Deltares - R&D Highlights 2009

Contact: frans.hamer@deltares.nl +31 (0)88 335 8093 Strength and Loading of Flood Defences Almost 3,500 kilometres of Dutch dunes and dikes along the coast, estuaries and rivers have to be assessed every fi ve years to check whether they meet statutory safety standards. The assessments clarify which fl ood defence structures require improvements. The social, political and economic interests are enormous. Extreme storm simulated by a waveovertopping simulator Dikes and dunes must be able to withstand extreme water levels and wave conditions. Compulsory safety assessments started in 1996 but they have not yet been completely successful. Predicting extreme conditions still involves too many uncertainties, while some specifi c dike failure mechanisms cannot be described accurately enough. The Strength and Loading of Flood Defences Programme involves the research needed to fi ll in the knowledge gaps and to help improve the safety assessments. Universities, institutes and consultants worldwide are involved in the programme. The research projects in the programme focus on both fl ood defence loads and strength. The Ministry of Transport, Public Works and Water Management conducted an extensive campaign measuring waves and currents. Improvements to the model for Wave measuring buoys and measurement posts in the complex geometry of the Wadden Sea 38

Field test set up at the Afsluitdijk wave prediction were successfully validated with the measured data (hindcasts). We can now predict extreme wave conditions more reliably, even for the complex geography of the Wadden Sea in the north of the Netherlands. To determine the strength of fl ood defence structures, possible failure mechanisms need to be identifi ed. First of all, of course, dikes need to be high enough to prevent too much wave overtopping. Wave overtopping can lead to the erosion of the inner slope of the dike, and possibly to the collapse of the entire structure. Extensive studies of the resistance of grass revetments subjected to wave overtopping and full size fi eld tests at several dike locations in the Netherlands produced impressive results. The grass revetments appeared to be much more resistant to erosion by wave overtopping than was previously believed. However, transition zones or construction elements in the dike proved to be weak spots. Other projects in the Strength and Loading of Flood Defences Programme also helped to reduce uncertainties in dike assessment. Piping studies are described in a separate highlight in this report. A guide for the schematisation of the subsoil based on soil investigations will help work to become more consistent and uniform. Progress has been made on producing better estimates of the macro-stability of fl ood defence structures. Instruments are being improved to assess dune erosion properly. Funding The Strength and Loading of Flood Defences Programme (Dutch title: “Sterkte & Belastingen Waterkeringen”) has been assigned by the Ministry of Transport, Public Works and Water Management in The Netherlands 39

frans.hamer@deltares.nl,
www.publitas.com, www.publitas.nl
Deltares - R&D Highlights 2009 main