Chandeleur Island chain before and after Katrina hit with 25 ft storm surge.
When a tropical cyclone makes landfall, the rising sea level is referred to as the storm surge. This can occur with tropical storms, but becomes more noticeable with hurricanes. The push of water can reach over 20 feet in extreme events. See the animation of average storm surge Cat 1-5 Hurricanes in the slide show below. Rainfall in the storm at rates of over 3 inches per hour can add to water levels. The very low air pressure can also create a bulging of the ocean. With less force pushing down near the center of the storm, the water can actually expand higher. Surge can be affected by three other main factors that will be discussed further below. These include:
- Slope of the continental shelf, or land near the coast.
- Normal tidal swing from the moon
- Topography, or lay of the land. A bay region for example can help to funnel the water in certain locations.
Since most of the populated Unites States along the Gulf and Atlantic coasts are less than 10 feet above sea level, the danger is very high.
Slope:
A gradual slope often means two things: The water is shallow of shore, and the coastal land is low lying.
These areas are more susceptible to flooding even from minor storm surge. This means that less water can reach farther inland to flood. In addition to structural damage, most fatalities from land falling storms occur from this.
A steep slope can provide more protection. This most often is located in areas that have rocky shorelines with bluffs or hills near the coast. Essentially the slope of the land near the coast will mirror each other above and below the water. In the United States, this is more coming on the northern shore of Long Island, NY and along the coast of New England. It takes more water or a higher surge to reach far inland to cause widespread problems. This does not eliminate the risk, but limits it to a small area.
Normal Tides:
This is in relationship to the moon and the sun. While the position of the moon in it's orbit is an important factor, the way the water moves along the coast, and sloshes in places like the Chesapeake Bay, there can be a wide range of high tide times in a local area. Other tings to note:
*Whenever the Moon, Earth and Sun are aligned, the gravitational pull of the sun adds to that of the moon causing maximum tides.
*Spring tides happen when the sun and moon are on the same side of the earth (New Moon) or when the sun and moon are on opposite sides of the earth (Full Moon).
*When the Moon is at first quarter or last quarter phase (meaning that it is located at right angles to the Earth-Sun line), the Sun and Moon interfere with each other in producing tidal bulges and tides are generally weaker; these are called neap tides.
*Spring tides and neap tide levels are about 20% higher or lower than average.
Local tide times can be found in the links at the bottom of this post.
Topography
Every location along the coastline is unique. Maryland itself has the longest coastline of any state in the US. That is if you include the entire border on the Chesapeake Bay. The inlets and harbors on both sides of the bay are a great example of how each location can be affected differently by a given storm.
This animation is from the SLOSH model, a tool designed to forecast how a storm surge would affect a coastline with a given storm strength and path. For slosh model images of the Chesapeake Bay, see the slide show below.
Storm Memory:
One important part that does not get much attention is how a weakening storm can actually generate a higher storm surge than expected. Katrina made landfall in Louisiana as a Category 3 storm with 120 mph winds, but produced much higher storms surge that that strength. Just a day before landfall, it peaked at a Category 5 storm with winds of 175 mph. This generated waves that did not settle as quickly as the winds weakened. So the maximum storm surge, was on the strong right/east side of the center of the storm. The water reached 27.8 feet higher than normal in Pass Christian, MS.
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