Flooding

What makes a river flood: the physical causes

A river floods when its discharge - the volume of water passing a point each second - becomes larger than the channel can carry, so the water spills out over the floodplain. Think of the drainage basin as a system: rain and snow are the inputs, the soil, rock, lakes and wetlands are the stores, and the river is the main output. Flooding is what happens when the inputs arrive faster than the stores can fill and the output can carry them away, so anything that adds more water, or speeds that water into the channel, raises the flood risk.

The rainfall itself is the first control. Heavy rainfall in a short time delivers more water than the soil can absorb, and prolonged rainfall over days or weeks is worse in a different way: the soil stores fill up, and once the ground is saturated every further millimetre of rain becomes surface run-off. Snowmelt is the same problem stored and then released - snow lies on the ground all winter and a sudden thaw, especially when warm rain falls onto the snow or the ground beneath is still frozen, sends the whole winter's precipitation to the river within days. This is why flooding on the Rhine and the Danube is often a spring event, while monsoon flooding in South and Southeast Asia is a summer one.

The ground decides how much of that water is stored on the way. Saturated soil cannot accept any more water, so the rain runs straight off the surface. Compacted soil, squeezed by machinery, by livestock or by the impact of raindrops on bare ground, has had its pore spaces closed, so it behaves almost like a pavement. Soil erosion makes it worse again: gullies and rills are channels that carry water downhill far faster than an intact soil surface would, and the eroded sediment is washed into the river, where it raises the bed and reduces the room the channel has for water.

Finally, the shape and the rock of the basin set the speed of the response. Impermeable rock such as clay or granite lets almost no water through, so nearly all of the rain becomes surface run-off, while a permeable rock such as chalk or sandstone absorbs it and releases it slowly. Steep relief gives run-off a fast route downhill, and a small drainage basin means the water has only a short distance to travel to reach the channel. A large basin with gentle slopes, permeable rock, deep soil and dense natural vegetation does the opposite: it stores water and releases it over weeks, so the river rises slowly and rarely floods.

• Heavy or prolonged rainfall: more water arrives than the soil and channel can deal with.
• Saturated soil: once the stores are full, every further millimetre of rain becomes surface run-off.
• Compacted soil: pore spaces closed by machinery, livestock or raindrop impact, so water cannot infiltrate.
• Soil erosion and gullies: fast routes for water to reach the river, plus sediment that raises the river bed.
• Snowmelt: stored winter precipitation released quickly by a sudden thaw or by rain falling on snow.
• Impermeable rock such as clay or granite: little infiltration, so most rain becomes surface run-off.
• Steep relief: water runs downhill quickly instead of soaking in.
• Small drainage basin: a short distance and a short time for water to reach the channel.

Flooding from the sea: storm surges, tsunamis and rising sea level

Not every flood comes down a river. Coastal flooding happens when the sea is driven over land that is normally dry, and the most common cause is a storm surge. Low air pressure above the sea allows the surface to rise, and the strong onshore winds of a tropical cyclone or a deep winter depression push that dome of water towards the coast; if the surge arrives at the same time as a high tide, the two add together. A surge is not a wave - it is a raised sea level that can persist for hours and reach several metres, which is why coastal defences are designed around the height of a surge at spring tide.

A tsunami is a different mechanism again. An earthquake, a volcanic eruption or an undersea landslide displaces a huge volume of water, which travels across the ocean as a series of waves at the speed of a jet aircraft and arrives with very little warning. Tsunamis are not weather-driven and they are not seasonal, but their effect on the coastline is the same as a flood: the sea comes in, and the low-lying land, ports, roads and settlements behind the shore are inundated and often destroyed.

Climate change makes both of these worse by raising the starting level. Thermal expansion of the ocean and the melting of glaciers and ice sheets have raised global sea level, and the rise is faster in some deltas and coastal cities because the land there is sinking as groundwater is pumped out and sediment is trapped behind dams. A surge of exactly the same size therefore reaches further inland than it did fifty years ago, and it overtops defences that were designed for a lower sea. The same warming also loads the dice on rainfall: warm air holds more water vapour, so the heaviest rainfall events carry more water and occur more often, even where the total annual rainfall has not changed.

• Storm surge: low air pressure raises the sea surface and strong onshore winds drive it onto the coast, worst when it coincides with a high tide.
• Tsunami: a wave set off by an earthquake, volcanic eruption or undersea landslide, arriving with little warning and flooding low-lying coasts.
• Sea level rise: a higher starting level means the same surge floods more land and overtops defences built for an older climate.
• More extreme weather events: warmer air holds more water vapour, so heavy rainfall events become more intense.

Human causes: how people make flooding worse

People rarely add water to a drainage basin; what they do is remove the places that used to store it and the time it used to take to reach the river. Urbanisation is the clearest example. Roofs, roads, pavements, car parks and compacted gardens are impermeable surfaces, so rain that would once have soaked into the soil runs off instead, and gutters, drains and sewers deliver that run-off to the river within minutes. At the same time the vegetation that intercepted rain and took it up through its roots is cleared, and the flat floodplain that used to store the flood is built over. The result is a shorter lag time and a higher peak discharge - a flashy river that floods quickly.

