Morphological evolution of river mouth spits: Wave effects and self-organization patterns

Abstract

Sand spits as a type of common landform features, associated with wave-dominated deltas, may be influenced by self-organization processes due to shoreline instability at the downdrift flank of the delta, which forms an asymmetric plan morphology for the delta. Here, we examine the spatial-temporal variations and their controlling processes of the sand spit system at the Changhua River mouth, northwest of the South China Sea, by combining 34-year (1986–2019) satellite-derived shoreline data with an ERA5 wave hindcast dataset. The spit system is characterized by extension towards the sea, as well as recurvature towards the land, in response to dominating N-NE waves, whilst, for the sheltered coastal embayment behind the spit, echelon reverse spits have developed due to the dominating S-SW waves. The wave action is governed by a combination of the short period local marine climate of the area (monsoonal regime) and large-scale weather cycles (i.e., El Niño and La Niña), resulting in inter-seasonal and inter-annual variations of the spit neck morphology. Furthermore, the river mouth spit shows self-organization patterns at different temporal-spatial scales, as indicated by the cyclical behavior of the updrift coast, together with the stepwise formation of the echelon reverse spits. Such behavior influences the spit morphology and its rate of change. The geomorphological self-organization patterns contain the information on local dynamic processes, as well as on large scale air-sea interactions as exemplified by the ENSO events. Thus, the analysis presented in this study is valid to describe and estimate the morphological evolution of highly dynamic barrier-spit systems elsewhere, based on wave energy flux estimated by available wave parameters.