Flooding: Hydrographs, Causes & Risk Factors
When a River Floods
A river floods when its discharge exceeds the capacity of its channel and water spills over onto the surrounding land. Flooding is a natural process — healthy rivers flood roughly every couple of years, depositing fertile silt across the flood plain. The problem comes when people build homes, farms, schools and infrastructure on those flood plains. UK floods now cost the economy over £1 billion in an average year, and the cost is rising as the climate warms.
Storm Hydrographs
A storm hydrograph shows how a river's discharge responds to a single storm. It plots rainfall as bars on one axis and river discharge as a line on the other, both against time. Several features matter:
Peak rainfall. The highest rainfall recorded during the storm.
Peak discharge. The highest river discharge recorded after the storm. The peak comes some time after the rain because water takes time to reach the river.
Lag time. The time gap between peak rainfall and peak discharge. Short lag time = water reaches the river quickly = high flood risk. Long lag time = water reaches the river slowly = lower flood risk.
Rising limb. The line of the graph as discharge climbs. A steep rising limb means water is reaching the river fast.
Falling limb. The line as discharge drops back to normal. A gentle falling limb means the channel is draining slowly — flood risk persists.
A short lag time, steep rising limb and high peak discharge all signal a flashy hydrograph — a drainage basin where water reaches the river fast. This is high flood risk. Urban basins, basins with impermeable rock and steep relief all produce flashy hydrographs.
Physical Factors That Increase Flood Risk
Four physical factors control how quickly water reaches the river after rain:
- Precipitation. Heavy or prolonged rainfall is the most obvious cause. Cumbria's flooding in December 2015 saw 341 mm of rain in 24 hours — far more than any drainage basin can absorb. Snowmelt during a sudden thaw can also produce huge flood discharges.
- Geology. Impermeable rocks like granite and clay force water to run off the surface rather than soaking in. Permeable rocks like chalk and limestone allow water to soak in, slowing the runoff and reducing flood risk.
- Relief. Steep slopes mean water runs off quickly. Upland UK basins like the Cumbrian Lakes have very steep relief and short lag times. Flat lowland basins like the Cambridgeshire Fens have long lag times.
- Saturation. When the ground is already wet from previous rain, it cannot absorb more — so all new rainfall runs straight off. November and December storms in the UK are often the most dangerous for this reason.
Human Factors That Increase Flood Risk
People can change a drainage basin's response to rainfall almost as much as the natural factors themselves.
- Urbanisation. Concrete, tarmac and roof tiles are impermeable. Rain that fell on grass or soil now flows straight off into drains, then into the river. Urban basins like the lower Thames have very flashy hydrographs. New housing on flood plains makes this worse.
- Deforestation. Trees absorb water through their roots and slow runoff. When woodland is cleared for farming or development, water reaches the river faster.
- Farming practices. Heavy machinery compacts soil; ploughing in autumn leaves bare fields exposed to winter rain; livestock paths channel runoff downslope. All increase how fast water reaches the river.
- Climate change. Warmer Atlantic waters hold more moisture, which means heavier rainfall events. The Met Office records show UK winters are now about 13% wetter than they were in 1961–1990. The frequency of major UK floods has roughly doubled compared to the 1980s.
Urbanisation increases flood risk in three ways: impermeable surfaces (concrete, tarmac, roofs) stop infiltration; storm drains carry water to the river much faster than natural flow; new housing on flood plains puts more property at risk.
Comparing Two UK Drainage Basins
The differences are starkest when you compare two basins side by side. The upper Eden in Cumbria has steep relief, impermeable slate and shale, high winter rainfall, and is rural with sheep grazing. Its hydrograph is flashy: short lag time, steep rising limb, very high peak. The upper Thames in Oxfordshire has gentle relief, much permeable chalk, lower rainfall and a mix of farmland and small towns. Its hydrograph is much smoother: longer lag time, gentle rising limb, lower peak. The Eden floods every couple of winters; the upper Thames much less often.
Three trends are pushing UK flood risk steadily upwards. First, climate change is making rainfall events more intense; the Met Office estimates climate change made a downpour like Storm Desmond about 40% more likely than a century ago. Second, ageing flood defences are not always re-sized for current conditions; some defences were built for once-in-100-year events that now happen every 25. Third, more building on flood plains has put roughly 5.2 million UK properties at risk — one in six homes. The combination is why floods that used to make headlines once a decade now happen most winters.