Why Do Rivers Flood? Rain, Catchments and the Floodplains We Built On
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A river's channel is sized by the river itself for the flow it carries most of the time, and once every year or two, on average, it receives more and spills across the flat land beside it. That land is the floodplain, built by the river out of the silt of past floods, and it is where towns were founded because the soil was rich and the water was near. Flooding is the river doing what made the plain; the disaster is that the plain is now full of houses, and the reasons it floods more often than it used to are the reasons the houses are there.
Where the water comes from
Every river drains a catchment, the area of land from which rain and snowmelt run to it, and a flood is the catchment delivering water faster than the channel can pass it on. Rain falling on saturated ground, on frozen ground, on steep slopes, on rock or on tarmac runs off within minutes; rain falling on dry soil, forest and marsh soaks in and reaches the river over days. The shape of the catchment matters too, since a round basin brings all its tributaries to the main river at once while a long one staggers them, and the timing of storms, a second downpour arriving as the first flood peak passes, can double a river. Snowmelt in spring, rain on snow, a blocked channel of ice, and a storm surge pushing the sea up an estuary against a river coming down each produce their own kind of flood.
The kinds
Floods differ in speed and in what can be done about them:
- •Fluvial floods: a river rising over days after prolonged rain or snowmelt, forecastable and slow, the floods of the Mississippi, the Rhine and the Thames
- •Flash floods: a torrent within minutes of intense rain over a small steep catchment or a city, which kill more people than any other kind because there is no warning; Boscastle in 2004 and the Ahr valley in 2021
- •Surface water floods: rain overwhelming drains in a town and pooling in streets far from any river, now the commonest flood in Britain
- •Coastal and estuarine floods: the sea driven inland by a storm surge, as in the North Sea flood of 1953 and New Orleans in 2005
- •Dam and levee failures: a stored flood released at once
Why they are getting worse
Three things have changed. The climate is warmer, and warm air holds about seven percent more water vapour per degree, so that the same storm drops more rain and the heaviest downpours have grown measurably heavier across the northern hemisphere. The catchments have been altered: forests cleared, wetlands drained, fields compacted by machinery, and towns covered in roofs and roads that shed every drop into the drains, so that the same rain reaches the river faster and higher. And the floodplains have been built on, so that a flood which would once have spread harmlessly over meadows now spreads over homes, and the embankments built to protect one town speed the water to the next. The insurance industry's losses from flooding have risen faster than any other peril, and the damage is mostly the third cause rather than the first.
What works
The engineering answer for a century was to confine the river: embankments, levees, straightened channels and walls that keep the water in and send it downstream faster. It works until it does not, since a levee that holds a river in also holds it up, and when a levee fails, as New Orleans's did in 2005, the water arrives all at once. The newer approach lets the river out where it does least harm. The Dutch programme called Room for the River, after the near-disasters of 1993 and 1995, moved dykes back, lowered floodplains and dug side channels so that the Rhine and its branches could spread, and Britain and Germany now build natural flood management, restoring meanders, planting trees on slopes, blocking drainage ditches and re-wetting peat so that the catchment holds water back. Flood warnings, which give hours or days of notice on large rivers, save more lives than any wall, and the cheapest measure of all is to stop building on the floodplain, which is also the hardest.
Living with it
A river's largest floods are described by their return period, and a hundred-year flood is one with a one percent chance in any year, which over a thirty-year mortgage is a one in four chance, a figure that surprises everyone who buys a house on the plain. The maps that show the risk are public in most countries, and the ground floor of a house in the zone is best built to be flooded and dried, with tiled floors, raised sockets and no carpet, as the Dutch and the Venetians have long done. The floodplain remains, as it was to the first farmers who settled on it, the best land in the valley, and the river remains what it was to them, a neighbour that visits.
The takeaway
Rivers flood when their catchments deliver more water than the channel can carry, which happens naturally every year or two and spreads over the floodplain the river built. Floods are worsening because a warmer atmosphere drops heavier rain, because cleared, drained and paved catchments shed water faster, and because the floodplains are now built on; embankments move the problem downstream, and the effective responses are to give the river room, hold water in the hills, warn early and stop building where it will come.