Tuesday, March 2, 2010
Is the intensifying wave climate of the U.S. Pacific Northwest of more concern than sea level rise?
Deep-water wave buoy data offshore from the U.S. Pacific Northwest (PNW, Oregon and Washington) document that the annual averages of deep-water significant wave heights (SWHs) have increased at a rate of approximately 0.015 m/yr since the mid-1970s, while averages of the five highest SWHs per year have increased at the appreciably greater rate of 0.071 m/yr. Histograms of the hourly measured SWHs more fully document this shift toward higher values over the decades, demonstrating that both the relatively low waves of the summer and the highest SWHs generated by winter storms have increased. Wave heights associated with higher percentiles in the SWH cumulative distribution function are shown to be increasing at progressively faster rates than those associated with lower percentiles. This property is demonstrated to be a direct result of the probability distributions for annual wave climates having lognormal- or Weibull-like forms in that a moderate increase in the mean SWH produces significantly greater increases in the tail of the distribution. Both the linear regressions of increasing annual averages and the evolving probability distribution of the SWH climate, demonstrating the non-stationarity of the Pacific Northwest wave climate, translate into substantial increases in extreme-value projections, important in coastal engineering design and in quantifying coastal hazards. Buoy data have been analyzed to assess this response in the wave climate by employing various time-dependent extreme value models that directly compute the progressive increases in the 25- to 100-year projections. The results depend somewhat on the assumptions made in the statistical procedures, on the numbers of storm-generated SWHs included, and on the threshold value for inclusion in the analyses, but the results are consistent with the linear regressions of annual averages and the observed shifts in the histograms.
The relative contributions of sea level rise and increasing extra-tropical storminess to the frequency with which waves attack coastal properties is assessed with a simple total water level model. We show that for the coast of the PNW over the period of wave-buoy observations (~30 years) wave height (and period) increases have had a more significant role in the increased frequency of coastal flooding and erosion than has the rise in sea level. Where tectonic-induced vertical land motions are significant, the impact of increasing wave heights has been two to three orders of magnitude more important than relative sea level change. While it is uncertain whether wave height increases will continue into the future at their present rates, it is clear that this process remains more important than or at least as important as sea level rise, and needs to be taken into account in terms of the increasing exposure to coastal hazards. These results confirm the need to incorporate climate-controlled processes in methodologies designed to assess the risks of enhanced coastal hazards to humans and infrastructure.