Estimation of Urban Imperviousness and its Impacts on Storm Water Systems

Imperviousness is an important indicator of the impact of urbanization on storm water systems. A hydrologic analysis was performed to evaluate long-term impacts from an apartment area in Miami. The result shows that the directly connected impervious area (DCIA), which covers 44% of the catchment, contributes 72% of the total runoff volume during 52 years. Few studies have actually measured the DCIA with a high level of accuracy for residential areas that constitute the largest proportion of urban land. A detailed analysis of urban imperviousness was performed using geographic information systems and field investigations on a 5.81 ha residential area in Boulder, Colo. For this study area, the total impervious area is 35.9% and the DCIA is 13.0%. Transportation-related imperviousness comprises 97.2% of the DCIA. Hydrologic modeling of this area shows about a 265% difference in estimates of peak discharge with imperviousness measured at five different levels of accuracy. These results suggest the need to focus on DCIA as the key indicator of urbanization's effect on storm water quantity and quality.