Hydrodynamic Separator Sediment Retention Testing

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 2-yr stormwater runoff from the drainage basins that they serve. However, during less frequent storm events, e.g. 10-yr storm events, device design treatment rates are exceeded and previously captured sediments can be scoured and washed out. At the St. Anthony Falls Laboratory, three devices have been studied and tested and, subsequently, a testing method has been developed to assess sediment retention in hydrodynamic separators under flow rates exceeding their maximum design treatment rates (MDTR). This new testing protocol has been used in controlled field and laboratory tests on full-scale commercial devices under high water flow conditions to determine sediment retention performance of hydrodynamic separators. In addition, velocity profiles have been taken to observe the complex flow patterns in these devices. Application of this work is primarily to establish the frequency of required cleaning for installed devices.