|Morfodynamica van het Zeegat van Texel = Morphodynamics of Texel Inlet|
Elias, E.P.L. (2006). Morfodynamica van het Zeegat van Texel = Morphodynamics of Texel Inlet. PhD Thesis. Technische Universiteit Delft: Delft. ISBN 1-58603-676-9. xviii, 262 pp.
Data analysis; Dynamics; Sand transport; Tidal inlets; ANE, Netherlands, Texel [Marine Regions]; Marine
Texel inlet, the largest inlet in the Dutch Wadden Sea, has undergone drastic changes in the morphology of its back-barrier basin, ebb-tidal delta and adjacent coastlines after the closure of the Zuiderzee (1926-1932). As a result large sand losses were observed along the adjacent coastlines and still until today the maintenance of this part of the coast is the most intensive of the entire Dutch coast. The processes controlling the sediment exchange between the North-Holland and Texel coasts, the ebb-tidal delta, tidal inlet and back-barrier basin are insufficiently understood despite intensive monitoring and analysis. The research reported in this thesis aims to acquire more understanding of this sediment exchange and of the morphodynamics of Texel Inlet in general.
The morphodynamics of Texel Inlet are studied over a wide range of temporal and spatial scales ranging from long-term descriptions of historic inlet evolution, from 1550 A.D. to present, to detailed analyses of hydrodynamics and morphodynamics on a tidal and seasonal process-scale. Although, the focus is on Texel Inlet, where possible, findings are generalized to contribute to the understanding of generic tidal inlet dynamics.
To a varying degree of accuracy Texel Inlet's ebb-tidal delta bathymetry has been monitored over the last 400 years. This series of regular bathymetric observations is unique in the world and allows the description of the inlets long-term morphodynamic evolution. An evolution governed by an increasing impact of major engineering works such as seawall construction (Helderse Zeewering) and damming of a main part of the basin (closure of the Zuiderzee). The well-monitored changes in the ebb-tidal delta morphology show the cumulative impact of inlet modification and of anthropogenic interventions in the back-barrier basin. Different stages of ebb-delta evolution, each characterized by specific orientations of the main channels and shoals, can be discerned. Prior to construction of extensive coastal defence works on the southern shore of the inlet in 1750 A.D. (predecessors of what is now known as Helderse Zeewering) the ebb-tidal delta showed a downdrift asymmetry. Periodic shoal breaching and downdrift channel relocation were the dominant mechanisms for sediment bypassing (major shoal bypassing). After construction of the coastal defence works a stable ebb-tidal delta with a westward stretching main ebb-channel developed over a period of approximately 60 years. Damming of the Zuiderzee, separating the major part of the back-barrier basin and completed in 1932 A.D., distorted this stable state and over a period of about 40 years the main channel switched to a southward, updrift course, remaining in position ever since. During the pre- and postdamming stable states the sediment bypassing took place as minor shoal bypassing; the main channel remained in position and smaller parts of the swash platform (periodically) migrated landward over the ebb-tidal delta.
The well-monitored large-scale changes on the ebb-tidal delta, which were initiated by the construction of the coastal defence works and closure of the Zuiderzee, show that incorporation of inlet modifications and back-barrier processes is vital for a correct description of the ebb-tidal delta dynamics and processes. The expression ‘backbarrier steering’ is introduced to describe this 'forcing' induced by the basin.
Analysis of observations significantly contributes to an improved understanding of the inlet behaviour and evolution on higher levels of aggregation. However, a major shortcoming is the lack of comprehensive descriptions of the underlying physics; observed morphological changes and expert judgement form the principal source of information. Knowledge of the underlying physical processes, and their interaction with sediments and sediment bodies is important for understanding ebb-tidal delta behaviour. Due to the non-linear interaction between water motion (wind-, wave-, density- and tide-driven) and variable channel and shoal structures compound (residual) flow and transport patterns arise that show a wide range in temporal and spatial variation. Suitable field data that provide detailed descriptions of water, flow and sediment transport variations on the intra-tidal and intra-event scales with the necessary spatial and temporal detail over the inlet domain are scarce, if not absent. Even at Texel Inlet, one of the most frequently monitored inlets worldwide with highquality observational datasets of water levels, wind, waves, currents and discharges, bathymetry, bedforms and sediment characteristics present the spatial and temporal data coverage is still limited.
Fundamental understanding of inlet dynamics is obtained by mathematical modelling. Recent advances in process-based modelling techniques include the computation of sediment transport and bed level change fully integrated in the flow module; the Delft3D Online Morphology model. Herein morphologic changes are calculated simultaneously with the flow calculations. One of the major assets of this model is the capability to increase the spatial and temporal resolution of point-oriented field observations. Point-oriented observations are used to force the model quasi real-time ’as realistically as possible’ by measured time-series of wind, waves and tides, and the model results provide synoptic, near-realistic data of high spatial and temporal resolution over the inlet domain. Analysis of these data provide valuable information on governing flow and sediment transport patterns both in the instrumented and the un-instrumented areas of the domain, and make identification of the dominant processes and mechanisms for flow and transport possible.
Fundamental understanding of the post-closure inlet dynamics and evolution is obtained by integrating analysis of field and model data, and by formulating a conceptual model describing the post-closure morphological adjustment in different stages of development. Initially, during the adaptation stage following the engineering works the ebb-tidal delta dynamics cannot be described in terms of the natural hydrodynamic processes, such as the ratio of wave versus tidal energy, alone. The asymmetrical ebb-tidal delta development with southward directed main channels was forced by the changed hydrodynamics in the back-barrier basin; the altered tidal characteristics, the northward displacement of the basin centre, the closing of southward basin channels and the amplification of the tidal prism. In the basin major sedimentation was observed, viz. over 200 Mm3 of sediment was imported during a period of approximately 40 years.
Tides are identified as the main process for these first-stage developments. Due to the large tidal prism and the corresponding large tidal transports involved, the channels regained equilibrium at a faster rate than the shoal areas (e.g. the abandoned ebbshield Noorderhaaks), and the present-day ebb-tidal delta development is best described as a second-stage self-organizing process of sediment redistribution, sediment recirculation and sediment exchange to obtain a natural equilibrium state adapted to the changed configuration of the main-ebb channels. Sediment is eroded from the ebb-delta (including adjacent shorelines) and deposited in the basin. Largest erosion prevails on the western margin of Noorderhaaks were tides and waves are important for the landward displacement of sediments. Locally, sedimentation and erosion patterns are governed by channel-shoal interactions; the interaction of the channels Molengat and Noorderlijke Uitlopers of Noorderhaaks induces sediment loss of the Texel coastline, while the Nieuwe Schulpengat-Bollen van Kijkduin channelshoal- system determines the development of the adjacent North-Holland coast. The presence of large flood-dominant channels along the coast induces a major sediment loss towards the basin. The sediment import in the basin is estimated to range at 5 to 6 Mm3/year. A number of aspects contribute to this large influx; (1) sediment deficit in the basin caused by the loss of intertidal shoal areas due to closure of the Zuiderzee and relative sea-level rise, (2) availability of a vast amount of sediment in the abandoned ebb-delta front Noorderhaaks, and (3) transport capacity due to the large tidal prisms through the inlet.
The large ongoing sediment import into the basin shows that the effects of closure of the Zuiderzee are far from damped out, and it will take many decades before a new equilibrium will be reached.