Long and Short-Term Morphologic Evolution and Stratigraphic Architecture of a Transgressive Tidal Inlet, Little Pass Timbalier, Louisiana

The majority of changes to barrier island shorelines can be attributed to tidal inlet processes, and therefore an understanding of these processes is important to effectively manage barrier systems. High rates of relative sea-level rise within the Mississippi River delta plain have resulted in rapid landwardmigration of barrier island and tidal inlet systems. Moreover, ongoing conversion of back barrier and interior wetlands to open water increases tidal exchange. Enlarging tidal prisms, together with the landward migration of the barrier systems, results in a dynamic environment within which tidal inlets undergo vast changes in position, geometry, and shoreline morphology. Historic bathymetric maps (dating to the 1880’s) and newly acquired bathymetric data for Little Pass Timbalier have been assembled to construct an evolutionary model for the region. The evolution of Little Pass Timbalier is complex and encompasses landward migrating ebb channel at rates of 33 m/yr, lateral channel migration at rates of 23 m/yr, and avulsion to new breaching sites along the adjacent barrier shoreline. Increasing backbarrier tidal prism results in seaward progradation of the ebb tidal delta as the inlet throat migrates landward. Following the active 2005 hurricane season, a post-storm bathymetric survey was conducted. A sediment volume change analysis determined that 10.6 x 106 m3 of sediment were removed from the study area. Vibracores and subbottom profiles taken along the inlet retreat path at locations where relic channel depth was determined to be deeper than the depth of wave ravinement reveal an inlet fill sedimentary package bounded by scour surfaces separating underlying and adjacent fluvio-deltaic deposits. The inlet channel fill consists of a coarsening upward interval of shelly clayey sands that are lenticular to wavy bedded and grade upward into ebb tidal delta shelly sands. The inlet fill geometry is in the form of an erosionally bounded dib-elongate channel fill that thins seaward and pinches out at the location of inlet formation (storm breach). The inlet fill developed as a result of landward migration (dip elongate geometry) and associated with a transgressive barrier system differs from inlet fills developed along stable coastlines at laterally migrating tidal inlets (strike elongate geometry)