Morphodynamics of the Wadden Sea: recent research and future challenges
The thematic meeting with almost 90 participants was jointly organized by the NCK, RWS and the Wadden Academy. Participants represent a range of institutes and organizations, including universities, research institutes, government agencies, consultancy firms, provinces and municipalities, NGO’s as well as Water Boards. Foreign representatives came from Germany and Denmark.
Session 1: Tidal inlets, ebb tidal deltas and adjacent coastal zone
The first session focused on the dynamics of ebb tidal deltas and tidal inlets, covering a range of scales from small-scale bedforms during storm events to tidal inlet dynamics on much larger temporal and spatial scales. Recent results of the SEAWAD project demonstrate that bedforms vary from small scale bedforms in shallow areas to megaripples in channels and saw-tooth bars at the downdrift (east)side of the ebb delta. Sonar images show the development of small-scale, low 3D ripples during storm due to bed shear stresses by both waves and currents. Present ripple predictors for sand transport models need further improvement. Another important observation of the SEAWAD project is the strong wind-and tide-driven flow across tidal watersheds during surges; measured velocities can be as high as 0.5-1.0 m/s at the “wantij”.
Cyclic developments on ebb tidal deltas are also a product of the interaction of wave- and current-driven processes. Waves appear to be important for the breaching process by the channels on the ebb tidal delta. Implementation of a nourishment on the ebb delta leads to an acceleration in the first part of the morphological cycle; the subsequent cycles though remain unaltered. The import in the tidal inlet also fluctuates and depends on the phase of the morphological cycle. The impact of basin shape on barrier coast development was evaluated by developing an idealized model for multiple tidal inlets. The total width of inlets depends on the ratio of the local width of the back-barrier basin and the tidal wave length.
The inlet session was concluded by a short intermezzo about the effect of grain sizes on by-passing of sediment along the German Wadden coast. According to modeling results fine sediments (fine sand) are in favour of by-passing but coarse sediments become more or less “trapped” in the tidal inlet system and contribute to a re-circulation of sediment on the ebb delta.
Session 2: Tidal basins, interaction channels and shoals
This session started with an introduction of the diversity in coastal zone management issues in the Wadden Sea and the modeling tools that are presently available to tackle problems. Process-based models still need further improvement, for example in relation to the spatial resolution and the simultaneous use of sand and mud fractions in morphological studies. At a much larger scale the response of tidal basins to sea level rise was discussed, demonstrating the role of transport capacity and accommodation space (= sediment demand), in particular for the development of intertidal areas. This approach though is entirely based on the role of sand.
The role of fine sediment (mud), based on field data (e.g. SIBES surveys 20Tidal basins, interaction channels and shoals08-2013) and the PACE model was discussed. There appear to be strong gradients in the percentage of fine sediments in the seabed of the Wadden Sea: the contribution of fine sediment increases from North to South and West to East. Suspended Matter (SPM) in the water column shows an interannual variability in concentrations that cannot be linked to fluctuations in the North Sea and seems to be the result of intrinsic properties of the Wadden Sea system itself. Fine SPM also plays an important role in the Ems-Dollard estuary. In the past fine SPM was deposited in intertidal areas which offered ample accommodation space. In the present estuary fine SPM is now artificially extracted to reduce turbidity and to use mud as a resource. Model simulations play an important role to determine whether enough fine-grained sediment remains available to cope with sea level rise in the estuary.
Session 3: Tidal flats and salt marshes
The “Mud Motor”was introduced, a project implemented North of the Port of Harlingen. The aim of the project is to use fine-grained dredged disposal from the port to enhance deposition in salt marshes North of Harlingen. It was assumed that local tidal currents in the area should be able to transport the SPM to the local marshes. Although relatively high sedimentation rates were observed in the salt marsh itself the increase in suspended sediment supply did not yet result in vertical accretion of the tidal flats nor an expansion of the vegetated areas.
Another example of the important role of wind-driven (episodic) events in the Wadden Sea was deduced from studies about the (net) transport of SPM through tidal inlets and across tidal watersheds. Field measurements may be useful to identify dominant processes and conditions for transport of SPM but to determine net transport rates clearly long-term records with landers will be necessary. Even a modeling approach, simulating the transport of SPM over a period of 3 years, was inconclusive. The effect of wind-driven flow in combination with tides – and in particular for intertidal areas – was further discussed based on field measurements. Wind has a direct impact by enhancing or reducing the tidal flow velocities and patterns. Indirectly, local wind-generated waves will stir up sediment and increase the concentration of SPM in the water column. Based on the observations windows of opportunity are defined for the supply, deposition and consolidation of fine sediment.
The last presentation of the day focused on long term deposition rates in salt marshes. Close to the mainland dikes annual deposition rates can be as high as 15-20 mm/year, although there is an interannual variability. Within a year there is the seasonal variation due to winter deposition and summer compaction; more recently the marshes have also been affected by grazing cattle.