All barrier islands are not "created equal"

[MiamiensisWx]

Firstly, I'll start the lede and state that evacuation of barrier islands is absolutely essential for safety, and all residents should heed evacuation orders.

However, it is also important to denounce the misnomer that ALL portions of a barrier island flood during a tropical cyclone. It is simply false. A barrier island's vulnerability is largely influenced by its location, topography/geology/elevation, the offshore seafloor topography, adjacent topographical/geographical features, and the closest approach/size/wind radii of a TC. For example, a barrier island on the Gulf Coast (from southwest Florida to Brownsville, Texas) is more vulnerable to complete inundation during the impact of a large tropical cyclone because of the expansive offshore fetch, shallow coastal waters, and "sloping" seafloor topography. This fact was demonstrated during the 1900 and 1915 Galveston hurricanes; Carla 1961; Camille 1969; Katrina/Rita 2005; Ike 2008; and other TCs. In this case, any location situated on a barrier island (featuring an elevation below a certain height) is completely submerged by the storm surge and higher waves. The town of High Island was the only town that escaped complete devastation on the Bolivar Peninsula during Ike, and the town was elevated on a considerably higher grade/elevation (in relation to sea level) than other portions of the area. Conversely, barrier islands on the southeast coast of Florida are more vulnerable to flooding from waves because of the deeper offshore waters; therefore, surge is reduced, but high waves cause significant beach erosion during large or intense TCs. Small sized TCs, "brushes", and extratropical/baroclinic/barotropic systems also induce significant erosion effects. The only exceptions are the islands adjacent to Biscayne and Florida Bays; they are vulnerable to waves from the Atlantic and a "back surge" from the shallower bay sides. Elliott Key, Soldier Key, etc. and the Florida Keys are examples, and Andrew of 1992 and the 1935 Labor Day hurricane are the representatives of the highest surge heights observed in southeast Florida.

However, as mentioned, "all barrier islands are not 'created equal.'" I have even witnessed some perplexing conundrums. For example, as Hurricane Floyd of 1999 moved over the northeastern Bahamas and (subsequently) passed safely off the eastern Florida coast, I made an excursion to the barrier island in Pompano Beach, Broward County, Florida. At this time, the tropical cyclone was paralleling the eastern Florida coastline; we were finally "all clear" and the TS/hurricane winds remained offshore. The barrier island that extends from Hillsboro Inlet to Port Everglades is nearly completely urbanized, and it is features a lower elevation and less defined central ridge (like other Broward/Miami-Dade barrier islands) than the barrier islands in Palm Beach and Martin counties. However, the underlying elevation of the initial dune facing the ocean is (historically) similar to the Palm Beach barrier islands and the barrier island that stretches from Hillsboro Inlet Light to the Boca Raton Inlet. The Boca Raton barrier island did not flood during Floyd's passage; however, that was not the case on the Pompano Beach barrier island, where waves and coastal flooding easily reached the parking lot at the extreme eastern terminus of Atlantic Boulevard. This was my personal observation. What factor(s) could enhance this discrepancy? It is possible that the dune elevation on the Pompano Beach island has declined since pre-development times. It is also possible that localized topographic/geographic factors may favor more coastal flooding on this island and others, unlike the Boca Raton island.

Regardless, I thought this was an interesting observation... does anyone have any explanations?

[jinftl]

I witnessed some amazing things such as you describe with Floyd....during high tide, the entire sand area of the beach in Fort Lauderdale (A1A and Sunrise Blvd.) was gone....water was making it to about the middle of A1A and it left so much sand on the roads that it looked like a brown snowstorm had hit. The same took place on Hollywood Beach. Water was standing on the boardwalk and crashing up against some buildings.

Another interesting thing was that parts of the west side...the Bay side...of Miami Beach flooded from Floyd during high tide. In particular, around the area of Alton Road and 10th Street. From what I recall hearing, the water was actually coming up through the sewers. The beach side did not see water hit the street...probably due in part to the wider expanse of sand than in other areas of south florida.

