Rivers and coasts

The discharge

HintMeasured in cubic metres per second, it rises fast after heavy rain.

The whyIt is the river's cross-sectional area multiplied by its speed, given in cumecs - cubic metres per second. A storm hydrograph plots how it climbs and falls in the hours after rain, and a river floods once it outgrows its channel.

Transpiration, infiltration, surface run-off - in that order.

HintLeaves breathe it out, the soil drinks it in, and the rest slides downhill over the land - name each in turn.

The whyNot every drop goes straight up and down: plants pump vapour back into the air, some rain sinks into the ground and creeps slowly towards the river, and the rest runs over the top and arrives fast. How much takes each route decides how quickly a river rises after a storm.

The lag time

HintA steep, bare basin shortens it; woodland and gentle slopes stretch it out.

The whyRain must travel to the channel before the river can rise, so the gap is short where the ground is steep, bare, clay or built over, and long where woodland and porous rock hold water back. A short gap with a high peak is what floods towns.

saltation

HintFrom the Latin for leaping - the grains hop rather than roll or float.

The whyA river shifts its load four ways: dissolved minerals travel in solution, fine silt is held up in suspension, sand hops along the bed, and boulders are rolled by traction. Which method dominates depends on the speed of the water and the size of the grains.

Abrasion

HintThe sea uses the stones it already carries as sandpaper.

The whyThis does most of the work of cutting a wave-cut notch at the foot of a cliff. Hydraulic action forces air into cracks, and attrition rounds the pebbles as they knock together. Some books use the older name corrasion for it.

A plunge pool

HintFalling water drills a hollow in the rock where it lands.

The whyThe falling water and the rocks it carries swirl at the base, deepening the hollow and undercutting the hard band above. When the overhang collapses the waterfall retreats upstream, leaving a gorge.

The outside

HintThe water on that side has furthest to travel round the curve.

The whyThe fast outer bend erodes into a river cliff while the slow inner bend deposits a slip-off slope of sand and shingle. In time the neck narrows until the loop is cut off as an oxbow lake.

A stack

HintOnce the roof of the arch falls, a lonely tower is left offshore.

The whyThe sequence runs crack, cave, arch, stack, stump. Old Harry Rocks in Dorset and the Needles on the Isle of Wight were shaped this way.

Faster

HintNothing soaks into a hard surface, so the rain runs straight off it.

The whyTarmac is impermeable, so almost all the rain becomes surface run-off and arrives in the channel at once. Trees catch rain on their leaves and their roots help it sink in, spreading the flow over days.

Soft engineering

HintWorking with the landscape instead of building against it.

The whyThese schemes slow water down where it falls - planting woodland, restoring meanders, letting washlands flood on purpose. They are cheaper and help wildlife, but a city centre still needs hard defences.

Groynes

HintThey stop the beach being dragged along the shore by the waves.

The whyLongshore drift shifts sand steadily along a coast, and these hold it in place so the beach itself absorbs wave energy. The cost is that the coast further along is starved of sand and erodes faster.

A U-shaped valley

HintIce is far broader than a stream, so it grinds out the sides as well as the floor.

The whyA river cuts a narrow V; a glacier is wide and heavy, so it planes the sides back and flattens the bottom. Snowdonia and the Lake District are full of them, cut in the last Ice Age.