Calculating Storm Surge

[Ptarmigan]

Just curious. Is there a way to calculate storm surge height? I know it is based on wind speed, pressure, size, and geography of the ocean. Hurricane Katrina produced 30 foot storm surge, even though it had sustained winds of 120 mph and pressure of 925 millibars.

[Aslkahuna]

There are computer models that can do that but like all computer models there are uncertanties. Katrina was a near worse case scenario for a surge in the most surge prone part of the Gulf Coast and it was mainly due to the track of storm and the size and prior history.

Steve

[wxman57]

The main factors that contribute to height and extent of a tropical cyclone's storm surge are:

Radius of Maximum Winds (RMW)
Average wind speed within the RMW
Coastal topography
Near-shore water depth
Central pressure
Speed of movement
Angle of impact on the coast

I am (still) in the process of writing an article for s2k about storm surge. I have a copy of the US Navy Shore Protection Manual next to me. There is a formula for calculating potential storm surge. Max winds aren't a big part of the equaiton. Coastal topography and radius of max winds are much more important than peak winds in a tiny part of a hurricane.

Depending upon where and how a Cat 3 hurricane exactly like Katrina were to hit, it could produce anything from a 5-6 foot surge to one over 30 feet. That makes the SS scale "9-12" ft. storm surge for a Cat 3 a bit off. Can't use peak wind to calculate surge. There are many other more important factors.

[wxman57]

There are computer models that can do that but like all computer models there are uncertanties. Katrina was a near worse case scenario for a surge in the most surge prone part of the Gulf Coast and it was mainly due to the track of storm and the size and prior history.

Steve

I agree with all but the "prior history" part. It was calculated that there was a negligible contribution to Katrina's surge due to the fact that it had a small area of Cat 5 winds well offshore. The extremely large area of 74+ mph wind was much more significant than a few square miles of Cat 5 wind as far as the volume of water moved toward the coast.

[Ptarmigan]

There are computer models that can do that but like all computer models there are uncertanties. Katrina was a near worse case scenario for a surge in the most surge prone part of the Gulf Coast and it was mainly due to the track of storm and the size and prior history.

Steve

I don't want to think about a storm surge from a full blown hypercane. I heard a rough way to estimate wave height in a hurricane is to divide the sustained winds by 2. For example, a hurricane with 100 mph winds, will produce 50 foot waves.

[TS Zack]

Many people say Katrina was only a Category 3, but look at the surge. You have to remember only a few hours before landfall the storm was a very strong Category 5 storm. This allowed a category 5 storm surge to develop and plow into the Ms/La Coast. Weakening storms at the coast will always have higher surges than the wind speed indicates like Ivan/Katrina. This is just due to momentum.

Another example that is opposite of Katrina/Ivan is Charley. That system ramped-up a few hours before landfall to a Category 4 Hurricane, still only a 4-8ft storm surge. The probelem with Charley was it did not have the time to develop a strong storm surge.

The ocean depth is another key factor in how high the storm surge will be. The shallow waters of the Northeastern Gulf makes the Big Bend of Florida very vulnerable to surge.

[Ptarmigan]

Many people say Katrina was only a Category 3, but look at the surge. You have to remember only a few hours before landfall the storm was a very strong Category 5 storm. This allowed a category 5 storm surge to develop and plow into the Ms/La Coast. Weakening storms at the coast will always have higher surges than the wind speed indicates like Ivan/Katrina. This is just due to momentum.

Another example that is opposite of Katrina/Ivan is Charley. That system ramped-up a few hours before landfall to a Category 4 Hurricane, still only a 4-8ft storm surge. The probelem with Charley was it did not have the time to develop a strong storm surge.

The ocean depth is another key factor in how high the storm surge will be. The shallow waters of the Northeastern Gulf makes the Big Bend of Florida very vulnerable to surge.

Charley was also a very small hurricane. Hurricane Katrina was a monster that was one of the most intense on record.

