Impervious area analysis using infra-red aerial photography

This study was completed in order to provide the City of West Lafayette, Indiana a basis by which to fairly and equitably bill property owners for stormwater collection and conveyance.  Since the amount of stormwater runoff generated is largely dependent on the impervious area of individual properties, initial impervious area estimates of all properties have been developed using satellite photographic technology (e.g., remote sensing techniques). The data used to generate these estimates includes visible spectrum and near-infra-red orthoimagery. The analysis method described herein resulted in a total impervious area estimate for all of West Lafayette with an estimated accuracy of +/-1.5%. It is recommended the city use these estimates as the basis of a stormwater billing program. Other municipalities can use a similar approach, particularly if the required imagery and georeferenced parcel data are readily available.

The City of West Lafayette, Indiana, currently covers an area of approximately 7.7 mi2 and a population of 30,600. The city currently does not bill its customers for stormwater collection and conveyance. However the increasing cost of providing storm drainage and the State/Federal Phase II Stormwater Regulations have prompted the city to examine the cost of stormwater collection and conveyance and how best to collect the revenue needed to address these costs. The city prefers that stormwater costs to be equitably allocated to property owners based upon the volume of stormwater generated by each property. This stormwater volume largely is dependent on the amount of impervious surface on each property. Therefore, this study has been conducted in order to create initial estimates of these areas. It should be emphasized that the impervious areas developed as a result of this study are estimates, created from the remote sensing data as described herein, and are not based upon actual field measurements. Therefore provisions to adjust and refine these values based on more accurate measurements should be provided.

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