Boulder-Cobble Beaches, Eastern Tasman Bay, New Zealand.

Dickinson, W.W.; Hartstein, N.D.; Davidson, R.J., and Bin Bakar, M.S., 2025. Boulder-cobble beaches, Eastern Tasman Bay, New Zealand. Journal of Coastal Research, 41(4), 551–565. Charlotte (North Carolina), ISSN 0749-0208. Boulder and cobble beaches are commonly associated with mid- to high-energy wave environments, but in eastern Tasman Bay, New Zealand, wave energy is too small to transport boulders and cobbles persistently along two boulder-cobble beaches. Using side-scan sonar, depth profiling, wave modeling, drone photography, and clast counts, the physical and geomorphic characteristics for these two beaches are presented. One beach forms a barrier at Greville Harbour on D'Urville Island, and the other forms a small spit extending eastward from Motuanauru Island in Croisilles Harbour. Boulders and cobbles on the Greville barrier form a shallow dipping platform that extends seaward below low tide for several hundred meters to depths of at least 12 m. Beaches comprise poorly to moderately sorted boulders, cobbles, and pebbles, but some boulders are greater than 1 m in diameter. Wave simulations, including cyclone scenarios in Tasman Bay, do not predict the longshore movement of boulder-sized clasts on either the beach face or the intertidal/subtidal area. These beaches do not fit a worldwide relationship between beach-face slope and particle size. The beaches appear to be the result of antecedent geology and topography. Essentially, they are the remnants of ridges that have eroded in situ, and they have been modified by sea-level rise and high-energy wave events, which can move boulders inshore but not along shore. This study has implications for other boulder-cobble beaches in Tasman Bay and other parts of the world. [ABSTRACT FROM AUTHOR]