G26: Flood Risk and Management
Physical and human factors affecting flood risk, storm hydrographs, hard and soft engineering flood management strategies, and the Keswick and Cockermouth floods case study.
🌊 Why Do Rivers Flood?
Definition: A flood occurs when a river's discharge exceeds its channel capacity, causing water to overflow onto the flood plain. Discharge is the volume of water flowing past a point in a given time, measured in cumecs (m³/s).
Rivers flood when the amount of water entering the river (from rainfall, snowmelt or tributaries) exceeds the channel's ability to carry it away. Whether a flood occurs depends on both physical factors (weather and drainage basin characteristics) and human factors (land use and management).
🌧️ Physical Factors Affecting Flood Risk
Rainfall Characteristics
- Intensity: Heavy, intense rainfall exceeds the infiltration capacity of soil, causing rapid surface runoff
- Duration: Prolonged rainfall saturates the soil, preventing further infiltration
- Frequency: Repeated storms prevent recovery between events
- Type: Snowmelt releases large volumes of water gradually; convective storms produce intense downpours
Drainage Basin Characteristics
| Factor | High Flood Risk | Low Flood Risk |
|---|---|---|
| Size | Large basin - collects more water | Small basin - less water collected |
| Shape | Circular basin - water reaches outlet simultaneously | Elongated basin - water arrives at different times |
| Relief (steepness) | Steep slopes - fast surface runoff | Gentle/flat slopes - slow runoff |
| Rock type | Impermeable rock (clay, granite) - no infiltration | Permeable rock (chalk, limestone) - water infiltrates |
| Soil type | Thin/clay soil - low infiltration | Thick/sandy soil - high infiltration |
| Vegetation | Bare/sparse - rapid runoff | Dense forest - interception slows runoff |
| Drainage density | High density - water reaches channel quickly | Low density - water moves slowly |
| Antecedent conditions | Already saturated soil | Dry soil - can absorb water |
Key Fact: Drainage density is the total length of river channels per unit area. It is highest in impermeable, steep basins where water runs off quickly rather than infiltrating. High drainage density means water reaches the main channel rapidly, increasing flood risk.
🏠 Human Factors Affecting Flood Risk
- Urbanisation: Impermeable surfaces (tarmac, concrete, roofs) prevent infiltration - up to 90% of rainfall becomes surface runoff in urban areas compared to 10% in rural areas
- Deforestation: Removing trees reduces interception and increases surface runoff; also reduces soil stability, increasing sediment in rivers
- Agriculture: Overgrazing and soil compaction reduce infiltration; arable fields with bare soil in winter increase runoff
- Drainage systems: Storm drains and culverts channel water directly into rivers, speeding up delivery
- Climate change: More intense rainfall events and rising sea levels increase flood frequency
Example: Urbanisation and Flooding
In urban areas, up to 95% of rainfall can become surface runoff compared to just 10% in natural woodland. Storm drains carry this water rapidly to rivers, creating a flashy hydrograph with a short lag time and high peak discharge. This is why urban flooding can happen quickly even with moderate rainfall.
📈 Storm Hydrographs
Definition: A storm hydrograph (or flood hydrograph) shows how a river's discharge changes over time in response to a rainfall event. It is a key tool for understanding flood risk and comparing the flood response of different drainage basins.
Key Features of a Hydrograph
- Rising limb: The steepness shows how quickly discharge increases - steep = flashy (rapid response)
- Peak discharge: The maximum discharge during the flood event
- Lag time: The time between peak rainfall and peak discharge - short lag time = higher flood risk
- Falling limb (recession limb): Shows how quickly discharge decreases after the peak
- Base flow: The normal discharge of the river from groundwater, without rainfall input
- Storm flow: The additional discharge caused by the rainfall event (total discharge minus base flow)
Lag Time = Time of Peak Discharge - Time of Peak Rainfall
Short lag time (<12 hours): Steep terrain, impermeable rock, urban areas, saturated soil
Long lag time (>24 hours): Flat terrain, permeable rock, rural areas, dry soil, dense vegetation
Short lag time (<12 hours): Steep terrain, impermeable rock, urban areas, saturated soil
Long lag time (>24 hours): Flat terrain, permeable rock, rural areas, dry soil, dense vegetation
| Hydrograph Feature | Flashy (High Flood Risk) | Subdued (Low Flood Risk) |
|---|---|---|
| Rising limb | Steep - rapid increase | Gentle - gradual increase |
| Peak discharge | High | Low |
| Lag time | Short (hours) | Long (days) |
| Falling limb | Steep - rapid decrease | Gentle - gradual decrease |
| Typical basin | Urban, steep, impermeable | Rural, flat, permeable, forested |
🧱 Hard Engineering Flood Management
Definition: Hard engineering involves building artificial structures to control water flow, contain floods and protect property. It is typically expensive, visually intrusive and requires ongoing maintenance.
