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Coastal policy in the Netherlands is characterised by three scale levels. The smallest scale is aimed at the preservation of safety agaisnt flooding by maintaining a minimum dune strength; the middle- and large scales at preservation of sustainable safety and of functions in the coastal zone by maintaining the coast line, respectively the sand volume in the coastal foundation. The definition of three scales basically implies a pro-active approach, based on the idea that the larger scale sets boundary conditions for the smaller scale. This pro-active approach appears to be succesful: nourishments since 1990 not only have resulted in maintaining the coast line, but also in an extension of the dunes, contributing to an increased safety against flooding. The succes has been achieved by a yearly nourishment volume of 12 Mm3. Recent studies have indicated the need to upscale the yearly averaged nourishment volume. A recent update of the sediment balance of the coastal foundation shows a negative total of ca. 20 Mm3 per year; so apparently 12 Mm3 nourishments per year is insufficient to maintain the total active sand volume of the system. Besides, in a study on future adaptation options to climate change in the Netherlands, the authoritive Deltacommissie (2008) even recommends a raise of nourishment budgets up to 85 Mm3/year until the year 2050, taking into account an ultimate worst case scenario of a sea level rise of 13 cm in 2100. To investigate innovative ways of nourishing larger volumes of sand, a Pilot project Sand Engine has been developed. The pilot implies a mega-nourishment of approximately 20 Mm3 (~ 10000 m3/m) on the coast of Holland, between Hook of Holland and The Hague. In the design phase of the pilot, a morphodynamic Delft3d model has been used to calculate morphological effects of several alternatives. The Environmental Impact Assessment EIA has decided on a shore attached cross shore bar with a length of ca. 1.5 km, as the preferent alternative. Implementation is planned for 2011, starting december 2010.

Position paper of the Netherlands Center for Coastal Research

Background, Aim and Scope: Coastal and river plains are the surfaces of depositional systems, to which sediment input is a parameter of key-importance. Their habitation and economic development usually requires protection with dikes, quays, etc., which are effective in retaining floods but have the side effect of retarding sedimentation in their hinterlands. The flood-protected Dutch lowlands (so-called dike-ring areas) have been sediment-starved for up to about a millennium. In addition to this, peat decomposition and soil compaction, brought about by land drainage, have caused significant land subsidence. Sediment deficiency, defined as the combined effect of sediment-starvation and drainage-induced volume losses, has already been substantial in this area, and it is expected to become urgent in view of the forecasted effects of climate change (sea-level rise, intensified precipitation and run-off). We therefore explore this deficiency, compare it with natural (Holocene) and current human sediment inputs, and discuss it in terms of long-term land-use options. Materials and Methods: We use available 3D geological models to define natural sediment inputs to our study area. Recent progress in large-scale modelling of peat oxidation and compaction enables us to address volume loss associated with these processes. Human sediment inputs are based on published minerals statistics. All results are given as first-order approximations. Results: The current sediment deficit in the diked lowlands of the Netherlands is estimated at 136 ± 67 million m3/a. About 85% of this volume is the hypothetical amount of sediment required to keep up with sea-level rise, and 15% is the effect of land drainage (peat decomposition and compaction). The average Holocene sediment input to our study area (based on a total of 145 km3) is ~14 million m3/a, and the maximum (millennium-averaged) input ~26 million m3/a. Historical sediment deficiency has resulted in an unused sediment accommodation space of about 13.3 km3. Net human input of sediment material currently amounts to ~23 million m3/a. Discussion: As sedimentary processes in the Dutch lowlands have been retarded, the depositional system's natural resilience to sea-level rise is low, and all that is left to cope is human countermeasure. Preserving some sort of status quo with water management solutions may reach its limits in the foreseeable future. The most viable long-term solutions therefore seem a combination of allowing for more water in open country (anything from flood-buffer zones to open water) and raising lands that are to be built up (enabling their lasting protection). As to the latter, doubling or tripling the use of filling sand in a planned and sustained effort may resolve up to one half of the Dutch sediment deficiency problems in about a century. Conclusions, Recommendations and Perspectives: We conclude that sediment deficiency – past, present and future – challenges the sustainable habitation of the Dutch lowlands. In order to explore possible solutions, we recommend the development of long-term scenarios for the changing lowland physiography, that include the effects of Global Change, compensation measures, costs and benefits, and the implications for long-term land-use options. Recommendations and Perspectives: We conclude that sediment deficiency – past, present and future – challenges the sustainable habitation of the Dutch lowlands. In order to explore possible solutions, we recommend the development of long-term scenarios for the changing lowland physiography, that include the effects of Global Change, compensation measures, costs and benefits, and the implications for long-term land-use options.