河川含砂濃度全洪程觀測與含砂濃度歷線推估模式建構(2/2)

Abstract

台灣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。本計畫主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，並配合時域反射法(Time Domain Reflectometry, TDR)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。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。

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. 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 related 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 under 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 information 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. New SSC probe with a carbon fiber central conductor and a minimized probe are proposed to raise the TDR SSC measurement accuracy. The open holes of field pipes are modified as well to reduce the sediment settling effect. Based on this developments, real time SSC hydrographs were provided during rainfall events. This study further reveals that the SSC variation is small and can be ignored between sampling near bridge piers and main river channel especially under the lower flow discharge condition. To extensively investigate the SSC variation in the cross section, a new design of TDR SSC monitoring setup combined with TDR scouring monitoring is suggested. Furthermore, modified modules to display advance of SSC hydrographs and vertical SSC profile are included in the information platform.
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. This study have already review current models and methods, then a combined model and correlated flow chart is recommended subsequently. However, this proposed approach need to consider the effect as construct the sub-region of watershed area in the first stage of simulating. Therefore, the TDR SSC field monitoring data from a small and region of watershed area in KaoPing River will be used for the SSC hyetograph modelling testing in the next step.