Flooding & Flood Management

Flooding is one of the most destructive natural hazards in the UK. This lesson examines the causes of river flooding, the factors that influence flood risk, and the strategies used to manage and reduce the impact of floods.

---

What Causes Flooding?

A river floods when its discharge (the volume of water flowing in the channel) exceeds the capacity of the channel. Water spills over the banks and onto the surrounding floodplain.

Physical (natural) causes

Cause	Explanation
Prolonged rainfall	Extended periods of rain saturate the soil, reducing infiltration. Surface runoff increases, feeding more water into the river.
Intense rainfall	Heavy, short bursts of rain (convectional storms) produce more water than the ground can absorb. Surface runoff is rapid.
Snowmelt	A sudden thaw after prolonged cold weather releases large volumes of water into rivers very quickly.
Impermeable rock	Areas underlain by impermeable rocks (e.g., clay, granite) do not allow water to infiltrate. Surface runoff is high.
Steep slopes	Water runs off steep hillsides quickly, reaching the river channel faster.
Saturated soil	If the soil is already waterlogged from previous rainfall, additional rain cannot infiltrate and runs straight into the river.

Human causes

Cause	Explanation
Urbanisation	Concrete, tarmac, and buildings create impermeable surfaces that prevent infiltration. Water runs off quickly into drains and rivers.
Deforestation	Removing trees reduces interception (trees catch rainfall on their leaves) and transpiration. More water reaches the ground surface and runs into rivers.
Agriculture	Ploughing compacts the soil, reducing infiltration. Field drains may channel water into rivers more quickly.
Climate change	Warmer temperatures lead to more evaporation, more moisture in the atmosphere, and potentially more intense rainfall events.
Building on floodplains	Construction on floodplains reduces the land available to absorb floodwater.

Exam Tip: In exam answers about the causes of flooding, always give both physical and human causes. A top answer will explain how human activities increase the risk by speeding up the transfer of water from the land to the river.

---

Hydrographs

A flood hydrograph (also called a storm hydrograph) shows how a river's discharge changes over time in response to a rainfall event.

Key features of a hydrograph

Feature	Definition
Peak discharge	The maximum discharge reached during the storm event
Peak rainfall	The point of maximum rainfall intensity
Lag time	The time delay between peak rainfall and peak discharge
Rising limb	The part of the graph showing discharge increasing as water enters the river
Falling limb (recession limb)	The part of the graph showing discharge decreasing after the peak
Base flow	The normal (background) discharge of the river between storms, fed by groundwater

Flashy vs flat hydrographs

Feature	Flashy Hydrograph	Flat Hydrograph
Peak discharge	High	Low
Lag time	Short	Long
Rising limb	Steep	Gentle
Falling limb	Steep	Gentle
Flood risk	High	Low
Typical conditions	Impermeable rock, urbanised area, steep slopes, deforested, saturated soil	Permeable rock, rural area, gentle slopes, forested, dry soil

Exam Tip: A "flashy" hydrograph (high peak, short lag time) indicates high flood risk. When explaining a flashy hydrograph, always link to factors that speed up surface runoff: impermeable surfaces, steep slopes, lack of vegetation, saturated ground.

---

Flood Management Strategies

Hard Engineering

Hard engineering uses man-made structures to control or contain floodwater.

Strategy	Description	Advantages	Disadvantages
Dams and reservoirs	A dam is built across a river, creating a reservoir behind it. Water is stored and released slowly, reducing peak discharge downstream.	Very effective at controlling flooding; reservoir provides water supply, recreation, and hydroelectric power	Very expensive (millions of pounds); floods a large area of land behind the dam; displaces communities; traps sediment
Channel straightening	Meanders are cut through to create a straighter, shorter channel. Water moves through the area more quickly.	Moves floodwater through towns quickly	Increases flood risk downstream (water arrives faster); removes natural habitats; can increase erosion
Embankments (artificial levees)	Raised earth or concrete banks are built along the river to increase the channel's capacity.	Relatively cheap; effective at containing floodwater	If breached, flooding is sudden and severe; can give a false sense of security; restrict access to the river
Flood relief channels	An artificial channel is built to divert excess water away from the main river during floods.	Effective at reducing flood risk in specific areas	Expensive; may transfer the flood problem elsewhere; takes up land
Flood barriers and gates	Moveable barriers that can be raised during flood events to prevent tidal or river flooding.	Very effective; can be opened/closed as needed	Very expensive (e.g., the Thames Barrier cost over £530 million)

---

Soft Engineering

Soft engineering works with natural processes to reduce flood risk.

Strategy	Description	Advantages	Disadvantages
Flood warnings and preparation	The Environment Agency monitors river levels and issues flood warnings to communities.	Saves lives; relatively cheap; allows people to prepare	Does not prevent flooding; relies on people taking action; may not give enough warning time
Floodplain zoning	Planning restrictions prevent building on floodplains. Different zones allow different land uses (e.g., parks and sports fields on the highest-risk areas).	Prevents new development in flood-risk areas; preserves natural floodplain	Difficult to apply to existing built-up areas; restricts development; politically unpopular
Afforestation	Planting trees in the upper catchment area to increase interception and transpiration, reducing surface runoff.	Cheap; natural; creates wildlife habitat; reduces soil erosion; absorbs CO₂	Takes many years for trees to grow and become fully effective; may not prevent flooding in extreme events
River restoration	Returning the river to its natural state by removing man-made features (e.g., straightened channels, concrete banks), allowing the river to meander and reconnect with its floodplain.	Natural; creates wildlife habitat; stores floodwater on the floodplain; cheap to maintain	May increase local flooding in the restored area; takes time; requires land
Managed flooding	Allowing certain areas of the floodplain to flood deliberately during high-water events, reducing the flood peak downstream.	Effective at reducing peak discharge; can create wetland habitats	Loss of agricultural land; compensation must be paid to landowners
Sustainable drainage systems (SuDS)	Permeable surfaces, green roofs, swales, and ponds designed to slow the flow of surface water into rivers in urban areas.	Reduces surface runoff; relatively cheap; can be attractive; reduces pollution in rivers	Requires retrofitting in existing urban areas; may not cope with extreme events

---

Case Study: Boscastle Flood (2004)

On 16 August 2004, the village of Boscastle in north Cornwall experienced a devastating flash flood.

Causes

Type	Details
Physical	75 mm of rain fell in 2 hours (a month's worth). The ground was already saturated from previous weeks of rain. The village sits at the confluence of the River Valency and River Jordan in a narrow, steep-sided valley, funnelling water into the village.
Human	Development on the floodplain had increased impermeable surfaces. The channel had been narrowed by buildings. A car park near the river acted as a dam when cars were swept into the river and blocked the channel.

Effects