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The Need to Reduce Impervious Cover to Prevent Flooding and Protect Quality

May, 2010 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality

Researched and Written by Contents: Peter Flinker, AICP Dodson Associates, Ltd. Page 3. Introduction Landscape Architects & Planners 463 Main Street, Page 4. The Connection Between Impervious Cover, Flooding, and Water Quality Ashfield, MA 01330 www.dodsonassociates.com Page 7. The Impervious Cover Model (413) 628-4496 Page 9. Predicting the Future Amount of Impervious Cover Edited by Scott Millar, Chief Page 10. Impervious Cover in Rhode Island Sustainable Watersheds Office Rhode Island Department of Page 11. Applying Impervious Cover to and Water Quality Regulations Environmental Management 235 Promenade Street, Suite 330 Page 11. Reducing Imperviousness Through Land Use Planning Providence, RI 02908 (401)222-3434 Page 14. Establishing Impervious Cover Limits Through Ordinances

Revised May, 2010, based on Page 15. Incorporating Impervious Cover Management into a Comprehensive Program materials developed as part of to Protect Watershed Health the Scituate Reservoir Watershed Greenspace Protection Strategy, Page 16. Conclusions Rhode Island Department of Envi- ronmental Management, 2008 Page 18. References

Original Project Funding: Providence Water Supply Board and the National Park Service

 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality Introduction and flooding are both evident. Above 25% impervious cover, water quality impacts can be Impervious cover, also referred to as impervi- so severe that it may not be possible to restore ous surface, refers to anything that water cannot water quality to pre-existing conditions. penetrate. Ranging from residential rooftops, patios and driveways to town , public build- For towns interested in protecting water quality, ings, commercial structures and parking lots, natural ecosystems, and the prevention of - impervious cover prevents rain and snow from ing, it is very hard to measure, let alone manage, soaking into the ground, turning it into storm- the dozens of separate factors that can impact water runoff. runoff carries organic small watersheds. However, because of the well matter, fertilizers, pesticides, oil and grease and documented correlation between impervious other contaminants into our ponds and streams. cover and stream health, there is an opportunity In addition to changing the quality of the water to use impervious cover as a surrogate for both running into our waterbodies , impervious measuring and managing water quality and cover changes the quantity of runoff, eroding watershed health. By keeping overall impervi- and changing the physical structure of existing ous cover below 10%, towns can ensure that the streams. Because water runs more quickly off land will be able to absorb and filter runoff from of an impervious area, flooding becomes both developed areas and prevent excessive flooding, more common and more intense downstream. ecosystem impairment and contamination of Meanwhile, because less water is soaking into water supplies. the ground, water tables can drop and streams The impact of impervious cover is obvious near a typi- and wells fed by groundwater begin to dry up. cal commercial area: both the quantity and quality of Approximately 75% of the stream miles that runoff are impacted, and is greatly reduced. feed our waterbodies with clean water are small All of these effects can be observed in develop- headwater streams that are often small enough the in the water? Are flash spoil- ing areas of Rhode Island towns, even if there to be straddled by a child. These streams are ing gravel spawning beds? Are water tempera- is enough open land remaining to absorb the extremely sensitive to land use changes and are tures rising? Or is it some combination of fac- extra runoff and dilute the pollutant load so that therefore very susceptible to contamination. If tors? As scientists worked to understand these impacts are minimized. But at some point, the the level of impervious cover rises too high in processes, it became clear that in most cases balanced is tipped and permanent damage to these areas, irreversible damage can occur to there is a direct correlation between the degree water quality and habitat can occur. The direct drinking water quality, to groundwater supply- of water impairment and the overall amount of cause of the damage depends on the nature of ing private wells, and to aquatic wildlife habitat. impervious cover in the watershed. While it is the runoff and the particular topography, As towns grow, there will be increasing pressure difficult to predict which factor will come into and vegetation on a site. When the native trout to replace natural areas with more impervious play in any particular situation, as impervious vanish from a stream, for example, it is often cover. Even when best management practices cover rises above 10% there is almost always a very difficult to pin down the cause. Is fertilizer are widely used when attempting to mitigate measurable loss in water quality. Between 10% in the runoff creating algal blooms that deplete the impacts of impervious cover, a threshold of and 25% these impacts increase, and pollution The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality  cover and to predict future impervious cover as development continues. It describes what a variety of communities are doing to incorpo- rate impervious surface management into local zoning ordinances. Finally, it details the extent of impervious cover in Rhode Island, reviews predictions of future impervious cover and its likely effects on water quality, and lays out some recommendations for what towns can do to limit impervious cover.

