| Coastal Georgia has numerous estuaries and tidal creeks surrounded by some of the largest intertidal (marsh) areas in the United States. These salt marshes are picturesque features of the coastal zone. They are also spawning and nursery habitats for a number of marine animals, providing nutrients for their use through twice-daily inundation by the tide. The efficiency that enables tides to furnish nutrients and remove wastes from a marsh is controlled, to a large extent, by marsh morphology, i.e., the distribution of high and low elevations. Defining the topography of a tidal channel and its estuaries, however, can be complicated. Over the past decade, two researchers at Skidaway Institute of Oceanography (SkIO) in collaboration with an international group of scientists have focused their attention on tidal circulation in the channels surrounding salt marshes.
In 1999, Dr. Jack Blanton and research coordinator, Julie Amft, began collaborating with fellow scientists in South Carolina, Massachusetts, Portugal and Brazil to measure the elevations of the intertidal areas typical of salt marshes. These high-resolution data sets are known as digital elevation models (DEMs). Such data are becoming increasingly important elements of numerical models designed to simulate flow in tidal channels. Examples of important information that can be derived from an area's DEM include its morphology, the tidal prism (related to the flushing of a tidal channel), and the hypsometric curve (the distribution of surface area to water depth).
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| Figure 1. Digital elevation model of the Okatee River, SC. The color bar represents elevations above mean low water. |
Existing data for the intertidal areas of marshes are usually limited since accessibility to them is generally restricted and difficult. The use of remote sensing from aircraft mitigates this difficulty. From a series of aerial images during a rising tide, measurements of the areal distribution were then converted to a grid of marsh elevations at resolutions as high as 1 m2. We refer to this technique as DIMITAP (digital measurements of intertidal topography from aerial platforms). The main objective of DIMITAP is to separate water from other reflecting objects in near-infrared images, so that detailed maps of the areal distribution of water, at the time of the image, can be determined. Applying DIMITAP to a series of aerial images obtained during the flood tide phase (elapsed time of 6-7 hours) leads to a high-resolution topographic map of the area (the DEM) (Fig. 1).
This MOVIE simulates the increasing water area as it follows the elevation contours during the rising tide. Professor Francisco Andrade and his assistant, Adelaide Ferreira, from the University of Lisbon (Portugal) designed DIMITAP.
The first DEM utilizing DIMITAP was constructed for a study site in South Carolina, on the Okatee River, as part of the Land-Use Ecosystem Research Program (LU-CES)(http://www.lu-ces.org/) and sponsored by NOAA's Coastal Ocean Program. The aerial mission was flown by Spectrum North Carolina, Inc. Two LU-CES projects have taken advantage of the Okatee River DEM. The first was a finite-volume circulation model (FVCOM) developed by Dr. Changsheng Chen at the University of Massachusetts. He and Dr. Haosheng Huang have computed detailed tidal currents in the upper reaches of the Okatee as part of the LU-CES project (Huang et al., in press) (Be patient, slow moving link!) (http://codfish.smast.umassd.edu/research_projects/LUCES/home.html). The high-resolution DEM of the Okatee is incorporated into the model grid (or mesh) that is the base of FVCOM's application to the Okatee. In a second project, the DIMITAP technique was used to compute flushing times and validate residence time models of the Okatee River (Moore, Blanton and Joye, 2006).
In 2000, tidal marsh areas in Georgia became the focus of the SkIO and Portuguese scientists as part of a larger national program, the Georgia Long-term Ecosystem Research program (G-LTER, http://gce-lter.marsci.uga.edu), sponsored by the National Science Foundation. The Duplin River (Sapelo Island, GA) was selected as a study site because with twelve main tributary creeks and little upland runoff, the tide is the primary generating force of the creek's circulation. Since only 15% of the high-tide area is covered at low water, the large intertidal expanse greatly affects the tidal circulation in the creek and the resulting fluxes of salt and other material.