| Strategy | How It Works | Advantages | Disadvantages |
|---|---|---|---|
| Dams and reservoirs | Store water upstream and release it slowly, regulating flow | Very effective; stores water for supply; generates HEP | Expensive (£20M+); floods valley upstream; sediment trapped |
| Flood walls | Concrete barriers alongside river to contain higher discharge | Protect specific areas; effective for small settlements | Expensive; may push water downstream; ugly |
| Levees (artificial) | Raised embankments increase channel capacity | Relatively simple; can use local material | If they breach, flooding is catastrophic; expensive to maintain |
| Straightening channel | Shortens river course, increasing velocity to move water away faster | Effective at moving water; reduces local flooding | Increases flood risk downstream; causes erosion |
| Culverts | Enclose river in underground channels through urban areas | Saves space; protects property above | Can block with debris; sudden surges; ecological damage |
🌱 Soft Engineering Flood Management
Definition: Soft engineering works with natural processes to manage flood risk. It is generally cheaper, more sustainable and more environmentally friendly than hard engineering, but less predictable in extreme events.
| Strategy | How It Works | Advantages | Disadvantages |
|---|---|---|---|
| Flood warnings and forecasting | Environment Agency monitors river levels; issues warnings via media | Gives time to prepare; saves lives; cheap | Doesn't prevent flooding; people may ignore warnings |
| Flood plain zoning | Restricts building on flood plains; keeps high-value development off flood-risk land | Prevents damage; cheap; allows natural flooding | Doesn't help existing buildings; limits development |
| Washlands | Designated areas allowed to flood intentionally, storing water and reducing peak flow | Natural; creates habitat; reduces downstream flooding | Requires available land; farmland may be lost |
| Afforestation | Planting trees increases interception and infiltration, slowing runoff | Natural; improves environment; carbon store | Takes years to be effective; needs large areas |
| River restoration | Re-meandering straightened rivers; reconnecting flood plains; removing hard defences | Natural; sustainable; creates habitat | May increase local flooding; needs space |
📍 Case Study: Keswick and Cockermouth Floods
Key Facts: In November 2009, severe flooding hit Cumbria. Cockermouth received 314 mm of rain in 24 hours - a UK record. The River Derwent and River Cocker both burst their banks, flooding over 900 homes and businesses.
Causes
- Exceptional rainfall: A slow-moving depression brought prolonged, heavy rainfall - over 300 mm in 24 hours at Seathwaite
- Steep catchment: The Lake District mountains have steep, impermeable surfaces that generate rapid runoff
- Saturated ground: Heavy rain in the preceding weeks had already saturated the soil
- Confluence: Cockermouth sits at the confluence of the Rivers Derwent and Cocker - both flooded simultaneously
- Narrow valley: The steep valley restricts the flood plain, channelling water through the town
Impacts
- Over 900 homes and businesses flooded in Cockermouth
- Bridge collapsed in Workington; PC Bill Barker died on the bridge
- Roads blocked by floodwater, debris and landslides
- Cost estimated at over £100 million
- Keswick flooded for the third time in 10 years
- Thousands evacuated; no electricity or clean water for days
Management Responses
- Cockermouth flood defences (2013): £4.4 million scheme including a 250-metre flood wall, embankments, a flood gate at Memorial Gardens, and raised river banks. Designed to protect against a 1-in-100 year flood.
- However: In December 2015, Storm Desmond brought even heavier rainfall (405 mm in 48 hours at Thirlmere), overtopping the new defences and flooding Cockermouth again. Over 500 properties were flooded.
- Revised approach: After 2015, the Environment Agency reviewed the scheme, planning enhanced defences including higher flood walls, additional storage upstream, and improved flood warning systems.
- Keswick defences: A £7.1 million flood defence scheme was completed in 2017, including raised embankments, flood walls and a pumping station.
Lessons from Cockermouth
The Cockermouth case study demonstrates that hard engineering has limits. The 2013 defences were designed for a 1-in-100 year event, but Storm Desmond in 2015 exceeded this. Climate change is making extreme rainfall more frequent, meaning what was once a 1-in-100 year event may now happen more often. This highlights the need for a combination of hard engineering, soft engineering (flood warnings, flood plain management) and resilience planning (making buildings more flood-resistant).
❓ Practice Questions
Q1: Explain how urbanisation increases flood risk. (4 marks)
Q2: Describe the features of a flashy storm hydrograph. (4 marks)
Q3: Compare hard and soft engineering strategies for managing flood risk. (6 marks)
Q4: Using a named example, assess the effectiveness of flood management strategies. (9 marks)
✅ Answers
- Urbanisation increases flood risk because impermeable surfaces like tarmac and concrete prevent rainwater from infiltrating into the soil (1 mark), causing up to 95% of rainfall to become surface runoff compared to 10% in natural areas (1 mark). Storm drains and culverts carry this runoff rapidly into rivers, shortening lag time (1 mark). This creates a flashy hydrograph with rapid rise and high peak discharge, increasing the likelihood of the river exceeding channel capacity and flooding (1 mark).
- A flashy hydrograph has a steep rising limb showing that discharge increases rapidly after rainfall begins (1 mark). The peak discharge is high, meaning the river reaches a large volume of flow (1 mark). The lag time between peak rainfall and peak discharge is short, typically just a few hours (1 mark). The falling limb is also steep, showing that discharge decreases quickly once rainfall stops (1 mark). This type of hydrograph is typical of steep, impermeable or urbanised drainage basins.
