GREEN UNIVERSITY TOWN SUCCUMBS TO BLACKTOP: QUANTIFYING THE INCREASE IN IMPERMEABLE SURFACES AND RUNOFF THROUGH TIME

High-resolution orthophotographs show a marked decrease over the past 20 years in greenspace in student neighborhoods surrounding the University of Vermont. Simple hydrologic modeling suggests that this paving of permeable areas increases run off volume and peak flow.

We determined land use changes in Burlington, VT neighborhoods using high-resolution, low-altitude orthophotographs taken in 1978 and 1999. Using these images, we identified and mapped 5 land use categories: buildings, paved parking areas, unpaved parking areas, sidewalks, and greenspace. Questionnaire surveys, field assessment, and analysis of Historic Preservation Studies provided demographic profiles of the past and present residents of the study area. The data generated from these maps and investigations showed a 10% loss of available greenspace over 21 years in the primary study area; 67% of this loss occurred on the 43% of parcels currently inhabited by students. Losses of greenspace in satellite study areas, as determined by teams of students in an undergraduate hydrology class, ranged upward toward 50% of available greenspace, particularly in neighborhoods with no owner occupants.

Loss of greenspace has occurred incrementally and in a predictable sequence. First, occasional lawn parking results in compaction, lowered infiltration rates, and muddy rutted areas. These areas are then "improved" with gravel, reducing detention storage. Finally, graveled areas are paved, reducing infiltration to zero. Curve number and rational runoff method modeling of the 10-year, 6-hour storm event suggest increases from 1978 to 1999 of approximately 10% for both runoff volume and peak flow. Conversion of lawn parking to formalized paved parking often escapes zoning controls and usually occurs without thoughtful planning or approval by the land owner.

Use of these methods illustrates a direct, quantifiable hydrologic impact that residents have on their immediate surroundings. Surveys of this kind can be done rapidly by students and provide data for urban environmental planning including urban catchment dynamics, increasing urban heat island effect, and possible remediation strategies.