The Narrowing Beach: Seawall, Nourishment or Retreat?
Te Arai Beach is a 2.4 km stretch of coarse sand backed by a 12 m cliff of soft sandstone. Council records show mean beach width fell from 34 m in 2004 to 17 m in 2024. Longshore drift is northward; a 1960s groyne at the southern end traps an estimated 8,000 m³ of sand per year, while the cliffs supply about 3,000 m³/yr and the nearest river 2,000 m³/yr. Offshore dredging for a marina removed 25,000 m³ in 2015. Three storm events since 2018 each removed 4–6 m of beach width, with recovery taking about 18 months. The tide gauge shows relative sea-level rise of 3.1 mm/yr, of which 0.9 mm/yr is land subsidence. Six houses sit within 15 m of the cliff edge; insurance liability is capped at $9 million. Options are: (A) a 400 m rock seawall, $12 million, design life 40 years; (B) relocate the six houses and set back the road, $9 million; (C) offshore nourishment of 60,000 m³ every 8 years at $6 million per campaign. The regional council must choose one option this year.
1. Construct a sediment budget for Te Arai Beach and use it to explain the long-term narrowing. Then predict the effect of option A on the beach in front of the wall.
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Sediment budget (approximate annual values):
- Inputs: cliff 3,000 m³ + river 2,000 m³ = 5,000 m³/yr.
- Outputs: groyne trapping 8,000 m³/yr (sand leaves the littoral cell and is not replaced).
- Net budget: 5,000 − 8,000 = −3,000 m³/yr, a persistent deficit even before storms and sea-level rise are considered.
A 3,000 m³/yr deficit sustained over 20 years is 60,000 m³ - enough to explain a large part of the 17 m width loss on a 2.4 km beach. The 25,000 m³ dredging loss and the three storms are superimposed shocks; because recovery takes ~18 months, closely spaced storms convert temporary erosion into permanent narrowing. Relative sea-level rise of 3.1 mm/yr (including 0.9 mm/yr subsidence) shifts wave energy landward and makes the beach more vulnerable.
Effect of a seawall: a hard wall locks the shoreline and removes the cliff as a sediment source, so the 3,000 m³/yr input is lost and the deficit grows to about −6,000 m³/yr. Wave reflection and toe scour move sand offshore, while erosion is transferred to the ends of the wall. The beach in front therefore continues to narrow and may disappear at high tide - the wall protects the houses but not the beach, and its design life assumes the toe is not undermined.
2. Evaluate options A, B and C using cost over 50 years, effectiveness, and uncertainty. Recommend one option and justify the trade-offs.
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A - seawall ($12M): cheapest headline cost, but design life is 40 years, so replacement or major repair is needed within the 50-year window. It protects six houses but removes cliff sediment input, accelerates beach loss, shifts erosion downdrift, and may fail by toe scour. Effectiveness is high for property, low for the beach.
B - retreat ($9M): a one-off cost that permanently removes the assets from the hazard zone, allows the cliff and dune to supply sediment again, and creates space for the beach to migrate landward with sea-level rise. Social and cultural costs (loss of homes, road relocation, consenting) are real but are the only costs that do not grow with time.
C - nourishment ($6M): if only one campaign is counted it looks cheapest, but it is needed every 8 years: roughly six campaigns over 50 years, about $36M, with no guarantee sand stays on the beach given the groyne and storms. It treats the symptom while the deficit continues.
Recommendation: B, supported by dune restoration and monitoring, because it is the only option that reduces long-term risk and restores the sediment budget. A is defensible only as a short-term emergency measure for the most vulnerable houses; C is defensible only if paired with removing the groyne so that nourishment is not immediately lost. The judgement is between certain but bounded social cost (B) and escalating engineering cost with environmental loss (A/C).
3. Identify two additional measurements the council should collect before committing, and explain how each would change the decision.
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Measured longshore transport and groyne bypassing, rather than the estimated 8,000 m³/yr. If actual trapping is 3,000 m³/yr, the deficit is close to zero and the case for retreat weakens; if it is 12,000 m³/yr, nourishment without groyne modification is futile. This directly changes whether C can work.
Storminess and wave-climate projections, plus a surveyed cliff-toe profile. If storm frequency increases, the 18-month recovery assumption fails and B becomes more urgent; if the cliff toe is already undercut, the seawall's design life and cost estimate are unreliable. A nearshore bathymetry survey would also show whether dredging removed an offshore sand reservoir that could naturally return.
A useful third would be an economic and cultural assessment of the six households, since the $9M figure is a liability cap, not the full social cost of relocation.