Technical Studies

The rivers and creeks in Taiwan have characteristics of small basin, high flow velocity, and rapid change between base flow and storm flow, most transportation of suspended sediment occurs in a few torrential rain incidents. While suspended-sediment concentration (SSC) can be observed by manual sampling and lab testing, but it is difficult to predict the right sampling timing and mobilize field crew during storms. The SSC in river has high spatial and temporal variability, but durable, efficient and automated techniques for high SSC monitoring are yet to be discovered. To overcome the aforementioned problems, a new method based on time domain reflectometry (TDR) was proposed and pilot SSC automatic monitoring stations were established to evaluate TDR performance and compare with other commercial instruments. Compared with traditional labor sampling, TDR SSC method has advantages on real-time automation monitoring and low-cost. Unlike optical and acoustic method, TDR SSC method is less affected by the soil particle size or type, and the water salinity, and it is more economical, easy to be maintained, and applicable for high SSC monitoring.

To improve the TDR SSC technique, further improvements on TDR SSC analysis method and probe design, and field verifications are conducted in this study. The new TDR frequency domain phase velocity analysis method was proposed. The main theory, numerical verification, temperature compensation, and calibration procedure were established and documented. The probe design was further modified to the new 25-cm coaxial probe to increase measurement stability and reduce debris interference. The laboratory and filed experimental results indicate that the accuracy of the improved TDR SSC measurement system is around 1500 ppm (half of the theoretical TDR instrument resolution). TDR SSC measurement remains independent of water salinity, cable length, and sediment particle size. Furthermore, the improved TDR SSC measurement system reduces the interference by ambient noise and debris around the probe, hence, increases measurement stability.

Four monitoring stations, including Neiwan, Shihgong, Chichi, and Kaoping, were maintained and updated with the improved TDR SSC probes. To further testing the long-term performance of the TDR SSC method, data of storm events will be collected and analyzed to evaluate the in-situ TDR SSC measurement stability and accuracy.