A Hindcast Study Of Wave Climate In The Northern North Sea

Abstract

Abstract Long-term wave statistics have been generated using a deep-water numerical wave model based on a parametric method first developed from the Joint North parametric method first developed from the Joint North sea wave Project (JONSWAP). The input to the model consists of wind data from a subset of severe storms in the northern North Sea over a period 1966–1976. The wave model was compared with wave measurements at two stations. Extreme value wave statistics have been derived using the statistics from the independent storm events during the hindcast period and taking into account possible secular changes in the wind field extremes. The results are compared with wave height estimates obtained in an independent study. Introduction In engineering studies concerned with the design and certification of offshore structures it is essential to have probability estimates of the most extreme wave conditions expected during the lifetime of the structure which is typically 20 years. Reliable estimates of extreme conditions require a time base of at least 10 years from which to extrapolate to return periods of 25 years or more. Measured wave data are available at only a few stations in the northern North Sea and furthermore the data are generally intermittent or only cover a few years (see Draper). The only method available at the present time for constructing wave statistics in the area being considered has therefore to be based on a wave hindcast approach using a data base of historical wind fields. Numerical hindcast procedures are being increasingly used in sea areas where the sparse nature of wave measurements, both in time and space, make it difficult to obtain reliable design wave information. For example, in the Gulf of Mexico hurricane wave conditions have been hindcast by Patterson, and by Cardone, Pierson and Ward. Resio and Vincents have used the formulation of Barnett's wave model to derive design wave information for the Great Lakes based on carefully estimated wind fields over the past two decades. A wave hindcast study is presently being carried out by the Norwegian Meteorological Institute (Haland and Smaland) for the Norwegian continental shelf area. The purpose of this study, the North Sea wave Model (NORSWAN) project, was to generate long-term wave statistics in the North Sea and adjacent areas using hindcast wave data from severe storms occuring during the period 1966 to early 1976. The 42 storms chosen for the hindcast study were representative of all gales occurring during the period under consideration so that a representative statistical base was available for extreme value analysis. A deep water hybrid parametrical wave model (Gunther et al.) was chosen to parametrical wave model (Gunther et al.) was chosen to hindcast the wave data the wave model was checked against wave measurements at two stations near the British Isles. Finally long-term wave statistics were estimated by a storm-model approach taking into account possible changes in the wind field extremes over the last one hundred years. Details of the NORSWAM project together with the derivation of extreme value wave height estimates have been given by Ewing, Weare and Worthington. WIND FIELD SPECIFICATION A most important part of all wave hindcast studies is that concerned with the accurate specification of the wind fields over the ocean. A full description of the way the wind fields were analysed has already been given by Harding and Binding. However, as this aspect of the study is of central importance in the accuracy of the hindcast results and in the estimation of Long-term extremes, a brief description of this work will be given here. The Daily Weather Reports of the British Meteorological Office were examined for the period 1966 to early 1976 for the occurrences of gales in the northern North Sea. (Synoptic weather charts are available for periods earlier than 1966 but the data were considered too limited for the accurate reconstruction of wind fields). P. 87