Environmental Engineering Reference
In-Depth Information
Oxygen
released by
vegetation
Tree plantation
Evapotranspiration decreases
Roads
destabilize
hillsides
Diverse
ecological
habitat
Evapotranspiration
Ranching accelerates soil
erosion by water and wind
Trees reduce soil
erosion from heavy
rain and wind
Winds remove
fragile topsoil
Agricultural
land
Agricultural land
is flooded and
silted up
Steady
river flow
Gullies and
landslides
Leaf litter
improves
soil fertility
Heavy rain leaches nutrients
from soil and erodes topsoil
Tree roots stabilize
soil and aid water flow
Vegetation releases water
slowly and reduces flooding
Silt from erosion blocks rivers and
reservoirs and causes flooding downstream
Rapid runoff
causes flooding
Forested Hillside
After Deforestation
Active Figure 11-23 Natural capital degradation: hillside before and after deforestation. Once a hillside has
been deforested for timber and fuelwood, livestock grazing, or unsustainable farming, water from precipitation
rushes down the denuded slopes, erodes precious topsoil, and floods downstream areas. A 3,000-year-old
Chinese proverb says, “To protect your rivers, protect your mountains.” See an animation based on this figure
and take a short quiz on the concept.
ing absorbed and released slowly, water from the mon-
soon rains runs off the denuded Himalayan foothills,
carrying vital topsoil with it (Figure 11-23, right).
This increased runoff of soil, combined with heav-
ier-than-normal monsoon rains, has increased the
severity of flooding along Himalayan rivers and
downstream in Bangladesh. In 1998, a disastrous flood
covered two-thirds of Bangladesh's land area for
9months, leveled 2 million homes, drowned at least
2,000 people, and left 30 million people homeless. It
also destroyed more than one-fourth of the country's
crops, which caused thousands of people to die of star-
vation. In 2002, another flood left 5 million people
homeless and flooded large areas of rice fields.
Living on Bangladesh's coastal floodplain also
means coping with storm surges, cyclones, and
tsunamis (caused when large earthquakes produce
huge waves). In 1970, as many as 1 million people
drowned in one storm. Another surge killed an esti-
mated 139,000 people in 1991.
In their struggle to survive, the poor in Bangladesh
have cleared many of the country's coastal mangrove
forests for fuelwood, farming, and aquaculture ponds
for raising shrimp. The result: more severe flooding,
because these coastal wetlands shelter Bangladesh's
low-lying coastal areas from the ravages of storm
surges, and cyclones, and tsunamis. Damages and
deaths from cyclones in areas of Bangladesh still pro-
tected by mangrove forests have been much lower than
in areas where the forests have been cleared.
Solutions: Reducing Flood Risks
We can reduce flooding risks by controlling river
water flows, preserving and restoring wetlands,
identifying and managing flood-prone areas, and,
if possible, choosing not to live in such areas.
We can use several methods to reduce the risk from
flooding. One is to straighten and deepen streams, apro-
cess called channelization. Although channelization can
reduce upstream flooding, it removes bank vegetation
and increases stream velocity. The increased flow of
water can promote upstream bank erosion, increase
downstream flooding and sediment deposition, and
reduce habitats for aquatic wildlife.
Another approach is to build levees or floodwalls
along the sides of streams. Levees contain and speed
up stream flow, but this increases the water's capacity
for doing damage downstream. They also do not pro-
tect against unusually high and powerful flood-
waters. In the massive floods of 1993, for example,
two-thirds of the levees built along the Mississippi
River in the United States were damaged or destroyed
(Figure 11-22).
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