Evaluation of the Proposal for Widening and Deepening the Matagorda Ship Channel

Principal Investigator
Paul Montagna, Ph.D.
Research
HydroEcology

The Calhoun Port Authority has initiated a study with the U.S. Army Corps of Engineers to acquire a permit for improving the Matagorda Ship Channel (MSC). The MSC is 26 miles long and extends from offshore in the Gulf of Mexico through Matagorda Baya and Lavaca Bay to the Port. This study will assess the effects on the natural system and human environment, including the economic development effects of existing inefficiencies.

The existing channel is used by vessels with drafts up to 38 feet deep at mean lower low water (MLLW). The channel bottom is 200 feet wide. The existing turning basin is 1000 by 1000 feet wide. The current alternative plan A is to deepen the channel to 47 feet MLLW, widen it to 350 feet in the bay and 600 feet in the Gulf, and expand the dimensions of the turning basin to 1,200feet.

The study predicts this would provide $6,539,000 in total net benefits, with a benefit-to-cost ratio (BCR) of 1.3. The study does identify environmental resources in Lavaca and Matagorda Bays in Section 2 and concludes no changes to any resources in Section 3. In Section 5, it states “Cumulative impacts due to past, existing, and reasonably foreseeable future projects, along with the Recommended Plan, are not expected to have significant adverse effects to resources in the study area.” However, there are concerns specific to Matagorda and Lavaca Bays that there could be the following potential changes:

Physical changes

  1. Circulation and storm surge changes due to changing the bathymetry of Matagorda and Lavaca Bays due to deepening the channel and placement of dredge spoil parallel to the channel, which will funnel tidal movement along the axis of the bay and restrict mixing perpendicular to the channel.
  2. Salinity changes due to the circulation changes and connecting with the groundwater lens.
  3. Puncturing the groundwater lens beneath the bay

Ecological changes

  1. Placement of dredge spoil could smother benthic habitats, such as oysters, seagrasses,      or muddy bay bottoms. This would affect forage potential for desirable fish species.
  2. Effects to primary producers or bottom dwelling organisms due to increase turbidity during the actual dredging.
  3. Mobilization of mercury by the dredging. Lavaca Bay is a National Superfund Site with considerable amounts of mercury in sediments of the bay.

Potential Environmental Assessments:

The assessment would be done in two parts: physical and ecological. Dr. James Gibeaut would lead the physical assessment, and Dr. Paul Montagna would lead the ecological assessment. The assessments would be based on literature review and reanalysis of existing data and information.

Tasks:

1. Physical Assessment:

  • 1.1. Circulation: Changing channel dimensions can change the pattern of circulation of water entering and exiting a bay driven by astronomical and wind tides and episodically by storm surge.  These impacts on circulation could cause changes in estuarine wetlands and increase storm surge hazard. A review of prior modeling results and available circulation data will assess the likelihood of adverse impacts and determine if a new modeling effort should go forward. 
  • 1.2. Salinity Change: Changes in circulation will also cause changes in the patterns and level of salinity in the bay. A review of prior salinity modeling results and available salinity data will assess the likelihood of channel deepening and widening affecting salinity and reveal if there is a need for further modeling. This assessment will also consider how much of a salinity change could adversely affect the ecosystem.
  • 1.3. Groundwater: The Gulf Coast Aquifer is a lens of freshwater beneath the bay, and it is possible that deepening the channel would puncture the containment layer such that a pathway for exchange of freshwater and seawater to mix. The threat is seawater incursion into the Coastal Aquifer or draining of the Coastal Aquifer into the bay. The location of the dredging will be evaluated for colocation with the aquifer.

2. Ecological Assessment:

  • 2.1. Dredge Spoil Placement: Habitat maps will be downloaded and compared to placement areas to identify resources at risk from burial or smothering.
  • 2.2. Turbidity: Sediment resuspension can lead to turbidity. This is often a temporary effect. A review of existing literature will compare turbidity effects on primary producers, which depend on light levels. Filter feeders, such as oysters, can also be affected by turbidity so these effects will be reviewed.
  • 2.3. Mercury: Mercury can be biomagnified in food webs, making fish dangerous to eat. Maps of the known mercury distributions in sediments will be overlaid with the location of the dredging to widen the channel and turning basin