Environmental Engineering Reference
In-Depth Information
• Presence of antecedent soil moisture
• Amount and type of vegetation present
• Soil type and texture
• Amount of impervious surface
High intensities of rainfall may fall on soils not fully capable of infiltrating the water. If
the soils are dry and permeable (e.g., larger-grained, such as sandy loam), then infiltration
rates will be higher. Some clay soils will initially infiltrate water at a high rate, but the rate
declines rapidly as the clay particles expand and reduce the porosity and permeability.
Urban soils tend to be compacted (Gregory et al. 2006) and have a generally low-organic
content that varies by their age. Soils in older urban residential areas tend to have higher
organic contents than those within newer developments (Scharenbroch et al. 2005). These
characteristics reduce infiltration and increase runoff. Tree canopies intercept and frag-
ment drops of precipitation and reduce its kinetic energy, lowering their erosive capacities.
The differences in volume (and mass) between a droplet of drizzle and a large drop from
a thunderstorm are quite significant:
3
Volume of a sphere
=
4/3 r (the 4/3 is a constant and can
b
π
e dropped for this comparison).
For a droplet of drizzle 0.02 mm in diameter:
3
3
Radius, r
=
0.01mm, volume
=
π
r
=
(3.14 * 0.01)
=
0.000031mm.
For a larger drop 2.0 mm in diameter:
3
3
Radius
=
1 mm, volume
.
0
=
π
r
=
(3 14 1
.
* . )
0
= 3 959144 or 3 1mm
0
.
0
.
.
The volume difference is
30.1/0.000031 = 1,000,000.
With a volume 1 million times greater, the 2.0 mm drop has more mass and more poten-
tial to dislodge soil.
Certain types of trees are very effective at intercepting precipitation, and vegetation can
reduce runoff significantly. In urban areas, species such as the Brisbane Box tree exhibit
interception rates as high as 66.5% (Xiao and McPherson 2002). A study in Dayton, Ohio,
found that a modest increase in the existing tree canopy cover could reduce surface runoff
by 5% (Sanders 1986).
In urban areas, there is more impervious surface created by roads and rooftops. These
constructions steer precipitation into a runoff path, rather than one of infiltration, and
replace the vertical movement of large quantities of water with horizontal movement.
The atmosphere is also a source of contaminants into watersheds. Significant atmo-
spheric deposition of lead and other metals such as arsenic, chromium, and mercury
occurs over many watersheds and is higher in urban watersheds (Murray et al. 1997;
Sabin et al. 2006). Acidic precipitation also acts as a contaminant to the surface waters
and groundwater within watersheds. As a result of the chemical reaction between water
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