Coastal Erosion and Sedimentation Processes at Waihi Beach, New Zealand

Waihi Beach is an example of chronic erosive tendency. The erosion problem is particularly bad at the northern end where a rock seawall was constructed in the 1960s in an attempt to protect coastal property at risk from coastal erosion and flooding hazards. Accordingly, this study was undertaken to identify and evaluate the fundamental coastal processes impacting upon erosion at Waihi Beach. Measured wave and current data combined with bathymetric and sediment data indicated that nearshore current velocities alone are incapable of inducing sediment movement and require wave motion to initiate entrainment. A net onshore flux of 2.6 m³/m/year was calculated for Waihi Beach, despite persistent rip currents at the far northern end of the beach producing offshore sediment transport at that location. The numerical wave model WBEND is used to examine the influence of wave refraction and convergence caused by Mayor Island and large shore-normal sand ridges (η=0.4–2.5 m, λ=300–1400 m) revealed by side-scan sonar, on the distribution of wave energy along the coast. Using average wave conditions, a zone of wave convergence caused by refraction around Mayor Island was found to produce enhanced wave heights along a particularly erosive sector of the beach fronting the seawall. Zones of wave convergence caused by wave refraction over the offshore sand ridges were found to produce variation in alongshore breaking wave heights.