Barrier Island Evolution - Geologic Analysis

Science Center Objects

Quantifying changes in morphology and sediment distribution over short time scales will demonstrate how geologic variability influences medium-term barrier island response and near-term barrier island trajectories and help to refine sedimentological boundary conditions for morphologic evolution models.

slide showing overwash and/or breaching processes in sediment cores

Sediment cores collected during the early part of 2012 show gradual increase in organic matter (OM) and porosity, decrease in bulk density upcore, which reflect longer term fair-weather deposition. Following Hurricane Isaac, sediment cores have sharp contacts of low OM and porosity and high bulk density reflecting the denser and presumably sandier, sediment transported across the island by overwash and/or breaching processes. (Public domain.)

Geologic variability (changes in stratigraphy, modern sediment distribution and composition, and morphology) has long been associated with barrier island evolution over centennial and millennial time scales. However, the relative importance of geologic variability over shorter time scales (days to years) remains poorly understood.

Regional-scale research, while helpful for establishing the geologic framework in which barrier islands evolve, lacks the finer-scale resolution necessary for addressing seasonal and interannual system response. Furthermore, models of morphologic evolution are often ill equipped to incorporate the complexity of natural geologic variability and often assume uniformity in sediment distribution, composition, and availability that may not exist. This leads to results that may not be consistent with observations.

In order to address medium-term relationships between geologic variability and storm and non-storm processes, high-resolution information from highly dynamic areas of the nearshore, surf zone, and back-barrier must be obtained, and observed geologic variability must be suitably parameterized for integration with predictive models. Quantifying changes in morphology and sediment distribution over short time scales will demonstrate how geologic variability influences medium-term barrier island response and near-term barrier island trajectories and help to refine sedimentological boundary conditions for morphologic evolution models.

Geomorphic series of the Chandeleur Islands

Geomorphic series of the Chandeleur Islands, Louisiana, depicting nearshore digital elevation models (50 m x 50 m) of merged single-beam and interferometric sonar soundings for the years 2011, 2012, 2013, and 2015 (small, focused survey with interferometric only). (Public domain.)

Objectives

  • Understand how offshore morphologic changes relate to topographic changes.
  • Improve estimates of cross-shore and alongshore variability of sediment sources and sinks.
  • Understand the influence of realistic morphology and sedimentology on predicted island response.
  • Determine how variations in back-barrier environments influence the long-term trajectory of barrier island evolution.
  • Determine how different back-barrier marsh and tidal flat environments respond to short-term (hurricanes) and long-term (sea-level rise) climate-related forcings.
  • Evaluate (pre)historic episodic events influence on long-term sedimentation trends in mainland and back-barrier marsh environments.
  • Continue to monitor natural land elevation loss (subsidence).

Methodology

Data collection

  • Geophysical observations (morphology, sediment distribution, subsurface stratigraphy) from the surf zone and nearshore to identify sediment transport pathways and relate changes in the submerged environment to topographic changes.
  • Sediment sampling (surface samples, short cores) from the berm and island to quantify horizontal (alongshore) and vertical (from surface to subsurface) variability in geology.
  • Evaluate sedimentation rates and chronology of back-barrier and mainland marsh via short-lived (e.g., lead-210, cesium-137, beryllium-7, and thorium-234) radionuclides.
  • Use ground-penetrating radar (GPR) to investigate along-barrier subsurface sediment distribution (focus on high, wide barrier islands).
  • Ground-truth GPR with land-based vibracoring and auger along axis of barrier island.
  • Evaluate (paleo)environments based on lithology, microfossils, and the geometry of depositional units.
  • Install shallow wells across the island and/or identify pre-existing irrigation wells (current or abandoned) that can be instrumented with pressure, temperature, and conductivity sensors.
  • Measure land elevations at established benchmarks at many locations around the northern Gulf of Mexico.

Data integration

  • Utilize project data resources and collaborators to characterize topographic changes to island/berm.
  • Utilize previously collected geophysical data as baseline for quantifying submerged changes.
  • Utilize previously collected sedimentological data to impose specific boundary conditions on morphologic models.
  • Incorporate geophysical data (i.e., bathymetry) into morphologic models as an update to model input and as a check on model output.
  • Parameterize (e.g., simplify) sedimentological observations for incorporation into morphologic models to determine role of sediment limitation on interannual barrier island evolution.
  • Extrapolation of marsh sedimentation rates using aerial imagery to a more regional scale.
  • Develop model of recent (paleo)environmental evolution of barrier island.
  • Develop a compartmentalized budget of dominant subsurface barrier island environments/units that may influence erodibility—ultimate inclusion in numerical models (to include, e.g., inlet fill, buried peat, and sand fractions).