Urban Stormwater Management in the United States (2009) / Chapter Skim
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3 Hydrologic, Geomorphic, and Biological Effects of Urbanization on Watersheds
3 Hydrologic, Geomorphic, and Biological Effects of Urbanization on Watersheds
Pages 129-256
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From page 129... ...
The chapter then discusses the characteristics of stormwater runoff, including its quantity and quality from different land covers, as well as the characteristics of dry weather runoff. Finally, the effects of urbanization on aquatic ecosystems and human health are explored.
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Parks include outdoor recreational areas such as municipal playgrounds, botanical gardens, arboretums, golf courses, and natural areas. Undeveloped lands are private or publicly owned with no structures and have a complete vegetative cover.
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The United Nations Population Division estimates suggest that the BOX 3-1 The Role of Tree Cover in Residential Land Use Figure 3-1 shows two medium-density residential neighborhoods, one older and one newer. Tree canopy is obviously different in each case, and it may have an effect on seasonal organic debris in an area and possibly on nutrient loads (although nutrient discharges appear to be more related to homeowner fertilizer applications)
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132 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES BOX 3-1 Continued FIGURE 3-1 Two medium-density residential areas (no alleys) ; the area below is older.
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The U.S. Geological Survey (USGS, 1999)
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Land-Cover Characteristics in Urban Areas As an area urbanizes, the land cover changes from pre-existing rural sur
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. Directly connected impervious area includes impervious surfaces which drain directly to the sealed drainage system without flowing appreciable distances over pervious surfaces (usually a flow length of less than 5 to 20 feet over pervious surfaces, depending on soil and slope characteristics and the amount of runoff)
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These directly connected roofs have much more of their runoff wa ters reaching the receiving waters than do partially connected roofs, which drain to pervious areas. A directly connected roof drain A disconnected roof drain (drains to pervi ous area)
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Contaminated paved storage area at vehicle Heavy equipment storage area on concrete junk yard surface Streets. Streets in municipal areas are usually paved and directly connected to the storm drainage system.
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Landscaped and Turf Areas. Although these are some of the only true pervious surfaces in municipal areas, disturbed urban soils can be severely compacted, with much more reduced infiltration rates than are assumed for undisturbed regional soils.
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when examining the effects of urbanization on stormwater quantity and quality. Runoff from disconnected impervious areas can be spread over pervious surfaces as sheet flow and given the opportunity to infiltrate before reaching the drainage system.
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of continuously recorded statistics on land cover and impervious surfaces derived from satellite images -- information which has been incorporated into the FIGURE 3-4 Example of a high resolution panchromatic aerial photograph of an industrial area used for measurements of ur ban surfaces.
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The general conclusion of many land-use and land-cover studies is that in urban areas, the amount of impervious surfaces has increased since the early years of the 20th century because of the tendency toward increased automobile use and bigger houses, which is associated with an increase in the facilities necessary to accommodate them (wider streets, more parking lots, and garages)
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, the major land cover was found to be landscaped areas, subdivided into front- and backyard categories, while 25 percent of this land-use area is covered by impervious surfaces broken down into three major subcategories: roofs, streets, and driveways. The subareas making up each land use show expected trends, with roofs and streets being the predominant directly connected impervious covers in residential areas, and parking and storage areas also being important in commercial and industrial areas.
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Impervious Cover (%)
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Therefore, information on the frequency distribution of storm events and properties is an important aspect of understanding the distribution of pollutant concentrations and loads in stormwater discharges. In northern climates, runoff production from precipitation can be significantly delayed by the accumulation, ripening, and melt of snowpacks, such that much of the annual load of certain pollutants may be mobilized in peak flow from snowmelt events.
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. The average intensity is 0.045 mm/minute, quite a bit lower than the peak intensity, since the storm duration is punctuated by periods of low and no measurable precipitation.
