Quantifying uncertainties of SWAN results : investigation of SWIVT cases

The probabilistic approach for the determination of Hydraulic Boundary conditions (now called Hydraulic loads) uses database with the results from waves calculations with SWAN to translate wave conditions from a measuring location (offshore) to a location near the toe of the flood defence. At present, this database consists of only one specific (deterministic) transfer function per condition (e.g. water level, wind speed and wind direction), neglecting the uncertainty in this transfer function. In fact, the assumption is made that the transfer of the wave conditions by means of this database is exact, thus assuming a “perfect” wave model used to fill the database.

Starting form 2011, the probabilistic calculations used for the derivation of Hydraulic Boundary conditions can take uncertainties into account. This means that the uncertainties in the basic stochastic variables and the uncertainties in transfer functions can be included into the hydraulic boundary conditions, provided these uncertainties are available.

The objective of this study is to derive a first estimate of the uncertainties in the transfer function for wave conditions used in the probabilistic derivation of the hydraulic boundary conditions. The uncertainties associated with the application of a data base with transfer functions (e.g. the discrete character of this database) are not part of this study. By performing a large number of hindcasts with measurements, the relative errors in the predicted wave height and wave period are derived. These relative errors are subsequently analysed and a normal distribution is fitted to this data both for individual regions (water systems, e.g. estuaries and the sea) and all regions combined

The results indicate that there is not a strong tendency to deviate from the normal distribution. It is therefore recommended to use the normal distribution to define uncertainties in the transfer functions for wave height and wave period. Since the measurement data is limited, it is also recommended to base the fit of the normal distribution on the combination of all regions and not to differentiate between regions. Uncertainty in wave direction has not been investigated due to lack of appropriate measurements.

Beschrijft de modelonzekerheden voor golven van het SWAN model voor golfhoogte en golfperiode. Voor golfrichting waren er onvoldoende veldmetingen om deze exercitie uit te voeren. Dit rapport heeft een plek gekregen in het WBI2017 (het wettelijke beoordelingsinstrumentarium 2017 voor het beoordelen van de primaire waterkeringen in Nederland.