USGS Proposal: Predicting the Resilience of the Chandeleur Island Chain as a Function of Restoration Options

Introduction:

Figure 1:  Location map of the Chandeleur Islands in relation to the Mississippi RiverDelta.  The islands, both Federally (green) and state owned (red) lie within the Breton National Wildlife Refuge

The Chandeleur Islands are the largest barrier island arc in Louisiana and are located in St. Bernard and Plaquemines Parishes (Figure 1). Remnants of the old St. Bernard delta, they trend north-south and provide the initial barrier to storms for the SE Louisiana wetlands and are a fundamental component of the geomorphologic features that protect the metropolitan New Orleans area.

The Breton National Wildlife Refuge (NWR) was established in 1904 to provide sanctuary for nesting and wintering seabirds, protect and preserve the wilderness character of the islands, and provide sandy beach habitat for a variety of wildlife species. Breton NWR is a combination of Federal and State land that includes Breton Island and all of Chandeleur Islands in St. Bernard Parish, LA. All of the Federally-owned lands, except for North Breton Island, in Breton NWR became part of the National Wilderness Preservation System on January 3, 1975 (Public Law 93-632). North Breton was excluded because an oil facility, owned by Kerr-McGee, Inc., was located on that island.

In recent years, restorative plantings through CWPPRA in 1999 and 2000 and the beneficial use of dredged material have been demonstrated on these islands in an attempt to stabilize the islands and enhance habitat for nesting birds. Initial results obtained in 2002 were encouraging because the plants generally thrived within the vegetated plots and some stands successfully trapped sediment in areas where island breaching had occurred during Tropical Storm Isidore and Hurricane Lili (M. Hymel, 2003).

The severity of damage by Hurricanes Katrina and Rita brings into question what the future configuration of the island chain will be, what protective function the islands will provide for the mainland wetlands and New Orleans, and whether the Breton NWR can continue to provide the same level of functional habitat for nesting birds and other wildlife as prior to the hurricanes.

Dramatic changes in land area and geomorphology as a result of repeated hurricane impacts suggest that the Chandeleur Islands are evolving toward a sub-aqueous shoal. Long-term geologic models suggest that this island chain will eventually disappear and become an inner-shelf shoal in response to the combined effects of sand loss during storms and continuing relative sea-level rise. Hurricane Katrina accelerated this process; at present the southern half of the chain is mostly submerged shoal. Elevations on Breton NWR prior to Hurricanes Katrina and Rita ranged from sea level to 6 m above mean sea level. Peak elevations are now are less than 3 m, and the marsh platforms that supported the islands’ sand before Hurricane Katrina have been reduced in width by more than half. Light Intensity Direction and Ranging (LIDAR) surveys before and after Hurricanes Katrina and Rita have documented a loss of roughly 85% of the island chain’s area and 92% of its volume (Figure 2, A. Sallenger, 2006). The land area of the Breton NWR is greatly reduced (Figure 3, S. Grace, pers. com.), and the average island width is now less than one half of the pre-storm width. Repeated surveys since September 2005 indicate that the island continues to change. In places the Gulf-side shorelines have retreated as much as 100 m in the presence of relatively low-energy waves. Continued monitoring of the system is essential to determining the future configuration of the islands.

Figure 2. Chandeleur Islands after Hurricane Lili in 2002 and (right) after Hurricane Rita in 2005.

Figure 3. Aerial photography of the northern portion of Breton NWR pre-Katrina in 2004 and post-Katrina in 2005 with refuge boundaries superimposed.

Historic information of physical geologic data provides points of ground truth on a graph of system response. USGS and UNO have a tremendous quantity of data, historical trends from the 1880’s, and field data collected during the last 30 years. Data on the Chandeleurs land area show a generally decreasing trend since the late 1800’s, with each storm that damages the island chain followed by a period of rebuilding. We don’t have data plotted yet for Hurricanes Katrina and Rita; however, we know from Sallenger (2006) that the 85% of the land area lost puts the 2005 point on the graph significantly lower than the point labeled Georges (Figure 4 top), and that the rate of change is undoubtedly higher than the preceding point (Figure 4 bottom).

Figure 4. Upper graph illustrates the general downward trend of remaining land following each storm since the late 1800’s. The dashed red line indicates a hypothetical threshold below which the islands won’t recover without intervention.  Precise location of the recovery threshold is unknown at present.  Bottom graph illustrates the variation in rate of change.

Figure 5. Boxes outline the three main units of the Chandeleur Island chain. Each unit will be treated and modeled as a separate segment.

Further, it has been noted that the Chandeleur Island chain responds not as a single unit but as three different units (Figure 5):

1. the northern segment (Chandeleur Island proper) that protects several smaller islands to the west,
2. a middle segment (Curlew to Grand Gosier), and
3. the southern segment (Breton Island).

It is apparent that in the northern segment of Chandeleur Island proper, the ends of the islands eroded significantly while the mid portions remained somewhat stable although migrating slowly landward. In the mid-section of the chain, the islands appear and disappear throughout historical time, undoubtedly responding to variable patterns of accretion and erosion resulting from storm events and calm weather conditions. In the southern portion of the chain, Breton Island showed significant recovery since Hurricane Georges (1998), tropical storm Isidore (2002) and Hurricane Lili (2002), the last storms to cause significant damage. Subaqueous shoals rebuilt to subaerial sand spits and the open water gap between the south and north portions of Breton Island narrowed significantly. What will happen without restoration subsequent to the 2005 storms is unknown.