GIS-based Planning Tools to Mitigate Channel Enlargement and Water Quality Impacts Resulting from Impervious Surfaces

摘要

Previous studies in Vermont alluvial streams have demonstrated statistical correlations between imperviousness and channel enlargement/ instability (CWP et al, 1999). Geomorphic stress has been demonstrated at total basin imperviousness of 2 % or more; active adjustment of channel form has been linked to imperviousness exceeding 9%. Increases in percent imperviousness have also been statistically correlated to decreases in biological diversity and overall macroinvertebrate health in receiving waters (CWP, 2000). Current estimates for two rural Vermont towns, based on 1991 to 1993 Landsat-derived land cover/ land use data, indicate imperviousness ranging from 2 to 5 % (Underwood and Davis, unpublished). In-stream geomorphic assessments conducted in these towns have confirmed geomorphic degradation in response to development-related stressors including floodplain encroachment, undersized bridge crossings, reduced riparian buffers, and conversion of agricultural and forested lands to residential and commercial use (VTDEC, 2003). Based on past development patterns, the towns expect build out under current zoning regulations to result in aggregate percent imperviousness exceeding the 9% threshold, similar to more urbanized townships in the Burlington area (4 to 25 %; Pease, 1997). Most residential development projects will not fall under State or Federal regulatory oversight, requiring stormwater management structures or practices. The authors have merged GIS-based, automated, land cover / land use-based calculations of percent imperviousness, with GIS-based community build-out analysis software to predict geomorphic condition resulting from increased development on a subwatershed basis. The methodologies / techniques serve as local planning tools to inform decision-making, facilitate consensus-building, and manage community development with the objectives of minimizing impacts to water quality and reducing future costs to the towns associated with channel erosion. Using the tools, the towns have explored specific planning and zoning alternatives (e.g., reduced development densities, stream corridor overlays) to minimize percent imperviousness and the consequences of stormwater runoff.