Urbanization - Stormwater Runoff
Stormwater Runoff and Impervious Surfaces
Perhaps the most defining characteristic of urban streams is the increased amount and rapidity of stormwater or surface runoff to those systems. Impervious surfaces associated with urbanization reduce infiltration and increase surface runoff (see Figure 16), altering the pathways by which water (and any associated contaminants) reach urban streams.
Common impervious surfaces include:
- Parking lots
- Driveways and sidewalks
- Compacted soils
How Does Stormwater Runoff Affect Streams?
- It alters natural hydrology, generally leading to more frequent, larger magnitude and shorter duration peak flows.
- It alters channel morphology, generally leading to changes such as increased channel width, increased downcutting and reduced bank stability.
- It alters in-stream hydraulics, affecting biologically important parameters such as water velocity and shear stress.
- It disrupts the balance between sediment supply and transport, generally leading to increased sediment transport capacity and channel erosion.
- It increases stream temperatures, due to the transfer of heat from impervious surfaces to stormwater runoff.
- It increases delivery of pollutants from the landscape to the stream. Pollutants commonly found in stormwater runoff include:
- Wear metals
- Organic pollutants
- Oil and grease
Effective vs. Total Imperviousness
- Total impervious area (TIA) = all impervious area in catchment.
- Effective impervious area (EIA) = impervious area in catchment that is directly connected to stream channels (i.e., precipitation falling on that area is effectively transported to stream).
- Geographic information system data combined with stormwater infrastructure overlays.
- Published empirical relationships between TIA and EIA (Alley and Veenhuis 1983, Wenger et al. 2008).
- Field assessments.
Many studies have found that EIA (also known as drainage connection or directly connected impervious area) is a better predictor of ecosystem alteration in urban streams. For example, Hatt et al. (2004) showed that % connection was more strongly related to water chemistry variables (e.g., conductivity, total phosphorus) than % total imperviousness, during both baseflows and stormflows (Figure 17).
The strength of EIA relationships suggests that stormwater management techniques aimed at disconnecting impervious areas from stream channels can improve urban water quality (Walsh et al. 2005b).
Imperviousness and Biotic Condition
Total or effective impervious cover has been linked to numerous changes in stream biotic assemblages. These changes include but are not limited to):
- Increased abundance or biomass.
[Walsh et al. 2006b, Busse et al. 2006]
- Other changes in assemblage structure.
[Walsh et al. 2005b]
- Decreased total abundance, richness or diversity.
[Walsh 2004, Moore and Palmer 2005, Utz et al. 2009]
- Decreased EPT abundance, richness or diversity.
[Walsh 2004, Walsh et al. 2005b, Schiff and Benoit 2007]
- Other changes in assemblage structure (e.g., functional feeding groups).
[Stepenuck et al. 2002, Wang and Kanehl 2003]
[Morley and Karr 2002, Walsh et al. 2005b, Schiff and Benoit 2007]
Decreased quality of biotic index scores.
- Decreased abundance, biomass, richness or diversity.
[Wang et al. 2001, Wang et al. 2003, Stranko et al. 2008]
- Other changes in assemblage structure (e.g., loss of individual species, changes in reproductive guilds).
[Wenger et al. 2008 (Figure 18), Helms et al. 2009]
- Decreased quality of biotic index scores.
[Wang et al. 2001, Wang et al. 2003]
Thresholds of ImperviousnessRelationships between impervious cover and stream condition measures, defined by physical, chemical or biological parameters, can take several forms (see Figure 19). When the relationship is linear, any increase in imperviousness results in a decrease in condition (see Figure 19, yellow and Figure 20). In other cases, there may be threshold values of impervious cover above which condition either decreases rapidly (Figure 19, green) or remains consistently low (see Figure 19, blue and Figure 21).
- Consistent channel instability when EIA > 10%.
[Booth and Jackson 1997]
- Different geomorphic response patterns (e.g., in terms of depth diversity, maximum pool depth) across sites with < 13% vs. > 24% TIA.
[Cianfrani et al. 2006]
- Consistently higher conductivity, dissolved organic carbon and filterable reactive phosphorus when EIA > 5%, 4%, and 1%, respectively.
[Walsh et al. 2005b]
- Uniformly low summer baseflow when TIA > 40%.
[Finkenbine et al. 2000]
- Consistently high algal biomass when EIA > 5%, low diatom index value when EIA > 2%.
[Walsh et al. 2005b]
- Sharp declines in macroinvertebrate diversity and richness when TIA between 8-12%.
[Stepenuck et al. 2002]
- Invertebrate taxa sensitive to impervious cover lost when TIA between 2.5-15% in Piedmont streams and between 4-23% in Coastal Plain streams.
[Utz et al. 2009]
- Brook trout absent when TIA > 4%.
[Stranko et al. 2008]
- Occurrence probability of three sensitive fish species approaches zero when EIA between 2-4%.
[Wenger et al. 2008]
- Sharp declines in fish IBI score and trout abundance when EIA between 6-11%, consistently low values when EIA > 11%.
[Wang et al. 2003]