[Ed Mahmoud]

High Island is actually a hill created by a salt dome. Jefferson and Avery Islands in Louisiana are similar.

I work with a geologist who believes 'The Heights' section of Houston is the last undrilled salt dome on the Gulf Coast.

East of Galveston, Texas, a thin barrier beach—Bolivar Peninsula—stretches along the coastline. Like Galveston, this barrier beach lay directly in the path of Hurricane Ike in September 2008. On the far eastern end of Bolivar Peninsula is High Island, so named for the salt dome that boosts the area some 11.6 meters (38 feet) above the rest of the Gulf Coast between Mobile, Alabama, and the Yucatán Peninsula.

These images show High Island before and after the destruction caused by Hurricane Ike. NASA’s Landsat 7 satellite captured the bottom image on October 22, 1999. In this image, the encircling Oilfield Road delineates High Island, which otherwise looks similar to the surrounding landscape, with its mixture of urbanized and vegetated areas. The white rectangle in this image outlines the area shown in the top image. A photographer with the U.S. National Oceanic and Atmospheric Administration (NOAA) took the top photo from an airplane on September 16, 2008. In the top image, High Island really has become an island. Off the edge of the salt dome, water has submerged the landscape, and an oil slick floats over Oilfield Road.

http://earthobservatory.nasa.gov/NaturalHazards/shownh.php3?img_id=15076

[MiamiensisWx]

I witnessed some amazing things such as you describe with Floyd....during high tide, the entire sand area of the beach in Fort Lauderdale (A1A and Sunrise Blvd.) was gone....water was making it to about the middle of A1A and it left so much sand on the roads that it looked like a brown snowstorm had hit. The same took place on Hollywood Beach. Water was standing on the boardwalk and crashing up against some buildings.

Another interesting thing was that parts of the west side...the Bay side...of Miami Beach flooded from Floyd during high tide. In particular, around the area of Alton Road and 10th Street. From what I recall hearing, the water was actually coming up through the sewers. The beach side did not see water hit the street...probably due in part to the wider expanse of sand than in other areas of south florida.

I can recall the effects on Lauderdale-by-the-Sea and Fort Lauderdale's oceanfront area as well... those areas are also situated on the same barrier island that encompasses coastal Pompano Beach east of the Intracoastal Waterway. I can vaguely (but definitely) remember the news coverage of the Biscayne Bay flooding on the western sides of Miami Beach, too. Honestly, the effects on coastal structures in southeastern Florida (via erosion/undermining) would have been disastrous if Floyd made a direct strike on the southeast Florida counties. We would have likely observed a repeat of the erosion effects of the 1926 hurricane on the Miami-Dade/Broward barrier islands. That TC was another large, intense hurricane. We were very fortunate in Floyd's case, but we did not escape its effects, and we received significant coastal effects from Frances and Jeanne in 2004.

I think another important factor that influences the vulnerability of a barrier island/coastal vicinity is the geology and soil content. Here is an excellent survey of the soils and related facets (drainage, composition, etc.) in eastern Broward County, Florida:

http://soildatamart.nrcs.usda.gov/manuscripts/FL606/0/broward.pdf

Note that the dredging, filling, and alteration of local deposits (for development) has changed the geology of the region, which has always been "in flux" for thousands of years as well... the effects of human settlement on soil/geology certainly could be contributing to greater flood hazards on the lower barrier islands with less well defined central ridges/dune elevations above mean sea level.

Interesting soil map from the preceding publication:

http://img388.imageshack.us/img388/6695/easternbrowardflsoil1iv0.png
Data is courtesy of the http://soildatamart.nrcs.usda.gov/manuscripts/FL606/0/broward.pdf report.

[Sanibel]

There was a category 3 that hit Sanibel in the past (I forget the year but I should research it and find the exact storm) that left a sea bottom sand and shell deposit across the island. The barrier island of Sanibel is just a ridge left over from rising sea levels that stayed above the bays behind it on the inland side. When I went to Useppa Island the Caloosa Indian Museum showed how Florida was 6 times wider than it is now 10,000 years ago or something like that. This area was well inland with hills etc, so we are the remaining high point that the sea reached on the last rise.

