InJanuary 1991 GeoSea produced its first repon for the Rijksinstituut voor Kust en Zee, Haren, which presented the results ofa Sediment Trend Analysis (STA TM) for the Eems/Dollard Estuary. Ah STA is a technique pioneered by GeoSea which uses the relative changes in the grain-size distributions ofbottom sediments to develop pattems of net sediment transport. In addition, the technique defnes the dynamic behaviour ofbottom sediments with respect to erosion, deposition and dynamic equilibrium. The data base is comprised of the complete grain-size distributions from sediment grab samples that ate collected over a predetermined grid which is typically at a 500 m spacing.Since the completion of the Eems/Dollard study (680 samples), an STA has been undertaken more or less annually in each of the Dutch Waddenzee tidal basins: Vliestroom (1,500 samples) 1992; Marsdiep (2,000 samples) 1994; Friesche Zeegat (850 samples) 1995; Borndiep (1,200 samples) 1996 and the Lauwers/Schild (1,100 samples) completed in 1997. Two smaller projects were also undertaken. One looked in detail at the Dollard (1992) and another at the NOURTEC beach nourishment site on Terschelling as part of the Bomdiep project (1995). The last time a grain-size data set on a regional scale was collected took place during the 1950's and covered only the intertidal areas.The STA for each tidal basin produced broadly similar results. Offshore ti'oro the barrier islands, sediment transport parallels the shoreline in a north-east and east direction. At each tidal inlet this regime is intermpted by the dynamics associated with its corresponding tidal basin. Over the ebb-tidal deltas there are typically complex transport gyres that originate in the deepest portion of the channel separating the barrier islands. Inside the tidal basins transport is generally landward fi'om the deeps and the sediment trends follow the dendritic network ofchannels with net accretion over the extensive intertidal flats. Fine sediments, often showing bimodal distributions, identified the wantijs of tidal null points that separate the tidal basins from each other.The distributions of the sediments and the interpretative findings of the STA's may be used for many coastal management considerations such as: (i) the best locations for dredged material disposal sites; (ii) optimum channel alignment to minimise dredging; (iii) understanding the effects ofcoastal stmctures; and (iv) determining the dispersal ofcontaminants contained in the sediments. In addition, the Rijksinstituut voor Kust en Zee use the textural data for correlation with numerical models and for generating GIS-derived bottom sediment maps.

This paper deals with the methods and results of monitoring sea level (tidal gauges) and with the analysis of depth sounding data. Possible future monitoring by means of remote sensing techniques will be presented. Some trends, based on water level and morphological monitoring, have been established in the western part of the Dutch Wadden sea: (a) the relative mean sea level is rising persistently by 15 cm/century, observed since the middle of the 19th century. The tidal range is increasing as well; (b) the cross-sectional areas of the most western Dutch Wadden tidal inlets have been increasing over the last two centuries; (c) the intertidal zones of the Texelstroom and Vliestroom tidal basins have shown an increase of surface area over the last decades. The Borndiep tidal basin shows a decrease of 4 % of the area shallower than 5 m -Dutch Ordinance Level (DOL) over the last two centuries. Several factors prevent serious predictions about future morphological developments of the tidal Wadden flats: ~a) man induced interfering factors, e,g, the construction of the harbour revetments of Den Helder, the construction of the Enclosure Dike and construction of sand drift dikes stabilising the Wadden islands; (b) the limited scope of the present analysis that deals only with the western part of the Dutch Wadden Sea.

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The rewatering location in the Eastern Scheldt at Yerseke has an important function for the Dutch mussel culture, safeguarding the keeping quality of the mussels. The mussels are relaid during 2–4 weeks on rewatering plots between production and marketing. It was feared that, after completion of the Storm-Surge Barrier in the Eastern Scheldt, the rewatering function would deteriorate as a consequence of accumulation of mussel-deposited mud (pseudo-faeces) and a decline of water quality during periods of low current velocity. A research program was drafted in order to make a prognostication of sedimentation and erosion and of the occurrence of adverse water quality and food shortage on the plots after the completion of the Storm-Surge Barrier. A study was made of erosion factors such as tidal currents, wave action and periodical dredging of mussels on the plots. Besides, food content and water quality parameters were measured. The measurements were carried out during 13-hours periods at half-hourly intervals. Periodic dredging of mussels on the plots appeared to be the most important among the erosion forces, while tidal currents and wave action have less impact. Sedimentation is almost entirely caused by the filtering activity and deposition of faeces by the mussels. It is expected that after the completion of the Storm-Surge Barrier the erosion forces will decrease slightly. Whether the erosion and sedimentation balance on the rewatering location, presently more or less in equilibrium, will change, will mainly depend on the future rate of faeces production. This, however, is difficult to predict.