Underground Stormwater Quality Detention BMP for Sediment Trapping in Ultra-Urban Environments: Final Results and Design Guidelines (0-4611-2)

Permanent stormwater quality structures in developed urban areas require the use of smaller footprint or underground structures. This project investigated if an underground treatment structure made from off-the-shelf precast concrete sections would exhibit the required sediment removal efficiency and involve minimum maintenance. The project consisted of a literature review of small footprint and underground stormwater treatment devices, their cost, performance, and maintenance considerations; and a physical model study to test the principles to be employed and construction of a full-scale prototype for proving the concepts. Research results of conceptual models, physical models and part of the prototype are documented in the first project report entitled "The Development of Nonproprietary Underground Stormwater Quality Structures, Report Number FHWA/TX-07/0-4611-1." These results demonstrate that extended detention can be used to remove suspended solids from stormwater. It also shows that maintenance will be needed to sustain the overall level of performance (in terms of sediment removal). While the efficiency of the structure is around 75 percent removal, approximately 17 percent of the solids discharged can be attributed to resuspension. This report documents experiments conducted on the prototype in an attempt to optimize sediment removal efficiency by reducing sediment resuspension. Design and maintenance guidelines are included in this report. In summary, several refinements show promise in significantly reducing the resuspension loading, which could increase the overall efficiency to over 80 percent with maintenance intervals in excess of twelve months.