Short period (1–4 h) sea level fluctuations on the Canterbury coast, New Zealand

Abstract

Data from six sea level sites on the Canterbury coast, New Zealand, were analysed for short period (1–4 h) waves. Persistent waves with periods of 3.4 h in Pegasus Bay and 2.4 h in the Canterbury Bight were found. Their amplitude and phase are highly variable. Edge waves of similar period were found in simulations using a 2‐dimensional, harmonic, shallow water, numerical model. Numerical and analytical modelling showed that the offshore decay in amplitude may be approximated by Stokes zero‐mode edge waves, but the numerical model revealed that the detailed structure of the offshore decay is more complicated than can be explained by analytical models using a semi‐infinite sloping beach. Rotation appears to have little effect. In Pegasus Bay the edge waves are trapped laterally between the northern extent of the bay and Banks Peninsula and offshore by the shelf. In the Canterbury Bight they are trapped laterally between Banks Peninsula and the curvature of the coast and offshore by the shelf. The origin of the waves and the reason for their variability are unclear, but may be the result of the non‐linear interaction between semidiurnal tides and meteorological effects.

Keywords:

• water waves
• edge waves
• coastal trapped waves
• seiche
• Canterbury