Put a LID on it
It’s time to think of stormwater as a natural resource. Low Impact Development (LID) offers an alternative to old drain-and-dispose techniques.
New Englanders do not generally think about water scarcity when they look around. Massachusetts receives about 45 inches of precipitation annually — an abundant water supply by national standards. In fact, people are likely to be concerned about too much water, especially when managing stormwater. Over the last century, as development grew more intensive, both in urban centers and suburban shopping areas, the major stormwater problem was how to get water off surfaces during major storms. It was, essentially, a waste-disposal problem. Accordingly, codes, technologies, and standards of practice all coalesced behind methods of moving stormwater offsite, quickly and in large quantities.
By the 1980s, people began connecting downstream flooding, erosion, and pollution problems with the high volumes of stormwater being flushed off developed landscapes. In response, new codes and regulations were established to reduce “peak flows” during storm events. Artificial ponds and other detention structures were built to temporarily store stormwater during heavy rains and release it at a pace that caused less flooding and erosion and filtered out some of the pollutants. These centralized structures relied on networks of drain pipes to collect the runoff from across the property.
Over the last decade, however, concern has been raised about stormwater’s role in a different problem: some of our rivers, streams, and wetlands are exhibiting new, dangerously low water levels between storms. In the Northeast, groundwater is the critical component of streams that enables them to be perennial (continuing to flow during dry weather). Groundwater is water contained in saturated layers of soil and bedrock below the land surface. Where these saturated layers intersect the surface of the land, water seeps out and either pools on the surface, forming wetlands, or runs downhill, forming streams. The frequent replenishment of groundwater by precipitation (“groundwater recharge”) enables a continual feed to streams (“baseflow”).
However, when forests and fields are replaced with roads, buildings, and parking lots, rain and snow have fewer places to soak into the ground. The proficient flushing of stormwater and meltwater off vast areas of pavement — a point of pride for engineers for decades — is now understood to contribute to a drop in groundwater levels. Hydrologists have long understood this connection, but as it has become clear to a wider audience, the importance of groundwater recharge has come to the fore of Massachusetts water policy. Suddenly, the paradigm of treating stormwater like waste is turned on its head. How do we treat stormwater coming off our built landscapes like the important resource it truly is?
Some answers come from looking back at how stormwater was managed before the emphasis on centralized collection. Picture a rural road — no curbs, no catch basins, no detention ponds. The road is simply crowned to shed water off to the sides, into the trees, shrubs, or grass. In retrospect, we call this, quaintly, “country drainage.” The two factors that make this design effective at groundwater recharge are decentralization and the use of planted areas as stormwater receptacles. Decentralizing the places where stormwater is directed makes maximum use of pervious area for recharge. Plants help keep soils loose, which aids infiltration. As it turns out, soils and plants are at least as good at filtering out pollutants as most structural devices designed for this purpose.
The problem, of course, is that this type of design becomes difficult to duplicate when development intensifies and the ratio of paved to unpaved surface increases significantly. But with some design and engineering ingenuity, these older practices form the underpinnings of a new approach to land development called “Low Impact Development” (LID): minimize the area of impervious surface (through cluster designs, narrower roads, shared parking areas, smaller setbacks); use permeable materials for paving (such as porous asphalt and grass pavers); and use open, decentralized planted drainage instead of curbs, catch basins, and detention ponds.
In greenfield development (conversion of forests or fields to developed use), the LID process begins by characterizing a site’s natural grading, laying out a design that uses existing low-lying planted areas for stormwater collection, and minimizing land disturbance overall. Avoiding soil compaction by heavy construction equipment is particularly important, to retain permeability of the soils. This approach contrasts with the conventional practice of beginning a project by clear-cutting and grading a site down to the known quantity of a flat, blank slate. It also means that LID projects are inherently harder to replicate in cookie-cutter fashion. This can add time and expense in the design phase, but often saves money in infrastructure and materials costs during construction. In redevelopment projects, the LID approach may have to rely more on imported soils, newly planted areas, and conversion of pavement to permeable alternatives in order to increase groundwater recharge.
In both greenfield and redevelopment contexts, studies comparing the cost of LID to conventional approaches attempt to balance the higher costs of design and specialized materials often associated with LID against the higher costs of stormwater infrastructure, land alteration, and overall area of impervious surface often associated with conventional development. A recent study from the US Environmental Protection Agency demonstrated that, project-for-project, the LID approach can usually hold its own from a profit perspective and is frequently a cost advantage for developers. As material availability, design know-how, and consumer awareness about the environmental advantages of LID expand, these cost advantages can be expected to grow.
