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Center Design for American Urban Landscape Design Brief, Number 3/ August 2003 Urban Green Space: Effects on Water and Climate Photo: DCAUL Regina Bonsignore, Research Fellow This design brief is part of a series entitled Taking Notice: Green Spaces in Urbanized Settings with a focus on the green spaces of center city and inner suburban communities in the Twin Cities region. Titles in this series are: The Diversity of Green Spaces Urban Green Space: Effects on Water and Climate People and Urban Green Areas: Perception and Use Plant and Animal Communities in Urban Green Spaces Mapping Green Spaces in the Center of the Twin Cities Region Design Briefs Urban Green Space: Effects on Water and Climate Regina E. Bonsignore, Research Fellow Overview By definition, an urban area has been altered and converted to accommodate people and our activities. As places like the Twin Cities grow, they become more built up, with fewer areas where soil remains and vegetation can grow. These remaining green areas, whether public parks or private lands, play a role in moderating the effects of human activities on the physical environment. This summary focuses on two areas of increasing concern, not only in the Twin Cities but nationally as well: water and climate, including air quality. Federal stormwater quality standards going into effect in 2003 will involve more cities and development sites than earlier Clean Water Act regulations. Not only quantity of runoff from sites, but also the quality of runoff coming from sites one acre and above will need to be addressed. As air quality alerts increase in frequency and duration, it is likely that this issue will be increasingly a local concern, rather than something that happens in other, far away cities. Green space has a role to play in addressing both of these issues in the Twin Cities. New types of green spaces are often part of water management strategies, as in this new boulevard through a reconstructed neighborhood in north Minneapolis. Overview of Key Points from Related Design Briefs Green spaces are outdoor settings that contain a significant amount of vegetation. The Diversity of Green Spaces includes a description of the twenty plus types of green spaces found in the center of the Twin Cities region. People share many perceptions and preferences about urban green spaces, but differ in important ways as well. Key Points Green spaces reduce the amount of water that typically overloads urban lakes and streams. Urban green spaces can enhance or protect water quality in a variety of ways, such as protecting soil from erosion while improving infiltration rates into the soil instead of the storm sewer. Trees and other vegetation ameliorate local urban heat island effects through shading, evapotranspiration, and reducing summer cooling demands. The foliage found in green spaces improves air quality by reducing particulate air pollutants and gases such as ozone. By reducing summer cooling demands, vegetation helps reduce emissions from power plants. Strategically planting, managing, and locating green spaces can extend the water and air quality benefits of urban green spaces. People and Urban Green Areas summarizes recent about perception and values. We share urban green spaces with plants and migrant or resident animals. Plant and Animal Communities in Urban Green Spaces describes important interactions between urban flora and fauna. For more detail, supporting facts, and references read on..... 2 Design Center for American Urban Landscape Photo: DCAUL Design Briefs Taking Notice: Green Spaces in Urbanized Settings Introduction to the Project This brief is one of a series and part of a larger project, Taking Notice: Green Spaces in Urbanized Settings. The goal of this project is to examine the location of green areas in relationship to community needs and the capacity of these green areas to provide ecological and social benefits. Based in the Twin Cities, the project has a focus on the core cities of Minneapolis and St. Paul, and on the first and second rings of suburban cities. However, the findings are meant to be relevant elsewhere and to help inform public discussions about how best to invest limited resources in this critical piece of the urban fabric. The Design Briefs written as part of this project each give an overview of key issues and controversies in a specific topic, illustrated by fine print facts culled from mainstream researchers in the wide variety of disciplines that study different dimensions of green spaces. They present the core findings of large bodies of research, rather than an exhaustive review. They are aimed