Hurricane Dorian Threatens to Inflict Long-Duration and Complex Flood Event

KEY MESSAGES

1) Hurricane Dorian threatens to inflict a long-duration flood event that will begin with saltwater storm surge flooding near and north of the landfall area;

2) Dorian's slow motion near and after landfall will exacerbate coastal flooding and erosion, while creating a "compound" flood event. Torrential rain will have nowhere to drain as a long-duration saltwater flood pushes inland;

3) Places that have never flooded before may experience deep water and a second flood hot-spot may develop several days after landfall in places like Jacksonville, Savannah and Charleston.

Infrared satellite image of Hurricane Dorian shows a strengthening storm on Friday morning. Source: CIMSS Tropical Cyclones

IN DETAIL

DORIAN'S COASTAL FLOOD POTENTIAL

Hurricane Dorian threatens to inflict a long-duration flood event beginning with saltwater storm surge flooding near and north of the landfall area. Massive ocean swells and waves should precede the arrival of destructive winds, and some coastal erosion could begin more than 24 hours before landfall.

With continued uncertainty in Dorian's track, we cannot pinpoint the exact region that will observe the worst coastal flooding. However, we can develop a general sense of storm surge potential by looking back at several hurricanes from the past.

BASIC OVERVIEW OF COASTAL FLOODING

Before we look at those historical hurricanes, let's touch base on the basics of storm surge and coastal flooding. A storm surge is an abnormal rise in saltwater at the coast caused by low air pressure and strong onshore winds in severe storms. 

The image below shows how storm surge and waves can inundate an area and push far inland. (Image: NOAA/ The Comet Program).

Water strikes objects with more force than wind, so as you can imagine, buildings quickly collapse under incessant pounding of sea water. In more severe cases, buildings can be gutted or washed away without a trace.

Consider the two pictures below that I took last October in Mexico Beach, Florida. I took these photos in exactly the same location (my feet are in the same position looking in the same direction). I took picture #1 on Tuesday morning and picture #2 on Saturday morning. In between these two photos, Hurricane Michael blasted Mexico Beach with a 15-foot storm surge that gutted coastal buildings.

"Before" picture in El Governor Hotel, Mexico Beach, FL. Tue Oct 9, 2018. Photo: Hal Needham

"After" picture in El Governor Hotel, Mexico Beach, FL. Sat Oct 13, 2018. Photo: Hal Needham

My post-storm view from the south-facing stairwell in the same building showed chewed-up roadway and concrete slabs where large houses stood just a few days earlier.

Another view of Mexico Beach, Florida, shows slabs where large homes once stood. Testimony to the power of storm surge. Photo taken October 13, 2018 by Hal Needham.

We often think of hurricanes as wind events, in part because it's difficult to document the destructive force of storm surges and live to tell the story! In reality, flooding from storm surge and heavy rain are responsible for 88% of deaths in hurricanes and tropical storms (Erdman 2019). 

Initially, Dorian is forecast to approach Florida's coast as a slow-moving, category-4 hurricane. The slow movement will prolong the storm surge flooding, enable saltwater to push far inland, and give angry waves more time to erode sand dunes and other coastal protection.

HISTORICAL STORMS FOR REFERENCE

A few historical storms provide context for Dorian's storm surge potential. Let's look at two major (cat 3+) hurricanes that both impacted Palm Beach County...the 1947 hurricane and Hurricane Jeanne in 2004.

THE 1947 HURRICANE

The 1947 made landfall as a category-4 hurricane with maximum sustained winds of 132 mph (115 knots).  The storm tracked to the west-southwest upon approach and made landfall between Pompano Beach and Hollywood.

Track of the 1947 Hurricane, courtesy NOAA.

 This storm generated an 11-foot storm surge at Hillsboro Lighthouse, Boynton Beach and Palm Beach (Barnes 1998). This means the saltwater rose to 11 feet higher than it would have been had there not been a hurricane.

This is the highest storm surge level on record in Palm Beach County. Dig deeper into this local flood history at the Palm Beach County U-Surge Page that I just launched this morning.

