Green Infrastructure/LID

Introduction to Stormwater Runoff

Stormwater runoff is that part of rainfall that reaches a stream channel quickly, usually within a day or so of first falling to the ground (Booth, 1991). In an undeveloped watershed or catchment, stormwater runoff percolates into the ground and recharges rivers/streams, lakes, and underground aquifers. Runoff also evapotranspirates through surface plant life into the atmosphere (USGS, 2011b).

Booth (1991) describes two types of stormwater runoff. The first is the “Horton Overland Flow” (HOF) in which precipitation falls on the soil surface more rapidly than the soil can absorb it, resulting in the excess water flowing over the surface of the land. The HOF is affected by a number of factors including topography and surface cover.

The second method in which stormwater runoff is produced is known as the “subsurface flow regime” where rainfall intensities are generally lower than the rate at which the soil can absorb precipitation. In areas of gentle rainfall and lush vegetation, precipitation is able to infiltrate where it lands, and flows beneath the surface (Booth, 1991).

Booth (1991) states that there are two key factors that control the magnitude of stormwater runoff: basin size and land use. Most fundamental of the factors that influence discharge of stormwater is the size of the drainage basin. In large basins, the amount of runoff depends primarily on the total volume of water that is precipitated over a number of days. However, in smaller basins such as urban catchments, runoff is influenced by the rate at which water is introduced and conveyed to the outlet stream.

The second key factor that controls the magnitude of stormwater runoff is land use. In small basins, land use plays a vital role in determining how much precipitation percolates into the surface. Booth (1991) says that typically, only a small fraction of precipitation reaches the stream channel with the remainder either: 1) never reaches the ground and is evaporated off the surfaces of vegetation, 2) enters the ground but is transpired by vegetation, or 3) percolates deeply into the ground, never reaching the stream. However, a variety of land uses can disrupt the natural hydrologic cycle and can have dramatic consequences such as increasing the volume and rate of stormwater runoff in small urban catchments.

Urban Hydrologic Changes

Impervious Surfaces

Stormwater runoff is a direct effect of impervious surface cover that characterizes urban environments.  Increased urbanization has resulted in increased amounts of impervious surfaces, including roads, parking lots, roof tops, among others (Brabec et al., 2002).  The National Oceanic and Atmospheric Administration (NOAA) estimates that there are over 25 million acres of impervious surfaces in the continental United States, and that it is not uncommon for over 45% of urban land cover to be impervious (Kloss and Calarusse, 2006).

Arnold and Gibbons (1996) defined four basic qualities of imperviousness that make it an important indicator of environmental quality: (1) although the impervious surface does not directly generate pollution, a clear link has been made between impervious surface and the hydrologic changes that degrade water quality; (2) an impervious surface is a characteristic of urbanization; (3) an impervious surface prevents natural pollutant processing in the soil by preventing percolation; and (4) impervious surfaces convey pollutants into the waterways, typically through the direct piping of stormwater.

Impacts from increases in impervious surfaces


Imperviousness leads to:

Resulting Impacts


Habitat loss

(e.g., inadequate substrate, loss of riparian areas, etc.)






Increased volume






Increased peak flow






Increased peak flow duration






Increased stream temperature


Decreased base flow


Changes in sediment loadings






From EPA (2011b)

Surface Runoff

Under natural and undeveloped conditions, surface runoff can range from 10 to 30 percent of the total annual precipitation, however depending on the level of development in a given area, the alteration of physical conditions can result in a significant increase of surface runoff to over 50 percent of the overall precipitation (DER, 1999). Figure 1 shows the changes of surface runoff given impervious surface conditions.

Runoff volumes are dramatically affected by impervious surfaces. According to NRDC (1999), a one inch rainstorm on a one acre natural meadow would typically produce 218 cubic feet of runoff (enough to fill a standard sized office to a depth of nearly two feet). However, when the same storm over a one acre paved parking lot occurs, 3,450 cubic feet of runoff is produced which is enough to completely fill three standard sized offices.

Surface runoff changes


From FISRWG (1998)

Urban Flooding

Alterations to site runoff characteristics can cause an increase in the volume and frequency of runoff flows. Changes in surface characteristics can disrupt the natural hydrologic balance resulting in higher runoff velocities that can cause flooding and compromise the ecological integrity of streams (DER, 1999). Where smaller storms prior to development did not produce runoff and flooding of local streams, due to development within stream valleys and urban watersheds they now do. Hydrographs from urbanized areas show a clear distinction between flow rate/quantity and time of pre-and post-developed sites. Paul and Meyer (2001) says that the lag time, the time difference between the center of precipitation volume to the center of runoff volume, is shortened in urban catchments which leads to floods that peak more rapidly. Figure 2 shows that the flow rates in urbanized areas peak sooner with greater volumes of precipitation.

