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Rivers: Opportunities and Hazards » Managing River Floods

What you'll learn this session

Study time: 30 minutes

Cambridge spec: 1.3.4

The difference between hard engineering, soft engineering and flood strategies
How each technique works, with its advantages, disadvantages, cost and sustainability
How to evaluate flood management and reach a clear judgement

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Strategies and techniques

You already know why rivers flood and what floods do. This lesson is about stopping floods, or at least making them do less harm. You will use two words in the exam, and they mean different things. A strategy is a plan or agreement about how to manage floods. A technique is the actual method used on the river or the land.

Techniques fall into two groups. Hard engineering uses large built structures. Soft engineering works with the river and the landscape. The best flood management often uses both.

Key terms:

Hard engineering: building large artificial structures to control a river.
Soft engineering: managing a river and its land in a natural way, with less building.
Sustainable management: managing floods in a way that meets the needs of people today without damaging the environment or the needs of people in the future.
Dredging: digging sediment out of the river bed to make the channel deeper.
Afforestation: planting trees on land that has no trees.

Hard engineering techniques

Hard engineering is usually expensive, but it can protect a town very well. For each technique, learn how it works, one advantage, one disadvantage, and the cost.

Technique	How it works	Advantages	Disadvantages	Cost and sustainability
Dams and reservoirs	A wall across the river stores water in a reservoir. Water is let out slowly, so the river downstream rises less.	Controls the flow all year. Water can also be used for supply and electricity.	Very high cost. Floods land upstream, can force people to move, and traps sediment.	Highest cost. Not very sustainable: it changes the whole river and harms habitats.
Embankments (artificial levees)	Walls of earth or concrete are built along the banks, so the channel can hold more water before it spills.	Fairly cheap for the protection. Quick to build.	Can fail and then flood suddenly. Water is held in the channel and sent faster downstream, so flooding can be worse there.	Medium cost, plus repairs. Low sustainability: it moves the problem downstream.
Channel straightening	Meanders are cut out so the river takes a shorter, straighter path and water flows away faster.	Water leaves the town quickly, so there is less flooding there.	Sends a faster flood downstream. Destroys wildlife habitats. More erosion of the bed and banks.	Fairly cheap to build, but costs rise later from bank repairs and downstream damage. Not sustainable.
Dredging	The bed is dug out to make the channel deeper, so it holds more water.	Easy to do. Also helps boats to navigate.	Must be repeated as sediment builds up again, so costs never stop. Harms plants and animals on the bed.	Cost repeats every few years. Not sustainable because the work never ends.
Flood relief channels	A second channel takes extra water around a town when the river is high.	Protects a town without changing the main river. Works well in a big flood.	Expensive and uses a lot of land. Only works if the channel is big enough.	High cost. Moderately sustainable if the channel is made to look and work like a natural stream.

Why hard engineering is not always sustainable

Hard structures cost a lot to build and to repair. They protect one place, but they often move the problem downstream. They also change the look of the river and harm habitats. They can give a false sense of safety, because a flood bigger than the design can still overtop them.

Soft engineering techniques

🌲 Afforestation

Trees intercept rain and take up water, so less reaches the river quickly. It is cheap and good for wildlife, but it takes years to work and needs a lot of land.

🏡 Floodplain zoning

Land use is controlled on the floodplain. Land at highest risk is left for parks or farming, and homes go on higher ground. It is cheap and sustainable, but it limits where a town can grow and does not protect buildings already there.

🌿 Wetland and river restoration

Straightened channels are put back into meanders and wetlands are recreated, so water spreads out and slows down. It helps wildlife and costs less than big walls, but it gives less protection in a very large flood and needs land.

📢 Flood warnings and preparation

Gauges and forecasts warn people early so they can move belongings and leave. It is low cost and saves lives, but it does not stop damage to buildings, and people must know what to do. Preparation means evacuation plans, flood kits and moving valuables upstairs, and it is also cheap and sustainable.

Flood strategies

Strategies sit above the techniques and decide how they are used.

Flood action plans: a plan that sets out what will be done before, during and after a flood, and who does it.
Land-use zoning: rules that decide what can be built where. It is the planning rule behind floodplain zoning.
Warnings: systems that tell people a flood is coming. In England the Environment Agency issues flood alerts and flood warnings.
Insurance: people pay regular fees, and the insurer pays for repairs after a flood. It helps recovery but does not stop floods, and cover can be costly or refused in high-risk areas.
International agreements for shared rivers: countries on the same river agree to work together. The Rhine flows through several countries. In 1998 the countries of the Rhine basin adopted an Action Plan on Floods through the International Commission for the Protection of the Rhine, so that they act together instead of passing the problem downstream.

The Netherlands shows soft engineering at scale. Its Room for the River programme moves flood walls back and lowers floodplains so the river has space to spread out in a flood, rather than only building higher walls.

Evaluating: how to reach a judgement

Evaluate means weigh up strengths and weaknesses and then decide. Use this checklist for any technique:

Cost: to build and to keep going.
Effectiveness: how much protection does it give, and where?
Who benefits: and who loses, for example people downstream.
Environment: does it help or harm wildlife and the river?
Sustainability: will it still work in the future without harm?

Hard engineering usually gives strong protection to a small area but costs more, harms the environment and can make flooding worse elsewhere. Soft engineering costs less and is more sustainable but gives less protection against the biggest floods. A strong judgement says which is best for a named place and why. For a dense city that needs strong protection now, hard engineering may be needed. For a rural basin, soft engineering is often the better choice.

Common mistakes

Saying a technique is "good" or "bad" with no reason. Listing advantages but no disadvantages. Forgetting that protecting one place can make flooding worse downstream. Mixing up the technique (a wall) with the strategy (a plan or agreement).

Exam-style question

Fig. 1.1 describes a made-up town, Ardale, beside a river. It has flooded three times in ten years. The town council can build a high embankment along the river, or plant trees and restore wetlands in the hills upstream.

(a) State one hard engineering technique used to manage river flooding. [1 mark]

(b) Explain how afforestation reduces flood risk. [3 marks]

(d) Evaluate the two options for Ardale and decide which is better. [6 marks]

Model answer

(a) Embankments (or dams, channel straightening, dredging, flood relief channels). (1)

(b) Trees intercept rainfall (1) and take up water from the soil (1), so less water reaches the river as overland flow and the river rises more slowly (1).

(c) The sediment builds up again, so dredging must be repeated (1), which means the cost never ends (1). (Or: it harms plants and animals on the river bed.)

(d) The embankment gives strong protection to the town straight away (1) and a high wall can contain big floods (1). However it is costly to build and repair (1), it can fail and flood suddenly (1), and it may push water downstream to other towns (1). Trees and wetlands cost less and are more sustainable, helping wildlife (1), but take years to work and give less protection in a very large flood (1). Judgement: because Ardale floods often, a mix is best, with the embankment now and trees and wetlands upstream for the long term, as this protects the town and is more sustainable (1, for a reasoned judgement). Maximum 6 marks: Level 1 (1-2) lists points, Level 2 (3-4) explains advantages and disadvantages of both options, Level 3 (5-6) explains both and gives a reasoned judgement.

Exam tip

In an "Evaluate" answer, give both sides for each option and finish with a clear decision that names the place. A decision with no reason scores no extra marks.

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