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
- Acoustic measurements of flow velocities in a sinuous macrotidal
mudflat channel show critical differences with channels in vegetated
intertidal plains
- Offset between streamwise and cross-stream velocity maxima limits
advection of secondary flows and hinders curvature-induced helical
flows
- High velocities and sustained seepage flows at late-ebb stages likely
exert stronger controls than helical flows on meander morphodynamics
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).