at people who deal with green space issues but who do not have time to delve deeply into the voluminous and often difficult to read literature in the area. Such people include members of city councils, planning commissions, non-profit board members, interested citizens, and staff of parks, planning and public works departments. For this project, nature is broadly defined as a variety of outdoor settings that have significant amounts of vegetation. This use of the term reflects work on the human dimensions of open space. However, we more often use the term green space. Green space does not include water bodies, but does include their surroundings. Urban refers to the urbanized areas, both central cities and suburbs. Diagrams on the following page show the relationship of impervious surface to runoff. A variety of negative impacts occurs when too much stormwater overloads streams and lakes. The rate and volume of stormwater converging on streams and lakes can mean flooding and bank erosion. Streams lose their ability to support fish and other aquatic species because the stream channel is flushed out and stripped bare of all its instream habitat. When entering a wetland, the fluctuations of urban runoff results in wetlands that are dominated by only the most tolerant of plant species, limiting the diversity that characterizes a wetland not impacted by urban flows. A monoculture of plants can then limit the diversity of wildlife that can be supported. Low impact development or redevelopment techniques include the strategic location of green spaces to infiltrate as much water as possible into the ground rather than a storm drain. Urban Green Space Effects on Lakes, Wetlands and Streams In the metro area, green space potentially benefits water bodies in several ways. This review highlights two key elements: runoff volumes and surface water quality. Runoff Volumes Green spaces are important for reducing the amount of water that typically overloads urban lakes and streams. Due to building and paving activities, urbanization tends to harden surfaces, causing more water to rush directly into local water bodies, rather than soaking into the soil, gradually releasing into lakes, rivers, or aquifers. Runoff volumes in vegetated areas are typically between 10-20% of the average annual rainfall. In urban areas, where surfaces are highly impervious, typical runoff volumes are 60-70% of the average annual rainfall (MPCA 2000, 4.20). Design Center for American Urban Landscape 3 Design Briefs Fine Print Facts Urbanization changes a watershed s response to precipitation. The most common effects are reduced infiltration and decreased travel time, which significantly increase peak discharges and runoff (USDA 1986, 1-1). Urbanization has a greater effect on runoff in watersheds with soils having high infiltration rates (sands and gravels) than in watersheds predominantly of silts and clays, which generally have low infiltration rates (USDA 1986, 2-1). Urban open spaces lawns, parks, golf courses, cemeteries produce less runoff than all other urban districts, including residential districts. Based on tables from USDA (1986, 2.1-2.7), in an area with moderately permeable soils (Type C) in a native wood-grass area, a 2 inch rainfall would generate .3 inches of runoff. The same rainfall would produce .35 inches of runoff in a park with turf grass. In the same space, developed with 1/4 acre residential lots, 0.9 inches would runoff. If the area was a commercial district, almost all, or 1.4 inches would end up in the storm sewers leading to lakes and streams. Better Site Design: A Handbook for Changing Development Rules in Your Community discusses twenty two principles for developments that minimize the effects of stormwater runoff by reducing impervious cover, conserving natural areas and reducing stormwater pollution from new development (Center for Watershed Protection 1998). 40% evapotranspiration 10% runoff All Green, Natural Ground Cover 50% infiltration 38% evapotranspiration 20% runoff 80-90% Green 42% infiltration 35% evapotranspiration 30% runoff 35% infiltration 50-65% Green 30% evapotranspiration Runoff diagrams. Green spaces, especially those with deeply rooted plants, such as native prairie species, or a healthy tree canopy, generate less surface runoff and provide shallow and deep infiltration. Shallow infiltration may be an important source of base flow to streams and lakes, cooled and filtered through the soil. Deep infiltration recharges aquifers, particularly important to communities relying on wells that draw from aquifers nearer the surface. Evaporation rates are also higher in green spaces, providing extra cooling in the summer heat. The numbers in these figures are from Stream Corridor restoration: Principles, Processes, and Practices (FISRWG, 1998). 