Hydrodynamic separators are widely used in urban areas for removal of suspended
sediments and floatables from stormwater due to limited land availability for the
installation of above ground stormwater best management practices (BMPs).
Hydrodynamic separators are often sized based on relatively frequent storm events.
However, during less frequent storm events, device design treatment rates are exceeded
and previously captured sediments can be washed out of the devices.
To study the potential for scour and washout of previously deposited sediments in the
sumps of hydrodynamic separators under high flow conditions during infrequent storm
events, sediment washout testing methods using mass balance were developed for both
controlled field testing and laboratory testing. The developed testing methods were
utilized to conduct sediment washout testing on three full scale hydrodynamic separators:
1) an Environment21 V2B1 Model 4, 2) a STC1200 Stormceptor and 3) a 6-ft
Downstream Defender. All three devices were tested in the laboratory. The
Environment21 V2B1 device was also tested in the field. In addition to full scale testing,
two scale models were constructed and tested in the laboratory.
Previous work by others on describing sediment washout in standard stormwater sumps
was applied to data obtained from the sediment washout tests to develop sediment
washout functions which incorporate non-dimensional parameters. Washout functions
were developed for each of the devices tested in the laboratory and the field, as well as
for a modified ecoStorm device previously tested by others. The washout functions that were developed can be incorporated, along with removal efficiency functions, into
continuous urban runoff models to predict maintenance schedules for hydrodynamic