Rising sea level increases hurricane risk

The hurricane of 1933, widely known as the "storm of the century" for Chesapeake Bay, generated a storm surge in Hampton Roads of 5.84 feet, more than a foot higher than the 4.76 ft storm surge recorded for Hurricane Isabel. Yet many long-time Tidewater residents say that the high-water marks left by Isabel equaled or exceeded those of the 1933 storm.

A new analysis of sea-level records by VIMS emeritus professor Dr. John Boon helps explain the apparent discrepancy and suggests that future storms will pose even greater flood risks. Boon also argues that changing the way storm tides are referenced could help coastal residents understand and minimize potential impacts.

Boon's analysis shows that Isabel's coastal flooding matched that of the August 1933 storm due to the long-term increase in sea level in Hampton Roads. Data from a tide monitoring station at Sewells Point show that sea level in tidewater Virginia rose 1.35 feet between August 1933 and September 2003. The National Ocean Service, a division of the National Oceanic and Atmospheric Administration (NOAA), runs the Sewells Point tide station located near the carrier piers at the Norfolk Naval Base.

The extent of coastal flooding during an individual hurricane or nor'easter depends largely on two interacting factors: the background astronomical tide and the surge generated by the storm's high winds and low atmospheric pressure. Together, surge and astronomical tide combine to form a "storm tide." Storm-tide flooding is maximized when the storm surge and a rising tide reach their peak at the same time.

Based on storm surge and astronomical tide, Isabel fails to measure up to the 1933 hurricane. "The 1933 storm produced a surge that exceeded Isabel's by more than a foot," notes Boon. "Its surge also occurred at the beginning of spring tides while Isabel's surge occurred in the middle of a neap tide."

But Hampton Road's long-term sea-level rise provided an equalizer. The increase in sea level at Hampton Roads in the seventy years between the two storms was enough to boost Isabel's storm tide to within an inch and a half of the level experienced during the 1933 storm.

Boon warns that sea-level rise will continue to exacerbate coastal flooding during future storms. "The inference from Isabel is very clear. Other things being equal, our present sea level trend will, over time, significantly increase the risk of coastal flooding during hurricanes."

Not content to simply sound a warning, Boon contends that a simple change in the way that forecasters and coastal residents reference predicted storm tides could in the long-term help them minimize potential impacts.

For navigational purposes, tides are traditionally referenced to Mean Lower Low Water (MLLW), a tidal datum calculated by averaging the lower of the two low water levels normally measured each day over a specific 19-year interval known as the National Tidal Datum Epoch. NOAA updates the interval periodically to account for ongoing changes in sea level relative to the land, the latest update including the years 1983-2001. Nautical charts use this datum to reference depths conservatively so that a mariner will know that the water depth shown on a chart can be counted on for safe passage even at the lowest levels of the tide.

Boon argues for the reverse to reference storm tides conservatively so that coastal residents will know how much additional rise to expect above the highest levels of the astronomical tide. The highest level of the tide at any one location is characterized by Mean Higher High Water (MHHW), a tidal datum calculated as the average of the higher of the two high water levels normally recorded each day during the current National Tidal Datum Epoch. Comparing extreme water levels to MHHW, says Boon, better indicates how storm tides will affect dry land at its beginning point, where people live and build structures.

"MHHW is a better datum for isolating and evaluating the risks associated with storm tides," says Boon, "because it removes the effect of tidal range--an independent factor that varies from place to place."

Boon notes that Hampton Roads residents live in a coastal region with one of the highest storm-tide risks relative to the datum that counts: Mean Higher High Water. "Points just above the MHHW line fall within a zone that appears dry most of the time. Coastal residents therefore tend to take advantage of it, adding infrastructure that enhances its monetary value and encourages its use. The next time the forecaster calls for a tropical storm or hurricane with possible record storm tides, we have to be especially concerned about any part of our property that lies near MHHW--and the first meter and a half above it."

"I'm living proof of that", says Boon, "My 30-foot sloop was on stands in exactly this zone in the York River Yacht Haven when Isabel knocked it over with no effort at all."