Sanibel was all barrier island Florida grass tussocks all the way across up until the 40's when some people got some dynamite and made some small ponds to hold water through the dry season. Before that there was no water sources on the island for mainland fuana to last through dry season. I think I saw an old Spanish map that referred to Sanibel as "Dry Island" in spanish. After the pond and ditching alligators and fish migrated to the island and trees took root. It was the grass Sanibel that the hurricane left sea floor across back when.

After seeing land dug out from under stilt houses on Bolivar any fantasy of riding out a storm has left my head. Sanibel is a bad place for hurricane enthusiasts to try to ride out a cyclone because the surge threat means early evacuation even for lower category hurricanes.

[MiamiensisWx]

There was a category 3 that hit Sanibel in the past (I forget the year but I should research it and find the exact storm) that left a sea bottom sand and shell deposit across the island. The barrier island of Sanibel is just a ridge left over from rising sea levels that stayed above the bays behind it on the inland side. When I went to Useppa Island the Caloosa Indian Museum showed how Florida was 6 times wider than it is now 10,000 years ago or something like that. This area was well inland with hills etc, so we are the remaining high point that the sea reached on the last rise.

Sanibel was all barrier island Florida grass tussocks all the way across up until the 40's when some people got some dynamite and made some small ponds to hold water through the dry season. Before that there was no water sources on the island for mainland fuana to last through dry season. I think I saw an old Spanish map that referred to Sanibel as "Dry Island" in spanish. After the pond and ditching alligators and fish migrated to the island and trees took root. It was the grass Sanibel that the hurricane left sea floor across back when.

After seeing land dug out from under stilt houses on Bolivar any fantasy of riding out a storm has left my head. Sanibel is a bad place for hurricane enthusiasts to try to ride out a cyclone because the surge threat means early evacuation even for lower category hurricanes.

Ultimately, the surge values on the southwest Florida coast (and other locales on the Gulf Coast) are determined by the wind radii, size, and angle of approach of the TC...

I believe I can recall accounts of the 1944 Florida hurricane, which was the eleventh TC of that season. It did cross the southwest Florida coast near Sarasota with winds of Category 3 strength, and its immense size was noted by the Monthly Weather Review and surface/reconnaissance observations. News accounts did mention reports that indicated the tropical cyclone submerged many portions of Sanibel Island, leaving deposits of shells/crustaceans, calcareous substances, mud/sand silt, and other sediments. I'm merely recalling this data from memory, so I'm not entirely positive in regards to the accuracy of the information. The 1910, 1921, 1926, and 1947 hurricanes also flooded Sanibel as well. Did you refer to the 1944 TC in the bolded portion of your post?

[MiamiensisWx]

I witnessed some amazing things such as you describe with Floyd....during high tide, the entire sand area of the beach in Fort Lauderdale (A1A and Sunrise Blvd.) was gone....water was making it to about the middle of A1A and it left so much sand on the roads that it looked like a brown snowstorm had hit. The same took place on Hollywood Beach. Water was standing on the boardwalk and crashing up against some buildings.

Another interesting thing was that parts of the west side...the Bay side...of Miami Beach flooded from Floyd during high tide. In particular, around the area of Alton Road and 10th Street. From what I recall hearing, the water was actually coming up through the sewers. The beach side did not see water hit the street...probably due in part to the wider expanse of sand than in other areas of south florida.

I can recall the effects on Lauderdale-by-the-Sea and Fort Lauderdale's oceanfront area as well... those areas are also situated on the same barrier island that encompasses coastal Pompano Beach east of the Intracoastal Waterway. I can vaguely (but definitely) remember the news coverage of the Biscayne Bay flooding on the western sides of Miami Beach, too. Honestly, the effects on coastal structures in southeastern Florida (via erosion/undermining) would have been disastrous if Floyd made a direct strike on the southeast Florida counties. We would have likely observed a repeat of the erosion effects of the 1926 hurricane on the Miami-Dade/Broward barrier islands. That TC was another large, intense hurricane. We were very fortunate in Floyd's case, but we did not escape its effects, and we received significant coastal effects from Frances and Jeanne in 2004.