55% runoff 0-25% Green 15% infiltration 4 Design Center for American Urban Landscape Design Briefs Enhance or Protect Water Quality Urban watersheds are characterized by streams and other water bodies exhibiting lower quality in terms of species diversity, water clarity, and chemical composition. Because of the amount of paving, rooftops, and other impervious surfaces, rainfall in the Twin Cities is funneled into storm sewers and into our streams, lakes, and rivers, after picking up a variety of pollutants and rising in temperature. In highly urbanized areas, with impervious surfaces making up over 15-30%, down stream water bodies will likely be impaired (Arnolds and Gibbons 1996, 246). Urban open spaces reduce the overall percentage of impervious surface, a key indicator in the health of urban streams. Vegetation can moderate urban effects on water bodies in several ways. Healthy stands of vegetation protect soil from erosion, particularly important along shorelines and during construction. Plants intercept and hold a portion of water on leaves and stems and can also act as a sediment trap, preventing transport of soil particles into storm sewers and eventually water bodies. This effect is enhanced in natural areas such as forests where fallen trees and leaf litter are allowed to accumulate and more water can be retained, absorbed, and infiltrated in the spongy layers of humus. Little if any sediment is filtered by water flowing through turf grass (Binford and Buchenau 1993, citing Karr and Schlosser 1977). Green spaces with trees help shade both water bodies and the urban area in general. Warm temperatures are a limiting factor for many aquatic species. Vegetation and green spaces help reduce this effect by shading streets, parking lots and other impervious surfaces, and absorbing water into the soil where temperatures can moderate it before reaching a water body (Schueler and Holland 2000). Leaf litter can also add to nutrient loads in lakes. This fact underscores the importance keeping leaves or grass clippings out of the streets and gutters where they can be transported to downstream water bodies. Fine Print Facts A recent U.S. Geological survey study of Twin Cities streams showed fair to very poor biotic integrity due to water quality and physical habitat (Talmage et al 1999, 16). Streams with poorer water quality tended to be correlated with the percentage of impervious surface. The more impervious surface, or less green space, the lower the water quality in terms of fish species richness and diversity as well as concentrations of chloride and sodium (Talmage et al. 1999, charts on 14,15). Two ways to manage streambank. In the upper image, trees, shrubs and native plants shade the stream and stabilize the banks. The same creek, further downstream has been managed as a ditch, with little aquatic or bankside habitat. Design Center for American Urban Landscape Photo: DCAUL 5 Design Briefs Urban Green Space Effects on Climate This review focuses on the relationship between green spaces, the urban heat island, and air pollution. Generally speaking, in Minnesota, sun and wind are two factors that greatly affect our local climate. Green spaces with vegetation can moderate these effects, if strategically located. Without vegetation, effects of wind and sun are exacerbated, confirmed by the experience of walking across a barren parking lot midsummer or midwinter. Fine Print Facts ...trees in a Davis, CA, parking lot reduced air temperatures .5-1.5 degree C (Scott et al., 1999) which in turn reduced HC emissions from gasoline that evaporated out of leaky fuel tanks and worn hoses (McPherson 2001, 106). Measurement on indivdual building materials, road surfaces, and parking lots shared albedo values as low as 5%. On the other hand, parkland in full leaf may reflect as much as 20 or 25 percent of the oncoming radiation (Landsberg 1981, 60). Albedo is the ratio of reflected light to the total amount of light falling on the surface. In areas with low albedo, more light and heat is stored in the surface, increasing local temperatures. The long term warming of cities due to the heat island effect is directly responsible for increases in peak electric loads. Each degree F increase in temperature is responsible for a 1 to 2 percent increase in peak cooling loads (Akbari et al 1990) (Sand 1991). Estimates indicate that in Minnesota, strategically placed shade tree could reduce an air conditioning bill up to 25% and a windbreak could reduce annual fuel bills by up to 10 to 20%. A tree-canopied neighborhood