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Sea-level-rise-induced threats depend on the size of tide-influenced estuaries worldwide

DOI:10.1038/s41558-019-0608-4
Authors:
Jasper R.F.W. Leuven at Wageningen University & Research
Jasper R.F.W. Leuven
Harm Jan Pierik at Rijksdienst voor Cultureel Erfgoed
Harm Jan Pierik
Maarten van der Vegt at Utrecht University
Maarten van der Vegt
Tjeerd J Bouma at NIOZ Royal Netherlands Institute for Sea Research
Tjeerd J Bouma
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Abstract and Figures

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.
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.
… 
Estuary size dependency on SLR-induced threats a, Possible trajectories of SLR-induced changes in maximum water level within the estuary against convergence length, which is a measure of estuary size. Lines indicate the effect of 1-m SLR without morphological adaptation (scenario 1). Shallow damping estuaries are probably most sensitive to future increases in water level and impose a flood risk, while deep amplifying estuaries are insensitive. b, Sediment need or sediment supply for adaptation with 1-m SLR over a period of 100 years against estuary convergence length. Vertical lines indicate the sensitivity to increased (scenario 2, a⁺) or decreased (scenario 2, a⁻) tidal amplitude. Squares indicate the required sediment when amplitude at the mouth remains constant; triangles indicate estimated fluvial sediment supply over 100 years. Blue triangles indicate a sediment supply less than the requirement under the a⁰ scenario. Orange triangles indicate a surplus of fluvial sediment input. Large amplifying estuaries are very sensitive to changes in tidal amplitude on required sediment for adaptation, while small damping estuaries are less sensitive. Large estuaries with increasing tidal amplitude have sufficient sediment input from the river for adaptation, while a decrease in tidal amplitude would lead to a larger requirement than supplied by the river. The grey shaded area indicates the generalized effect of estuary size on future water levels and sediment need.
Estuary size dependency on SLR-induced threats a, Possible trajectories of SLR-induced changes in maximum water level within the estuary against convergence length, which is a measure of estuary size. Lines indicate the effect of 1-m SLR without morphological adaptation (scenario 1). Shallow damping estuaries are probably most sensitive to future increases in water level and impose a flood risk, while deep amplifying estuaries are insensitive. b, Sediment need or sediment supply for adaptation with 1-m SLR over a period of 100 years against estuary convergence length. Vertical lines indicate the sensitivity to increased (scenario 2, a⁺) or decreased (scenario 2, a⁻) tidal amplitude. Squares indicate the required sediment when amplitude at the mouth remains constant; triangles indicate estimated fluvial sediment supply over 100 years. Blue triangles indicate a sediment supply less than the requirement under the a⁰ scenario. Orange triangles indicate a surplus of fluvial sediment input. Large amplifying estuaries are very sensitive to changes in tidal amplitude on required sediment for adaptation, while small damping estuaries are less sensitive. Large estuaries with increasing tidal amplitude have sufficient sediment input from the river for adaptation, while a decrease in tidal amplitude would lead to a larger requirement than supplied by the river. The grey shaded area indicates the generalized effect of estuary size on future water levels and sediment need.
… 
Relative upstream sediment input from the catchment versus the range of required sediment for 1-m SLR depending on the change of tidal amplitude Required sediment is an indicator for estuary size when all boundary conditions remain constant (scenario 1). Rivers with a relatively large catchment area (Acatchment) compared to estuary area (Aestuary) deliver sufficient sediment to estuaries, which can therefore adapt to rising sea level, while other estuaries may depend on marine sediment input, peat formation or sediment nourishment for morphological adaptation. The sensitivity to changes in tidal amplitude (see Fig. 2) are indicated for a small selection of all systems indicated in Fig. 2. Holocene sediment budgets from geological cases are added as a reference for minimal sediment supply potential. Mid-Holocene cases experienced SLR comparable to future SLR (see also Supplementary Table 1).
