Exhumation of sequestered carbon by the Mid-Barataria Sediment Diversion

Reasonable estimates of the patterns of erosion caused by flow velocity from the Mid-Barataria Sediment Diversion can be made from flow velocity rates determined by the Delft 3-D modeling for the MBSD Environmental Impact Statement. The pink outline encloses an area where the model predicts velocities greater than about 25 centimeters per second. These velocities are adequate to cause erosion of sand and silts, and the highly peat layers in the area are likely to be even more erodible.

The size of the area of erosion nearest the river channel at MBSD can be compared to the area of land loss caused by erosion at the Fort St. Philip crevasse on the east bank of the river just down stream. The flow at the MBSD is likely to create channels that remove marsh material all the way to the open waters of Barataria Bay.

The material removed by erosion will be entirely brackish and saline marsh, which contain thick deposits of highly organic clays and peats.

The thickness of the peat and the percent organic content of the layer can be measured with data from the geotech borings from the MBSD project. A profile of borings shows the peat layer with percent organic values in black.

Using the thickness contours of the highly organic clays and peats within the outline of the area of erosion caused by flow velocity a accurate estimate of 7.6 billion cubic feet or 274 million tons of highly organic clays and peats will be removed by erosion. Using an average organic content of 40% from the geotech borings a reasonable estimate of 110 million tons of organic carbon will be removed by erosion from the flow velocity of the Mid-Barataria Sediment Diversion.

This organic material will be dispersed as particulate matter into the open waters of Barataria Bay where it will be subject to much higher rates of oxidation than when it was sequestered in the marsh. This will mean that there will be a significant increase in the rate of CO2 production from the eroded material.

The Delft3D model also projects the deposition of mineral sediment that will create emergent land across much of the area of erosion. A comparison suggests that in a 50-year time frame there will be a net loss of land area. This is consistent with the ratio of loss to gain at the Fort St. Philip crevasse over a 50-year time frame.