|Holocene evolution of the coast of Holland|In: Marine Geology. Elsevier: Amsterdam. ISSN 0025-3227; e-ISSN 1872-6151, meer
|Auteurs|| || Top |
- Beets, D.J.
- van der Valk, L.
- Stive, M.J.F.
The Holocene evolution of the coast of Holland was controlled by the complex interaction of such diverse parameters as wave and tidal climate, the rate of sea-level rise, and the morphology of the pre-transgressional surface. The latter, in combination with the rate of sea-level rise, mainly determines the location of sediments sources and sinks, while the hydrodynamic parameters mainly determine the rate and direction of sediment transport. In the early Holocene the low sea level in the shallow southern North Sea strongly affected wave climate and tidal regime. However, the barrier and back-barrier sedimentary record since 5000 14C yrs B.P. gives us no reason to assume major changes in these parameters since then, so we conclude that breaks in barrier development are due to variations in rate of sea-level rise inl combination with the morphology of the pre-transgressional surface.
The transformation from an open “tide-dominated” to a closed “wave-dominated” coast, which occurred in the Subboreal period, and the concomitant change in barrier movement from transgressive to regressive around 5000 yrs B.P. are the main events in the development of the coast of Holland up to the Middle Ages. The morphology of the pre-transgressional surface gives a shoreline in the late Atlantic which consists of two protruding headlands separated by a large tidal basin.
The southern headland is the alluvial plain of the Rhine and the Meuse, the northern headland constitutes the moraines of the Texel High. The tidal basin in between is connected to the North Sea by a large number of inlets. The rate of sea-level rise at that time (1 m/century) outran the supply of sediment to the tidal basin. After 6000 yrs B.P. the rate of sea-level rise decreased gradually, whereas the rate of sediment supply remained constant. This led to a gradual decrease in the tidal prisms of the inlets as the tidal basin was filled in with sediment. Shortly before 5000 yrs B.P. the first channels silted up and closed; the last tidal channel disappeared around 3300 yrs B.P., leaving two inlets along the coast which were both connected to the river Rhine. The closure of the first inlets occurred at the same time as the barrier began prograding because (1) the rate of sea-level rise diminished considerably, (2) not all the sand supplied by longshore and cross-shore transport disappeared into the tidal basin, but instead could be used for barrier progradation; and (3) ebb-tidal deltas of the closed inlets provided major sand sources.
The prograding barrier sequence enclosed between the two headlands roughly forms a closed system. Using the relationships between tidal prism, cross-sectional area of inlets and volume of the ebb-tidal delta, and the results of modelling of longshore transport along the Subboreal coast under present-day wave conditions, a simple sand budget for the coast of Holland is given which shows that a large part of the sand now stored in the barrier sequence was obtained by cross-shore transport from the North Sea.