Report: Bayou Chene project’s damage understated

A report from a wetlands permitting consultant says the Bayou Chene Flood Control and Diversion Project will hurt Avoca Island more than the St. Mary Levee District determined. Avoca LLC, which owns much of the land on Avoca Island, hired the consultant to do the report.
Senior Permitting Analyst Gregg Fell of Natural Resource Professionals in Baton Rouge submitted the report to the U.S. Army Corps of Engineers May 20 and to the Louisiana Department of Natural Resources May 31.
St. Mary Levee District officials are in the permitting phase of the estimated $107.5 million Bayou Chene Flood Control and Diversion Project in Amelia that is aimed at protecting up to six parishes from flooding with a permanent flood control structure. The structure would allow the levee district to simply close a floodgate when the Atchafalaya River reaches 7 feet in Morgan City and is forecast to rise.
Officials installed a temporary structure in January 2016 and May 2011 when water was high in the Atchafalaya River.
The report determined that the permanent structure “will result in the degradation of 371 acres of coastal wetlands” on Avoca Island in addition to the area beneath the footprint of the structure. St. Mary Levee District Executive Director Tim Matte said the district’s modeling has shown that the damages to wetlands on Avoca Island are limited to 8 acres of direct impact and 24 acres of total impact.
The permanent project consists of about a 10-mile levee, 258-foot floodgate anchored to the southeastern part of Avoca Island, construction of the Bayou Chene channel and a weir on Tabor Canal.
Eight miles of the levee will be constructed on Avoca Island using 420,000 cubic yards from a proposed borrow area on Avoca Island, the report said.
Levee district officials used a 2-D hydrodynamic model to estimate the coastal wetlands impacts to Avoca Island as part of applying for permits with the Department of Natural Resources and Corps of Engineers.
The report said that adverse impacts from the levee district’s model were limited to the footprint of the proposed levee and control structures.
DNR issued a permit to the levee district for the project in September 2015. In November 2015, Avoca filed a lawsuit in district court against DNR requesting the department rescind the levee district’s permit. The levee district has not yet received any permits from the Corps of Engineers for the project.
In its lawsuit against DNR, Avoca said that DNR did not have the information needed to properly evaluate the Bayou Chene project’s impacts to surrounding property when it issued the permit. The court has not yet ruled on the lawsuit.
Avoca hired John Andrew Nyman, an expert in hydrodynamic models and wetlands ecosystems, to review the model. Nyman considered the 371 acres of impact to be a conservative estimate, the report said.
Based on Nyman’s assessment, Avoca rejected the levee district’s impact assessment, concluding that the levee district’s model “lacks the resolution and coverage needed to adequately assess adverse impacts to coastal wetlands within Avoca Island” the report stated.
Matte said the report portrays some effects as consequences of the Bayou Chene project, when the effects really are just consequences of flooding as a result of the river being high.
The 10-mile levee to which the report refers consists of using high ground as part of the levee and raising the road on Avoca Island, Matte said.
To determine the Bayou Chene project’s impact to the island, Avoca used modeling to figure out how many acres the project would have affected in 2008, 2009 and 2011, taking the average of those results to arrive at the 371 acres of impact.
Matte said that’s not an “appropriate way” to determine the impact, in part, because the levee district likely would not have even operated the Bayou Chene floodgate one of those years.
Avoca hired engineering firm Moffat & Nichol to develop the Avoca Island Regional Hydrodynamic Model. Moffat & Nichol collected hydrodynamic data within the island’s open water areas during the January 2016 flood and for about three months thereafter, the report said.
The Avoca Model was then used to do a comparative analysis of with and without Bayou Chene Project features based on the levee district’s proposed floodgate operational plan using regional U.S. Geological Survey gage data from October 2007 through October 2012, the report stated.
Engineers determined that three Bayou Chene floodgate closures would have occurred during this time period, once in 2008, 2009 and 2011 with an average closure of 33 days, the report said.
Moffat & Nichol also determined the average increase in maximum water elevations would have been 0.57 feet, the report said.
Levee district officials continue to work toward getting the Bayou Chene project to construction with a goal date of April 2017 when Restore Act funds from the BP oil spill settlement become available.