Contrasting sand-mud transition migrations in estuarine and bay beaches and their potential morphological responses

Abstract

Morphological variations in a sandy-muddy transitional beach (SMT-beach) are expected to coincide with sand-mud transition (SMT) boundary migrations. SMT migrations of SMT-beaches should also differ in river estuaries and semi-enclosed bays due to their contrasting dynamic sedimentary environments. However, the anticipated diversity of SMT migrations of estuarine and bay beaches and their potential morphodynamic responses are still poorly understood. Thus, four measurements of SMT spatial distributions and typical profiles of 6 estuarine beaches and 6 bay beaches located along the macrotidal coasts of the Taiwan Strait, East Asia are presented, and these sites were studied in April and August of 2018 and 2019, respectively. The results show that the SMT boundary of the estuarine beaches continuously migrates landward with increased elevations. Simultaneously, the SMT boundary of bay beaches synchronously remains relatively stable with minor fluctuations and relatively stable elevations. The SMT migration amplitudes present a significantly positive linear correlation with SMT elevation changes, which are strongly controlled by the sediment supply. Following the SMT migration processes, both estuarine and bay beaches display clear erosion during the summer monsoon period that is mainly associated with strong river discharge and monsoon rainfall, respectively. In contrast, all beaches present distinct accretion and recovery during the winter monsoon period that are mainly due to the decreased monsoon rainfall and river discharge and/or the strengthened onshore sediment transport. Despite limitations, the results of this study have clarified the diverse SMT migrations of estuarine and bay beaches and aid in a better understanding of their possible morphodynamic responses.