The causes of river flooding

Overview

River flood and physical and the human causes of a flood.

What you need to know

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Questions from this subtopic

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1. 1

   What is a river flood?

   Model answer

   A river flood occurs when the discharge of a river exceeds the capacity of its channel, so that water rises above bankfull and spills out over the surrounding land. It is a natural process rather than an abnormal one - most rivers exceed bankfull discharge roughly once every year or two - and it becomes a hazard only where people, property and productive land occupy the floodplain onto which the water spreads.

2. 2

   What are the main meteorological causes of river flooding?

   Model answer

   Prolonged rainfall over days or weeks, which saturates the soil so that all further rain becomes runoff, is the commonest cause of large lowland floods. Intense rainfall, from convectional summer storms, tropical cyclones or a monsoon, delivers water faster than the ground can absorb it and causes rapid flash flooding. Rapid snowmelt, especially when a sudden thaw or warm rain falls on a deep snowpack lying over frozen ground that cannot absorb it, releases a very large volume at once. Storm surge at the coast, particularly when it coincides with a high spring tide, can hold back the discharge of a river at its mouth so that water backs up the estuary; the surge is the meteorological part of that, the tide is astronomical.

3. 3

   How can the physical characteristics of a drainage basin make flooding more likely?

   Model answer

   Steep relief speeds overland flow to the channel; a circular basin delivers water from all parts at the same time; impermeable geology such as granite, shale or clay prevents percolation into groundwater storage; thin or clay-rich soils have a low infiltration capacity and saturate quickly; high drainage density means water reaches a channel almost immediately; and extensive flat, low-lying land at the basin mouth allows any water that escapes the channel to spread widely and drain away only slowly. A basin with several of these characteristics converts a high proportion of its rainfall into a sharp peak discharge.

4. 4

   Why do antecedent conditions matter so much in explaining a particular flood?

   Model answer

   Because they decide how much of the incoming rain the basin can absorb. If the weeks before the storm have been wet, soil moisture and groundwater storage are already full, so there is no capacity left and almost all the new rainfall becomes rapid runoff. Frozen ground has the same effect, since ice seals the pores. Ground baked hard by a long drought also sheds water, because the surface crusts and cracks and the soil becomes water-repellent. A storm that would pass harmlessly in other circumstances can therefore produce a major flood simply because of the state of the basin when it arrives.

5. 5

   How does urbanisation increase flood risk?

   Model answer

   Tarmac, concrete and roofs are impermeable, so infiltration is virtually eliminated and almost all rainfall becomes overland flow. Gutters, drains and sewers are designed to remove that water as fast as possible and deliver it straight to the river, bypassing the slow soil and groundwater routes entirely. Urban channels are often straightened, culverted or lined, which speeds the flow further, and bridges and culverts can be blocked by debris and act as dams. The result is a much shorter lag time and a much higher peak discharge. At the same time urbanisation puts far more people and property in the path of the water, so the same discharge causes much greater damage.

6. 6

   How do agricultural practices affect flood risk?

   Model answer

   Ploughing up and down a slope creates channels that funnel water directly downhill. Heavy machinery and livestock compact the soil, forming a pan that drastically reduces infiltration. Removing hedgerows and field boundaries takes away the barriers that slowed overland flow and the roots that held the soil. Under-drainage of fields with pipes deliberately removes water quickly and delivers it to the river. Leaving fields bare over winter exposes the soil to raindrop impact, crusting and erosion. Each of these shortens lag time, raises peak discharge and increases the sediment washed into the channel, which reduces its capacity.

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