From Scaling Theory to Stormwater Practice: A Predictive Framework for Low Impact Development Planning

Low impact development (LID) is gaining popularity as a methodology to address increased runoff associated with urbanization. However, there is significant uncertainty about how effective LID is in reducing peak flow at the watershed level. This uncertainty affects public trust and acceptance of LID solutions. To increase LID adoption, we must better understand why such discrepancies exist according to underlying hydrological process controls. To address this uncertainty, we expanded a theoretical time-space scaling framework into a quantitative tool for LID planning. This framework classifies LID performance into distinct hydrological regimes based on the interaction of three critical timescales: rainfall duration ((Formula presented.) ​), catchment time of concentration ((Formula presented.)), and LID storage time ((Formula presented.) ​). We applied the framework to a case study using over 500 SWMM models to assess LID efficiency across a wide range of design storms, real-world storm patterns, and LID siting configurations. A key innovation was the introduction of a normalized performance index ((Formula presented.)) that quantifies peak flow reduction relative to storm size. This index helps establish clear, performance-based boundaries for the governing hydrological regimes, showing the specific conditions where peak flow reduction is dominated by storm characteristics, catchment properties, or spatial location. By providing a robust, process-based understanding of what drives LID performance, this improved framework moves beyond complex, site-specific modeling. It serves as a powerful diagnostic and planning tool that can help build confidence in LID solutions, optimize investments, and ultimately accelerate the adoption of more sustainable and resilient urban stormwater solutions.