Refining Measurements of Effective Impervious Area in Urban Watersheds to Support Targeted Green Infrastructure Retrofits

Urbans stream worldwide are degraded due to the intensity of development and activity in our most populated watersheds. These streams that flow amongst our most thriving cities have been recognized as valuable assets to restore for the natural and cultural values they provide. In order to undo the factors that have led to their decline, we must understand these drivers better so that we can create the most affordable and sustainable solutions possible. Impervious surfaces are widespread in urban areas and their role on shifting urban hydrology has been well-studied. While the total impervious area (TIA) can be quantified readily, the subset of the TIA that is more closely connected hydrologically to the stream network, the effective impervious area (EIA) is not as easy to quantify nor locate. This is due to complex urban drainage patterns, ambiguous runoff pathways, connective underground infrastructure that may not be easily inventoried, and influences on hydrology that are spatially and temporally variable. Recent advances in geospatial assessment capabilities and high-resolution data availability have shown promise in updating and improving the measurement of EIA. This is important because many studies have shown that it is the EIA that is a greater determinant of urban stream health as measured by a variety of different metrics. Additionally, restoration efforts to disconnect the EIA using distributed at-source stormwater control measures (SCM) have become popular in many cities. Therefore, the determination of EIA in quantity and location across a watershed can greatly assist these types of watershed management approached that are attempting to build in green infrastructure (GI) within our cities to achieve a balance between effective stormwater management and natural hydrologic functioning. The sum of this work represents the effort to provide rapid assessment capabilities for urban watershed managers that can target efforts in GI placement so that the impacts of EIA can be diminished for the benefit of urban streams everywhere.