Page 402 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

Low-Impact Development and Stormwater Issues 363

Percent Impervious Cover Effect on Downstream Receiving Waters
<10 Sensitive
10–25 Impacted
>25 Nonsupporting

TABLE 10.1.2 Downstream Effect Rankings on Receiving Waters from Nonmitigated
Impervious Cover for Some Example Watersheds (CWP, 2000)

Why BMPs Were Developed: Stormwater Impacts of Urbanization
The Center for Watershed Protection (CWP) in Ellicott City, MD, has compiled, reviewed,
and synthesized extensive research and data on the impacts of urbanization on
stormwater runoff and watershed protection methods. It has been shown that one of
the main parameters that increase stormwater runoff and downstream impacts is an
increase in impervious areas caused by development. These increases can be both on
the ground (pavements) and from roofs. Each watershed is different and responds
uniquely to the amount and location of impervious cover, so there is no single simple
model based only on impervious cover. Table 10.1.2 shows a range of impacts that was
found to occur for a number of watersheds studied.
In these, the impacts of urbanization are usually categorized into four areas—
hydrological, geomorphological, water quality, and habitat-related. Some of the effects
and impacts in each of these categories are listed in Table 10.1.3.
The Center for Watershed Protection also summarized some of the methods or tools
which can be used to protect the watershed and puts them into eight approximate
groupings. These are summarized in Table 10.1.4.

Hydrological Geomorphological Water Quality Habitat/Wildlife
Increased frequency Widened streams Increased temperature Decreased
of flooding habitat value
Increased volumes Modified access for Increased beach closures Decreased large
of flooding fish passage woody debris
Higher peak flows Loss of riffle pools Increased pollutants Increased fish
- SS (suspended solids) barriers
Increased variation Fragmentation of - Nutrients (N and P) Shift in food
in storage needs tree cover - Metals (Cu, Zn, Pb, Cd) chain
- Oil and greases
Modified Silt embedded in Decreased
channelization banks and bottom - Bacteria speciation
- Various toxics (pesticides/
Lower dry weather Degraded bottom Decreased
herbicides)
flows substrate quality weather impact
buffers
Eroded stream Increased algal
banks problems

TABLE 10.1.3 Some Impacts of Urbanization and Increased Impervious Cover (CWP, 2000)