Some Aspects Of Extreme Wave Prediction Related To Climatic Variations
Authors: Resio Donald T. 
Published: October 2008

Results from recent climate investigations indicate that significant climatic variations occur over periods of time which may be small compared to the design life of a structure. A procedure is developed for quantifying the effects such variations produce in the expected distribution of extreme wave heights. Wave height distribution from hindcasts indicate that this modification to the conventional treatment of extremes can lead to significantly different estimates for wave heights associated with a given recurrence interval.

INTRODUCTION

Prior to 1970, there was only minimal interest in the United States focused on climatic variability. Normal values of precipitation, temperature, wind, and other weather elements were based on a 20-yr record that was assumed, to represent the long-term mean climate. The time-scale of climate change was popularly believed to be on the order of 10,000's of years and related to advances and retreats of major ice sheets in the Pleistocene. However, during the last 20 years or so researchers investigating different aspects of climate change have begun to amass a large set of evidence which indicates that climatic variability can occur on time scales which significantly affect the activities of man. Since 1970, frequently occurrences of what might once .have been termed "abnormal" weather has led to a much more widespread acceptance of the importance of short-term climatic variability. Droughts in Africa, North America, and Europe, widespread flooding in the Mississippi Valley, the dearth of landfall Atlantic hurricanes, and the extreme cold of the winter of 1976-1977 in the eastern United States are but a few facets of the shifting patterns of world-wide weather in the 1970's.

In spite of the general acceptance of the existence of short-term climate fluctuations, practically all planning and design for offshore and onshore structures is done with no direct allowance for this variability, Indirectly, of course, it is expected that, by accumulating a sufficient record length, one can approximate the statistical population of wave heights and water levels, since various past climate states will be included in the sample. However, extrapolations based on data from mixed populations can lead to significant problems in the estimation of extremes (Borgman and Resio, 1977; Resio, 1975). Consequently, it is desirable to examine interrelationships between climate and wave extremes and to base long-term wave height estimates on some combined physical statistical model, if possible. This is complicated by uncertainty of climate forecasts and the increased complexity of the statistical analyses in such an approach. An alternative suggested by Goldman (1977) is to rely more heavily on deterministic estimates of wave heights from dynamical models; but this approach cannot be incorporated into an analysis of risk in a straight forward manner. It is therefore advantageous to follow through an analysis of tile relationship between extreme wave heights and climate fluctuations in spite of increased complexity and uncertainty.