Bayou Caddy Restoration Site Shore Protection Evaluation and Conceptual Design


Bayou Caddy restoration site is an active beneficial use disposal area for the creation and restoration of marsh habitat. The marsh restoration site is currently confined with geotubes; however, prior to upgrades in 2013, there had been significant damage to the tubes (i.e., shifting and elevation loss) that resulted in reduction of restored marsh function. Structure damage from Tropical Storm Lee in 2011, and again from Hurricane Isaac in 2012, produced a general lowering of portions of each geotube, resulting in inadequate containment of dredged material used for salt marsh substrate. Tropical storms pushed the sand fill to the north end of each tube, packing and swelling that end while leaving the other end under-filled. Even during non-storm conditions, additional shifting of the geotubes occurred. As a result of the U.S. Army Corps of Engineers, Mobile District (USACE) replaced the geotube containment dike in 2013 and plans to construction of an offshore breakwater to reduce wave energy in its lee and enhance marsh restoration resilience. The intent is for the breakwater to provide protection to the geotube structure during critical design conditions (i.e., wave breaking at the structure crest) for dredged material containment purposes, and to provide an additional level of protection to the created marsh wetlands after the geotubes have degraded.

The basis for the design depended on substantial data collection/analysis efforts, as well as numerical modeling of waves to ensure that the wave attenuation structure meets the minimum design requirements. Specifically, historical shoreline and bathymetry data were used to document trends in sediment transport within the study area. Derivation of representative wave, wind, and tide/storm surge conditions, including hurricane conditions to evaluate structural performance during critical design conditions, were completed to support numerical modeling and design activities. The breakwater design was targeted to reduce wave heights to no greater than 3 feet acting on the geotube structure during storm events. All coastal processes analyses were used for evaluating various alternatives for breakwater project design, including the use of living shorelines as a beneficial ecosystem component of the project. Based on the alternatives analysis, two different structural alignments were recommended as part of the conceptual design. The structure design process considered alignment, height, width, and construction volume requirements of the two recommended alternatives. The preliminary design includes engineering plans, cost estimates for each alignment, and guidelines and criteria for construction specifications. CDM Smith personnel provided an independent technical review of engineering analysis procedures and project design.