5State Key Laboratory of Estuarine and Coastal Research, School of Marine Sciences, East China Normal University, Shanghai, 200062, China
*Corresponding authors:
Ya Ping Wang (ypwang@nju.edu.cn );
Alvise Finotello (alvise.finotello@unipd.it)
Key Points
Keywords
Tidal Meanders; Mudflat; Hydroacoustic; Helical flow; Secondary circulations; Flow Separation
Abstract
Meandering channels are ubiquitous features in intertidal mudflats and play a key role in the eco-morphosedimentary evolution of such landscapes. However, the hydrodynamics and morphodynamic evolution of these channels are poorly known, and direct flow measurements are virtually nonexistent to date. Here, we present new hydroacoustic data collected synchronously at different sites along a mudflat meander located in the macrotidal Yangkou tidal flat (Jiangsu, China) over an 8-day period. The studied bend exhibits an overall dominance of flood flows, with velocity surges of about 0.8 m/s occurring immediately below the bankfull stage during both ebb and flood tides. Unlike salt-marsh channels, velocities attain nearly-constant, sustained values as long as tidal flows remain confined within the channel, and reduce significantly during overbank stages. In contrast, curvature-induced cross-sectional flows are more pronounced during overbank stages. Thus, a phase lag exists between streamwise and cross-stream velocity maxima, which limits the transfer of secondary flows and likely hinders the formation of curvature-induced helical flows along the entire meander length. Our results support earlier suggestions that the morphodynamics of intertidal mudflat meanders does not strongly depend on curvature-induced helical flows, and is most likely driven by high velocities and sustains seepage flows at late-ebb stages, as well as by other non-tidal processes such as waves and intense rainfall events. By unraveling complex flow structures and intertwined morphodynamic processes, our results provide the first step toward a better understanding of intertidal mudflat meanders, with relevant implications for their planform characteristics and dynamic evolution.
1 Introduction
Tidal mudflats are among the most extensive coastal ecosystems worldwide (Murray et al., 2019; Murray et al., 2022). They are low-gradient intertidal landforms typically occurring in sediment-rich environments (Gao, 2019; Klein, 1985; Rogers & Woodroffe, 2015) characterized by large tidal oscillations relative to characteristic wind-wave heights (e.g., Friedrichs, 2011; Klein, 1985; Morales, 2022). Tidal mudflats are extremely important from both ecological and economic perspectives thanks to the broad range of ecosystem services they provide (Passarelli et al., 2018), including, nutrient cycling, carbon sequestration, water filtering, habitat provision for wildlife, food production, recreational activities, and cultural services (Choi, 2014; Friedrichs & Perry, 2001; Kim et al., 2000; Kirwan & Megonigal, 2013; Pilkey & Cooper, 2004; Shi et al., 2018; Temmerman et al., 2013; Vousdoukas et al., 2020; Wang et al., 2012).
The morphosedimentary evolution of tidal mudflats is intimately linked to the morphodynamics of the extensive networks of tidal channels that cut through them (Figure 1). These channels are typically meandering in planform to a greater or lesser degree (Choi, 2014; Friedrichs, 2011; Gao, 2019; Hughes, 2012), and play a primary role in regulating the exchanges of water, sediments, nutrients, and biota with the open sea (Coco et al., 2013; D’Alpaos et al., 2005), thus exerting a prominent control on the eco-geomorphology of the tidal-flat ecosystem as a whole (Choi, 2014; Hughes, 2012; Wells et al., 1990). Besides, lateral migration of meandering channels critically affects both the sedimentology and stratigraphy of tidal-flat systems, especially in terms of preservation potential (Choi, 2011; Choi et al., 2013; Ghinassi et al., 2019; Kleinhans et al., 2009). Indeed, mudflat tidal channels are typically preserved in the fossil record either as laterally-accreting, heterolithic point bars or through the infilling of abandoned channels generated either from meander cutoff or channel avulsion (Brivio et al., 2016; Choi, 2010; Cosma et al., 2020; Hughes, 2012; Sisulak & Dashtgard, 2012).