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| Figure 2. Digital elevation model of the Duplin River, GA. The color bar represents elevations above mean low water. |
Another aerial mission, similar to the one flown over the Okatee River, was completed over the Duplin River from low to high water, in April 2004, to obtain the needed infrared aerial images to compute a DEM for this particular area (Fig. 2). (view MOVIE) Two high-resolution numerical models are being developed for the Duplin River area that will utilize this DEM. The first is based on the 3-dimensional version of the TELEMAC model, a proprietary finite-element model developed in Europe and being used by Dr. Elisa Fernandes at the Federal University of Rio Grande (Brazil). The second is a 3-D finite difference model (ALGE), designed by Dr. Alfred Garrett at DOE's Savannah River National Laboratory (SRNL). Both investigators will use the DEM to accurately portray the elevations of the intertidal area and its distribution of tributary tidal creeks to raise numerical simulations of tidal channel circulation to state-of-the-art detail.
An additional project is underway to take further advantage of the DIMITAP technique. The ALGE model, designed at SRNL, is also being used in Groves Creek, a small tidal creek adjacent to SkIO. Plans are underway to acquire the necessary flood-tide sequence of aerial infrared images and then to use the DIMITAP technique to provide high-resolution elevations of the intertidal area in Groves Creek. The Groves Creek marsh presents new challenges because the drainage patterns in the center of the marsh are unusually complex. However, there are new opportunities because the marsh is small enough to cover the entire domain from a helicopter. Moreover, being in SkIO's own backyard decreases the magnitude of the logistical problems associated with providing facilities for an airplane.
The use of remote sensing from aircraft, as described here, is being motivated by attempts to understand how transport processes in complex intertidal areas can be linked to marsh biogeochemistry, organic matter fluxes, and plant distributions. The tidal channels and marshes here in the southeastern US are prototypes of similar areas found throughout the planet. The attempt centered here at SkIO is attracting oceanographers from regions encompassing the Northern and Southern Hemispheres and Europe.
Acknowledgments: Several agencies have supported these efforts over the past 10 years. We thank the following:
Georgia Coastal Zone Management Program (Grant No. RR100-279/9262764)
National Science Foundation (LMER Grant No. DEB-9412089, LTER Grant No. OCE-9982133, and SEI+II Grant No. 0429644)
NOAA Coastal Ocean Program (Grant to South Carolina Sea Grant Consortium entitled "Tidal Circulation and Salt Transport in a Tidal Creek-Salt Marsh Complex")
Office of Naval Research (the SEACOOS program, N00014-02-1-0972)
Department of Energy Grant to the Savannah River National Laboratory, SR06-COL073
Luso -- American Foundation
Federal University of Rio Grande, Rio Grande, Brazil
Read more about the GCE-LTER and the Duplin River project:
Exchange Processes between Intertidal Areas and Tidal Creeks: The Duplin River Study by Blanton et al. (http://www.lternet.edu/news/images/spring05/GCE_Duplin_River_Study.pdf)
LTER Network News, Vol 18 (1), Spring. See pages 11 & 12.
Additional reading:
Blanton, J.O., G. Lin and S.A. Elston (2002) Tidal current asymmetry in shallow estuaries and tidal creeks. Continental Shelf Research 22: 1731-1743. pdf
Blanton, J.O., H. Seim, C. Alexander, J. Amft and G. Kineke (2003) Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA. Estuarine, Coastal and Shelf Science., 57:993-1006. pdf
Blanton, J.O., F. Andrade and M. Adelaide Ferreira (2006) The relationship of hydrodynamics and morphology in tidal-creek and salt-marsh systems in South Carolina and Georgia. In: Implications of changing land use patterns to coastal ecosystems. (Eds. Kleppel, G.S., M.R. DeVoe and M.V. Rawson, Jr.) p. 93-107. Springer-Verlag, New York, USA. Abstract
Moore, W. S., J. O. Blanton, and S. B. Joye (2006) Estimates of flushing times, submarine groundwater discharge, and nutrient fluxes to Okatee Estuary, South Carolina, J. Geophys. Res., 111: C09006, doi:10.1029/2005JC003041.
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Seim, H.E., J. O. Blanton and S. Elston (2006) Tidal circulation and energy dissipation in a shallow, sinuous estuary, Ocean Dynamics 56 (3-4): 360-375.
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Posted: 2007/10/26 10:32:11 |