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are typically not recorded except at primary weather service meteorological stations, while daily precipitation records are more extensively collected and available through the Cooperative Weather Observer Program (http://www.nws.noaa.gov/om/coop/)
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For example, the second highest value on record at an observing station may occur in the same year as the highest value on record but will not be included in the annual maximum series. The design precipitation depths determined from the annual exceedance series can be adjusted to match those derived from an annual maximum series using empirical factors (Chow et al., 1988; NOAA Atlas data series, see http://www.weather.gov/oh/hdsc/currentpf.htm, e.g., Bonnin et al., 2006)
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An increased frequency of intense precipitation events will shift depth-frequency-duration curves for a given location, with a given return period being associated with a more intense event. Alternatively, the return period for a given intensity (or depth)
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Precipitation Depth-Duration-Frequency - Phoenix Airport 3.5 3 Precipitation Depth (in) 2.5 min 5 2 min 15 min 60 1.5 hr 6 hr 24 1 0.5 0 0 10 20 30 40 50 60 70 80 90 100 Return Period (years)
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BOX 3-5 Climate Change and Stormwater Management An ongoing report series issued by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research summarizes the evidence for climate change to date and expected impacts of climate change, including impacts on the water resources sector (http://www.climatescience.gov/)
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Examples of design storms include the 24-hour, 100-year event for flood control and the 24-hour, 2-year event for channel protection. The magnitude of the design storm can be derived from data at a single gauge, or from synthesized regional data published by state or federal agencies.
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Watershed characteristics that influence the relative dominance of surface versus subsurface flowpaths include infiltration capacity as affected by land cover, soil properties, and macropores; subsurface structure or soil horizons with varying conductivity; antecedent soil moisture and groundwater levels; and the precipitation duration and intensity for a particular storm. The distribution and activity of flowpaths result in changing patterns of soil moisture and groundwater depth, which result in patterns of soil properties, vegetation, and microbial communities.
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For example, restored streams typically lack any gauged streamflow or water quality information prior to or following restoration. This makes it very difficult to assess both the potential for successful restoration and whether project goals are met.
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As the percent of the landscape that is paved over or compacted is increased, the land area available for infiltration of precipitation is reduced, and the amount of stormwater available for direct surface runoff becomes greater, leading to increased frequency and severity of flooding. Reduced infiltration of precipitation leads to reduced recharge of the groundwater reservoir; absent new sources of recharge, this can lead to reduction in base flow of streams (e.g., Simmons and Reynolds, 1982; Rose and Peters, 2001)
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Thus, a functional way of partitioning urban areas is by the nature of the impervious cover and by its connection to the drainage system, underlying the differentiation of total impervious area and effective impervious area discussed in Box 1-2. As examples of how runoff changes with urbanization, Figure 3-12 shows
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shows a much greater variability in flow that is dominated by impervious surface runoff, and a dampened response to the drought because natural groundwater flow is a much smaller component of the total discharge. The percentage of time a discharge level is equaled or exceeded is displayed by flow duration curves, which show the cumulative frequency distributions of flows for a given duration.
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It is interesting to note that the Rv values are relatively constant until values of directly connected impervious cover of 10 to 15 percent are reached (at Rv values of about 0.07 for sandy soil areas and 0.16 for clayey soil areas) -- the point where receiving water degradation typically has been observed to start (as discussed later in the chapter)
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FIGURE 3-14 Relationships between the directly connected impervious area (%) and the calculated volumetric runoff coefficients (Rv)
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. This decrease in the importance of parking areas as a source of runoff volume is associated with an increase in runoff contributions from streets and directly connected roofs.
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These two major rains would be in the category of design storms for conventional drainage systems. These plots indicate that these very large events, in the year they occurred, caused a measureable fraction of the annual pollutant loads and runoff volume discharges, but smaller events were responsible for the vast majority of the discharges.
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These rains occur on average about every two weeks from spring to fall and subject the receiving waters to frequent high pollutant loads and moderate to high flows. 1.5 to 3 inches.
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Natural surface water pathways are often interrupted or reversed, as shown by the blue lines in Figure 3-19 for a drainage system in Baltimore. Understanding how the system operates as a whole can often require knowledge of the history of construction conditions and field verification of the actual flow paths.
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EFFECTS OF URBANIZATION ON WATERSHEDS 165 FIGURE 3-19 Dead Run drainage system, Baltimore, Maryland. Black lines indicate surface (daylighted)
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Alteration of Travel Times The combination of impervious surface and altered drainage density provides significantly more rapid hydraulic pathways for stormwater to enter the nearest receiving waterbody compared to a natural landscape. This is illustrated quantitatively by Figure 3-21, which shows that the lag time -- the difference in time between the center of mass of precipitation and the center of mass of the storm response hydrograph -- is reduced for an urbanized landscape compared to a natural one.