However, LID developers currently face an additional set of hurdles that, in Massachusetts and elsewhere, can trump these advantages. They often have to factor in increased project costs associated with obtaining waivers from local boards and conducting sometimes extensive engineering studies to demonstrate to local boards and officials the advantages of the LID alternative over the “by-right” approach dictated by codes.
In recognition of the substantial obstacles posed by existing laws and regulations, Massachusetts state government has been working with stakeholders over the last several years to revise state stormwater regulations to promote LID, implement incentives, and fund demonstration projects. Government agencies have also been working with nonprofits and private-sector advocates to develop educational materials and provide technical assistance to local communities interested in becoming more LID-friendly.
Outreach efforts, especially those targeting municipal boards and decision-makers, will remain important, as Massachusetts land-use practices are still primarily determined at the local level. Simultaneously, general education is needed to help shift the public aesthetic away from some of the conventions that have come to characterize typical development, such as large lots and setbacks, wide roads, and extensive curbing.
A recent study demonstrated that the LID approach can usually hold its own from a cost perspective and is frequently a cost advantage for developers.
Perhaps just as important, however, is pursuing answers to some of the outstanding scientific questions about LID — especially those that might refine the message itself. How significant are impervious surfaces compared to other factors contributing to low flows in streams and rivers, such as over-pumping of groundwater wells and structural barriers such as dams? Are there places or conditions that are more and less appropriate for the LID approach? How will various LID techniques function in the extreme climate conditions of New England? And perhaps most importantly, how can we be sure that we are not inadvertently creating new problems as we are fixing the old, much as we discovered with the stormwater management philosophies that drove development through most of the 20th century?
New studies are starting to address some of these questions. For example, a recent state-funded analysis by the Horsley Witten Group (with Bridgewater State College) of the Taunton River watershed found that loss of groundwater recharge due to impervious surfaces accounts for a 4 percent drop in annual baseflow over the entire watershed, but caused up to a 25 percent drop in baseflow for some of the small tributaries surrounded by substantial development. A recent study by the US Geological Survey similarly identified scale as an important factor in the impact of impervious surfaces on streamflow. Using a model to simulate the impacts of extensive implementation of LID throughout the Ipswich River watershed, researchers found that even converting half of the impervious surface runoff from all developed areas of the watershed back into soil infiltration would not appreciably improve flows in the river and large tributaries. However, they found that LID could significantly improve flow in small streams in the immediate vicinity of development.
Meanwhile, new research from the University of New Hampshire Stormwater Center has reduced concerns about the effects of cold climate conditions by monitoring a variety of LID features, including porous asphalt, at a demonstration site in Durham, New Hampshire over several winters of freeze-thaw conditions, conventional road sanding/salting regimes, and normal wear-and-tear. Other studies are looking at pollutant removal rates, infiltration rates, groundwater quality impacts, and the effects of varying levels of maintenance.
Paradigm shifts are slow, but momentum is a big factor. As LID starts to enter the mainstream consciousness, the ideas will gain increasing traction and in turn be tested by time, research, and practice — a case study in science shaping politics and policy.
U.S. Environmental Protection Agency, 2007, Reducing Stormwater Costs through Low Impact Development (LID) Strategies and Practices, EPA 841-F-07-006. http://www.epa.gov/owow/nps/lid/costs07/
2 Horsley Witten Group, Inc. (forthcoming, 2010). Taunton River Watershed Management Plan – Phase II: Implementation and Demonstration. Under contract to Bridgewater State College, Bridgewater, MA with funding from the Commonwealth of Massachusetts. http://www.horsleywitten.com/tauntonwatershed.
3 Zimmerman, M. J., Barbaro, J. R., Sorenson, J. R., and Waldron, M. C., 2010, Effects of Selected Low Impact Development Techniques on Water Quality and Quantity in the Ipswich River Basin, Massachusetts: Field and Modeling Studies, U. S. Geological Survey Scientific Investigations Report, 2010-5007.
4 Roseen, R. M., et al., 2009, Seasonal Performance Variations for Storm-Water Management Systems in Cold Climate Conditions, Journal of Environmental Engineering. 135:3(128).