Morphodynamics of intermediate beaches: a video imaging and numerical modelling study

Four years of time-exposure images of Palm Beach, Sydney, Australia, are used to: (a) develop a phenomenological scheme based on surf zone properties for the classification of the four intermediate beach states [low tide terrace (LTT), transverse bar rip (TBR), rhythmic bar beach (RBB), and longshore bar trough (LBT)] identified by Wright, L.D., Short, A.D., 1984. Morphodynamic Variability of Surf Zones and Beaches. Marine Geology, 56 93-118; and (b) determine environmental conditions associated with the different beach states. The overall agreement between beach state classifications obtained by visual classifications of images and those predicted by the phenomenological scheme exceeds 90%. Analysis of time series of intermediate beach states and wave data indicates that the nearshore morphology continuously cycles through the four beach states in response to variations in the incident wave energy. The results also indicate that LBT morphology is produced by high-energy wave conditions (erosive), while the progression towards LTT occurs under lower waves (accretive). A depth-averaged morphodynamic model is employed to determine the morphodynamic processes governing the RBB to TBR and TBR to LTT transitions. Model results indicate that the RBB to TBR transition is governed by the erosion of the original longshore bars (in the initial RBB morphology) and the formation and subsequent welding of shoals in the lee of the original longshore bars. Model results also indicate that RBB to TBR transitions can occur under shore normal and oblique wave conditions. The TBR to LTT model simulations indicate that, under obliquely incident waves, progressive erosion of the transverse shoals and deposition of the eroded sediment in rip channels gradually diminishes the longshore variability in morphology, resulting in an LTT morphology. (C) 2004 Elsevier B.V. All rights reserved.