Background
Texel from the airOuter deltas or ebb tidal deltas are complex and highly dynamic morphological structures situated at the seaward side of tidal basins. They play an important role in the exchange of water and sediment between the tidal basin and the coastal zone. In turn the hydrodynamic conditions (waves, tides, etc.) determine the horizontal extent, shape and sand volume of the outer deltas. This interaction also results in the presence of channels and shoals, which migrate in the direction of mean currents along the barrier coasts. These bars and channels provide for the sediment bypassing over outer deltas and thereby affect the stability of barrier islands. For example, the Southwestern coast of Texel, the Netherlands, was consistently eroding at a rate of max. 15 m/yr. Constructing breakwaters in the area reduced this rate, but erosion still continued. In addition, through sand supply since 1990, the sand volume of the sub tidal area remains stable.
Objectives
Qualify and quantify the processes steering the morphodynamics of ebb tidal deltas on different space- and timescales.
Problems
To Quantify the morphodynamic processes in the ebb tidal delta of Texel, a qualitative research for different space- and timescales is necessary. Which morphological phenomena are typical for the different timescales? Are they related to specific processes? In addition, the behavior of the sediment dynamics in the area, is the feedback mechanism which determines the morphodynamics. What are the sediment transport modes, what are the transport paths and what are the sediment sources and sinks?
Questions
The following research questions are addressed:
- Which morphological phenomena are observed in the ebb tidal delta, what are their characteristics? What is the extend of their morphological changes (migration rates, changes in slope) and how does the sedimentation-erosion pattern look like?
- What are the characteristics of the hydrodynamics of the ebb tidal delta: velocity distribution in the channels of the ebb tidal delta and within a channel, tidal- and wave characteristics, secondary and residual currents? How do they vary over a spring-neap cycle or during a storm?
- How can the observed morphological phenomena be coupled to the observed hydrodynamics in the ebb tidal delta?
- What are the characteristics of the hydrodynamics on the channel-shoal transition: currents, tidal- and wave characteristics?
- What are the sediment fluxes, how can they be related to the local hydrodynamic conditions?
Methods
Instruments and methods are described in the table below.
| Scale | Instrument | Method | Result |
|---|---|---|---|
| 2 | Echosounder: “vaklodingen” |
Morphological analysis | Estimate the value of changes in the research area: slopes, bedforms, erosion-sedimentation patterns and rates, high- and lowdynamic areas, migrationrates of channel and shoals. |
| 2 | Discharges with an ADCP | Analysis of the current-patterns on the ebbtidal delta scale and channel scale | Estimate the value of the discharge through each channel of the ebb tidal delta and the tidal inlet, velocity distribution in the vertical and within a channel transect, ebb- or flood dominance for each channel, location of maximum current velocities within a tidal cycle |
| 3 | Multibeam and side scan sonar | Analysis of bedforms | Determine the relative current velocities and current directions |
| 3 | Tripods | Analysis of currents, waterheight displacements and (relative) sediment concentrations on the channel-shoal transition | Determine the wave- and current field under different conditions, like spring-neap tide, calm and stormy weather; coupling of sedimentfluxes to wave- and current signals; shoaling, refraction of currents and waves due to the channel-shoal transition. |
Fig. 1: Morphological time scales and measurement methods
This research is part of the Outer Delta Dynamics project, financed by NWO/ALW
More info can also be found on the Tidal Inlets website.
Please contact Sandra Vermeer for more info on this project.