The Connection between Impervious Cover, Flooding, and Water Quality

The expansion of impervious cover is part of a greater set of changes that occurs as land is converted from agriculture or forest to residen- tial or other uses. Each of these changes has an impact on what happens to rainwater once it reaches the ground. Even rural large-lot development can create long impervious roads and driveways, and mowed lawn areas can These impacts can be placed in four categories: create almost as much runoff as pavement. hydrological, biological, chemical, and physical, each of which has implications to watershed impervious cover is eventually crossed, beyond it. Furthermore, by reducing impervious cover health and our own well being. which predevelopment water quality cannot be using a range of better site design and construc- maintained. (CWP 2000) tion practices, the health of the watershed can be protected and flooding minimized even as Hydrological Now is the critical time to evaluate the impacts growth continues. of potential future development and associated Under natural forested conditions, only about impervious cover on the health of the Rhode The purpose of this report is to provide an over- 10% of precipitation runs off the surface of the Island’s Watersheds. . It is important to note that view of the scientific research on impervious site, 50% soaks into the ground, and a surprising it is not growth itself that is the problem; instead cover and to recommend tools for managing 40% is taken up by trees and other vegetation and it is the way that communities require growth to development in sensitive watersheds so that sent back into the atmosphere through the pro- occur that is the issue. By adopting more creative impervious cover is minimized. It reviews the cess of evapotranspiration (Figure 1). As roads land use techniques and development standards, literature documenting the connection between and houses are built, this ratio starts to change, towns can guide growth away from sensitive impervious cover and stream health and with runoff increasing as the amount of impervi- areas to those sites that can better accommodate describes efforts to measure existing impervious  The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality ous cover increases. For example, the total runoff volume for a one-acre is about 16 times that produced by an undeveloped one-acre meadow (CWP 2000). Therefore it’s understand- able why suburban and urban communities have more severe flooding than undeveloped areas. Because the water is spending less time on site, infiltration declines dramatically. This is a particular concern in many urban and suburban regions, where groundwater has been reduced because there is not enough rainfall soaking into the ground. The increase in runoff that occurs during this process, combined with the loss of recharge to groundwater, has dramatic impacts on streams.

Biological

The decline of biological indicators is the first sign of stream degradation, and has been the most commonly studied result of increased impervious cover. As a result of a high percent- Figure 1: Changes in site hydrology with increasing impervious cover (US EPA) age of impervious cover, naturally occurring aquatic insects, plants, and amphibians with stream degradation, ranging from 4% to Chemical decline and are gradually replaced by spe- 50%, but that biotic integrity seems to be the cies that are adapted to pollution and flooding most sensitive: “aquatic communities change Impervious surfaces such as roads, driveways (Schueler 2000). The most common standard at a level of impervious surface much lower and parking lots collect a variety of chemical measure of biological degradation is the Index than that which affects water quality measures” pollutants and hydrocarbons and discharge of Biotic Integrity (IBI) (Karr, 1987, quoted (Brabec, et al, 2002). Looking at a range of them to aquatic systems with every heavy rain. in Brabec, 2002). The IBI measures species studies, they found that impacts on overall biotic Fertilizer, herbicides and pesticides run off of richness and composition, local indicator spe- measurements were seen within a range of 3.6% lawns, and household cleaning products and cies, trophic composition, fish abundance and to 15% impervious cover; the threshold for fish even pharmaceuticals find their way into ground- fish diversity. Brabec’s team looked at several population health ranged from 3.6% to 12%, water from septic systems. The most common dozen studies and found that there was a broad and macroinvertebrate health declined between disturbance in the chemical make-up of streams range of impervious cover thresholds associated a range of 8% to 15%. is the elevated levels of nitrogen and phosphorus. Other common contaminants include calcium, The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality  sodium, potassium, magnesium and chloride and sends organic matter and silt downstream, Health Risks from salt. Metals found in streams due to creating further damage (Schueler 2000). Figure runoff include nickel, chrome, lead, copper and 2 illustrates how a stream dramatically changes There is a strong correlation between increased zinc (Paul 2001). Meanwhile, the only chemical as a watershed increases impervious cover. impervious cover and increased risk to human that should be found in healthy streams is dis- health. A variety of chronic and acute illnesses solved oxygen, which declines due to warming In another review of stream data, Paul et al. are caused by microorganisms that either are and eutrophication, or nutrient enrichment. (2001) found that channel enlargement begins swept into water bodies by increased runoff, or to be observed at only 2% impervious cover. flourish because of increased nutrient pollution. In Connecticut, Clausen et al. surveyed 15 Temperature, critical to the success of native People can contract these illnesses through direct streams and collected data on the effects of trout and other species, is also affected. A study contact or through the consumption of tainted impervious cover on stream quality. The study on Long Island found urban streams to be 5-8oC seafood. Elevated levels of nitrogen in drinking found a strong correlation between water quality higher in the summer and 1.5-5o lower in the water are associated with a number of health and percent impervious cover across a range of winter than in undeveloped watersheds (Paul risks, including methomoglobineamia, shortness contaminants, including organic residue, nitro- et al. 2001). of breath, blueness of skin (especially in infants), gen, phosphorus, dissolved chloride, and fecal along with various threats to pregnant women coliform. In each case, as impervious surface (Carson 2005). increases so does the contaminant of concern.

