Assessment of beach states at the Sand Motor on the basis of alongshore variability in bathymetry data

Abstract

Beach state classification, though simple in its concept, is difficult to be distinctly and accurately applied in some real life situations. The issue is further compounded when it is applied to mega- nourishments which, in essence, are morphologically different in terms of an artificially-formed beach versus a naturally-forming beach. Yet, another contributing factor could be due to the difference in geographical locations, with varying wave heights, wave periods and sediment sizes, which result in different permutations of the dimensionless fall-velocity (?) values. The six beach state classification scheme presented by Wright and Short (1984) has been the widely-accepted guidelines in the coastal engineering community. Even when references are made to these diagrams, there is an issue of subjectivity as not all coastal researchers may reach the same conclusion when the same image of a particular nearshore morphology is used. This has been found through a questionnaire finding. This paper thus attempts to present a conceptual quantification technique which analyses the alongshore variability along the coast of the Sand Motor as a basis to classify beach state indiscriminately and objectively. Three beach states, namely Longshore Bar Trough (LBT), Rhythmic Bar and Beach (RBB) and Transverse Bar and Rip (TBR), have been observed at the Sand Motor from bathymetric surveys which spanned a monitoring period of two years. It has been found that the success rate of executing the objective beach state classification via the conceptual quantification technique is 94.0% for LBT, 66.6% for RBB and 54.4% for TBR features. Interestingly, LBT has been found to be the persistent and occasional beach state at the updrift and downdrift of the Sand Motor respectively. Furthermore, it has been observed that these occurrences of beach state along the Sand Motor can be divided into three temporal stages namely the formative, the dormant and the developed phases. Stage I has been associated with the formative period of the Sand Motor since its completion in August 2011. It has largely been affected by the formation of a spit and its subsequent dynamics while advancing towards the downdrift of the Sand Motor. Stage II has been a relatively dormant phase where there were no significant changes to the observed beach states. Stage III is considered the developed phase where beach states could be observed along the whole analysed length of the Sand Motor. These three stages have been found to be sensitive to seasonal variations, particularly the increased number of storm events during winter period. The development of the beach states is also found to be highly dependent on the spit dynamics in the case of the Sand Motor.