Beach Narrowing at Il-Bajja s-Sewda: Choosing a Coastal Defence

A sediment budget compares inputs and outputs to a beach cell.

Inputs here are cliff erosion and any longshore supply. Cliff retreat of roughly 0.3 m a year along 1.8 km, even with a 15 m cliff, yields about 8,000 m³ a year, but most of that clay and rubble is washed offshore or broken down, so the usable sand input is far smaller. Outputs are losses offshore, losses alongshore and wind-blown loss to the backshore.

The groynes altered the budget: each trapped sand on its up-drift side but starved the cell down-drift. Now two are breached, so sand leaks past them and the traps no longer work, while the down-drift cell is still deprived. The February storm removing 4,500 m³ in one event shows that a single high-energy episode can remove a fifth of the proposed replenishment volume.

Coarsening from 0.6 mm to 1.1 mm is the signature of winnowing. Waves and onshore winds selectively remove the finer fraction and leave a coarser lag, so the beach armour-plates. That looks like a 'healthier' sand but actually signals net loss: there is less material, and what remains is the part the sea cannot easily move. Sediment starvation, not a change in supply of coarse material, is the likely cause.

Replenishment (€1.2m for 25,000 m³) costs about €48 per cubic metre. If losses run at roughly 4,000–5,000 m³ a year (based on the storm loss and the observed narrowing), the beach survives five to six years, so it is a recurrent cost, not a one-off. It is, however, the only option that actually adds sediment to the budget and it directly protects the 90,000 visits that underpin the local economy.

Rock groynes are durable, perhaps 30+ years, and cheaper per year of life, but they do not add sediment. Rebuilding them simply redistributes the existing, shrinking supply and will further starve the down-drift beach. They also fix the shoreline in place, which is questionable where the cliff behind is retreating and a watchtower is only 11 m from the edge.

An offshore breakwater is the most expensive and the most uncertain: it changes wave refraction, may form a tombolo, and can damage the offshore meadows that damp wave energy.

The defensible recommendation is a package: spend most of the €1.2m on replenishment, remove rather than rebuild the two breached groynes so sediment can move, and use the remainder for monitoring. Replenishment buys time, adds to the budget and supports the beach economy; hard structures lock in a sediment deficit. The watchtower should be dealt with separately - relocation or managed realignment is honest where retreat is 0.3 m a year.

Monitoring must test the sediment budget, not just the view.

1. Beach profiles at fixed benchmarks every 100 m along the 1.8 km, surveyed in spring and autumn (after the storm season) - this gives volume change in m³, not just width, which is the number that matters.
2. Grain-size sampling at the same points, because coarsening is an early warning of winnowing.
3. Cliff-edge survey using a total station or drone at the watchtower and at three other fixed points, to detect retreat beyond the 0.25–0.35 m/yr baseline.
4. Bathymetry of the nearshore every two years, since sand lost offshore may or may not return.
5. Wave and wind records from a local logger so that storm losses can be separated from slow trends.
6. Visitor counts and beach-club occupancy, to link physical change to the economic case.

The key design principle is a control: monitor an unmanaged neighbouring bay so that change can be attributed to the scheme rather than to a regional stormy decade. A written trigger - for example, 'replenish again if central volume falls below X m³' - converts monitoring into a decision rule instead of a report on a shelf.