KTA 2207 [23] defines the reference water level for coastal sites and sites on tidal flows as a storm tide water level with an exceedance probability of 10~4/a. This storm tide water level SFWH(10-4) can be obtained using suitable but highly laborious probabi­listic methods, which can also be used to determine flood runoffs (cf. [45]). Alterna­tively, according to the annexe to KTA 2207, a probabilistic based extrapolation method can be used, taking the storm tide water level SFWH(10-4) as the total of a base value BHW(10-2j and an extrapolation difference ED as follows:

SFWH(10-4} = BHW(10-2) + ED.

The design basis water level BHW(10 2) with an exceedance probability of 10~2/a is calculated here based on a quantitative statistical extreme value analysis. The spread of the results with the usually long, good-quality time series of water levels on the coasts and in tidal flows is relatively low.

Determining the extrapolation difference ED calls for detailed studies of the coastal or estuary levels of the tide flows concerned. At the water gauge sites of Cuxhaven and

Brokdorf on the river Elbe, for example, this gives an extrapolation difference ED of the order of 100-150 cm.

With dykes, as well as the storm tide water level SFWH(10-4) the wave run-up must also be taken into account (Figure 5.6) and, having superposed these two variables, the dykes must be designed without waves breaching them or a possible breaching wave putting the stability of the dyke at risk. The wave run-up height at the dyke depends not only on the wave height and wave period, but also on the characteristics of the dyke itself, such as its slope or surface area. When calculating wave heights, it should be borne in mind that these are particularly subject to local wind speed and direction and to the topography of the foreshore.