is cooler in the summer winter and winds are cut in half. When summer temperatures are cooler, fewer air pollutants form. Thus trees create more comfortable and cleaner places for people to live (Sand 1994). Ameliorating Local Urban Heat Island Effects The well known phenomena of the urban heat island is a result of solar radiation on buildings and paved surfaces. Urban vegetation can reduce the severity of the urban heat island in several ways. Generally, vegetation absorbs some solar energy for photosynthesis and greater amounts of solar energy in order to evaporate water and cool leaf surfaces (Miller 1997). By moderating temperatures as well as shading buildings, the demand for summer air conditioning is lessened and human comfort is increased, both literally and perceptually. As discussed earlier in the water section, trees shade pavements, which in turn reduces ozone concentrations by lowering emission of hydrocarbons that are involved in the formation of ozone (McPherson 2001). Energy Conservation When we feel too hot or too cold, we use more energy to moderate temperatures in homes and businesses. Not only does this cost more for individuals, increased energy use exacerbates the heat island effect. Shade trees on the west and east of a building s windows and trees planted throughout a neighborhood are some of the most effective ways to reduce peak energy demands (Sand 1994). The HB Fuller Company, in Vadnais Heights, Minnesota, uses wide swales between parking lanes to plant vegetation that both shades the parking lots and provides filtration of stormwater runoff. 6 Design Center for American Urban Landscape Photo: DCAUL Design Briefs Air Pollutant Reduction Most information known about urban green space and pollution relates to trees and less is known about the role of other vegetation. Trees intercept particulates and the soil below absorbs these pollutants. Through several mechanisms, trees cause sedimentation of particulates onto the leaf and branch surfaces. Particulates are then washed into the soil, a sink where some may undergo decomposition and some may remain in the soils (Miller 1997). In order for this to occur, the ground below the trees must be green rather than paved, as is common in many plazas or downtown street tree situations. Gasses can be removed through absorption in the leaves of plants. Ozone, in particular, can be removed from the atmosphere by plants at a rapid rate (Miller 1997). Researchers have observed that the upper portions of a forest canopy are more effective at removing pollutants than the understory (Miller 1997 citing DeSanto et al. 1976). Recently, researchers have highlighted the role of trees in the storage of CO2, a gas that has been increasing in concentration in the atmosphere and contributes to the greenhouse effect. Trees take up CO2 during their growth processes, effectively removing it from the atmosphere (Nowak 1994). As noted in the section on the urban heat island, when strategically placed, trees can lower cooling demands in the summer. By reducing demand, power plant emissions are reduced, thereby reducing the pollutants that contribute to air pollution. Some researchers have noted that herbaceous species such as grasses and flowers absorb more gaseous pollutants than woody species such as trees and shrubs (Miller 1997 citing DeSanto et al. 1976). Photo: DCAUL In these two approaches to a business park landscape, the more diverse planting arrangement in the lower photo provides ecological benefits that also create a pleasing environment for visitors and employees. Fine Print Facts: Smith and Dochinger (1976) stated, in theory a forest could remove about one-eighth of the 03 [ozone] content of the atmosphere in about one hour (Miller 1997, 67). In the summer of 1991 the urban forest of Cook and DuPage counties (Chicago region) removed an average of 1.2 metric tons (t)/day of carbon monoxide, 3.7 t/day of sulfur dioxide, 4.2 t/day of nitrogen dioxide, 10.8 t/day of ozone, and 8.9 t/day of particulate matter smaller than 10 m (Nowak, 1994) (Miller 1997, 67) Design Center for American Urban Landscape Photo: DCAUL 7 Design Briefs Implications of Review This brief brings together several strands of research to highlight the effects of green spaces in urbanized areas. The following section describes the implications of this research for people who are involved with community planning and development, as an interested citizen or local official. These considerations are not exhaustive, but are intended to provoke thought about the many ways that green spaces can be designed or managed to improve the quality of water and climate in urbanized areas. Water quality