Figure1. Examples of meandering channels in tidal mudflats along the
World’s coast. (a) Baegmihang Port, South Korea (37°09′N,
126°40′E; ©Google, TerraMetrics;
imagery date: March 14, 2019). (b) Boseong Bay, South Korea (34°52′N,
127°30′E; ©Google, Maxar Technologies; imagery date: August 30, 2020).
(c) Cardiff Flats, England (51°28′N, 3°08′W; ©Google, Maxar
Technologies; imagery date: July 11, 2013). (d) Fundy Bay, Canada
(45°45′N, 64°38′E; ©Google, Maxar Technologies; imagery date: May 21,
2017). (e) Mühlenberger Loch, Germany (53°32′N, 9°48′E; ©Google,
CNES/Airbus; imagery date: April 22, 2020). (f) The Wadden Sea, Germany
(53°41′N, 8°02′E; ©Google, Maxar Technologies; imagery date: September
25, 2016). (g) I’Épinay Estuary, France (47°31′N, 2°36′W; ©Google, Maxar
Technologies; imagery date: March 19, 2011). (h) Lanveur Bay, France
(48°21′N, 4°17′W; ©Google, Landsat/Copernicus; imagery date: January 01,
2005). (i) Morlaix Bay, France (48°38′N, 3°51’W; ©Google, TerraMetrics;
imagery date: January 01, 2005).
In spite of their prominence and ubiquity, however, meandering channels
in tidal mudflats are still poorly studied especially from a
hydrodynamic standpoint. Previous field measurements of flow fields in
tidal meanders focused primarily on either tidally-influenced fluvial
reaches, where flow dynamics are largely influenced by river discharges
and density-stratification effects (Chant, 2002; Keevil et al., 2015;
Kranenburg et al., 2019; Somsook et al., 2020), or on intertidal
channels dissecting vegetated salt marshes and mangrove swamps
(Finotello, Ghinassi, et al., 2020; Horstman et al., 2021). In contrast,
field studies on tidal meanders wandering through unvegetated intertidal
mudflats are still scarce (Choi et al., 2013; Kleinhans et al., 2009),
and flow velocity measurements are virtually nonexistent to date. This
is a critical knowledge gap because significant differences might exist
in terms of flow fields between tidal channels wandering through
vegetated and unvegetated intertidal plains, especially concerning
overbank stages (i.e., water levels that exceed the channel bankfull
capacity). Overbank velocities in vegetated settings dominated by
turbulence and friction are typically a magnitude lower than those
observed on unvegetated mudflats (Bouma et al., 2005; Christiansen et
al., 2000; D’Alpaos et al., 2021; Friedrichs, 2011; Hughes, 2012;
Rinaldo et al., 1999a; Sullivan et al., 2015). Besides, overbank stages
are more frequent in mudflats than in salt marshes, owing to the
relatively lower position occupied by mudflat channel banks within the
intertidal frame. As such, stage-velocity relations in mudflat tidal
channels can differ greatly from those observed in vegetated marshes and
mangrove forests, and overbank stages might have stronger control on
tidal channel morphodynamics (D’Alpaos et al., 2021; Hughes, 2012;
Kearney et al., 2017; McLachlan et al., 2020; Sgarabotto et al., 2021),
potentially justifying the observed morphological differences of tidal
channel networks in distinct vegetational settings (Geng et al., 2021;
Kearney & Fagherazzi, 2016; Schwarz et al., 2022; Wang et al., 1999a,
1999b). These differences in landforming hydrodynamic processes are also
likely to affect the development of curvature-induced helical flow that
is typically related to the development and growth of meander bends in
both rivers and salt-marsh tidal channels (Azpiroz-Zabala et al., 2017;
Finotello, Ghinassi, et al., 2020; Keevil et al., 2015; Kranenburg et
al., 2019; Nidzieko et al., 2009; Thorne et al., 1985). Such helical
flow forms as a consequence of secondary (i.e., cross-sectional)
circulations, oriented toward the inner and outer bank in the near‐bed
and near‐surface zone, respectively, which result from the imbalance
between the upward-increasing centrifugal forces and the lateral
pressure gradients created by the curvature‐induced superelevation of
the water surface at the outer bank (Engelund, 1974; Prandtl, 1926;
Rozovskiĭ, 1957; Solari et al., 2002). The downstream advection of
secondary circulations operated by the main streamwise flow produces a
helical flow, as extensively documented in a variety of field (Dietrich
& Smith, 1983; Dinehart & Burau, 2005; Frothingham & Rhoads, 2003),
laboratory (Blanckaert, 2011; Liaghat et al., 2014), and numerical
studies (Blanckaert & de Vriend, 2003; Bridge & Jarvis, 1982; Ferguson
et al., 2003).