Deforestation removes the interception of the canopy and the roots that held the soil open and in place, so infiltration falls and surface run-off rises. Where the forest is cleared on steep tropical hillsides the effect is worse than a flood alone: the saturated soil loses its grip and moves as a landslide, adding sediment to the channel, blocking it, and releasing a burst of muddy water when the blockage gives way. The flood that follows may be smaller in the river but far more destructive in the hillside villages below.

Poor farming practice does the same job in a quieter way. Heavy machinery and the trampling of livestock compact the soil so that rain cannot infiltrate; leaving fields bare between crops exposes the surface to raindrop impact, which seals it; ploughing up and down the slope gives run-off a channel to follow; and field drains and ditches that are dug to dry the land deliver the water straight into the river. Removing hedgerows, field margins and riverside strips takes away the last places where run-off could be slowed and stored.

Drainage and engineering complete the list. Draining wetlands, floodplain meadows and upland bogs removes natural storage that used to hold millions of cubic metres of water and release it slowly. Building in the channel, culverting streams and allowing bridges to be blocked by debris all reduce the capacity of the river to carry water away. Dams are a two-edged case: while a reservoir has space it can cut the flood peak downstream, but when it is full the operators must release water and add to the flood, and a dam that fails releases a surge far more destructive than any natural flood.

The flood hydrograph and what a short lag time tells you

A flood hydrograph is the graph that links the rain to the river. The bars show rainfall in millimetres for each hour or day of the storm, and the line shows the discharge of the river in cubic metres per second (cumecs, m3/s). Read it from left to right: the rainfall rises, then the river rises along the rising limb, reaches its highest point at peak discharge, and falls away along the recession limb towards the base flow that the river keeps even in dry weather. The lag time is the gap between the peak rainfall and the peak discharge - the time the basin takes to pass the water from the storm to the river. The rain falling on the slope is the cause; the flood peak is the response.

A short lag time means the basin responds quickly. Water reaches the channel soon after it falls, so the rising limb is steep, the peak discharge is high and it arrives with little warning: this is the flash flood, the kind that sweeps through a town an hour or two after a storm and gives people no time to move their belongings or themselves. A long lag time gives the opposite - a gentle rising limb, a lower peak, and hours or days in which a warning can be issued and an evacuation carried out. The same total rainfall can therefore produce a harmless river rise in one basin and a disaster in another, which is exactly why the lag time is the number examiners want you to read off and explain.

Everything in this topic can be expressed as a change to the hydrograph. Steep relief, impermeable rock, saturated or compacted soil, bare ground, urban surfaces, drains and a small drainage basin all shorten the lag time and raise the peak. Gentle slopes, permeable rock, deep porous soil, dense vegetation, wetlands and an intact floodplain lengthen the lag time and lower the peak. In a hydrograph comparison question, check what is different between the two catchments (surface, relief, vegetation, rock, soil) and say what that does to the speed of the water, not just what the graph looks like.

The impacts of flooding on people and the environment

The first impacts are physical and immediate. Floodwater damages buildings and infrastructure: homes are made uninhabitable, bridges and roads are washed away or undermined, railway lines are closed, and power, water and sewage works are flooded so that electricity, clean water and sanitation all fail at once. Crops are lost when the harvest is submerged or the seed and stored grain are ruined, livestock drown or have to be moved, and habitats are damaged when silt, sewage and pollution smother the gravel beds in which fish spawn and the vegetation along the bank. On saturated slopes the flood can also trigger landslides, which cut roads and bury settlements, as the monsoon floods in the Indian state of Kerala in 2018 showed.

The second wave of impacts is about water and health. Drinking-water supplies are contaminated when sewage treatment works, septic tanks and pit latrines overflow and when wells, boreholes and hand pumps are submerged in floodwater carrying sewage and farm waste. Bacteria spread as water-related disease - cholera, typhoid and dysentery are the classic flood diseases - and the risk is greatest where people are crowded together in temporary shelters with poor sanitation. Standing water left behind after the flood dries becomes a breeding ground for mosquitoes, so malaria and dengue fever rise in the following weeks. Drowning and injuries kill during the flood itself, and the loss of homes, income and security produces long-term mental health problems afterwards.

The third group is economic. Financial losses include the cost of emergency response and repairs to roads, bridges, water works and power lines, the loss of business while factories and shops are shut, the damage to machinery, stock and vehicles, insurance claims, and the loss of a whole year's income for farmers whose crop and seed are gone. The same flood costs a low-income country far more in proportion, because homes are often informal, insurance is rare and the government has less money to rebuild; and the people with the least savings recover most slowly. Flood damage to a major industrial area can also be felt internationally, when a disrupted factory interrupts a supply chain on another continent.

It is worth remembering that the same water also does good. Flooding deposits nutrient-rich silt across the floodplain, which is why the flattest and most fertile farmland in the world is the land beside rivers: the Nile floodplain, the Mekong floodplain and the Ganges plain were farmed for thousands of years precisely because the flood renewed the soil. Floodwater also recharges surface water and ground water stores, refilling the wetlands, lakes and aquifers that keep villages supplied and rivers flowing through the dry season. The problem is not the water but the size and unpredictability of the flood, which is why management aims to keep the benefits while reducing the damage.

