攔河堰對河道沖淤影響之研究－以集集攔河堰為例

Abstract

為瞭解集集攔河堰建造後對濁水溪河道沖淤的影響，並研擬可行的防治沖刷工法，本研究蒐集濁水溪歷年大斷面資料及現場觀測資料，作一系統性分析，包括河道流路、河床坡降、河道橫斷面及河床質變遷等。研究結果顯示，濁水溪受集集攔河堰軸向及施工過程因素對河道流路及河道橫斷面擺盪產生影響，受攔河堰坡降變化及阻水效應因素影響，使得河道產生一般沖刷、局部沖刷、束縮沖刷、橫向沖刷、向源沖刷等，造成下游河道束縮下降，河床覆蓋層砂石流失，進而形成河道深槽化，甚至危及集集攔河堰與富州堤防安全，因此需要研擬一連串河道保護措施，以確保河防安全。 由水流特性得知，當水流由集集攔河堰高速流下時，由於消能效率僅達0.42～0.56，因此高流速的水流將以直線方向流向左岸深水槽，又因地質、地形因素產生束縮沖刷與局部沖刷，造成深水槽峽谷化。本文研議以跌水工方式解決或改善河道峽谷化現象，利用不同堰壩高度H d及不同跌水數D 等試驗條件進行堰壩投潭渠槽試驗，並於渠底埋設壓力量測系統，以不干擾流場下量測自由跌水下游之壓力水頭分佈，求得水流沖擊位置Lp與堰高Hd、跌水數D之經驗式，且獲致單寬垂向沖擊力試驗值與理論式相關性良好，可運用於集集攔河堰下游河道跌水工實地投潭現況。 本研究建議在NO.114 及NO.115 斷面處各佈置1 座跌水工，並利用經驗公式求取，投潭水沖擊位置Lp(=Hd×1.821D0.173)，以及單寬沖擊力F''e(=1/2ρgH''pd B''n，其中ρ為水密度、g為重力加速度、H''pd為最大沖擊壓力水頭推估值、B''n為水舌厚度推估值)，作為設置跌水工參考之用。另於跌水工下游端以逆坡道方式佈置，以符合局部沖刷形態，同時深槽渠道上填補大塊石，以抵抗水流拖曳力及沖擊力，讓河道自然回復穩定坡度，確保集集攔河堰及富州堤防安全。

This paper aims to investigate the influence of the Chi-Chi division weir on riverbed evolution of the Chou-Shui River. To develop feasible protection works, the information of historical cross-section and field observation is collected to analyze the changes on flowing route, channel slope and riverbed materials. The results reveal the riverbed significantly affected by the Chi-Chi weirs. The sands and stones in the upper-zone are flushed away, and the riverbed has a great change of slope and becomes much deeper. In addition, the riverbed scouring marching upstream may endanger the safety of the division weir. Thus, a simulation of the scouring phenomenon was performed to examine the scouring situation with the existing formulas and the laboratory experiments. Based on the analysis, a series of riverbed protection works were also proposed to assure the weir safety. The experimental results indicate that the energy dissipation efficiency of the weir is between 0.42 and 0.56. Therefore, high-speed streams flow directly toward the deep channel near the left bank downstream of the Chi-Chi weir. Moreover, a series of free overfall drop tests for different drop heights H d and Drop Number D were experimented. The pressure transducers without flow disturbance were installed under the channel bed to measure the pressure distributions downstream of the drop. According to the test results, the relationship among the flow impact position LP , weir height Hd and the Drop Number D can be expressed as LP / Hd =1 82 D0.17. These results correlate highly between the experimental figures and theoretical formula, and was then applied to design the in-situ overfall drop works of downstream course of Chi-Chi weir. In practical, it is propose to set up the drop construction in sections No.114 and N0.115 of Chou-Shui River. Through the empirical formula, the impact position L p(= H d ×1.821D0.173 ) and the unit width of impact force F''e(=1/2ρgH''pd B''n , where ρ=denisity of fluid, g=gravity acceleration, H''pd =the estimated maxium impact pressure head, n B''=the estimated effective width of impacting jet) of drop were computed for the reference in the design of construction dimension. It is suggested to install the inverse slope construction in the downstream to protect the Chi-Chi weir and Fu-Zhou embankment.