Although secondary currents akin to those found in river meanders have
been observed and modelled in meandering salt-marsh creeks and large
estuarine tidal channels (Finotello, Canestrelli, et al., 2019;
Finotello et al., 2022; Finotello, Ghinassi, et al., 2020; Kranenburg et
al., 2019; Nidzieko et al., 2009; Pein et al., 2018; Somsook et al.,
2020; Somsook et al., 2022), their presence in sinuous mudflat channels
has yet to be demonstrated. In fact, previous studies (e.g., Choi, 2011;
Choi & Jo, 2015; Ghinassi et al., 2019; Kranenburg et al., 2019)
suggested that the morphodynamic processes governing meander evolution
in intertidal mudflat settings can differ greatly from the classic
secondary-current-driven lateral channel migration mechanism acting in
vegetated fluvial and intertidal plains. For instance, Kleinhans et al.
(2009) argued that owing to the higher thresholds for erosion that
characterize mudflat deposits, bank erosion is primarily due to bank
undercutting caused by backward-migrating steps along the channel bed
driven by hydraulic jumps that form during ebb tides. They also
demonstrated that bank migration occurs preferentially in very sharp
bends, where flow separates from the meander inner (convex) bank and
impinges directly against the outer (concave) bank. Choi (2011) noted
enhanced tidal channel migration in association with episodic and
seasonal increase of discharge due to, for example, heavy
precipitations, pointing to a strong control of these non-tidal
processes on the morphodynamic and sedimentology of tidal mudflat
meanders. Accordingly, Choi and Jo (2015) measured pronounced meander
migration in the Yeochari macrotidal flat (South Korea) during the
summer rainy season, when point bars were observed to migrate as fast as
40 m per month due to increased runoff discharge caused by heavy
rainfalls in the order of tens to hundreds of millimeters per hour,
possibly compounded by monsoon precipitations. Finally, Ghinassi et al.
(2019) suggested that wave winnowing of mudflats during high-tides
modulates meander morphosedimentary evolution, leading to widespread
bank collapses within the channel.
In view of the above, the structure of tidal flow fields in mudflat
tidal meanders appears to be worth investigating. Here we present novel
hydroacoustic data from a meandering tidal channel dissecting a
macrotidal mudflat located along the Jiangsu coast (China). The aim of
the study is threefold, as we intend to: (i) highlight the
characteristics of tidal flows within a meander bend developed in an
unvegetated tidal mudflat; (ii) unravel possible differences in meander
hydrodynamics among below-bankfull and above-bankfull (i.e., overbank)
water stages; and (iii) disclose the characteristics of secondary
circulations and their relations with the overbank flows. To the best of
our knowledge, this study represents the very first attempt to directly
measure tidal flows in meandering mudflat channels.
2 Geomorphological setting and study-case
Our study case is found in the Yangkou tidal flat (YTF), an extensive
mudflat system located on the southern Jiangsu coast, northward of the
Yangtze River Delta, which is bordered by the Yellow Sea to the East and
North and by the East China Sea to the South (Figure 2a). The YTF was
formed by abundant sediment supply input from both the Yangtze River and
the Yellow River, which historically allowed for seaward expansion of
the whole Jiangsu province coastline (Shi et al., 2016; Wang & Zhu,
1990). Sediments consist mainly of silty-muddy material, with average
grain sizes ranging between 10 and 45 μm (i.e., 4.5 ~
6.6 φ) (Shi et al., 2016; Wang & Ke, 1997). In the last 2 centuries,
however, the seaward extent of the YTF has decreased from 5
~ 11 km to about 5 ~ 8 km as a
consequence of changes in sediment transport regime driven by
anthropogenic interventions, the latter including the diversion of the
Yellow River to the Bohai Sea in 1855 (Ren & Shi, 1986), and the
construction of the Three Gorges Dam in 2003, which significantly
decreased sediment supply from the Yangtze River (Yang et al., 2014). In
addition to this, land reclamation projects, the building of oceanic
outfalls, aquaculture, and the construction of wind farms have further
contributed to increasing anthropogenic pressures in the YTF area (Xu et
al., 2019; Zhao et al., 2020; Zhao & Gao, 2015). Nowadays, the whole
intertidal area in the YTF covers approximately 100
km2, extending seaward from the shoreline with gentle
slopes ranging between 0.5‰ and 1.2‰ on average (Wang & Ke, 1997; Zhu
et al., 1986).
