Sediment transport in the Ameland tidal inlet system: Insights from field data and model simulations
Maarten van der Vegt
Co-authors: K. J. H.Lenstra1, L. B. Brakenhoff1, M. C. G. van Maarsseveen1, B. C. van Prooijen2, M. F. S. Tissier2, F. P. de Wit2, S. G. Pearson2,3, K. den Heijer2,3,6, J. J.van der Werf3, P. K. Tonnon3, C. A. Schipper3,4, B. Grasmeijer3, J. W. Mol4, H. de Looff4, H. Holzhauer5
1.Utrecht University, 2. Delft University of Technology, 3. Deltares, 4. Rijkswaterstaat, 5. Twente University, 6. Data2Day
Ebb-tidal deltas are shallow features located seaward of tidal inlets. They can be a temporary source of sediment and they play an important role in limiting the wave energy into the Wadden Sea. Observations show that many ebb-tidal deltas of the Dutch Wadden Sea are losing sediment. Therefore nourishments of ebb-tidal deltas are considered. To this end, we need a better understanding of the sediment transport pathways on the ebb-tidal delta and in the basin, and how these depend on wind, waves and tides.
As part of the SEAWAD project we measured hydrodynamics, sediment transport and bed form dynamics in the Ameland tidal inlet system. The main questions addressed were: what are the wave-current interactions and how do these determine sediment transport and bed form dynamics? What is the sediment grain size distribution and what are the transport pathways of the different fractions? What is the benthic species distribution and how does this depend on hydrodynamic conditions? What is the relative role of exchange of water and sediment over the tidal divides, compared to the transport at the tidal inlet? In September and October 2017 at four locations state-of-the-art frames were placed, measuring at a high vertical and temporal resolution water levels, flow velocities, sediment concentrations, sediment size distributions and bed forms. At the tidal divides ADCP’s were placed. Furthermore, cross-sections were sailed to determine larger scale bed forms and the tidal prism. Sediment grain size distributions were determined and marked tracer sediments were released and recovered. The data is now being analyzed and first results will be presented.
In addition to field data also numerical models are being used and developed. Here I will present results obtained with the Delft3D/SWAN models. Model simulation show that the periodic variations in the channel positions of the ebb-tidal delta significantly influence the tidal asymmetry in the inlet and mean flow characteristics; as a result, the net sediment exchange between basin and sea is cyclic. Winds have a strong influence on the sediment pathways, especially during storm surges. The effects strongly depend on the wind direction and the model results help us to interpret the data from the field campaign.
PDF presentation Maarten van der Vegt
Ebb-tidal deltas on the move: cyclic channel-shoal dynamics
Co-author: Maarten van der Vegt, Utrecht University
Natural cyclic behavior is a common feature of the ebb-tidal deltas in the Wadden Sea: channels rotate and breach while shoals periodically form, migrate and attach to the Wadden island. The aim of this presentation is twofold: (1) to unravel the long-term mechanisms that cause the cyclic channel-shoal dynamics; and (2) to test how the cyclic behavior would be affected by ebb-tidal delta nourishments. These nourishments have been proposed as part of the Dutch coastal defense strategy. In fact, in 2018 a nourishment of ~5 Mm3 has been implemented at the ebb-tidal delta of Ameland.
Firstly, the cyclic behavior was modeled in an idealized model setup in Delft3D/SWAN. We show that the relative roles of tides and waves change throughout the phase of the cyclic behavior. For example, wave-induced currents and sediment concentrations force the shoal formation and channel rotation, while tidal currents cause the ebb-tidal delta to breach as the channel deflection continues. However, this breach takes less time if waves entrain additional sediment to be transported by the tidal currents. Additionally, cyclic variations in mean flows and sediment transport through the inlet are an inherent feature of the modeled cyclic behavior.
Secondly, we implemented and tested a wide range of possible nourishments, varying in size, location and phase of the cyclic behavior. Our results show that the nourishments could potentially reduce the first period of the natural cyclic behavior by 30%, whereas the time scale of the subsequent cycles are unaffected. Furthermore, a nourishment impacts the long-term ebb-tidal delta volume and the sediment exchange through the inlet.
