Deltaic morphodynamics and stratigraphic evolution of Middle Barataria Bay and Middle Breton Sound regions, Louisiana, USA: Implications for river-sediment diversions

River-sediment diversions have been recognized as a key strategy for offsetting land loss in coastal Louisiana. Recently, much attention has been focused on sediment capture from the main stem river and conveyance of that material through diversion outflows. Yet, the performance and long-term feasibility of diversions as land building agents also hinges upon the subsurface stratigraphic architecture of receiving basins, a concept that has largely been unexplored. A major core collection and analysis program was undertaken to study the geological properties of substrates in the Middle Barataria Bay and Middle Breton Sound diversion receiving basins, Louisiana, USA. Over a region of similar to 200 square kilometers in both basins, 50 vibracores up to 5.5 m in length were collected in spring and summer of 2015. Cores were logged for bulk density and imaged with a Geotek Multi Sensor Core Logger. Split cores were then subsampled for granulometry, organic content, and C-14 dating. Both receiving basins are characterized by 1-2 m of organic-rich surficial strata, underlain by > 2 m of mineral rich sand and silt beds that display a greater bulk density. Additionally, some cores contain deeper peaty strata intercalated within muds and sands. C-14 dates of these buried peats range between 1910 and 3203 calendar years before present demonstrating that relatively old deltaic strata exist at shallow depths. Age comparisons of stratigraphically-similar peat beds from this study and previous delta-lobe chronological models indicate that the St. Bernard lobe prograded eastward at a time-averaged rate of 75 m/yr. From a geotechnical standpoint, the surficial, uncompacted (< 2 m depth, average 1.15 g/cc bulk density) peaty strata in both basins are likely to erode and/or compact under the influence of diversion flows, but deeper consolidated mineral-rich strata (average 1.85 g/cc bulk density) are likely to be more resistant. Hydrodynamic models that predict the erosion associated with river-sediment diversion outflows in this region would benefit from incorporating a two-layer subsurface configuration, with a weaker, peat-like layer overlying a stronger, mineral-rich layer. The results of our study suggest that regions with an abundance of clastic strata at depth should be sought out when planning coastal restoration methods such as river-sediment diversions.