Environmental Engineering Reference
In-Depth Information
higher water quality. Stormwater may also contain high levels of coliform bacteria, oil, and
grease and particles with adsorbed heavy metals (USEPA 2004).
Overland flow. . This item encompasses the ephemeral drainage network created during wet-
weather events. Roads, rooftops, parking lots, and other surfaces acting to intensify the
drainage network are included.
Result: This system has similar impacts as stormwater. The density and pattern of roads in
urban areas often creates small patches of habitat incapable of supporting a wide diversity
of birds (Ortega-Álvarez and MacGregor-Fors 2009).
Navigation/Flood control . Any measure used for channelization applies: locks, straighten-
ing, deepening, or widening as well as dams, reservoirs, dikes, levees, or rip-rap.
Result: Channelization often involves the removal of bankside vegetation, causing
a litany of environmental and ecosystem impacts to ensue (Brooker 1985). Dams can
increase erosion and have significant impacts on fish migrations by creating environ-
ments more conducive to nonnative and exotic plant, fish, snail, insect, and animal spe-
cies (WCD 2000).
This analysis demonstrates how a finer delineation of watershed impacts is obtained
when major systems are decomposed into subsystems. For example, contamination
emerges as a critical concern, since it spawns several other impacts including substitu-
tion, simplification, reduction, and overgrowth. Navigation and flood control measures
that straight-jacket streams with concrete also have many negative consequences. There
are also additional on-site (within the watershed) and external impacts. On-site, the pro-
cess of building infrastructure has ecosystem consequences, because there is accelerated
erosion when building roads, laying pipe, or constructing buildings. Outside of the water-
shed, the need to supply the electricity for the pumps used for water transfers also cre-
ates impacts. For instance, the California State Water Project is the largest single user of
energy in California. In the process of delivering water from the San Francisco Bay-Delta
to Southern California, the project uses 2%-3% of all electricity consumed in the state
(Cohen et al. 2004). How much coal is burned to support this activity?
12.4.2 Stormwater
Stormwater is nonpoint pollution occurring in urban areas. During wet weather events,
particles containing contaminants from the soil and road surface are carried overland to
the nearest water body or released by storm drains downstream. The problem is exacer-
bated by urbanization, which increases the amount of impervious surface (Walesh and
Videkovich 1978).
Stormwater is best characterized as an interrelated problem of water quantity and water
quality. From a quantitative perspective, the extra volumes of runoff have two major
impacts: (1) the creation of impervious surfaces such as concrete and asphalt lowers the
surfaces' resistance to flow and increases the competence (ability to carry particles) of
the flowing stormwater. Surface resistance is incorporated as the roughness coefficient in
Manning's equation used to compute the average open-channel flow velocity (Equation
12.1). As the equation demonstrates, when smooth pavement replaces rough ground, the
value of the roughness coefficient (n) decreases, and average velocity increases.
= 100
2 3
1 2
V
*
R
*
S
(12.1)
n
Search WWH ::




Custom Search