SEDIMENT RETENTION EFFICIENCIES OF SEDIMENTATION BASIN FILTERED OUTLETS

Principal spillway filter envelopes of expanded polystyrene chips (EPS ) and 2-B gravel were compared to a no-filter control to assess their effectiveness in capturing sediment from a laboratory scale sedimentat ion basin. Each filter was evaluated with two perforated risers, one d esigned to dewater the basin in 1.5 h and the other in 3.0 h. The aver age sediment retention efficiencies for the no-filter treatments were 59.8 and 71.0% for the 1.5- and 3.0-h outlet configurations, respectiv ely. Perforated risers encased in 508-mm-diameter envelopes of EPS chi ps and gravel had sediment retention efficiencies, averaged over dewat ering time, of 84 and 82%, respectively. The 1.5- and 3.0-h dewatering times had sediment retention efficiencies, averaged over the EPS- and gravel-filter treatments, of 78.3 and 87.5%, respectively. Both EPS a nd gravel envelopes show promise in enhancing the sediment captured fr om sedimentation basins.