Sarah Madden

4 April 2010

MIT-DUSP Urban Sustainability Evaluation

Green Infrastructure: Stormwater Management Technical Memo

What Is Stormwater Management, and How Does it Work?

Urban stormwater runoff is an important contributor to water pollution; it both contaminates and physically harms aquatic environments. According to the U.S. Environmental Protection Agency (EPA), urban pollutants[dM1] are the most important contributor to contamination of the nation’s waters (National Research Council 2009)[dM2].

Urban stormwater problems arise because urban development typically involves converting undeveloped areas to impervious, often paved, surfaces, thereby transforming surfaces that would have soaked up heavy rains into impermeable land cover that generates runoff. In natural landscapes, less than 10 percent of the rainfall volume converts to runoff; instead, rainfall and snowmelt filter slowly into the ground (EPA 2003). By contrast, in urban landscapes, roads, parking lots, rooftops, and compacted soils seal 45 percent to 90 percent of land cover, preventing the infiltration of rainwater and altering the hydrology of surrounding ecosystems (Paul and Meyer 2001; Booth, Hartley, and Jackson 2002; Kloss et al. 2006). In fact, imperviousness is used as a key indicator for measuring the impacts of urbanization, and many studies reveal negative effects on biological diversity in aquatic ecosystems at and above watershed imperviousness levels of 10 percent (Brabec, Schulte, and Richards 2002; Clar, Barfield, and O'Connor 2004; Jacob and Lopez 2009).

During storm events, impervious surfaces cause runoff to rapidly flow towards receiving pipes or water bodies, all the while picking up pollutants—including sediment, metals, bacteria, nutrients, pesticides, trash, and polycyclic aromatic hydrocarbons—from the surfaces of the city. Those pollutants can either infiltrate into the ground, potentially degrading the groundwater, or flow into surface waters, depositing pollutants, sediments, and debris, while also eroding stream channels, altering sediment loads, and affecting stream temperature (Paul and Meyer 2001; Roy et al. 2008). Furthermore, impervious surfaces—especially roads and parking lots—increase the risk of flooding for downstream regions by increasing water volume and decreasing travel time, resulting in rapid flood peaks (NRC 2008; Jacob and Lopez 2009). In sum, there is a direct relationship between land cover and downstream water quality, and an effective management strategy to mitigate the Urban Stream Syndrome requires addressing the cumulative impacts of urbanization, including hydrology, water quality, and habitat considerations (NRC 2008).

Cities have a variety of infrastructure systems to control the flow of rainwater and runoff in the city. Many older U.S. cities use conventional combined sewer systems, which collect both sewage and stormwater runoff in the same underground pipes. In such gray infrastructure systems, Combined Sewer Overflow (CSO) events are a serious problem: during heavy rains, when the sewer system reaches capacity, excess flows are released directly into rivers or receiving water bodies. CSOs release an estimated 850 billion gallons per year of a mix of untreated sewage and stormwater, which contains microbial pathogens, viruses, and oxygen-depleting substances, in addition to normal stormwater pollutants. These releases present significant environmental and health risks (USEPA 2004).

Since the mid-1990s, cities have been installing green-infrastructure systems to address stormwater in a more distributed pattern, using landscape interventions instead of underground pipes. Green infrastructure involves taking a more environmentally sensitive approach to prevent runoff rather than treating it down the line—using green roofs, rain gardens, rain barrels, and porous pavements to filter and absorb rainwater (Spirn 1984; Hill 2007; Berghage 2009). These more natural drainage systems are of a comparable cost to underground concrete sewers, but also provide above-ground benefits and take much less time to install than gray infrastructure projects. Each of these engineered solutions or Best Management Practices (BMPs) has different performance (some are better than others at removing pollutants), variable water holding capacity, longevity, cost, climate suitability, and maintenance requirements.

Laws and Institutions to Regulate Stormwater

The federal regulatory history of stormwater control in the U.S. dates to the 1972 Clean Water Act and the 1990 Phase I Stormwater Rule, which regulate the discharge of pollutants into water bodies (NRC 2008; Booth and Bledsoe 2009). Part of the challenge of stormwater management is that by the time the stormwater regulations were enacted, cities already had extensive water infrastructure networks and laws focused more on flood control and protecting structures than water quality.