I think another important factor that influences the vulnerability of a barrier island/coastal vicinity is the geology and soil content. Here is an excellent survey of the soils and related facets (drainage, composition, etc.) in eastern Broward County, Florida:

http://soildatamart.nrcs.usda.gov/manuscripts/FL606/0/broward.pdf

Note that the dredging, filling, and alteration of local deposits (for development) has changed the geology of the region, which has always been "in flux" for thousands of years as well... the effects of human settlement on soil/geology certainly could be contributing to greater flood hazards on the lower barrier islands with less well defined central ridges/dune elevations above mean sea level.

Interesting soil map from the preceding publication:

http://img388.imageshack.us/img388/6695/easternbrowardflsoil1iv0.png
Data is courtesy of the http://soildatamart.nrcs.usda.gov/manuscripts/FL606/0/broward.pdf report.

Bump for JinFTL and other interests...

[jinftl]

Great info! Thanks...very interesting subject for sure....and educational.

I am determined to find the photos i took on fort lauderdale beach when floyd made his closest approach.....instead of the ocean meeting the sand, and then the usual expanse of sand between the beach and A1A, the water meets the yellow median on A1A and the beach is 'gone'. I packed those photos when i moved....but they are worth digging out and posting. Shows what a storm of Floyd's size and strength could do 200 miles away. I shudder to imagine what a direct hit would have done.

I witnessed some amazing things such as you describe with Floyd....during high tide, the entire sand area of the beach in Fort Lauderdale (A1A and Sunrise Blvd.) was gone....water was making it to about the middle of A1A and it left so much sand on the roads that it looked like a brown snowstorm had hit. The same took place on Hollywood Beach. Water was standing on the boardwalk and crashing up against some buildings.

Another interesting thing was that parts of the west side...the Bay side...of Miami Beach flooded from Floyd during high tide. In particular, around the area of Alton Road and 10th Street. From what I recall hearing, the water was actually coming up through the sewers. The beach side did not see water hit the street...probably due in part to the wider expanse of sand than in other areas of south florida.

I can recall the effects on Lauderdale-by-the-Sea and Fort Lauderdale's oceanfront area as well... those areas are also situated on the same barrier island that encompasses coastal Pompano Beach east of the Intracoastal Waterway. I can vaguely (but definitely) remember the news coverage of the Biscayne Bay flooding on the western sides of Miami Beach, too. Honestly, the effects on coastal structures in southeastern Florida (via erosion/undermining) would have been disastrous if Floyd made a direct strike on the southeast Florida counties. We would have likely observed a repeat of the erosion effects of the 1926 hurricane on the Miami-Dade/Broward barrier islands. That TC was another large, intense hurricane. We were very fortunate in Floyd's case, but we did not escape its effects, and we received significant coastal effects from Frances and Jeanne in 2004.

I think another important factor that influences the vulnerability of a barrier island/coastal vicinity is the geology and soil content. Here is an excellent survey of the soils and related facets (drainage, composition, etc.) in eastern Broward County, Florida:

http://soildatamart.nrcs.usda.gov/manuscripts/FL606/0/broward.pdf

Note that the dredging, filling, and alteration of local deposits (for development) has changed the geology of the region, which has always been "in flux" for thousands of years as well... the effects of human settlement on soil/geology certainly could be contributing to greater flood hazards on the lower barrier islands with less well defined central ridges/dune elevations above mean sea level.

Interesting soil map from the preceding publication:

http://img388.imageshack.us/img388/6695/easternbrowardflsoil1iv0.png
Data is courtesy of the http://soildatamart.nrcs.usda.gov/manuscripts/FL606/0/broward.pdf report.

Bump for JinFTL and other interests...