Physical Hydrologic, Physical, Water Quality, and Biological Impacts Associated with IC (ENSR, USEPA, 2005) In undeveloped watersheds, precipitation is slowed by vegetation and organic matter, and Hydrologic Impacts Biological Impacts stormwater runoff volumes are reduced and • Increased runoff volume (flooding) • Reduced aquatic insect diversity • Increased peak flow rates • Reduced fish diversity delayed before reaching streams and waterbod- • Increased bankfull flow • Reduced amphibian diversity ies. As impervious cover increases, so does both • Decreased baseflow • Reduced wetland plant diversity the volume and the velocity of runoff, resulting in a large volume of water reaching the stream Physical Impacts Water Quality Impacts system soon after a storm. Undeveloped wet- • Modified sediment transport • Increased sediment concentrations lands and stream systems can generally absorb • Channel enlargement • Increased nutrient concentrations flooding without extensive erosion or loss of • Channel incision • Increased trace metal concentrations • Stream embededness • Increased hydrocarbon concentrations vegetative cover. Development of impervious • Loss of large woodsy debris • Increased bacteria and pathogens cover in a watershed can happen so quickly that • Changes in pool/riffle structure • Increased organic carbon concentrations stream systems can’t adjust, resulting in erosion • Loss of riparian cover • Increased MTBE concentrations of stream banks and alteration of the stream bed, • Reduced channel sinuosity • Increased pesticide concentrations which tends to become straighter, deeper, and • Warmer in-stream temperatures • Increased deicer concentrations more U-shaped. This degrades in-stream habitat  The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality In watersheds with less than 5% impervious cover, streams are typically stable At 10% impervious cover, this stream has doubled its original size and the pool and pristine, maintaining a good pool and riffle structure. and riffle structure is lost.

This stream is approximately 20% impervious cover and shows erosion due This stream has a 30% impervious cover. The size of the stream channel has to an absence of vegetation to stabilize the bank. increased by five to ten times. Figure 2: Relationship between percent imperviousness and stream morphology (All images Copyright 1999, Center for Watershed Protection). The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality  The most common health risk is fecal coliform 50 bacteria, which are a normal occurrence within the intestinal tracts of animals. While acceptable at very low levels in recreational , coli- 40 form bacteria is very dangerous when introduced into drinking water. Coliform bacteria find Percent 30 their way into our water by way of substandard Watershed septic systems, agricultural runoff, and animal Impervious waste. The rapid runoff from impervious sur- Coverage 20 faces increases the likelihood that every heavy rain will wash bacteria-laden waste directly into streams. Similarly, waterborne parasites such 10 as Giardia and cryptosporidium become much more mobile as flooding increases in developing watersheds. 0 Protected Impacted Degraded The Impervious Cover Model

