Measured wind-wave climatology Lake IJssel (NL) + Errata

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Abstract

In the period 1997-2007, an extensive wind and wave measuring campaign has been carried out on Lake IJssel and Lake Sloten in The Netherlands. The aim of this campaign was to gather and analyse welldocumented wind and wave measurements of high quality, for a range of fetch, depth and (strong) wind conditions. The data should serve a number of purposes, and dike design in particular. The campaign has resulted in a good indication of wind and wave climatology of both lakes, while enhancing the knowledge on various aspects of wind and waves. However, the gap between measured conditions (up to 9 Beaufort inclusive) and dike design conditions (with 12 Beaufort winds) has only slightly decreased due to an exceptionally long storm-free period from 1990 to at least mid 2007. Hence, it is recommended to continue (part of) the present measurements until at least one event with 10 Beaufort winds is suitably measured. In the following, a brief overview of the contents of this report is given. Chapters 1 and 2 give some introductory details like relevance and aim of the present project, the measuring locations, instrumentation, data processing and data validation. Detailed overviews of experimental techniques and measuring errors are given in Appendix A-B. Appendix C shows that step gauge, capa probe and log-a-level instruments all (can) agree excellently, but that the latter is sensitive to wind from 6 Beaufort (12 m/s) winds on. In Chapter 3, the availability and range of the data is discussed. In the last 5 years, data availability during gales was excellent (Table 3.1). Some gales yielded wave periods that nearly equalled the (12 Beaufort) dike design values. However, water levels, wave heights and wave-run-up levels all remained well below the design values (section 3.3). Chapter 4 is about wind and temperatures. A key result is the fact that during gales, wind speed differences between land and water largely disappear; a feature that can not yet be explained by any of the existing models and theories (section 4.4; Appendix E). Chapter 5 is about water levels and its wind-induced set-up (storm surge). Rapid wind changes may also cause overshoots and oscillations up to a metre, larger than the stationary storm surge (section 5.3). Chapter 6 discusses wave climatology and several features relevant to wave modelling. Key uncertainties in the latter are related to the way waves scale with the wind and to depth limited wave growth (section 6.5-6.6). Without these shallow water effects, design wave heights at Lake IJssel would have been order 60% higher. Chapter 7 presents a number of test and calibration cases for wave models; time-dependent cases show that waves can grow very rapidly. Chapter 8 discusses the present data of wave run-up against dikes. The main result is that run-up reduction by berms, dike roughness etc. (typically 50-75%) is wave height dependent. Finally, conclusions and recommendations are given in Chapter 9.