Cities typically cede responsibility for water management—providing drinking water, removing and treating wastewater, and managing wet weather flows—to a specialized water utility. These utilities can be a regional water and/or sewage district, multi-functional watershed district, or groundwater management district, and usually manage several, but not all, aspects of the urban water environment (Baker, Shanahan, and Holway 2009). Some utilities do house all municipal water-related functions, an arrangement that may ease the path towards promoting innovative green infrastructure programs: the Seattle Public Utilities and the Philadelphia Water Department, for example, operate combined drinking water, wastewater, and stormwater utilities, and both have undertaken ambitious green-infrastructure projects (PWD 2009; City of Seattle 2010)[dM3].

Furthermore, cities rely on land-use zoning, building codes and standards, and infrastructure standards and practices to shape the built environment, and the “overlapping and conflicting maze of codes, regulations, ordinances, and standards [have] a profound influence on the ability to implement stormwater control measures” (NRC 2008, 72). Because such regulations affect activities and material choices for land cover in the public and private realms, they also affect stormwater and efforts to mitigate its impacts. In order to effectively address pollution and water quality issues, then, local governments must coordinate the work of multiple departments, reconnecting land-use planning, regulation, and stormwater management (NRC 2008).

Zoning

Zoning influences stormwater runoff regulations because it impacts the amount of impervious area in a development and opportunities for on-site stormwater management (NRC 2008). Euclidean zoning segregates land uses into geographic districts and dimensional standards, and could be updated to establish a reward-based incentive system that incorporates stormwater control measures. As an alternative, performance zoning offers a more flexible alternative to the rigid Euclidean zoning framework, requiring development to meet performance standards, which could easily include stormwater. Form-based zoning combines prescriptive and discretionary criteria, which can be designed to encourage small impervious footprints and preserve open space, yet often favors shorter buildings that leave less area for on-site stormwater management (NRC 2008)[dM4].

Building Codes

Building codes establish standards for construction, and sometimes incorporate materials standards that prevent the use of landscape-based green infrastructure (such as porous pavements) because of traditional concerns about protecting structures from water. As technical engineering expertise with green infrastructure practices improves, cities can update building codes that currently impede the adoption of innovative stormwater approaches (such as rules that specify a minimum distance between a building and an infiltration area, for example).

Groups such as the American Society of Landscape Architects (ASLA) are considering how to encourage sustainable site design among the design and engineering professions. The ASLA devised a voluntary land- and water-oriented rating system for sustainable landscapes, The Sustainable Sites Initiative: Guidelines and Performance Benchmarks 2009 (Sustainable Sites Initiative 2009) as a landscape parallel to the Leadership in Energy and Environmental Design Green Building Rating System™ (LEED®) model for buildings. The system rates criteria related to the site selection, assessment and planning, design for water, design for vegetation and soils, materials selection, human health and well-being, construction, operations and maintenance, and monitoring and innovation, and outlines a precise system for sustainable landscape design.[dM5]

Engineering and Infrastructure Standards and Practices

In the public right-of-way, engineering standards and practices define how cities address surface drainage, road construction, grading, pipe size, landscaping, and other requirements that affect stormwater. Local codes also establish requirements for stormwater drainage from private homes into the municipal drainage system, and may conflict with green approaches to reduce runoff (NRC 2008). Stabilizing dynamics such as path dependence reinforce the status quo: the many layers of uses in the modern built environment, including networks of underground infrastructure, size requirements for emergency vehicles, and years of refinement and maintenance experience tempt cities to stay the course, to the detriment of innovative stormwater management approaches.

