6. Conclusions
There have been few field studies of sediment transport on coarseclastic beaches. This study adopts and validates the use of RFID and PIT
technology for tracking cobbles and presents measurements of cobble transport onanopen-coastmixedsand and gravel beach over an 8-month period at Hawke Bay, New Zealand. Variability in recovery rates were associated with higher energy events, which have the potential to strip cobbles from the beach-face during cross-shore transport, and bury cobbles on the beachface during post-storm re-deposition. Over the study period all cobble tracers moved northward. The median cobble transport rate was close to 500 m over 207 days, indicating long-term net northward transport rates on the order of 2–2.5 m/day. The results are consistent with theory on wave-driven alongshore sediment transport, but also highlight that several other factors are also important for understanding cobble transport patterns, particularly the possibility for burial and cross-shore transport of cobbles during storms. In addition to tracking movement, cobble tagging allowed direct in situ measurements of abrasion rates demonstrating the viability of PIT-tags as an approach for quantifying cobble abrasion rates in a field setting. At the study site the integrated (net transport) abrasion measurements ranged from ~1 to 5 g over 64 days, and 5 to 14 g over 207 days.
Acknowledgements
Funding was provided by Hawkes Bay Regional Council. We thank Neil Daykin for assistance with fieldwork and resourcing, and the Port of Napier for providing access to wave buoy data. Thanks also to two anonymous reviewers for their detailed comments and suggestions, and to Jon Allan for methodological advice getting this project started. Tri-plot (Graham and Midgley, 2000) was used in production of Fig. 4.