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The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estuaries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adaptation, whereas it slightly reduces tidal amplification in small estuaries.
SLR effects on boundary conditions of estuaries a–c, Flowcharts along the top of panels indicate main morphological effects. The red dashed line indicates the scenario effect on water level and estuary width. Red arrows indicate changes in tidal amplitude at the mouth; am, tidal range at the mouth; Qr, river discharge. The cumulative distribution functions (CDFs) indicate hypsometric curves that summarize cross-sectional bed elevations in cumulative profiles. a, In the simple case (scenario 1, an increase in MSL), all boundary conditions remain equal, which means that the required sediment (Qs,req) for adaptation is the estuary surface area (A) multiplied by the increase in MSL (ΔMSL). b, Tidal range either increases (scenario 2, a⁺), decreases (scenario 2, a⁻) or remains the same (scenario 1, a⁰) depending on the location of the estuary in relation to its amphidromic point (tidal node). Changes in tidal amplitude at the mouth modify the tidal prism and thereby alter the equilibrium channel volume (Vch). This increases the required sediment for adaptation for decreasing amplitude (a⁻) and decreases the required sediment for increasing tidal amplitude (a⁺). c, The estuary widens (scenario 3, W(x)⁺) if surrounding land is drowned or by managed realignment. Increased planform width (W(x)) has a similar effect as scenario 2, but it additionally alters the cross-sectional distribution of bed levels, which means that salt marsh sediments become available for redistribution.
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Articles
https://doi.org/10.1038/s41558-019-0608-4
1Faculty of Geosciences, Utrecht University, Utrecht, the Netherlands. 2Royal Netherlands Institute for Sea Research, Yerseke, the Netherlands.
*e-mail: j.r.f.w.leuven@uu.nl
Estuaries are highly dynamic wetland zones at the transition
from the river to the ocean, susceptible to future climate
change and especially to sea-level rise (SLR)1,2. In estuaries
such as the Western Scheldt, Elbe and Yangtze, the channels provide
access to inland harbours3 and the intertidal bars form a valuable
ecological habitat4. The surrounding land is often densely popu-
lated; in fact, 21 of the world’s 30 largest cities are located next to
estuaries5. Potential SLR-induced threats are increased flood risk6,7,
reduced navigability8 and drowning of the intertidal habitat area3,9
12. Estuary size and shape may affect their future response to SLR13,14,
but SLR-induced changes in morphology have so far received little
attention. Here we study changes in morphology and the resulting
tidal dynamics for estuaries of different sizes worldwide.
Estuarine bars, tidal flats and salt marshes have the potential
to grow with SLR if they import sufficient fluvial or marine sedi-
ment to adapt the morphology to the new boundary conditions15.
Three key boundary conditions for estuary morphology and their
potential to adapt are (1) planform shape, (2) tidal amplitude at the
estuary mouth and (3) sediment supply. Together these parameters
control the overall volume of water (tidal prism) and sediment
moving in and out of the estuary14,16,17, which determine channel
volume and the space available to form intertidal bars18. However,
under SLR, tidal amplitudes at estuary mouths are likely to change
because of shifting amphidromic points19,20. If the distance between
the estuary mouth and amphidromic point increases, the tidal
amplitude increases and vice versa. It remains unknown how the
combined future SLR and changes in tidal amplitude will affect the
tidal propagation and equilibrium morphology of bar-filled estu-
aries worldwide. Their future equilibrium morphology determines
whether present-day fluvial sediment supply could be sufficient for
adaptation of the morphology.
The balance between bed friction and channel-width conver-
gence determines whether the tidal range amplifies (becomes
larger), remains constant (an ‘ideal estuary’) or dampens (becomes
smaller) in the landward direction14,2124. A future increase in mean
sea level (MSL) reduces bed friction, which means that the tidal
range can become increasingly amplified. The consequence is
flood risk (higher high waters) and reduced navigability (lower low
waters).
Human exploitation has largely affected the natural processes
occurring in deltas and estuaries in the past centuries25. In par-
ticular, dyke construction and land reclamation3 have cut off the
ecologically valuable flanking mudflats and salt marshes from the
channels that supply sand and mud during inundations26,27. These
intertidal areas provide storage space and friction for the tidal
wave21, thereby naturally reducing flood risk. However, the chan-
nels are dredged for harbour accessibility, which reduces friction for
the tidal wave, thereby enhancing flood risk3,10,11. Additionally, dam
construction in rivers has largely reduced fluvial sediment supply to
estuaries2830. Here we evaluate how flood risk and drowning threats
can be mitigated by managed realignment. Managed realignment31
means removing coastal protection to expose additional, currently
terrestrial, areas to tidal flooding, which widens the estuary.
We calculate the morphological and hydrodynamic response of
36 estuaries worldwide to SLR and assess the potential for increased
space by managed realignment. Estuaries varied from very small
to very large (0.1–1,000 km2), which allowed us to test the effect
of estuary size on SLR-induced threats. The required sediment for
adaptation and flood water levels within the estuary are used as
main indicators for coping with SLR.
Approach and scenarios
The effect of future SLR was studied in three scenarios (Fig. 1).
To do so, a semi-empirical morphological tool32 was coupled with
a one-dimensional (1D)-hydrodynamic model (see Methods) to
estimate the present-day situation and the scenario effect. Here we
briefly summarize the approach.
