Technical Studies

Due to the steep topography and week geology conditions of watershed areas in Taiwan, the amount of the suspended sediment yielded from mountains is dramatically increasing during torrential rainfall events. To support the related design and management of engineering or environment issues, it is much essential and urgent to access the continuously temporal and spatial variation of suspended sediment concentration (SSC) in a river.Development of SSC measurement method based on Time Domain Reflectometry (TDR) technique was then initiated by Water Resource Agency (WRA) since 2006 and kept improved till now. Compared with commercial optical turbidity meter and acoustic sensor, TDR SSC method is not affected by the soil particle size, and it is more economical, easy to be maintained, and applicable for high SSC conditions. The measurement accuracy was updated to 1000 ppm after the modifications in 2012. A TDR SSC monitoring information platform was established to provide the real time SSC hydrographs of demonstrated stations consequently. But, some practical concerns, such as interference of debris and sedimentation in the protection pipe, were still observed in field and brought uncertainties for TDR SSC monitoring.

To effectively solve these problems, one of objectives of this study is to continue maintaining TDR SSC demonstration stations and improving the accuracy of TDR SSC method. A new minimized SSC probe with a carbon fiber central conductor inside and a modified signal processing are proposed to raise the TDR SSC measurement accuracy. Protection pipes with new open-hole type are implemented as well to reduce the sedimentation of all demonstration stations. In addition, a concept of SSC measurement at common water depth supported by a floating device is provided and constructed in Li-Lin bridge, and it is also proved via measured SSC during typhoon events, 2015. Based on the aforementioned developments, real time SSC hydrographs of each stations are provided this year accordingly.

To extensively investigate the SSC variation in the cross section of a river and obtain the discharge-representative average SSC, a new design of TDR SSC monitoring combined with TDR scour wires is suggested last year and then constructed in Li-Lin bridge in this study. However, woods and weeds are easily trapped on TDR scour wires such that the SSC probe with a shielded case cannot be mounted on wires in practice.To access the feasibility of the SSC probe with the shielded case, verifications are proceeded under tests in the laboratorial physical model and open channel flow in field. Results show measured SSC data are consistent with real ones. Therefore, the cross section SSC monitoring system is suggested operate continuously as possible.

In addition to TDR monitoring technique, we have also developed a framework which enables the determination of sediment concentration hydrograph at the catchment outlets during a rainstorm event. The framework integrates the mechanisms of rainfall-runoff, landslide, soil erosion, and sediment delivery. The framework was applied to estimate the sediment concentration hydrographs at Lofou, Shihmen Reservoir Catchment during Fingwong, Jangmi, and Tanmei Typhoons. The comparison in sediment concentration between TDR measurement and model simulation indicates that the developed framework is accurate and applicable. Furthermore, the sensitivity analysis was adopted which showed that the soil cohesion, friction angle, CN value, and Manning’s n are the most sensitive parameters in estimating the sediment concentration hydrograph. The first-order second-moment method was applied to account for the uncertainties from the four uncertain parameters and estimate the prediction interval of sediment concentration.