A better way to view storm tides in Chesapeake Bay

Researchers at the Virginia Institute of Marine Science have added forecast capabilities to their network of Tidewatch water-level stations, giving residents of the lower Chesapeake Bay region a new on-line tool for gauging the magnitude of coastal flooding in a given location and minimizing its potential impacts.

The public launch of the Tidewatch forecast system (vims.edu/tidewatch) comes just in time for the height of the Atlantic hurricane season, which typically peaks on September 10. A string of hurricanes and strong nor’easters during recent years, coupled with rising sea level, have brought several episodes of significant flooding to Tidewater Virginia and other areas of the Bay.

Tidewatch is a system developed by VIMS emeritus professor John Boon that measures and forecasts the difference between the predicted astronomical tide and observed water levels at 10 locations within Chesapeake Bay and along Virginia’s seaside Eastern Shore. This difference—the residual—represents non-tidal, weather-induced change in water level including storm surge.

Tidewatch charts provide an effective way to measure, visualize, and predict the magnitude and impacts of coastal flooding in a given location. The use of Tidewatch charts can help forecasters and coastal residents better prepare for storm tides and minimize their potential impacts. They are particularly useful for comparing storm tides in areas with different tidal ranges.

Tidewatch Stations

Tidewatch operates at 10 stations. Access observations and forecasts for individual stations using the links or Google Map below, or view a single graph that compares water-level forecasts among all the stations.

The Tidewatch system now generates 36-hour public forecasts for 9 water-level stations within Chesapeake Bay and a single station on Virginia’s seaside Eastern Shore. The forecasts, updated every half hour, were previously only available on an experimental basis.
Testing and Refinement
Boon and his colleagues at VIMS—associate professor John Brubaker and assistant research scientist David Forrest—decided to make the forecasts available to the public after testing and refinements increased their confidence in the forecasts’ accuracy.

“We worked with Virginia Sea Grant, the National Weather Service, emergency managers, and selected waterfront property owners to test and refine the system over the last three years,” says Boon. “Now we think it’s good to go.”
Their testing included performance checks in real-time during Hurricane Irene, tropical storm Ernesto, and the November 2009 nor’easter, as well as “hindcasts” made by feeding data from 2003—the year of Hurricane Isabel—into their prediction model. The average expected error for the Tidewatch forecasts is now about 3.5 inches at 36 hours forward in time, and about 2.5 inches at 12 hours. However, during a tropical storm or hurricane the error can be several times as large due to rapidly changing wind speed and direction. The potential for large errors is offset by half-hourly updates made with the latest observed water levels. 

"Extratidal" Water
Tidewatch is also unique in referencing the height of storm tides to the “highest astronomic tide.” This value, abbreviated in Tidewatch as “HAT83,” is the highest predicted tidal level in the current version of the tide tables for a given location. The “83” refers to 1983, the initial year of the 19-year interval (1983-2001) that NOAA currently uses as a baseline for its official tide tables. (This baseline, the “National Tidal Datum Epoch,” is periodically updated as required to account for the ongoing change in sea level observed at U.S. tide stations.)
Comparing extreme water levels to HAT, says Boon, best indicates how storm tides will affect coastal residents, as it provides a natural benchmark that people have used to site waterfront structures such as docks, boathouses, and dwellings.
"Points just above HAT fall within a zone that appears dry most of the time,” says Boon. “Coastal residents therefore tend to use that land, adding infrastructure that enhances its monetary value and encourages its use. A Tidewatch forecast that calls for water levels to exceed the HAT line—what we call ‘extratidal’ water—is thus of special concern, as those conditions are likely to affect vulnerable property.”
Boon says another benefit of referencing storm tides to HAT is that doing so removes the effect of tidal range—an independent factor that varies from place to place. “Only extratidal high water is a true index of flooding severity at two locations with different tidal ranges,” he says.
Boon will explore the public launch of the Tidewatch forecast system during VIMS’ After Hours Lecture on Thursday, August 23rd. The lecture begins at 7:00 pm and takes place in Watermen’s Hall on the VIMS campus in Gloucester Point. Visit https://events.wm.edu/event/view/vims/9580 to register online.