Based on a review of hundreds of studies, Tom Figure 3: Relationship between percent imperviousness and environmental quality (Arnold & Schueler and others at the Center for Watershed Gibbons, 1996, Schueler, 1992). Protection (CWP) in Maryland developed what they called the “Impervious Cover Model” (Model). The Model is based on the average considered suitable for restoration. Obviously Measuring Impervious Cover percentages of impervious cover at which stream these ranges are part of a single continuum, and quality declines, and classifies the impacts to there is much variation between regions and Using the Impervious Cover Model (Model), streams into three categories. Sensitive streams even individual streams. But the Model allows towns can evaluate the present health of their are those within watersheds which are still below streams and watersheds to be generally catego- rivers and streams, as well as predict future 10% impervious cover, where impacts are gener- rized, making management decisions clearer. impacts as growth and development continue. ally minor. These streams are relatively pristine In their “Rapid Watershed Planning Handbook” In both cases, sound decision-making requires and can be preserved through a variety of con- and other publications, CWP provides a detailed accurate measurements of impervious cover. servation measures. Impacted streams are found methodology for using the Model to more An enormous amount of research effort over within watersheds with between 10% and 25% effectively plan for the restoration of damaged the last ten years has contributed to the develop- impervious cover, and can still be restored. Non- streams and in protecting those that are still ment of efficient ways to measure impervious supporting streams are found within watersheds below the key thresholds. cover. The most accurate is a ground survey of with over 25% impervious cover, and are not existing roads, buildings, driveways, ,  The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality etc. However, this may be very expensive at of impervious cover was calculated for typical the watershed scale. Alternately, impervious development types and densities. The standards Assumptions of the Impervious Cover Model surfaces can easily be traced using aerial photog- were based on a series of research projects con- The Impervious Cover Model has proven to be raphy, but this can be incredibly time-consuming ducted in the 1960s and 1970s, many of which reliable when applied correctly – but it includes when applied to a large area. Since most states used direct measurement to establish the basic a number of assumptions (Center for Watershed have compiled detailed land-use maps that relationships. Under TR-55, residential devel- Protection, 2002): delineate various land use types in a fair amount opment at a ¼ acre lot density results in 38% of detail, planners can calculate the acreage of impervious cover; ½ acre lot, 25%; 1 acre lot, • It should be applied only to smaller 1st, 2nd land in various categories, such as commercial, 20%, and 2 acre lot, 12%. Several studies have and 3rd (low) order streams, since these are industrial, and low-, medium-, and high-density extrapolated from these percentages levels of the types of streams involved in most of the residential, and multiply it by typical averages impervious cover for larger lots, with three acres research that supports the model Most of for impervious cover. The averages, in turn, are corresponding to 8% impervious cover, 5 acres, the streams in the Scituate Reservoir Water- developed based on detailed surveys of real- 5%, and 10 acres, 2.4% (Huron River Watershed shed are low order streams. • It requires accurate estimates of percent world examples of each cover type. Council 2003). While it is difficult to reconstruct impervious area. the methods used to develop the original TR-55 • It predicts potential rather than actual steam The most common reference for relating the numbers, various recent studies have produced quality, so field monitoring is recom- percentage of impervious cover to land use type similar results (Table 1). The range of impervi- mended to confirm any predictions made. is Technical Release 55: Urban Hydrology for ousness most likely reflects differences in local • It does not predict the precise level of any Small Watersheds (TR-55; USDA 1986), which requirements for development in each zoning individual stream indicator, but rather the was developed to help engineers calculate the category (lot frontage, road width, average of a group of indicators. amount of stormwater runoff from develop- requirements, building setbacks, etc.) as well as • 10% and 25% thresholds are not sharp ment sites. As part of TR-55, the percentage differences in calculating residential density. breakpoints, but reflect a range where tran- sitions in stream quality become apparent. Minimum Lot Size Density in Dwelling Units/Acre Percent Impervious Cover • It should only be applied within the areas where it has been tested, which includes the 10 Acres 0.1 2.4b Northeast. 5 Acres 0.2 5b, 8e • It has not been validated for non-stream 3 Acres 0.333 8b, 7e (2-3 acres/unit) conditions like reservoirs and groundwater 2 Acres 0.5 12a, 15c, 13-16d (1-2 units/acre) aquifers. 