Innovative Approaches to Stormwater

The optimal approach to stormwater runoff would be comprehensive, addressing land-use planning and stormwater management[dM6], as well as…. According to the National Resource Council (NRC),

In an ideal world, stormwater discharges would be regulated through direct controls on land use, strict limits on both the quantity and quality of stormwater runoff into surface waters, and rigorous monitoring of adjacent waterbodies to ensure that they are not degraded by stormwater discharges. Future land-use development would be controlled to prevent increases in stormwater discharges from predevelopment conditions, and impervious cover and volumetric restrictions would serve as a reliable proxy for stormwater loading from many of these developments. Large construction and industrial areas with significant amounts of impervious cover would face strict regulatory standards and monitoring requirements for their stormwater discharges. Products and other sources that contribute significant pollutants through stormwater—like de-icing materials, urban fertilizers and pesticides, and vehicular exhaust—would be regulated at a national level to ensure that the most environmentally benign materials are used when they are likely to end up in surface waters. (NRC 2008, 101)[dM7]

The task is to link water management with land-use: “We now understand, for example, that groundwater and surface water are really one resource that should be managed conjunctively, the pollutants should be managed at their source, and that we can predict, within limits, the consequences of urban development on the ecological health of urban streams,” and the connections between these systems adds technical expertise to more effectively design with nature (Baker, Shanahan, and Holway 2009, 288)[dM8].

Cities’ options for innovative approaches to stormwater management initially may be constrained by the existing water infrastructure, competing land and water uses, water rights, laws, and institutions. Green infrastructure is an especially promising approach because it provides an array of environmental and social benefits to the community that are not provided by the gray infrastructure approach. The green approach is attractive to cities because the combination of benefits may ease some of the inter-departmental and cost-benefit challenges cities face[dM9].

The NRC (2008) suggests ways that cities can update their zoning, building codes, and infrastructure and engineering standards to improve stormwater management:

  • Separate ordinances for new and infill development. Because redevelopment is more challenging than new development, and strict stormwater requirements might hinder redevelopment in the city core at the expense of undeveloped land at the fringe, separate ordinance for the two development types make [dM10]
  • Integrated stormwater management and growth policies. [dM11]San Jose, CA adopted an approach that combines attracting high-density infill development with performance-based water-quality goals (i.e. minimizing impervious area, prioritizing swales).
  • Unified development codes. Consolidate layers of code to surface any inefficiencies and encourage innovation.
  • Design review incentives to speed permitting.Incentives include reduced development fees, preferential review and approval for innovative plans, reduced stormwater fees, and other incentives. Chicago’s Green Permit Program and Portland’s Green Building Program offer developers ways to save time and money by meeting environmental goals.

Common Program Types

Cities have adopted a variety of different approaches to stormwater. Because this memo focuses on green-infrastructure approaches, with an emphasis on the structural investment by the city, we have categorized these programs as infrastructure investment (by the city), codes and regulations (including incentives and deterrents), watershed protection and restoration, and education.

Table 1 summarizes the programs in the 25 project cities, and assigns an initial rating to the cities’ stormwater efforts relative to a sustainable stormwater management system. (The rating system is described in more detail in the Analysis section.)

Table 1. Components of Cities’ Stormwater Programs.
***DRAFT—April 2010 – if a cell row is blank, that means I still need to investigate that [dM12]place or category

regulation

/ infrastructure
CITY / Used for Direct CSO Control? / Watershed Department or Regional Sewerage District / Design Guidelines for New Contstruction & Retrofits / Density bonus or requirements for developers, incentives for homeowners / Stormwater Utility and Fees based on Impervious Area / Gray infrastructure: Deep storage tunnels / Greenways/Wetlands/Riparian Protection/Urban Forests / Green Roofs / Rain gardens/Vegetated Swals & Landscape features / Permeable Pavement / Downspout Disconnection/Rainwater Collection / Overall rating: 1(beginning stages) - 5 (strong, comprehensive approach)
Austin, TX / ? / Y / Y / Y / N / Y / N / Y / Y / Y / 2
Boston, MA / Y / N / Y / N / N / N / Y / Y / Y / Y / N / 1
Boulder, CO
Cambridge, MA / Y / N / Y / N / N / N / N / 1
Chatanooga, TN / Y
Chicago, IL / Y / N? / Y / Y / ? / Y / Y / Y / N / N / N / 1.5
Denver, CO
Detroit, MI
Houston, TX / Y / N / N / N / N / N / N / N / N / N / N / 1
Jacksonville, FL
Los Angeles, CA / ? / N / Y / Y / N / N / N / N / N / N / Y / 1
Milwaukee, WI / Y / Y / Y / Y / Y / Y / N / Y / 1.5
Minneapolis, MN
New York, NY / Y / Y / 1
Philadelphia, PA / Y / Y / Y / Y / Y / N / Y / Y / Y / Y / Y / 2
Pittsburgh, PA / Y / N / N / N / N / N / Y / Y / Y / N / Y / 1.5
Portland, OR / Y / N / Y / Y / Y / Y / Y / Y / Y / Y / Y / 2
Salt Lake City, UT
San Diego, CA / N / N / N / N / N / N / N / N / N / N / N / 1
San Francisco, CA / Y? / Y / Y / Y / N / N / Y / N / N / N / N / 1
Santa Monica, CA / N / N / N / N / N / N / N / N / Y / Y / N? / 1
Seattle, WA / Y / N / Y / Y / Y / N? / Y / Y / Y / N / Y / 2
Washington DC / N / proposed / N / Y / Y / Y / N / 1

