Rivers and coasts, Year 10: flood risk, hydrographs and managing floods (River landscapes option)

8

HintSubtract one clock time from the other.

The whyLag time is the gap between peak rainfall and peak discharge. Here 11:00 − 03:00 = 8 hours. A short lag time means water is reaching the river quickly, so the risk of a flood is higher.

rising

HintA word for going up, as the sun does at dawn.

The whyA steep limb shows discharge increasing fast, which is typical of a river likely to flood. After the peak the line drops back more gently towards the normal flow; that part is the falling limb.

The soil becomes saturated, so more rain cannot soak in and runs off into the river

HintA sponge that is already full takes up no extra drops.

The whyProlonged rainfall fills the spaces in the soil with water. After that, extra rain flows over the surface straight to the river. A cloudburst causes flooding differently: the rain falls faster than it can soak in.

Rain cannot soak into the rock, so more runs over the surface to the river

HintPour a jug onto a pavement, then onto a gravel drive.

The whyPermeable rocks such as chalk let water pass down through pores and cracks, so it reaches the river slowly or is stored underground. Impermeable rocks keep the water at the surface, where it runs off quickly and gives a short lag time.

They shorten it

HintThink of rain on a pitched roof compared with rain on a level lawn.

The whyOn a steep slope water runs downhill fast and has little time to soak in. It reaches the river quickly, so discharge climbs steeply to a high peak. On gentle slopes more water soaks in and the response is slower.

They raise it, by carrying rainwater to the river very quickly

HintTheir whole job is to get water off the streets fast.

The whyUrban land use is a human factor in flood risk. Hard surfaces stop rain soaking in, and the drains then deliver it to the river in minutes. Cutting down trees has a similar effect, because less rain is caught by leaves and taken up by roots.

It holds the water back and releases it slowly

HintThink of a bath filling while the plug is only partly open.

The whyThe reservoir stores the storm water, and the dam's gates control how fast it goes on downstream. The stored water can also supply homes and generate electricity. The costs are high: dams are very expensive, and the reservoir drowns farmland, habitats and sometimes homes.

Water flows through faster, so it leaves before it can build up

HintA shorter, smoother route means less time spent on the journey.

The whyStraightening shortens the channel and steepens it slightly, so the water speeds up. The town is protected. The cost is that the water arrives sooner and faster further downstream, where flooding and erosion may get worse.

An extra channel that carries surplus water around a town when the river is high

HintA bypass does the same job for traffic.

The whyWhen the river rises, some of the flow is diverted along the second channel. It rejoins the river below the town. Relief channels are expensive to build and take up land, and the place where the water rejoins may itself be put at risk.

It gives people time to move belongings, protect their homes and get to safety

HintWhat could you do with a few hours' notice?

The whyIn England the Environment Agency watches river levels and rainfall and issues a flood alert, a flood warning or a severe flood warning. People can then use sandbags and flood gates, move valuables upstairs and leave if they must. It is cheap, but it does not stop the flood, and it fails if people do not receive or act on the message.

Flood plain zoning

HintThe last word is how planners talk about dividing a town into areas for different uses.

The whyThe land nearest the river is kept for uses that can cope with being flooded, such as playing fields, parks and grazing. Housing, hospitals and power supplies go on higher ground. It is cheap and leaves room for flood water, but it cannot help towns that are already built beside the river.

restoration

HintThe same word is used for repairing an old painting or building.

The whyA natural, winding channel slows the water down, and the wetlands beside it store flood water. This lowers the flood peak for places downstream and creates habitat. It needs little upkeep, but some land beside the river has to be allowed to flood.