The Disappearing Shoreline
A coastal town measures its beach width every spring. Records show the mean high-tide line moved inland by 11 m between 2005 and 2015, and by a further 19 m between 2015 and 2025. The town has also built a rock groyne 400 m to the north in 2012 and dredged its harbour channel every year since 2010. Sea-level records from a nearby tide gauge show a rise of about 3 mm per year over the same period. A council member says the erosion is 'entirely due to sea-level rise' and proposes building a seawall along the whole frontage. A resident says the groyne is the real cause and wants it removed.
1. Estimate how much of the 30 m retreat could be explained by sea-level rise alone, and assess whether that fully accounts for the observations.
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Over 20 years at 3 mm per year, sea level rose about 60 mm, or 0.06 m. On a gently sloping beach, horizontal retreat is roughly the vertical rise divided by the beach slope. Even with a very gentle slope of 1:100, 0.06 m of rise gives only about 6 m of retreat; with a steeper 1:50 slope it gives about 3 m. The observed retreat is 30 m, five to ten times larger. So sea-level rise can explain only a small part of the total. The council member's 'entirely due to' claim is not supported by the arithmetic.
2. Explain how the groyne and the dredging could each contribute to retreat, and why the acceleration after 2012 matters.
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A groyne interrupts longshore drift: sand accumulates on its up-drift side and the down-drift side is starved of sediment, so the beach to the south can erode faster. The town is 400 m south of the groyne, in the likely starved zone. Dredging removes sediment from the harbour channel and can deepen or shift the nearshore profile, changing wave energy reaching the beach and cutting off a sand supply. The retreat rate roughly doubled after 2012, the year the groyne was built, which is consistent with a new human cause being added to the slow background rise. Neither cause alone is proven, but both are plausible and together better explain the acceleration than sea level alone.
3. Evaluate the seawall proposal as a response, using the evidence, and suggest one monitoring step before committing.
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A seawall protects the land behind it but does not restore the beach; it can reflect wave energy and accelerate erosion of the sand in front, and it locks the shoreline in place. Because the evidence points to a sediment-supply problem rather than only sea-level rise, a wall may treat the symptom and worsen the cause. Removing the groyne is also risky, since accumulated sand would be released and could shift erosion elsewhere. A better first step is to survey beach profiles north and south of the groyne and measure longshore sediment movement for a year, so the town can tell whether the groyne or dredging dominates. That evidence should guide a decision such as beach nourishment or adjusting dredging, rather than a wall built on an unsupported single cause.