Infrastructure Investment

Infrastructure investment refers to the physical structures that cities build to manage stormwater. Historically, cities have used gray infrastructure to rapidly convey water away from the city; now, more cities are looking towards natural drainage systems as a more sustainable system. In theory, green stormwater management works because the aggregate of strategically located systems detain, filter, and slowly release rainwater, thereby alleviating the rapid conveyance of polluted runoff towards sewers and, during heavy storms, water bodies. In other words, green infrastructure for stormwater involves applying principles of landscape ecology to the urban built environment (Ahern 2007).

One of the problems that cities struggle with is how to design for both frequent storms with relatively light levels of rainfall, and extreme storm events, which test the capacity of the sewer system and potentially lead to CSO events or flooding. In many cities that have long deferred investment in sewer infrastructure, the prospect of installing larger sewers is an expensive and slow undertaking. By contrast, green infrastructure approaches provide above-the-ground benefits and, because the scale of construction is smaller, come on line sooner. Common green infrastructure interventions include surface infiltration practices (e.g., infiltration basins), subsurface infiltration systems (e.g., infiltration trenches), gravel wetland systems, bioretention systems, water quality swales, porous pavement systems, wet ponds, and extended dry detention ponds, all of which the EPA has evaluated for performance in particular climatic conditions (EPA 2008). Although green infrastructure approaches, as an alternative, are ideal for mitigating runoff from small, frequent storm events, they generally are not sufficient to absorb runoff from extreme storm events. Under NPDES requirements for long-term CSO mitigation plans, cities must eliminate 85 percent of CSO events. To meet this requirement, many cities are exploring hybrid approaches that mix the known engineering solutions – increasing storage capacity with pipes or tunnels – to address the extreme storm events, along with natural drainage systems to soak up runoff from routine smaller storms.

Many cities are undertaking massive infrastructure projects oriented towards updating aging sewer infrastructure, and some are embracing green-infrastructure projects. On one end of the spectrum is the construction of deep tunnels and reservoirs to detain stormwater during extreme events. These systems are designed to hold excesses of stormwater runoff until the treatment facility is able to treat and release it. One city that took this more traditional route to mitigate the impact of large-scale storms is Chicago. Begun in the mid-1970s and due to be fully operational in 2019, Chicago’s Tunnel and Reservoir Plan (TARP) represents approximately $4 billion invested for the long-term management of stormwater. This solution was noteworthy for the scale of the infrastructure investment, but represents a gray or engineered approach to address CSOs, and one that took over 40 years to complete. Simultaneously, Chicago targeted local sites as points for improvement and initiated a Green Roof Program, featuring a green roof installed on City Hall. The City has established design guidelines for other on-site stormwater management approaches including permeable pavements, rain barrels, green buildings, and green roofs[dM13].

Portland, OR has a similar hybrid approach to manage CSOs and promote the health of the Willamette River. In an effort to update the sewer system, Portland built “the Big Pipe” to address the extreme weather events that cause the majority of CSOs, and has installed green infrastructure throughout the city. Supported by a climate that features regular levels of rainfall, green infrastructure projects have been very successful in the Pacific Northwest. Portland has undertaken numerous street-level projects, such as the Southwest 12th Avenue Green Street Project, which manages runoff from the street in a series of stormwater infiltration planters. In 2006, the American Society of Landscape Architects awarded the project a General Design Award of Honor, commending the design for integrating effective water management with a thoughtful and aesthetically beautiful design.