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. precipitation and a waterbody's response give rise to greater velocities and volumetric discharges in receiving waters.
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However, for 100 percent sewered areas, the ratio of peak discharges ranges from 1.7 to 8 for 0 to 100 percent impervious area. Clearly both impervious surfaces and the presence of a storm-drain system combine to increase discharge rates in receiving waters.
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Increases in peak discharge are certainly among those changes, and they will always gather attention because of their direct impact on human infrastructure and potential for more frequent and more severe flooding. The extended duration of flood flows, however, also affects natural channels because of the potential increase in erosion.
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Although these impacts are commonly ignored in efforts to improve "water quality," they are inextricably linked to measured changes in water chemistry and must be part of any attempt to recover beneficial uses that have been lost to upstream urbanization. Geomorphology Watershed geomorphology is determined by the arrangement, interactions, and characteristics of component landforms, which include the stream-channel network, the interlocking network of ridges and drainage divides, and the set of hillslopes between the channel (or floodplain)
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Because this may influence the set of stream channels that are regulated by the Clean Water Act (CWA) , it is the subject of current legal arguments in courts up to and including the Supreme Court (e.g., Solid Waste Agency of Northern Cook County v.
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Riparian zones provide important ecosystem functions and services, such as reducing peak flood flows, transforming bioavailable nutrients into organic matter, and providing critical habitat. In humid landscapes, a functioning riparian area commonly is an area where shallow groundwater forms discharge seeps, either directly to the surface and then to the stream channel or through subsurface flowpaths to the stream channel.
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Alternatively, channels may fill with sediment derived from farther upstream to produce a braided form where a single-thread channel previously existed. The clearest single determinant of urban channel change is the alteration of the hydrologic response of an urban watershed, notably the increase in streamflow discharges.
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MacRae (1997) , reporting on other studies, found that channel cross-sectional areas began to enlarge after about 20 to 25 percent of the watershed was developed, commonly corresponding to about 5 percent impervious cover.
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(2005) as "Urban Stream Syndrome," which includes not only the visible alteration of the physical form of the channel but also the consequent deterioration of stream biogeochemical function and aquatic trophic structures.
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As a watershed shifts from having mostly natural pervious surfaces to having heavily disturbed soils, new impervious surfaces, and activities characteristic of urbanization, the runoff quality shifts from relatively lower to higher concentrations of pollutants. Anthropogenic activities that can increase runoff pollutant concentrations in urban watersheds include application of chemicals for fertilization and pest control; leaching and corrosion of pollutants from exposed materials; exhaust emissions,
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, animal operations that are major sources of bacteria in runoff, and forestry operations. Indeed, urban stormwater may actually have slightly lower pollutant concentrations than other nonpoint sources of pollution, especially for sediment and nutrients.
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The hydrologic connectivity of nonpoint pollutant source areas to receiving waterbodies is also a critical control on loading in developed catchments (Nadeau and Rains, 2007) and is dependent on both properties of the pollutant as well as the catchment hydrology.
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, medium-density suburban development on separate sewers (Glyndon) , and higher-density residential, commercial, and highway land cover (Dead Run)
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. runoff are controlled largely by the increase in volume and the washoff of pollutants from impervious surfaces.
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Extensive evaluations of urban construction site runoff problems have been conducted in Wisconsin for many years. Data from the highly urbanized Menomonee River watershed in southeastern Wisconsin indicate that construction sites have much greater potentials for generating sediment and phosphorus than do other land uses (Chesters et al., 1979)
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Unfortunately, NURP was limited in that it did not represent all areas of the United States or all important land uses. More recently, the National Stormwater Quality Database (NSQD)
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The NSQD makes it possible to statistically compare runoff from different land uses for different areas of the country. A number of land uses are represented in MS4 permits and also the database, including industrial stormwater discharges to an MS4.
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184 TABLE 3-4 Summary of Selected Stormwater Quality Data Included in NSQD, Version 3.0 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES
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NOTE: The complete database is located at: http://unix.eng.ua.edu/~rpitt/Research/ms4/mainms4.shtml. SOURCE: National Stormwater Quality Database.
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More detailed discussions of land use and regional differences in stormwater quality can be found in Maestre et al.
