USGS: Prehurricane Survey of Barrier Islands 2004

2004 PREHURRICANE SURVEY OF BARRIER ISLANDS (Louisiana, Mississippi, Alabama)

By:

Thomas C. Michot (Pilot/Biologist)
Christopher J. Wells (Observer/Geographer)

Aerial survey date:	11 August 2004, 1030-1700 CDT
Storm name:	Tropical Storm Bonnie
Storm date (landfall):	12 August 2004
Max. sustained winds	81 kmph (50 mph)
Aircraft:	N727 (DOI 08), Cessna 185 Amphibian
Route of flight:	Lafayette – Breton Island (Chandeleur chain) – Redfish Point – North/New Harbor Island – Freemason Island – Redfish Point – Chandeleur Lighthouse – Ship Island – Horn Island – Dauphin Island – Ship Island – Cat Island – New Orleans – Lafayette (flight path)

Background

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.

Number of overwashes on the Chandeleur Islands

Click on graph for larger version.

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.

With two storms (Bonnie and Charley) either already in the gulf or moving into it the week of 9 August 2004, we conducted the current survey to establish prestorm baseline conditions and thus to be in a good position to document damages from these two or any other subsequent storms this year.

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.

Methods

Observations and photography are taken from a Department of the Interior Cessna 185 amphibious aircraft. Observation altitude is 300 m (1,000 ft) with the observer's side nearest the shore of interest so that high-oblique photography or videotape may be acquired. Concomitantly, the pilot transmits his voice observations to computer, where it is recorded. Each entry to the computer is linked with the aircraft’s Global Positioning System (GPS) location and time. The time-stamp allows correlation between the voice log and the photography. On this reconnaissance trip, digital still photos were taken of all islands including both tips, any locations in between that could serve as landmarks, portions that appeared to be vulnerable, and portions that appeared to have changed from previous observations. Pilot’s data log recorded latitude and longitude and notes for each photo. Notes presented in this report are largely from memory and may contain some inaccuracies, but more detailed information (including latitude and longitude references) will be available once the pilot’s data have been transcribed from the computer. Photos and other data are available on request (tommy_michot@usgs.gov).

Results

General: Much healing and recovery has occurred since Hurricane Georges (September 1998), Tropical Storm Isidore (September 2002), and Hurricane Lili (October 2002). Those were the last storms to cause significant damage to the Chandeleur Islands and other islands in the northern Gulf of Mexico. Island portions that were reduced by Georges to subaqueous shoals have now built back up to subaerial sand spits or unvegetated islands. Overwash channels formed by Georges, especially on the main chain of the Chandeleurs, have mostly closed up (see description of Chandeleurs below).

Breton Island:
The open water gap between the south and north portions of Breton Island has closed in significantly though there is still an opening of about 100-300 m (300-1000 ft) of shallow shoals. At the northwest end, near the old Kerr-McGee facility, the beach front has become well developed in front of the emergent marsh, and a long narrow spit has formed that extends to the south to become the new island tip (see photo). Sand fences are visible on the north end.

Grand Gosier Island: Formerly a single island, this island had been cut into two portions (North Gosier and South Gosier) by previous storms. During Georges, North Gosier became completely subaqueous; now it has resurfaced to become subaerial as several small islands, though it remains unvegetated except for a few sprigs of vegetation. South Gosier remains vegetated, has now formed substantial sand spits on both ends, and has a well developed arc on the north end and some shoaling on the south end; the island is wider on the north end. Numerous brown pelicans were noted on South Gosier.

Curlew Island: Formerly a single island, this island had been cut into two portions by previous storms. During Georges, South Curlew became completely subaqueous and North Curlew was barely subaerial with little to no vegetation. Now both are subaerial. South Curlew is an unvegetated hook-shaped sand spit. North Curlew has somewhat reformed as a long linear subaerial island, largely unvegetated (but with a few small patches of vegetation near the middle). There were several unconnected islets on the north end of North Curlew.

Chandeleur Islands, main chain: Overwash channels formed by Georges, and reopened by Isidore and Lili, have mostly closed up. We found only six major and one minor channel now cut through the main chain, as opposed to 101 (60 major, 41 minor) channels after Georges (see graph). By March 2002 the island had recovered so that there were only 15 channels through the island, but after Isidore and Lili we found 65 channels. Now that number is down to seven. Four overwash channels remain south of Redfish Point (one north of an area known locally as “Degeau Hole,” one at “Monkey Bayou,” and two south of that). The middle portion of the main chain, from just north of “Degeau Hole” to a few miles south of the lighthouse, is intact. There are two major channels on the north end, south of the lighthouse, and one small minor channel (about 5-10 m or 15-30 ft in width) at our Transect A location (known as the “Big Overwash” site) about 2 km (1.2 miles) south of the lighthouse. The lighthouse is still intact, in open water about 15-20 m (50-65 ft) from the existing shoreline. The lighthouse has been situated in open water since Georges, but the land behind it, subaqueous after Georges, has recovered since then to a well developed spit, though west of its original position (see photo).

New Harbor, North, and Freemason Islands: These islands, known as the “back islands” of the Chandeleur chain, were surveyed and photographed. They are largely protected from wave energy by the main chain and thus not as susceptible to storm damage. North Island is currently in three sections, New Harbor is also in three sections, and Freemason is in two sections but connected by a shallow shoal in a gap of about 20-25 m (65-80 ft) in width. Freemason is largely unvegetated shell substrate, whereas North and New Harbor are predominantly emergent marsh and mangrove.

Cat, Ship, Horn, Petit Bois, and Dauphin Islands: These islands are largely intact, with some low elevation beach habitat (e.g., east end of Ship Island), recovered from previous storms, seemingly susceptible to new storm impacts.

Impacts from TS Bonnie

On 11 August, at the time of our observational flight in that area, the storm was still about 321 km (200 miles) radial distance from the southern tip of the Chandeleurs. Since the tropical storm effects extended about 112 km (70 miles), we suspect there was no significant effect of the storm on Chandeleur Islands at that time. NOAA maintains an anchored buoy (Number 42007) northeast of the northern tip of Chandeleur Islands. A review of wind and wave action from 11 August compared to 12 August revealed that mean sustained wind velocity and direction were largely the same until the predawn hours of 12 August when the winds shifted 90 degrees. On 11 August the winds were out of the SSW all day, with sustained wind velocities of 9-25 kmph (6-15 mph) (gusts to 28 kmph or 18 mph). On 12 August the winds remained SSW until 02:50 AM CDT when the wind shifted abruptly and the hourly recorded direction was NNW to northerly at 03:50 AM CDT, from which direction it has remained as of 12 August 11:50AM CDT. Wind velocities remained about the same as on 11 August, i.e., 8-25 kmph (5-15 mph).

Wave heights and direction were fairly unchanged (SSE, 20-90 cm or 0.7-2.6 ft) over 11 August to 12 August. However, there was a slightly higher wave height maintained between 8:00 PM CDT 11 August and 9:00 AM CDT 12 August than either before or after those times. Swell heights and direction remained steady.

Conclusions

Based on the above buoy data, we believe that impacts from TS Bonnie would be minimal to negligible. Therefore, it does not appear that a post-storm survey flight would be warranted. We should keep an eye on Hurricane Charley, but if it continues on its projected path to the gulf coast of south Florida, and then on to the Atlantic coast, impacts to our study area from that storm will likely be minimal as well. We have successfully updated our dataset for the Chandeleurs and other islands and are now set to accurately assess impacts of any storms that may occur during the current hurricane season (through November 30). Data collection should continue at least annually to establish prestorm baseline conditions that are current for a given year.