Large-scale sand extraction on sand ridges offshore of the Netherlands. Inventory of instruments to predict physical effects of sand extraction on the Zeeland ridges

Hommes, Saskia (2004) Large-scale sand extraction on sand ridges offshore of the Netherlands. Inventory of instruments to predict physical effects of sand extraction on the Zeeland ridges.

Abstract:8 Large-scale sand extraction on sand ridges offshore of the Netherlands. This Master thesis is accomplished at the Rijksinstituut voor Kust en Zee (RIKZ) as part of KUST2005, which is a morphological research programme of Rijkswaterstaat (RWS). The management question of RWS (Directie Noordzee, DNZ) concerns the effects of large-scale (> 10x106 m3) sand extraction on the Zeeland ridges. DNZ wants to gain knowledge on the physical effects of sand extraction on the Zeeland ridges. Therefore, the objective of this study is to make an inventory of instruments that can be used to predict the long-term physical effects of large-scale sand extraction on the Zeeland ridges, in order to support the Dutch decision-making process. The Zeeland ridges is a group of ridges located in front of the coast of the Dutch provinces Zeeland and Zuid-Holland, seaward from the continuous -20 m NAP depth contour line. The characteristics of the Zeeland ridges are: 5-15 m height, 9-39 km length, transverse spacing 3-7 km, oriented 0-20 degrees clockwise with respect to the tide, (partly) covered with sand waves of 2-8 m height. The origin of the ridges is not clear. First, they were considered as drowned dunes, but others concluded that they were partly formed by erosion of older forms and partly by sand accumulation. Furthermore, there are no data that point to migration of the Zeeland ridges. At first sight, the Zeeland ridges seem to be tidal sand ridges. However, the ridges are oriented like shoreface-connected ridges, clockwise with respect to the tide. Except for the fact that the ridges are not connected to the shore. Therefore, no clear conclusion can be drawn on the classification of the Zeeland ridges. To achieve the objective as formulated, we determine the information need of decision-makers and other stakeholders, using the `Rugby ball method¿ from `Meetstrategie 2000+¿ of RWS. An important aid in this process is the definition of a set of Coastal State Indicators (CSI¿s); a quantitative concept of the actual state of the system as a basis for objective and transparent decision-making. Each indicator is related to a specific coastal user function. In this research, we restrict ourselves to the user functions: coastal safety and maintenance, offshore infrastructure and navigation. For each of the selected coastal user function, research questions, CSI¿s and if possible assessment criteria are formulated. The result of the Rugby ball is the inventory of instruments (objective Master thesis). We selected state-ofthe-art models: three analytical (non)-linear stability-type models (Twente model, Utrecht model, Amplitude-evolution model) and four numerical models (Delft3D, SUTRENCH, Telemac, mu-SEDIM) to include in this inventory. The current situation of these instruments on the modelling of sand extraction is investigated and the future possibilities of each model are estimated. The information on the models is gathered through interviews with model developers, literature study and organizing a workshop for model developers. Furthermore, with the Twente model and the Amplitude-evolution model some calculations are done. In the inventory of instruments it is indicated for which CSI¿s the instruments are applicable and with what reliability. From this inventory it was clear that a couple of instruments are capable to directly support the decision-making process on a few CSI¿s (but only on short-term effects) and on the remaining part of the CSI¿s qualitative and quantitative insight into a schematised situation is possible at this point. We conclude that the most important information missing are measurements on sand transport and knowledge on the long-term (morphological) evolution of the sea bottom. The Kwintebank on the Belgian Continental Shelf can form a good test case. From 1979 till 2003, sand was extracted on this ridge. During this period the Kwintebank was also monitored intensively. Since February 2003, there is a temporary extraction closure of three years, because two depressed areas on the ridge exceeded the permitted extraction depth. During the closure period intensive monitoring will take place, to evaluate the natural potential of restoration of the ridge. It would be worthwhile to propose an idealised configuration of the Kwintebank and its depressed areas. Then, we could run models on this idealised case and compare the results in between and the magnitude orders with the field observations.
Item Type:Essay (Master)
Rijksinstituut voor Kust en Zee (RIKZ) in The Hague
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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