What are extreme water levels as used in the report?

The 2022 Technical Report uses the term extreme water levels to refer to water levels experienced during a wide range of flooding events, from common events that happen ten times a year to rare events such as a flood with a 1% annual chance of occurring. The extreme water levels are used to assess current and future flood exposure within the coastal floodplain considering future sea level rise using NOAA’s height-severity categories of minor, moderate, and major high tide flooding.

In addition to long-term sea level rise, many different physical processes can affect coastal water levels on much shorter time scales, such as winds and storm surge, tides, and waves. Extreme water levels in this report are specifically those measured by NOAA tide gauges in mostly protected areas, and therefore reflect still water levels without direct wave influences. The extreme water levels generally relate to when coastal water levels exceed specific elevation thresholds related to local flooding hazards. These elevation thresholds are often reflective of flooding events ranging from bathtub-like “nuisance” flooding that is tidally driven to destructive storm-surge flooding whose impact footprint is specific to that storm.

The extreme water levels as defined in the 2022 Technical Report have probabilities, or likelihoods, of occurring in a given year that are based on statistical analysis of regional sets of historical tide gauge measurements, called a regional frequency analysis. Results based on regional frequency analysis typically suggest that higher water levels are more probable than results based upon a single tide gauge data record. This is because regional sets of data observation better capture spatially the overall probability of a high-water event occurring from a passing storm. The perspective from a single gauge can be rather limited due to storm track variability (storm missed the tide gauge) or short data records (important storms occurred prior to the gauge installation).

The regional frequency analysis extreme water level probabilities are different from those produced and used by FEMA, since FEMA-based estimates often include synthetic storm simulations (i.e., they consider storms that could happen under today’s climate but might not yet have happened) and high water marks not necessarily directly measured by a local tide gauge. The different methods produce different probabilities for low frequency flooding (e.g., a 100-year flood), but for more frequent events such as high water levels occurring every few years, the two sets of probabilities are quite similar.