Temporal and spatial variability of transport and mixing mechanisms using heat and salt in the Duplin River, Georgia
McKay, James Paul
MetadataShow full item record
A study of the Duplin River, a shallow, sinuous, tidal creek which connects the salt marshes of Sapelo Island, on the central Georgia coast, with the waters of Doboy Sound and the coastal Atlantic Ocean, was conducted to quantify the physical processes which regulate the flux and zonation of heat and salt throughout the creek system. Three water masses are identified with differing temperature and salinity regimes. Hourly scale heat budgets are constructed for the upper (warmer) and lower (cooler) areas of the Duplin River showing the diminishing importance of tidal advection away from the mouth of the creek along with the concomitant increase in the importance of both direct atmospheric fluxes and of interactions with the marsh and side creeks. The heat budget is re-examined on daily averaged scales revealing the decreased importance of advective fluxes relative to direct atmospheric fluxes on this scale but the constant importance of marsh/creek interactions regardless of time scale or season. Tidally averaged along channel salt fluxes are calculated and a contrast is drawn between the lower and the upper Duplin. The main channel of the lower Duplin is bordered by creekless marsh, marsh hammocks and hard upland and salt fluxes are largely constrained to the main channel with salinity in the lower Duplin closely tracking observed salt fluxes. The upper Duplin is isolated from the lower Duplin by a sinuous channel and is subject to significant local fresh groundwater input. The upper Duplin acts as a reversing estuary on a fortnightly time scale. Salt fluxes are not constrained to the main channel but show a significant influence of the marsh. Vertical mixing is shown to be modulated on both M4 and fortnightly frequencies with turbulent stresses being generated near the bed and propagating into the water column on periods of max flood and ebb and being significantly greater on spring tide than on neap. Horizontal mixing is driven by tidal dispersion, which is modulated by the fortnightly spring/neap cycle. Net export of salt from the lower Duplin is shown to be due to residual advection modified by upstream tidal pumping which, in the absence of external forcing, exhibits a pulsating character with net export taking place for a short period on spring tide followed by a longer period of net import of salt. A box model is developed to explore subtidal inputs of groundwater and salt into the three water masses of the Duplin River. The results of this model are examined to draw insights into the magnitude and spatial distribution of these processes and their effect on the Duplin River water masses.