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Flood safety is determined through the combination of the loads on a flood defense and the strength of this flood defense. In this configuration, both components are considered. Up until now, it has been common practice to study both loading and strength separately. This new approach aims to establish a balanced approach of both aspects. Balanced, in the sense that an increase in knowledge and insight into one aspect is not overshadowed by the uncertainties of the other. The main issues in this approach is to name, and where possible quantify, the uncertainties in the two different parts.
Flood defense systems comprise of several parts, such as dunes, dikes and constructions. Eventually the safety of the system is determined by the weakest link. Therefore, not only a balanced approached to strength and loading components is necessary, but also knowledge relating the different types of flood defense systems needs to be brought to a comparable level.
The research within this approach is divided into sea defenses (dunes and sea dikes), river dikes and constructions. The research connects to the findings of the independent research ‘Safety against flooding’ (‘Veiligheid tegen overstroming’), conducted by the Environment and Nature Planning Agency, part of the Federal Institute for National Health and Environment (Milieu en Natuurplanbureau, RIVM) of which the prime publication ‘Dike risks – a thematic evaluation of the Dutch safety policy against flooding’, may 2004 (‘Risico’s in bedijkte termen – een thematische evaluatie van het Nederlandse veiligheidsbeleid tegen overstromingen’).
I Sea defences
For flood defenses along the coast, the determination of the effects of a sterner wave climate in combination with sea level rise on the safety against flooding is an important research issue. This is a topical theme, as during the last few years it has become clear that hydraulic conditions along the coast have been underestimated in the scheme of flood safety. Next to this, the influence of sea depth variations during storms, fluctuations over the seasons and more long term changes, are more prominent than previously assumed. The combination of a sterner wave climate and uncertainties in the influence of sea depth fuel the need for improved knowledge of hydraulic loading. It is expected that the hydraulic conditions will increase with respect to the current conditions for design and testing, which in turn will increase loading.
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Aforementioned alterations in hydraulic loading mean that its magnitude shall be larger than previously assumed. However, the flood defenses are also suspected to embody an extra strength that contributes to a higher level of safety (greater resistance). Given these two partly compensating aspects, it is not clear what the actual level of safety along the coast is. To this end, more insight into prevailing failing mechanisms is necessary.
In the case of dunes, this translates to a knowledge requirement with respect to the influence of larger wave periods on dune erosion. In the case of sea dikes, this translates amongst other things, to a knowledge requirement with respect to the processes that cause inner slope failure. (scour/slip mechanisms during constant or wave overtopping)
Integration of knowledge on loading and strength produces insight into the safety against flooding along the coast.
II River Dikes
Research on river dikes is aimed at reviewing assumptions that govern the current safety philosophy. These assumptions, which comprise of requirements, fitting parameters and test results date back to 1960’s research on the strength of secondary levees. A thorough revision of these assumptions and of geotechnical aspects in particular shall lead to a greater transparency of the design and testing processes for dikes.
In this respect, a first step has been Delft Cluster’s cooperation with Rijkswaterstaat (Water Municipality) on a large scale failure test on an old river dike near Bergambacht. This test has resulted in many insights that have been able to be drawn out in successive research and have thus contributed to a renewal of the current safety philosophy.
For an optimal benefit of this large scale test, an international review commission has cast its light over the research results. One of the more important findings of the commission is the recommendation to use more internationally common practices to test flood defense stability next to traditional ways.
The method is not immediately applicable to Dutch sea defenses. To comply with the wish to review the current safety philosophy, this work package shall do the following: draw out the international common practice, so that this is able to be applied next to the current practice for the evaluation of the stability of the Dutch water defenses.
III Constructions
Constructions are often critical links within flood defense systems. There is a multitude causes for the failure of constructions as a flood defense. (insufficient strength of the bearing construction, the water resisting elements, the foundations of transition constructions, insufficient height, late closure and undermining due to groundwater flow. It is current practice to make coarse schematizations, also in more advanced risk analyses, which limits the quality of the risk assessment.
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Another development in the same field and the field of measuring, i.e. testing the actual state of different elements such as under and hind- scour screens is also a necessity for a balanced system approach within a dike ring area and to make available on a more overall level. The research aims at the translation of the hydraulic conditions into loads on constructions and on the determination of the actual strength of the elements. Furthermore, specific attention shall be paid towards societal, spatial and technical allocation of constructions in city areas.
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