The sediment removal capacity in catchbasin sumps and other hydrodynamic separators does not necessarily imply the ability to prevent the sediment from being scoured, especially when the remaining sediment capacity volume of the device is small and the flow rates are high. The sediment removal ability of catchbasin sumps and hydrodynamic separators must be balanced with their ability to retain the captured material by preventing scour. Therefore, understanding and quantifying scour processes in catchbasin devices is an important need when considering these devices as part of a stormwater management plan. This study investigated the sediment scour from a conventional catchbasin sump for different particle sizes, sediment depths, and flow rates. Full-scale physical experimentation and Computational Fluid Dynamics (CFD) modeling were conducted to determine a wide range of conditions at which different particles sizes are scoured.