Pocket beach wave processes and current systems investigated via field and numerical modelling studies: A case study of Okains Bay
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Confined coasts in general, and pocket beach environments in particular, are under huge development pressures worldwide, not least due to their sheltered nature and perceived shoreline stability. However, understanding of their physical functioning is poor in comparison to that of open coast beaches. This study aims to improve understanding in terms of the existing gaps in knowledge of wave processes and nearshore currents, and also to examine the importance of local wind and tide factors in generating nearshore currents, in micro-tidal pocket beaches. The boundaries of embayments are generally recognized as important controls of their beach processes and responses, yet little detailed knowledge exists of how the exact embayment dimensions and characteristics influences these processes. One key embayment feature the influence of which is poorly understood is the downcoast headland. In this thesis, field observations plus Zanuttigh and Van der Meer’s (2008) approach, and the SWAN wave model were used to evaluate the downcoast headland effects on wave processes within Okains Bay, an example pocket beach environment. The results showed that incident wave heights and directions were significantly influenced by wave reflection processes from the downcoast headland inside the bay. The intensity of reflection effects on wave characteristics inside the pocket beach varied according to approaching wave direction. Reflection effects reduced when waves approached from angles close to parallel to the headlands, increasing towards headland-perpendicular wave approaches.
Field observations and the XBeach model were used to examine whether or not tides can significantly influence nearshore currents within example and model pocket beach environments. Results indicated that tides can be the primary driver of nearshore currents close to the bed inside micro-tidal pocket beaches, depending on incident wave conditions. In areas of micro-tidal pocket beaches exposed to direct approaching waves, currents were wave driven, while in areas further into the bay that experienced headland filtering of their wave environment, currents were mainly tide generated.
The results of this study demonstrated how the current circulation system within micro-tidal pocket beaches is related to the incoming directions of offshore waves. If high energy waves approach oblique or normal to the shoreline (with the assumption that the shoreline is at 90° to the headlands), the current system was found to consist of longshore currents influenced by headlands, plus a rip current in the center of the shoreline or a toporip in proximity to headlands. The location of the rip current or toporip was determined by the direction of approaching incident waves.
This study also examined the behavior of local winds in a pocket beach environment and their consequent effects on nearshore currents. Results for Okains Bay show that local winds tended to blow in offshore and onshore directions, as the bay is located in a valley, so orographic effects channel and shift the wind directions to angles close to offshore and onshore directions inside the bay. Results also indicated that local winds influence the hydrodynamic currents of pocket beaches that are confined by elevated topography, producing semi-cross shore influences since the winds are topographically channelled to blow in predominantly offshore and onshore directions.
This research significantly refines our understanding of micro-tidal pocket beach wave and current processes, including quantification of the filtering effects of headlands on their wave environments, revealing the various and variable influences of tides and winds compared to in open coast beaches; and, significantly, highlighting the role of downcoast headland wave reflection effects. With regard to the latter, this research elucidates some key process differences between pocket and embayed beaches and clarifies reasons why the application of embayed beach models that include refraction and diffraction but exclude reflection effects to the study of pocket beaches is inappropriate for studying pocket beaches. This research also provides methodological and topic suggestions for future research on pocket beach environments, including how to use the improved hydrodynamic knowledge of this study in future studies seeking to better understand pocket beach sediment systems, a topic that was beyond the scope of the current research.