Relative upstream sediment input from the catchment versus the range of required sediment for 1-m SLR depending on the change of tidal amplitude Required sediment is an indicator for estuary size when all boundary conditions remain constant (scenario 1). Rivers with a relatively large catchment area (Acatchment) compared to estuary area (Aestuary) deliver sufficient sediment to estuaries, which can therefore adapt to rising sea level, while other estuaries may depend on marine sediment input, peat formation or sediment nourishment for morphological adaptation. The sensitivity to changes in tidal amplitude (see Fig. 2) are indicated for a small selection of all systems indicated in Fig. 2. Holocene sediment budgets from geological cases are added as a reference for minimal sediment supply potential. Mid-Holocene cases experienced SLR comparable to future SLR (see also Supplementary Table 1).
… 
Effects of managed realignment a,b, Change in water level within the estuary (ΔWL, a) and change in required sediment for adaptation normalized by surface area plotted against degree of estuary widening (b). Dashed lines indicate idealized model results for scenarios 3A–D for a small estuary (length of 15 km) and solid lines indicate a large estuary (length of 70 km). Widening was implemented on a fixed length and thus with variable local width. The type of widening in scenario 3 has a large effect on required sediment for large estuaries and on water levels in small estuaries. Increased estuary width increases tidal prism at the mouth, but only when the widening is located seaward of the tidal excursion length, which is the distance a particle of water travels in half a tidal cycle. Increased tidal prism increases channel depth and volume, and therefore reduces required sediment. However, increased depth also reduces friction, which increases water levels in the estuary.
Effects of managed realignment a,b, Change in water level within the estuary (ΔWL, a) and change in required sediment for adaptation normalized by surface area plotted against degree of estuary widening (b). Dashed lines indicate idealized model results for scenarios 3A–D for a small estuary (length of 15 km) and solid lines indicate a large estuary (length of 70 km). Widening was implemented on a fixed length and thus with variable local width. The type of widening in scenario 3 has a large effect on required sediment for large estuaries and on water levels in small estuaries. Increased estuary width increases tidal prism at the mouth, but only when the widening is located seaward of the tidal excursion length, which is the distance a particle of water travels in half a tidal cycle. Increased tidal prism increases channel depth and volume, and therefore reduces required sediment. However, increased depth also reduces friction, which increases water levels in the estuary.
… 
Main responses of estuaries to SLR Large, deep, amplifying estuaries are susceptible to sediment starvation if tidal amplitude at the mouth decreases. This can be counteracted by increased space for the estuary along the entire channel, which could increase tidal prism and available erodible sediment for adaptation, or in the tide-dominated zone at the expense of higher water levels in the estuary. Small, shallow, damping estuaries are susceptible to future tidal amplification when sediment supply is insufficient to adapt the morphology to SLR. Managed realignment can potentially reduce amplification, but only when it is implemented in the most landward part of the estuary or along the entire estuary (Fig. 4). The benefit of the latter is that it also strongly decreases the required sediment for adaptation. Blue (red) text and symbols represent positive (negative) effects.
Main responses of estuaries to SLR Large, deep, amplifying estuaries are susceptible to sediment starvation if tidal amplitude at the mouth decreases. This can be counteracted by increased space for the estuary along the entire channel, which could increase tidal prism and available erodible sediment for adaptation, or in the tide-dominated zone at the expense of higher water levels in the estuary. Small, shallow, damping estuaries are susceptible to future tidal amplification when sediment supply is insufficient to adapt the morphology to SLR. Managed realignment can potentially reduce amplification, but only when it is implemented in the most landward part of the estuary or along the entire estuary (Fig. 4). The benefit of the latter is that it also strongly decreases the required sediment for adaptation. Blue (red) text and symbols represent positive (negative) effects.
… 
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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 direction14,21–24. 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
estuaries28–30. 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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Full-text available
  • Sep 2022
Coastal restoration is often distrusted and, at best, implemented at small scales, which hampers its potential for coastal adaptation. Present technical, economic and management barriers stem from sectoral and poorly coordinated local interventions, which are insufficiently monitored and maintained, precluding the upscaling required to build up confidence in ecosystem restoration. The paper posits that there is enough knowledge, technology, financial and governance capabilities for increasing the pace and scale of restoration, before the onset of irreversible coastal degradation. We propose a systemic restoration, which integrates Nature based Solutions (NbS) building blocks, to provide climate-resilient ecosystem services and improved biodiversity to curb coastal degradation. The result should be a reduction of coastal risks from a decarbonised coastal protection, which at the same time increases coastal blue carbon. We discuss barriers and enablers for coastal adaptation-through-restoration plans, based on vulnerable coastal archetypes, such as deltas, estuaries, lagoons and coastal bays. These plans, based on connectivity and accommodation space, result in enhanced resilience and biodiversity under increasing climatic and human pressures. The paper concludes with a review of the interconnections between the technical, financial and governance dimensions of restoration, and discusses how to fill the present implementation gap.
