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Deltares | R&D Highlights 2015
Extreme wind-field
evolution in time and space
The Dutch Water Act requires the periodical assessment of the strength of the Dutch primary water defences. In order to improve the accuracy of this assessment, Deltares and the KNMI (Royal Netherlands Meteorological Institute) launched a wind-modelling project in 2011 to determine wind strength above open water and to incorporate the dimensions “time” and “space” in the hydraulic boundary conditions used in the assessment.
The project calibrated and validated the high-resolution atmospheric model HARMONIE before using it to produce accurate wind velocities above open water for all Dutch water systems. One of the primary results of the project is a unique time series of wind fields covering the period from 1979 to 2013 with a time resolution of 1 hour on a grid with a 2.5 km resolution. The HARMONIE resolution is significantly finer than the resolutions of models used so far, enabling better modelling of strong gradients in atmospheric variables due to factors such as roughness variations at land-water transitions.
In parallel with this activity, the statistical Combined Max-Stable (CMS) lifting method was developed to determine extremes in evolution over space and time. Roughly speaking, the method uses time series with the variables of interest and location-specific distributions of extreme values in order to “lift” observed extreme events into yet unobserved and even more extreme events. The lifted events provide descriptions of what could happen in time and space in a future extreme event with a very small probability of occurrence.
The 1979-2013 dataset was used to determine directional distributions of extreme wind speeds on open water. These distributions complete our knowledge about winds on open
water since the distributions used at present are based on wind velocity measurements at land stations, which require a parametric, error-prone, transformation to open water. The CMS lifting method was then applied using the distributions of extreme wind speeds and the original fields to determine the evolution of extreme winds in space and time in all Dutch water systems. The extreme storm winds can be used as input for hydrodynamic and wave models to determine temporal and spatial variations in the hydraulic boundary conditions used to assess Dutch primary water defences.
As well as being useful in the assessment of primary water defences, the data and knowledge gained in this study can be applied more broadly. The lifting method developed is generic and can be used to lift other data, such as water levels and the associated hydraulic loads, elsewhere in the world. The HARMONIE data and lifting methods can be used to explore a number of storm scenarios in the Dutch waters, such as the consequences for safety of increasingly severe classical storms due to climate change. Furthermore, the extreme wind statistics and lifted winds can be used as standards for the assessment and design of Dutch offshore platforms, wind farms and piles, and for structures and floating bodies in lakes, rivers and estuaries. sofia.caires@deltares.nl T +31(0)6 4691 1205 jacco.groeneweg@deltares.nl T.+31(0)6 4691 1161 Further reading: Van Nieuwkoop et al (2015). Towards time- and space-evolving extreme wind fields, 36th IAHR World Congress, The Hague < Original (HARMONIE) and lifted time series showing wind speeds at a reference location
10,000-year return values for wind speed from western direction (arrow)
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< Snapshot of a lifted wind field
Woodcut from the book of W. Scheidig: Die Holzschnitte des Petrarca
