Beach erosion and a proposed groyne at Maspalomas
Playa de Maspalomas on Gran Canaria is a 2.7 km sandy beach backed by a dune field and, at its northern end, by a promenade and hotels. In February a storm with offshore waves of 5.5 m removed an estimated 35,000 m³ of sand and cut the dune scarp back by 12 m; one hotel is now 40 m from the scarp. The littoral drift is dominantly toward the southwest at about 50,000 m³/yr. The Cabildo proposes a 180 m rock groyne at the northern end plus 20,000 m³ of beach nourishment dredged from an offshore sand bank. Local hoteliers support the groyne; environmental groups oppose it because the dune field is a protected reserve and the beach is a nesting site for loggerhead turtles in summer. You are asked to advise the coastal authority on the proposal before the next budget cycle. The offshore sand bank has a median grain size of 0.35 mm; the beach sand is 0.50 mm.
1. Evaluate whether the proposed groyne will solve the erosion problem. Use the sediment budget and longshore drift in your answer.
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The groyne will not solve the erosion problem; it will redistribute it. With a southwest-directed drift of about 50,000 m³/yr, a 180 m groyne at the northern end will trap sand on its upcoast side and starve the downdrift beach to the south. The hotel near the northern scarp may gain temporary protection, but the southern two-thirds of the 2.7 km beach will erode faster. The sediment budget is also inadequate: the storm removed 35,000 m³, while the proposed nourishment is only 20,000 m³, less than half the annual drift. The nourishment sand is finer than the native beach (0.35 mm versus 0.50 mm), so it will be mobilised and lost more quickly. The groyne therefore does not create new sediment; it interrupts a transport system. My conclusion is that the proposal is a short-term fix for one frontage at the expense of the wider beach and the dune reserve.
2. Compare the groyne-plus-nourishment option with alternatives and recommend one, taking account of the protected dune field and turtle nesting.
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I would reject the groyne and recommend a sediment-budget strategy based on a larger, better-matched nourishment plus dune restoration and a managed setback. A nourishment of at least 70,000–100,000 m³ from a coarser offshore source would replace the storm loss and create a buffer, but it must be monitored because finer sand will erode faster. Dune restoration-fencing, replanting and controlling foot traffic-would rebuild the natural buffer and protect the reserve. A managed setback of the most exposed hotel, or a temporary removable defence, avoids locking the coast into hard engineering. The turtle nesting constraint favours moving nourishment and works to outside the summer nesting season and avoiding beach lighting changes. The trade-off is cost and political resistance: a groyne is visible and appears decisive, while nourishment and setback require recurrent funding and land-use decisions. On sediment-budget grounds, however, the groyne transfers erosion downdrift and is likely to require further groynes, a classic coastal-engineering cascade.
3. What monitoring would you require before and after any intervention, and what would make you stop?
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Before intervention I would require a baseline of monthly beach profiles along the full 2.7 km, bathymetry, grain-size analysis of native and borrow sand, wave and current records, and a tracer or numerical study of longshore drift. I would also map turtle nesting and dune vegetation. After any nourishment or defence, I would continue the same profiles and add aerial or drone surveys after storms, sediment traps, and water-quality checks for turbidity during dredging. Stop criteria should be explicit: if downdrift shoreline retreat exceeds an agreed threshold, if the nourished sand is lost faster than predicted, if nesting habitat is disturbed, or if the dune scarp retreats behind the pre-project line. Monitoring must include the whole cell, not just the hotel frontage, because the main risk is erosion transferred elsewhere. Without these data, the authority cannot distinguish a successful scheme from a temporary redistribution of sand.