Flooding » Land Relief and Flooding

What you'll learn this session

Study time: 30 minutes

How land relief affects flooding patterns
The relationship between slope steepness and flood risk
How drainage basin shape influences flood hazards
The impact of valley shape on flood extent
Case studies of flooding related to land relief features
Management strategies for flood-prone areas with challenging relief

Introduction to Land Relief and Flooding

Land relief (the shape and features of the Earth's surface) plays a crucial role in determining how water flows across landscapes and influences flooding patterns. When heavy rain falls, the way water moves downhill and collects in rivers depends greatly on the shape of the land.

Key Definitions:

Land Relief: The physical shape, height and features of the land surface.
Drainage Basin: An area of land where precipitation collects and drains into a common outlet like a river or lake.
Watershed: The boundary between drainage basins.
Floodplain: Flat land around a river that gets covered by water when the river floods.

📍 How Slope Affects Flooding

The steepness of slopes has a major impact on how quickly water reaches river channels:

Steep slopes = faster runoff = quicker river response = higher flood risk
Gentle slopes = slower runoff = more time for infiltration = lower flood risk

In mountainous areas, heavy rainfall can lead to flash floods because water rushes downhill rapidly, giving little warning time.

🏠 Drainage Basin Shape

The shape of a drainage basin affects how quickly rainwater reaches the main river:

Circular basins tend to concentrate water quickly, causing rapid rises in river levels
Elongated basins spread the water's arrival time, reducing peak flood levels
Fan-shaped basins with tributaries converging at one point create high flood risks at that location

Valley Characteristics and Flooding

The shape of river valleys significantly influences how floods develop and spread:

📏 V-Shaped Valleys

Common in upland areas

Steep sides confine floodwater

Floods move quickly downstream

Limited lateral flooding but higher water depths

Higher erosive power

📏 U-Shaped Valleys

Formed by glacial erosion

Broader floor allows water to spread

Slower flood movement

Settlements in these valleys often at risk

Floodplains may be extensive

📏 Wide Floodplains

Found in lower river courses

Allow extensive lateral flooding

Slower flood velocities

Longer flood duration

Often heavily developed despite risks

Specific Land Relief Features and Flood Risk

Confluence Points

Where two or more rivers join together, flood risk increases significantly. This is because:

Water volume combines from multiple sources
If peak flows from tributaries arrive simultaneously, severe flooding can occur
Towns historically developed at confluences for transport reasons, putting them at risk

Examples include Carlisle (UK) where the Rivers Eden, Caldew and Petteril meet, creating a flood hotspot.

Constriction Points

When valleys or river channels suddenly narrow, water backs up behind the constriction, causing flooding upstream:

Gorges and narrow valleys act as bottlenecks
Bridges and other structures can create artificial constrictions
Areas upstream of constrictions are particularly vulnerable

Case Study Focus: Boscastle Flood, Cornwall, UK (2004)

The village of Boscastle experienced devastating flash floods due to its challenging land relief:

Steep-sided valley: The village sits in a narrow, steep valley where the River Valency flows
Steep surrounding slopes: The drainage basin has steep slopes that rapidly channelled rainfall
Confluence point: The River Valency and Jordan River meet in the village
Constriction points: Narrow bridges and the valley itself created bottlenecks
Funnel-shaped catchment: The drainage basin channelled water to a single point

On 16th August 2004, 200mm of rain fell in just 5 hours. The valley's shape concentrated this water, causing a 3-metre wall of water to surge through the village, destroying buildings and cars. The land relief amplified what would have been a serious flood into a catastrophic one.

Other Relief Factors Affecting Flooding

🌋 Altitude and Orographic Rainfall

Mountains and high ground force air to rise, cool and release moisture as rain or snow:

Upland areas typically receive more rainfall than lowlands
Western UK uplands can receive 2-3 times more rainfall than eastern lowlands
This increased precipitation creates greater flood potential in mountainous regions
Example: Lake District receives over 3000mm of rain annually, compared to less than 600mm in parts of eastern England

💧 Soil and Rock Permeability

The underlying geology interacts with land relief to affect flooding:

Impermeable rock (like clay) prevents infiltration, increasing surface runoff
Permeable rock (like chalk) allows water to soak in, reducing immediate flood risk
Steep slopes with impermeable surfaces create the highest flood risk
Example: Dartmoor's granite bedrock and steep slopes create rapid runoff during storms

Managing Flood Risk in Areas with Challenging Relief

Areas with flood-prone land relief require specific management approaches:

🛠 Hard Engineering

Channel straightening to move water quickly through constricted areas

Flood walls designed for narrow valleys

Upstream dams to control water release

Bypass channels around constriction points

🌲 Soft Engineering

Afforestation on steep slopes to slow runoff

Creating upstream storage areas

Land use controls in high-risk areas

Natural flood management in upland areas

📋 Planning Approaches

Avoiding development in high-risk confluence areas

Flood-resistant building design in valley bottoms

Early warning systems for flash-flood prone areas

Evacuation planning for communities in steep valleys

Post-Boscastle Management

After the 2004 flood, Boscastle implemented several measures to address its challenging relief:

The river channel was widened and deepened
A new lower bridge was built with a wider span to reduce constriction
The car park was raised to act as a flood defence
Buildings were flood-proofed with water-resistant materials
An improved flood warning system was installed
Upstream land management changes to slow water flow

These measures acknowledge that the village's location and surrounding land relief cannot be changed, so adaptation is necessary.

Summary: Key Points About Land Relief and Flooding

Steep slopes accelerate runoff and increase flood risk
Drainage basin shape affects how quickly water concentrates in rivers
Valley shape determines how floods develop and spread
Confluence points and constrictions create flood hotspots
Orographic rainfall increases flood potential in upland areas
Management strategies must be tailored to the specific relief challenges of each location

Understanding how land relief influences flooding is essential for predicting flood risk, planning settlements and designing effective flood management strategies. While we cannot change the fundamental shape of the land, we can work with it to reduce the impact of floods on communities.

🧠 Test Your Knowledge!