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Title: Supplemental Environmental Assessment #555: Terrebonne Non-Federal Levee Mitigation, Terrebonne Parish, Louisiana
Authors: United States. Army. Corps of Engineers. Mississippi Valley Division.
Keywords: Levees
Environmental management
Terrebonne Parish (La.)
Publisher: United States. Army. Corps of Engineers. New Orleans District.
Abstract: The U.S. Army Corps of Engineers, Mississippi Valley Division, Regional Planning and Environment Division South, has prepared this Supplemental Environmental Assessment for New Orleans District to evaluate the effectiveness of the mitigation plan implemented for impacts incurred during improvement to non-Federal levees in Terrebonne Parish, LA, described in EA #450 titled “Terrebonne Parish Non-Federal Levee System Repairs, Replacements, Modifications, and Improvements Terrebonne Parish, Louisiana” and to explore corrective actions to ensure full satisfaction of the mitigation requirement. The Finding of No Significant Impact for EA #450 was approved by the CEMVN Commander on 14 January 2009. The project area is located approximately 8.5 miles south of Houma, LA and 2 miles north of Dulac, LA on the west shore of Lake Boudreaux. In 2006, through Public Law 109-234 (4th Supplemental), Congress authorized the repair, replacement, modification, and improvement of non-Federal levees and associated protection measures in Terrebonne Parish at full Federal expense. In response, the USACE, in coordination with Terrebonne Parish, raised 6 miles of levees to an elevation of +8.5 feet North American Vertical Datum of 1988. These improvements resulted in environmental impacts that required mitigation. A "habitat-based methodology" in the form of the wetland value assessment model was used to assess both the environmental impacts incurred during construction of the Terrebonne Non-Federal Levee improvements and the future benefits that would be obtained through the compensatory mitigation projects. The WVA model computes the difference in the habitat value over the period of analysis between the future with and future without project conditions. The difference is expressed as net average annual habitat units. For example, if the net change between the future without project condition and future with project over the 50-year period of evaluation is a +0.2 over 100 acres, then that project would produce 20 AAHUs of ecological benefit. The same version of the model was used to calculate both the impacts from Terrebonne NFL work and future benefits to be obtained through the implementation of the mitigation. For further information regarding WVA models see Section 1.9. Improvements to the NFL project impacted 12.1 acres (8.01 AAHUs) of bottomland hardwoods and 25.7 acres (17.21 AAHUs) of brackish marsh. The mitigation plan for these impacts was presented in EA #450 and involves the purchase of BLH mitigation bank credits and the construction of a brackish marsh creation project (see EA #450 for details). To satisfy the BLH mitigation requirement, approximately 13.2 BLH-Wet credits were purchased from Upper Bayou Folse and Ponderosa mitigation banks in March of 2017. To satisfy the brackish marsh mitigation requirement, a 74 acre marsh creation project was planned. However, following construction, the 74 acres included retention dikes without plans for future dike degrade. This resulted in only approximately 71 acres of actual marsh creation. Additionally, upon review of the project’s final surveys, it was discovered that portions of the marsh creation site never achieved elevations conducive to brackish marsh establishment (some too high, some too low). A site inspection conducted by the USACE and the Interagency Environmental Team in the fall 2013 verified the results of these surveys. In coordination with the IET, the project modeling was re-run to assess the benefits of the created project, which showed the site fell short of satisfying the brackish marsh mitigation requirement by 6.73 AAHUs. During the fall 2013 inspection, the IET also discovered damage to the mitigation project from all-terrain vehicles traversing the site, which incurred an additional 2.48 AAHUs of damage to the site. SEA #555 presents the remedial actions evaluated to ensure full satisfaction of the outstanding brackish marsh mitigation requirement, taking into account the shortfall at the created site and the damages to the marsh creation project from ATVs. Additionally, SEA #555 presents the final monitoring requirements for the functional portions of the marsh creation project as anticipated in EA #450’s FONSI.
Description: Supplemental Environmental Assessment / Finding of No Significant Impact
Rights: Approved for Public Release; Distribution is Unlimited
Size: 41 pages / 3.462 Mb
Types of Materials: PDF/A
Appears in Collections:Environmental Documents

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