Groin Expansion on a Retreating Barrier Island
A barrier-island town has tracked shoreline position along three shore-normal transects using georeferenced aerial photographs since 1950. Transect A sits 200 m south (downdrift) of a 1962 rock groin at the island's north end and moved 210 m landward between 1950 and 2020. Transect B, 3 km farther south, moved 40 m landward over the same period. Transect C, 7 km south near a tidal inlet, moved 85 m seaward. The town's consultant proposes extending the existing groin by 150 m and building a second groin at transect C, arguing that the shoreline is retreating everywhere and that hard structures are the only permanent fix. Regional tide gauges show relative sea level rising about 4 mm/yr. The island's long axis runs north-south, and waves approach predominantly from the northeast. The town council asks the science teacher on the planning board to review the proposal.
1. Infer the direction of net longshore drift and explain how the groin produced the pattern at the three transects. Include average shoreline-change rates for A, B and C in m/yr.
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Rates (change divided by 70 years): A = 210/70 = 3.0 m/yr landward; B = 40/70 = 0.6 m/yr landward; C = 85/70 = 1.2 m/yr seaward.
The drift direction comes from the wave approach and from the asymmetry around the groin. Waves from the northeast striking a north-south shore drive net transport toward the south. A groin traps sand on its updrift (north) side and starves the downdrift (south) side, so the strongest erosion should occur immediately south of it and weaken with distance as other sediment sources recharge the beach. That is exactly the observed gradient: 3.0 m/yr at A just below the groin, dropping to 0.6 m/yr at B 3 km south.
Transect C does not fit a simple 'everything is eroding' story. Accretion near an inlet is common where flood-tidal deltas or recurved spits accumulate sand, or where the inlet has migrated. The consultant's claim of uniform retreat is contradicted by C, and the alongshore pattern is best explained as groin-induced starvation superimposed on a slowly rising sea level.
2. Evaluate the two-groin proposal. Identify which claims in the consultant's argument are contradicted by the data, and recommend at least two alternatives the council should weigh.
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The proposal rests on two faulty premises. First, 'retreating everywhere' is false: C is accreting at 1.2 m/yr. Second, the consultant treats the groin as a fix, when the data show it is a cause - the 3.0 m/yr loss at A is downdrift starvation produced by the 1962 structure.
Extending the groin by 150 m would trap more sand updrift and push the starvation zone farther south, shifting the erosion problem toward B rather than eliminating it. A second groin at C would interrupt supply to the inlet and to the southern shoreline, and because groins hold a shoreline in place rather than add sand, neither structure addresses the 4 mm/yr relative sea-level rise, which will continue to force landward migration of the whole system.
Better options: beach nourishment with sand matched to the native grain size, coupled with dune rebuilding and a setback line that keeps new construction out of the erosion zone; relocation or hardening of specific at-risk infrastructure; and, if a structure is unavoidable, a tapered groin field with periodic sand bypassing plus a before-and-after monitoring program. Monitoring data should be required before any permit is granted.