1 Acre 1 20a • The relationship between impervious a d (2-4 units/acre) cover and stream quality is weakest in .5 Acre 2 25 , 22-31 watersheds with less than 10% impervious a .25 Acre 4 38 cover. Under 10%, other factors such as Table 1: Percentage of Impervious Cover Associated With Various Residential Lot Sizes the amount of forest cover, continuity of vegetated stream buffers, soils and land use a d Source: USDA Conservation Service (USDA, 1986). Source: Alley and Veenhuis, 1983. activity can play a significant role. b Source: Huron River Watershed Council (Wyckoff, et. al, 2003). e.Source: Prisloe, et al, 2000. c Source: Taylor, 1993. The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality  Fortunately, help is available for Rhode Island ordinance and development regulations. Build- towns needing more accurate measurements of out analyses help towns plan for additional impervious cover. Rhode Island contracted with schools and town services and used to model the Sanborn Map Company to develop a state- the amount of impervious cover that is likely to wide impervious cover dataset that is available be created. through RIGIS. Sanborn used color photogra- phy acquired as part of a statewide orthophoto However, most build-out analyses do not have mapping initiative. A semi-automated process the level of detail where new roads, houses and was used where the computer is programmed driveways are shown. Therefore, researchers to recognize impervious surfaces.. The great have attempted to define how much impervi- advantage of this process is its speed, cost, and ous cover is likely to result from development high resolution (two feet), which allows roads, under the common range of lot sizes and zoning buildings and parking lots to be shown in great densities, using averages from TR-55 and other detail. sources as multipliers. Logically, the results are most accurate where future development is The Providence Water Supply Board is using an similar to the studies from which the original even more accurate survey process with a resolu- numbers were developed. Towns can look at tion as high as six inches. Called LIDAR, for development within their own borders and calcu- “light detection and ranging”, it uses the same late their own percentages. For example, TR-55 principal as radar. Housed in an airplane, the predicts that a development with two acre lots LIDAR instrument sends pulses of laser light, will result in 12% impervious cover. We can test at up to 100,000 pulses per second, down to the this with a simple example: if zoning requires ground surface. The light, which can penetrate LIDAR survey showing impervious roads, houses and a two acre lot and 200 feet of frontage, and we clouds and foliage, reflects back to a collector other structures in a corner of Scituate. assume that the dwelling is built 100 feet from which registers the differences in each pulse. the front lot line, we could predict the following The result is an accurate survey of surface fea- predict, based on hundreds of scientific studies, amount of impervious cover: tures, from which it is fairly easy to separate out whether a particular stream or subwatershed impervious cover. is likely to decline in health in the near future. House and garage: 3000 square feet. However, in order to manage that watershed, you Patios, pool, outbuildings, etc: 2000 s.f. Predicting the future amount of impervious also need to know what is likely to happen as (100’ x 12’): 1200 s.f. cover development continues in the watershed. Many Sidewalk (200’x 5’): 1000 s.f. towns have prepared “build-out” analyses, for Roadway (30’x 200/2): 3000 s.f. The measurement of existing impervious cover example, which predict the number of future Total: 10,200 s.f. is relatively straightforward if you have the right homes and businesses that are theoretically information and enough time to use it. Knowing possible if all of the buildable land is developed 10,200 s.f./ 2 acres (87,120 s.f.) the amount of impervious cover allows you to to the maximum allowed by the town’s zoning = 11.7% impervious cover 10 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality This demonstrates that there are multiple factors achieve water quality objectives beyond simple lot size/density that affect the in the larger rivers and coastal amount of impervious cover. Because the size waters they feed. These headwater of houses and any additions tend to be driven by streams are extremely vulnerable the marketplace rather than regulations, towns to direct and indirect impacts of will have little control over the amount of imper- land use. Most headwater streams vious cover they create. Moreover, lawn area is are located in areas outside of the considered to be semi-impervious and does not USB in subwatersheds that have a function as well as undisturbed natural areas in small amount of impervious cover infiltrating precipitation into the ground (Schuler and are considered to be healthy. 2000). What towns can control are elements However, headwater streams codified in zoning and regulations, are less likely to be adequately such as lot width, setbacks, roadway width and protected from new growth by sidewalk standards. To define the percentage of Federal, State and local regula- impervious cover associated with each zoning tions than larger stream and river district, towns can use a similar calculation to systems (CWP, 2007). Moreover, predict the impact of required road width and Federal and State regulations lot frontage on the amount of impervious cover provide no authority for state associated with each building lot. For towns agencies to regulate impervious with GIS capability, this can be compared with cover at a watershed scale, nor the actual amount of impervious cover measured to effectively control many of in areas that are already fully developed. the diffuse non‑point pollution sources directly related to land Impervious Cover in Rhode Island use.