and green space Green spaces in urbanized areas are one of the few places that rain water can infiltrate into the ground, rather than being shunted in the storm drain systems. The water quality benefits of green spaces, whether private yards or public parks, can often be enhanced, without sacrificing the other benefits they offer. These opportunities are especially possible when renovations or redevelopments are planned. Some examples are land management decisions, some are policy decisions: Planting deeply rooted vegetation, especially natives that will not require extra watering or fertilizing. Requiring only the amount of road paving or parking areas necessary for average use rather than peak times. Directing water to infiltration areas where feasible and safe. Mowing vegetation along the edges of shorelines is kept to a minimum or avoided. Shading streets, parking lots, and rooftops. Providing as much soil area as possible for planting areas to promote vigorous tree growth (McPherson 2001). Allowing leaf litter to collect under trees and shrubs, but keeping this material out of the streets where it can flow into the storm sewers. Climate Plants in urban areas can moderate the urban heat island effects, need for energy at peak times, and levels of pollutants in the air. Strategies that shade and minimize paving are also good for reducing energy needs and the urban heat island. When planning or reviewing plans for new projects, consider the following specific recommendations for improving the climate of a site, or at least not exacerbating local conditions: Siting trees to avoid solar gains in the summer and allow solar gain in the winter. Therefore, in northern climates, plant deciduous trees to shade east and west windows and walls, and avoid shading south windows (Sand 1994). Selecting medium height trees to the south, southeast, or southwest of air conditioners to shade air conditioning units or condensers (Sand 1994). Planting bands of trees perpendicular to winter s prevailing winds, typically the north and northwest (Sand 1994). The interactions between plants and the different dimensions of air pollution are complex. However, researchers have demonstrated that plants have an important role to play in moderating the air quality impacts of human activities. The following list is from W.H. Smith s Urban Vegetation and Air Quality (1978) as cited in Miller (1997, 67-68). Local ecologists favor following these recommendations using native species and plant communities where possible. 8 Design Center for American Urban Landscape Design Briefs 1. For particulate removal, species with high ratios of leaf circumference to area and surface to volume, and with leaf surface roughness, should be favored. 2. Coniferous species and deciduous species with abundant branch and twig structure provide particulate removal during winter months. 3. For gaseous pollutant removal, tree species with high tolerance for urban environments should be favored. These species have the highest probability of maximum metabolic rates and, therefore, stomatal opening. It is particularly critical to favor species with a high resistance to drought. 4. Only relatively large natural areas have significant potential to improve air quality in urban areas. The minimum width of a greenbelt may approximate 150 m but varies greatly about this depending on local conditions. 5. Urban forest density and structure influence pollutant removal capability. A balance must be struck between a stratified forest and a forest impervious to air mass movement. A multilayered forest soil, herb, shrub, and tree layers is a more effective pollutant sink than an unstratified forest. If the edge strata are overlapping and dense, however, and the stand of trees may force the air mass up and over and be a relatively ineffective sink..... 6. Mixed plantings of coniferous and deciduous species provide maximum insurance against sudden loss of function due to adverse environmental, entomological, or pathological stress imposed on one species. Additional recommendation from DeSanto et al. (1976, cited in Miller 1997): a belt of vegetation of medium density is best for gaseous pollutants, while dense vegetation is best for removing particulates. People and Wildlife Effects Fortunately, many of the ideas described above result in places that people prefer as homes or work settings and places that support native wildlife and habitats. The related publications, People and Urban Green Areas: Perception and Use and Plant and Animal Communities in Urban Green Spaces discuss these topics. Often communities make decisions about where to site public facilities such as stormwater management areas. The publication Mapping Green Spaces in the Center of the Twin Cities Region discusses how a community to locate places most in need of additional green spaces. References Akbari, H., A.H. Rosenfeld, and H. Taha, 1990. Summer Heat Islands Urban trees, and White Surfaces. ASHRAE Transactions 96:1. Arnold, Chester L., James C. Gibbons. 