Rivers and coasts, Year 9

hard

提示This approach relies on concrete and steel to control the river.

为什么Dams, embankments and straightened channels hold water back or move it on quickly. They can protect a town very effectively, but they are costly to build and to maintain, and they can pass the problem downstream.

They intercept rain and slow the water's journey to the river

提示Picture a downpour hitting leaves and roots instead of bare ground.

为什么Leaves catch rainfall, roots help it soak into the soil and the trees take some up themselves. Less water reaches the channel, and it arrives more slowly, so the river's peak is lower and later.

The fields hold water back, lowering the river's peak in the town

提示Think of the flooded land as a temporary reservoir.

为什么At Pickering in North Yorkshire, the 'Slowing the Flow' project (finished in 2015) paired a flood storage area on farmland upstream with new woodland and small woody dams. It cut the town's chance of flooding in any year from about 25% to under 4%.

8

提示Compare what is saved with what is paid by dividing one by the other.

为什么Benefit divided by cost gives 8, so each pound spent saves eight. Planners use this kind of sum to decide which schemes go ahead: a town with many homes scores far higher than a few fields, which is why some places are defended and others are not.

It sends the water on faster instead of letting it spread out

提示The flood has to go somewhere; ask where it can no longer spill.

为什么Before the wall, some of the flood would have spread across the floodplain and been stored there. Hemmed in, it races on to the next settlement. Flood management has to look at the whole river, not one town at a time.

It reflects the waves' energy back out to sea

提示Think what happens to a ball thrown at a solid, shaped surface.

为什么The wall is a solid barrier between the waves and the land, and its curved face turns each wave back on itself. Sea walls protect valuable seafronts well, but they are among the most expensive defences and the rebounding waves can scour away the beach in front.

They starve the beaches beyond them of sand

提示Longshore drift was carrying that material somewhere before the fences trapped it.

为什么Sand and shingle normally travel along the coast by longshore drift. Trap them in one place and the next stretch gets no fresh supply, so its beach thins and waves reach the cliffs. Rock groynes built at Mappleton on the Holderness coast in 1991 hold a beach in front of the village in just this way.

Managed retreat (managed realignment)

提示The first word shows it is planned; the second is what the defence line does.

为什么At Medmerry in West Sussex, finished in 2013, the Environment Agency built a new bank inland and breached the old shingle one. The wetland this created now soaks up wave energy and shields more than 300 homes.

Whether the land is worth more than the defences would cost

提示Compare a seafront full of homes and shops with a few fields.

为什么Defending every metre of coast is unaffordable, so money goes where it protects the most homes, jobs and roads. That is hard on the people whose farms and houses lie in the gaps, which is why coastal defence is argued over, not simply calculated.

Ice fills the whole valley, eroding its sides as well as its floor

提示A river only touches the very bottom; think how much rock a thick tongue of frozen water would touch.

为什么A river is a thin ribbon cutting downwards, so the slopes above it stay V-shaped. A glacier hundreds of metres thick grinds away everything it touches, straightening and deepening the valley. Great Langdale in the Lake District is a classic British example.

A corrie (also called a cirque or cwm)

提示Its steep back wall and deepened floor often hold a small round lake known as a tarn.

为什么Snow collected in a sheltered hollow, turned to ice and rotated as it slid out, deepening the floor and steepening the back wall. Cwm is the Welsh name and cirque the French; Red Tarn below Helvellyn sits in one.

plucking; abrasion

提示One word is what you do to a guitar string; the other is what sandpaper does to wood.

为什么The two work together: the first supplies loose rock, and that rock then becomes the grit that grinds the bed. Scratches called striations, left on bare rock in Snowdonia and the Lake District, show which way the ice moved.

Lake District

提示It is in Cumbria, in the north-west of the country.

为什么During the Ice Age, glaciers flowed outwards from the central fells, deepening the valleys. When the ice melted, water filled the long hollows to make ribbon lakes. Windermere is the largest lake in England.