Data Management Plan (Deprecated)

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.

Lineage Statement:
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.

Process Steps:

• Verified water level data from a water level station (the verified 6-minute data, and the secondary datasets of highs and lows (except for Great Lakes stations), hourly heights, daily means (Great Lakes stations only), and monthly means) are made available roughly one month after collection. This is a 2-step process where one person reviews the data, and if necessary, removes erroneous data or fills gaps by either using data from the station's back-up sensor, predictions for that station, or similar data from proximal stations. With exception of the corrected errors or filled gaps, most preliminary (raw) 6-minute data become the verified 6-minute data. Secondary datasets are also generated during this step. Then a second person reviews the work of the first, and when satisfied, marks the data in the database as "verified." The order in which the 300+ water level stations are processed each month is random, to prevent the same stations or geographical locations from always being first or last. Verified water level data are made available on a monthly basis for the previous month's data.