Wave Characteristics and Transformations on Sub-Horizontal (Type B) Shore Platforms on the East Coast of the North Island, New Zealand

Abstract

This study presents the results of the most detailed hydrodynamic experiments conducted on sub‐horizontal ‘Type B’ shore platforms to date. The aims of this study were to: provide detailed descriptions of wave transformation processes; identify morphological controls on wave processes; and investigate possible morphological implications. Six platforms with a range of widths (80–270 m) and gradients (0.2– 1.3°) were selected from Gisborne and Auckland, North Island, New Zealand. Synchronised high frequency sea‐surface records were obtained using electronic wave gauges. The Gisborne experiments were conducted under fair‐weather swell conditions to characterise temporal and spatial characteristics of wave transformation. The Auckland experiments were conducted under storm conditions to explore the interaction of storm waves with the platform surface. Results showed that breaking was forced on the platform edge when the relative water depth (depth/significant wave height) at the edge was less than 2.5; whereas waves shoaled onto the platform surface under lower energy conditions. Gravity‐wave height was depth limited on the inner platform surface and independent of incident wave conditions. The maximum wave height on very flat (< 0.23°) platform surfaces was limited to 55% of platform water depth, but this limit increased with increasing platform gradient. Platform width was an important control on wave attenuation at low tide due to increased frictional energy loss: the attenuation rate of gravity waves was 1.5 times higher at low than high tide. In general, gravity waves progressively attenuated across platforms, but infragravity‐wave energy increased toward the cliff toe. The relative importance of infragravity waves at the cliff toe increased with increasing platform width and decreasing water depth. The proportional increase in infragravity‐wave height at the cliff toe was found to be a function of the platform energy window index (ψ) and it was proposed that Type B platforms may be characterised as ‘gravity‐wave’ and ‘infragravity‐wave’ dominated platforms. Collectively, the results presented within this thesis demonstrate the importance of platform water depths (hence elevation), width and gradient in controlling wave energy characteristics across shore platforms. Existing models of platform morphodynamics and evolution present a ‘black box’ in respect to wave processes. The quantitative field data on wave processes presented here provide an opportunity to reconsider some of the basic assumptions in existing models.