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Sustainable estuarine governance based on artificial island scheme for a highly anthropogenic influenced Shenzhen Bay, China
Article
Morphodynamics is essential to the sustainable development of estuaries, and plays an importance role in sediment movement, material and energy exchange, and ecosystem health. The divergent morphology of estuary commonly traps sediment and causes sediment deposition, which threats flood control, navigation and ecological environment, and finally leads to the unsustainability of estuarine development. Shenzhen Bay, China is highly anthropogenic influenced waters between Shenzhen and Hong Kong, two densely populated and highly urbanized cities. Due to an expansion in the middle of the Bay, Shenzhen Bay has been experiencing persistent siltation. In order to keep the sustainability of the Bay, an artificial island scheme (AIS) was proposed on the tidal flat of the expansion area, which aimed to fundamentally control or reduce sedimentation by changing the morphology of the Bay, and at the same time, made ecological compensation for habitat loss. The hydrodynamic and sediment transport models were applied to evaluate the impact of the AIS on tidal current, sediment control and siltation. The numerical results indicated that the proposed AIS increased the tidal current around the island, thus reducing the sediment transport into the inner bay and effectively mitigating the sedimentation in the siltation areas. On the other hand, the island was proposed to be designed as an appropriate ecological site to provide multiple functions, such as foraging, nesting, and breeding of estuarine species, especially important migratory birds, to improve health and sustainable development. This idea of estuary governance is also applicable to other estuaries with similar conditions to promote the sustainable management of estuaries.
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Exploring the three-dimensional flow-sediment dynamics and trapping mechanisms in a curved estuary: The role of salinity and circulation
Article
Full-text available
  • Sep 2022
Combined with the observed data in the wet season in June 2015, structures of longitudinal and lateral residual current and characteristics of the estuarine turbidity maximum (ETM) in the Yongjiang estuary (YE) are studied using a three-dimensional baroclinic flow and sediment numerical model. The mechanisms of residual current and sediment trapping are investigated according to the momentum balance analysis and sediment transport decomposition. The results show that at spring tide, the outflowing longitudinal residual current is dominated by longitudinal advection and barotropic pressure gradient. At neap tide, a remarkable baroclinic effect emerges at the bottom of the river mouth area, driving the landward residual current and forming the estuarine circulation. Lateral residual current at upstream bends with lower salinity is dominated by longitudinal advection and barotropic pressure gradient. The flow directs toward the concave bank at the surface and toward the convex bank near the bottom at these sections. At downstream bends with higher salinity, the lateral residual current is greatly affected by the baroclinic gradient, which will shift the lateral flow circulation structure. In transition straight reaches located at Qingshuipu and Zhenhai, the lateral residual current presents a double-cell circulation with surface convergence and bottom divergence. During spring tide, the ETM is located near Qingshuipu, driven by landward tidal pumping transport due to the strong tidal energy. During neap tide, a strong exchange flow generates landward circulation transport around the river mouth, and the ETM moves downstream to Zhenhai. At bends, sediment along the cross section is laterally trapped on the convex bank, driven by bottom lateral flow induced circulation transport. While in transition straight reaches, high turbidity is still concentrated in the deep groove, caused by bottom divergent flow and circulation transport.
