Extreme Water Levels

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Extremely high or low water levels at coastal locations are an important public concern and a factor in coastal hazard assessment, navigational safety, and ecosystem management. Exceedance probability, the likelihood that water levels will exceed a given elevation, is based on a statistical analysis of historic values. This product provides annual and monthly exceedance probability levels for select CO-OPS water level stations with at least 30 years of data. When used in conjunction with real time station data, exceedance probability levels can be used to evaluate current conditions and determine whether a rare event is occurring. This information may also be instrumental in planning for the possibility of dangerously high or low water events at a local level. Because these levels are station specific, their use for evaluating surrounding areas may be limited. A NOAA Technical Report, "Extreme Water Levels of the United States 1893-2010" describes the methods and data used in the calculation of the exceedance probability levels.

The extreme levels measured by the CO-OPS tide gauges during storms are called storm tides, which are a combination of the astronomical tide, the storm surge, and limited wave setup caused by breaking waves. They do not include wave runup, the movement of water up a slope. Therefore, the 1% annual exceedance probability levels shown on this website do not necessarily correspond to the --> Base Flood Elevations (BFE) defined by the Federal Emergency Management Administration (FEMA), which are the basis for the National Flood Insurance Program. The 1% annual exceedance probability levels on this website more closely correspond to FEMA's Still Water Flood Elevations (SWEL). The peak levels from tsunamis, which can cause high-frequency fluctuations at some locations, have not been included in this statistical analysis due to their infrequency during the periods of historic record.

September, 2013 -- In late October 2012, Hurricane Sandy struck the coast of New Jersey. The storm's large area, direction of motion relative to the coastline, strong winds, and low barometric pressure caused a large storm surge along the Mid-Atlantic coast from Cape Cod to Cape Hatteras. At most long-term NWLON stations, the peak water levels were high, but not the historically highest levels, with the exception of two stations in New York Harbor (The Battery and Sandy Hook) and at Bridgeport on Long Island Sound. Because these new maxima are so far above the previous maxima, they cause a significant change in the shapes of calculated exceedance probability curves especially at the longer return periods. Therefore, the high water exceedance probability levels for these three stations have been recalculated with the inclusion of the highest levels for 2011 and 2012, which were the peak levels reached by Hurricane Irene and Hurricane Sandy at all three stations. As a result, the 1% annual exceedance probability level was raised by 0.22 meters at The Battery, 0.34 meters at Sandy Hook, and 0.37 meters at Bridgeport.

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