Multifunctional Flood Defences

Abstract

Multifunctional use of the flood defences is as old as the flood defences themselves. Historically infrastructure, housing and livestock are located on the flood defences. Over the years, the cities along the rivers became bigger and bigger. With the still increasing demand for spatial development it is inevitable to also start combining other functions with the flood defences. The underground space of the flood defences is not yet used. Constructing a structure inside a dike is a solution to create more space. With multifunctional use of the flood defences are a lot of different parties involved with each its own interest and purposes. This results in a difficult and devious design process. The technical difficulties are researched in the master thesis in order to optimise this design process. The Dutch guidelines for flood defences as well as the Eurocodes are relevant for the design of multifunctional flood defences. Dealing with the hydraulic loads on the structure leads to uncertainties for the reliability of the flood defence. Applying partial factors on forces resulting from design water level with already a very small exceedance probability seems to introduce excessive amount of reliability into the calculations. A case study is performed to research the issues during the design process of a multifunctional flood defence. The Grebbedijk is chosen as location for the case study because of the demand for spatial development as well as the demand for flood safety. The Grebbedijk is a relatively small dike protecting a relatively large area with a high economic value. The Grebbedijk is often named as a possible first Delta dike. Together with the demand for parking close to the city centre of Wageningen, this location meets both criteria for a multifunctional flood defence. The assessment of the design made for the Grebbedijk confirmed the idea that the forces resulting from the water level are treated in a too conservative way by applying partial factors over design water levels with already small exceedance probabilities. This effect is obtained in the calculations for the failure mechanisms overall stability and structural strength. In order to analyse the reliability of the multifunctional flood defence, first order reliability method (FORM) analyses are carried out. Three failure mechanisms are considered: horizontal stability, overturning stability and strength of the wall. The failure probability of each failure mechanism is calculated. All three failure mechanisms have a lower failure probability than the required failure probability. The partial factors used in the semi probabilistic approach are calibrated in order to find the correct partial factors. The calibrated partial factors for the force related to the water level showed a variation equal to the proportionality of the force to the water level. The partial factors are in this case not useful. The exceedance probabilities of the design water levels resulting from the FORM analyses are very similar and in the same order as the failure probabilities. This resulted in the conclusion that using a design water level with an exceedance probability equal to the failure probability introduces enough reliability into the calculations. The partial factors for the forces not related to the water level were all very close to one. Resulting in the conclusions that using partial factors of 1.1 for unfavourable force and 0.9 for favourable force, would be sufficient to obtain the target failure probability. The applicability of these partial factors is not unlimited. The water level is the most important parameter in these calculations. Other forces might become more important in other cases. Further research has to be carried out to say something about the extent to which the partial factors are applicable.