Manage the Flood

Two towns, two answers

Banbury in Oxfordshire sits on the River Cherwell and has a long history of being under it. In 1998 the town flooded badly enough to close the railway station and the A361, with damage of the order of 12 million pounds, and it flooded again in July 2007. The response, completed in 2012, cost around 18.5 million pounds: an earth embankment roughly 2.9 km long, a flood storage area able to hold about 3 million cubic metres of water upstream of the town, a pumping station, and the A361 raised clear of the water. It protects something in the region of 440 properties. Pickering in North Yorkshire took a different route entirely. Instead of building a wall, the Slowing the Flow partnership spent around 3.2 million pounds working on the catchment above the town: more than three hundred leaky timber dams in the becks to hold water back briefly, roughly 29 hectares of new woodland to intercept rain and help it soak in, and a bund storing about 120,000 cubic metres. On 26 December 2015, when large parts of Yorkshire flooded, Pickering did not. Neither town is simply right. The point of this topic is that flood management is a set of trade-offs, not a ranking, and the trade-off that catches people out is geographical. A scheme that moves water away from a town faster has moved it towards whoever lives downstream, which is exactly what happened after the Jubilee River relief channel was built near Maidenhead. This module covers the defences, what they cost, and how to weigh them for a specific place.

Five flood management terms

The first two are the families of approach, the third and fourth are specific measures worth naming, and the fifth is the idea that decides most judgements.

Why the town floods

These six stages explain why a settlement downstream of a developed catchment floods after heavy rain. Put them in order.

• Prolonged heavy rain falls across the catchment
• Tarmac, roofs and compacted ground stop most of it soaking in
• Drains and straightened channels carry the water to the river quickly rather than slowly
• A great deal of water arrives in the channel over a short period, so discharge peaks sharply
• The channel through the town cannot hold that peak
• Water spills onto the floodplain, which is where the streets and houses are

Why can a defence make things worse?

The Jubilee River relief channel protected Maidenhead, Windsor and Eton, and communities downstream complained that their flooding had worsened. Which answer explains how that can happen?

• A relief channel and a straightened channel both work by moving water past a place more quickly. The water is not removed from the system, only delivered somewhere else sooner, so the flood peak arriving downstream is both larger and earlier than it would have been. Measures that store water instead, such as a flood storage area or leaky dams high in the catchment, reduce the peak rather than relocating it, which is why they do not produce the same complaint.
• The relief channel added extra water to the river that had not been there before.
• The channel silted up and stopped working, so the water backed up downstream.
• Downstream communities simply happened to receive more rainfall in those years.

The menu, and two real schemes

The specification note asks for hard and soft engineering to be weighed for the specific scheme named rather than in general, so the first two columns are the menu and the third is what happens when two real towns choose differently.

Match each measure to what it changes

Store it or shift it

The specification note asks you to weigh hard against soft engineering for the specific scheme named, not in general, which rules out the two commonest answers: a list of measures with their pros and cons, and a verdict that soft engineering is better because it is natural. The most useful distinction to carry into the exam is not hard against soft at all. It is whether a measure stores water or shifts it. Anything that speeds water past a settlement, such as straightening a channel or cutting a relief route, relieves that settlement by handing the flow to whoever is next in line sooner and in greater quantity. Anything that holds water back, whether a built storage area or trees and leaky dams high in the catchment, lowers the peak for everybody downstream. Some hard engineering stores and some soft engineering does not, so this cuts across the usual categories, and noticing that is a reliable way into the top band. Then ask the same questions as for any management scheme. What is being protected and what is it worth? A town with hundreds of properties and a main road justifies expenditure that a few isolated houses do not. How certain does the protection need to be? A built defence gives a level of certainty that planting trees cannot. And who pays for it, including the people who never chose it? For a judgement question, say plainly which criterion you are using. Judged on cost per property protected, a catchment scheme usually wins. Judged on certainty for a large town, built defences usually win. A conclusion that names its measure is worth far more than one that declares a winner and leaves the reasoning unstated.

Complete the flood management facts

Banbury flooded in 1998 and again in 2007, and the scheme completed in _____ cost around 18.5 million pounds. It combined an earth embankment roughly 2.9 km long with a flood _____ area holding about 3 million cubic metres upstream, which lowers the peak reaching the town rather than simply hurrying the water through it, and it protects around 440 properties. Pickering took the opposite approach for about 3.2 million pounds, using leaky dams and roughly 29 hectares of new _____ across the catchment, and the town stayed dry in December _____ when much of Yorkshire flooded. What separates the real options is not the hard and soft labels but whether the water is held back or merely _____ downstream to somebody else, because anything that speeds it past one place delivers it to the next sooner.

