- Commentary on Gowanus Canal Sponge Park™
"Even with care-fully graded lawns there are still many depressions that must be filled before surface runoff begin. Where blocks contain vacant lots, some, or all, of the lots may be depressed below the sidewalks, thus creating large pools of depression storage. For storm sewer design, however, it will be generally assumed that these larger areas will be filled and graded at some later date (Tholin and Keifer 1960)."
At one time, it was thought that full build-out of the urban environment was a desirable outcome and the way to plan for it for urban hydrologists was to assume that all surface runoff needed to be conveyed by sewers either to a wastewater treatment plant or the nearest receiving water. But we have always known that even little area of green will provide some storage.
In 1994, the U. S. Environmental Protection Agency (EPA) Combined Sewer Overflow (CSO) Control Policy (EPA 1994) changed the way municipalities were required to manage CSO. Prior to this policy, municipalities could discharge untreated overflows without consequence. The policy directed municipalities to address CSOs. Some of the first measures for municipalities with CSOs were to institute "Nine Minimum Controls" (EPA 1995a) and to come up a "Long Term Control Plan" (EPA 1995b). In planning for control and minimization of CSOs with traditional infrastructure (i.e. sewers, storage tanks and treatment plants), it soon became clear to many communities that this was going to be a large and expensive task. Therefore many communities are beginning to implement "green infrastructure" along with traditional infrastructure approaches to mitigate CSOs.
Green infrastructure is the application of a suite of land management practices (aka stormwater best management practices (BMP)) as opposed to the traditional, buried below the street, approaches. The suite of BMPs most suited to the urban environment is low impact development (LID), which seeks to control and treat stormwater at the source. Low impact development includes bio-retention or rain gardens and swales. This is the approach which is presented by Drake and Kim in their Sponge Park™. This type of initiative has been encouraged nationally by EPA (EPA 2007).
The design appears adequate given the obvious limitations of only addressing a portion of the sewershed. The authors note that the Sponge Park™ will only address a fraction of the calculated water quality volume of runoff control. The authors also note that if the approaches presented here were instituted throughout the Gowanus Watershed, a significant reduction in the number of CSOs could be achieved. While the benefits in reduction of CSOs may be modest, the Sponge Park™ potentially offers access and aesthetic improvements for the public. Furthermore, the stormwater treatment component at the ends of the streets will help reduce the pollution load to the Gowanus Canal from the direct runoff .
New York City (NYC) through PlaNYC is attempting to incorporate more green infrastructure approaches like the proposed Sponge Park™. Nationally, many municipalities with combined sewers are implementing green infrastructure approaches similar to the Sponge Park™. In the case presented, the swales and bioretention units are limited to the Right of Way of the municipality. Further reductions in runoff from impervious cover could be realized by also installing controls on private property. New York City offers incentives for other efforts like green and blue roofs, which reduce discharge to sewers. Drake and Kim refer to the use of cisterns in the watershed as an additional control option.
The authors note that due to potential contamination of the soil at the location they may not be able to infiltrate the stormwater. However, the addition of green infrastructure could potentially reduce some stormwater runoff through evapotranspiration alone. This can be a significant help in reducing runoff, especially if woody vegetation with vertical structure is added to the planting palette.
It is encouraging to see the installation of a monitoring program. While green infrastructure projects are being implemented around the nation, there is still a great need for [End Page 401] reliable operational data of these controls. Reliable data from a variety of urban settings and climates are needed to demonstrate that green infrastructure approaches like the Sponge...