City of Portsmouth, Virginia - Emergency Preparedness

What is Storm Surge?

"The greatest potential for loss of life related to a hurricane is from the storm surge."

Storm surge is simply water that is pushed toward the shore by the force of the winds swirling around the storm. This advancing surge combines with the normal tides to create the hurricane storm tide, which can increase the mean water level 15 feet or more. In addition, wind driven waves are superimposed on the storm tide. This rise in water level can cause severe flooding in coastal areas, particularly when the storm tide coincides with the normal high tides. Because much of the United States' densely populated Atlantic and Gulf Coast coastlines lie less than 10 feet above mean sea level, the danger from storm tides is tremendous.

The level of surge in a particular area is also determined by the slope of the continental shelf. A shallow slope off the coast (right picture below) will allow a greater surge to inundate coastal communities. Communities with a steeper continental shelf (left picture below) will not see as much surge inundation, although large breaking waves can still present major problems. Storm tides, waves, and currents in confined harbors severely damage ships, marinas, and pleasure boats.

Coastline with a gentle slope

The National Hurricane Center uses a modeling program called SLOSH (Sea, Lake, and Overland Surges from Hurricanes) to estimate the amount of storm surge to be expected from a hurricane. The model takes into account wind speed, forward speed, barometric pressure, track, and size of the hurricane to provide the estimate for a given area. The parameters entered into the model for the storm combine with the characteristics of the particular area, referred to as a basin which will provide expected results. The results of the SLOSH model is accurate within 20%. For example, if the model indicates 10 feet of flooding, the actual amount of flooding will normally be between 8 to 10 feet. The amount of storm surge is verified after each land falling hurricane and compared to the estimates that the SLOSH model provided for the storm.

You can learn more about the National Hurricane Center's SLOSH modeling program by going to: http://www.nhc.noaa.gov/HAW2/english/surge/slosh.shtml.

A map has been produced for all areas in coastal Virginia which indicates the potential flooding caused by storm surge. This map is a composite of many storm scenarios with various intensities, forward speeds, and directions of approach. The model is based on the Chesapeake Bay Basin. No single storm would result in the flooding indicated on the map. Storm surge models do not take rainfall into consideration. Rainfall flooding will be in addition to storm surge flooding. In many cases, areas that normally do not have a rainfall flooding problem may have unprecedented flooding.

The maps below require Adobe Reader to view. You may download the free software by going to : http://www.adobe.com/products/acrobat/readstep2.html

Will a Storm Surge affect you?

Category 5 Storm (No information available)

City of Portsmouth GIS Interactive Map

You may also view specific properties in the city of Portsmouth using the Department of Information Technology Geographical Information System (GIS) Interactive Map. Click on the link below and follow the instructions on the main page to get started. This interactive map offers a wealth of information for residential property and business owners including flood zones for specific neighborhoods.

Geographical Information System (GIS) Interactive Map

What kind of damage can you expect?

Category	Wind Speed	Storm Surge	Damage
1	74 - 95 mph	4 - 5 feet	Primarily to unanchored mobile homes, shrubbery, and trees. Some damage to poorly constructed signs.
2	96 -110 mph	6 - 8 feet	Some roofing material, door, & window damage to buildings. Considerable damage to shrubbery and trees.
3	111-130 mph	9 - 12 feet	Some structural damage to small residences & utility bldgs, large trees blown down, mobile homes & poorly constructed signs destroyed.
4	131-155 mph	13 - 18 feet	Some complete roof failures on small residences. Shrubs, trees, and all signs blown down. Complete destruction of mobile homes. Extensive damage to doors & windows.
5	>155 mph	>18 feet	Some complete building failure with small utility buildings blown over or away. All shrubs, trees, & signs blown down. Complete destruction of mobile homes. Severe & extensive damage to doors & windows.

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