Introduction 
5 
1 Introduction 
In many marine harbours around the world, shipping restrictions apply due to shallow water depths. 
Exact knowledge and reliable predictions of meteorological and oceanographic conditions are 
indispensable to ensure safe navigation in pilotage waters. The physical factor most relevant in this re 
gard is the water level, which fluctuates periodically about a mean value due to tidal influence, and 
aperiodically mainly due to meteorological factors. The water depth available to a vessel is computed 
by addition of the current local water level referred to local chart datum and the water depth shown in 
the navigational chart, allowing for some under-keel clearance. Water level predictions for deep draught 
vessels must be very accurate in order to be able to define precisely the tidal window that is required, 
for example, to navigate a shoal area. Water level predictions for Cuxhaven are of extreme importance 
for the harbour of Hamburg, located 100 km upstream, and for navigation between these harbours. The 
tidal range at the Cuxhaven gauge station is 3 m; the time of tidal high water is 11 hours and 49 minutes 
after the moon's transit at Greenwich. Meteorological factors may cause water levels to rise by more 
than 3.5 m or to fall by more than 2.5 m. The influence of freshwater discharge from the upper reaches 
of the Elbe is just a few centimetres, even in case of extremely large discharge volumes (RUDOLPH, 
2005). Storm surges causing water levels to rise more than 1.5 m above mean high water (MHW) occur 
in less than 0.6% of all cases (period from 1959 to 2008). 
Deviations of tidal high or low water levels from the astronomically computed levels are called wind 
surge or wind set-up. The measured times of occurrence of high water and low water never correspond 
exactly to the astronomically predicted times. In this context, surge is defined as the difference between 
the astronomically computed high and low water levels and the relative extreme values of the measured 
water level curve. This value is also called “skew surge” (GERRITSEN et al., 1995). The numerical 
method uses an analogous approach; here, too, one model variation uses both tidal and meteorological 
forcing to simulate measured water levels, the other one is identical but excludes meteorological forcing. 
The skew surge value is determined for each grid point of the model area and is used as DMO in the 
BSH-MOS process. Surge values at Cuxhaven depend primarily on wind conditions in the central German 
Bight. 
Frequency distributions of wind speeds and wind directions measured in the area have shown that 
wind speeds from 4 to 10 m/s (8 - 20 kn) occur in more than 50% of all cases, and that the prevailing 
wind directions are SW - W. Simple wind surge graphs can be prepared (MULLER-NAVARRA and 
GIESE, 1999) using only wind speed and wind direction as input (Fig. 1). 
Fig. 1: Wind surge graph for Cuxhaven (onshore winds, after Müller-Navarra and GlESE, 1999)