Flooding and flood mitigation practice questions.

After the 2004 Boscastle flood in Cornwall, the channel through the village was widened and deepened and a culvert under the car park was removed. What does this combination of works change about the flood risk?

• It raises the discharge the channel can carry before water spills onto the streets
• It reduces the total volume of rain falling on the catchment upstream
• It lengthens the lag time by slowing water down on the valley sides
• It removes the need for any flood warning system in the valley

Answer: It raises the discharge the channel can carry before water spills onto the streets

Channel enlargement and culvert removal are conveyance measures: they raise bankfull capacity at the point of risk. They do nothing to the rainfall or to the runoff generated upstream, which is why Boscastle also kept an upstream warning system.

In the Dutch Room for the River programme, the dyke at Nijmegen was moved inland and a side channel was dug across the former floodplain. Which principle does this represent?

• Giving the river more space to store and convey water at high flow
• Raising every embankment to the height of the largest recorded flood
• Transferring water out of the basin into a neighbouring catchment
• Replacing river transport with road freight to justify narrower channels

Answer: Giving the river more space to store and convey water at high flow

Room for the River reverses two centuries of narrowing: widening the floodway lowers peak water levels rather than confining a higher and higher flood behind a taller dyke. Raising dykes alone concentrates the hazard and raises the cost of eventual failure.

Two catchments receive the same storm. Catchment X is steep with thin soils over granite; catchment Y is gentle with deep permeable sandstone. How will their storm hydrographs differ?

• X has a shorter lag time and a higher peak discharge than Y
• X has a longer lag time and a lower peak discharge than Y
• The two hydrographs will be identical because rainfall is identical
• Y has the steeper rising limb because deep soils release water faster

Answer: X has a shorter lag time and a higher peak discharge than Y

Steep slopes and thin impermeable soils force water to reach the channel as overland flow, which is fast; permeable rock routes it through slow throughflow and groundwater. Lag time and peak height are set by the pathway, not the rainfall total.

A student compares two neighbouring basins after the same storm and writes: "Basin A's peak discharge was twice Basin B's, so Basin A must have received twice as much rain." Which statement identifies the error in this reasoning?

• Peak discharge is fixed by the size of the channel, so it cannot be compared between two different basins.
• Peak discharge depends on how fast a basin delivers water as well as on the rainfall total, so basin characteristics could explain the gap.
• Peak discharge is always larger than the rainfall total, so the two quantities can never be set against each other.
• Peak discharge is measured only after a storm has ended, so it cannot be linked to the rainfall of that storm at all.

Answer: Peak discharge depends on how fast a basin delivers water as well as on the rainfall total, so basin characteristics could explain the gap.

Rainfall total is only one control: slope, soil depth, geology, drainage density and land use decide how much of the rain reaches the channel and how quickly, so identical rainfall can produce very different peaks.

The Three Gorges Dam on the Yangtze, China, can hold back part of a flood wave arriving from the upper river, yet the city of Wuhan, several hundred kilometres downstream, still suffers serious flooding. Which reason best explains this?

• The reservoir is emptied completely before every flood season, so it can hold nothing back.
• The dam raises the bed of the river downstream, so that channel can no longer carry an ordinary flood.
• Water released for hydroelectric generation always reaches Wuhan exactly at the flood peak.
• Large tributaries and lakes join the river below the dam, and their flood water is not regulated by it.

Answer: Large tributaries and lakes join the river below the dam, and their flood water is not regulated by it.

A dam can only regulate the part of the basin above it; the middle Yangtze takes in huge unregulated inflows from tributaries and from lake systems downstream of the dam, so the peak at Wuhan is only partly the upper river's.

A national agency proposes to use farmland in an upper valley as a washland that is allowed to flood deliberately, in order to protect a town 20 km downstream. Which statement best describes how the costs and the benefits of the scheme are distributed?

• The costs fall on the upstream landowners whose fields are flooded, the benefits on downstream property owners.
• Costs and benefits both fall on the upstream community, because the washland lies on its own land.
• The costs fall on downstream residents, who pay for the lost farmland through higher water charges.
• Neither costs nor benefits can be located, because flood risk is shared equally along the whole river.

Answer: The costs fall on the upstream landowners whose fields are flooded, the benefits on downstream property owners.

Flood management usually moves risk rather than removing it, so the decisive question is who carries the cost of the transfer; compensation payments exist precisely because the losers and the winners live in different places.