PDF presentation Klaas Lenstra
Modelling the interaction among the inlets in a meso-tidal barrier coast
Pieter C. Roos
University of Twente
Co-authors: Koen R.G. Reef1), H.M. Schuttelaars2), Suzanne J.M.H. Hulscher1)
1. University of Twente,2. Delft University of Technology
Understanding the long-term dynamics (decades to centuries) of barrier coasts is important for coastal management. However, due to the coupling of large spatial and temporal scales, interaction among the multiple inlets of such a system must be taken into account. Here we present a new idealized model for the long-term morphological evolution of multiple tidal inlets. As a specific novelty, we investigate the influence of the back-barrier basin’s plan view shape on the equilibrium configuration of the inlets (size and spacing). Results show that this configuration is affected by the local width of the back-barrier basin, generally showing more (less) inlets where it is wide (narrow). This is in agreement with observed relationships in the literature. Next, varying the width in only one part of the basin also influences the inlet size and spacing in the other part (where the width is unaffected), demonstrating interaction among inlets. Further increasing basin width may lead to tidal resonance, which is also reflected in inlet size and spacing. This last result shows that estimating tidal prism as a product of basin surface area and seaward tidal range is only valid for narrow basins. These results help to understand the influence of large-scale human intervention on multiple inlet dynamics, such as the Zuiderzee closure leading to a deepening of Texel and Vlie inlets.
PDF presentation Pieter Roos
Bedform dynamics on the Ameland ebb-tidal delta
Co-authors: Gerben Ruessink, Maarten van der Vegt and Maarten Kleinhans
Ebb-tidal deltas are subject to both waves and cross-shore and longshore currents. Interaction between these hydrodynamics and the sandy bed creates bedforms on a variety of scales. The largest bedforms on ebb-tidal deltas are sandy shoals, which have been studied thoroughly in the past. On the smaller scales, not much is known yet. For example, along most of the ebb-tidal deltas and shores of the Wadden Islands, so-called saw-tooth bars are found, but until now, no studies had analysed their characteristics and behaviour. When we zoom in even further, to the ripple and megaripple scale, a lot of studies were done, but none of them regarded ebb-tidal deltas. Yet, the complex hydrodynamic situation on ebb-tidal deltas makes it almost impossible to predict (mega-)ripple characteristics based on flume or other field studies. Still, these bedforms impose roughness, so it is very important to know when and where they occur. This can help in improving models like Delft3D. Therefore, in the context of the SEAWAD/Kustgenese2.0 project, a field study was executed, in which both bedform dynamics and hydrodynamics were measured on the Ameland ebb-tidal delta. In this presentation, the three bedform types (saw-tooth bars, megaripples and ripples) will be compared.
PDF presentation (Laura Brakenhoff)
Aspect multiple sediment fractions (Gerard Herrling)
Overview of sand transport- and morphodynamic models for the Wadden Sea and it’s tidal inlets; recent research and future challenges
Pieter Koen Tonnon
An overview of process-based sand transport- and morphodynamic models for the Wadden Sea and it’s tidal inlets developed at- or in cooperation with Deltares will be presented. Improvements in schematization, detail and results including application for management will be summarized. Initial ideas in general for future process-based model application, development and challenges will be listed for discussion afterwards.
PDF presentation Pieter Koen Tonnon
Sediment feeds the Wadden Sea. Rising sea levels, sediment budgets and the future of intertidal flats
Ad van der Spek
Deltares, University Utrecht
Co-authors: Edwin Elias2, Zheng Bing Wang1,4, Quirijn Lodder4,5
1.Deltares, 2. Deltares-USA, 3. University Utrecht, 4. TU Delft, 5. RWS-WVL
Tidal basins along the North Sea shores tend to import sediment. Without relative sea-level rise (SLR), the combination of a rise in mean ocean level and subsidence of the sea bed that creates new space for sediment to be stored, the basins will fill in. Hence, relative SLR keeps the basins ‘alive’. These are long-term processes.
However, an acceleration in the rise of mean sea level will cause a rapid increase in sediment demand that needs to be satisfied to maintain the intertidal flats in the basins. A deficient supply will lead to disappearance of the flats in the long run.
A recent assessment of the sediment budget of the Dutch Wadden Sea shows that the eastern and western parts are in different states of development and that the western Wadden Sea seems more vulnerable for a sediment deficit caused by accelerated SLR than the eastern part. The pace and details of drowning of the tidal flats in the basins are not clear yet.
PDF presentation Ad van der Spek, Deltares
Mud transport and morphology in the Wadden Sea
Thijs van Kessel
Co-author: Peter Herman
Mud dynamics is relevant for local and short-term issues such as turbidity and sedimentation, but also for more long-term and large-scale issues such as morphological and ecological development.
