Manage the Flood

You cannot stop every flood

Rivers flood. It is what they have always done, spilling onto the flat flood plain they built. Flood management starts from the fact that this will keep happening. So the question is never "how do we stop the river?" It is which places are worth defending, with what, and what happens downstream once we do. Every answer in this topic is a cost-benefit argument.

What a flood takes

Some words you need, and they are not the same thing:

The town that floods itself

A town paves over the fields in its river's catchment with roads, roofs and car parks. Downstream flooding then gets worse. Why?

• Impermeable surfaces stop water soaking in, so more rain runs off quickly and the river's peak discharge rises
• Building a town makes more rain fall on the catchment
• It is a coincidence, since towns do not affect rivers
• The town makes the river channel itself narrower

Two philosophies

Flood management splits into hard engineering, which builds something to resist the river, and soft engineering, which works with natural processes instead.

Match each scheme to how it actually works

The price of concrete

Select the TWO genuine drawbacks of hard engineering on a river.

• It has a very high build cost, so no authority can defend every settlement
• Moving water downstream faster can worsen flooding further along the river
• It has to be rebuilt every single year
• It is banned everywhere in the UK

Giving the river room

Soft engineering can mean deliberately allowing low-value land to flood instead of defending it. It sounds like giving up, and examiners reward students who explain why it is not. A washland or restored flood plain stores water during the peak, lowering the flood downstream where the town is, permanently and for very little. The honest costs are real too: farmers lose the use of the land, and someone has to compensate them.

The rational case

When is letting a stretch of flood plain flood the rational choice?

• When the cost of defending it is greater than the value of what is behind it
• Whenever the authority has run out of money for defences
• Never, because losing any land is always the worst outcome
• In town centres, where the most people would benefit from a new wetland

The River Severn

This is the named river to write about. Britain's longest river rises in the Welsh mountains and flows through Shrewsbury, Worcester and Tewkesbury before reaching the sea. It floods often, and it is managed very differently along its length.

You are the flood officer

You advise the authority managing the Severn, on a limited budget, about three places along the river. Take them one at a time.

• Place one is the historic town centre: shops, homes and a tourist trade, flooded several times in recent years. What do you recommend?
• Place two is low-lying grazing farmland upstream, behind an old earth bank. Defending it would cost more than the land is worth. What do you recommend?
• Place three is the upland catchment, where fast runoff feeds the flood. How could you reduce flooding here?

Plan the answer

A flood-management question is really a decision process. Put its stages into the order that produces a full-mark answer.

• Identify what is actually at risk in this specific place
• Judge its value: a town and its economy, or open farmland
• Set out the realistic options, hard and soft, with their costs
• Work out the knock-on effects downstream
• Decide, justify it against the alternative, and say how it will be monitored

Discharge and the barriers

The volume of water passing a point each second is the _____. A barrier that stores water and releases it slowly is a _____, and raising the banks so the channel holds more uses _____. Deliberately flooding a field to store water creates a _____, and restricting building on the highest-risk land is flood plain _____.

Evaluate the strategy

'Hard engineering is the best way to manage flooding on a river.' Using the River Severn, evaluate this statement and reach a conclusion.

• Give one place where hard engineering IS the right answer, and say why in cost-benefit terms
• Give one place where it is not, and explain what is done instead and why
• Explain one knock-on effect that hard engineering can have downstream
• Refer to the named river throughout, not to rivers in general
• Finish with a conclusion that answers "best" rather than listing both sides