GM6.10

Fluvial sediments have collectively formed about 900,000 km² of deltaic land since Holocene sea-level rise slowed down. The rate at which deltas have retained fluvial sediment to build deltaic land, however, has varied greatly between different deltas. Here we quantify sediment retention in the delta topset and foreset for 3,556 deltas globally. We estimate retention from data on delta morphology and cross-sectional area, combined with WBMSed data on fluvial suspended sediment supply. Deltas, on average, retain 25±2% (standard error of the mean) of the fluvial sediment in their topset and 31±2% in their foreset. Because topset sediment retention reduces the sediment delivery to the river mouth, it sets up a feedback with processes that build delta morphology. Waves reduce topset sediment retention whereas tides increase it. Tide dominated deltas retain 61±24% on their topset, on average, compared to 21±3% and 24±2% for river- and wave-dominated deltas, respectively. Larger deltas trap more sediment, but not in comparison to their larger sediment loads, making them relatively inefficient sediment traps.

How to cite: Nienhuis, J., Paniagua-Arroyave, J., Dunn, F., Cohen, S., and Tornqvist, T.: Sediment retention in river deltas and feedbacks with delta morphology, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-967, https://doi.org/10.5194/egusphere-egu21-967, 2021.

Coastal wetlands lie at the interface between submerged and emerged environments and therefore represent unique yet delicate ecosystems. Their existence, resulting from complex interactions between hydrodynamics and sediment dynamics, is challenged by increasing rates of sea-level rise, lowered fluvial sediment input as well as an increasing anthropogenic pressure. The future survival of these peculiar morphologies is becoming even more complicated, because of the construction and planning of coastal defence structures designed to protect urban areas from flooding. Important examples are the flood protection systems built to protect New Orleans (USA), the river Scheldt Estuary (The Netherlands) and Venice (Italy). In this context, understanding the physical processes on which coastal marshes are grounded and how engineering measures can alter them is of extreme importance in order to plan conservation interventions.

To understand marsh sedimentation dynamics in flood-regulated environments, we investigated through field observations and modelling the effect of the storm-surge barrier designed to protect the city of Venice, the so-called Mo.S.E. system, which has in fact become operational since October 2020.

Sedimentation measurements in different salt marshes of the Venice lagoon carried out in the period October 2018-October 2020 show that more than 70% of yearly sedimentation accumulates during storm-surge conditions, despite their short duration. Moreover, the sedimentation rate displays a highly non-linear increase with marsh inundation intensity, due to the interplay between higher water levels and greater suspended sediment concentration. Barrier operations during storm surges to avoid flooding of urban areas will reduce water levels and marsh inundation. Therefore, we computed sedimentation in a flood-regulated scenario for the same observation period, using the relation we obtained between tidal forcing and sedimentation rate. Our results show that some occasional closures during intense storm surges (70 hours/year on average) suffice to reduce the yearly sedimentation of the same order of magnitude of the relative sea-level rise rate experienced by the Venice lagoon during the last century (2.5 mm/y).

We conclude that storm-surge barrier operations can dangerously reduce salt-marsh vertical accretion rate, thus challenging wetland survival in face of increasing sea-level rise.

How to cite: Tognin, D., D'Alpaos, A., Marani, M., and Carniello, L.: Coastal flooding protection will change salt-marsh sedimentation dynamics, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1639, https://doi.org/10.5194/egusphere-egu21-1639, 2021.

Riverine sediment yield changes by human-induced catchment alterations can have important implications for river delta morphology. Here, we assess the potential response of 99 tropical deltas along the Caribbean and Pacific coasts of Colombia to projected human land use intensifications by deforestation and river damming. We assess delta morphology through the balance of wave, tidal, and (modern and future) river sediment transport processes at their mouths. We find that most Colombian deltas along the Caribbean coast are wave-dominated, except for large catchments with high riverine sediment load, which are river-dominated. Most deltas are wave-river dominated along the Pacific coast, with few examples of river-tide and wave-tide dominance. We predict Colombian deltas to become more wave and tide-dominated under river damming scenarios. In contrast, deforestation scenarios suggested virtually no future morphological changes.

