Submitted to: American Society of Agricultural Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/8/2004
Publication Date: 3/1/2005
Citation: Starr, G.C. 2005. Basal sediment concentration measurement in southern arizona runoff waters using a time domain reflectometry method. American Society of Agricultural Engineers. Interpretive Summary: In the deserts of southeastern Arizona, erosion rates by water runoff are high, despite the fact that monsoon erosion events are fairly rare. It has not been possible to measure sediment concentrations near the base of these flows where the concentrations are at a maximum. This paper describes a method for accomplishing the needed measurements and shows the progression of sediment concentration through several events including a very large scale monsoon. Sediment concentrations observed were some of the highest ever reported. The results suggest that erosion rates are very high not just at the peak of the flooding event, but also throughout the receding portion of the flow. These results will be of interest primarily to erosion scientists and engineers.
Technical Abstract: Sediment concentration measurement in high concentration runoff waters is highly problematic. A method was developed with the goal of characterizing a sediment concentration measurement system for monitoring bed material concentration during monsoon runoff events at the USDA-ARS Walnut Gulch Experimental Watershed in Southeast Arizona. Using a model and the empirical function describing pure water, the sensor's concentration output was calculated. Data were obtained using a 55 cm, three-prong, embedded time domain reflectometry probe and sediments in a range of size classes under laboratory conditions; then installing the probe in the base and center of a flume. Basal (0-2 cm depth) sediment concentrations were monitored in three flows with peak discharges of 5, 90, and 130 m3/s representing small, intermediate, and large events. Without sediment specific calibration, laboratory validation indicated agreement between sensor and gravimetric concentrations within about 0.02 Kg/L for suspended sediment and within 0.2 Kg/L when the probe was completely buried in coarse (<2.5 cm) channel bed material. For the large and intermediate events, basal sediment concentrations rose from 0.4 - 0.12 Kg/L to a plateau of 0.15-0.19 Kg/L after the flow peak. The plateau extended through much of the tail of the hydrograph before falling to 0.04-0.12 Kg/L. The small event had a similar progression but lower overall concentrations of 0.02-0.08 g/L. 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 and bed transport environments where traditional methods fail.