Whakapirau Beach: seawall or foredune after a sediment budget change
Whakapirau Beach is a 2.1 km sandy beach on the west coast of the North Island. Since 2012 the dune toe has retreated an average of 18 m, and a 2018 storm cut a 2.5 m scarp directly in front of the surf club. The beach sediment budget has changed: the river that once delivered about 40,000 m3 of sand a year now delivers roughly 24,000 m3 because of an upstream dam, while longshore drift still moves sand north at an estimated 15,000 m3 a year. The district council has $1.2 million for the next ten years. A 400 m rock seawall in front of the 34 most at-risk properties is quoted at $1.1 million. Dune restoration and planting along the same 400 m is quoted at $180,000, with an expected 6-8 year lag before it traps measurable sand. Residents closest to the scarp want the seawall built immediately; the surf club and the iwi landowner north of the wall support restoration.
1. Recommend how the council should spend its $1.2 million, justifying your choice with the sediment budget and the erosion record.
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Reasoning steps
- Read the budget as a net figure, not a total. Inputs are about 24,000 m3/yr from the river; longshore output is about 15,000 m3/yr. That leaves roughly 9,000 m3/yr, which sounds positive, yet the dune has retreated 18 m in 12 years. This tells you the loss is concentrated in the storm season and in the subaerial beach, not spread evenly, so an annual average hides the real risk.
- Test the seawall against the budget. A rock wall fixes the shoreline position but adds no sediment. It removes the dune as a sand store and reflects wave energy, so the beach in front usually narrows and erosion shifts to the ends of the wall. It also costs $1.1 million of a $1.2 million budget, leaving almost nothing for monitoring or for the properties outside the 400 m.
- Test restoration against the timing. Dune restoration is cheap ($180,000) and works with longshore drift by trapping sand, but the 6-8 year lag is a real problem when a scarp is 2.5 m high and storms are unpredictable.
Recommendation: fund dune restoration first and spend roughly $700,000-$800,000 of the remainder on targeted protection or ratepayer-assisted relocation for the four to six sections with the least dune buffer, keeping the rest for monitoring. A seawall for all 34 properties locks in a defence that the sediment budget cannot sustain.
2. Explain why building the seawall could increase erosion on the coast north of it.
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Longshore drift moves sand north at about 15,000 m3/yr, so the beach north of the wall depends on sand arriving from the south. A rock seawall interrupts this in two ways.
First, it fixes the sediment that would otherwise be released from the eroding dune and scarp. Under natural conditions, storm erosion feeds the beach and is gradually carried north. Once the wall is in place, the dune behind it cannot erode, so that supply stops.
Second, the wall reflects wave energy instead of dissipating it in the dune and beach. Reflected waves scour sand from the base of the wall and move it offshore, and wave energy is deflected around the ends of the structure. The result is usually a narrower, lower beach at the wall and a scour lobe at the downdrift (northern) end.
Because the north end is where the surf club and iwi land sit, the wall would protect the 34 properties at the direct expense of the land downdrift: the erosion problem is moved, not solved. The size of that effect would need monitoring, but the direction is predictable from the drift direction.
3. Identify one further piece of evidence the council should collect, and explain how it could change the decision.
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Evidence needed: a surveyed shoreline record extending back at least 30-40 years (aerial photos, cadastral boundaries, LiDAR), rather than the 12-year record quoted.
Why it matters: twelve years is short compared with the cycle of El Nino and La Nina storminess. The 18 m retreat may include a single storm cluster, in which case the long-term rate could be much lower and dune restoration alone may be enough. Alternatively, the rate may be accelerating as sea level rises, in which case even a hybrid response is only buying time and the council should plan for managed retreat.
How it changes the decision: if the long-term rate is low, the seawall is an expensive overreaction and dune restoration plus monitoring is clearly justified. If the rate is high and accelerating, spending on a wall for 34 properties may be throwing money at an unwinnable position, and the $1.2 million would be better used to relocate the most exposed sections and keep the beach functioning for everyone else. The same evidence also sets the trigger point for when to act, which the current data cannot do.