A high-resolution hydrodynamic model of the Duplin River Estuary, Sapelo Island GA with groundwater as the major buoyancy forcing

A high-resolution three dimensional hydrodynamic model for the Duplin, a tidally dominated elongated bay, is developed as part of the Georgia Coastal Ecosystems Long Term Ecological Research (GCE LTER) project. The major goal of the project is to quantify salinity and inundation patterns within the domain dominated by Spartina Alterniflora salt marsh. Extensive bathymetric mapping was carried out within the Duplin using LiDAR, swath multi beam mapping, and echo soundings of small creeks to create a 4m high-resolution digital elevation model. Observations of temperature, salinity, and sea surface height are forced at the open boundary, which takes into account the large scale offshore forcing and seasonal river influences from adjacent estuaries separated by the intracoastal water way. Groundwater is introduced in the upper section of the Duplin and is the major driver for fresh water in the system as a whole. Groundwater is difficult to gauge and our model/data comparisons of salinity have led to estimates of the discharge within the domain. To emphasize the importance of groundwater as the major driver of the dynamics in this system, we will present an analysis of residence times and horizontal transport processes via a salt flux analysis. One characteristic of the groundwater input together with the spring/neap modulation of horizontal mixing shows a reversing horizontal salinity gradient in the upper section. On spring tide, tidal transport is high and tidal energy mixes salty oceanic water into the upper Duplin (a positive estuary). On neap tide, fresh groundwater input in the middle section, combined with lower penetration of salt from the lower Duplin into these waters, establishes a negative salinity gradient in the upper section (a negative estuary).