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Impacts of Urbanization - Part 1
 

Impacts of Urbanization - Part 1

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  • The land development process has a documented impact on the quality of our watersheds. One of the best ways to mitigate these impacts is to control the way that development sites are designed. Better site design is a process by which local governments can review their zoning and ordinance codes to minimize impervious cover and promote conservation of natural areas. This presentation outlines 22 model principles for land development that focus on streets, parking lots, lot design, and conservation of natural areas in new developments.

Impacts of Urbanization - Part 1 Impacts of Urbanization - Part 1 Presentation Transcript

  • The Impacts of Urbanization Presented by
  • Impacts of Land Development
    • Hydrology
    • Geomorphology
    • Water Quality
    • Habitat
    The effects of urbanization on aquatic resources can be organized into four categories:
  • Impervious cover, such as this parking lot, is impervious to rainfall, creating large amounts of runoff . Photo Copyright 1999, Center for Watershed Protection
  • Hydrological Effects of Urbanization
    • Disruption of natural water balance
    • Increased flood peaks
    • Increased stormwater runoff
    • More frequent flooding
    • Increased bankfull flows
    • Lower dry weather flow
  • This diagram shows how development and its corresponding increase in impervious cover disrupts the natural water balance. In the post-development setting, the amount of water running off the site is dramatically increased.
  • This graph shows that as the percentage of watershed imperviousness increases, the volumetric runoff coefficient increases as well.
  • This hydrograph shows the effects of urbanization on the flow of the Anacostia River.
  • The large amount of runoff entering an urban stream can frequently result in flooding, such as this moderate overbank flooding event. Photo Copyright 1999, Center for Watershed Protection
  • Catch basins or inlets collect stormwater and direct it through pipes to a downstream stormwater facility or to the receiving waters. Curbs and gutters are designed to deliver stormwater away from the road surface in an efficient and timely manner. Photos Copyright 1999, Center for Watershed Protection
  • Often, the runoff is directly discharged into nearby waterbodies without adequate water quality treatment. Photo Copyright 1999, Center for Watershed Protection
  • This graph illustrates the effects of urbanization on hydrograph peak discharge . In pre-developed conditions, flow gradually increases to a relatively flat peak and gradually descends to a low flow condition. In the urbanized condition, flow rapidly increases to a peak and just as sharply descends, often to a lower flow condition than pre-development.
  • Bankfull flooding (or the condition of the flow that "fills up" the channel) occurs much more frequently in highly urbanized areas and has the potential to be extremely erosive and damaging to the natural morphology of the stream. Photo Copyright 1999, Center for Watershed Protection
  • This figure illustrates how upstream development can widen and lengthen the floodplain limit.
  • Decline in Streamflow Due to Diminished Groundwater Recharge An increase in impervious surface often decreases the amount of rainfall available for infiltration in humid climates. Without infiltration, the groundwater will not be recharged and the stream will lose this potential source of water, thus low flows tend to be lower in urbanized watersheds.
  • This often means that a headwater stream that was once perennial now becomes intermittent . One example is a set of streams in the Long Island, New York region. The graph shows the predominantly agricultural watersheds that have been able to sustain flow during dry weather.
  • Geomorphological Effects of Urbanization
    • Stream widening & erosion
    • Reduced fish passage
    • Degradation of habitat structure
    • Decreased channel stability
    • Loss of pool-riffle structure
    • Fragmentation of riparian tree canopy
    • Embeddedness
    • Decreased substrate quality
  • In watersheds with less than 5% impervious cover, streams are typically stable and pristine, maintaining good pool and riffle structure, a large, wetted perimeter during low flow, a good riparian canopy coverage. Photo Copyright 1999, Center for Watershed Protection
  • While this stream at 8-10% impervious cover is still relatively stable, signs of stream erosion are more apparent and include loss of the wetted perimeter, more eroded material in the banks, and debris. Photo Copyright 1999, Center for Watershed Protection
  • At 10% impervious cover, the stream is slightly more visibly impacted. The stream shown here has approximately doubled its original size, tree roots are exposed, and the pool and riffle structure seen in sensitive streams is lost. Photo Copyright 1999, Center for Watershed Protection
  • Active erosion becomes much more evident at 20% impervious cover with decreased substrate quality due to more material "flushing" through the system. Photo Copyright 1999, Center for Watershed Protection
  • The surrounding area of this stream is also approximately 20% impervious cover and shows stream erosion that is much worse than in the previous slide due to an absence of vegetation to hold together bank structure. Photo Copyright 1999, Center for Watershed Protection
  • This stream has a surrounding area of approximately 30% impervious cover. The large amount of impervious cover has increased the size of the stream by a factor of five to ten. The manhole in the middle of the diagram was originally in the floodplain and is an indicator of the degree to which channel erosion has occurred. Photo Copyright 1999, Center for Watershed Protection
  • In a study conducted by Booth and Reinholt (1993), documented changes in channel stability occurred as impervious cover passed the 10% threshold.
  • In many highly urbanized areas, natural streams have been channelized to speed runoff along, but these fail to provide any habitat value. Photo Copyright 1999, Center for Watershed Protection
  • The stream pictured here has downcut several feet in elevation because of the increased stormwater flow. In this case, the forested wetland in the floodplain is now hydraulically disconnected from the stream that sustained it. Photo Copyright 1999, Center for Watershed Protection
  • The surrounding area of this stream is approximately 50% impervious cover, and in these situations streams are often piped. Photo Copyright 1999, Center for Watershed Protection
  • Embeddedness is another impact of urbanization and is characterized by the packing of pebbles or cobbles with fine grained silts and clays. This slide illustrates a relatively clean substrate with an isolated area of fouling. Photo Copyright 1999, Center for Watershed Protection
  • In extreme examples of embeddedness, habitat, water and dissolved oxygen exchange become greatly reduced. Photo Copyright 1999, Center for Watershed Protection
  • Effects of Urbanization on Water Quality
    • Increased stream temperature
    • Increased pollutants
    • Increased risk of shellfish bed/beach closure
  • Urbanization causes the temperature of streams to rise, both during low flow and storm events. Photo Copyright 1999, Center for Watershed Protection
  • This graph shows the results of a study conducted by John Galli in Montgomery County, MD. The temperature increases dramatically in an urban stream as compared to a rural one.
  • Urban and Industrial Stormwater: Typical Pollutants
    • Suspended solids/sediments
    • Nutrients (nitrogen & phosphorus)
    • Metals (copper, zinc, lead, and cadmium)
    • Oil & greases (PAHs)
    • Bacteria
    • Pesticides & herbicides
    • Temperature
  • An increase in pollutants can adversely affect aquatic organisms and can result in beach closure, safety concerns, and/or activity limitations for people. Photo Copyright 1999, Center for Watershed Protection
  • Numerous types of pollutants can enter a stream from the street surface.
  • Nutrients and pollutants associated with stormwater runoff can increase the amount of algae activity in streams. Photo Copyright 1999, Center for Watershed Protection
  • This graph shows that as impervious cover increases, there is a corresponding increase in phosphorus delivered by stormwater as well.
  •