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Effects of Roofing Materials on Stormwater Quality The extensive rooftops of industrial areas can be a significant pollutant source area. A summary of the literature on roof-top runoff quality, including both roof surfaces and underlying materials used as subbases (such as treated wood)
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Effects of Pavement and Pavement Maintenance on Stormwater Quality Pavement surfaces can also have a strong influence on stormwater runoff quality. For example, concrete is often mixed with industrial waste sludges as a way of disposing of the wastes.
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Stormwater quality from asphalt-paved surfaces seems to vary with time. Fish kills have been reported when rains occur shortly after asphalt has been installed in parking areas near ponds or streams (Anonymous, 2000; PerezRivas, 2000; Kline, 2002)
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are also very high compared to that of most outfall industrial stormwater. OTHER SOURCES OF URBAN RUNOFF DISCHARGES Wet weather stormwater discharges from separate storm sewer outfalls are not the only discharges entering receiving waters from these systems.
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This is particularly true in arid areas, where dry weather discharges can occur daily. For example, despite the fact that rain is scarce from May to September in Southern California, an estimated 40 to 90 million liters of discharge flow per day into Santa Monica Bay through approximately 70 stormwater outlets that empty onto or across beaches (LAC DPW, 1985; SMBRP, 1994)
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FIGURE 3-30 Dry weather flows from Toronto industrial area outfall.
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FIGURE 3-32 Failing sanitary sewer, causing upwelling of sewage through soil, and draining to gutter and then to storm drainage system. SOURCE: Robert Pitt, University of Alabama.
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Snowmelt In northern areas, snowmelt runoff can be a significant contributor to the annual discharges from urban areas through the storm drainage system (see Figure 3-34)
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EFFECTS OF URBANIZATION ON WATERSHEDS 197 sion is reduced or eliminated during winter snow-cover conditions. However, erosion of bare ground at construction sites in the spring due to snowmelt can still be very high (Figure 3-35)
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After it falls to the ground and accumulates, the snow can become further contaminated by dry atmospheric deposition, deposition of nearby lost fugitive dust materials (usually blown onto snow packs near roads by passing vehicles) , and wash off of particulates from the exposed ground surfaces as it melts and flows to the drainage system.
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Also, the large volume of melt plus rain can wash off pollutants that have accumulated on various surfaces such as roads, parking lots, roofs, and saturated soil surfaces. The intensity of runoff from a rain-on-snow event can be greater than a summer thunderstorm because the ground is saturated or frozen and the rapidly melting snowpack provides added runoff volume (Oberts, 1994)
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Small midwinter melts accounted for less than 5 percent of the total loads. Box 3-8 shows mass pollutant discharges for a study site in Toronto and emphasizes the significance of snowmelt discharges on the total annual storm drainage discharges.
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Warm weather flows were also the predominant sources of phosphorus for the industrial area. One of the interesting observations is that, at these monitoring locations, warm weather stormwater runoff only contributed about 20 to 30 percent of the total annual flows being discharged from the separate stormwater outfalls.
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were much less than the TSS concentrations observed during warm weather runoff (250 mg/L) for these same areas.
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TABLE 3-10 Median Pollutant Concentrations Observed at Toronto Outfalls during Cold 1 Weather Baseflow Snowmelt Measured Parameter Residential Industrial Residential Industrial 3 Stormwater volume (m /ha/season) -- -- 1800 830 3 Base flow volume (m /ha/season)
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The relationships between atmospheric deposition and stormwater quality are, however, not well understood and difficult to determine. Following are a few illustrative examples.
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from atmospheric deposition to stormwater runoff in a small impervious urban catchment in the Los Angeles area. Dry deposition contributed 90 percent or more of the total deposition inside the catchment, indicating the dominance of dry deposition in semi-arid regions such as Los Angeles.
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* Relationships between atmospheric deposition rates and the quality of urban stormwater are complex and cannot be generalized regionally or temporally.
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. The results of the modeling of copper in San Francisco and its watershed demonstrate the feasibility of modeling the impact of a source, in this case copper input by atmospheric deposition, on water quality in a receiving waterbody.
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. The goals of this work were to improve national consistency in the rating and application of biological assessment tools for all types of waterbodies and to provide a baseline for the development of tiered aquatic life uses.
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FIGURE 3-38 The Biological Condition Gradient (BCG) and summaries of biological condi tion along tiers of this gradient.
