Boulder beaches: a sedimentological study
thesisposted on 01.08.2022, 07:31 authored by Helen Lorraine Oak
Five boulder beaches along the central New South Wales coast were selected for the study of their sedimentary properties, form, and sediment transport. Each beach is aligned obliquely to the approaching waves and is composed of local sediment. One beach, which appears to have little or no recent sediment input, is considered to be a closed sediment system and the other four beaches, which appear to have recent sediment supply, are considered to be open sediment systems. -- On the open system beaches, boulder size fines towards the embayment in the direction of transport, and on all beaches size fines up-beach (contrasting with the up-beach coarsening of pebble and cobble beaches). During transport, breakage and chipping diminish boulder size, and the products of these forms of abrasion constitute a subordinate fine population causing the distribution of size to be positively skewed, contrasting with most fine-sediment beaches which exhibit negative size skewness. -- More boulders are oblate than prolate, but this may reflect geology rather than coastal weathering processes. In contrast to pebble beaches, no longshore or up-beach shape zoning exists, and boulder shapes are believed to be largely determined by geology. Boulder shape is not related to boulder size. Sphericity varies little within each beach and nowhere does it increase seaward as is common for pebble and cobble beaches. -- Boulder roundness tends to increase longshore towards the embayment, and decrease up-beach. The relationships between boulder roundness and size may be influenced by sediment supply. Roundness and shape of boulders do not appear to be related. -- Overall beach form is consistent and no rhythmic features could be identified. Surface packing or armouring occurs on all beaches and may contribute to beach stability. Foreshore slopes tend to be concave upward and range between 7° and 12°, significantly lower than the slopes of >= 24° predicted in the literature for boulder-sized sediment. This anomaly may be explained by the fact that only very high-energy waves, which produce low beach slopes, are competent to transport boulders. -- Boulder mobility is evident on all beaches and was monitored on one beach. Wave competency appears to determine the maximum size of transported boulders, and a competency model is proposed in which it is predicted that there exists a power relationship between transported particle diameter and significant wave height. Since boulder beaches and rubble coastal protection structures have environmental and compositional similarities, beach-boulder movement is examined in the light of engineering studies of protection-structure stability. Two no-damage design formulae were found to over-predict the movement of the smaller-sized beach sediment and underpredict the movement of the larger-sized sediment. This effect may be due to the packing of beach boulders. -- Up-beach fining, positive size skewness, the absence of shape zoning, much particle breakage, the absence of sphericity grading, and low foreshore slope are all characteristics of the five studied boulder beaches which contrast markedly with the characteristics of pebble and cobble beaches. These findings, combined with the development of a reasonable predictive transport model, suggest that the studied boulder assemblages are organized and distinct coastal deposits, which may properly be termed beaches.