Shades of Green
Natural Concepts for Stormwater Management
by Shohreh Karimipour
The phone rings in one of the New York City Department of Environmental Conservation's (NYSDEC) regional offices and the frustrated voice on the other end booms, "My property is disappearing!" It is not unusual to receive complaints concerning stormwater runoff impacts on individual properties, which are frequently the result of uncontrolled runoff from new development. However, the long term, cumulative impacts of stormwater are much more detrimental than individual drainage issues. They shift our streams! The National Academy of Sciences examined this phenomenon and recommended development that mimics the natural hydrologic cycle. The New York State Stormwater Management Design Manual (Design Manual), the state's guidance on proper stormwater design, also stresses the green infrastructure approach to managing stormwater.
Why is Stormwater a Problem?
As precipitation falls in natural and undeveloped areas, it is intercepted by vegetation, returned to the atmosphere by evapotranspiration, or infiltrated into the ground. As precipitation falls in a developed area, it follows a different pattern. The impervious cover reduces infiltration and evapotranspiration and increases runoff. That increased runoff degrades streams and other water bodies. In the natural environment, rain that infiltrates to the ground plays an essential role in maintaining stream health. For example, if this pattern is disrupted, the rainfall does not contribute to base flow and the stream has a greater chance of drying up between storms. Also, impervious cover increases the volume and rate of water running off the land and into streams. The increased discharges scour a stream, deposit sediment, change the shape and hydraulics of the stream, increase temperature and deliver pollutants. They collectively damage the habitat of fish and macro-invertebrates. The sight of such damage can alarm the citizen into making the above mentioned phone call.
Making Green Mainstream
The field of stormwater science is adapting to target this problem by utilizing green infrastructure. In the context of stormwater management, green infrastructure includes a wide array of practices that maintain and restore natural hydrology using vegetative or engineered systems. The Design Manual, which sets forth the post-construction technical standards for New York State's stormwater permitting program, adopted this approach in August 2010. In recognition of the emerging concepts of low impact development and environmental site design, the Design Manual was updated to define a step-by-step planning and design process where "flow" is addressed as a pollutant of concern and innovative green techniques are utilized to reduce runoff and maintain the natural hydrologic cycle.
Parallel to the regulatory framework, public acceptance is crucial to the success of implementing green infrastructure for stormwater management statewide. Let's surf the channels and airwaves to explore the "Top Ten List" of elements that led to this approach, and build a better understanding of the effort to make green mainstream.
The History Channel:
Stormwater design criteria historically have focused on flood control and chemical pollutant treatment. Traditional stormwater management techniques designed solely based on treatment objectives do not address the increased volume of runoff. The Design Manual (2003-2008) required the capture and treatment of the 90 percentile storm using structural practices that met the performance criteria of 80 percent total suspended solids, 40 percent phosphorus removal and traditional methods of detention for larger, less frequent storms (e.g. detention of 10-year and 100-year storms) to prevent flooding. At the time, national studies recommended methodologies that target reduction of runoff volume by implementing better site design, preservation of natural areas, onsite retention techniques, and watershed planning. The latest findings in the stormwater field recommended the use of green infrastructure approaches by employing evaporation, infiltration and reuse of stormwater.
In response to a need for non-structural practices, NYSDEC developed the document, Credits for Stormwater Management through Non-Structural Practices and Better Site Design - 2005, to incorporate environmentally sound planning. Innovative site design and runoff reduction was a voluntary method and was hardly used. According to NYSDEC construction permit records, as of 2010, only 22 percent of the sites used infiltration to treat stormwater and alternative practices (green infrastructure) were as low as 2 percent. In the absence of a permit requirement, few designers proposed plans to maintain pre-development hydrology.