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Projections of declining fluvial sediment delivery to major deltas worldwide in response to climate change and anthropogenic stress
Article
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Deltas are resource rich, low-lying areas where vulnerability to flooding is exacerbated by natural and anthropogenically induced subsidence and geocentric sea-level rise, threatening the large populations often found in these settings. Delta 'drowning' is potentially offset by deposition of sediment on the delta surface, making the delivery of fluvial sediment to the delta a key balancing control in offsetting relative sea-level rise, provided that sediment can be dispersed across the subaerial delta. Here we analyse projected changes in fluvial sediment flux over the 21st century to 47 of the world's major deltas under 12 environmental change scenarios. The 12 scenarios were constructed using four climate pathways (Representative Concentration Pathways 2.6, 4.5, 6.0, and 8.5), three socioeconomic pathways (Shared Socioeconomic Pathways 1, 2, and 3), and one reservoir construction timeline. A majority (33/47) of the investigated deltas are projected to experience reductions in sediment flux by the end of the century, when considering the average of the scenarios, with mean and maximum declines of 38% and 83%, respectively, between 1990-2019 and 2070-2099. These declines are driven by the effects of anthropogenic activities (changing land management practices and dam construction) overwhelming the effects of future climate change. The results frame the extent and magnitude of future sustainability of major global deltas. They highlight the consequences of direct (e.g. damming) and indirect (e.g. climate change) alteration of fluvial sediment flux dynamics and stress the need for further in-depth analysis for individual deltas to aid in developing appropriate management measures.
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Empirical Assessment Tool for Bathymetry, Flow Velocity and Salinity in Estuaries Based on Tidal Amplitude and Remotely-Sensed Imagery
Article
Full-text available
  • Nov 2018
Hydromorphological data for many estuaries worldwide is scarce and usually limited to offshore tidal amplitude and remotely-sensed imagery. In many projects, information about morphology and intertidal area is needed to assess the effects of human interventions and rising sea-level on the natural depth distribution and on changing habitats. Habitat area depends on the spatial pattern of intertidal area, inundation time, peak flow velocities and salinity. While numerical models can reproduce these spatial patterns fairly well, their data need and computational costs are high and for each case a new model must be developed. Here, we present a Python tool that includes a comprehensive set of relations that predicts the hydrodynamics, bed elevation and the patterns of channels and bars in mere seconds. Predictions are based on a combination of empirical relations derived from natural estuaries, including a novel predictor for cross-sectional depth distributions, which is dependent on the along-channel width profile. Flow velocity, an important habitat characteristic, is calculated with a new correlation between depth below high water level and peak tidal flow velocity, which was based on spatial numerical modelling. Salinity is calculated from estuarine geometry and flow conditions. The tool only requires an along-channel width profile and tidal amplitude, making it useful for quick assessments, for example of potential habitat in ecology, when only remotely-sensed imagery is available.
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Sediment budget and morphological development of the Dutch Wadden Sea: Impact of accelerated sea-level rise and subsidence until 2100
Article
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The Wadden Sea is a unique coastal wetland containing an uninterrupted stretch of tidal flats that span a distance of nearly 500km along the North Sea coast from the Netherlands to Denmark. The development of this system is under pressure of climate change and especially the associated acceleration in sea-level rise (SLR). Sustainable management of the system to ensure safety against flooding of the hinterland, to protect the environmental value and to optimise the economic activities in the area requires predictions of the future morphological development. The Dutch Wadden Sea has been accreting by importing sediment from the ebb-tidal deltas and the North Sea coasts of the barrier islands. The average accretion rate since 1926 has been higher than that of the local relative SLR. The large sediment imports are predominantly caused by the damming of the Zuiderzee and Lauwerszee rather than due to response to this rise in sea level. The intertidal flats in all tidal basins increased in height to compensate for SLR. The barrier islands, the ebb-tidal deltas and the tidal basins that comprise tidal channels and flats together form a sediment-sharing system. The residual sediment transport between a tidal basin and its ebb-tidal delta through the tidal inlet is influenced by different processes and mechanisms. In the Dutch Wadden Sea, residual flow, tidal asymmetry and dispersion are dominant. The interaction between tidal channels and tidal flats is governed by both tides and waves. The height of the tidal flats is the result of the balance between sand supply by the tide and resuspension by waves. At present, long-term modelling for evaluating the effects of accelerated SLR mainly relies on aggregated models. These models are used to evaluate the maximum rates of sediment import into the tidal basins in the Dutch Wadden Sea. These maximum rates are compared to the combined scenarios of SLR and extraction-induced subsidence, in order to explore the future state of the Dutch Wadden Sea. For the near future, up to 2030, the effect of accelerated SLR will be limited and hardly noticeable. Over the long term, by the year 2100, the effect depends on the SLR scenarios. According to the low-end scenario, there will be hardly any effect due to SLR until 2100, whereas according to the high-end scenario the effect will be noticeable already in 2050.
