河川含砂濃度全洪程觀測與含砂濃度歷線推估模式建構(1/2)

Abstract

台灣因地形影響坡陡流急河川挾砂能力強，加以地質條件脆弱集水區產砂量高，每逢颱風豪雨河川泥砂濃度即迅速飆升，而高濁度之水流往往對下游地區之水資源利用、河道穩定、河中構造物安全與生態系統等產生嚴重衝擊，因此瞭解颱洪期間河川泥砂濃度於時間與空間之變化實為重要課題，以做為擬定流域土砂管理策略或評估高濁度水流對水土災害影響之參考。傳統上河川泥砂濃度多是利用以統計法所建立之流量與輸砂量經驗曲線進行推估，惟此法因未考慮流域泥砂生產與遞移之物理機制，無法解析集水區降雨於空間分布與雨型變化等對集水區泥砂遞移之影響，更無法適用於集水區環境發生變化後(如流域管理或河川治理)之河川泥砂濃度推估，因此實有必要建立以物理概念為基礎之颱洪期間河川泥砂濃度歷線推估架構與方法，以解決上述統計法所面臨之問題。
無論是統計法之經驗曲線或利用物理概念建立之河川泥砂濃度歷線推估架構，皆須有準確之河川泥砂濃度觀測資料以利於相關模式之建立、檢定與驗證，而自動化且即時之泥砂觀測數據更可做為颱洪期間下游土砂災害預警與水資源設施即時操作之參考。目前河川水文站之泥砂觀測多仰賴人工取樣，常缺乏高流量下之泥砂濃度資料，因此水利署於2006~2009年度曾辦理以時域反射法(Time Domain Reflectometry, TDR)方式進行自動化泥砂觀測之先期研究，以同時獲得即時性、自動化與節省作業人力等特性，後續並於2011~2012年持續針對現地應用所發現之實務問題進行改善與提昇量測精度與穩定度，並建置對應之「泥砂濃度量測平台」，以提供觀測資料之查詢及展示，但由目前實務觀測經驗發現，現場感測器避免雜物干擾與落淤等限制需要克服，此外對於提供河川橫斷面泥砂濃度量測規劃與驗證仍須進一步探討。
綜合以上所述，本計畫目的在以既有之固定式自動化泥砂觀測示範站為基礎，擬定持續維護既有固定式自動化泥砂觀測示範站穩定運作外，並針對實務應用所面臨之問題持續改善相關設備。此外，為進一步將泥砂觀測資料實務運用，將探討橫斷面泥砂濃度空間變異與量測機制評估。另一方面本計畫亦將探討流域泥砂生產與遞移之分析方法，提出以物理概念為基礎之颱洪期間河川泥砂濃度歷線推估架構，期使本計畫之即時監測系統與數據以及泥砂濃度歷線推估架構，做為後續集水區土砂管理策略擬定或流域綜合治理等相關計畫之參考。

Due to the steep topography and week geology conditions of watershed areas in Taiwan, the amount of the suspended sediment yield is increasing during torrential rainfall events. The corresponding problem of high turbidity water is critical to the water resources, flood control engineering, river artificial constructions, and river ecology in downstream areas. Therefore, accessing the continuously temporal and spatial variation of suspended sediment concentration (SSC) in river is essential and urgent for the evaluation and management of the aforementioned issues. In the traditional way, the total amount of sediment in river is often estimated using the rating curve between sediment load and river flow discharge, which is based on the statistics of observed data. This approach cannot provide the continuous hydrograph of the sediment yield and delivery from watershed during the runoff, especially with the large variation of the rainfall and renovation in field. To solve the limitation, an appropriate model which consider the physical mechanism should be developed to reveal the real time SSC hyetograph or hydrograph in advance under different rainfall or other scenarios. Furthermore, this model should be calibrated and verified with the monitoring SSC data in field. However, the most existing SSC data were observed manually under low flow discharge circumstances, leading large uncertainty in high flow discharge conditions. 

A new suspended sediment concentration (SSC) measurement method based on Time Domain Reflectometry (TDR) technique had been developed since 2006 with the support from Water Resource Agency (WRA). TDR SSC method is not affected by the soil particle size or type, and it is more economical, easy to be maintained, and applicable for high SSC monitoring. The measurement accuracy is updated to 1000 ppm after the field modifications in 2012, also a TDR SSC monitoring platform was established to provide the real time SSC data of demonstrated stations. But, some practical problems, such as interference of debris and sedimentation in the protection pipe, were still observed in the field monitoring program.

To effectively solve these problems, one of objectives of this study is to continue maintaining and improving the field construction method of the TDR SSC measurement, and investigate SSC variation in the cross section for further requirements of obtaining the discharge-representative average SSC. The other objective is to develop a feasible mode based on the physical mechanism to provide the continuous SSC hyetograph or hydrograph during the rainfall events. The TDR SSC field monitoring data and the SSC hyetograph modelling can be provided as references for getting management strategy of sediment yield from watershed and comprehensive improvement of the river basin.