Stormwater Symposium

Oral

398080 - Establishing an Impervious Definition in a Stormwater Utility Context

Monday, June 4
10:30 AM - 12:00 PM
Location: Greenway AB
Co-Authors: Tiffany Neubig, Ann ARbor, MI – Tetra Tech

In many cities throughout the United States, stormwater utility charges are based on the amount of impervious surface on the parcel. Most communities define impervious surfaces to include roofs, roads, sidewalks, patios, and gravel or compacted soil. In a stormwater utility that treats an area as either impervious or pervious, those surfaces which are not viewed by the public as paved may result in dispute. This presentation will outline various testing methodologies that were used in supporting an impervious classification process in a stormwater utility context, along with their results and challenges. The primary objective of the investigation was the desire to identify a testing method that could be standardized and easily executed, and would resolve disputed impervious surface determination.
The initial approach involved using standard test methods or modified infiltration testing to classify impervious and pervious areas. Tests ranged from use of double ring infiltrometer to optional testing apparatuses to perform the testing. Lessons learned included challenges with either inserting the double ring infiltrometer or sealing alternative testing apparatus. Material being tested was also non-uniform, occasionally included larger rock and often was highly compacted. Alternative methods used included bulk density testing, which had its own limitations and challenges in applicability due to the materials being evaluated.
This presentation will focus on the intersection of science and policy, and various methods used to assess whether a surface should be classified as impervious or pervious. As we determined from a wide range of methods, a simple question may be difficult to answer in a simple testing methodology.

Carol Lynn Hufnagel


Tetra Tech

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