River Flooding: Causes and Effects

Flooding is one of the most significant natural hazards in the UK. Understanding why rivers flood and what happens when they do is essential for managing flood risk and protecting people and property. This lesson examines the physical and human causes of flooding, the effects on people and the environment, and how hydrographs are used to analyse flood events.

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What Is a Flood?

A flood occurs when a river's discharge exceeds the capacity of its channel, causing water to overflow onto the surrounding floodplain. Floods can be caused by natural processes, human activity, or (most commonly) a combination of both.

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Physical (Natural) Causes of Flooding

Cause	How It Increases Flood Risk
Prolonged rainfall	Soil becomes saturated — it cannot absorb any more water. Additional rainfall flows over the surface as surface run-off directly into rivers, rapidly increasing discharge.
Intense rainfall	Very heavy rain falls faster than the soil can absorb it, even if the ground was previously dry. Surface run-off is high.
Snowmelt	A rapid thaw following heavy snowfall releases large volumes of water into rivers over a short period.
Impermeable rock	Rocks like granite and clay do not allow water to pass through. Rainfall runs off the surface quickly rather than soaking in, leading to rapid increases in river discharge.
Steep slopes	Water runs off steep slopes quickly, reaching the river channel in a short time. This produces a rapid rise in discharge.
Saturated or frozen ground	If the ground is already waterlogged (from previous rain) or frozen, it cannot absorb more water. Run-off is increased.
Thin soils	Thin, rocky soils in upland areas absorb less water, increasing run-off.
Drainage basin shape	A circular drainage basin delivers water to the river from all directions simultaneously, producing a rapid peak in discharge. An elongated basin delivers water more gradually.
High drainage density	Many tributaries funnel water into the main river quickly, increasing discharge.
Tidal surges	In coastal and estuarine areas, a high tide combined with low atmospheric pressure and onshore winds can push seawater upstream, blocking river outflow and causing flooding.

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Human Causes of Flooding

Human activities have significantly increased flood risk in many parts of the UK.

Cause	How It Increases Flood Risk
Urbanisation	Buildings, roads, car parks, and pavements create impermeable surfaces that prevent infiltration. Rainwater runs rapidly into drains and then into rivers. Urban areas can produce run-off rates 5-10 times higher than rural areas.
Deforestation	Trees intercept rainfall on their leaves (interception), absorb water through their roots (uptake), and release it gradually through transpiration. Removing trees removes these natural buffers, increasing the speed and volume of run-off.
Agricultural practices	Ploughing compacts subsoil, reducing infiltration. Draining wetlands removes natural water storage. Overgrazing compacts soil.
Channel straightening	Artificially straightening a river channel speeds up the flow, delivering flood water downstream faster. This may protect the straightened area but increases flood risk downstream.
Building on floodplains	Floodplains are naturally designed to flood — they act as natural storage for excess water. Building on them reduces storage capacity and puts people and property at risk.
Climate change	Increasing global temperatures are leading to more intense rainfall events in the UK, increased winter precipitation, and potentially more frequent storms — all increasing flood risk.
Drains and sewers	Urban drainage systems channel water directly into rivers, bypassing the natural infiltration process and reducing lag time. During extreme events, the drainage system can be overwhelmed, causing surface water flooding.

Exam Tip: In exam questions about flood causes, the best answers link physical and human causes together. For example: "The flood was caused by intense rainfall (physical) falling on an area that had been heavily urbanised (human), meaning the impermeable surfaces prevented infiltration and dramatically increased surface run-off, leading to a rapid rise in river discharge."

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Flood Hydrographs

A flood hydrograph (also called a storm hydrograph) is a graph that shows how a river's discharge changes over time during and after a rainfall event. It is one of the most important diagrams in GCSE Geography.

Key Components of a Hydrograph

Component	Description
Rainfall bar chart	Shown at the top of the graph (often as blue bars); shows the amount and timing of rainfall
Discharge line	The curve showing river discharge (m³/s) over time
Base flow	The normal, day-to-day discharge of the river before the storm (fed by groundwater)
Rising limb	The part of the curve where discharge is increasing as rainwater reaches the river
Peak discharge	The maximum discharge reached during the flood event
Falling limb (recession limb)	The part of the curve where discharge is decreasing as the flood subsides
Lag time	The time delay between the peak rainfall and the peak discharge — this is the most important measurement on the graph

What Lag Time Tells Us

Lag Time	Meaning	Likely Conditions
Short lag time	Water reaches the river quickly → high flood risk	Impermeable surfaces, steep slopes, urbanisation, deforestation, intense rainfall
Long lag time	Water reaches the river slowly → lower flood risk	Permeable rock, gentle slopes, vegetation, rural land use, less intense rainfall

Flashy vs Flat Hydrographs