Abstract:
Gravel beaches provide an effective means of natural sea defence as they are usually stable under sustained wave attack. There has been growing interest in the application of coarse grained sediments as a form of coastal protection but there is a deficit in knowledge on the fundamental processes of sediment transport on gravel beaches. Unlike sandy beaches, mixed sand and gravel (MSG) beaches only deal with a single breaker which occupies the same position under all wave conditions. The reflective nature of gravel beaches enables them to adapt dynamically to disturbance and buffer the impacts of incident wave energy. MSG beaches are prevalent on the southern Hawke’s Bay coastline, much of which experiences chronic erosion and faces ongoing coastal management concerns. Model predictions are required for management decisions, but there is a lot of uncertainty in model assumptions due to lack of field measurements of sediment transport rates and sediment mass loss during transport. In this thesis RFID technology was used to directly measure in-situ abrasion loss and sediment transport rates at two locations, Clifton and Awatoto, in southern Hawke’s Bay. These sites were selected because the southern beaches are chronically eroding and northern beaches exhibit accretion. 1000 cobbles were tagged in total with 400 released at Clifton and 600 released at Awatoto. Observations confirm that alongshore sediment transport rates are eight times faster in the south (0.8 m/day at Clifton) than in the north (0.1 m/day at Awatoto). Abrasion loss was found to vary considerably with no correlation to the size or shape of cobles, or the net transport distance. On average cobbles abraded at a rate of 0.29 %/km at Clifton and 22%/km at Awatoto. The results validated the estimates of the UNIBEST shoreline prediction model and draw attention to the link between rapid alongshore sediment transport and coastal erosion versus slow alongshore sediment transport and coastal accretion. The results also reinforce the applicability of RFID experiments as a tool for measuring in-situ abrasion loss. This study addressed gaps in the knowledge on the fundamental processes of sediment transport on gravel beaches. Rates of alongshore sediment transport validated predictions of the UNIBEST model and revealed that while alongshore sediment transport is the dominant mode of cobble movement in fair-weather conditions, storm events drive cross-shore transport. The abrasion experiment was the biggest of its kind in the southern Hawke’s Bay. The results identified the applicability of RFID technology as a means of measuring in-situ abrasion and provide a foundation for future abrasion measurements.