Proprietary underground devices have become a popular means of stormwater treatment in dense urban areas due to tight space constraints. Several technologies are employed to remove sediment and other debris from stormwater runoff prior to discharge into lakes, rivers, and streams, including hydrodynamic separation, gravitational separation, and screening. Many laboratory tests and field monitoring studies have been undertaken which have shown these devices to be capable of removing a significant amount of coarse sediments contained in surface runoff. However, results of field monitoring studies depend upon accurate sampling of the runoff, upstream and downstream of the device, and obtaining representative samples at each location is a challenge. This research investigates the feasibility of controlled field tests of proprietary devices under a variety of treatment flowrates and influent sediment concentrations in order to gain a better understanding of each device’s performance. Each device is carefully cleaned prior to the testing and then a bulk solids analysis is performed on sediment captured by the device so as to eliminate errors due to sampling of influent and effluent water. After data analysis is complete, a revised sizing criterion is proposed which will improve overall performance and sizing of such devices. The resulting approach refined through field experiments will be incorporated into an assessment (monitoring and field testing) protocol that will be used by consultants, local governments, and state agencies to assist in selecting, designing, and evaluating stormwater treatment technologies for public infrastructure improvement projects.