Field assessments yield opportunity
The project team began by analyzing mapping data for the watershed: state hydrography and topography, the university's infrastructure and building footprints, and the town's stormwater infrastructure. They determined that 18%, or 218 acres, is impervious cover — higher than the 11% target. Most is concentrated in the highly urbanized section of the university's campus. On the other hand, the town's portion of the watershed is primarily composed of rural residential development.
In July 2009, the team conducted field work to identify opportunities to disconnect impervious cover using the Retrofit Reconnaissance Inventory (RRI) developed by the Center for Watershed Protection.
Members evaluated the retrofit potential of 51 sites by analyzing drainage patterns, drainage areas, impervious cover, available space, and other constraints such as conflicts with utilities and land uses, site access, and potential impacts to natural areas. They also sought to verify subwatershed drainage boundaries and identify impervious cover that was already disconnected. They found:Discrepancies in the original watershed boundary as contained in the state hydrography data layer; the watershed is actually 26 acres smaller51 acres of impervious cover are already disconnected via sheet flow to a large forested area, undetected diversion to another watershed, or being treated by a best management practiceSeveral impervious surfaces in the center of campus drain to highly compacted pervious areas with reduced ability to infiltrate stormwater. So although they were considered pervious when determining the original estimates, the team also considered impervious portions draining to compact pervious areas without a best management practice to be directly connected to the watershed.
Unless there were obvious constraints and/or evidence that a retrofit would offer few or no benefits, a stormwater retrofit concept was developed. Of the 110 potential retrofits the team identified, most are on the university campus. The team then identified 10 priority projects based on pollutant removal capability, runoff reduction, integration with other improvements, and cost.
Although impervious cover will be used to measure progress in this TM-DL, the ultimate goal is to restore the watershed's biological communities by improving the brook's water quality.
Thus, several questions still need to be addressed:How are discrepancies in impervious cover estimates and watershed boundaries addressed in regard to the TMDL regulatory framework?Is the “effective” watershed impervious cover comparable to actual watershed impervious cover, and what should be the process for accounting for each in development and implementation of maximum pollutant levels?How should stormwater managers account for “partial” or “ineffective” treatment, such as undersized or under-managed stormwater management practices, of impervious cover? Do these practices get partial credit?What happens if there aren't enough on-the-ground opportunities to meet target pollutant levels due to the lack of publicly-owned properties and uncooperative land owners?
Moving forward, the DEP's Bureau of Water Management will collect surface water flow and benthic macro-invertebrate data to measure the TMDL's impact on the watershed's aquatic life. The bureau and the Conneticut Inland Fisheries Division also will gather and analyze data regarding fish populations. The data will be incorporated into a watershed-based action plan that's in the draft phase.
Overall, accounting for impervious cover when developing water quality objectives makes sense because it typically is easier to generate a community response than with many other pollutants, such as bacteria or heavy metals. An impervious cover TMDL is easy to understand and measure, and it can result in a quick path to implementation.
Although not yet quantified, the progress so far in Eagleville Brook supports this view. Based on this experience, combining an integrative indicator like impervious cover with an accounting system like a TMDL provides a promising approach for helping communities design land use plans that protect water resources.
— Arnold (email@example.com) is associate director for the extension department of the Center for Land Use Education and Research at the University of Connecticut; Collins (firstname.lastname@example.org) is water resources engineer for the Center for Watershed Protection; Caraco (email@example.com) is senior watershed engineer for the Center for Watershed Protection; Kitchell (firstname.lastname@example.org) is senior environmental planner for the Horsley Witten Group; and Lilly (email@example.com) is watershed ecologist and planner for the Center for Watershed Protection.WHAT IS IMPERVIOUS COVER?
According to the U.S. EPA, it is the amount of land cover in roads, buildings and parking lots, and turf grass cover in a watershed, which can seriously impact biotic integrity (fish community health) in associated streams.