BASAL SEDIMENT CONCENTRATION MEASUREMENT USING A TIME DOMAIN REFLECTOMETRY METHOD

Sediment concentration measurement in high-concentration runoff waters is highly problematic. A novel device was developed for high-concentration measurements. This device was then used to measure sediment concentrations during monsoon runoff events at the USDA-ARS Walnut Gulch experimental watershed in southeast Arizona as an example application. Data were obtained using a 55 cm, three-prong, embedded time domain reflectometry probe and sediments in a range of size classes from the site under laboratory conditions. In the laboratory, the sensor’s concentration output was calculated using a model and the empirical function describing pure water. Without sediment-specific calibration, laboratory validation indicated agreement with known concentrations within about 0.02 kg L-1 for sediment and water mixtures and within 0.2 kg L-1 when the probe was completely buried in 0 to 2.5 cm channel bed material. In the field application, the probe was installed in the base and center of a flume. Basal (0 to 2 cm depth) sediment concentrations were monitored during three flows with peak discharges of 5, 70, and 130 m3 s-1, representing small, intermediate, and large events. For the large and intermediate events, basal sediment concentrations rose from 0.4 to 1.2 kg L-1 to a plateau of 1.5 to 1.9 kg L-1 after the flow peak. The plateau extended through much of the tail of the hydrograph before falling to back to 0.4 to 1.2 kg L-1. The small event had a similar progression but lower overall concentrations of 0.2 to 0.8 kg L-1. These observations are consistent with a period of high sediment transport and channel erosion in the tail of monsoon runoff hydrographs. The dielectric method provides in-situ measurements in high-concentration environments where traditional methods fail.