The effect of suspended sediment and discharge on natural infiltration of ephemeral streams.

Abstract

Studies were conducted to evaluate the effects of suspended sediment and stream discharge, and their interrelationships, on intake rates in the Santa Cruz River. A lysimeter-flume, 100 feet long, 2 feet wide, and 5 feet deep, was constructed at the Field Laboratory. The interior boundaries of the unit were lined with butyl rubber to prevent leakage. A perforated pipe line was installed in the bottom of the lysimeter-flume to permit drainage of percolating water. The unit was filled with representative materials from the contiguous river bed. Cortrol sections were installed immediately before and after the flume to promote stabilization of flow conditions through the unit. A six-inch Parshall flume was located at the inlet of the upstream control section for measuring application rates during testing. A steel hopper, constructed and installed near the upstream control section, was used to store and meter sediment into the applied water in controlled amounts. The water supply for testing was obtained from the Field Laboratory. Three flow rates, 300 gpm, 450 gpm and 600 gpm, and three sediment concentrations, 10,000 ppm, 20,000 ppm, and 30,000 ppm, were selected for experimentation. The nine possible treatment combinations from these application rates were replicated twice, so that a total of 18 trials were conducted, The combinations were tested in random order. During each trial, water and suspended sediment were metered onto the lysimeter-flume at the required rates. Infiltration rates were determined by measuring the discharge from the interior drain line. Recharge rates varied from 4.2 feet/day to 5.0 feet/day in the streambed model. In general, the effects of discharge and suspended sediment in Santa Cruz materials were similar to those reported for Rillito Creek materials. Increasing discharge rates appeared to increase infiltration rates indirectly through the effect of stream velocity on bed erosion and sedimentation. Increasing suspended sediment concentrations were found to be highly significant in reducing infiltration rates.