Underground hydrodynamic separators are widely used in urban areas for removal of suspended solids and floatables from stormwater due to limited land availability for the installation of above ground stormwater best management practices (BMPs). Hydrodynamic separators are sized for applications based on design runoff from the drainage basins that they serve. However, during less frequent storm events, device maximum treatment rates are exceeded and previously captured sediments can be scoured and washed out of these devices. A methodology has been developed to assess sediment retention in hydrodynamic separators under water flow rates exceeding their maximum design treatment rates. 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. Application of this work is primarily to establish the frequency of required cleaning for installed devices.