A New Monitoring System for Right-of-Way Bioswale Performance Reveals Factors Controlling Runoff Delivery and Retention

A portable system for measuring stormwater flow in a street gutter leading into and out of a right-of-way bioswale (ROWB) at high time resolution was developed. The system was deployed at ROWB 9B in the Bronx, New York City, and measurements taken over six storms were used to determine factors influencing bioswale stormwater performance. The measurements show that runoff delivery to the bioswale’s upstream inlet and retention of that delivery are controlled by sub-storm level rainfall pulse events (RPEs) represented by rising then falling limbs of rainfall intensity during a storm. Observations associated with 15 RPEs across the six storms were used to develop data-driven, empirical models to predict bioswale performance. A rain depth-dependent runoff coefficient provided a strong basis for predicting runoff delivery from the bioswale’s catchment, while the antecedent moisture state of the deeper bioswale soil was a predictor of retention. Across the six analyzed storms, the bioswale retained in aggregate an observed 37.7% of the rainfall volume falling on its catchment area, compared to a predicted value of 37.8%. Since ROWB 9B occupies only 0.57% of its catchment area, this level of retention highlights the stormwater benefits of installing ROWBs within the limited physical space available in dense urban environments.