@Jungbl I reply myself. I found it: It is the maximum force of gusts within the next 10 days.

What exactly it wind accumulation?

Here at the germany authority BSH (Bundesamt für Seeschifffahrt und Hydrographie) you will find all the stations for which one can obtain tide Tables for the german north-sea-coast:
https://www.bsh.de/DE/DATEN/Gezeiten/gezeiten_node.html

The level of the water depends not only on the tides but also on the wind. So here also at BSH you can look for a prognosis of the water level with the parameters of tide and Wind. And this prognosis is for 24 hours, for 0-3 days and for 3-6 days. In this example for Schulau on the river Elbe, where a huge marina is located.