Pre- and post-development hydrograph

From Ryerson (n.d.)

The National Resources Defense Council (1999) state that “the most dramatic consequence of increases in the volume and rate of stormwater runoff is flooding and property damage.” NRDC (1999) also found that due to the increase of impervious surface covers in watersheds, a flood event that should be expected once in 100 years could occur once every five years when impervious cover reaches 25 percent, and could become an annual event in a watershed comprised of 65 percent impervious surfaces. FEMA in 1997 stated that, “about 90% of our natural disasters involve flooding in one way or another [and] as more and more land is cleared for development or paved over, there is less and less available to soak up excess water” (“No One Safe”, 1997).

Stormwater Pollution


Urban stormwater runoff is recognized as one of the greatest sources of water quality impairment for downstream waterways and aquatic ecosystems due to its high concentration of numerous pollutants (Hsieh et al., 2007; Eckley and Branfireun, 2009; Greenway, 2010).

The most common heavy metals found in stormwater runoff are zinc, copper, lead, and cadmium (EPA, 1997; Paul and Meyer, 2001). Paul and Meyer (2001) say that in addition to industrial discharges, metals found in urban catchments come from brake linings, tires, and engine parts that all contain nickel, chromium, lead, copper, and manganese. Mercury, iron, cobalt, molybdenum, and tin have also been found in elevated concentrations in urban stream sediments (Paul and Meyer, 2001). These metals accumulate on roads and parking lots and are carried with stormwater during heavy rain events. Table 1 shows the general categories of pollutants in stormwater runoff.

Other pollutants found in stormwater runoff include pesticides from lawn chemicals, hydrocarbons from oil and grease spills, and in some instances polychlorinated biphenyls (PCBs) (EPA, 1997; Paul and Meyer, 2001; Kloss and Calarusse, 2006). Though outlawed due to their carcinogenic effects, PCBs are still found in fish at concentrations exceeding consumption-level guidelines in urban rivers such as the Chattahoochee river near Atlanta, Georgia and the Willamette Basin in Oregon (Paul and Meyer, 2001).

Urban stormwater pollutants

Pollutant Source
Bacteria Pet waste, wastewater collection systems
Metals Automobiles, roof shingles
Nutrients Lawns, gardens, atmospheric deposition
Oil and grease Automobiles
Oxygen-depleting substances Organic matter, trash
Pesticides Lawns, gardens
Sediment Construction sites, roadways
Toxic chemicals Automobiles, industrial facilities
Trash and debris Multiple sources

From Kloss and Calarusse (2006)

First Flush Phenomenon

“First flush” is a term that has been used to refer to the initial period of a storm event which contains a large percentage of the total pollution found in stormwater runoff (Deletic, 1998; Taebi and Droste, 2004; Mitchell et al., 2010). Generally, the first flush is influenced by many parameters such as watershed/catchment area, rainfall intensity, impervious area, and antecedent dry weather period (Lee et al., 2002). For these reasons, Deletic (1998) finds that the first flush phenomenon is complex and site specific. Mitchell et al. (2010) say that by characterizing and treating the first flush, adverse water quality effects of runoff can be minimized while requiring less volume to be treated.

Normalized cumulative pollutant loads curve

                                                                        From Taebi and Droste (2004)

Green Infrastructure

Concept and Definition

“Green Infrastructure” at first glance may appear to be a contradiction, with infrastructure being defined as, “the substructure or underlying foundation, especially the basic installations and facilities on which the continuance and growth of a community depends” (Benedict and McMahon, 2002).  This usually conjures images of “gray” infrastructure (i.e. roads, sewers, utility lines), yet green infrastructure is instead an alternative form of infrastructure that seeks to produce economic, social, and most importantly, environmental benefits.

Green infrastructure (GI) can formally be defined as, “an interconnected network of green space that conserves natural ecosystem values and functions and provides associated benefits to human populations” (Sheladia, 1998; Benedict and McMahon, 2002; Carroll, 2007).  Because this definition is broad and all encompassing, GI can be broken down into discipline-specific contexts such as landscape ecology, wildlife biology, landscape architecture, and civil engineering.  Green infrastructure can apply to non-structural best management practices (BMPs) such as wildlife corridors, wetland preserves, and riparian buffers, and structural BMPs  including tree box filters, rain gardens, vegetated swales, open space, and urban gardens.  Engineering and architectural techniques such as green roofs and permeable pavements are also structural BMPs (Dunn, 2010).