Rhode Island is one of the most densely popu- Extensive research has enabled scientists and engineers to empiri- lated states in the Country resulting in a state- Figure 4: Percent Impervious Cover in the State of Rhode cally relate how watersheds with wide average impervious cover of 12% (Figure Island with Urban Service Boundary 4). However, Figure 4 also shows large areas high percentages of impervious achieve more efficient (“smarter”) future growth. outside of the urban services boundary that are cover exhibit negative impacts to hydrology and The area within the USB has an impervious 4% impervious cover, which is below the 10% water quality, reducing or destroying aquatic cover of 25%. Therefore from a water quality threshold. The urban services boundary (USB) habitat quality and biodiversity (CWP 2000, and smart growth perspective it makes sense to was established by the Statewide Planning Pro- Coles 2004, Morse 2003). It has been docu- encourage future growth within the USB. gram as part of the State Land Use Plan to recom- mented that, for a given amount of new devel- Research clearly indicates that the protec- mend where future growth should be directed to opment, higher density can significantly reduce tion of small headwater streams is essential to impervious cover on a watershed basis compared The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality 11 to the same amount of new homes and businesses and specific zoning densities was difficult to pin laid out in a low‑density development pattern A recent study in Chesapeake Bay determined down. The many variables of soil, topography that new development was increasing nutrient (EPA 2006). Therefore the current Rhode Island and land use introduce many uncertainties in and sediment loads at rates faster than restoration trend of large-lot, low-density development efforts were reducing them. Developed lands establishing regulations that are fair to all land- with segregated uses is creating significantly within the Chesapeake Bay Watershed contribute owners in a town. However, as we have seen, more impervious cover than necessary to meet less than one third of the Bay loading of pol- advances in data collection and mapping are our growth needs. More creative land use tech- lutants, but would require approximately two making it easier to arrive at more defensible con- niques such as conservation development and thirds of the overall restoration costs. This study clusions about what level of impervious cover is compact mixed use (village-style growth) can concluded that the most cost-effective approach acceptable while water quality as well as public significantly reduce impervious cover. to reverse the trend of increasing pollutant loads health, safety and welfare is still protected. from new development was by forming strong Applying Impervious Cover to Land Use and partnerships with communities to encourage Reducing Impervious Cover through Land Water Quality Regulations them to adopt and implement more environ- Use Planning mentally sensitive development techniques and reduce impervious cover. -EPA 2007 Given the clear usefulness of impervious cover Community zoning and subdivision regula- as a predictor of water quality and ecosystem tions and the patterns of land use they forge are and sedimentation. Therefore, communities do health, why is it not used more often in land directly responsible for the amount of impervi- have the authority, if they choose to exercise use regulations? The answer is that in order to ous cover in a watershed. Generally speaking, it, to establish reasonable standards via zoning comply with constitutional protections of private as density increases, the amount of impervious regulations to protect water quality. property rights, zoning and other regulations cover also increases. However, the overall pat- must justify the use of the “police power” of tern of development is also important. Although In order to use zoning and other regulations to the state, which is “the power of government to a few rural communities have residential zoning directly control the amount of impervious cover, intervene in the of private citizens for the densities that would, at build-out, keep impervi- towns will have to demonstrate that defensible, protection of public health, safety and welfare” ous cover below a 10% threshold, the large-lot objective standards have been established to (The Practice of Local Government Planning, zoning currently used to accommodate growth guide the administration of such regulations. 2000). The State Zoning Enabling Act specifies requires houses to be far apart, creating unnec- Therefore, any standard for impervious cover exactly how towns may exercise their police essary impervious cover and encouraging more must be backed up with the objective data cited power. Section 45-24-30 of the Rhode Island off-site impervious infrastructure, such as roads in this report. Zoning also needs to be supported Zoning Enabling Act allows communities to and parking lots (US EPA 2006). Moreover, by the town’s comprehensive plan, which com- provide for orderly growth and development many of the surfaces remaining after large-lot bines data analysis and mapping with a process by recognizing in part “the natural characteris- development that are believed to be pervi- of public deliberation and collective decision- tics of the land, including its suitability for use ous actually behave like impervious surfaces. making. based on soils characteristics, topography, and Research indicates that the volume of runoff susceptibility to surface or groundwater pol- from highly compacted lawns is almost as high Until very recently, the science behind impervi- lution”. It also allows communities to control as from paved surfaces (Shueler 1995, 2000; ous cover analysis was extensive, but the con- surface and groundwater pollution, USDA 2001). Lawns clearly do not function as nection between the amount of impervious cover 12 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality well as undisturbed natural areas and can contain in Figure 5, that if contaminants such as nutrients, herbicides and a watershed is built pesticides. Unlike undisturbed natural areas, out at a one housing development of a typical large lot subdivision unit per acre density, changes pre-existing hydrology due to site grada- the total impervious tion, the removal of topsoil, and cover will be 20%, by heavy equipment (Schueler 1995, 2000). significantly above Due to these development impacts, even if you the 10% threshold reduce impervious cover by limiting the number for protecting water of houses, you cannot compensate for the loss quality. However, if 1 unit per acre 4 units per acre 8 units per acre of watershed services that were provided before the same number of development. The is to maintain the houses that would overall density allowed by current zoning while be allowed at the encouraging the use of more compact growth one acre density techniques that can reduce impervious cover on were clustered on a per unit basis. smaller lot sizes of Figure 5: Overall impervious cover for the watershed decreases as site four and eight units density increases given the same amount of growth (USEPA). Village Development per acre, the imper- vious cover for the watershed is reduced to 9.5% tional scenario. Development or redevelopment within a compact and 8.1%, respectively. Although the impervious Continuing this analysis at the town and water- mixed-use village is the most efficient means to cover for the particular clustered development is shed scales, the study found that the results were reduce impervious cover on a per unit basis. In higher than the one dwelling per acre scenario, consistent regardless of the size of the area: their 2006 report “Protecting Water Resources the overall watershed impervious cover is greatly With Higher-Density Development”, the US reduced and is maintained at a level that protects • “For the same amount of units, higher- Environmental Protection Agency reinforces water quality. Compact development yields density development produces less runoff this common-sense approach to accommodat- less impervious cover on a per unit basis since and less impervious cover than low-density ing development in sensitive watersheds (www. most of the impervious cover is related to the development; and epa.gov/smartgrowth/pdf/ protect_water_higher_den- transportation infrastructure (roads, driveways • For a given amount of growth, lower-density sity.pdf). There is no need to stop development and parking lots) needed to support growth. large-lot development impacts more of the outright, or even to reduce the overall number Transportation-related impervious cover typi- watershed.” of proposed dwelling units. Rather, towns can cally comprises 65-70% of the total impervious have the benefits of both growth and conserva- cover associated with development (CWP 2000). These findings show that large lot development, tion if development is directed to the areas best Obviously, the key to taking advantage of this even though it can reduce impervious surface suited for it. reality is to increase densities in some areas, on an individual property, it is not the preferred while maintaining the same overall number of strategy for reducing flooding or protecting water The EPA study demonstrated, as can be seen new units that could be built under the conven- resources on a town or watershed level. There The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality 13 will be far less negative impact on the watershed if the same number of units are concentrated together in compact neighborhoods and village centers. Moreover, by greatly reducing roads, utilities and other infrastructure costs, this approach can be profitable for developers while reducing house prices for consumers.