- Hard engineering uses artificial structures like dams and flood walls to control water and protect property (1 mark). Dams are very effective at regulating flow but are expensive (£20M+) and flood valleys upstream (1 mark). Soft engineering works with natural processes like flood warnings, flood plain zoning and afforestation (1 mark). Flood warnings save lives cheaply but don't prevent damage (1 mark). Hard engineering is more immediately effective but can create problems downstream and is costly to maintain (1 mark). Soft engineering is more sustainable and environmentally friendly but may be insufficient in extreme events like Storm Desmond (1 mark). A combination of both approaches is most effective.
- Cockermouth in Cumbria has experienced repeated flooding, most severely in 2009 and 2015. After the 2009 flood, a £4.4 million hard engineering scheme was built in 2013, including a 250-metre flood wall and embankments designed for a 1-in-100 year event (1 mark). This was initially effective for moderate floods but was overtopped by Storm Desmond in 2015, which brought 405 mm of rain in 48 hours, flooding over 500 properties (1 mark). This shows that hard engineering has limits when extreme events exceed design capacity (1 mark). Following 2015, a revised approach was adopted combining enhanced hard defences (higher walls) with soft engineering including improved flood warnings, upstream water storage and flood resilience measures for properties (1 mark). Keswick's £7.1 million scheme (2017) combined embankments with a pumping station (1 mark). The effectiveness of management in Cockermouth is mixed - defences reduce the impact of moderate floods but cannot eliminate risk from extreme events (1 mark). Climate change is increasing the frequency of severe rainfall, making 1-in-100 year events more common (1 mark). The most effective long-term approach is an integrated strategy combining hard defences where needed, soft engineering to slow runoff, flood warnings to protect life, and planning to restrict development on flood plains (1 mark). However, complete protection is impossible, and communities must also build resilience to accept and recover from flooding (1 mark).
🎯 Exam Tips
- Know the difference between physical and human causes of flooding
- Practise reading and interpreting hydrographs - identify rising limb, peak, lag time, falling limb
- Explain what makes a hydrograph "flashy" vs "subdued" using specific basin characteristics
- Use Cockermouth as your case study - know both the 2009 and 2015 events
- For evaluation, always argue that a combination of hard and soft engineering is most effective
- Link climate change to increasing flood frequency - this shows higher-level understanding
📝 Exam Technique
Geography Exam Tips — Flood Risk and Management:
1. For Flood Risk and Management questions, always name specific case studies with factual detail
2. Use geographical terminology precisely (e.g. specific processes, not vague descriptions)
3. Consider social, economic and environmental perspectives in your evaluations
4. Support your points about Flood Risk and Management with data, statistics or named examples
5. For 'assess' or 'evaluate' questions, reach a clear judgement supported by evidence
1. For Flood Risk and Management questions, always name specific case studies with factual detail
2. Use geographical terminology precisely (e.g. specific processes, not vague descriptions)
3. Consider social, economic and environmental perspectives in your evaluations
4. Support your points about Flood Risk and Management with data, statistics or named examples
5. For 'assess' or 'evaluate' questions, reach a clear judgement supported by evidence
⚠️ Common Errors
Watch Out!
Students often write vague answers without specific geographical evidence. Wrong: Writing generalised statements like 'it causes problems' Correct: Using specific data and named examples, e.g. 'the 2010 Haiti earthquake killed over 200,000 people due to poor building quality'
Students often confuse causes and effects. Wrong: Mixing up what caused the event with what resulted from it Correct: Clearly separate causes (why it happened) from effects (what happened as a result)
Students often describe rather than evaluate. Wrong: Listing strategies without assessing their effectiveness Correct: Weighing up strengths and weaknesses of each approach and reaching a supported judgement
✍️ Model Answer
Full-Mark Response
6 marks: Explain the key factors affecting flood risk and management.
Flood Risk and Management involves multiple interconnected factors that geographers must understand. The key concepts include the processes that create and change flood risk and management, the impacts on both people and environment, and the strategies used to manage associated challenges. For a comprehensive answer, specific case study evidence should be used throughout, with named examples and data to support each point. Geographical terminology should be used precisely, and the interrelationship between physical and human factors should be demonstrated. Top-level responses evaluate the relative importance of different factors and consider how the situation varies between locations.
Mark scheme: 2 marks for identifying key factors, 2 marks for explaining processes with detail, 2 marks for using specific evidence
📊 AO Deep Dive
Assessment Objective Analysis
AO1 requires knowledge of the key facts and processes related to flood risk and management. AO2 demands understanding of how and why these processes operate, and their implications. AO3 asks you to analyse, evaluate and make judgements — this is where grade 9 answers stand out by weighing up competing perspectives and reaching supported conclusions. AO4 may involve interpreting maps, graphs or data related to this topic. To move from grade 5 to grade 9: use precise geographical terminology, support every point with specific case study evidence, and always evaluate rather than just describe.
📝 Exam Questions by Topic
🎬 Video Resources
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