Rivers and coasts, Year 9

hard

HintThis approach relies on concrete and steel to control the river.

The whyDams, 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

HintPicture a downpour hitting leaves and roots instead of bare ground.

The whyLeaves 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

HintThink of the flooded land as a temporary reservoir.

The whyAt 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

HintCompare what is saved with what is paid by dividing one by the other.

The whyBenefit 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

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

The whyBefore 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

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

The whyThe 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

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

The whySand 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)

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

The whyAt 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

HintCompare a seafront full of homes and shops with a few fields.

The whyDefending 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

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

The whyA 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)

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

The whySnow 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

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

The whyThe 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

HintIt is in Cumbria, in the north-west of the country.

The whyDuring 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.