Interacting processes on a mixed sand–gravel beach
A 4 km stretch of mixed sand–gravel beach on the east coast of the South Island is experiencing uneven shoreline change. Longshore drift moves sediment northward at about 12,000 m³ per year. A nearby river supplies gravel during flood events, but gravel extraction upstream has reduced supply by an estimated 15%. Storm waves from the south-east, with a 4 m swell, strike during spring tides. The foredune, 3.5 m high, is vegetated with spinifex and marram grass. A recent storm removed 8 m from the dune toe. The district council proposes a 300 m rock seawall at the northern end to protect a coastal road. Residents living 500 m south of the proposed wall have already noticed faster erosion after a temporary rock wall was trialled. The council needs a geomorphic assessment before consent is granted.
1. Explain how interacting natural processes have shaped the beach–dune system in this location. In your answer, refer to at least three processes and their feedbacks.
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Wave refraction and longshore drift are the dominant processes. South-east swell approaches the coast at an angle, creating a northward longshore drift of about 12,000 m³/yr. This moves sediment from south to north, tending to build a wider beach in the north and a narrower one in the south. Fluvial supply from the river adds gravel during floods, but reduced upstream extraction (15%) means the sediment budget is now negative, so erosion occurs. Storm waves during spring tides remove sediment from the dune toe; the 8 m loss shows the foredune is being cut back. Vegetation on the foredune (spinifex and marram) traps sand and builds the dune, but when storms remove the toe, the vegetation is undermined and the dune becomes less stable. A key feedback is that as the dune erodes, the beach profile steepens, which increases wave energy at the shore and causes more erosion. The system is therefore shaped by the interaction of marine processes (waves, tides, drift), fluvial supply, and biological processes (vegetation), with feedbacks that amplify change when the sediment budget is disturbed. The present shoreline change is not caused by one process alone but by the combination of reduced supply and storm-driven erosion.
2. Evaluate the likely geomorphic consequences of the proposed 300 m rock seawall for the sediment budget and for the coast south of the wall. Use the evidence in the setup to support your judgement.
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A seawall is a hard defence that fixes the shoreline and prevents wave attack on the land behind it. In the short term, it protects the road. However, it will have three geomorphic consequences. First, it cuts off sediment from the dune and cliff behind the wall, so that sediment is no longer available to feed the beach. Second, because longshore drift is northward, the wall will trap sediment moving northward, causing accretion at the wall's northern end but starvation immediately downdrift to the south. The residents south of the wall have already observed faster erosion after a temporary wall, which supports this ‘end effect’. Third, wave reflection from the rock wall will increase turbulence and scouring at its base, deepening the nearshore and increasing erosion of the beach in front of the wall. The wall will therefore exacerbate the negative sediment budget and shift erosion downdrift. My judgement is that the seawall will likely increase erosion south of the wall, making it a maladaptive response unless accompanied by sand replenishment or managed retreat. The council should consider the wider sediment compartment, not just the road asset.