Sea Level rise example

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Vanderbilt/University of South Carolina

1. Project Description/Goals

The goal of this project is to understand and predict ecological and morphological responses to sea-level rise. Development of a two-dimensional landscape model for the Pamlico coastal wetlands is aimed at forecasting long-term changes in plant community composition, sediment accretion and supply, and morphological evolution in response to tidal forcing and sea-level rise.

Characterization of salt marsh macrophyte dynamics will help define the relationship between inundation ratio and biomass, as well as competition among species. Prediction of flow velocity is a surrogate for inundation time and advection of sediments adjacent to tidal channels. Sedimentation patterns can be extracted by documenting the role of turbulent energy and dissipation as a function of biomass. The significance of drag from macrophyte stems, which effects the rate at which water floods and drains salt marsh platforms, has yet to be appropriately tested.

3. Background

Tidal marsh lands are sensitive to long-term climatic change, with survival related to a balance between surface slope, sedimentation, and rates of sea level rise. In general, the elevation of salt marsh platforms strongly influences plant productivity, and this productivity has a positive feedback on the rate of accretion of the marsh surface. Marsh surface vegetation, on the other hand, controls flow (velocity and turbulence) within the canopy through drag and enhanced sediment deposition, which ultimately determines change in elevation (Fagherazzi et al., 2004).

Future changes along the marsh-terrestrial boundary are expected to be a consequence of sea-level rise, and it is important to know how the geometry of the shoreline will change as a result of intertidal marshes transitioning to subtidal habitat.

4. Research Design and Methods

This study will take advantage of existing long-term data, as well as establish a comprehensive data set using field and experimental laboratory methods. See milestone chart

5. Field Work

  1. Salt Marsh Macrophyte Dynamics
    Biomass distribution on salt marsh platforms is both directly and remotely observed with aerial photographs and hyperspectral data. Establishment of several marsh planters provides belowground production of Juncus in response to changes in relative elevation and tidal forces.
  2. Sedimentation
    Marsh elevation is monitored with the deep and shallow rod surface elevation (SET), a portable, mechanical leveling device designed to attach to a benchmark pipe driven into the ground (Cahoon & Turner, 1989). Vertical accretion in fertilized and unfertilized marsh plots within the edge, middle, and interior zones of the marsh platform are monitored with artificial soil marker horizons. Sedimentation is being analyzed with radioisotope methods ( 210 Pb changes and 137 Cs accumulations) within different plant communities and distances from channel networks to determine subsidence due to sediment autocompaction, constrain organic and inorganic deposition, and determine past rates of deposition. Short-term (biannual) sedimentation and grain size is monitored using tile plates for both Juncus and Spartina species at varying distances from the tidal channel.
  3. Long-term and synoptic measurements of flow, suspended sediment and water-surface elevation during ebb and flood cycles will be conducted over different stages of growing seasons.
    1. Pressure transducers

      Pressure transducers will record surface water depths along transects within Spartina and Juncus . At the completion of a growing season they will be moved to cover greater spatial coverage, i.e. increased distance from the mouth of the estuary. The continuous measurements will provide for a time-space data set that will for the first time reveal the two-dimensional form of flooding-ebbing platform flow and associated variations in water surface that drive this flow. Additionally, the rate of flood-ebb propagation will be monitored over several tidal cycles to obtain information about the spatial path of platform inundation.

    2. Suspended sediment concentrations (SSC)

      Suspended sediment concentrations (SSC) will be measured along transects parallel to flooding-ebbing flow away from the tidal creek. SSC will be used to evaluate gravity-driven settling and trapping by vegetation (which is necessary for the hydrodynamic-sedimentation model).

    3. Flow velocity

      Flow velocity measurements with an acoustic Doppler velocimetry meter will be used to characterize turbulence intensities in relation to vegetation structure. Such measurements will clarify the relationship between sedimentation rates and suppression of turbulent kinetic energy by vegetation.

  4. Field Work Products to Date:
    • Water level recorders installed at both the Cedar Island and PKS aquarium sites
    • GPS with RTK the PKS aquarium site, June 2006
    • Survey elevation of marsh platform at the PKS aquarium site, October 2006
    • Collect sediment from sediment tiles from 1 st grain size analysis, October 2006
    • Analyzed surface water data from July to October 2006 and created animation of flood-ebb flow
    • Dismantle and sample marsh plants, September 2006
  5. Model Formulation

    The model that will be used to predict flow, sedimentation, and vegetation dynamics in the Pamlico Sound is currently under development, with several milestones having been achieved. The two-dimensional flow routing algorithms are now functioning. We will calibrate the appropriate drag coefficients and fluid flux laws that will govern flow on the marsh by comparing model predictions with field measurements. In addition, the model now has data structures in place that can incorporate arbitrary marsh geometry; the geometry of the field sites will be incorporated into the model upon processing of the GPS surveys recently collected in the field. Over the next several months, we anticipate completion of the sedimentation and biomass components of the model, as well as a module that tracks the evolution of marsh stratigraphy.

  6. Expected products
    The end products of this study will provide a coupled physical and ecological model (1-D and 2-D) that demonstrate marsh surface response to sea-level rise.

    Empirical products will include:

    1. Above and belowground production of Juncus roemerianus and Spartina alterniflora as a function of relative elevation (marsh organ experiments).
    2. Time series measurements of surface water elevations across the marsh surface and subsequent drag coefficients.