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UBC Theses and Dissertations

The direct joint probability method for estimating extreme sea levels Liu, Joan C. H.


The design of coastal structures includes the key element of estimating crest elevation. A crest height designed to protect against specified return periods avoids damages due to overflowing and overtopping. In order to avoid overflowing, the design sea levels should be at least at the design flood level, also referred to as the extreme flood level, which is usually composed of tides and storm surges. The extreme flood level can be determined by several approaches, such as the Annual Maxima, Simple Addition, Joint Probability, and Revised Joint Probability Methods. These methods have various limitations in terms of the required amount of data, the representation of contributing factors in sea level fluctuations, the ability to assess the joint probability of these factors, and the degree of independence required of the data. To minimize overtopping, in addition to considering tides and storm surges, the design sea levels should also include wave run-up. The design sea level, also referred to as the extreme sea level, includes the effects of tides, storm surges, and wave run-up. Wave run-up estimates are generally based on the design flood level and design wave climate, data for which are often dependent. This thesis develops the Direct Joint Probability Method for estimating extreme sea levels which simultaneously considers tides, storm surges, and wave run-up. This method has fewer limitations than the previously mentioned methods in terms of the assumption of independent variables and the required amount of data. Data for the City of Richmond, British Columbia, Canada, are used to demonstrate the Direct Joint Probability Method, and results show that the method provides a reasonable estimate of extreme sea levels, that is, the resulting estimates are within the same range as other traditionally applied methods. The results also indicate a large difference between design sea levels required for preventing overflowing and those for preventing overtopping. The sea levels at Richmond are also increasing due to the climatic and geologic effects. A hybrid of the Direct Joint Probability and the Simple Addition Methods is also applied in this thesis and is used to estimate extreme sea levels for regions facing long-term increases in sea levels. The results of the hybrid approach indicate that the contribution to extreme sea level due to wave run-up increases with long-term increases in sea levels. This can dramatically affect estimates of extreme sea levels.

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