Maritime Modelling

Numerical Sea Level Forecasts

Forecasts for storm surges at the coast of the Netherlands are made in close cooperation between Rijkswaterstaat, Deltares and KNMI, with the use of state-of-the art numerical models for the atmosphere and the sea.

The Continental Shelf Model is being used for day-to-day sea level forecasts by KNMI and Rijkswaterstaat since 1990. The model calculates the sea level and the depth averaged current on the Northwest European Continental Shelf using wind and pressure forecasts from KNMI's limited area model HIRLAM or ECMWF's global IFS. The current operational version, WAQUA-in-Simona/DCSM98, uses a grid with cells of approximately 8km x 8km. Until September 1999 the CSM-16 model was used with a resolution of 16km x 16km.
A new model is being developed which covers an extended area of the North Sea and North Atlantic Ocean with a resolution which ranges from 1.6km far at sea to a few hundred metres in the coastal zone.

Short-range forecasts

Deterministic sea level forecasts are produced by the model 4 times per day for 48 hours ahead, closely following the available meteorological input. A Kalman filter is used for real-time data assimilation of sea level observations.
Real-time assimilation of observed sea levels along the British and Dutch coasts gives a significant contribution to the high accuracy which is especially required for the management of the storm surge barriers in the Oosterschelde and the Rotterdam Waterway (Maeslantkering). Unlike in atmospheric modelling, however, it is not a vital part of sea level forecasting.

Medium-range forecasts

Probability forecasts for sea level are produces 2 times per day for 48 – 240 h ahead with forecasts from the ECMWF Ensemble Prediction System.
The ensemble of sea level forecasts is transformed into probability forecasts for high and low tides with a calibration which has been derived from model results in previous periods.

Model performance

Generally the model produces sea level forecasts with a standard deviation of less than 15 cm along big parts of the Dutch and British coasts. For forecasts less than 12 h ahead, the Kalman filter brings this even down to less than 10 cm.

It is difficult to quantify the performance of the model for extreme surges accurately, due to the fact that they are relatively rare. Every (severe) storm presents a new challenge for the models (and the meteorologists). Nevertheless, experience over the past few years has given confidence in the model results.

Contact information

dr. J.W. de Vries