Spot the three accurate flood claims

Select the THREE statements that are true.

• What really matters is not the hard and soft labels but whether a defence holds the flow back or hands it along, since a built storage area lowers the peak for everyone below it while a straightened channel does not.
• Flood warnings save lives and property without stopping a single litre of water, which is why they are judged on how many people act on them rather than on what happens to the river.
• Pickering stayed dry on 26 December 2015 when much of Yorkshire flooded, having spent roughly a fifth of what Banbury spent, although it is protecting a considerably smaller town.
• Soft engineering is always preferable because it works with natural processes.
• An embankment is entirely reliable, because once the banks are raised the river cannot leave its channel.
• A flood relief channel removes water from the river system altogether.

A 9-mark evaluation, marked

Using a named example, assess the costs and benefits of a flood management scheme. [9 marks] Banbury on the River Cherwell flooded seriously in 1998, closing the railway station and the A361 and causing damage of the order of 12 million pounds, and again in July 2007, so the scheme completed in 2012 had a clear and quantified problem to solve. It cost around 18.5 million pounds and combined an earth embankment roughly 2.9 km long with a flood storage area upstream holding about 3 million cubic metres, a pumping station, and the A361 raised clear of flood level. The benefits are substantial and measurable: in the region of 440 properties are protected along with the road and the railway, and because the scheme stores water rather than simply hurrying it through the town, it lowers the peak discharge instead of handing the problem to communities downstream, which is a genuine advantage over a relief channel such as the Jubilee River near Maidenhead, where downstream communities complained that their flooding had worsened. The storage area also doubles as farmland and habitat for most of the year, and new footpaths were created, so some of the land serves more than one purpose. The costs are the price itself and its permanence. Eighteen and a half million pounds is a great deal for one town, the embankment is a visible structure in the landscape, and once built, the maintenance continues indefinitely. It is worth comparing with Slowing the Flow at Pickering, which achieved protection for around 3.2 million pounds using leaky dams, roughly 29 hectares of woodland and a bund holding about 120,000 cubic metres, and where the town stayed dry in December 2015 while much of Yorkshire did not. Judged on cost per property protected, the catchment approach at Pickering is clearly better value. Judged on the certainty required for a town of Banbury's size with a main road and a railway to defend, the built scheme is justified, because tree planting cannot offer that guarantee. The honest conclusion is that the two towns faced different problems and that the Banbury scheme is good value for what it had to be certain of doing. What earned the marks: a named scheme with dates, cost, components and the problem it answered; benefits and costs both quantified rather than asserted; a comparison with a genuinely different approach and with a scheme that transferred its problem downstream; and a conclusion that judges on two stated criteria instead of picking a favourite.

Find the two that store rather than shift

Five flood measures. Two of them reduce the peak by holding water back; the rest either move it along faster or do not touch the water at all. Tap those TWO.

• Farmland upstream of the town is set aside so that it can be deliberately flooded and drained slowly once the river has fallen.
• Meanders are cut out of the channel so that the water passes the town more quickly.
• Hundreds of leaky timber barriers are placed in the streams above the town to hold water back for a few hours.
• Residents are signed up for text alerts giving several hours of notice before the river peaks.
• A second channel is dug to carry surplus water around the edge of the town and back into the river beyond it.

Build the downstream sentence

Assemble a sentence that explains why a faster channel can create a new problem.

Four catchments, four budgets

You advise on flood management. Each situation has different assets, different money and different neighbours.

• A market town of several hundred properties with a railway station and a main road has flooded twice in a decade. The council needs protection it can rely on.
• A small upland town has a modest budget and a steep catchment of moorland and beck above it. Built defences through the settlement would be disruptive and expensive.
• A proposed relief channel would protect the town that commissioned it. Parishes a few miles downstream have written pointing out that the same water will reach them sooner and in greater quantity.
• A developer applies to build two hundred homes on flat ground beside the river, immediately downstream of the existing town. The land is cheap because it floods.

Compare the two approaches

Assess the extent to which soft engineering is a more effective way of managing flood risk than hard engineering. Use named examples. [9 marks]

• A visible distinction between soft and hard engineering
• What each scheme cost, what it built, and how many properties or what assets it protects
• Why some measures create problems downstream and others do not
• Catchment measures work gradually and cannot guarantee protection for a large town
• Cost per property protected or certainty of protection, not "better in general"