Monitoring and Design of Louisiana Barrier Island Elevations
Whitney C. Thompson1, P.E., Program Manager
Whitney.Thompson@CBI.com
225-932-2568
Gordon Thomson1, P.E.
Barry Richard2, P.E.
Melissa Hymel2
1.
2.
CB&I Coastal Planning & Engineering
2481 N.W. Boca Raton Blvd.
Boca Raton, FL 33431
Louisiana Coastal Protection and Restoration Authority
New Orleans Field Office
Louisiana recognizes that restoring barrier islands is a critical element in coastal restoration plans.
However, only one of these barrier islands is inhabited (Grand Isle) and thus the dune elevation for the
remaining islands was based primarily on environmental benefits since protection of upland property is
not the primary consideration. This presentation provides new dune settlement data that supports the
consideration of increasing the constructed dune elevation to meet 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, NAVD88. 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, NAVD88 and subsidence impacts are minimal, there was a reluctance to construct the
dune higher than +6 feet, NAVD88. However, including impacts due to modeled and measured dune
settlement suggests that increasing the construction elevation may be necessary to realize project goals.
Several recent barrier island projects (Pelican Island, Shell Island East) have constructed dunes to an
elevation of +8 feet, NAVD88, and monitoring data for these projects is now available.
A geotechnical analysis of the soils at Shell Island East suggested that a dune constructed to +6 feet,
NAVD88 would settle and subside to an elevation just under +2 feet, NAVD88 in 20 years, without
consideration of any other coastal processes, such as overwash. This analysis was compared with
settlement and survey data from recently constructed Barataria barrier islands (Shell Island East, Pelican
Island, Scofield Island, Chaland Headland, East Grand Terre, Pass Chaland to Grand Bayou Pass and
Louisiana Sand Berms).
A typical project objective when designing barrier islands is to maintain dune elevation above +4 feet,
NAVD88 over the project life (CPE, 2005, 2009, and 2011). This may not be possible at historic dune
construction elevations at +6 feet, NAVD88 given the higher than previously anticipated settlement rate
and expected storm impacts. Therefore, several recent barrier island dunes have been constructed to +8
feet, NAVD88 in an effort to realize updated project objectives. This presentation will provide recent
Barataria dune settlement data along with the modeling analysis of the various constructed dune
elevations.