Reconsidering Louisiana Island Barrier Island Construction Elevations
Gordon Thomson1, P.E., D.CE, Director (gthomson@coastalplanning.net)
Whitney Thompson1, P.E., Sr. Civil Engineer
Anna Wojtanovich, Engineer Intern
Andrew Wycklendt1, P.E., Coastal Engineer
David Swigler1, Coastal Engineer
1. Coastal Planning & Engineering, Inc., A Shaw Group Company
2481 N.W. Boca Raton Blvd.
Boca Raton, FL 33431
2. Louisiana Office of Coastal Protection and Restoration, Baton Rouge
Louisiana recognizes that restoring barrier islands is a critical element in their coastal restoration
plans. However, only one of these barrier islands is inhabited (Grand Isle) and thus the dune
elevation for the remaining islands was and is based primarily on environmental benefits because
protection of upland property is not the primary consideration. This presentation presents new
dune settlement data that suggests that consideration should be given to increasing the
constructed dune elevation to meet the CWPPRA’s evaluation criteria.
The Wetland Value Assessment (WVA) model (Environmental Working Group, 2002) is
typically used in the design and performance evaluation of Louisiana’s barrier islands. This
model is largely reliant upon acreage between various elevations, including dune acreage, which
is defined as acreage above +5 feet, NAVD. While it is recognized that subsidence results in
lowering of the dune elevation and thus can negatively impact the apparent performance of the
island, the impact of dune settlement has previously been considered negligible. As Louisiana’s
barrier islands do not tend to have elevations higher than +6 feet, NAVD and subsidence impacts
are minimal, there has been reluctance to construct the dune higher than +6 feet, NAVD.
However, including impacts due to modeled and measured dune settlement suggest that
increasing the construction elevation may be necessary to realize project goals.
A geotechnical analysis of the soils at Chenier Ronquille suggested that a dune constructed to +6
feet, NAVD would settle and subside to an elevation of +3.6 feet, NAVD in 20 years, without
consideration of any other coastal processes, such as overwash. This finding was compared with
newly available settlement and survey data from recently constructed barrier islands (Chaland
Headland, East Grand Terre, Pass Chaland to Grand Bayou Pass and Louisiana Sand Berms).
Given that the projects were adjacent to Chenier Ronquille, the data suggests that the rate of
settlement predicted for Chenier Ronquille may even be an underestimate of the rate of dune
crest lowering.
A typical project objective when designing barrier islands is to maintain dune elevation above +4
feet, NAVD over the project life (CPE, 2005, 2009, and 2011). This may not be possible at
Chenier Ronquille given the higher than previously anticipated settlement rate and expected
storm impacts. Therefore, it is proposed to raise the constructed dune elevation at Chenier
Ronquille to +8 feet, NAVD in an effort to realize updated project objectives. This presentation
will present this data along with the modeling analysis of the various constructed dune
elevations.