1996. Impervious Surface Coverage: The Emergence of a Key Environmental Indicator. Journal of the American Planning Association 62:243-258. Binford, Michael W. and Michael J. Buchenau. 1993. Riparian Greenways and Water Resources. Pp. 69-104 in Ecology of Greenways, ed. Daniel S. Smith and Paul Cawood Hellmund. Minneapolis: University of Minnesota Press. Center for Watershed Protection. 1998. Better Site Design: A Handbook for Changing Development Rules in Your Community. Ellicot City MD: Site Planning Roundtable. DeSanto, R.M. et al. 1976. Open Space as an Air Resource Management Measure, Vol. 2, Design Criteria, E.P.A.450/3-76-028b. U.S. Environmental Protection Agency. Federal Interagency Stream Restoration Working Group (FISRWG). 1998. Stream Corridor Restoration: Principles Processes, and Practices. GPO Item No. Design Center for American Urban Landscape 9 Design Briefs 0120-A. (Web location-www.usda.gov/ stream_restoration) Karr, J.R., and I.J. Schlosser. 1977. Impact of Nearstream Vegetation and Stream Morphology on Water Quality and Stream Biota. Ecological Research Series. EPA-600/3-77-097. U.S. Environmental Protection Agency. Landsberg, Helmut E. 1981. The Urban Climate. Academic Press: New York. McPherson, E. Gregory. 2001. Sacramento s Parking Lot Shading Ordinance: Environmental and Economic Costs of Compliance. Landscape and Urban Planning 57:105-123. Miller, Robert W. 1997. Urban Forestry: Planning and Managing Urban Greenspaces-2nd Edition. Upper Saddle River NJ: Prentice Hall, Inc. Minnesota Pollution Control Agency (MPCA). 2000. Protecting Water Quality in Urban Areas: Best Management Practices for Dealing with Storm Water Runoff from Urban, Suburban and Developing Areas of Minnesota. St. Paul: Minnesota Pollution Control Agency. Nowak, D. J. 1994. Atmospheric Carbon Dioxide Reduction by Chicago s Urban Forest. In E.G. McPherson, D.J. Nowak, and R.A. Rowntree, eds., Chicago s Urban Forest Ecosystem: Results of the Chicago Urban Forest Climate Project. Gen. Tech Rep. NE-186. Radnor, PA: USDA Forest Service, NEFES. Sand, Margaret. 1991. Planting for Energy Conservation in the North. St. Paul: Minnesota Department of Natural Resources. Sand, Margaret. 1994. Energy Saving Landscapes: The Minnesota Homeowners s Guide. St. Paul: State of Minnesota. Schueler, Thomas R. and Heather Holland. 2000. The Practice of Watershed Protection: Techniques for Protecting Our Nation s Streams, Rivers and Estuaries. Ellicott City MD: Center for Watershed Protection. Scott, K.I., J.R. Simpson, E.G. McPherson. 1999. Effects of Tree Cover on Parking Lot Microclimate and Vehicle Emissions. Journal of Arboriculture 25:129-141. Talmage, Philip J., Kathy E. Lee, Robert M. Goldstein, Jesse P. Anderson, Jesse D. Fallon. 1999. Water Quality, Physical Habitat, and Fish-Community Composition in Streams in the Twin Cities Metropolitan Area, Minnesota 1997-98. Water-Resources Investigation Report 99-4247. Mounds View MN: U.S. Geological Survey. US Department of Agriculture (USDA). 1986. Urban Hydrology for Small Watersheds: TR-55. Technical Release 55. Washington D.C.: USDA. Acknowledgments I would like to thank reviewers Peggy Booth, Kate Drewry, Ellen Fuge, Steve Hobbs, Jay Riggs, and Julie Westerlund for their helpful comments on earlier drafts of this publication. Design Center for American Urban Landscape 1 Rapson Hall 89 Church Street Minneapolis, MN 55455 612.625.9000 www.designcenter.umn.edu Funding for this project was provided by The McKnight Foundation. 2003 Design Center for American Urban Landscape (DCAUL) College of Architecture and Landscape Architecture University of Minnesota. Photos and drawings by DCAUL. Permission is granted for non-profit education purposes for reproduction of all or part of written material or images, except that reprinted with permission from other sources. Acknowledgment is required and the Design Center requests two copies of any material thus produced. The University of Minnesota is committed to the policy that all persons shall have equal access to its programs, facilities, and employment without regard to race, color, creed, religion, national origin, sex, age, marital status, disability, public assistance status, veteran status, or sexual orientation. 10 Design Center for American Urban Landscape

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