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POSTHURRICANE IVAN SURVEY OF CHANDELEUR ISLANDS, LOUISIANA

By: Thomas C. Michot (Pilot/Biologist), USGS - NWRC
Scott Wilson (Electronics Engineer), USGS - NWRC
Aerial survey date: September 17-18, 2004
Storm name: Hurricane Ivan
Storm date (landfall): September 16, 2004
Max. sustained winds 212 kmph (132 mph)
Aircraft:
N727 (DOI 08), Cessna 185 Amphibian
Route of flight: Chandeleur LighthouseRedfish PointMonkey BayouNorth/New Harbor IslandsFreemason IslandCurlew IslandGosier IslandBreton Island
Chandeleur Lighthouse Redfish Point New Harbor Island Monkey Bayou Freemason Island Curlew Island Gosier Island Breton Island Breton Island Chandeleur Map
Map showing various landmarks in the Chandeleur Island chain that were surveyed before and after Hurricane Ivan (click on landmark name to go directly to the text and pictures for that area).

In coastal Louisiana, no event has such immediate and widespread economic, human, and ecological effect as a tropical weather system. The high winds, extreme rain, storm surge, and flooding may damage infrastructure, disrupt and destroy lives, and impact the natural environment. There is a well developed and understood emergency system in place for rapid assessment of storm impacts on the human population and infrastructure (i.e., FEMA). A similar system has not yet emerged for assessing the effects on the biota.

Researchers at National Wetlands Research Center (NWRC) have been monitoring the barrier islands of the Gulf of Mexico coast for almost two decades. Scientific studies of biological processes related to aspects of the barrier islands have been an important part of research activities. Additionally, we have produced numerous seagrass and terrestrial maps over the years allowing habitat trends to be followed at a very detailed

scale in space and time. Finally, we have an active aircraft observation methodology that allows rapid

qualitative and quantitative assessment of marsh and barrier island impacts of various human-caused and naturally occurring events. We have data from 13 flights conducted from September 1998 to present; from those data we can document changes in shoreline characteristics and island integrity.

Number of overwashes on the Chandeleur Islands

Click on graph for caption and larger version.

The first line of defense that may weaken a tropical storm is a barrier island system. Those islands are also usually the least resistant lands to the full force of violent weather. The use of low-altitude aircraft observations has allowed NWRC staff scientists to quickly respond to major events, informing policymakers and public land managers of major storm effects on the natural environment.

Chandeleur Lighthouse
Chandeleur Lighthouse 8/11/2004
Chandeleur Lighthouse 9/18/2004
The lighthouse was situated on land until Hurricane Georges (September 28, 1998). After that the island had eroded from under the lighthouse such that the lighthouse appeared to be in open water. Since Georges, although the island had reformed behind the lighthouse, the lighthouse remained in open water. The pre-Ivan photo (August 11, 2004) shows the lighthouse in open water about 30 m from the shoreline, and the northern tip of the island was relatively broad and extended several hundred meters north of the lighthouse. Note in the background, beyond the lighthouse, two large vegetated islands (1, 2), connected by beach, then an overwash channel that still had not closed up since Hurricane Lili. That channel is one of the seven overwash channels that were still open in our pre-storm survey (see graph). Beyond the aforementioned overwash channel, one can see two small vegetated islands (3, 4) followed by a larger island (5). Note that the land behind the lighthouse has completely washed away, and the first of the two large vegetated islands that were beyond the lighthouse has disappeared as well. Consequently, the closest vegetated island south of the lighthouse is now what was the second large island (2) before the storm. Beyond that island one can see the two smaller vegetated islands (3, 4) followed by the larger island (5), as in the previous photo. Note that the water behind the lighthouse has no whitecaps or breaking waves, which indicates an absence of shallow shoals. Such shoaling was evident immediately after Hurricane Georges. It was probably the cumulative effect of four hurricanes in 7 years that resulted in the deep erosion (evidenced by lack of shoaling) seen now after Hurricane Ivan.
Redfish Point, looking west to east from Chandeleur Sound to Gulf of Mexico.
Redfish Point is the largest peninsula in the Chandeleur chain and is the most prominent land feature of the main chain when seen from the air or in aerial photographs. It is located in the middle of the main chain
Redfish Point 8/11/2004
Redfish Point 9/18/2004

Note that the beach at this time was completely closed, with no exchange of water between the gulf and the sound. Overwash channels from previous storms had closed up. Green vegetation in foreground is smooth cordgrass (Spartina alterniflora, lighter green) and black mangrove (Avicennia germinans, dark green).

