Tuesday, September 12, 2017

Four Surges in One - Perspective on Irma's Wild Coastal Flooding

Although Hurricane Irma made landfall in the Lower Florida Keys as a category-4 hurricane, the massive storm generated wild coastal flooding patterns throughout the Florida peninsula, Georgia and South Carolina. Here's a recap on Irma's storm surge.



Peak Surge Levels

The map below depicts Irma's peak storm surge levels, with storm surge representing the difference in observed and predicted water levels based on normal astronomical tides. The highest observed storm surge levels exceeded 6 feet in places near the Florida/ Georgia border, like Fernandina Beach and Mayport, while water levels in a wider area from east-central Florida through South Carolina, as well as portions of southwest Florida, observed surge levels exceeding 4 feet.


Map of maximum storm surge data. Map created by Dr. Hal Needham (Marine Weather and Climate/ U-Surge), data provided by NOAA Tides and Currents.


As people are just returning to the worst-hit areas of the Florida Keys and southwest Florida, maximum storm surge areas are still being reported.  The table below provides the mapped data in tabular form.


Storm surge observations for Hurricane Irma
Effective Tue Sep 12 at 06:00AM Eastern Time

Storm Surge (water level above normal astronomical tides)

NOAA Tides and Currents Tide Gauges (preliminary water levels)

Fernandina Beach, FL:      7.75 ft

Mayport, FL:                       6.42 ft
Fort Pulaski, GA:               5.63 ft
Naples, FL:                         5.14 ft

Charleston, SC:                 4.87 ft
Trident Pier, FL:                 4.43 ft
Fort Myers, FL:                  4.25 ft
Virginia Key, FL:                3.91 ft
Key West, FL:                    3.30 ft
Old Port Tampa, FL:          3.17 ft

Springmaid Pier, SC:         3.04 ft
Vaca Key, FL:                     2.76 ft
Cedar Key, FL:                   2.50 ft
​Lake Worth, FL:                 2.23 ft
St. Petersburg, FL:            1.96 ft    



Irma's large size and shore-parallel track through southwest Florida, enabled the storm to generate four distinct storm surge events in different regions. Here's a summary below:


Florida Keys


Irma made landfall at Cudjoe Key, in the Lower Florida Keys, as a category-4 hurricane with maximum sustained winds of 130 mph on Sun Sep 10 at approximately 9:10AM Eastern Time.

NOAA tide gauges to the west (Key West) and east (Vaca Key) Irma remained more-or-less operational throughout the storm's passage, although the gauge at Key West has been reporting data intermittently. Surge levels at both sites increased substantially after Irma's eye passed and winds changed direction. As Irma approached, prolonged winds were blowing from east-to west across the Keys, but winds suddenly reversed from west-to-east after the eye passage. Maximum surge levels at both Key West and Vaca Key remained less than 4 feet.

Storm surge levels are highly localized, particularly in island chains, where two locations just several miles apart border water in different directions. For example, the NOAA tide gauge at Key West is located near the western end of the island, making it most susceptible to storm surge generated by a strong west wind.

 
NOAA's Key West tide gauge is located near the western end of the island, making it most susceptible to strong winds approaching the island from the west.

Based on Irma's track, the highest storm surge levels in the Keys, would have likely occurred near Cudjoe Key, Big Pine Key and Marathon. Initial rumors of a 15-foot storm tide (storm surge + tide) circulating social media remain unverified and were likely exaggerated. However, numerous images of sail boats swept onto the Overseas Highway (Hwy 1) provide evidence of a powerful storm surge that swept through the islands.

While spending time on Monday evening on social media groups focused on connecting locals from the Florida Keys, I noticed that numerous posts suggested a peak storm surge in the range of 5-6 feet at Cudjoe Key. However, this information was still speculative, as evacuees were not allowed back into the region yet and information from the Keys to the outside world was limited, often relying on satellite phones.


Although storm surge levels at Key West remained below 4 ft, the force of water pounding over the seawall was powerful enough to peel the paint off the famous "Southernmost Point" monument. 