The photo below depicts damage from this storm to the Seacrest Hotel in Delray Beach. Note the effect of scouring from the storm surge and powerful waves.

The 1947 hurricane was not only a powerful hurricane, but also geographically large. This large wind field enabled the hurricane to generate a high and extensive storm surge.

The Monthly Weather Review from 1947 reads, "Winds of 100 m.p.h. or over were experienced generally along the Florida east coast from the northern portion of Miami to well north of Palm Beach, a distance of about 70 miles, while winds of hurricane force prevailed from approximately Cape Canaveral to Carysfort Reef Light, a distance of about 240 miles."

I created the map below to show the track of this hurricane, as well as the swath of peak storm surge levels, shown on the map in orange, reaching from 8-12 feet (peak surge was 11 feet).

HURRICANE JEANNE (2004)

Hurricane Jeanne approached the Florida coast as a category-3 hurricane with maximum winds of 121 mph (105 knots). The storm made landfall north of Palm Beach, near Port St. Lucie.

Not only was Jeanne less powerful than the 1947 hurricane, but the wind field was considerably smaller. Whereas the diameter of hurricane-force winds reached 240 miles in the 1947 Hurricane, they only reached 138 miles (120 nautical miles) in Hurricane Jeanne (Demuth et. al 2006).

Lawrence and Cobb (2005) comment on Jeanne's relatively small size by stating, "Maximum winds at landfall are estimated at 105 kt [121 mph] over a very small area north of the center and it is not clear whether these strongest winds reached the coast or remained over water."

As expected, Jeanne's storm surge levels were considerably lower, reaching an estimated six feet from Melbourne to Ft. Pierce (Lawrence and Cobb 2005).  The map below show's Jeanne's track and storm surge observations.

So what do these storms tell us about the storm surge potential for Hurricane Dorian? Well, we have a few clues...

The latest NHC advisory only provides wind field size forecasts for 74-mph wind out to Sun Sep 1 at 200AM EDT and landfall may occur as late as Tuesday morning. The forecast predicts Dorian to have an unusually small area of hurricane-force winds for such a powerful storm, only reaching to 46 miles (40 nm) by early Sunday morning. However, it should be on a trend of an expanding the wind field at that time. Even if the wind field doubled by early Tuesday morning, it would still only reach 92 miles (80 nm).

The table below compares wind speed and the area of hurricane-force wind speed for these storms. 

Note that storm surge prediction is complex and depends on many factors, including coastal shape, profile and offshore water depth. Generally speaking, the farther north we travel along Florida's East Coast, the higher the storm surge potential.

That said, the world needs a "back of the envelope" storm surge estimate right now...something to give general guidance. So I'm going to step out and make a prediction for a peak surge of 8-10 feet based on the best available data right now.

Keep in mind this is water depth above Normal Astronomical Tides (NAT). We are just coming down from a king tide cycle, so water levels in most communities should already be elevated well above Mean Sea Level (MSL) at time of high tide.

Consider the tide forecast at Trident Pier in Port Canaveral (see graph below). High tide is forecast to reach 2.47 feet above MSL on Monday night around 11PM. This prediction is based on the Earth- Moon relationship and does not account for impacts from Hurricane Dorian. A storm surge level of 7.53 feet would push salt water to 10 feet above MSL at this site.

Tide forecast for Trident Pier in Port Canaveral shows a forecast high tide of 2.47 feet above Mean Sea Level (MSL) on Monday night. 

Please keep the following in mind:

1) This forecast could change substantially given changes in Dorian's the track, intensity and size.

2) Even if this forecast is right on the money, it only tells a small part of the story. If Dorian was moving through quickly and exiting the stage door 12 hours after landfall, this storm would be easier to comprehend. But latest model runs show Dorian slowing down considerably and tracking north along the Peninsula.

This creates the following high-impact angles to Dorian's flood story:

EXTENSIVE COASTAL FLOODING AND EROSION

If Dorian does track north up the Florida Peninsula, the potential for extensive coastal flooding and erosion along the entire Florida coastline north of Palm Beach increases substantially.

