Extreme Water Levels from CO-OPS

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. The probability that water levels will exceed a given elevation is based on a statistical analysis of historical extremely high and low water levels. This product provides annual exceedance probability levels for select Center for Operational Oceanographic Products and Services (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.

The extreme levels measured by the CO-OPS tide gauges during storms are called storm tides, which are the total observed seawater level during a storm, resulting from the combination of storm surge and the astronomical tide. They do not include wave runup, the movement of water up a slope. Therefore, the 1% annual exceedance probability levels 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 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.

This dataset provides statistical analysis showing the likelihood of water levels exceeding a given elevation for select NOAA stations, given the difficulty in recreating the same series from observed data.

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. Additionally, Section 3.2 of the "2022 Sea Level Rise Technical Report" discusses regional frequency analysis of tide gauge data.

The datasets that were analyzed consist of a series of verified monthly maximum and minimum water levels. The series were detrended before selecting annual maximum and minimum water levels. Additionally, while most of the results are based on verified data, there are situations where a station malfunctions during a storm, but post-storm visits allow to discern how high the water reached from a mud/debris line on site or at nearby buildings. If the debris lines are leveled to the station bench marks or to a known datum such as NAVD88, that was added to the instrument-observed levels as the maximum for that month. None of that information is captured in any of our other published products. An accurate exceedance probability analysis strongly depends on not missing the most extreme levels during the period of observations.

Sensors used at our non-Great Lakes stations are acoustic or microwave water level (MWWL) with pressure sensor (bubbler) as the back-up sensor. The type of sensor used depends on the station's environment. The lack of infrastructure may prevent the use of the desired sensor type at that location.

Latest detailed information on sensors and accuracy can be found at: https://tidesandcurrents.noaa.gov/publications/CO-OPS_Measurement_Spec.pdf

While most of the results are based on verified data, there are situations where a station malfunctions during a storm, but post-storm visits allow to discern how high the water reached from a mud/debris line on site or at nearby buildings. If the debris lines are leveled to the station bench marks or to a known datum such as NAVD88, that was added to the instrument-observed levels as the maximum for that month. None of that information is captured in any of our other published products. An accurate exceedance probability analysis strongly depends on not missing the most extreme levels during the period of observations.

None.

While most of the results are based on verified data, there are situations where a station malfunctions during a storm, but post-storm visits allow to discern how high the water reached from a mud/debris line on site or at nearby buildings. If the debris lines are leveled to the station bench marks or to a known datum such as NAVD88, that was added to the instrument-observed levels as the maximum for that month. None of that information is captured in any of our other published products. An accurate exceedance probability analysis strongly depends on not missing the most extreme levels during the period of observations.

Observed data are recorded and transmitted, then go through a quality control procedure and are loaded into a database. For Extreme Water Levels, typically the verified monthly and annual maxima/minima are used. Prior to the extreme analysis being completed, the long term sea level trend is removed so that maximum or minimum levels reached by an event in the past can be compared to an equivalent event today.

Observed data are recorded and transmitted, then go through a quality control procedure and are loaded into a database. For Extreme Water Levels, typically the verified monthly and annual maxima/minima are used. Prior to the extreme analysis being completed, the long term sea level trend is removed so that maximum or minimum levels reached by an event in the past can be compared to an equivalent event today.

While most of the results are based on verified data, there are situations where a station malfunctions during a storm, but post-storm visits allow to discern how high the water reached from a mud/debris line on site or at nearby buildings. If the debris lines are leveled to the station bench marks or to a known datum such as NAVD88, that was added to the instrument-observed levels as the maximum for that month. None of that information is captured in any of our other published products. An accurate exceedance probability analysis strongly depends on not missing the most extreme levels during the period of observations.

While most of the results are based on verified data, there are situations where a station malfunctions during a storm, but post-storm visits allow to discern how high the water reached from a mud/debris line on site or at nearby buildings. If the debris lines are leveled to the station bench marks or to a known datum such as NAVD88, that was added to the instrument-observed levels as the maximum for that month. None of that information is captured in any of our other published products. An accurate exceedance probability analysis strongly depends on not missing the most extreme levels during the period of observations.

Observed data are recorded and transmitted, then go through a quality control procedure and are loaded into a database. For Extreme Water Levels, typically the verified monthly and annual maxima/minima are used. Prior to the extreme analysis being completed, the long term sea level trend is removed so that maximum or minimum levels reached by an event in the past can be compared to an equivalent event today.