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Environmental Investigation of the Long-Term Use of Trinity Shoal as Sand Resources for Large Scale Beach and Coastal Restoration in Louisiana

$700K

FY 2007-2009

Louisiana State University

Roger Amato

Ongoing

August 2009

http://www.mms.gov/sandandgravel/Louisianastudies.htm

January 2008

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Environmental Studies Program Information System

MMS ENVIRONMENTAL STUDIES PROGRAM: ONGOING STUDIES
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Description:
Background
MMS recognizes the potential for the long-term use of Outer Continental Shelf sand resources for coastal erosion management, a critical challenge to Louisiana's ecosystems and economies. MMS is currently negotiating agreements with the State of Louisiana and other Federal agencies for planned barrier shoreline and coastal restoration projects. Many large-scale projects are in the planning stages in coastal Louisiana, some of which will require enormous amounts of nourishment material. The potential physical and biological impacts of mining offshore shoals are being explored on a regional and local scale through the MMS ESP; this study focuses on the potential use of Tiger and Trinity shoals located off Vermillion Parish. Past physical field and numerical modeling studies conducted offshore Louisiana indicate that inner shelf morphology exerts a significant influence on regional hydrodynamics, reducing wave energy and modulating current velocity, particularly during storm events (Stone and Xu, 1996; Stone et al, 2001). The study area is a unique section of coast due to the influence of the Atchafalaya River and its debouching of fine sediment (silts and clays) into the Gulf coastal zone. Subsequent effects of fine grained sediment on the inner shelf must be considered in determining trends in wave propagation and nearshore wave-current properties. These shoals also support major demersal fisheries for white and brown shrimp. These shrimp are likely dependent upon the system's benthic macro- and meiofaunal (infaunal) communities. Sand mining is expected to adversely affect the existing benthic communities and to result in altered communities for an unknown period of time after initial recovery. As such, sand mining may have adverse impacts on the system's shrimp and the important fisheries (i.e., shrimp and spotted sea trout). Given the current understanding of the benthic-based shrimp food chain as it pertains to the shoal system, it is not possible to forecast the recovery rate of benthic animals or any impacts on the food chain that might occur after sand mining.
Objectives
The objectives of the study are to provide biological, physical, and other pertinent information which can be used by the MMS to evaluate the impacts associated with large-scale, cumulative extraction of sand from Tiger and Trinity Shoals.
Methods
Site-specific numerical wave modeling, using fine-mesh sub-grids, will be performed using a state-of-the-art spectral model coupled to hydrodynamic and sediment transport modules to evaluate the cumulative effects of large-scale sand extraction from within the most likely targets. These models will be calibrated and verified using bottom boundary layer observations obtained from multiple tripod deployments. Faunal surveys (e.g., trawls), in situ water sampling, and sediment sampling will be used to collect samples for the determination of abundance, taxonomic composition, community structure, and habitat conditions of macrofaunal and meiofaunual communities. Water and sediments samples will be processed using HPLC to measure primary production. PAR and water column characteristics will be measured through the water column at sampling stations for comparison with biological data. Stable isotope analysis (nitrogen, carbon, sulfur) of consumer and primary producers (benthic microalgae and phytoplankton) will be used to investigate food web dynamics.
Importance to MMS
Relatively little information relative to physical conditions, processes, and pelagic/benthic biology is available for this area. The collection and assessment of biological information, as well as site-specific numerical modeling in the areas most likely to be utilized, is critical if MMS is to assess the potential consequences of long-term use of the shoals.
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