Optional theme: Freshwater - drainage basins practice questions.

In a steep upland reach of the River Tees in northern England, turbulent flow forces water and compressed air into joints in the resistant dolerite, and the repeated pressure prises whole blocks out of the bed. Which erosional process is described?

• Abrasion (corrasion) by the bedload
• Attrition between transported particles
• Hydraulic action
• Solution (corrosion) of the bedrock

Answer: Hydraulic action

Hydraulic action is the mechanical force of water and trapped air alone; abrasion needs sediment as a tool, attrition only wears down the load itself, and dolerite, unlike limestone, is effectively insoluble on the timescale of channel erosion, so solution removes a negligible amount here.

The table shows measurements taken at three sites along one river during a single day of fieldwork. Which statement is supported by the data?

• Mean velocity falls downstream because bed roughness increases with distance
• Mean velocity increases downstream, and discharge rises far more steeply
• Discharge falls at the two downstream sites as the channel becomes wider
• Velocity and discharge both roughly double between site A and site C

Answer: Mean velocity increases downstream, and discharge rises far more steeply

Discharge is cross-sectional area multiplied by velocity, so the large downstream gain in area magnifies a modest gain in velocity: velocity roughly doubles while discharge rises about a hundredfold. The usual error is to assume upland rivers flow fastest because they look turbulent.

In 1998 a flood on the Ganges-Brahmaputra-Meghna delta in Bangladesh inundated roughly two thirds of the country, and standing water remained for about two months. Which factor best explains the exceptional duration of the flooding?

• The delta is underlain by impermeable granite, so no infiltration occurs
• Himalayan snowmelt reached the delta long before any monsoon rain fell
• Peak flows of three large rivers coincided on an almost flat delta
• Tectonic uplift of the delta blocked the distributary channels during the flood

Answer: Peak flows of three large rivers coincided on an almost flat delta

With a gradient of only centimetres per kilometre, water on the delta drains extremely slowly, so simultaneous peaks from the Ganges, Brahmaputra and Meghna pond instead of passing through. Duration, not depth alone, is what makes delta floods so damaging.

A local authority is choosing between four flood-management measures for a town on a floodplain. Which of them is non-structural (soft) engineering?

• Raising and reinforcing the earth embankments along both banks
• Straightening and deepening the channel to move water away faster
• Building an underground flood-relief tunnel beneath the town centre
• Banning new housing in the highest-risk floodplain zone

Answer: Banning new housing in the highest-risk floodplain zone

Soft engineering changes what people do rather than what the river does. The other three alter the channel or add built structures, and each of them passes water, and therefore risk, on to settlements downstream.

The table compares one basin's response to a 55 mm storm before and after housing was built on 30 % of its area. Which statement is supported by the data?

• Storm rainfall increased, which is why the peak discharge is higher
• Water now reaches the channel faster, so the same rain gives a higher peak
• The rise in baseflow shows that more water infiltrated after the change
• The drainage density of the basin must have fallen, lengthening every flow path

Answer: Water now reaches the channel faster, so the same rain gives a higher peak

Rainfall is identical in both columns, so only the pathway can have changed: impermeable surfaces and storm drains more than halve the lag time and raise the peak, while the lower baseflow shows that less water reached the groundwater store.

In August 2014 the city of Toledo, Ohio, told residents not to drink tap water after a cyanobacteria bloom in the western basin of Lake Erie released the toxin microcystin. Which chain of events best explains the bloom?

• Acid rain lowered the lake's pH, letting cyanobacteria outcompete other algae
• Chloride from winter road salting raised water density and trapped nutrients
• Warm cooling water from power stations raised dissolved oxygen and fed the bloom
• Phosphorus in runoff from farmland enriched the warm, shallow water

Answer: Phosphorus in runoff from farmland enriched the warm, shallow water

Blooms need a nutrient supply, mainly phosphorus from fertiliser and manure, plus warm, shallow, slow-moving water, and the western basin of Lake Erie provides both. Treating the symptom at the water works does nothing about the source in the catchment.