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Extreme water levels of the United States 1893-2010
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Extreme water levels of the United States 1893-2010
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    "Extreme monthly highest and lowest water levels at 112 long-term stations of the National Water Level Observation Network (NWLON) operated by the Center for Operational Oceanographic Products and Services (CO-OPS) are analyzed to quantify probabilities of exceedance and the return periods of extreme events. All data through 2010 are used from stations on the U.S. Atlantic and Pacific coasts, the Gulf of Mexico, Hawaii, Alaska, and islands in the Pacific and Atlantic Oceans. Since there are statistically significant trends in the data, the values are linearly detrended using the mean sea level trend (or the mean high or low water trend in a few cases) prior to analysis. A set of the annual maxima and annual minima are then derived for each station. The Generalized Extreme Value (GEV) approach was chosen to characterize the distribution of extreme values of a long-term water level time series. A GEV distribution is fitted to each station's annual maxima or minima by solving for the location, scale, and shape parameters. The solution defines an exceedance probability curve and its 95% confidence interval as a function of the return period, which is the average length of time between exceedances of a given extreme water level. The shapes of the derived GEV curves show large variations at the longer return periods depending on whether the GEV shape parameter is positive (the curve rises sharply) or negative (the curve flattens out). A positive shape parameter indicates a distribution with a higher probability of a rare event in the extreme tail of the distribution. A GEV curve with a positive shape parameter also has much wider 95% confidence intervals than one with a negative shape parameter. The stations with occasional large hurricane storm tides in the data generally have positive shape parameters. Stations on coasts that do not experience hurricanes generally have GEV curves which flatten out at longer return periods and have narrow 95% confidence intervals at the longer return periods. In the Appendices, the GEV exceedance levels for each station are displayed as a set of plots showing their relationship with the observed annual maxima or minima versus return period, with the tidal and geodetic datums as a stick diagram relative to mean sea level, and with the observed monthly highest and lowest water levels versus time. Extreme events which exceeded the 0.01 exceedance probability level, which can be considered as a 100-year event, are identified. The monthly data are also separated into sets of the highest and the lowest water levels for each month of the year. A GEV analysis is then performed on each of the 24 subsets to show how the likelihood of exceedance changes seasonally. Hurricane Sandy struck the coast of New Jersey in October 2012. Its peak water level at The Battery was over a meter higher than any previously recorded water level. Levels reached at Bridgeport and Sandy Hook (before the station was destroyed) were also far above previous records. As a result, the GEV exceedance probability levels for high water levels at these three stations were recalculated and the results are discussed in Appendix VIII"--Executive Summary.
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