This short presentation discusses a conceptual model for mud dynamics at different time scales, with hydrodynamic forcing, availability and properties of mud as determining factors.
With regard to availability, two examples are given on observed long-term variations of SPM (from MWTL database) and of bed composition (from SIBES database).
The long-term SPM variations in the Wadden Sea cannot be linked statistically to long-term SPM variations in the North Sea.
The variations in the Wadden Sea are therefore likely to have a local origin, possible causes are variations in hydrodynamic forcing (tide, wind, waves, river discharge), morphological evolution or variations in mud properties (e.g. due to biological factors). This needs further investigation.
The bed composition is shown to be persistent, with limited changes during the past decades. A bimodal distribution is observed: either the seabed is sandy with little mud or muddy with little sand.
The cause for this bimodal distribution will be further investigated. It is also shown that there is a strong correlation between mud content and microfytobenthos. The question is whether this is a causal relationship and if so, whether mud content steers microfythobenthos content or vice versa.
For further investigations the conceptual model will be extended towards numerical models (with both schematised and realistic geometries).
PDF presentation Thijs van Kessel
Mud as a resource: reducing turbidity while keeping up with sea level rise
Bas van Maren
Deltares & Delft University of Technology
The Ems estuary has become more turbid in the past decades as a result of various human interventions deepening, port construction) and change in dredging strategies. Over longer timescales (centuries) a mechanism for the increase in turbidity is the loss of natural sediment sinks: historically sediment deposited on extensive intertidal areas and marshes. This effectively resulted in extraction of sediment from the system. The present-day coastline is a closed system in which fine sediments cannot deposited, leading to an increase in turbidity. In order to reduce turbidity, measures (such as the Kleirijperij) are presently designed and implemented to extract sediment from the system in a sustainable way. However, sediment is also needed to keep pace with sealevel rise. An important question therefore is to what extent the Ems Estuary can keep pace with sealevel rise, and how much this depends on sediment availability. This presentation will provide an overview of the current state of knowledge and tools on determining the effect of extraction, and the capacity of the estuary to keep pace with the expected sealevel rise.
PDF presentation Bas van Maren
Beneficial use of dredged sediment to enhance salt marsh development by applying a ‘Mud Motor’
Martin Baptist Wageningen University and Research
Co-authors: Martin J. Baptist1, T. Gerkema2, B.C. van Prooijen3, D.S. van Maren3,4, M. van Regteren1,
K. Schulz2, I. Colosimo3, J. Vroom4, T. van Kessel4, B. Grasmeijer4, P. Willemsen2,4,8, K. Elschot1,
A.V. de Groot1, J. Clevering1,5, E.M.M. van Eekelen6, F. Schuurman7, H.J. de Lange1,
M.E.B. van Puijenbroek1
1.Wageningen University & Research, Wageningen, The Netherlands, 2. NIOZ Netherlands Institute for Sea Research, Department of Estuarine and Delta Systems and Utrecht University, Yerseke, The Netherlands, 3. Hydraulic Engineering Section, Faculty of Civil Engineering and Geosciences, Delft University of Technology, Delft, The Netherlands, 4. Deltares, Department of Ecosystems and Sediment Dynamics, Delft, The Netherlands, 5. Arcadis, Zwolle, The Netherlands, 6. Van Oord Dredging and Marine Contractors, Rotterdam, The Netherlands, 7. Royal HaskoningDHV, Nijmegen, The Netherlands, 8. Water Engineering and Management, University of Twente, Enschede, The Netherlands
We tested an innovative approach to beneficially re-use dredged sediment to enhance salt marsh development. A Mud Motor is a dredged sediment disposal in the form of a semi-continuous source of mud in a shallow tidal channel allowing natural processes to disperse the sediment to nearby mudflats and salt marshes. In the Mud Motor pilot a total of 470,516 m3 of fine grained sediment (D50 of ?10 ?m) was disposed over two winter seasons, with an average of 22 sediment disposals per week of operation. The salt marsh demonstrated significant vertical accretion though this has not yet led to horizontal expansion because of hydrodynamic stress. In carrying out the pilot we learned that the feasibility of a Mud Motor depends on an assessment of additional travel time for the dredger, the effectiveness on salt marsh growth, reduced dredging volumes in a port, and many other practical issues. Our improved understanding on the transport processes in the channel and on the mudflats and salt marsh yields design lessons and guiding principles for future applications of sediment management in salt marsh development that include a Mud Motor approach.