How to cite: Paniagua-Arroyave, J. F., Nienhuis, J. H., and Dunn, F. E.: Potential Morphologic Responses of Tropical River Deltas in Colombia to Future Sediment Supply Scenarios, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3302, https://doi.org/10.5194/egusphere-egu21-3302, 2021.

As one of the largest deltas in the world, the Mekong delta is home to over 17 million people and supports internationally important agriculture. Recently deposited sediment compacts and causes subsidence in deltas, so they require regular sediment input to maintain elevation relative to sea level. These processes are complicated by human activities, which prevent sediment deposition indirectly through reducing fluvial sediment supply and directly through the construction of flood defence infrastructure on deltas, impeding floods which deliver sediment to the land. Additionally, anthropogenic activities increase the rate of subsidence through the extraction of groundwater and other land-use practices.

This research shows the potential for fluvial sediment delivery to compensate for sea-level rise and subsidence in the Mekong delta over the 21st century. We use detailed elevation data and subsidence scenarios in combination with regional sea-level rise and fluvial sediment flux projections to quantify the potential for maintaining elevation relative to sea level in the Mekong delta. We present four examples of localised sedimentation scenarios in specific areas, for which we quantified the potential effectiveness of fluvial sediment deposition for offsetting relative sea-level rise. The presented sediment-based adaptation strategies are complicated by existing land use, therefore a change in water and sediment management is required to effectively use natural resources and employ these adaptation methods. The presented approach could be an exemplar to assess sedimentation strategy feasibility in other delta systems worldwide that are under threat from sea-level rise.

How to cite: Dunn, F. E. and Minderhoud, P. S. J.: Feasibility of sedimentation strategies for the Mekong delta to counterbalance relative sea-level rise, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-5140, https://doi.org/10.5194/egusphere-egu21-5140, 2021.

Channel beds in estuaries and deltas often exhibit a local depth maximum at a location close to the coast. There are two known causes of large-scale (i.e. >10 river widths along-channel) channel bed scours: width constriction and draw down during river discharge extremes, both creating a local flow acceleration. Here, we systematically investigate a potential third mechanism. We study the effect of tidal dominance on the equilibrium channel bed in estuaries with a 1D-morphodynamic model. In estuaries, a morphodynamic equibrium is reached when the net (seaward) transport matches the upstream supply along the entire reach. The residual (river) current and river-tide interactions create seaward transport. Herein, river-tide interactions represent the seaward advection of tide-induced suspended sediment by the river flow. Tidal asymmetry typically creates landward transport. The main reason for scour formation is the amplification of tidal flow through funnelling of tidal energy. Only for a scouring profile the drop in river induced current magnitude reduces the river-tide interaction term, so that the net sediment transport matches the upstream sediment transport. When tidal influence is relatively large, and when channel convergence is strong, a equilibrium is only obtained with a scouring profile. We propose a predictor dependent on the width convergence, quantified as S_B, and on the ratio between the specific peak tidal discharge at the mouth and the specific river discharge at the landward boundary (q_tide/q_river). Scours develop if (q_tide/q_river)/S_B exceeds 0.3. These results are independent of scale and allow the prediction of scour in estuaries under future changes.

How to cite: Leuven, J., van Keulen, D., Nienhuis, J., Canestrelli, A., and Hoitink, T.: Large-scale scour in response to tidal dominance in estuaries, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8290, https://doi.org/10.5194/egusphere-egu21-8290, 2021.

Future mean sea level rise will influence tidal dynamics and storm surge events in estuaries. The bathymetry in estuaries and coastal areas will also be affected by mean sea level rise, since it is in a morphodynamic equilibrium with hydrodynamic forces. Tidal flats, which are an important component of coastal protection, will grow to a certain extent with mean sea level rise in case of sufficient sediment availability.

With the help of a highly resolved hydrodynamic-numerical model of the German Bight (North Sea), we analyse the potential influence of mean sea level rise and vertical growth of tidal flats on tidal dynamics and storm surge events in the Elbe estuary.

