Longshore sediment transport in a mixed sand and gravel foreshore, South Canterbury

Abstract

This thesis examines the processes, patterns and rates of longshore sediment transport in the mixed sand and gravel foreshores (particle sizes 1 mm to 200 mm) of the South Canterbury coast. The beach system that is studied presents a very different situation from that most commonly studied on sand beaches. The flows of water and sediment are dominated by breaking waves and swash rather than ‘surf’ and interrelated subzones of distinctive processes, responses and sediment transport regimes occur across the foreshore. A variety of methods for measuring water flows and sediment movements are assessed and a set of daily beach observations made over four months is analysed. Long- and short-term net rates of longshore transport in the study area are estimated to average 51,288 m³ .yr-¹from measurements of the historic accumulation of beach material updrift of structures at Timaru Harbour. This estimate is then used with deepwater wave data to ‘calibrate’ a widely used linear relationship between the transport rate and the longshore component of wave power, for use in a mixed beach situation. Short-term measurements of the transport rate and wave power from shore-based observations are also used to calibrate the relationship. The values obtained are 14 to 94 times lower in magnitude than the accepted relationship for sand beaches, and can be used with greater certainty for other locations in the study area. Finally, a new method of estimating net transport based on longshore variations in shore morphology over time is developed using data from a 10-year profile survey program. Results suggest that ‘slugs’ of beach sediment are moved alongshore as collective units, at rates of about 1.4 km.yr-¹. Rates of movement are dependent on the prevailing angle of wave approach.