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Controls on mud distribution and architecture along the fluvial-to-marine transition
Article
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The interaction of marine (tides and waves) and fluvial processes determines the sedimentary fill of coastal systems in the fluvial-tomarine (FTM) transition zone. Despite frequent recognition of tidal and wave influence in modern and ancient systems, our understanding of the relative importance of marine processes and their impact on mud deposition and reservoir architecture is limited. This study combined subsurface field observations and numerical simulations to investigate the relative importance of river flow, tides, waves, and mud input in governing the sedimentary fill in funnel-shaped basins along the FTM transition. Model simulations show a self-forming bar-built estuary with dynamic channels and sandy bars flanked by mud flats, which is in agreement with trends observed in nature. From three-dimensional virtual sedimentary successions, statistical tendencies for mud distribution and thickness were derived for the spectrum of marine and fluvial processes, and these values provide quantitative information on the net-to-gross ratio and mud architecture. The relative influence of marine and fluvial processes leads to a predictable facies organization and architecture, with muddier and more heterogeneous sediments toward the flanks. For the first time, our simulations allow the sedimentary fill in basins along the FTM transition to be related explicitly to hydrodynamic conditions, providing new insights into the morphosedimentary evolution of coastal systems, with implications for sequence stratigraphy.
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Future response of global coastal wetlands to sea-level rise
Article
Full-text available
The response of coastal wetlands to sea-level rise during the twenty-first century remains uncertain. Global-scale projections suggest that between 20 and 90 per cent (for low and high sea-level rise scenarios, respectively) of the present-day coastal wetland area will be lost, which will in turn result in the loss of biodiversity and highly valued ecosystem services1-3. These projections do not necessarily take into account all essential geomorphological4-7 and socio-economic system feedbacks8. Here we present an integrated global modelling approach that considers both the ability of coastal wetlands to build up vertically by sediment accretion, and the accommodation space, namely, the vertical and lateral space available for fine sediments to accumulate and be colonized by wetland vegetation. We use this approach to assess global-scale changes in coastal wetland area in response to global sea-level rise and anthropogenic coastal occupation during the twenty-first century. On the basis of our simulations, we find that, globally, rather than losses, wetland gains of up to 60 per cent of the current area are possible, if more than 37 per cent (our upper estimate for current accommodation space) of coastal wetlands have sufficient accommodation space, and sediment supply remains at present levels. In contrast to previous studies1-3, we project that until 2100, the loss of global coastal wetland area will range between 0 and 30 per cent, assuming no further accommodation space in addition to current levels. Our simulations suggest that the resilience of global wetlands is primarily driven by the availability of accommodation space, which is strongly influenced by the building of anthropogenic infrastructure in the coastal zone and such infrastructure is expected to change over the twenty-first century. Rather than being an inevitable consequence of global sea-level rise, our findings indicate that large-scale loss of coastal wetlands might be avoidable, if sufficient additional accommodation space can be created through careful nature-based adaptation solutions to coastal management.
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Morphology of bar-built estuaries: Empirical relation between planform shape and depth distribution
Article
Full-text available
  • Sep 2018
Fluvial–tidal transitions in estuaries are used as major shipping fairways and are characterised by complex bar and channel patterns with a large biodiversity. Habitat suitability assessment and the study of interactions between morphology and ecology therefore require bathymetric data. While imagery offers data of planform estuary dimensions, only for a few natural estuaries are bathymetries available. Here we study the empirical relation between along-channel planform geometry, obtained as the outline from imagery, and hypsometry, which characterises the distribution of along-channel and cross-channel bed levels. We fitted the original function of Strahler (1952) to bathymetric data along four natural estuaries. Comparison to planform estuary shape shows that hypsometry is concave at narrow sections with large channels, while complex bar morphology results in more convex hypsometry. We found an empirical relation between the hypsometric function shape and the degree to which the estuary width deviates from an ideal convergent estuary, which is calculated from river width and mouth width. This implies that the occurring bed-level distributions depend on inherited Holocene topography and lithology. Our new empirical function predicts hypsometry and along-channel variation in intertidal and subtidal width. A combination with the tidal amplitude allows for an estimate of inundation duration. The validation of the results on available bathymetry shows that predictions of intertidal and subtidal area are accurate within a factor of 2 for estuaries of different size and character. Locations with major human influence deviate from the general trends because dredging, dumping, land reclamation and other engineering measures cause local deviations from the expected bed-level distributions. The bathymetry predictor can be used to characterise and predict estuarine subtidal and intertidal morphology in data-poor environments.