The NYSDEC Water Quality Assessment Program, which identifies the impaired waters of New York State, lists urban stormwater runoff as the most prevalent cause of water quality degradation for segments that were assessed as impacted or impaired. The frequency of problems due to "Habitat/Hydrologic Modification" ranked as high as 30 percent (Summary of 2010 Priority Water List-PWL). The biannual assessment of waters of New York in 2010 identified urban stormwater runoff and its hydrologic impacts as the primary contributor to 25 percent of impaired rivers and streams, 16 percent of impaired lakes, 84 percent of impaired estuaries, and 16 percent of impaired coastal shorelines.
You don't need to look too far to find numerous examples where conventional methods of stormwater management have not addressed this critical issue in New York. Local examples of rapid degradation of the headwaters and first order streams, bank destabilization, and erosion occur throughout the state.
The cumulative impact of upstream development results in severe geomorphic changes in receiving waters. Such impact is highly visible on many urban streams and can be detected by simple observation of the lack of natural habitat, sedimentation on streambeds, stream bank erosion, and frequent flooding during small storms
All Things Considered:
The green infrastructure approach includes three primary components:
- Avoiding the Impacts: Consider minimizing disturbance, preserving natural features and using conservation design techniques.
- Reducing the Impacts: Look at ways to reduce impervious surfaces, such as parking areas, the road length and width, and building footprints.
- Managing the Impacts: Utilize natural features and runoff reduction practices to slow down the runoff, promote infiltration and evapotranspiration, and, consequently, minimize the need for the structural "end-of-pipe" practices. This approach reduces runoff, increases its travel time, and reduces peak discharge.
The Design Manual provides a menu of options that offers a variety of solutions from the planning phase to control techniques, which collectively help reach the goals of the standards. The wide spectrum of preventative measures and design techniques provide flexibility for incorporating options that suit the site. Where runoff reduction to the full capacity is not feasible, the standards allow reduction to a minimum level. This provides flexibility based on the site's hydrologic soil groups. Where such runoff reductions cannot be met, the designer can still propose conventional treatment methods to meet the standard
Green infrastructure practices target the flow by a simple concept: slow it down, spread it out, and soak it in . This family of practices has a combination of traditional techniques along with younger innovative methods. Buffer strips, planters, cisterns or grass swales have been around for decades. Some other techniques such as green roofs, rain gardens and porous pavements, although new in some areas, have been around long enough to prove their effectiveness. Some old design elements may be subject to change to provide flexibility for particular applications. Application of some of the practices in ultra urban settings may demand alternative pretreatment or setbacks. Innovation is the key. So, as hybrid and modified designs are researched, they are considered for acceptance in the standards. As design specifications advance so that engineered media are mass produced and prefabricated units manufactured, technologies become mainstreamed and more affordable.
America's Top Model:
The computation methods associated with green infrastructure are not necessarily complex. The Design Manual allows the use of a simple method to estimate the runoff from small, frequent storms. This method relies on a few variables such as impervious cover, contributing area and precipitation for modeling small storm hydrology. Most designs use simple concepts such as storage within a practice or subtraction of contributing areas. One may argue that more scientifically verified modeling tools and design methods offer more in-depth analysis. The Design Manual allows for the use of advanced tools and continuous simulation modeling. These can enable the designer to simulate the hydrologic response of the site by factoring in storm intensity, soils, slopes, existing moisture and evaporation rates, and provide more meaningful analysis of the proposed development, assisting more informed decision making. The designer can balance simplicity with complex methods that require more extensive input variables, detailed analysis and rigorous computation.
Among the menu of options provided to address runoff reduction, there are practices that are less costly than conventional methods. Many of the requirements of the green infrastructure approach are applied as a part of environmentally sound planning and erosion control but do not involve additional expenses (e.g., limiting disturbance). Several requirements are based on using the green space available on the site and do not involve advanced design features, structural controls, hardscape and construction materials (e.g. rooftop disconnection). Some of the practices also use very basic design features and are constructed on a small scale which provide an opportunity to reduce or eliminate the need for larger centralized treatment systems. Practices that are more costly are those that are installed underground, involve structural design, such as weight bearing capacity and insulation issues, as well as those with extensive excavation and fill media. Generally, such designs are geared toward ultra urban projects. Such projects are often redevelopment projects in which runoff reduction is an option, not a requirement.