Intertidal mudflats in the YTF are dissected by extensive networks of
tidal channels. These channels serve as the main conduits for the
propagation of both the East China Sea progressive tidal wave and the
southern Yellow Sea rotary tidal wave, which converge nearby the town of
Yangkou giving rise to complex coastal circulations (Liu et al., 1989).
The tidal regime in the study area is semidiurnal macro-tidal, with
average and spring tidal ranges equal to 4.6 m and 8 m, respectively.
Morphodynamic processes are also affected by the East Asian Monsoon,
which blows with a mean winter wind speed of 4.2 m/s toward the
southeast and a mean summer wind speed of 2.8 m/s toward the northwest,
respectively (maximum measured wind speed is 34 m/s; (Li et al., 2011;
Xing et al., 2012). As wave conditions in this region are mainly related
to wind speeds, wave heights are smaller in the summer and larger in the
winter, with annual average values ranging between 0.5 and 1.5 m (Chen,
2016). The annual precipitation is about 900 ~ 1000 mm
on average, with the summer season accounting for more than 40% of the
whole yearly rainfall (Wang & Ke, 1997; Xing et al., 2012).
Our study site is a blind tidal channel found within a natural reserve
facing the Xiaoyaokou Scenic and the Xinchuan port, both located nearby
the city of Yangkou (Figure 2b). The studied channel is 1.9 km long and
is characterized by average width of 8 m. With an overall channel
sinuosity equal to 1.5, it represents a well-developed meandering reach.
The channel originates from a fringing salt marsh, which borders the
Xiaoyangkou Scenic and is covered by Spartina alterniflora Loisel(Figure 2c,e), and extends seaward wandering through an unvegetated
intertidal mudflat. Freshwater fluxes from the Beiling river to the
North and the Bencha canal to the South do not interfere with the
hydrodynamic regime of the studied channel, which is always submerged at
high tide and drains out almost completely at low tide.
In this study, we focused specifically on a meander bend located in the
central portion of the channel and surrounded by unvegetated tidal flats
(Figure 2c,d,f). The studied bend is characterized by a cartesian
wavelength (i.e., the linear distance between bend inflections)\(L_{xy}\)=37 m, whereas the along-channel bend length (\(L_{s}\)) is
equal to 56 m. Hence, the bend attains a sinuosity\(\chi\)=\(L_{s}/L_{\text{xy}}\)=1.5. The average meander radius of
curvature is \(R\)=19 m, and the amplitude, measured as the maximum
distance from the line passing through both bend inflections, is equal
to \(A\)=18 m. The cross-sectional width (\(W\)) decreases from 8.8 m to
8.3 m in the landward direction (average width\(\overset{\overline{}}{W}\)=8.5 m). Being the bankfull depth
(\(Y_{B}\)) equal 1.20 m on average, the studied bend is characterized
by an average
width-to-depth
ratio (\(\beta=\overset{\overline{}}{W}/Y_{B}\)) of about 7.1. All
these morphometric parameters are in line with typical values observed
for tidal channels worldwide (D’Alpaos et al., 2005; Finotello,
D’Alpaos, et al., 2020; Hughes, 2012). While many regularly-spaced small
erosional gullies cut through the channel banks (Figure 2f), a 4 m wide
and 0.5 m deep side tributary, meandering in planform, is found landward
of the apex of the studied bend (Figure 2d).