地形劇烈變化河道之沖蝕特性及其對河道穩定性的影響及治理策略-子計畫:河道多功能沖蝕監測纜之研發與應用(I)

Abstract

橋墩沖刷之監測已有許多方法提出，例如常見的磁性感應、聲納及新 近發展的壓電薄膜、光纖光柵等，這些方法原理上都可行，但國內河川的 流況惡劣，夾帶木石之瞬間水流衝擊力甚大，現有的監測儀器設置方式難 適用在高流速環境下之即時監測；河道沖刷之監測因無橋墩可供設置固 定，更形困難。有鑑於此，前期研究提出以預力鋼鍵作為電磁波導波器之 TDR (Time domain reflectometry)傳輸線式自動化沖蝕監測技術，達到無 須埋入任何電子元件之極致作法，並以類似地錨工法的方式設置，提高耐 衝擊性。TDR 方法需事先標定纜線在空氣、水及地層的電磁波速，但若以此 方法監測橋墩之間的河道沖刷，現地實測發現空氣段（位於水面上之感應 纜線）會受到降雨水潑的影響，此外，現地實測亦顯示感測器之施工性有 待改良，因此有必要提出改良研究，使之可適用於河道一般沖刷之監測， 並改善感測纜線之施工性。另一方面，在沖蝕機制與沖蝕力學模型的研究 中，除了河床沖蝕量，尚需要水位、流速、含砂濃度等物理量之觀測。本 子計畫之主要目的在於持續改進傳輸式沖刷監測技術，使其適用於河道沖 刷之監測，並延伸擴充其他監測功能（如水位、含砂濃度及流速之量測）， 與結合其他子計畫，進行沖刷試驗及現地沖刷機制觀測。本研究首先將研 製施工性較良好的導波鋼纜，並以此鋼纜結構為基礎，研發可適用於河道 一般沖刷之監測技術，本研究所發展的TDR 河道沖刷監測鋼纜，預期能同 時監測水位，本研究將透過全波形反算分析，評估同時監測含砂濃度的可 行性，或研發可外掛在監測鋼纜上之含砂濃度感測器與旋轉式流速感應 器，以達到河道的多功能水文監測。

There are a few automatic scour monitoring methods, such as sonar, magnetic sliding collar and newly-developed piezo-film and fiber optical grating techniques. These instruments all work in principle, but are not practically feasible in Taiwan’s high-velocity harsh fluvial environment. It is even more difficult for monitoring of general scouring where there is no structure to protect the instrument. To optimize field durability, a previous study proposed a new method based on TDR (Time domain reflectometry) transmission line technique which uses steel strand to form a sensing waveguide, hence, completely avoiding any embedded electronic sensors. The sensing steel wire is constructed similar to soil anchors, optimizing the resistance against flow impacts. The TDR method requires calibration of EM wave speeds along the wire in air, water, and ground. Field testing revealed that the section above water may be affected by splash from rainfall and there is a room for improving the workability of the sensing wire. Furthermore, other physical parameters, such as water level, flow velocity, and suspended sediment concentration are required for investigation of souring mechanism and development of scouring model. The main objectives of this study are to develop new TDR sensing wire suitable for general scouring of river bed and extend the sensing wire to accommodate measurements of water level, flow velocity, and suspended sediment concentration. This study will also collaborate with other studies to facilitate erosion testing and collect scour data for understanding scour mechanism and developing scour model for rock riverbeds. The study will start with improving the structure and workability of the TDR sensing wire. The technique for monitoring of general scour will be developed based on the improved structure. It is anticipated the new sensing wire will measure scour depth and water level simultaneously. A full waveform inversion technique will be studied to investigate the feasibility to measure suspended sediment concentration as well. Studies will also be carried out to attach suspended sediment probes and velocity meters to the sensing wire to establish a multi-function hydrological station in the middle of the river.