Note that the a major overwash channel [see graph] has opened between the gulf and the sound just south of Redfish Point; a minor channel can be seen just north of Redfish Point. Note also that, between overwash channels, the sand from the beach has been pushed back into the adjacent marsh. Marsh vegetation and mangroves seem to have survived the storm impacts so far, though a delayed response is possible. Seagrass beds can be seen in the foreground, though some are not visible because of the high turbidity in the water. The beds seem to be intact along the island except for where they were buried by sediment (for instance, 25% of the beds were buried during Hurricane Camille). Increased turbidity from the overwash channels could cause a decrease in seagrass viability in the near future.
Monkey Bayou (main chain, Chandeleur Islands) looking west to east from Chandeleur Sound toward the Gulf of Mexico
Monkey Bayou 8/11/2004
Monkey Bayou 9/18/2004
Note that a small cut through the beach remained open from the gulf to the sound since Hurricane Lili. Green vegetation in foreground is smooth cordgrass (Spartina alterniflora, lighter green) and black mangrove (Avicennia germinans, dark green).
Note that the opening between the gulf and the sound at Monkey Bayou is now much wider than it was before Ivan. Additional "major" overwash channels can be seen (see graph) north and south of the Monkey Bayou cut. Note also that between overwash channels, the sand from the beach has been pushed back into the adjacent marsh. Marsh vegetation and mangroves seem to have survived the storm impacts so far, though a delayed response is possible.
New Harbor Island, East (Chandeleur Islands)
E New Harbor 8/11/2004
E New Harbor 9/18/2004
Looking north to south, the eastern portion of New Harbor Island is shown; the tip of the western portion is barely visible on the right border of the photo. Black mangroves (Avicennia germinans, dark green) are well established along the shoreline of New Harbor Island, with smooth cordgrass (Spartina alterniflora, lighter green and brown) growing on most of the land mass. The mangroves on this island serve as roosting habitat for thousands of magnificent frigatebirds Fregata magnificens), a species that nests in the Caribbean region. The birds forage in the waters of Chandeleur Sound throughout the summer, which is the nonbreeding season for these birds.
In a view from south to north, here you can see that the island is intact after the storm, probably because it is situated directly behind the middle of the main chain, which is the most protected part. The marsh and mangroves seem unaffected at this time, though latent effects are possible. We did not observe any magnificent frigatebirds over the island on this flight, but during Hurricane Ivan birders reported more than 1,000 magnificent frigatebirds sighted over New Orleans, which is well inland of their normal foraging area. Note several areas of floating wrack (darker brown), mats of dead marsh vegetation (such as Spartina alterniflora and Phragmites australis) that was probably broken from marsh areas of the Mississippi Delta during Hurricane Ivan. Floating mats have commonly been observed in this area immediately after hurricanes. Seagrass beds (apparently intact) can be seen adjacent to the floating wrack mats.

New Harbor Island, West (Chandeleur Islands)
W New Harbor 8/11/2004
W New Harbor 9/18/2004
The western portion of New Harbor Island is shown. As in the eastern portion of New Harbor Island, black mangroves (Avicennia germinans, dark green) are well established along the shoreline and smooth cordgrass (Spartina alteriflora, lighter green) is growing the rest of the land mass. The mangroves on both portions of New Harbor Island serve as roosting habitat for magnificent frigatebirds. The island is intact after the storm, probably because it is situated directly behind the middle of the main chain, which is the most protected part. The marsh and mangroves can be easily seen in this image; they seem unaffected at this time, though latent effects are possible. Seagrass beds and wrack can be seen adjacent to the island.
Freemason Island (Chandeleur Islands)
Freemason Island is the southernmost of the "back islands" of the Chandeleur chain. It is a subaerial island with a substrate composed of 100% shell fragments from numerous mollusk species common to the area. This island is a favorite fishing spot for recreational anglers.
Freemason Island 8/11/2004
Freemason Island 9/18/2004
Looking east to west from Chandeleur Sound the island is intact except for a shallow opening of approximately 30 m that separates the two portions of the island. Two small patches of herbaceous vegetation (green) are visible in the photo. Looking west to east from Chandeleur Sound the island has been reduced to two small islets, one at each tip of what was previously a much longer island. Note several areas of floating wrack (darker brown), mats of dead marsh vegetation (such as Spartina alterniflora and Phragmites australis) that were probably broken from marsh areas of the Mississippi Delta during Hurricane Ivan. Floating mats such as these have commonly been observed in this area immediately after hurricanes.
South Gosier Island (Chandeleur Islands)
S. Gosier Island 8/11/2004
S. Gosier Island 9/18/2004
Prior to 1969, Grand Gosier Island was a single island, but Hurricane Camille split it into two. After Hurricane Georges (1998), North Gosier disappeared, but it came back so that just prior to Hurricane Ivan it was a long sand spit, largely unvegetated. South Gosier remained subaerial after Georges, and just prior to Ivan (pictured) it was a vegetated island with a well-developed sand spit on each end. North Gosier Island was completely washed away with no shoaling evident. South Gosier was also completely washed away, but with a small shoal in evidence.
North Curlew Island (Chandeleur Islands)
North Curlew Island 8/11/2004
North Curlew Island 9/18/2004
Curlew Island was a single island prior to Hurricane Georges (September 28, 1998). After Georges it was split into two islands. Just prior to Hurricane Ivan, South Curlew was only a subaerial spit, and North Curlew (pictured) was a long sand island with no vegetation except for a few small shrubs in the middle.

Both the north and south portions of Curlew Island were completely washed away, with no shoaling evident.

 

 

Breton Island
Breton Island 8/11/2004
Breton Island 9/18/2004
Breton Island was a single island prior to Hurricane Opal (1995), but it was divided into two during that storm. There was some recovery, but during Hurricane Georges (1998) it was further divided into three portions. Just prior to Hurricane Ivan, Breton Island (pictured) was again two portions separated by a shallow gap Breton was cut into three portions by Hurricane Ivan.

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