Page 139 - Biosystems Engineering

120 Cha pte r T h ree

maintain bank stability as the roots of vegetation hold the soil. Further-
more, riparian buffers are known to reduce nutrients and bacteria from
runoff and septic system effluents in rural and agricultural areas. Because
vegetation provides shade, it helps to reduce stream temperatures.
Terraces Terracing is commonly used to reduce runoff and soil erosion.
Because terracing requires an additional investment, it should be con-
sidered only when conventional nonstructural BMPs are found to be
inadequate. Terraces reduce soil erosion by decreasing the slope length
of the hill slope. They also prevent formation of gullies by attenuating
overland flow (Fangmeier et al. 2005). Water retained behind terraces
might serve as a supplemental water supply during dry periods.
Vegetated Waterways and Dry/Wet Swales These are shallow vegetated
ditches. Runoff is sent trough a grass filter strip and may be sent
through a second BMP for further treatment before being discharged.
By slowing down flowing water due to increased roughness, ditches
increase infiltration and reduce peak flow. Portion of suspended sol-
ids also settle due to reduced flow velocity. Vegetated waterways are
used in channels where the soil is very susceptible to soil erosion due
to concentrated flow (Fangmeier et al. 2005). Wet swales are preferred
when the water table is high. They are most efficient when the drain-
age area is small, typically less than 4 to 5 hectares.

Detention Basins/Ponds These structures hold water for a certain
duration and release. They are helpful in controlling floods, down-
stream erosion, and sedimentation, and reducing various pollutants.
In the United States, areas with large impervious surfaces such as
commercial areas, parking lots, airport runways, and the like, are
required to have detention ponds to control flow and reduce levels of
various water quality constituents, including sediment. Detention
basins could be either dry or wet. Wet detention basins drain much
slower and often retain a certain amount of pooled water.

Wetlands Wetlands are the kidneys of nature. They are excellent nat-
ural filters. By acting like a sponge, wetlands help to mitigate flood-
ing events (even hurricane-generated storms). Wetlands also help in
cleaning polluted waters and detaining sediment. In a sense, they are
sinks for many chemicals and pollutants. Unfortunately, their bene-
fits have not been realized until the second half of the 20th century.
Today, not only are wetlands under protection by law in many coun-
tries, they are constructed to treat wastewater.
The American Society of Civil Engineers keeps a database of
BMPs where they report various BMP-related statistics (input flow,
concentration; output flow, concentration) from some of the studies
around the United States (http://www.bmpdatabase.org). Table 3.2
provides a portion of the summary table showing how some of the
selected BMPs perform in reducing pollutant.