Geomorphological development of the Cable Bay gravel barrier

Abstract

A study was undertaken to investigate the geomorphological development of the Cable Bay Gravel Barrier. The study aims to improve our understanding of gravel barriers and in particular, to apply the findings to the neighbouring Nelson Boulder Bank, which acts as a breakwater for the Port of Nelson. Several cores were taken within the bay and sampled for microfossils (diatoms, ostrocods and foraminifera), grain size changes, and eight samples were radiocarbon dated. Wave recorders were used to investigate the wave climate within the bay and compared with wind data. Several clast counts and some surveying was also taken within the bay. From the sediment cores six depositional environments were identified. Changes in depositional environment indicate that the Cable Bay Gravel Barrier has migrated from the mouth of the bay, 800m inland to its present position today over the last 8,000 years. The largest wave recorded in this study was 2.6m while the largest hindcasted wave for this area is 4.7m. Entrainment calculations show that neither of these waves can entrain clasts greater than 0.15m in diameter. The boulder lag, left behind by the migrating barrier averages between 0.26m to 0.41m which is immobile in the present day wave climate. The boulder lag was found to get larger towards the mouth of the bay. The lag ends 235m from the present day barrier and has been attributed to a possible overstepping event where the barrier migrated 235m in one large storm event. Given the boulder lag within Cable Bay is immobile in today's wave climate it seems unlikely that longshore drift is the mechanism transporting boulders along the neighbouring Nelson Boulder Bank.