Experimental study of debris capture efficiency of trash racks

Trash (debris) racks are the main line of defense to keep submerged aquatic plants such as “Egeria” and other non-floating debris out of fish screen facilities. This study evaluates the debris capture efficiency under 16 different debris rack configurations, which include 4 bar spacings and 4 angles of inclination, under two flume velocities. For each flow regime, first the velocity fields were measured at three transects near the debris rack with no debris. While under 0.30 m/s velocity the debris rack did not affect the flow field, under 0.61 m/s velocity it slowed the flow velocity around the rack near the surface and increased it near the flume bottom. The debris that was collected in front of the debris racks at Tracy Fish Collection Facility, California was analyzed for its composition. The debris composition was found to change through the year, and Egeria of less than 15.24 cm was about 50% of the debris loading. Six replicates of Egeria run were conducted for each combination of debris rack configuration and flow regime. In the presence of debris, velocity profiles at locations upstream from the debris rack were not significantly affected by the debris or the rack, while downstream from the debris rack velocities at the flume bottom were significantly larger than those at the top. The experimental results show that when average velocity is 0.30 m/s, in general the debris capture efficiency increases as the angle of inclination decreases for bar spacing greater than 3.81 cm. However, for 0.61 m/s velocity, the same trend only holds for 7.62 cm bar spacing, with no discernible trend for other bar spacings. Meanwhile, debris capture efficiency increases with decrease in bar spacing for all angles of inclination and all velocities.