Mixed sand and gravel beaches: accurate measurement of active layer depth to develop predictive models of sediment transport

This thesis addresses the need to calibrate longshore sediment transport equations for use on heavily managed mixed sediment beaches, which are becoming increasingly common but are still relatively understudied. This is achieved through the use of active layer measurements, tracer pebble experiments, and monitoring of morphodynamics over a total period of 16 months. It is unique in investigating a complex, artificially replenished, groyned mixed sand and gravel beach in this way. The groynes have a significant impact on beach morphology and sediment transport at Eastoke. Beach levels were monitored using repeated profiles at three main locations within a groyne cell, allowing for the effect of these structures on beach profile shape and response to changing wave climates to be assessed. This provides a more accurate representation of morphodynamics than would otherwise be available from semi-annual profile surveys. The beach response to hydrodynamic conditions was shown to be rapid, and not always indicative of drift direction. Sediment sorting was observed, but was not strongly correlated with wave conditions. A vast dataset of active layer measurements indicated significant variability, which has not been made clear by previous studies of active layer depth. The key finding of this thesis is that active layer depth as a percentage of wave height can be predicted from significant wave height using the equation ln(y) = -0.797x + 3.565 (R2 = 0.578), and this value can then be converted into a depth measurement. The prediction can be applied at a daily scale to the combined upper and mid beach as an estimate of the average active layer depth. Further field sites should be investigated to determine whether this equation can be applied to other mixed sediment beaches. Short- and medium-term pebble transport patterns were observed using passive integrated transponder (PIT) tagged pebbles, deployed in stages throughout the research. Sediment transport volumes were estimated based on field data, and hydrodynamic data used to calculate a range of drift coefficients (k) for the commonly used CERC transport formula. Low detection rates ultimately limit the confidence which can be applied to final calculations of longshore transport volumes and values of k, but this study provides insight into the complexities associated with studying a site like this, and suggestions for improvements which could be made to future research.