PDF presentation Martin Baptist
Transport in tidal channels: the Wadden Sea as an event-driven system
Royal Netherlands Institute for Sea Research
Co-authors: Kirstin Schulz1,2, Matias Duran-Matute3,1
1.NIOZ Netherlands Institute for Sea Research, Department of Estuarine and Delta Systems, and Utrecht University, 2. AWI, Bremerhaven, 3. Department of Applied Physics, Eindhoven University of Technology
In the Wadden Sea, tidal currents exchange large volumes of water and loads of suspended sediment between tidal basins and the North Sea, and among tidal basins across watersheds. The main pathway is via the tidal channels. However, in spite of the recurring nature of the tides, net (i.e. tidal averaged) exchanges turn out to be enormously variable. For example, the standard deviation of net flows through inlets is typically one order of magnitude larger than their annual mean or median value. Moreover, even annual mean transports vary strongly from year to year. Recent research has demonstrated that this variability can be ascribed to the wind. Being much more than just a perturbing factor, it fundamentally stamps the character of the hydrodynamics of the Wadden Sea as an event-driven system.
The complexity of the dynamics is further amplified by the fact that different inlets have different preferential wind directions for optimal exchange. Strong winds are also able to enhance the transport of water (along with its dissolved and suspended constituents) across the shallow watersheds due to heightened local water level (wind surges) and wind-driven flows. Although intermittent, the resulting annual cumulative transport is very significant.
This depence on wind climate carries over to transport of suspended sediment, examples of which are shown for inlets and the Kimstergat channel.
PDF presentation Theo Gerkema
Winds of Opportunity: the Influence of Wind on Tidal Flat Morphology
Delft University of Technology
Co-authors: Bas van Maren1,2, Lodewijk de Vet1,2, Bram van Prooijen1
1.Delft University of Technology, 2 Deltares
The Wadden Sea is a multi-inlet tidal system characterized by a complex pattern of interconnecting channels, that transport water and sediment from the North Sea to the inner intertidal zones (and vice versa). The complexity of the system makes the sediment transport - and therefore the morphological evolution - difficult to predict. With three long-term (1 or 2 months) field campaigns on a tidal flat nearby Harlingen (Friesland), we explore the effect of the interaction between the deterministic (tide) and stochastic (wind) forcing, on the intertidal flat sediment transport.
Although the tide is the main forcing regulating the filling/infilling of the tidal basins, it controls the sediment transport only in the deeper (subtidal) zone. The wind is the forcing regulating the transport in the most elevated part of the flats.
The effect of the wind on the tidal flow increases for decreasing tidal flow amplitudes. This finding, based on a tide-wind interaction analytical model, shows good agreement with the observations.
On the higher mudflat zone the bed level change over a tide directly depends on the residual transport, which is a function of wind speed and, particularly, of wind direction. Here, the wind affects the occurrence of the Windows of Opportunity for the sediment to deposit, and to consolidate. The bed level increase on the higher flat is hence controlled by the stochastic component of the hydrodynamic input
PDF presentation Irene Colosimo
Sediment deposition in marshes along the Dutch Wadden Sea coast
Wageningen University & Research
Co-authors: M. van Regteren1, G. Lagendijk1, W. van Duin2, C. Sonneveld1 & M. van Puijenbroek1
1.Wageningen Marine Research, Wageningen University & Research, ankerpark 27, 1781 AG Den Helder, The Netherlands, 2. Artemisia, Graaf Willem II straat 258, 1785 KL Den Helder, The Netherlands
A major concern for the Wadden Sea is the predicted sea-level rise. Almost 10.000 ha of salt marshes is present in the Dutch Wadden Sea, and they all have to keep pace with the increasing sea level by accumulating sediment and increasing in surface elevation, i.e. the marsh accretion rate. The marshes along the Dutch main coast are manmade and originate from reclamation works performed in the first half of the 20th century. In the late 1980’s the focus changed from land reclamation towards conserving them for their high nature values.
A long-term monitoring program by Rijkswaterstaat, 1960-now, shows that marsh accretion rates along the mainland coast are high. They range from ~2 cm in the West to ~1 cm per year in the East. Within marshes, the sediment deposition rate varies and depends on many different biotic and abiotic factors, such as surface elevation, distance to the sediment source, vegetation structure and management regime. Identifying which factors control marsh accretion rates is key to predict the adaptability of marshes to future enhanced sea-level rise.
This study is part of the program WOT- Natuur & Milieu, funded by the Dutch ministry of Agriculture, Nature and Food Quality (LNV).
PDF presentation Kelly Elschot