When a critical wetland prairie was threatened by encroaching erosion from Lake Pontchartrain west of New Orleans, ABMB was called in to help rescue the irreplaceable marshland. Separated only by a narrow landbridge, the wetland was deemed valuable enough that four non-federal entities partnered to fund the project, including the Louisiana Department of Natural Resources, the Louisiana Department of Wildlife and Fisheries, Southeastern Louisiana University and the Richard King Mellon Foundation.
The Manchac Prairie is a critical open water area surrounded by intermediate to brackish marsh. With only the narrow landbridge separating the marsh from destruction by Lake Pontchartrain, the Corps of Engineers, New Orleans District had constructed segmented breakwaters to reduce wave-induced erosion. DNR had also constructed a small gabion wall structure near the most vulnerable part of the landbridge to protect the prairie. However, settlement of the Corps’ breakwaters, combined with scour between the breakwaters, have again put the Manchac Prairie in jeopardy.
ABMB, working closely with a local coastal engineering specialty firm, have evaluated the existing conditions and made recommendations for a solution that should provide the necessary protection for the Manchac Prairie. Before the wave and wind data could be analyzed in terms of its hydraulic impact on the new breakwaters, a complete hydrographic survey of the site was performed. Aerial photography was also reviewed to determine the effectiveness of DNR’s gabion structure compared to the breakwater structures.
Using historical wave and wind data obtained from the Coastal Engineering Section, the shoreline hydraulics were modeled using the Automated Coastal Engineering System (ACES) suite of coastal hydraulic models to simulate hurricane force winds up to 90 m.p.h.
With these models, ABMB was able to determine the optimum length of breakwaters, gap width and distance offshore to encourage sediment accretion landside of the breakwater without causing new scour from trapped lake water flowing back into the lake. ABMB prepared plans and specifications to build these breakwaters in 2006. As Hurricane Katrina has shown, coastal preservation and restoration is more critical than ever, and ABMB is pleased to play a role in this vital effort.