The latest Euro model run below shows the potential for hurricane-force winds to move up the coast even as Dorian's eye could track inland. A slow-moving hurricane taking this path could generate an extensive coastal flood event with massive erosion and property losses.

 COMPOUND FLOOD EVENT

A slow-moving hurricane or tropical storm greatly increases the risk of compound flooding from both storm surge and heavy rain.

The latest GFS precipitation forecast, copied below, shows the potential for 14 or more inches of rain over an extensive area from South Florida through South Carolina.  

Prolonged saltwater inundation will oppose, and effectively "dam" fresh water runoff, preventing this rain from draining to the coast. So when it rains 18" in an urban area next week, where is that rain going to drain to when saltwater is vigorously pushing inland?

It is becoming more likely that places that have never flooded before could experience deep inundation. Seven years ago this week, Hurricane Isaac's (2012) compound surge/ rain flooding inundated parts of southeast Louisiana that did not flood in Hurricane Katrina. The picture below shows the inundation in Laplace, Louisiana.

Inundation in Laplace, Louisiana, from Hurricane Isaac (2012). 

The ironic fact about compound flooding is that the worst impacts and greatest loss to life and property often happen after the storm has made landfall and the category has been "downgraded". 

Compound floods are long-duration events and humans cannot process long time frames well. We've watched too many movies when all the action happens in a quick explosion. It's much harder for us to comprehend that our flood threat is just beginning after the news says a storm has been downgraded or weakened.

LONG-DURATION FLOOD EVENT

Hurricane Dorian threatens to generate a long-duration flood event, which will exacerbate impacts. A long-duration coastal flood event enables saltwater to push farther inland. A longer-duration rain and wind event will bring down many more trees.

I observed a lot of tree falls last year from Hurricane Florence in North Carolina. When I talked to locals about the storm impacts, many people told me they thought the long-duration of the wind and rain enabled Florence to bring down so many trees, because the soil gave way after days of inundation. See pic below.

Wilmington, NC observed massive tree falls in Hurricane Florence last September because of the long-duration of strong winds blowing on trees in wet soil. Photo: Hal Needham

SECOND FLOOD "HOT" SPOT

The concaved-shaped coastline from northeast Florida through southwest South Carolina, including the entire Georgia Coast, is a "hot spot" for coastal flooding. The shape of the coastline and the shallow offshore water depth effectively enhance salt water floods if a hurricane is centered just off the northeast Florida coast or even located over the Peninsula.

Georgia has a brutal history of saltwater floods that struck in the late 1800s. The photo below, from the 1898 hurricane, shows widespread destruction at Brunswick, where a 16-foot storm surge slammed the coast. This storm also produced an 18-foot storm tide at Sapels Lighthouse and killed 179 people.

For more reading, see my blog post from October 7, 2016, "The Protected Coast is Now the Most Dangerous Place of All..."

Hurricane Irma's (2017) storm surge map shows how efficient this area is for generating storm surge. Along the east coast, tide gauges near the Florida/ Georgia border observed the highest water levels, as seen on the U-Surge storm surge map below. This surprised many people who expected the highest surge on Florida's east coast to be closer to Miami.

U-Surge map of Hurricane Irma's storm surge flooding. Note the localized maximum near the Florida/ Georgia border.

The latest Euro model run shows Dorian tracking just inland up the Florida Peninsula and then following the coastline along Georgia and South Carolina. The model run shows the wind field increasing in size dramatically compared to the initial landfall in southeast Florida.

Euro model for Thursday evening, forecasts Dorian to be centered along the coast near the Georgia/ South Carolina border with an increasing wind field.

If this forecast comes close to verifying, places like Jacksonville, Savannah (Tybee Island), Hilton Head and Charleston could observe substantial flooding, especially if the wind field increases in area.

Long story short, we are potentially looking at a multi-dimensional, long-term flood event that includes:

1) An initial storm surge in southeast or central Florida;

2) Extensive coastal flooding and erosion along most of Florida's East Coast;

3) Severe compound flooding that inundates areas with deep water that have never flooded on record;

4) A potential second coastal flood hotspot from St. Augustine/ Jacksonville through Charleston, including all of coastal Georgia.