This approach can be the foundation of a “smart growth” strategy for our communities, in which a mix of residential, commercial and community uses are combined in traditional towns and vil- lages instead of sprawling over the landscape. The result is a walkable, liveable community surrounded by open space, with low energy costs and assured water supplies. As the market for large houses in the countryside decreases, developers and consumers are both returning to traditional neighborhoods, which can offer Conservation Development allows for growth to continue, even while important features of a site are protected. In a range of house styles and prices, along with reducing the amount of pavement per unit, it also allows for significant reductions in impervious cover. various shared amenities. Furthermore, travel land use technique that allows a community The Conservation Development design process to work, school, stores, and recreational areas to guide growth to the most appropriate areas is an ideal way to continue development while is reduced, saving gas and further helping the within a parcel of land to avoid impacts to the protecting the headwater streams that are most environment. These are the communities that environment and character-defining features of vulnerable to the impacts of runoff from imper- will be able to weather the impacts of climate the property. Typically, the same number of units vious surfaces. Just as important, by reducing change and energy costs, and adapt to changing as permitted under conventional development the overall footprint of development, these proj- economic circumstances. are built on a portion of the property, leaving ects can significantly lower the total amount of at least 50% of the site as permanent, protected impervious cover across the watershed. As part Conservation Development open space. Conservation Development is a of the Scituate Reservoir Source Water Assess- step-by-step design process that begins with ment, University of Rhode Island Cooperative Zoning that limits the overall percentage of analysis of the site and its context. The most Extension compared predicted impervious cover impervious cover is useful, but should be com- environmentally sensitive areas as well as those under conventional development to that created bined with Conservation Development ordi- areas most valuable for public recreation, etc. by more compact conservation development nances that provide design flexibility and other are identified first, then streets and building sites projects. Their projections showed an increase incentives to encourage village-style develop- are laid out in a way that avoids these important in impervious cover across the watershed from ment. Conservation Development is a creative resource areas. 3% (existing) to 7% (future), enough to cause 14 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality impairment of aquatic in many streams and (principal and accessory uses) including Resource Protection Area ordinances that include other waterbodies. If 50% of future development paved, impervious, or traveled surfaces shall overall limits on impervious cover (Kaufman & follows the Conservation Development process, not exceed: Brant 2000). Functioning as zoning overlay overall impervious cover can be reduced to 5%, a. 15 percent of the total land area for lots districts, these ordinances limit the amount of a significant benefit, particularly if Conservation less than 2 acres, impervious cover to a maximum of 10% to 20% Development is applied in the most sensitive b. 0.3 acres for lots between 2 acres and 3 in (sensitive aquifers, wellhead, recharge and areas. This does not include the additional ben- acres [about 12%], and reservoir water resource protection areas- ). In efit of reducing lawn area, which would be an c. 10 percent for lots 3 acres and larger.” the reservoir district, new single-family devel- anticipated result of smaller house lots. opment is limited to a maximum impervious This recognizes that while smaller lots have a cover of 10%, which is calculated to equate to a Establishing Impervious Cover Limits higher percentage of impervious cover, they have gross density of two to three dwellings per acre. through Zoning Ordinances a lower average percentage of impervious cover Based on this model, Kaufman & Brant proposed per dwelling unit. watershed zoning for the entire Christina River Historically, community zoning regulated the Basin, covering parts of Maryland, Pennsylva- amount of development that could be located in The ordinance limits impervious cover in nia and Delaware (the latter containing the only a given area, but ignored the transportation com- business districts to a maximum of 25%. In all drinking water supply streams in Delaware, its ponent needed to support development. Since cases, lot coverage is defined as: only six trout streams, and a growing popula- impervious cover associated with transportation tion of 400,000 people). The zoning ordinance (roads, driveways and parking) can be 60-70% “the percentage of the lot, which is covered would characterize each portion of the watershed of the total impervious cover, towns and county by structures including (but not limited as urban, suburban or open space district, and governments around the country have started to to) buildings, swimming pools, swimming limit impervious cover to corresponding ranges incorporate limits on impervious cover into their pool equipment, decks, porches, patios, of 40-55%, 20-35%, and 10-15%. regulations. Most commonly, these have been sports courts, chimneys, air conditioning adopted in specific overlay districts, typically equipment, generators, utility meters, In an example from North Carolina, Moore to protect public water supplies or groundwater transformers, above ground propane tanks, County has established impervious cover limits recharge areas. Increasingly, towns are adopt- and most man made impervious surfaces. for zoning overlay districts associated with water ing zoning regulations to limit impervious cover Driveways, parking areas, and parking lots supply watersheds. (http://www.co.chatham.nc.us/ across entire communities. are included in the lot coverage calculation dept/planning/planning_dept/watershed_review_board/ whether or not they are paved. Pedestrian supporting_documents/10-70_Rule/Planning_Board_10- 70/moore_cnty_watershed.pdf) The Town of Washington, Connecticut has walkways are included unless they are established maximum lot coverage requirements made of pervious materials such as gravel, The county has four different densities within within its zoning regulations to limit impervious pea stone, or randomly spaced stones set in these overlay zones: cover. The ordinance states: grass.” (http://www.washingtonct.org/zoning.pdf) • One dwelling unit per 80,000 s.f., or 6% “In residential districts, the maximum land On a larger scale, New Castle County and the maximum built-upon area coverage for all buildings and structures City of Newark in Delaware have adopted Water • One dwelling unit per 40,000 s.f., or 12% The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality 15 maximum built-upon area sensitive locations. The system has withstood strategies often involve partnerships with • One dwelling unit per 20,000 s.f., or 24% numerous legal challenges, and seems to have landowners, towns, non-profit conservation maximum built-upon area strong support in an area where natural springs groups and state agencies. • One dwelling unit per 14,000 s.f., or 36% and watercourses can disappear quickly when maximum built-upon area their recharge areas are paved over. 3. Aquatic Buffers: Stewardship of the area where land and water meet is critically The “built-upon area” includes “that portion of Incorporating Impervious Cover Manage- important, especially in rural watersheds. a development project that is covered by imper- ment into a Comprehensive Program to Pro- vious or partially impervious cover including tect