Vegetative Succession Model Development

 The following outline for Task TWMSM, Vegetative Succession Model Development, is being presented as a task statement for review, modification, and approval. The Scope of Work detailed below was developed as a cooperative effort between the U.S. Fish and Wildlife Service (FWS) and Applied Technology & Management, Inc. (ATM).  Both the FWS and ATM are involved in collection of field data as detailed in TASK Family TWSA and TASK Family TWSU.  The data will be used to development a wetland succession model that will provide predictive capability regarding the future wetland vegetation distribution within and around the Savannah National Wildlife Refuge.  Applied Technology and Management, Inc. will develop the model in cooperation with the U.S. Fish and Wildlife Service.

I. TASK GOAL

The goals of this task are to integrate data collected under the field data collection tasks into wetland impact prediction model.  This model will be a geographic information system (GIS)-based spatial model that will predict changes in wetland vegetation distribution caused by salinity and water level changes associated with harbor deepening.  A further goal of the model will be to study potential long-term (50-year) changes in vegetation distribution resulting from sea level rise.

II. PROJECT NEED

Based on input received from the Tier I EIS, GPA is planning to develop an improved methodology for the application of salinity modeling results to an assessment of salinity impacts on the tidal freshwater wetlands of the Refuge.  This impact assessment will need to include the development and use of a set of hierarchically nested spatial tidal freshwater wetland succession models as a predictive tool to assess salinity impacts under various design alternatives.

III. PROPOSED SCOPE

This scope will compile all data generated by the FWS and ATM and incorporate it into a spatial tidal freshwater wetland impact prediction model.  Development of the model will be a collaborative effort between the FWS and ATM.   Development of the Tidal Wetland Succession Model will be ongoing for the duration of the project.  The model will be a spatially based model, meaning that it will receive input and produce output in the form of maps as per Pearlstine et al. (1990).  It will be designed to provide acreage calculations of marsh changes over time.  The principle input variables of concern are spatial hydrologic parameters, including hydroperiod, timing, and salinity regimes either modeled or measured to reflect pre- and post-project conditions as well as substrate characteristics.  The model will be calibrated using data generated from the field data collection tasks outlined above and verified against field data collected in the late-1980s during the time the tide gate was in operation. 

Fundamental to any discussion of marsh succession is the availability and viability of seeds within the marsh sediments.  This study will determine what seeds are found at what locations within the sediments of the study area and under what salinity conditions they will germinate and grow.  This study will be conducted in a greenhouse using sediments collected from a number of locations within the study area.  The study design will provide for salinity treatments ranging from fresh to brackish.  In addition to the seed bank study, seeds will be collected from selected tree species.  In a greenhouse-based study, the collected seeds will be germinated and growth rates assessed under different salinity treatments.

 Another important component of model development will be the consideration of the affect of historic water level changes within the study area and future changes that may be expected.  This task will be conducted by the hydrodynamic modeling group and will provide data regarding historic water levels in the study area.  It is anticipated these data will be generated by both compilation and review of historic records and through modeling.  An estimate of long-term sea level rise over the past 150 years and a prediction of continued rise for the next 50 years are to be produced.  Water level predictions are to be accompanied by estimates of changes in salinity.  Specific factors to be considered in this task are water level changes and discharge volumes associated with the construction the upstream dams and reservoirs, the affect of the tide gate on water levels (including the residual effect of the remaining concrete structure), and tidal surge and post-storm river discharges resulting from Hurricane David in 1979.

IV. DELIVERABLES

The deliverable for this task will be a working computer model.  The model will be calibrated and verified as discussed above and ready for subsequent use in tidal freshwater wetland impact prediction and mitigation option assessment.  A report documenting the development of the model will be submitted.