Morpho-sedimentary dynamics of a megatidal, mixed sand-gravel beach

Abstract

Sediment dynamics on mixed sand-gravel (MSG) beaches have received much less attention in the literature than on sandy beaches. The steep slopes characteristic of MSG beaches result in an energetic shorebreak, accompanied by ballistic transport of gravel- and cobble-sized grains. The associated risks of damage to \textit{in situ} instrumentation have contributed to the relative scarcity of observational data. A central goal of this thesis is to contribute new knowledge and understanding of morphodynamic responses to wave forcing on MSG beaches through the use of innovative, inexpensive sensing systems not exposed to the rigours of the shorebreak. The studies were carried out at Advocate Beach, Nova Scotia, a 1:10 slope megatidal MSG beach at the head of the Bay of Fundy. The principal results are presented in four chapters. First, the vertical structure of surface gravity wave-induced pore pressure in the intertidal zone is investigated using a coherent array of buried pressure sensors. A key finding is that the phase of the pore pressure lags the pressure at the sediment surface. This phase lag is shown to be due to the presence of bubbles within the sediment column, which has implications for using buried pressure sensors for surface gravity wave measurement in the intertidal zone on MSG beaches. Second, video observations are used to characterise beach cusp morphodynamics at high temporal resolution. The timescale of cusp evolution is shown to be O(10) minutes. Importantly, the cusps exhibit pronounced bay/horn size segregation, indicating strong feedback between the hydrodynamics and cusp formation on MSG beaches. In the third main chapter, correlations between bed level and the surficial mean grain size are investigated using GPS and photographic surveys of the intertidal beach. Finally, the coevolution of bed level and grain size is investigated in the swash zone using an array of collocated acoustic range sensors and cameras, and Lagrangian tracking of painted cobbles is utilised to study grain size segregation at the swash scale. The results are discussed in the context of a morpho-sedimentary dynamics framework, emphasising the intrinsic interrelationships between morphology, flow, and the broad surficial grain size distribution.