IMPERVIOUSNESS: A PERFORMANCE MEASURE OF A DELAWARE WATER RESOURCE PROTECTION AREA ORDINANCE¹

Abstract

The New Castle County Resource Protection Area Technical Advisory Committee (RPATAC) requested that the University of Delaware utilize impervious cover estimates to evaluate the performance of the Water Resource Protection Area (WRPA) ordinance. This 1991 ordinance was the first in Delaware to protect the quantity and quality of drinking water supplies by limiting new development in WRPAs ‐ such as areas of ground water recharge, wellhead protection, drainage above reservoirs (reservoir watersheds), and limestone aquifers ‐ to a maximum 20 percent impervious cover. The research used geographic information system (GIS) techniques to evaluate the effectiveness of the ordinance in attaining its objective. The analysis indicated that 138 new developments were proposed in WRPAs since the ordinance was approved in 1991. The composite impervious cover of the 231 square kilometers of WRPAs in New Castle County is 15 percent, less than the 20 percent code requirement, ranging from 7 percent in recharge areas to 41 percent in several wellhead protection areas. To further protect drinking water supplies, the study results indicate, New Castle County should discourage code variances for applications in the more developed WRPAs, those where impervious cover already exceeds 20 percent, and should acquire parks and open space to protect the healthier WRPAs where impervious cover is currently less than 20 percent.

Hydrological transit times in nested urban and agricultural watersheds in the Greater Toronto Area, Canada

“…Note. Mimico values are from GIS spatial overlays using data from DMTI Spatial (2014); Rouge values are estimates based on common scaling factors (30% for residential areas, 80% for industrial area) used in the literature (Brabec, Schulte, & Richards, 2002;Han & Burian, 2009;Kauffmann, Corrozi, & Vonck, 2006;Shuster et al, 2005).…”

Challenges in Mechanistic and Empirical Modelling of Stormwater: Review and Perspectives

“…The runoff coefficient is defined as the ratio of runoff to rainfall over a given time period and depends on the percentage of impervious surfaces, slope, and soil conditions (Chow et al ., ). Imperviousness of small urban river basins can be directly measured by field surveys and analysis of aerial photographs (Han and Burian, ); for large river basins, imperviousness can be indirectly determined through analysis of rainfall–runoff data, assigning specific total impervious area (TIA) values to different land‐use types (Kauffman et al ., ; Han and Burian, ). A common approach to calculate imperviousness is to use typical values of runoff coefficient following the basic literature (Urban Drainage and Flood Control District (UDFCD), ) for different land uses and compute the imperviousness as the area‐weighted average of the runoff coefficients for all land uses in the sub‐basin or study area.…”

Climate, Land Use and Hydrologic Sensitivities of Stormwater Quantity and Quality in Complex Coastal Urban Watersheds

“…The runoff coefficient is defined as the ratio of runoff to rainfall over a given time period and depends on the percent impervious surfaces, slope, and soil conditions (Chow et al, 1988). Imperviousness of small urban watersheds can be directly measured by field surveys and analysis of aerial photographs (Han & Burian, 2009); for large watersheds imperviousness can be indirectly determined through analysis of rainfallrunoff data, assigning specific total impervious area (TIA) values to different land use types (Kauffman et al, 2006;Han & Burian, 2009 …”