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Cumulative impacts are a characteristic of urbanization, and biological organisms typically integrate the effects of many small insults to the landscape. Additionally, most natural systems often have strong "connectance," such that aquatic life often has stages that rely on migrating across multiple types or sizes of waterbodies.
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As such, it has strong application to understanding stormwater impacts and to communicating where a goal is located along the gradient of biological condition. While most urban goals may be distant from "pristine" or "natural," the BCG process can dispel misconceptions that alternate urban goals are "dead streams" or unsafe in some manner.
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In each of these areas, no or extremely low urban development, substantial forest cover, and minimal disturbance of riparian zones characterized sites with the highest biological scores, but these conditions did not guarantee high scores because other impacts could limit biology even with these "natural" characteristics. In all three regions, high urbanization and loss of natural cover always led to biological degradation (Figures 3-39 and 3-40)
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EFFECTS OF URBANIZATION ON WATERSHEDS 213 BOX 3-9 Continued FIGURE 3-39 Plots of a measure of urbanization (TIA + Wetland & Forest Cover + IRI) versus B-IBIs for Austin, Texas (top)
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and versus the ratio of coho salmon to cutthroat trout for Puget Sound (bottom)
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They made detailed observations of two subbasins in the Puget Sound lowland area, one with a greater degree of stormwater management than the other (although neither had what would be considered comprehensive stormwater management with a focus on water quality issues)
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, although concentrations at base flows may not routinely exceed water quality benchmarks (Sprague et al., 2007)
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. Recent work has found that salmonids show substantial behavioral changes from olfactory degradation related to copper at concentrations as low as 2 g/L, well below copper water quality criteria and above levels measured in most stormwater-affected streams (Hecht et al., 2007; Sandahl et al., 2007)
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, such that nearly all states have developed water quality criteria for these parameters. For example, nutrient enrichment in stormwater runoff has been associated with declines of biological condition in streams (Miltner and Rankin, 1998)
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Hydrologic simplification and stream straightening that occur in urban streams, often as a result of increased peak flows or as a local management response, typically remove habitat used as temporary refuges from high flows, such as backwater areas, undercut banks, and rootwads. There is a large literature relating populations of fish and macroinvertebrates to various habitat features of streams, rivers, and wetlands.
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. These streams had historical fish collections when they were primarily influenced by agricultural land use; sampling after the onset of suburban development documented the loss of many of these species attributable to land-use changes and habitat degradation along these urban streams.
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Although suburban areas had high rates of production in organic-rich debris dams and gravel bars, higher storm flow effects in urban streams may make these features less stable and able to be maintained (Groffman et al., 2005)
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222 URBAN STORMWATER MANAGEMENT IN THE UNITED STATES FIGURE 3-42 Simplified diagram of a lotic food web showing sources and major pathways of organic carbon. Dotted lines indicate flows that are a part of the microbial loop in flowing water but not in planktonic systems.
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The initial focus of water quality standards was on developing chemical criteria that could serve as engineering endpoints for waste treatment systems (e.g., NPDES permits)
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have been widely used because direct biological measures were poorly developed and these surrogates were assumed to be important to pollutant delivery to urban streams. However, biological assessment has rapidly advanced in many states and can be readily applied or if needed modified to be sensitive to stormwater stressors (Barbour et al., 2008)
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Human Health Impacts Despite the unequivocal evidence of ecosystem consequences resulting from urban stormwater, a formal risk analysis of the human health effects associated with stormwater runoff is not yet possible. This is because (1)
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The decline in stream quality is greatest toward the higher end of the IC range. Stream segments that range between 25 and 60 percent subwatershed impervious cover are classified as NonSupporting Streams (i.e., no longer supporting their designated uses in terms of hydrology, channel stability habitat, water quality, or biological diversity)
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(IC > 60%) Runoff as a Fraction of 1 10 to 20% 25 to 60% 60 to 90% Annual Rainfall Frequency of Bankfull 2 1.5 to 3 per year 3 to 7 per year 7 to 10 per year Flow per Year Fraction of Original Stream Network 60 to 90% 25 to 60% 10 to 30% Remaining Fraction of Riparian 50 to 70% 30 to 60% Less than 30% Forest Buffer Intact Crossings per Stream 1 to 2 2 to 10 None left Mile Ultimate Channel 2.5 to 6 times 6 to 12 times 3 1.5 to 2.5 larger Enlargement Ration larger larger Typical Stream Habitat Poor, often Fair, but variable Consistently poor Score absent Increased Stream 4 2 to 4 °F 4 to 8 °F 8+ °F Warming 5 1 to 2 times 4 to 6 times Annual Nutrient Load 2 to 4 times higher higher higher Wet Weather Violations of Frequent Continuous Ubiquitous Bacteria Standards Potential risk of Should be Fish Advisories Rare accumulation presumed 6 Aquatic Insect Diversity Fair to good Fair Very poor 7 Fish Diversity Fair to good Poor Very poor 1 Based on annual storm runoff coefficient; ranges from 2 to 5% for undeveloped streams.
