Morphological and sedimentary features of sandy‐muddy transitional beaches in estuaries and bays along mesotidal to macrotidal coasts

Abstract

Sandy‐muddy transitional beaches (SMT‐Beaches), representing the transition from sandy beaches to tidal mudflats, should theoretically develop very different morphological and sedimentological characteristics in river estuaries and in semi‐enclosed bays due to their contrasting dynamic sedimentary environments. Evidence, however, is rare in the scientific literature. To reveal these morphological and sedimentary differences, the sand–mud transition (SMT) boundary distribution, beach profiles, and surface and downcore sediment grain‐size compositions of 27 SMT‐Beaches located along mesotidal to macrotidal coasts of the western Taiwan Strait, southeastern China, were investigated. The results show that typical estuarine SMT‐Beaches are mainly characterized by an ambiguous SMT, a long distance between the SMT and the coastline (31–302 m), lower SMT and inflection point altitudes (average –0.76 m and –0.04 m), and lower upper beach gradients (~0.068) with fine sand. Estuarine SMT‐Beach sediments display clear interbedded mud and sand layers, implying potential SMT migrations over various timescales. By contrast, typical bay SMT‐Beaches are characterized by distinct SMT, a short distance between the SMT and the coastline (11–52 m), higher SMT and inflection point altitudes (~0.24 m and ~0.35 m), and narrower upper beaches with higher gradients (~0.095) and coarse sand. Bay SMT‐Beaches present relatively stable sedimentary sequences and a narrow gravel belt surrounding the inflection point and/or SMT. These morphological and sedimentary differences between the two SMT‐Beach types are initially constrained by sediment supply and transport and are further affected by tide conditions and wave climate. Sediment supply and transport predominately control the sediment structures, while the tidal range strongly influences spatial variations in SMT distances. Wave climate normally drives SMT altitude variations. This study highlights the morphological and sedimentary differences in SMT‐Beaches in estuaries and bays, providing important knowledge for further revealing their morphodynamic processes and potential future nourishment. © 2020 John Wiley & Sons, Ltd.

Sand-mud transition dynamics at embayed beaches during a typhoon season in eastern China

“…These two beaches are more natural than Dongsha, suggesting that SMTs of beaches with less human intervention are more stable. In addition, SMTs of embayed beaches are known to be more stable than estuarine beaches (Zhao et al, 2020b), which is confirmed in this study with overall small SMT migrations.…”

“…10b and Table 3). In line with Zhao et al (2020b), the SMT offshore distance at embayed beaches decreases with increasing tidal range. The three embayed beaches in this study have the same mean spring tidal range (2.6 m) and similar nearshore significant wave height, but show different SMT locations (Fig.…”

“…Generally, all coasts have one or more SMTs while progressively moving offshore, but their offshore distance and water depth vary considerably in different seas (George and Hill, 2008). The location of the SMT and its water depth are closely related to the hydrodynamic forcing (Dunbar and Barrett, 2005;George and Hill, 2008;Stanley and Moore, 1983), sediment supply (Chang et al, 2017;Edwards, 2002;George and Hill, 2008;Zhao et al, 2020a), and beach morphological changes (Zhao et al, 2020b).…”

“…Changes in SMT depth of sandy-muddy beaches are mainly controlled by nearshore waves (Xu et al, 2018) and sediment supply (Zhou et al, 2019). The distribution and migration of SMT on sandy-muddy beaches in different estuaries and bays in southeastern China were further summarized by Zhao et al (2020aZhao et al ( , 2020b.…”

Foredune height variations along the western coast of Taiwan Strait

“…The strong, northest waves prevail in autumn and winter, while the weak, southwest waves dominate in summer (Chien et al, 2014;Wu, 2012Wu, , 2021. However, the wave height maxima usually occur in summer due to typhoon events (Chien et al, 2014;Zhao et al, 2020).…”