Keep in mind that along island chains, storm surge values are extremely localized. Water displaced from a distance can pass between islands, but where a hurricane's eyewall is engaged on an island, surge levels will quickly spike, sometimes reaching levels twice as high as areas just 10-15 miles away.

Authorities are letting residents back into the Lower and Middle Keys today, so we should know more about the wind and storm surge impacts soon.


Video posted by Anthony Paul Davis shows storm surge flooding in Marathon, Florida.


Florida West Coast

Florida's West Coast observed the most unusual storm surge patterns, as Irma moved from south to north, approximately following a shore-parallel track. Due to counterclockwise winds around hurricanes in the Northern Hemisphere, substantial "negative" storm surges, when water is pushed away from the coast, occur when hurricanes take shore-parallel tracks and have a large body of water to the left of their forward path.

This occurs because prolonged winds of increasing intensity blow offshore along the coast, while the storm cannot draw any new water to replace the displaced water because strong winds are blowing offshore.

Numerous images of extensive mudflats emerged on Sunday afternoon in places like Tampa Bay. In some cases, humans were able to rescue marine animals, like dolphins and manatee, that were stranded by the receding water.

Marine animals, like manatee, became stranded as Irma pushed water away from Florida's west coast on Sunday, producing negative storm surges. Image: Michael Sechler/ Facebook.

People should use caution in such situations, as rapid changes in wind direction as the eye approaches, can lead to the water rushing back into the bay, along with additional water from an onshore storm surge.

The hydrograph below shows a "negative" storm surge of approximately 5.3 ft was observed at Old Port Tampa on Sunday afternoon, followed by rapid water level rises leading to a storm surge that exceeded 3 ft by Monday. Loss of data near the peak "negative" storm surge may indicate that the water level was too low for the gauge to record.

Hydrograph for Old Port Tampa depicts a substantial "negative" storm surge followed by a "positive" storm surge. Red = observed water level, blue = normal astronomical tide, and the difference is the storm surge.

Negative storm surges of this magnitude are somewhat rare but not unprecedented. The U-Surge Project, which provides more than 9,000 storm surge observations for the United States since 1880, places Irma's negative storm surge at Old Port Tampa in the top-5 all-time recorded negative storm surges. Three of the top five events occurred at Tampa.

Notable "Negative" Surges in the U-Surge Database

Year        Storm Name       Water Level (ft)          Datum              Location
1926        Great Miami         -10.30                       MSL                Mobile, AL
1910        Unnamed              -8.00                        Unknown        Tampa, FL
1926        Great Miami         -6.00                         MLT                Tampa, FL
1943        Unnamed              -5.70                         NAT                Galveston, TX
2017        Irma                      -5.32                         NAT                Old Port Tampa

Datums: MSL = Mean Sea Level; MLT = Mean Low Tide; NAT = Normal Astronomical Tide

The Great Miami Hurricane of 1926 generated the most extreme negative storm surge level on record, as the storm took a shore-parallel track along the northern Gulf coast, which generated powerful offshore winds near Mobile, Alabama, reducing water levels in Mobile Bay by more than 10 feet. This storm did not take a shore-parallel track along Florida's west coast, but Tampa stayed north of the storm's track, enabling powerful offshore winds to persist through the most intense part of the storm.

The Great Miami Hurricane of 1926, generated a "negative" storm surge of 10.30 ft at Mobile, Alabama, and 6.0 ft at Tampa. Powerful offshore winds pushed water away from the coast in both cases. 

Although negative storm surges may seem benign, and appear harmless compared to positive storm surges, such events can have negative impacts as well. Rapid water loss in marinas can leave fiberglass boats on hard surfaces as powerful winds push them around. Of greater impact to the general public would be loss of water for critical cooling processes at large facilities, like power plants. Given the susceptibility of negative storm surges at Tampa, local planners and utility operators should review potential impacts from negative surges in the region.

Storm surge flooding was evident in Everglades City during reconnaissance on Mon Sep 11.

Storm surge levels in southwest Florida, near Naples and Fort Myers, are not as severe as forecast, as Irma tracked farther inland than feared on Sunday morning.