The results show an increase of tidal amplitude and storm surge water levels due to mean sea level rise. A bathymetric rise of tidal flats in the German Bight and the mouth of the Elbe estuary leads to a decrease in storm surge water level and tidal amplitude compared to the scenario with sole mean sea level rise without a change in bathymetry. Further analyses show, how geometric parameters of the Elbe estuary are changing due to mean sea level rise and tidal flat growth. These changes in geometry influence tidal dynamics and can therefore be an explanation for the observed changes in tidal amplitude and storm surge water levels.

These findings enable a better understanding of future changes in the Elbe estuary and support coastal managers in decision making processes concerning adaptation options to reduce the impacts of climate change.

How to cite: Mahavadi, T., Rudolph, E., Seiffert, R., and Winkel, N.: Effects of mean sea level rise and tidal flat growth on tides and storm surge events in the Elbe estuary, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9498, https://doi.org/10.5194/egusphere-egu21-9498, 2021.

Global flood modelling requires standardised global flood-protection levee datasets. Current datasets, however, are generally confined to territorial boundaries (national datasets) and are scarcely made public. Here we report on our effort to collect and standardise flood-protection levee data for river deltas from various sources to create a single, open source and FAIR-aligned global dynamic evolving river levee data environment (openDELvE).

openDELvE aggregates data from national databases (including the USACE National Levee Database, and the UK EA Asset Information Management System, amongst others) as well as data collected from reports, maps, and satellite imagery. We report primarily the land areas  that the levees have been designed to protect, and where additional data is available, the location of levees and unified attributes.  openDELvE currently contains 1601 mapped leveed area polygons distributed over 152 deltas, covering 28% of globally defined delta area. Out of the 152 deltas, which cover a total delta area of 239,043 km², the levees registered in the database protect a land area of 42,342 km². Additionally, more extensive data has been collected from a selection of freely accessible public national databases (mostly the UK and USA, and some of Australia) spanning 5,089 km of levees with additional unified attributes (e.g. levee height, crest width, construction material), and a semi-automated process is being used to extend and develop this layer.

The data is published aligned to FAIR-standards and is open-source, with an interactive viewing platform to supplement the data which is targeted for use in global river delta modelling and research. The viewing platform for the database incorporates a community-driven revision tool to encourage ongoing improvement and refinement of delta levee data, which can be extended to future projects as required. 

How to cite: O'Dell, J., Nienhuis, J. H., Cox, J. R., Edmonds, D. A., and Scussolini, P.: A global database of flood-protection levees on river deltas (openDELvE), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13180, https://doi.org/10.5194/egusphere-egu21-13180, 2021.

Intertidal areas disappear in deltas worldwide because of land reclamations. This greatly impacts delta morphology because the presence and physiological characteristics of intertidal areas determine the tidal regime and, as a result, residual sediment transport patterns in tidal basins. Understanding how the interaction between intertidal areas and channels in tidal basins influence morphodynamics is therefore important to predict morphological development and to assess the resilience of delta’s to changing boundary conditions.

In the Ems estuary (The Netherlands), the gradual embankment of a large intertidal embayment altered the planform and cross-sectional geometry of the estuary, leading to changes in the tidal regime and associated residual sediment transport patterns. As early as 1952, it was already suggested that these changes eventually caused a shift towards an alternative historical development of the geometric configuration of the channels; from a multiple to a single channel system.

This study shows through centennial-scale morphological modelling that the observed system shift can be hind-casted, while conserving model validity by comparison to the observed gross morphodynamics trends. The results indicate that the system shift is indeed driven by land reclamations. This provides a unique case to study the processes leading to the observed developments and evaluate the value of tidal-asymmetry based stability relationships to predict regime shifts in estuarine development.

How to cite: Schrijvershof, R., van Maren, B., Vermeulen, B., and Hoitink, T.: Intertidal floodplain controls on centennial-scale morphological channel development, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16435, https://doi.org/10.5194/egusphere-egu21-16435, 2021.