Subtask: Mississippi River Delta Plain: Controls on Coastal Evolution, Interior Wetland Sustainability, and Sediment Dynamics

Subtask Leader: Michael D. Miner - Pontchartrain Institute for Environmental Sciences (PIES), University of New Orleans, Louisiana

Subtask Aim

Figure 1. Coastal land loss time-series map for the Caminada Quadrangle from the USACE by Britsch and Dunbar (2007a). [Enlargement]

The Mississippi River delta plain (MRDP) is the most dynamic region in the Northern Gulf of Mexico (NGOM) due to rapid rates of relative sea level rise (RSRL), decreasing sediment supply associated with the abandonment phase of the delta cycle, and anthropogenic alterations to delta plain hydrology. These drivers produce a transgressive regime within which interior wetlands are converted to open water at rates of up to 100 km2/yr for select time periods (Barras et al., 2003), and barrier shorelines migrate landward at rates of up to 23 m/yr (Williams et al., 1992). A concise understanding of: 1) controls on coastal evolution, 2) resilience of interior wetlands and barrier shorelines to storm impacts and sea-level rise, and 3) system-wide sediment dynamics are crucial if management attempts are to be successful. This proposed task aims to resolve and quantify all three of these critical issues. In order to predict future trends, the relationship between interior wetland loss and coastal evolution must be better constrained and quantified for both long and short-term time periods. We presently do not understand how wetland loss increasingly more distant from tidal inlets will contribute to the tidal prism. Moreover, there may be wetland areas within backbarrier bays that when converted to subtidal regions would redistribute tidal prisms among the interconnected tidal inlets (i.e. Barataria Bay). Historical analyses combined with results from existing hydrodynamic models will allow us to investigate these types of relationships and predict future bay area-barrier island tidal inlet dynamics and evolution. Wetlands serve a similar purpose in reducing storm surge elevation and dampening wave energy (Day et al., 2007). While it has been recently shown by Barras et al. (2005; in prep) that storm events may act as a catalyst for subsequent interior wetland loss at locations where less-resistant marsh becomes torn or denuded of vegetation, one of the benefits of major storms is that they suspend mud and blanket marsh regions with a layer of inorganic sediment, thereby aiding their vertical accretion and ability to keep pace with RSLR (Reed, 2002). It appears that an important factor in determining marsh resilience to the storm-associated destruction described and documented by Barras et al. (2005) is high concentrations of inorganic sediment within the marsh substrate. If that is the case, areas of marsh that are more frequently inundated by marine waters during smaller tropical storms and the passage of frontal systems and contain higher concentrations of mineral sediment will be more resilient to sea-level rise than those marshes that are only inundated during large magnitude hurricanes and characterized by highly organic soils.

This task uses a two-pronged approach to better constrain and quantify the relationship between interior wetland evolution and coastal/inner shelf evolution for the Northern Gulf of Mexico, with the overall goal of establishing a sediment budget for the regional transgressive system. The first part of this task focuses on quantifying the relationship between interior wetland loss and coastal evolution and sediment dynamics over long (1800s to present) and short-term (annual) timescales. The second focuses on classifying the physical characteristics of marshes, estimating accretion rates, and quantifying relative contributions of inorganic sediment through time with a specific focus of quantifying the role of storms in sediment delivery to the marshes. Results from the two efforts will be merged to assess Northern Gulf of Mexico interior marshes and coastal zone resilience to future hurricanes and ongoing sea-level rise.

Subtask Activities

This task will focus on better understanding linkages between coastal evolution and interior wetland evolution and MRDP sediment dynamics. The following generalized approach will be employed to quantify these relationships:

• Historical charts and satellite imagery acquired through NGOM Subtask 3.3 (John Barras, PI), dating from the 1780s to present will be used to delineate bay shorelines and quantify bay areas. Tidal prisms will be calculated based on the cubature method (Jarrett, 1976) and information concerning attenuation of the tidal wave within the bays. The signature of tidal forcing in the bays will be determined from co-range and co-tidal lines that will be established from historical tide/stream gage data, new measurements, and existing hydrodynamic models for these regions. Historical changes in tidal prism for different time period will then be related to coastal geomorphology (ebb tidal delta volume and inlet cross sectional area) in order to quantify the relationship between interior wetland loss and coastal evolution through time.
• Annual surveys and monitoring along the coast in key areas focused on quantifying sediment dynamics, current velocities, tidal prism, and inlet and shoreface geometry will be used to assess coastal processes and dominant forcing mechanisms in order to develop short term coastal evolutionary models. These data will also provide input for hydrodynamic and sediment transport modeling efforts as part of UNO NGOM Task 3.
• Transects through the three major estuarine basins within the MRDP (Terrebonne/Timbalier, Barataria, and Pontchartrain/Breton Sound), which correspond to vignette focus areas highlighted in NGOM Subtask 3.3, will be sampled using short marsh cores and vibracores. Transects will extend from the barrier shoreline inland to the upper freshwater-dominated zones of the estuary. Samples will be used to classify the physical, geotechnical, and geochemical characteristics of marsh sediments; accretion rates will be determined using Pb-210 and Cesium-137 isotopic analyses. The goal of this part of the project will be to better identify key factors that contribute toward marsh resilience to hurricane impacts, understand why some marshes respond differently than others, and assess their ability to accrete vertically and keep pace with sea-level rise. This part of UNO Task 1 will be used to answer key questions raised in NGOM Subtask 3.3 about marsh response to storms and their near term ability to recover. UNO will work closely with USGS personnel from Subtask 3.3 to identify these questions, select sampling sites, and relate the results from physical, geotechnical, and geochemical analysis to the storm response captured in satellite imagery analysis.

Subtask Publications