Nonetheless, the eye remained offshore while passing the community of Everglades City, and reports are just surfacing about extensive storm surge flooding in this region. Everglades City likely observed the highest storm surge level on Florida's west coast.


Southeast Florida


Southeast Florida remained on Irma's "strong side" from Friday through much of Sunday, enabling storm surge to gradually increase over a several day period. At Virginia Key, near Miami, storm surge levels unfortunately peaked around midday Sunday, near the time of high tide. This enabled the NOAA tide gauge at this site to report a maximum storm tide (storm surge + tide) level of 3.88 ft above NAVD88 datum.

The U-Surge Project launched a Miami website as Irma approached, enabling us to compare Irma's maximum water level with 27 observed events since 1880. Irma's storm tide ranked 10th all-time and was the highest reported saltwater level since Hurricane Wilma in 2005. This water level was high enough to flood much of Miami's financial district with flood water on Sunday. 

Time series of observed storm tide events at Miami, Florida, from 1880-2017 (138 years). Irma's storm tide ranked 10th all-time and highest since Hurricane Wilma in 2005. 


Approximately 10-12 inches of sea-level rise in the Miami area over the past century enabled Irma to flood thousands of additional properties, even though the peak storm surge remained around 4 feet. In fact, if Irma hit one century ago, the maximum storm tide in Miami would have been around 2.88 ft, which would have ranked it 12th all-time (instead of 10th), as total water levels would have been lower than the storm tides from Hurricane Donna (1960) at 3.74 ft and Hurricane King (1950) at 3.67 ft.


NE Florida, Georgia, South Carolina


As of Tuesday morning, Irma's highest credible storm surge observations have been reported from northeast Florida through South Carolina, including the Georgia Coast. Fernandina Beach, Florida, near the Georgia border, leads the pack with a 7.48-ft storm surge observation.

Due to high tidal ranges in this region, storm surge observations only tell part of the story. The worst flood impacts usually occur near the time of high tide.

As this region stayed on Irma's "strong" side throughout the entire event, a prolonged storm surge in this region ensured moderate storm surge levels would persist through multiple high-tide cycles.

Although storm surge levels at Fort Pulaski, Georgia, near Savannah, peaked at more than 5 feet near the time of low tide, before dawn on Monday, persistent onshore winds kept storm surge levels elevated through the day. Astronomical tide levels rose faster than storm surge retreated on Monday morning, enabling peak water levels to exceed 12 ft above Mean Lower Low Water, which generated the second-highest water level on record at the gauge in an 82-year period, barely missing the maximum level set last October by Hurricane Matthew, according to Jill Gambill, with Georgia Sea Grant.

The Savannah River reached extreme levels on Monday, overflowing its banks and inundating portions of downtown Savannah. 

On their Category 6 blog post yesterday evening, Jeff Masters and Bob Henson provided excellent insights on storm surge observations in Georgia and northeast Florida, including a record water-level set in Jacksonville. Irma's persistent storm surge, heavy rainfall runoff and high astronomical tides converged in this region to provide widespread flooding on Monday.

Both Jacksonville and Savannah were victims of compound flooding, when prolonged onshore winds elevate storm surge levels for multiple high tide cycles and impede the drainage of heavy inland rainfalls.

This is my last update on Hurricane Irma's storm surge. Feel free to contact me with questions or interview requests at: hal@marineweatherandclimate.com.

Let's hope the Gulf Coast and Southeast States can dry out in the next couple of weeks and that Hurricane Jose stays out to sea!

Take care and stay safe, everyone!

-Hal


3 comments:

  1. Just a final thanks for your excellent coverage and summaries. Probably many people share this but do not comment.

    Just for the record, my parents lived on Big Pine Key for a number of years before moving to Tampa/Clearwater area where they spent their last years. They experienced Donna while on Big Pine.

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    1. Thank you so much for your encouragement! I appreciate the kind messages you have posted. I wanted to reply sooner but everything has been so crazy! I thought your insights on how we keep building more along the coast were interesting. We cannot necessarily stop cat-4 hurricanes, but we can plan better to minimize impacts!

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  2. Hal, I couldn't help reading this in your voice. It's an incredible account of Irmas surge journey. Thank you for this informative contribution!!

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