The concept of green infrastructure was born out of a recognition that stand-alone green patches, or what we call parks, are not going to provide the full benefits of nature anytime soon.  Even in 1903, the landscape architect Frederick Law Olmstead stated that “no single park, no matter how large and how well designed, would provide the citizens with the beneficial influences of nature”, alluding to the notion that greenspaces need to be connected, rather than fragmented islands, to be effective (Benedict and McMahon, 2002).

Benedict and McMahon (2002) offer seven essential principles of green infrastructure.  The first principle states that that GI should function as the framework for conservation and development.  Principle number two says to design and plan green infrastructure before development, meaning that whenever possible, incorporate GI techniques prior to development, as it can become difficult to modify an existing development in the future.  The third principle states that linkage is key, referring to Olmsteads recognition that greenspace needs to be an ecological network.  The fourth principle acknowledges that GI functions across jurisdictions and at different scales.  The fifth principle states that GI is grounded in sound science and land-use planning theories and practices, and that initiatives should incorporate the expertise of the appropriate context, be it landscape ecology, civil engineering, or wildlife biology.  The sixth principle states that GI is a critical public investment, and that it should be funded in the same way as our current built infrastructure.  The final principle states that GI engages key partners and involves diverse stakeholders.  Community involvement is key to seeing that green infrastructure is realized and maintained for the future.

Green Infrastructure Structural BMPs

Green infrastructure BMPs are intended to treat, reduce, and delay non-point source stormwater pollution.  The EPA defines non-point source stormwater pollution as pollution that comes from many diffuse sources, unlike point source which comes from industrial and sewage treatment plants (EPA, 2011).

The purpose of structural BMPs is to collect, convey, or detain stormwater to improve water quality and/or provide a reuse function.  Lloyd et al (2002) identify numerous structural BMPs, including: diversion of runoff to garden beds, rainwater tank, sediment trap, biofiltration system, native vegetation with drip irrigation systems, porous pavements, buffer strips, dry retention system, and swales.  They say, however, that in built-up catchments, land availability often limits the BMP that can be used to control stormwater.

Bioretention systems are among the most recognized and popular alternative to conventional stormwater management. Bioretention BMPs such as bioswales and rain gardens are shallow topographic depressions filled with engineered soils and vegetation that retain, treat, and infiltrate water and successfully remove pollutants through increased contact time with soils and plant materials (HUD, 2003). Compared with conventional storm water management systems, bioretention areas more closely mimic the natural hydrologic cycle. Bioretention systems are site-solutions that treat stormwater runoff from rooftops, streets, and parking lots. Bioswales can be situated alongside streets to absorb low flows or carry runoff from heavy rains to storm sewer inlets. They can improve water quality by being designed to infiltrate the first flush of stormwater runoff and treat storm flows of pollutants. Bioswales should contain deep-rooted native plants, be designed with infiltration rates greater than one-half inch per hour, and be able to convey at least a 10-year storm event (NRCS, 2005).

Rain gardens are most commonly found on residential property, but can be used in commercial parks as well. On the surface, rain gardens look like attractive gardens that can support habitat for birds and butterflies, yet below the surface, natural processes are occurring that mimic the hydrologic action of a healthy forest. Rain gardens should be designed to drain within four hours of a one inch rain event (LIDC, 2007).

Costs and benefits vary based on size, but can range from $3 to $15 per square foot of bioretention area (EPA, 2000). Much of the costs incurred come from purchasing plants and soils for the system (HUD, 2003). Maintenance consists of changing the soil mixture and replanting vegetation.

Rain Garden


Permeable Pavement/Concrete/Asphalt

Pervious pavements are green infrastructure BMPs that allow stormwater to infiltrate into the ground or engineered medium preventing the ponding of water and other adverse effects of stormwater runoff. Pervious pavements, asphalt, or concrete provide a hard surface that can be used in parking lots, sidewalks, bike lanes, or low-congested side streets or driveways. According to CRWA (2008), permeably paved areas are typically designed to infiltrate runoff from at least a two-year storm, reducing runoff from most storms by 100%.

Pervious pavements/pavers are commonly made up of a matrix of concrete blocks or a plastic web-type structure with voids filled with sand, gravel, or soil (Brattebo and Booth, 2003), whereas pervious asphalt consists of coarse stone aggregate and asphalt binder with very little fine aggregate (PBES, n.d.). Some systems can contain an under-drain to convey to storage tanks or an outlet stream.

Costs are normally the major barrier for the use of pervious pavements/asphalts (Gunderson, 2008). Permeable paver stones and blocks range from $2 to $4, whereas pervious asphalt can range from $0.50 to $1 per square foot (EPA, 2000). When the full cost includes the underground infiltration bed, installation costs range from $7 to $15 per square foot (CRWA, 2008). Gunderson (2008) says that while pervious asphalt costs 20-25% more than conventional asphalt, a life cycle cost-analysis shows that pervious pavement systems are more economical in the long run as conventional pavement typically lasts 12 to 15 years, while pervious pavement can last more than 30 years.