Watershed Health 4. Better Site Design: Innovative approaches building, pavement, gravel areas (e.g. roads, to the design of subdivisions, parking lots, parking, lots, paths), recreation facilities (e.g. It is important to keep in mind that like any regu- streets and buildings can greatly reduce tennis courts), etc.” The interesting thing about latory strategy, zoning limits on impervious cover stormwater impacts. this ordinance is that it ties watershed protec- must be supported by larger planning, education tion to traditional lot size/zoning density with and outreach efforts, as well as specific design 5. Erosion and Sedimentation Control: Con- a cap on impervious cover. Within the 40,000 practices that can mitigate the impacts of devel- trol of erosion and unnecessary site distur- and 80,000 s.f. zones particularly, developers opment. The Center for Watershed Protection bance during the relatively short period when will have to substantially decrease road widths (CWP) has compiled a useful list of Watershed a project is under construction can prevent and other impervious surfaces to remain under Protection Strategies ranging from watershed permanent impacts to water quality. the 12% and 6% impervious caps. These per- planning to site design (CWP 1998): lot impervious cover ratios must be used with 6. Stormwater Best Management Practices: caution since they would preclude a mixed-use 1. Watershed Planning: Establishing the A range of BMPs, from simple grass swales compact growth development that would sig- location and amount of existing and poten- to complex filtering systems, are increas- nificantly decrease the percentage of impervi- tial future development, and the impervious ingly applied to reduce the impacts of new ous cover on a per unit basis and help to reduce surface that results, is the foundation of any construction and to mitigate existing prob- overall impervious cover within a watershed. watershed protection effort. The planning lems. process involves data collection and analysis, Since the early 1980’s, the City of Austin, Texas testing and selection of management alterna- 7. Non-Stormwater Discharges: Discharge has been adopting watershed ordinances that tives, and creation of policies and strategies from private septic systems, municipal are customized for 45 watersheds within the to guide implementation. wastewater plants and other sources is regu- city planning area (www.ci.austin.tx.us/news/ lated by state and federal agencies, but often sg_history.htm). Impervious cover limits range 2. Land Conservation: Conservation efforts without reference to an overall plan for the from 15% in sensitive resource areas to up to include identifying critical habitats and watershed. 30-40% for more urban watersheds. A provi- aquatic corridors and the water sources and sion for transfer of impervious cover rights recharge areas that sustain them, as well (TICR) allows for increased impervious cover as cultural and historic landscapes that are in appropriate areas through transfers from more important to a sense of place. Protection 16 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality Conclusions the specific characteristics of their streams and Finally, towns should require the use of design watersheds and adopt thresholds appropriate for and construction practices that reduce runoff at While the immediate cause of stream degrada- their community, ranging from 4% in sensitive the source. Known as “Low Impact Develop- tion varies from place to place, including physi- areas to 10% where there are more favorable soil ment” (LID), these include a suite of techniques cal changes and flooding, loss of habitat and conditions. However, on the watershed scale, the that catch stormwater as it flows off roofs and biodiversity, and chemical contamination, there 10% threshold can be a valuable benchmark. driveways and divert it to areas where it can soak is a consistent correlation between rising levels back into the ground. Combined with low over- of impervious cover and stream degradation. As a practical matter, using impervious cover all density and the use of Conservation Develop- While you may not be able to predict exactly as a surrogate for all of the various water qual- ment to provide flexibility in locating new roads what will happen to a stream when impervious ity impacts allows towns to avoid the cost of and homes on a site, LID has the potential to cover exceeds 10%, you can be fairly certain that tracking dozens of indicators independently. keep runoff from most storms entirely within negative impacts will result. By keeping impervious cover below the thresh- the boundaries of a site, mimicking the site’s old where flooding occurs or water quality is pre-development hydrology. Close attention to impervious cover when for- impacted, towns can avoid having to pursue mulating planning and management strategies costly mitigation when it may be too late to Recommendations for Rhode Island Towns: has great potential for helping Rhode Island provide much benefit. towns prevent flooding, protect low-order or • Encourage growth to be located within the small streams and ultimately the health of its Keeping the level of impervious cover in a Urban Services Boundary wherever pos- watersheds. Lacking data about physical or watershed below 10% will establish an over- sible. biotic health of small streams, communities have all safeguard for the watershed, but it won’t • Promote compact mixed-use development to a hard time knowing which are the most sensi- prevent some degradation of the most sensitive accommodate larger numbers of dwellings in tive or most important to protect. Towns can use resources, especially if development occurs a traditional village context, which reduces impervious cover to assess threats to sensitive in unsuitable areas. Therefore, towns should the amount of impervious cover per unit of streams and to prioritize conservation activities. combine overall limits on impervious cover with development. Managing the impact of incremental growth on policies and regulations that promote Conserva- • Adopt 10% as the upper limit of impervious those resources may be harder, but by predicting tion Development, which allows developers the cover as a policy to be incorporated into the future levels of impervious cover at build-out, flexibility to cluster new homes and businesses Comprehensive Plan, where applicable. towns can predict whether future growth will on the portion of the site that is best suited for • Make a strong policy connection in the likely impact ecosystems or water supplies. development, while providing conservation Comprehensive Plan between the need to buffers around sensitive areas. Another benefit limit impervious cover and the designated The inclusion of impervious cover in the regula- of Conservation Development is that it reduces zoning density for various areas. tory process at the municipal level must be used road, sidewalk and driveway lengths, further • Adopt Conservation Development ordi- carefully. Towns need to establish a clear link limiting the amount of impervious cover on a nances that reference the town’s impervi- between specific impervious cover thresholds per unit basis. ous cover policy and make it clear that and the protection of public health, safety and calculations for conservation developments welfare. Indeed, communities should look at should be based on the average percentage The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality 17 of impervious cover across the entire site. Require the amount of impervious cover per dwelling unit be available when comparing conventional and conservation subdivision plans. • Adopt the better site design practices from the Rhode Island Community LID Guidance Manual (DEM 2010) to more effectively reduce impervious cover from all develop- ment.

18 The Need to Reduce Impervious Cover to Prevent Flooding and Protect Water Quality References

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