The Relationship between Nearshore Wave Conditions and Coarse Clastic Beach Dynamics
Brayne, Ralph Philip
Thesis or dissertation
University of Exeter
Beaches composed of pebble to boulder-sized material are a common feature of rocky coastlines globally and provide effective protection against wave attack. However difficulties with the proximal deployment of wave gauges and measuring the transport of coarse clasts in the field has limited research in this area. The overarching aim of this thesis is to use contemporary ‘off-the-shelf’ technology in a series of interlinked, field-based experiments to improve quantitative understanding of relationship between coarse clastic beach dynamics and the nearshore wave conditions. In the first of the empirical chapters (Chapter 3), indigenous clasts implanted with motion loggers provided a unique insight into the threshold-entrainment within the littoral zone on: (1) an exposed macro-tidal field site in Abbotsham, UK and; (2) a fetch-limited system in Flathead Lake, USA. The threshold-entrainment was positively related to the power of waves that prevailed over the hour-long experiment and accurately predicted with the equations of Lorang (2000). The relationship between clast mass and power of individual incident waves linked to each entrainment event was more complex. There was the propensity of large clasts to occasionally become entrained by exceedingly weak waves. The processes behind this result remain unclear. Comprehensive experiments measuring variability in the concurrent wave conditions along the inner shore platform and offshore are presented in Chapter 4, along with daily longshore clast displacement and depth of activation measurements across the adjacent coarse clastic beach at Abbotsham. The longshore transport rate varies in concert with the longshore component of wave power, which is dependent on interactions between incident waves and shore platform morphology. These findings are used in Chapter 5 to model the (positive) longshore flux of clastic material towards the economically-valuable natural barrier, which has diminished in recent years. It was found that a disproportionately large fraction of positive transport was generated by storm events, the effectiveness of which was moderated by water depth. Net annual positive transport was predicted to increase with sea level rise and storminess. However, the increased influx was small in comparison to the volume of the barriers itself, and therefore unlikely to halt, or ameliorate future degradation. The experiments presented in this thesis provide new insight into nearshore wave transformations and the relationship between wave properties and clast transport at a time when sea level rise and the potential increase in storminess is threatening coastlines globally.
PhD in Geography