Another barrier for using pervious pavements/asphalts is the perception that they do not perform well in cold climates. The literature shows that this concern is unfounded. Studies done at the University of New Hampshire Stormwater Center (UNHSC) show that the voids in pervious pavement/asphalt allow for the infiltration of snow and reduce the need for winter maintenance as permeable pavers can create the “igloo effect” (insulation) upon the subsurface medium and in turn keeps the ground warm enough to melt snow faster than conventional impervious pavement/asphalt (Gunderson, 2008).

Porous Pavers

Tree-box Filters

Tree-box filters or stormwater tree pits consist of an underground structure and above ground plantings (shrubs, trees, etc.) which collect and treat stormwater runoff from streets and parking lots. These filters can be placed alongside streets and be designed to resemble the surrounding sidewalk. They typically include a ready-made concrete box containing an appropriate soil mixture and plantings (CRWA, 2008).

Tree-box filters are designed to capture and treat smaller storms, but should be used in conjunction with other stormwater BMPs to be effective in larger storm events. Costs are generally high for tree-box filters due to their prefabricated design. Prices can range from $8,000 to $10,000 for one prefabricated system which includes filter material, plants, and sometimes maintenance (depending on the provider), and $1,500 to $6,000 for installation (CRWA, 2008).

Tree-box Filter

Green Roofs

Green roofs, also known as vegetated roofs, ecoroofs, or nature roofs, are structural components that capture, filter, and detain rainfall (MWS, 2009a). A top layer of vegetation (e.g. grass, shrubs, plants, etc.) absorbs and transpires rainfall with the remaining water infiltrating through an engineered soil.

There are two types of green roofs: intensive and extensive. Intensive green roofs have thicker soil layers (greater than 6”) and can support large plants such as shrubs and trees (CRWA, 2008). Extensive green roofs have minimal shallow layers (less than 6”) and can only support smaller vegetation such as grass, succulents, herbs, and/or mosses. Extensive green roofs require less maintenance and are the most common for pilot and retrofit projects (CRWA, 2008; MWS, 2009a).

Best BMPs

Deciding on the appropriate structural BMP is contingent on a variety of factors.  The literature tells us that BMPs can be selected based on available space and landscape characteristics (Lloyd et al., 2002), scale of government jurisdiction (Martin et al., 2007), cost of implementation (Sample et al., 2003), economic benefits (Wossink and Hunt, 2003), and social/community benefits (Wolf, 2003)

Lloyd et al (2002) say that the selection of structural BMPs should be based on maximizing flow control and/or water quality benefits relative to the costs incurred over the life of the asset(s), along with achieving premium treatment effectiveness with minimal design issues.

Battiata et al (2010) identify three steps in their Runoff Reduction Method (RRM) that help identify which BMPs should be used in a stormwater management project or plan.  They say that practices to help reduce runoff (with percent reduced) such as permeable pavements (45-75), bioretention (40-80), and dry swales (40-60) should be used.  Also, they say that in order to remove pollutants from stormwater (percent removed), practices such as grass channels (15-20), permeable pavements (25), and water quality swales (20-45) should be explored.  Lloyd et al (2002) confirm many these findings, and add that retarding basins and biofiltration systems can assist in flood management, flow attenuation, and reduce flow volume.

Martin et al (2007) used a French national study on stormwater control to illustrate a multicriteria approach at identifying appropriate BMPs.  They found that priorities varied by level of government and interest group.  They French study showed that local governments weighted capital and maintenance costs the highest, regional governments prioritized the impact on groundwater quality and pollution retention, and homeowners’ associations preferred BMPs that contributed to sustainable development and improved amenities.

Low Impact Development (LID)

“LID is a site design strategy with a goal of maintaining or replicating the predevelopment hydrologic regime through the use of design techniques to create a functionally equivalent hydrologic landscape. Hydrologic functions of storage, infiltration, and ground water recharge, as well as the volume and frequency of discharges are maintained through the use of integrated and distributed micro-scale stormwater retention and detention areas, reduction of impervious surfaces, and the lengthening of flow paths and runoff time (Coffman, 2000). Other strategies include the preservation/protection of environmentally sensitive site features such as riparian buffers, wetlands, steep slopes, valuable (mature) trees, flood plains, woodlands and highly permeable soils.”- EPA (2000)


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2 responses

14 02 2012

Please excuse the formatting. WordPress…

28 05 2012

Missing sources available upon request.

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