[wxman57]

Many people say Katrina was only a Category 3, but look at the surge. You have to remember only a few hours before landfall the storm was a very strong Category 5 storm. This allowed a category 5 storm surge to develop and plow into the Ms/La Coast. Weakening storms at the coast will always have higher surges than the wind speed indicates like Ivan/Katrina. This is just due to momentum

Zack, the fact that Katrina was a Cat 5 the day before landfal had very little to do with the large surge. Peak wind speed is a small factor in storm surge. Storm surge is a function of the volume of water driven into the coast. Katrina's Cat 5 winds were located only in a very small section of the right front quadrant, so they accounted for very little additional volume. I heard an expert address that question at the NHC this past spring. He calculated that the higher winds 24 hours before landfall may have accounted for an additional 1-2 feet of surge at landfall. That's it. That's why the Saffir-Simpson scale should not be used to estimate potential storm surge. You need to know the size of the stronger wind field to better-estimate storm surge, not the peak wind in a small part of the hurricane.

[MGC]

I also think that Katrina's Cat-5 winds didn't contribute much if at all to the surge. The main factors that made Katrina's surge was the large wind field, the shallow waters and the lay of the land. Louisiana and Mississippi form a kind of 90 degree angle trapping the water pushed ashore. Had Katrina hit further east say where Ivan hit it would have produced a surge simulate to Ivan. I shutter to think what the coast would look like had Ivan hit say Gulfport followed by Katrina. The Charlie vrs Katrina example is quite valid in terms of small storm- large storm differences......MGC

[Aslkahuna]

Prior history of Katrina also includes the expansion of windfield while it was out in the GOM and not just the intensity.

Steve

[Jim Cantore]

They nailed Katrina

from the 4pm advisory on August 28th

COASTAL STORM SURGE FLOODING OF 18 TO 22 FEET ABOVE NORMAL TIDE
LEVELS...LOCALLY AS HIGH AS 28 FEET

Highest surge I heard was 28 feet in Bay Saint Louis

[SouthAlabamaWX]

Highest surge I heard was 28 feet in Bay Saint Louis

The NWS Jackson survey team estimated a storm surge of 37 feet at Point Henderson.

[f5]

Highest surge I heard was 28 feet in Bay Saint Louis

The NWS Jackson survey team estimated a storm surge of 37 feet at Point Henderson.

doesn't matter until NHC says its offical

[SouthAlabamaWX]

Actually, the NHC and NWS Jackson are working to confirm it and make it official. The 28 feet surge is still higher than hurricane Camille. It will also remain the highest recorded storm surge for that area even if the NHC decides that the will discount the 37 foot surge and the numerous 34 and 35 feet storm surge reports.

[f5]

if that 37 ft surge is confirmed Katrina will be up there beside the lenendary Baythurst bay storm that struck australia in 1899 with a 43 ft surge.she will be regarded as one of the biggest baddest meanest surge producing storm ever to form on the planet

[wxman57]

Prior history of Katrina also includes the expansion of windfield while it was out in the GOM and not just the intensity.

Steve

Interesting you should mention the wind field size farther offshore, Aslkahuna. We've done extensive research on hurricanes over the past 25 years (wind fields). What we do is we use HRD post-storm analysis data (for recent years), calculate the radius of the 39, 58, 74, and 100 mph winds in each quadrant then calculate an "effective wind radius" that accurately reflects the areal coverage of each wind field. Basically, what radius circle would = the area from each quadrant.

Katrina was fairly symmetrical when it was at peak intensity, but strongest winds concentrated on the right side at landfall. Here are the value we measured for Katrina at max intensity 18Z the 28th and then at landfall at Buras:

Max Intensity Aug. 28th:
39 mph = 174 nm. radius
58 mph = 111 nm. radius
74 mph = 82 nm. radius
100 mph = 39 mph radius

Now, at Buras:
39 mph = 167 nm. radius
58 mph = 112 nm. radius
74 mph = 89 nm. radius
100 mph = 40 nm. radius

Now, considering that Katrina had more strong winds in the left side at peak intensity (offshore-blowing winds), the data suggest that winds on the right side (onshore-blowing winds) increased in coverage as Katrina made landfall. It's the coverage of these SE-S winds east of Katrina's center which produced the storm surge. These winds covered a larger area when Katrina weakened to a Cat 3 than when Katrina was a Cat 5.

One interesting thought -- it's quite possible that if Katrina had hit with the Cat 5 intensity and structure it had on August 28th that the surge height may have been similar in the right eyewall, but the surge farther east of the point of landfall may have been lower (Biloxi, Pascagoula).