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The effect of geometry and tidal forcing on hydrodynamics and net sediment transport in semi-enclosed tidal basins: A 2D exploratory model
Article
  • Jul 2018
A new depth-averaged exploratory model has been developed to investigate the hydrodynamics and the tidally averaged sediment transport in a semi-enclosed tidal basin. This model comprises the two-dimensional (2DH) dynamics in a tidal basin that consists of a channel of arbitrary length, flanked by tidal flats, in which the water motion is being driven by an asymmetric tidal forcing at the seaward side. The equations are discretized in space by means of the finite element method and solved in the frequency domain. In this study, the lateral variations of the tidal asymmetry and the tidally averaged sediment transport are analyzed, as well as their sensitivity to changes in basin geometry and external overtides. The Coriolis force is taken into account. It is found that the length of the tidal basin and, to a lesser extent, the tidal flat area and the convergence length determine the behaviour of the tidally averaged velocity and the overtides and consequently control the strength and the direction of the tidally averaged sediment transport. Furthermore, the externally prescribed overtides can have a major influence on tidal asymmetry in the basin, depending on their amplitude and phase. Finally, for sufficiently wide tidal basins, the Coriolis force generates significant lateral dynamics.
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Anthropogenic Modifications Can Significantly Influence the Local Mean Sea Level and Affect the Survival of Salt Marshes in Shallow Tidal Systems
Article
  • Apr 2018
In a shallow dissipative tidal system, interventions that modify the local morphology in one location of the basin may quickly affect the surrounding areas, promoting strong changes in their morphology. If the localized modification persists over time, it may produce far-field morphological modifications at delayed times. These modifications may have an impact on the local mean sea level and on tidal range, affecting the fate of salt marshes with implications for their survival. In this study, we investigate the effect of two anthropic interventions performed between the end of the nineteenth century and the beginning of the twentieth century in the northern Venice lagoon (Italy): the construction of jetties at one inlet and the removal of reed barriers protecting a fish farm in the inner lagoon. Using a 2-D numerical model to reproduce the hydrodynamics of different historical lagoon configurations and a zero dimensional model of marsh vertical accretion, we investigate the effect of these interventions on the salt marshes of the northern lagoon basin. Interestingly, our results show that the increased depth at the inlet induced by the jetties lowered the local mean sea level of nearby areas and increased the tidal range, producing a temporary positive feedback on the stability of the marshes in proximity of the inlet. On the contrary, in the inner lagoon areas characterized by extremely low marshes, the removal of reed barriers delimiting a fish farm may have reduced the sediment fluxes thus contributing to the drowning of large marsh surfaces.
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Estuarine Channel Evolution in Response to Closure of Secondary Basins: An Observational and Morphodynamic Modeling Study of the Western Scheldt Estuary
Article
  • Jan 2018
The fringes of estuaries are often characterized by the presence of side embayments (secondary basins), with dimensions in the order of hundreds of meters to tens of kilometers. The presence of secondary basins significantly alters the hydrodynamic and sediment characteristics in the main estuary, implying that loss of secondary basin area due to human interventions might affect the estuarine morphodynamics. Analysis of historical bathymetric data of the Western Scheldt Estuary (Netherlands) suggests that closure of its secondary basins has triggered the observed lateral displacement of the nearby channels. This analysis motivated investigation of the impact of secondary basins on decadal evolution of estuarine channels, using the numerical model Delft3D. Model results show that channels that form near a secondary basin are located further away from the bank of the estuary with respect to their positions in the case without a basin. Overall, results in cases with two or three basins are similar to those in case with one single basin. The wider the basin, the further away the nearby channel forms. Removing a secondary basin causes a lateral displacement of the nearby channel towards the bank, indicating that the observed lateral displacement of channels in the Western Scheldt is triggered by closure of its secondary basins. The physical explanation is that tidal currents in the main estuary are weaker and more rotary near secondary basins, favoring sediment deposition and shoal development at these locations. Model results are particularly relevant for estuaries with moderate to high friction and converging width.
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