Hard engineering: holding back the water

Hard engineering means building structures to control the river. Dams and reservoirs are the largest option: the dam stores water during the wet season and the operators release it at a controlled rate, so the flood peak downstream is cut. A dam usually pays for itself in more than one way, because the same reservoir supplies water and irrigation and can generate hydroelectric power. The limitations are heavy, though. A dam is enormously expensive to build and maintain, the reservoir floods farmland, forest and villages so people are displaced and their livelihoods are lost, and the protection lasts only as long as there is space in the reservoir to store floodwater. Trapping silt behind the dam starves the floodplain of the sediment that built it and removes the nutrient supply from the fields, the released water erodes the channel downstream, and a dam that fails releases a surge far worse than the flood it was built to prevent.

Levees (embankments) and flood walls raise the banks so that the channel can hold a larger volume before it overflows, and a flood wall can be built along the length of a city where there is no space for an earth embankment. They are cheaper per metre than a dam, can be built quickly and protect exactly the land behind them. But a levee is only as good as the flood it was designed for: it can be overtopped by a larger event and it can be breached, and because it makes the channel narrower and the water deeper, it also speeds the flow and raises the level, passing the flood peak on to the next community downstream. It stops silt reaching the floodplain, needs constant maintenance, and by making people feel safe behind it, it encourages even more building on the floodplain - so when it does fail, the damage is far greater.

Channel straightening, together with dredging and the cutting of new relief channels, works on the channel itself. Straightening removes the bends so that water moves faster and the flood passes through the city more quickly, and a relief channel diverts part of the flood around the built-up area altogether. Both can reduce the local flood level without losing land to a reservoir, and both can be done in stages as money allows. The catch is that the water which has been hurried through the city arrives sooner and in a sharper peak at the next town downstream; the new channel also erodes and needs dredging to stay effective, and the natural bends, pools and riffles that made the river a habitat are destroyed.

Soft engineering, and managing a flood before, during and after it

Soft engineering works with the natural system instead of building against it. Afforestation and planting along watercourses in the upper basin increase interception, keep the soil open and slow the run-off, which lengthens the lag time and lowers the peak; the limit is that trees take decades to grow, need land that could be farmed, and cannot hold back an exceptional storm by themselves. Floodplain zoning uses planning controls to keep housing and industry off the land that floods and to keep it as park, playing field or pasture that can be flooded without much loss - cheap, effective and good for wildlife, but politically very difficult where land is scarce and it only works where the rules are enforced. Wetland restoration and the approach known as Room for the River - lowering floodplains, moving dikes inland, reconnecting side channels and old meanders - deliberately gives the river somewhere to spread out and store water, which lowers the peak, recharges groundwater and restores habitat, but it needs a great deal of land and takes years to complete. Permeable surfaces and sustainable urban drainage (permeable paving, swales, ponds, green roofs, water butts and rain gardens) make new development behave more like the fields it replaced, though retrofitting them into an existing city is disruptive and expensive, and the storage is small compared with a major storm. Warning systems and insurance are also soft measures: rain gauges, river levels and weather radar feed forecasts, and sirens and mobile alerts give people time to act.

Management is usually described in three stages. Before a flood: monitoring and warning systems, land use zoning, building design that raises floor levels and keeps services out of basements, disaster preparation with plans, drills, emergency supplies and trained rescue teams, flood risk maps, and insurance. During a flood: issuing the warning, evacuating people and livestock along planned routes, opening shelters, putting up temporary barriers and sandbags, and carrying out rescues. After a flood: rebuilding in a way that is more resistant, restoring water, power and roads, compensating the people affected, and international aid in the form of money, equipment, food and specialist teams. The stages are interdependent - a warning is worth nothing without transport, shelters and somewhere safe to go, and insurance is worth nothing to a household that cannot afford the premium or lives in an area the insurers have refused to cover.

The judgement you are expected to make is that no single method is sufficient, and that each has a cost. Hard defences protect expensive assets immediately, which is what a city with hospitals, power stations and factories needs, but they are costly, need maintenance for ever, and transfer the risk downstream. Soft measures lower the peak for everyone in the basin, are usually cheaper and benefit habitats and groundwater, but they need land, take years and cannot cope with an extreme event alone. Monitoring and warning is the cheapest way to save lives, but it saves far less property. The practical answer for most basins is a combination: slow the water and store it upstream with trees, wetlands and permeable surfaces, keep the floodplain free of new building through zoning, defend the assets that cannot be moved, warn and evacuate for what remains, and decide openly who carries the residual risk and how it is paid for.

• Before: monitoring and warning, land use zoning, flood-proof buildings, plans, drills, emergency supplies and rescue teams.
• During: warning, evacuation of people and livestock, shelters, temporary barriers, emergency rescue.
• After: rebuilding stronger, restoring services, compensation and international aid.
• Soft methods: afforestation, floodplain zoning, wetland restoration, permeable surfaces and sustainable urban drainage, warning systems and insurance.