Siting a Coastal Management Strategy on a Retreating Shoreline
A local council manages a 4.2 km sandy beach backed by a low dune system. Survey pegs show the high-water mark has retreated an average of 1.8 m per year over the last 20 years, but retreat is uneven: the northern 1.5 km lost 45 m total, the central 1.2 km lost 12 m, and the southern 1.5 km lost 3 m. Behind the northern section sit 60 houses, a surf club and the only road access; the central section has a caravan park; the southern section is a council reserve with endangered coastal vegetation. The council has a budget of AUD 6 million over 10 years. Hard engineering (a rock seawall) for the northern section is quoted at AUD 5.2 million plus AUD 80,000 per year maintenance. Beach nourishment for the central section costs AUD 350,000 per 100 m of beach and lasts about 6 years. Planned retreat (buyback and relocation) for the northern houses is estimated at AUD 2.4 million in total.
1. Identify which section of the coast is experiencing the most rapid erosion and explain two plausible reasons why erosion rates differ along this single beach.
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Most rapid section: the northern 1.5 km.
- Rate calculation: 45 m ÷ 20 years ≈ 2.25 m/yr, compared with the central section (12 m ÷ 20 years = 0.6 m/yr) and the southern section (3 m ÷ 20 years = 0.15 m/yr). The northern section is retreating roughly 15 times faster than the southern section.
Reason 1 - Sediment supply and alongshore drift. If the dominant alongshore drift moves sediment from north to south, any interruption to supply (such as a groyne, harbour wall or river-mouth training works up-drift of the northern section) starves the northern beach of sand. A narrower beach dissipates less wave energy, so storm waves reach the dune toe and remove it. The southern section, by contrast, receives the sediment that has moved down the coast, explaining its slower retreat.
Reason 2 - Exposure and wave energy. The northern section may face a longer fetch or a more oblique angle to prevailing swell, increasing wave energy and the rate of longshore transport. Local bathymetry (deeper water close to shore) also allows larger waves to break nearer the dune. The southern section may sit in the lee of a headland, reducing wave energy and allowing vegetation to stabilise the dune.
A strong answer links the numbers to a process (sediment budget or wave energy) rather than simply restating the survey data.
2. Evaluate the three management options against the council's constraints. Recommend a strategy and justify it with reference to spatial variation and cost.
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Option 1 - Seawall (northern section). Cost AUD 5.2 million + AUD 800,000 over 10 years = AUD 6.0 million, consuming the entire budget. It protects the 60 houses and road but causes terminal scour at the ends of the wall, accelerating erosion in the adjacent central section where the caravan park sits. It also locks the shoreline in place, so the beach narrows and eventually disappears in front of the wall. Poor value: it solves one section while transferring the problem.
Option 2 - Nourishment (central section). At AUD 350,000 per 100 m for 1,200 m, one full cycle costs AUD 4.2 million, and because it lasts only 6 years, a second cycle is needed within the 10-year window - total AUD 8.4 million, over budget. Nourishment is soft and reversible but cannot be sustained at this price across the whole central section.
Option 3 - Planned retreat (northern houses). AUD 2.4 million, leaving AUD 3.6 million for other works. It removes the assets at risk, allows the dune to migrate landward and maintains the sediment budget, but has social costs (relocation, loss of community) and political risk.
Recommended strategy (spatially differentiated):
- North: planned retreat (AUD 2.4 m) - cheaper than the seawall and avoids downdrift impacts.
- Central: targeted nourishment of the most vulnerable 400–600 m only (AUD 1.4–2.1 m per cycle), not the full frontage.
- South: no engineering; protect the endangered vegetation with fencing and boardwalks (low cost).
This keeps total spend within AUD 6 million, matches intervention intensity to erosion rate and asset value, and avoids the downdrift harm a seawall would cause. A weaker answer picks one option for the whole coast and ignores the spatial variation the data reveal.