Flooding Causes, Management, and Flood Defenses
A flood is the overflow of water onto land that is normally dry, caused when a river’s discharge exceeds the capacity of its channel, or when the sea or heavy rain inundates the land; flood management combines hard engineering (dams, levees, channel changes) with soft engineering (floodplain zoning, wetland restoration, warnings). In brief: physical causes include heavy or prolonged rainfall, rapid snowmelt, saturated or frozen ground and steep, impermeable terrain; human causes include deforestation, urbanization (more paved surfaces), poor drainage and the failure of defenses. Floods are the most common natural hazard in the world. Management tries to reduce both the likelihood of flooding and its consequences.
Flood risk is increasing as more people live on floodplains and rainfall extremes grow with a warming climate. This guide explains the causes of floods, how a storm hydrograph shows a flood, the main methods of flood defense and management including dams and reservoirs, compares them in a table, works through a hydrograph and reservoir calculation, outlines the history of flood control and corrects common mistakes.
What causes floods?
Physical causes: prolonged or intense rainfall (including monsoons and tropical cyclones), rapid snowmelt (spring thaw, or rain on snow), saturated, frozen or impermeable ground that cannot absorb water, steep slopes and a high drainage density that speeds runoff, and a basin shape that is round, so that the water from all parts arrives together. Other types of flood are coastal (storm surges and high tides), flash floods (very fast, from intense rain over small basins), and floods from dam or levee failure, glacial lake outbursts or tsunamis. Human causes: urbanization replaces soil with roads and roofs, so more rain runs off quickly into drains and rivers; deforestation reduces interception and infiltration; draining wetlands and building on floodplains removes natural storage and puts people in the way; inadequate drains and poorly maintained defenses; and climate change, which increases the intensity of heavy rain and raises sea levels.
What does a storm hydrograph show?
A storm hydrograph plots the discharge of a river against time after a rainstorm, together with the rainfall. Its main parts are the base flow (the normal flow from groundwater), the rising limb (discharge increases as runoff arrives), the peak discharge (maximum flow), the falling (recessional) limb and the lag time (the delay between the peak rainfall and the peak discharge). A “flashy” hydrograph has a short lag time, a steep rising limb and a high peak, and is typical of small, steep, impermeable, urban or deforested basins with high drainage density. A “flat” hydrograph with a long lag time and a low peak is typical of large, permeable, vegetated and gently sloping basins.
How are floods managed?
Hard engineering uses structures: dams and reservoirs, levees or embankments, flood walls, channel straightening or deepening (dredging), diversion channels (floodways) and tidal barriers such as the Thames Barrier. These give a high level of protection but are expensive, change the river, may increase floods downstream and can give a false sense of security. Soft engineering works with natural processes: floodplain zoning (preventing building in high-risk areas), afforestation and upland land management to slow runoff, wetland and floodplain restoration, managed realignment (allowing the sea to flood coastal land), sustainable urban drainage (permeable pavements, green roofs, retention ponds), flood warnings and evacuation plans, flood-proofing buildings and flood insurance. Modern management is integrated: it combines measures across the whole basin and accepts that some flooding cannot be prevented.
What do dams and reservoirs do?
A dam holds back water in a reservoir. For flood control, the reservoir is kept partly empty to store floodwater and release it slowly, which reduces the peak flow downstream. Dams also supply drinking and irrigation water, generate hydroelectric power and can improve navigation. They also have costs: they flood land and displace people, trap sediment (starving deltas and floodplains of silt and causing erosion downstream), change river ecosystems and block fish migration, release methane from decaying vegetation, and carry the risk of failure. Examples: the Three Gorges Dam on the Yangtze (main structure completed in 2006), the Hoover Dam (1936) and the Aswan High Dam (1970).
Flood management methods compared
| Method | Type | Advantage | Disadvantage |
|---|---|---|---|
| Dam and reservoir | Hard | Controls flow; water and power | Costly; traps sediment; displaces people |
| Levees and flood walls | Hard | Protect a town directly | Fail suddenly; raise levels elsewhere |
| Channel straightening | Hard | Carries water away quickly | Increases floods downstream |
| Floodplain zoning | Soft | Cheap; reduces damage | Limits land use |
| Wetland restoration | Soft | Stores water; habitat | Needs land; slower effect |
| Warnings and evacuation | Soft | Saves lives | Does not stop property damage |
Worked example: hydrograph, reservoir and cost
- Lag time: rainfall peaks at 12:00 and the river reaches its peak at 18:00 on a rural basin: the lag time is 6 hours. After urbanization the peak occurs at 14:00, so the lag time falls to 2 hours, one-third of the previous value.
- Peak ratio: the base flow is 20 m3/s and the peak discharge is 120 m3/s, which is 120 ÷ 20 = 6 times the base flow.
- Reservoir filling: a flood reservoir has an empty storage space of 50 million m3. The inflow is 200 m3/s and the controlled release is 80 m3/s, so it fills at a net rate of 200 − 80 = 120 m3/s. The time to fill is 50,000,000 ÷ 120 = 416,667 s = 115.7 hours = 4.8 days.
- Effect of the reservoir: without the reservoir, the flow downstream would be 200 m3/s. With it, 80 m3/s, a reduction of (200 − 80) ÷ 200 = 60 percent for 4.8 days.
- Cost–benefit: flood defenses cost 20 million (in any currency) and avoid an expected 1.5 million of damage a year. The payback period is 20 ÷ 1.5 = 13.3 years, ignoring maintenance and the discounting of money.
How has flood control developed?
Early flood control was by levees and diversion channels, such as the dikes along the Yellow River in China and the Dutch dikes in the Middle Ages. Major 20th-century floods drove national programs: the 1927 Mississippi flood led to the Flood Control Act of 1928 in the United States; the 1953 North Sea flood caused more than 1,800 deaths in the Netherlands and led to the Delta Works; and flooding in London led to the Thames Barrier (operating since 1982). In recent decades, the focus has shifted to “working with nature” and risk-based management, such as the Dutch “Room for the River” program.
Which mistakes are common?
Mistake 1: Thinking floods are caused only by heavy rain
Land use, soil, slope, snowmelt, and the state of defenses all matter. Human changes can make the same rain cause a worse flood.
Mistake 2: Believing that hard engineering is always the best solution
It is effective locally, but may transfer the problem downstream and may not suit extreme floods. Soft methods can be cheaper and have other benefits.
Mistake 3: Mixing up lag time and the time of the peak
Lag time is the difference between the time of the peak rainfall and the time of the peak discharge. It is not the total duration of the flood.
Mistake 4: Seeing dams only as benefits
They also trap sediment, change ecosystems, and displace people, and they can fail.