Cliff collapse at Barrow Bay
Barrow Bay is a coastal village of 1,800 people. Its 28 m high cliffs are made of jointed sandstone resting on impermeable clay. The beach is 30–40 m wide and narrows during storms; offshore wave fetch is about 180 km. Cliff retreat measured from aerial photographs averaged 0.6 m per year between 1990 and 2010, but 1.4 m per year since 2015. Fourteen houses, the only access road and a sewage pipe lie within 20 m of the cliff edge. The council is comparing two schemes. A 900 m rock revetment would cost £6.8 million, last about 40 years and protect the houses and road. Managed retreat would cost £2.4 million to buy the 14 houses plus £1.1 million to reroute the road inland; the cliffs would continue to erode. Cliff sediment currently feeds a beach that supports summer tourism worth about £900,000 per year. A recent storm cut back the cliff by 3.5 m in one night.
1. Explain why the average rate of cliff retreat rose from 0.6 m to 1.4 m per year after 2015.
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The cliff is not uniform. Jointed sandstone lets waves force air and water into cracks (hydraulic action), while the clay beneath is weaker and saturated, so it is eroded by abrasion and may fail when undercut. This creates a positive feedback: a small fall removes support from the sandstone above, so the next storm removes a larger block. The 2015–present rate is an average of larger, less frequent events rather than steady loss; the recent 3.5 m storm cut shows that one night can produce several years' 'average' retreat. The narrowing beach matters: during storms the beach is thinned, so wave energy reaches the cliff instead of being dissipated. Also, averages from aerial photographs can change because photographs are taken at different states of tide and season, so some of the apparent acceleration could be measurement timing. To test the explanation, compare storm frequency and beach width with retreat; if retreat peaks after storms and after beach lowering, the marine explanation is strong.
2. Recommend either the revetment or managed retreat. Use economic, environmental and social criteria, and state one assumption.
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Managed retreat is the better default. Costs: revetment £6.8m versus retreat £2.4m + £1.1m = £3.5m, saving £3.3m. Environmentally, retreat keeps cliff sediment feeding the 30–40 m beach, which supports tourism worth £900,000 per year; a revetment locks the cliff and can cause coastal squeeze, lowering the beach and damaging the very tourism asset. Socially, retreat is disruptive for 14 households and the road, but the sewage pipe must be relocated anyway, and buying houses allows planned compensation; a revetment protects only those 14 houses for 40 years while creating a long-term liability. Assumption: the 14 houses can be bought at the stated average and residents accept relocation. If the houses are worth more than £6.8m, or if the road is a lifeline to more than the village, revetment could be justified. Before deciding, compare the 40-year present cost of revetment maintenance with relocation and tourism loss. Recommend retreat with a compensation and sewage relocation plan.
3. Identify one unintended consequence of building the revetment and explain how it could be monitored.
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One unintended consequence is beach lowering and loss of sediment supply. The revetment fixes the cliff line, so erosion cannot supply sand and gravel; waves may steepen and lower the beach in front. That reduces storm protection and may make the beach less attractive, cutting tourism. Monitoring should measure beach profiles along five transects perpendicular to the shore at the same tide level every three months, and repeat drone surveys annually; compare with a control stretch of unprotected cliff. Also record cliff-top retreat pins and visitor counts. If beach width falls below, say, 20 m for two consecutive summers, trigger a review and possibly beach nourishment. The key is to set a threshold before construction, not after.