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There is robust debate as to whether there is a sharp initial threshold or merely a continuum of degradation as IC increases, although the latter is more favored. There is much less debate, however, about the dominant role of IC in defining the hydrologic, habitat, water quality, and biodiversity expectations for streams with higher levels of IC (15 to 60 percent)
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Numerous researchers have evaluated the relative impact of riparian forest cover and IC on stream geomorphology, aquatic insects, fish assemblages, and various indices of biotic integrity. As a group, the studies suggest that indicator values for urban streams improve when riparian forest cover is retained over at least 50 to 75 percent of the length of the upstream network (Booth et al., 2002; Morley and Karr, 2002; Wang et al., 2003; Allan, 2004; Sweeney et al., 2004; Moore and Palmer, 2005; Cianfrina et al., 2006; Urban et al., 2006)
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In particular, it helps define general thresholds where water quality standards or biological narrative conditions cannot be consistently met during wet weather conditions (see Table 6-2)
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Although not every degraded waterbody is a product of intense urban development, all highly urban watersheds produce severely degraded receiving waters. Because of the close and, to date, inexorable linkage between land cover and the health of downstream waters, stormwater management is an unavoidable offshoot of watershed-based land-use planning (or, more commonly, its absence)
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They constitute as much as 70 percent of total impervious cover in ultra-urban landscapes, and as much as 80 percent of the directly connected impervious cover. Roads tend to capture and export more stormwater pollutants than other land covers in these highly impervious areas because of their close proximity to the variety of pollutants associated with automobiles.
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Standardized watershed assessment methods to identify the sources of human pathogens and indicator organisms in receiving waters need to be developed, especially for those waters with a contact-recreation use designation that have had multiple exceedances of pathogen or indicator criteria in a relatively short period of time. Given their difficulty and expense, epidemiological studies should be undertaken only after careful characterization of water quality and stormwater flows in the study area.
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Journal of the American Water Resources Association 40(2)
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Journal of the American Water Resources Association 33:1077–1090. Booth, D
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2006. Sources of heavy metals and polycyclic aromatic hydrocarbons in urban stormwater runoff.
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Journal of the American Water Resources Association 42(4)
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1996. Fish assemblages as indicators of water quality within the Apalachicola-Chattahoochee-Flint (ACF)
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2005. Draft: Use of Biological Information to Better Define Designated Aquatic Life Uses in State and Tribal Water Quality Standards: Tiered Aquatic Life Uses -- August 10, 2005.
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1984. Nationwide assessment of urban runoff impact on receiving water quality.
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2004. Hydrologic Monitoring of the Seattle Ultra-Urban Stormwater Management Projects: Summary of the 2000-2003 Water Years.
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2003. Metals Removal Technologies for Urban Stormwater.
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Journal of the American Water Resources Association 40:1645–1658. Kunkel, K
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In National Symposium on Water Quality, American Water Resources Association. Maxted, J
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Journal of the American Water Resources Association 6:565–580. McClain, M
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Journal of the American Water Resources Association 43(2)
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2001. Assessing the TMDL Approach to Water Quality Management.
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1995. Urban stormwater toxic pollutants: assessment, sources, and treatability.
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Journal of the American Water Resources Association. Ramcheck, J
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2007. A sensory system at the interface between urban stormwater runoff and salmon sur vival.
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Journal of the American Water Resources Association 43(5)
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United Engineering Foundation, Environmental and Water Resources Institute of ASCE. Suter, G
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Water Resources Research 39(10)
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Journal of American Water Resources Association 43(1)
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2005. Castro Valley Atmospheric Deposition Monitor ing.
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