Determination of Effective Impervious Area for an Urban Indian Catchment

Determination of imperviousness, which is defined by total impervious area (TIA) and effective impervious area (EIA), is mandatory for hydrological modeling of water quantity and quality in urban areas. The determination of TIA is relatively easy and preferred to determination of EIA, even though the latter is considered to be more appropriate for hydrological studies. Accurate and representative determination of EIA requires knowledge of drainage network connectivity with the impervious surface, which cannot be ascertained from remote sensing data alone. A more realistic semiautomated direct method is suggested in this study to determine EIA by integrating the remote sensing data, the digital format of the drainage network, and a digital elevation model (DEM) of the study area. Based on the results obtained from the proposed method, a power relationship $[\mathrm{EIA}=0.0035\times {(\mathrm{TIA})}^{2.17}]$ relating easily measurable TIA and hydraulically relevant EIA was determined for an ungauged urban catchment in northeast India. There are no such relationships available in the literature for an urban Indian catchment. EIA was also determined using two indirect methods reported in the literature, and the results were compared. The present study indicates that there is a distinct difference between the EIA values obtained by the direct method and those obtained by the indirect methods. Among indirect methods, Sutherland’s equation was observed to predict EIA values close to the directly determined EIA. The maximum overestimation of EIA by the Sutherland equation was 4 times, that by the Alley and Veenhuis equation was 20 times and, TIA was observed to be 56 times more than the directly estimated EIA. If EIA determined by indirect methods or easily measureable TIA were used as input for hydrological studies like flood modeling, it would result in considerable overestimation of peak discharge. The design of hydraulic structures or flood management strategies based on the overestimated peak discharge would have far-reaching undesirable impacts on efficient planning and management, causing overexpenditure.