? about storm surge

[Dave C]

I was curious, does height of storm surge respond to pressure of hurricane or to the wind speed or both? Even though wind speed was about 100mph with Isabel the central pressure was @957 at landfall which was as low as Fabian when it hit Bermuda Thanks!!

[weatherluvr]

Part of it depends on wind speed and pressure, but a lot also depends on the topography of the continental shelf offshore. Places with a shallow or gently sloping shelf are prone to the worst storm surges, because the water cannot flow away and piles up higher and higher near and to the right of the eye. Places like the Gulf coast get the worst surges because of this. Places like Jamaica, where the water is much deeper offshore, are spared the worst surge heights.

[Rainband]

The direction the storm comes in also plays a part. The worst case scenario for Tampa bay would be a Cane moving in from the sw. All the surge would come into Tampa bay and have no where to go..but into Pinellas county..which is basically is an Island and Hillsborough county..which after that would be one :o

[wxman57]

I would refer you to the U.S. Navy Shore Protection Manual, which has a chapter devoted to the calculation of storm surge. There are several factors to consider, the least of which is the low pressure itself. The actual ocean rise due to the lower pressure in the center of a storm is at most a few inches. So here is what to look for:

1. The difference in pressure from the storm center to a point outside the storm circulation. This is used to estimate surface wind speed. It is this wind speed that drives the storm surge, not the low pressure (directly, anyway). This delta-P would NOT have given a proper storm surge for Isabel, as winds were less at the surface (and over a much smaller area) than with a typical storm with a pressure that low.

2. Speed of movement of the storm. Slower movement = higher storm surge.

3. Angle of incidence - the angle at which the storm intersects the coast. 90 deg = max storm surge.

4. Sea floor topography. Each section of the U.S. coast is given a numerical rating that is used in the equation to calculate the surge. Miami may be 0.8, Tampa may be 1.7, etc. Basically, the more gradual the slope of the sea bed, the greater the storm surge risk.

There's an actual equation in the Shore Protection Manual where you input the 4 variables above and come up with a rough figure. I say rough because other factors can significantly affect storm surge. When the storm surge is driven into a bay, for example, it can be up to twice as high as the calculation would indicate. This would be true of a place like Tampa Bay or Galveston Bay. A 12 foot storm surge on the coast may translate to 20-25 feet inside the bay if the wind is blowing into the bay.

The NHC FTP Site has some neat surge graphics up for Isabel:

ftp://ftp.nhc.noaa.gov/pub/users/surge/ ... 46_CHP.GIF

ftp://ftp.nhc.noaa.gov/pub/users/surge/ ... 9_EHAT.GIF

Drop down to the root folder(s) for more neat stuff. These files are 8-9MB, so be patient.

[weatherluvr]

2. Speed of movement of the storm. Slower movement = higher storm surge.

Forgive me, but I would have thought that faster forward speed would mean a higher surge?

[Dave C]

Thanks for the info everyone!!!

[JetMaxx]

The wind damage in Isabel was consistent with a cat-1/2, but the storm surge damage looked like what I'd expect from a 957 mb hurricane (cat-3).....very damaging.

[AussieMark]

How much damage do u think Isabel did in $'s?

Could it exceed Floyd's $4.5 Billion.

[JetMaxx]

I personally won't be surprised when all is added up if it totals $2-3 billion in damage...possibly more.

[wxman57]

2. Speed of movement of the storm. Slower movement = higher storm surge.

Forgive me, but I would have thought that faster forward speed would mean a higher surge?

Slower movement = prolonged onshore flow in advance of the center = higher storm surge.

[wxman57]

The wind damage in Isabel was consistent with a cat-1/2, but the storm surge damage looked like what I'd expect from a 957 mb hurricane (cat-3).....very damaging.

I believe that the measusred storm surge was in the 6-8 foot range - higher than that area has seen in proably 100 years, but clearly below a Cat 3 surge of 9-12 feet on the outer banks and and 15-20 feet in northern parts of bays. Most of the time, that area escapes the worst part of hurricanes because they come in from the south and are recurving as they hit the outer banks.

http://www.erh.noaa.gov/mhx/Images/IsabelSurge.gif

From the Morehead City report. Note that a 6-8 foot storm surge at the coast can easly increase to 10-15 feet if the wind blows into a shallow bay. Hurricane Alicia hit just west of Galveston Bay in 1983 with 115 mph winds, producing about a 10-12 foot storm surge on Galveston Island - consistent with a Cat 3 storm. However, in northern Galveston Bay, the surge was 20-25 feet above MSL, equal to a Cat 5 storm. So a Cat 3 storm can produce a storm surge equal to the coastal surge of a Cat 5 storm. But if a Cat 5 storm had hit where Alicia did, then the surge in northern Galveston Bay could have been 40+ feet above normal. It's the same with Isabel, the general surge was well within Cat 2 levels, but some areas got a little higher surge than what a Cat 2 storm would produce at the coast. A Cat 3 storm would have produced a considerably higher surge in NC. Notice the highest storm surge below was not at the Outer Banks, but in an inland bay (where the 6-8 foot surge was magnified slightly.

D. STORM TIDES

THE HIGHEST STORM TIDES OCCURRED ACROSS THE LOWER REACHES
OF THE NEUSE AND PAMLICO RIVERS WHERE WATERS LEVELS ROSE FROM 6 TO
10.5 FEET ABOVE NORMAL. ON THE NEUSE RIVER THE HIGHEST SURGE WAS
BETWEEN CLUBFOOT CREEK AND ADAMS CREEK WHERE THE WATER ROSE TO 10.5
FEET ABOVCE NORMAL. CLUBFOOT CREEK ROSE TO 9.5 FEET, 1.5 FEET HIGHER
THAN DURING HURRICANE DENNIS IN 1999. SIGNIFICANT FLOODING WAS
REPORTED IN THE HARLOWE AREA IN CRAVEN COUNTY, AND IN THE TOWN OF
ORIENTAL IN EASTERN PAMLICO COUNTY. WATER LEVELS ROSE BETWEEN 6 AND
7 FEET ON THE PAMLICO RIVER WITH FLOODING IN THE CITY OF WASHINGTON.
STORM TIDES OF 6 TO 8 FEET OCCURRED ALONG THE OUTER BANKS WITH MAJOR
OCEAN OVERWASH. FLOODING ON OCRACOKE WAS SIGNIFICANT WITH WAIST
HIGH WATER IN MANY AREAS AT THE HEIGHT OF THE STORM.