The Reef, the Beach and the Groynes
A 3 km beach on a small island has narrowed by about 25 m in 20 years. Behind it are hotels, a road and a freshwater well field. Offshore lies a fringing coral reef. In 2010, three rock groynes were built at 200 m intervals to hold sand. Since then, the beach has widened by 15 m updrift of the northernmost groyne but narrowed by 20 m immediately downdrift, where a hotel has lost its beach. Sediment samples show the beach sand is mainly carbonate (shell and coral fragments). Reef health surveys show 40% live coral cover in 2000 and 15% in 2023, with more algae.
1. Explain the links between the reef, the beach and the groynes.
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The beach and reef form one sediment system.
- Sediment source: the sand is carbonate, so much of it comes from reef organisms and shells. A healthy reef produces and traps sediment; a degraded reef supplies less.
- Wave energy: coral cover fell from 40% to 15% and algae increased, so the reef is rougher and less able to absorb wave energy. More energy reaches the shore, increasing erosion.
- Groynes: they interrupt longshore drift. Sand builds up on the updrift (northern) side and is starved downdrift, which explains the 15 m gain and the 20 m loss.
So the narrowing has at least two causes: a weakening reef and the groynes trapping sand. The groynes treat a symptom and shift erosion along the coast.
2. A developer proposes extending all three groynes seaward. Assess this proposal.
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Extension would trap more sand, widening the updrift beach, but the downdrift loss would probably worsen because less sediment passes each groyne. With three groynes in 600 m, the coast is already segmented, and the hotel downdrift has lost 20 m. Extending them increases the starvation effect and may push erosion further along the shore.
It also does not address the reef decline, which is likely increasing wave energy. Structures can also damage reef and seagrass and interfere with sediment supply.
Judgement: extension is a short-term fix with a high risk of moving the problem. Better options are:
- beach nourishment using compatible carbonate sand;
- reducing runoff and overfishing to help coral recover;
- removing or redesigning groynes if downdrift loss continues;
- setback rules so buildings are not rebuilt in the eroding zone.
Any nourishment must be monitored, because repeated dredging is costly and can smother reef.
3. How would you decide whether the reef decline or the groynes is the main cause of the downdrift erosion?
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Use evidence that separates the two causes.
- Timing: compare beach width before 2010 and after. If the downdrift loss began sharply after groyne construction, groynes are the likely cause. If narrowing was already happening, reef decline contributes.
- Pattern: groyne effects are local and directional, strongest next to the structures. Reef effects should be more widespread along the whole 3 km.
- Sediment budget: measure sand volume and longshore drift rates. If drift is high and trapped, groyne effect is strong.
- Reef surveys: check whether coral loss matches periods of faster erosion.
- Controls: compare an unmodified stretch of coast nearby.
A reasonable conclusion is that both operate, but the sharp downdrift loss next to the groyne points to the groynes as the immediate cause, while reef decline is a longer-term background factor.