Urbanization and the Disruption of Matter and Energy Flows within Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 432

the landscape, and the ponds did not have the capacity to hold the extra runoff generated
by urbanization. Regional detention as the second response provided more storage and
stopped the floods, but at some cost to the sensitive wetland ecosystems included as part
of the plan. Neither method captured stormwater before it left its source—the rooftops and
lawns on individual parcels.
12.4.3 Erosion and Sedimentation
The construction of new roads, buildings, and other infrastructure accompanies urbaniza-
tion. When land is disturbed by construction activity, soil erosion increases dramatically.
Erosion rates as high as 40,000 times the preconstruction rate have been observed. The
typical magnitude is between 20 and 40, which equates to a loss of 27-64 tons (30-70 tons)
of soil per acre per year (Goldman et al. 1986).
Formal laws mandating erosion and sedimentation control exist in many countries.
Sedimentation is the deposition of eroded material in a sink. In urban streams, where
erosion rates at nearby construction sites have accelerated erosion, more sedimentation
occurs (Wolman and Schick 1967). At the federal level in the United States, sediment is
considered the principal contaminant in the water systems, a point explicitly recognized
by the Clean Water Act of 1972. Many states have also adopted erosion and sedimentation
control laws and ordinances. Unfortunately, enforcement of their provisions is not always
carried out.
In 2000, a study of 30 construction sites in the urbanizing region of east-central Michigan
found that procedures used to stabilize slopes, stabilize soil, and manage water were not
sufficient to control soil from leaving the site. Only four of the sites successfully imple-
mented the Best Management Practices recommended by the state and local erosion
and sedimentation control ordinances (Kaufman 2000). None of the sites studied were
inspected after rain events, which would have provided the best indication of the efficacy
of the erosion control measures used. As seen with stormwater management, the timing
of human interventions and the selection and placement of the proper controls are critical
to avoiding more damage.
Soils take a beating in urban areas. In part due to the removal of topsoil during con-
struction activity, the top layer of the urban soil horizon may have low organic content
(Scharenbroch et al. 2005). This lack of organic material inhibits their ability to contain
certain contaminants and prevent their migration. As noted earlier, urban soils are also
compacted and as a result exhibit lower infiltration capacities. In addition, they are often
overfertilized and contribute significant levels of nitrogen and phosphorus to stormwater
(Stow et al. 2001; Collins et al. 2010).
In some countries, erosion is a catastrophe. Tanzania has reported losses of one-third of
its Gross Domestic Product from soil erosion (Beyadi 2010). In one major study of soil ero-
sion, it was found that, during the last 40 years, nearly one-third of the world's arable land
has been lost by erosion and continues to be lost at a rate of more than 10 million ha/year
(Pimental et al. 1995). In many countries, soil erosion is similar to high blood pressure—
a silent killer. The geographic scale of erosion and its gradual nature make it difficult to
observe on a daily basis. Quite often the damage is done before anybody notices.
12.4.4 The Urban Heat Island
An urban heat island (UHI) results from the excess energy input into the atmosphere
from anthropogenic activities. The primary source of the additional energy comes from