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, it is difficult to predict the right sampling timing and mobilize field crew during storms. The SSC has high spatial and temporal variability. Efficient and automated techniques for high SSC monitoring are yet to be discovered.

This study performed an exhaustive literature review on SSC monitoring techniques. Optical turbidity, acoustic attenuation, and spectral reflectance methods were further evaluated. A new method based on time domain reflectometry (TDR) was proposed. The results show that spectral reflectance requires on-site calibration and has low spatial and temporal resolution, making it unsuitable for fluvial environment. Optical turbidity meter has the highest measurement sensitivity but the relationship between turbidity and SSC is highly affected by sediment particle size. The measurement range is typically less than 5000 ppm. It is suitable for high-resolution (below 100 ppm) SSC monitoring but site-specific calibration is important. Acoustic attenuation technique has a medium measurement sensitivity, suitable for resolution around 1000 ppm. But the relationship between acoustic attenuation and SSC is affected by sediment particle size the most among all methods evaluated. The measurement range is around 60,000 ppm for sediment with 30% clay content. New probe type, waveform analysis method, and temperature compensation were proposed for TDR-based method. The accuracy increases from 30,000 ppm to 10,000 ppm. The measurement range is practically unlimited. Sediment particle size has a limited influence on TDR method which is related to the bound water near fine particles. Particle size analysis based on TDR may be feasible by dielectric spectroscopy using TDR.

SSC monitoring techniques differ in measurement sensitivity, measurement range, and effect of sediment particle size. Appropriate instrumentation depends on the site conditions and requirements. TDR method possesses several advantages over other methods including low-cost sensors, durability, maintainable, multiplexing capability, and multi-functions (e.g. water level, rain gauge, and soil moisture, etc). But the sensitivity to low SSC needs further improvements.

A pilot automatic monitoring station incorporating optical turbidity meter, acoustic attenuation meter, TDR instrument, and automatic pump sampler was established at Nei-Wan hydrology station. Data collected by this pilot station will be used to demonstrate continuous monitoring during a storm event and compare the performance of different monitoring techniques.