Hydraulic model test of wave overtopping on crest drainage dike

The risk of dike failure along the coasts worldwide is continuing to increase due to theeffects of climatic changes such as sea level rise, severity of storms, etc. Hence, thereis an urgent need for more appropriate technical and managerial solutions to mitigatethis risk. Most research has focused on understanding the failure of the shorewardslope which was mainly induced by heavy wave overtopping. Principal mitigationmeasures undertaken up to now have led to increased dike heights. However, due tospace limitation and high costs, heightening is no more feasible for most of the seadikes along the North Sea.With framework of the European ComCoast project a Dutch company, DHV (2005),proposed a method to reduce wave overtopping without altering the cross section ofthe dike. This innovative idea was called 'Crest Drainage Dike' (CDD). main idea ofthis innovative crest design is to reduce the wave load and wave overtopping of a dikeby installing an ""Overtopping Buffer Basin"" on top of the dike crest where parts ofthe overtopping water is discharged to the shoreward side.The Crest Drainage Dike (CDD) was theoretically analysed by Nieuwenhuis et al(2005). However, the study was based on many assumptions taken from existingknowledge about standard dike profiles. Existing theories for wave overtoppingparameter are not applicable to a crest drainage dike, since the crest basin changes thebehaviour of the overtopping water on the dike crest. Therefore, the CDD wasproposed to be tested in hydraulic model tests to improve the understanding ofprocesses linked to the CDD and to propose overtopping formula for the design of theCDD.2D hydraulic model tests were conducted at Leichtweiss Institute (LWI) of theTechnical University Brauschweig, Germany. The research mainly focused to find outthe feasibility of using the crest drainage dike concept to enhance the safety of theexisting dikes. More specifically, research focused on (i) optimising the overtoppingbuffer basin for a 1:4 seaward slope, (ii) identifying the best drainage system,(iii) identifying undesired flow condition on the landward slope, and (iv) developingnew overtopping formulae for the crest drainage dike.During the experimental study, wave conditions in front of the structure, waveovertopping volumes per wave, flow depths and the flow velocities at the seawardslope, the crest and the landward slope of the dike, were performed. After analysing atotal of 275 tests a dimensionless coefficient, CDD ã was found to calculate thereduction in overtopping discharge. CDD ã can be used as a reduction factor whichincludes the overall effect of the crest basin on the mean overtopping discharge.Furthermore, suitable dimensions of the crest basin were proposed to reduce the meanwave overtopping discharge to the desired quantities. Apart from that, the influence ofthe crest basin on overtopping layer thicknesses and overtopping flow velocities onthe landward side of the dike were assessed