,Wave overtopping discharges at coastal structures are well described in the EurOtop Manual (2007), including the distribution of overtopping wave volumes. Each volume that overtops a dike or levee will have a certain flow velocity and depth record in time, often given by the maximum velocity and flow depth. This paper describes some further development of the theory on flow depth and velocities on the crest, but will also show an inconsistency with respect to the mass balance. The second part of the paper gives an analysis of measured values on real dikes, simulated by the Wave Overtopping Simulator. It gives also the method of "cumulative hydraulic load" to compare overtopping discharges for different wave conditions. A large wave height with less overtopping waves, but larger overtopping wave volumes, is more damaging than a small wave height with more, but smaller overtopping volumes, even if the overtopping discharge is similar. The reasons to develop the cumulative hydraulic load have been compared with the recently in the US developed method of erosional equivalence.

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One of the main objectives within the EU-project CLASH (www.clash-eu.org) was to create a generic prediction method for wave overtopping at coastal structures by means of the Neural Network technique. An extensive and homogeneous database on wave overtopping was set up within CLASH, mainly with the aim to be used for the training process of the Neural Network (NN). A total number of 10,532 tests from 163 independent test series were screened and included in the database. The final database consists of far more information than needed for the training of the NN: 31 parameters are included to describe each overtopping test of which only 17 are used for the NN development. This explains the possible use of the overtopping database on its own. Plotting various parameters of the database together in graphs gives a clear view on the contents of the database. Also the ranges covered by the parameters can be detected in this way. The creation of the database, the analysis of the database, and the possible use of the database on its own are described in this paper.

In 2009 tests have been performed at the Afsluitdijk, the closure dike of Lake IJssel in the Netherlands. This dike has a sand core covered by layers of clay and boulder clay, and a grass cover. In 2010 succeeding tests were performed at a river dike near Zwolle. This dike was a sand dike with a grass cover. In all of these tests research was performed on the behaviour of the grass cover on the landward slope of the dikes. Furthermore, the influence on erodibility of the grass cover caused by damage of the grass cover by mice, moles or other influences like tractor tracks were investigated. Also the influence of obstacles or other elements, which may be present at dikes, was part of the investigations (tree, stairs, fence, maintenance road). It was observed that all of these influences are to some extent contributing to a decrease of the resistance against failure.