弱且降雨強度大，往往一場颱風即可使集水區產生崩塌及河道輸砂量遽增，嚴重的淤砂對水庫壽命造成威脅，以桃園縣石門水庫為例，為了阻擋水庫上游的砂石進入石門水庫，水利署在石門水庫的上游集水區蓋了122座攔砂壩，巴陵壩是一座高38公尺的攔砂壩，位於大漢溪上距離石門水庫約三十五公里，設計淤積量為1047萬立方公尺，由於巴陵壩基礎早已遭沖刷掏空且溢洪道鋼筋亦有裸露狀況，經過韋帕颱風的侵襲，巴陵壩體結構禁不起洪水一再的侵襲，致使巴陵壩於民國九十六年九月十八日潰壩，部分上游攔蓄的砂石被帶往下游，上游因此產生嚴重掏刷。 為了瞭解巴陵壩潰後上游河床的變化，本研究係利用擬似二維輸砂演算模式(NETSTARS)，進行動床輸砂模擬及演算，此外，亦利用擴散方程式之理論，推導出一適用於本研究案例的解析解，利用此解析的數學模式來模擬並解釋河床之變動，觀察其向源侵蝕的行為及趨勢；且利用選定的穩定指標將模式輸出值先行判斷為穩定後再做討論，最後發現，巴陵壩潰壩後至民國98年底，河床穩定雖增加，但推移載持續在移動，整個河段呈現動態平衡，除此之外，利用觀察侵蝕點的移動行為，可推論出向源侵蝕現象是呈現冪次分佈且向上游移動的行為。

Balin Dam break cause the sediment goes down, and the upstream channel severe erosion. This is a big concern about head-cutting behavior after Balin dam break. Balin Dam is a 38 m-height dam, which is one of the dams on the upstream reaches of Shihmen reservoir. Balin dam broken because of the ruin of dam foundation and damage by flood events. In this paper, we will focus on simulating upstream channel evolution by using a quasi-two-dimensional model (NETSTARS) and analytical solution to compare with field observation, further to explain the river-bed evolution after Balin dam break. Owing to the transition of flow situation and severe landform variation, the output of model will be shaken. In case the simulation is more meaningful, choosing a stability index not only to evaluate the output of model, but also to estimate the river bed stability. In addition, Diffusion equation is developed as an analytical solution of riverbed evolution model so as to explain here the river bed evolution hourly after Balin dam break. Furthermore, to evaluate the head-cutting behavior by observing the motion of knick-point (or erosion threshold position) and finally find out head-cutting behavior is a power-rule distribution.