ENVIRONMENT GLOBAL WARMING

CAUGHT ON CAMERA: Seaside town's pier ripped up by 'Violent Waves' at Aberystwyth

How Can We Reduce the Threat of Flooding?

CONCEPT 11-4 We can improve flood control by protecting more wetlands and natural vegetation in watersheds and by not building in areas subject to frequent flooding.

Some Areas Get Too Much Water from Flooding

Whereas some areas have too little water, others sometimes have too much because of natural flooding by streams, caused mostly by heavy rain or rapidly melting snow. A flood happens when water in a stream overflows its normal channel and spills into the adjacent area, called a floodplain. Floodplains, which usually include highly productive wetlands, help to provide natural flood and erosion control, maintain high water quality, and recharge groundwater. People settle on floodplains because of their many advantages, including fertile soil, ample water for irrigation, and availability of nearby rivers for transportation and recreation. Floodplains provide flat land suitable for crops, buildings, highways, and railroads.

To reduce the threat of flooding and thus to allow people to live in floodplains, rivers have been narrowed and straightened (channelized), equipped with protective levees and walls, and dammed to create reservoirs that store and release water as needed. But in the long run, such measures can greatly increase flood damage because they can be overwhelmed by prolonged rains, as happened along the Mississippi River in the Midwestern United States during the summer of 1993.

Floods provide several benefits. They have created the world’s most productive farmland by depositing nutrient-rich silt on floodplains. They also recharge groundwater and help refill wetlands. But floods kill thousands of people each year and cause tens of billions of dollars in property damage. Indeed, floods annually affect more people than the combined numbers affected by drought, tropical cyclones, famine, earthquakes, tsunamis, and volcanic eruptions. Floods usually are considered natural disasters. Since the 1960s, however, human activities have contributed to the sharp rise in flood deaths and damages.

One such activity is removal of water-absorbing vegetation, especially on hillsides, and replacing that vegetation with farm fields, pastures, pavement, or buildings that cannot absorb rainwater. Another is draining wetlands that normally absorb floodwaters and building on the land. For example, Hurricane Katrina struck the Gulf Coast of the United States in August 2005 and flooded the city of New Orleans and surrounding areas. The damage was intensified because of the degradation and removal of coastal wetlands that had historically buffered the land from storm surges. Living on floodplains increases the threat of damage from flooding. Many poor people have little choice but to live in such risky areas, as discussed in the following

■ CASE STUDY

Living Dangerously on Floodplains in Bangladesh

Bangladesh is one of the world’s most densely populated countries, with 147 million people packed into an area roughly the size of the U.S. state of Wisconsin. It is very flat, only slightly above sea level, and it is one of the world’s poorest countries. The people of Bangladesh depend on moderate annual flooding during the summer monsoon season to grow rice and help maintain soil fertility in the delta basin. The annual floods deposit eroded Himalayan soil on the country’s crop fields.

In the past, great floods occurred every 50 years or so. But since the 1970s, they have come roughly every 4 years. Bangladesh’s flooding problems begin in the Himalayan watershed, where rapid population growth, deforestation, overgrazing, and unsustainable farming on steep and easily erodible slopes have increased flows of water during monsoon season. Monsoon rains now run more quickly off the denuded Himalayan foothills, carrying vital topsoil with them. This increased runoff of soil, combined with heavier-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 9 months, 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 starvation. In 2002, another flood left 5 million people homeless and flooded large areas of rice fields. Yet another major flood occurred in 2004.

Living on Bangladesh’s coastal floodplain at sea level means coping with storm surges, cyclones, and tsunamis, such as the one in 2004 caused by earthquakes under the Indian Ocean. In 1970, as many as 1 million people drowned as a result of one tropical cyclone. Another cyclone in 2003 killed more than a million people and left tens of millions homeless. 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 had sheltered Bangladesh’s low-lying coastal areas from storm surges, cyclones, and tsunamis. Damages and deaths from cyclones in areas of Bangladesh still protected by mangrove forests have been much lower than in areas where the forests have been cleared.

A 3,000-year-old Chinese proverb says, “To protect your rivers, protect your mountains.”

THINKING ABOUT

Bangladesh

What are three things that could be done to help reduce the threat of flooding in Bangladesh?

We Can Reduce Flood Risks

To improve flood control, we can rely less on engineering devices such as dams and levees and more on nature’s systems such as wetlands and natural vegetation in watersheds. Straightening and deepening streams (channelization) reduces upstream flooding. But it also eliminates aquatic habitats, reduces groundwater discharge, and results in a faster flow, which can increase downstream flooding and sediment deposition. In addition, channelization encourages human settlement in floodplains, which increases the risk of damages and deaths from major floods.

Levees or floodwalls along the sides of streams contain and speed up stream flow, but they increase the water’s capacity for doing damage downstream. They also do not protect against unusually high and powerful floodwaters, such as those occurring in 1993 when two-thirds of the levees built along the Mississippi River in the United States were damaged or destroyed. Dams can reduce the threat of flooding by storing water in a reservoir and releasing it gradually, but they also have a number of disadvantages. Another way to reduce flooding is to preserve existing wetlands and restore degraded wetlands to take advantage of the natural flood control they provide in floodplains.

On a personal level, we can use the precautionary approach to think carefully about where we live. Many poor people live in flood-prone areas because they have nowhere else to go. Most people, however, can choose not to live in areas especially subject to flooding or to water shortages caused by climate factors, increased population, and economic development.

THINKING ABOUT

Where to Live

Do you now live in a flood-prone area? Have you thought about moving to or away from such an area? Do the attractions of living there outweigh the risks for you?

SOLUTIONS

Reducing Flood Damage

Prevention

Preserve forests on watersheds

Preserve and restore wetlands in floodplains

Tax development on floodplains

Use floodplains primarily for recharging aquifers, sustainable agriculture and forestry

Control

Strengthen and deepen streams (channelization)

Build levees or floodwalls along streams

Build dams