Evaluation and enhancement of the effectiveness of sediment trapping and retention devices installed on the Nevada side of Lake Tahoe.

摘要

The gradual decline in the clarity of Lake Tahoe in recent decades has been well documented. Suspended solids in stormwater runoff may contribute to the decline in clarity. Some of the suspended solids originate from sand applied to roadways around the lake during the winter months in order to increase vehicle traction. Another source of suspended solids is the erosion of soil at the base of cut slopes along roadways. In an attempt to reduce the quantities of solids entering the lake, the Nevada Department of Transportation (NDOT) has installed sediment trapping devices along the highways within the Lake Tahoe basin. The devices work by intercepting stormwater runoff and allowing time for particles in the water to settle to the bottom of the traps. Two specific types of sediment trapping devices which are widely used in the basin are the drop inlet and the double can sediment traps. Settled solids are stored in the traps and the water exits and continues to the lake. Research was conducted at the University of Nevada, Reno (UNR) in an attempt to enhance the performance of these sediment trapping devices. Full-scale laboratory models of each trap were constructed and tested in the fluid mechanics laboratory at UNR. Each of the various modifications that were tested to enhance the performance of the traps improved particle removal to varying degrees compared to the performance of the standard traps. The most effective enhancement was the addition of multiple layers of vertically-oriented filter fabrics which resulted in trap efficiencies of greater than 50% for the removal of total suspended solids (TSS). Sand media filtration increased the removal efficiency for TSS in the double can sediment trap to greater than 40%. Arranging additional sediment traps in series was found to increase the removal efficiency of TSS to above 30%. The results and recommendations should help NDOT devise and implement techniques to economically and efficiently improve the performance, operation, and maintenance of existing sediment traps and to enhance the design of future sediment traps in an effort to help keep Lake Tahoe blue.