Inundation is the amount of water that occurs above normally dry ground as a result of flooding. Along the coast, there are a few common sources of inundation including abnormally high tides, storm surge, persistent onshore winds and waves. In rivers and tidal estuaries, runoff from excessive rainfall can provide another source of inundation. The combination of all of these potential factors makes up the total water level.
Inundation is most commonly referenced to the average daily highest tide, or Mean Higher High Water (MHHW) tidal datum. Inundation typically begins when water levels reach above this level. In most locations across the U.S., a relationship exists between tidal datums and the land elevation.
Coastal Inundation Dashboard provides real-time water levels, 48-hour forecasts of water levels and historic flooding information at a majority of coastal water level stations operated by the National Ocean Service (NOS) Center for Operational Oceanographic Products & Services (CO-OPS). The product features both a map based view where users can see which stations across the U.S. may be flooding, and a more detailed station view where real-time water levels and historical data for a specific location are highlighted. Emergency managers and other coastal decision makers can use this information to understand and prepare for the impacts of coastal flooding. The coastal planning community can use this information to gain a better understanding of past peak water level events and the increased frequency of days when flooding is possible as sea levels change.
Most stations in Alaska and all Great Lakes stations are not yet available in this product, however CO-OPS is working with partner agencies on establishing relationships between measured water levels and flooding in order to provide the same level of information for all locations in the near future. By default, water levels are displayed relative to Mean Higher High Water (MHHW), the average highest tide in a day, but can be adjusted to other Tidal Datums as needed.
When the National Weather Service issues a tropical storm or hurricane watch or warning for the U.S. or its territories, CO-OPS will begin to monitor water levels in the path of the storm. This will be available as a customized view on the Coastal Inundation Dashboard map that will show the latest tropical cyclone forecast track and intensity information, active storm surge watches or warnings, and coastal flood watches, warnings or advisories issued by NWS. A summary of coastal water level conditions will be updated three times a day. Tropical cyclones and other strong coastal storms, such as nor’easters, can elevate water levels by several feet or more above normal tide levels, which can cause flooding along the coast.
In most cases, the threshold at which these impacts may occur is defined by local National Weather Service (NWS) forecast offices. In some instances, this threshold is derived from an empirical relationship between known flood elevations and tidal range outlined in NOAA Technical Report NOS CO-OPS 086 - Patterns and Projections of High Tide Flooding.