Law and Order:
Green infrastructure, like any new technology, may present challenges within the structure of existing laws. Some local laws, such as zoning and building standards, codes and ordinances, have construction requirements that can conflict with some of the planning principles. Road width and parking spaces widths are two common examples. The green infrastructure planning principles should be examined and applied where circumstances and local codes allow. Sometimes seeking a variance from the municipality is a way to address the issue. However, for long term implementation, NYSDEC encourages municipalities to review their local codes and address the obstacles to green infrastructure as a one-time investment. It is also very effective when multiple municipalities perform an area-wide evaluation through a collaborative effort. The NYSDEC has awarded grants to municipalities interested in initiating such efforts, and supports collaboration with the Department of State to address this need at the state level.
Tell Me Something I Don't Know:
Many of the principles of green infrastructure are already commonly practiced. Preservation of natural resources and avoiding construction on steep slopes are commonly considered in many municipal master plans and conservation plans. Several communities within the state, such as Long Island or the Lake George basin, require more stringent stormwater recharge and retention within their jurisdictions. Some communities have requirements on the ratio of green space to impervious cover.
Most of green infrastructure design details are simply common sense and similar to considerations in conventional stormwater management. In both designs, precautions are needed for recharge in sole source aquifers, and infiltration in hotspot or brownfield areas. Soil testing is also needed to verify capacity of proposed infiltration systems, and all systems require provisions for overflow and bypass of larger storms.
One element common among all the green practices - and conventional stormwater systems for that matter - is maintenance. Maintenance is the most essential and relevant factor in operation of stormwater systems. No matter how advanced or primitive a system, there can't be too much emphasis on this single factor to ensure the success and longevity of the practice. The advantage of many green infrastructure practices is that their maintenance can be incorporated into the routine landscaping practices.
Wait, Wait … Don't Tell Me:
There are always new questions when implementation of a new concept begins. For that reason NYSDEC has conducted a series of workshops across the state. More than 1,000 professionals from Long Island to Buffalo have attended these events. Feedback from these workshops has resulted in the preparation of a frequently asked questions (FAQs) document and other revisions that will be used to improve the Design Manual (available at: http://www.dec.ny.gov/chemical/8694.html). Upcoming training efforts will focus on the needs of zoning and planning board members, for whom an online training module has been developed and posted at: ftp://ftp.dec.state.ny.us/dow/stormdocuments/GIUpdateWorkshop/. Adoption of green infrastructure practices into New York State standards was not done in a vacuum. Local impacts and supporting science clearly identified a need to address pollution caused by stormwater runoff by minimizing the runoff volume. The NYSDEC held a series of public meetings on stormwater in 2008 and 2009. It was decided at those meetings to add green infrastructure to the Design Manual. The update of seven chapters of the Design Manual was public noticed in the Environmental Notice Bulletin in Fall 2009. Most of the comments submitted to NYSDEC were incorporated in the final Design Manual. The concepts incorporated in these updates have been supported both at the national and international levels for more than a decade. Many states have implemented runoff reduction requirements and green infrastructure practices. New Jersey, West Virginia, Virginia, California, Washington, Oregon and Pennsylvania, are just a few examples.
Green is a Multi-Faceted Approach
Bringing it all together, green infrastructure is more than a single solution. It is a multi-faceted approach for planning and inventive management of stormwater as a resource. The shades of green extend from simple measures to complex techniques, engage cultural acceptance, promote creative designs, and encourage natural environment and smart protocols. Changes are on the horizon. Soon private citizens will take pride in maintaining the buffer along the stream that runs through their land instead of worrying that it encroaches on their property.
Shohreh Karimipour, PE, is Senior Engineer, New York State Department of Environmental Conservation in Albany, NY. She may be contacted at: email@example.com.
The DOW article is reprinted here courtesy of Clearwaters. For subscription information, contact NYWEA directly. http://nywea.org/clearwaters