Depth distribution was estimated based on the estuary plan-
form shape, tidal amplitude at the mouth and river discharge
(Supplementary Fig. 4 and Supplementary Table 2). Average depth
at the landward boundary and seaward boundary were obtained
from hydraulic geometry relations that were developed for rivers
Sea-level-rise-induced threats depend on the size
of tide-influenced estuaries worldwide
Jasper R. F. W. Leuven 1*, Harm Jan Pierik 1, Maarten van der Vegt1, Tjeerd J. Bouma1,2 and
Maarten G. Kleinhans 1
The effects of sea-level rise on the future morphological functioning of estuaries are largely unknown because tidal amplitudes
will change due to combined deepening of the estuary mouth and shifting amphidromic points at sea. Fluvial sediment supply is
also globally decreasing, which hampers infilling necessary to maintain elevation relative to sea level. Here we model 36 estu-
aries worldwide with varying sizes, shapes and hydrodynamic characteristics, and find that small shallow estuaries and large
deep estuaries respond in opposite ways to sea-level rise. Large estuaries are threatened by sediment starvation and therefore
loss of intertidal area, particularly if tidal amplitude decreases at the mouth. In contrast, small estuaries face enhanced flood
risks and are more sensitive to tidal amplification on sea-level-rise-induced deepening. Estuary widening can partly mitigate
adverse effects. In large estuaries, expanded intertidal areas increase tidal prism and available erodible sediment for adapta-
tion, whereas it slightly reduces tidal amplification in small estuaries.
NATURE CLIMATE CHANGE | VOL 9 | DECEMBER 2019 | 986–992 | www.nature.com/natureclimatechange
986
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... Nonetheless, they provide several hypotheses for how hydrodynamics of estuaries may change due to SLR. First, SLR increases the cross-sectional area of tidal channels by enlarging channel volume as a response to the change in tidal prism (Hijma & Cohen, 2011;Leuven et al., 2019). How much and in what direction (vertically/deepening or laterally/widening) channel volume will be altered is dependent on several factors, primarily: the presence or absence of lateral/vertical constraints on channels, including bank stability and overflow depth , and how much SLR decreases channel bed friction and resulting changes to flow velocity (Wachler et al., 2020). ...
... Meanwhile, SLR is predicted to decrease friction and therefore slow down erosion (Wachler et al., 2020), something we see in the downstream parts of our SLR experiments (deltas gain elevation rapidly). This matches the predictions of Leuven et al. (2019) that in the mouth area where width is constrained and sediment supply is limited, channels become shallower and bars become subdued. It is possible that a certain threshold depth exists to balance the effects of SLR and channel deepening, but this hypothesis and potential threshold depth that balances sedimentation and erosion with dredging requirements needs to be further explored. ...
... Undredged estuaries with shallow multichannel systems convey the additional water relatively evenly during ebb and flood, which can advance onto and drown intertidal areas. They have more subdued morphology (as proposed in Leuven et al. (2019)) with lower surrounding areas and shallower channels. The presence of connecting and side channels redistributes water and sediment evenly over the estuary. ...
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... This becomes even more complex as SLR plays an important role in morphological changes of estuaries. It has been shown that shallow and deep estuaries will respond in different ways (Dissanayake et al., 2012;Leuven et al., 2019;Van Goor et al., 2003). While this study ignored changes in the bathymetry over long terms, this should be considered as another source of uncertainty for MWE predictions in future. ...
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... Tidal amplitude is likely to decrease in short estuaries but increase in long estuaries. Leuven et al. (2019) found that estuarine shape and size significantly influence their hydrodynamic responses to SLR. Also, simulations that capture inland inundation experience a decrease in tidal amplitude toward the coast due to a change in the magnitude and spatial distribution of tidal energy and resonance effects (Carless et al., 2016;Pelling & Green, 2014). ...
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... Estuaries are energetic environments where tidal and fluvial currents interact and shape a dynamic landscape with continuously migrating bars and channels and are therefore sensitive to changing boundary conditions such as river discharge, tidal amplitude and sediment supply Dalrymple & Choi, 2007). Understanding the long-term effects of these changes on sediment dynamics and estuary morphology is important, since these systems are often of large and sometimes conflicting economic and ecological value (Ashworth et al., 2015;Barbier et al., 2011;De Vriend et al., 2011) and are under pressure from human interference and climate change (Du et al., 2018;Ensing et al., 2015;Leuven et al., 2019). Moreover, a better understanding of the driving forces of estuarine morphodynamics is needed to improve reconstructions of paleoenvironments (Dalrymple & Choi, 2007). ...
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... The responses of estuaries to RSLR will be complex and context dependent (Khojasteh et al., 2021), however it is projected that increases in saline intrusion will be most extreme in low gradient (e.g. coastal plain), shallow estuaries with depths <10m (Krvavica and Ružić, 2020;Leuven et al., 2019;Mulamba et al., 2019;Prandle and Lane, 2015;Williamson and Guinder, 2021). In estuaries that are heavily modified, saline intrusion is exacerbated by human activities such as channelisation (i.e. the deepening and narrowing of estuarine channels and removal of flood storage areas) for navigation and flood defence, which work to amplify and propagate Fig. 2. Estuarine squeeze schematic showing hypothetical spatial changes in estuarine salinity zones (based on the Venice system) through time in an A) unbounded and B) bounded estuary in response to saline intrusion driven by relative sea level rise (RSLR) and/or decreased river flow. ...
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