THE CONTROL OF OVERWASH AND SOUND-SIDE FLOODING ON MIXING-ZONE MORPHOLOGY

Conceptual models of the mixing zone in coastal aquifers describe a sharp interface between fresh and saline waters or a mixing zone that forms in response to tidal oscillations, and other factors. While some studies describe more complex conditions in coastal aquifers, few describe the influence of storm events. We use calibrated simulations with the USGS finite-element code SUTRA to examine the evolution of mixing zone morphology within a barrier-island aquifer in response to such influences.

Our study site lies on Hatteras Island, a barrier island in North Carolina. Existing data from this site includes salinity, water level, aquifer parameter, and electrical resistivity soundings. In August 1993, Hurricane Emily created a three meter storm surge in Pamlico Sound. The resulting sound-side flooding infiltrated a water-supply wellfield, causing a dramatic increase in chloride levels that persisted for more than three years.

We calibrated a 2D cross-sectional simulation to these chloride breakthrough data in order to infer model dispersivity values while using known permeability and storativity values. Specified pressure boundary conditions were applied to each side of the cross-section representing Pamlico Sound and the Atlantic Ocean; however, only those corresponding to Pamlico Sound varied during the simulated storm event.

We performed predictive simulations along another cross-sectional transect using the calibrated dispersivity values and 56 years of hurricane records. Peng et al. (Est., Coast and Shelf Sci. 59, 2004, 121-137) modeled water levels in Pamlico Sound under the ten most common hurricane tracks to affect this region. We used their water-level predictions to create specified-pressure time series on the sound side of the simulated cross section based on the track of each hurricane. We used the hurricane track history for the appropriate timing of the storm events.

Our results suggest that it is rare for the mixing zone to approximate the conceptual morphology. Even during quiescent periods, such as occurred between 1966 and 1976, chloride levels do not have time to return to theoretical levels. Thus, while tidal oscillations are important to mixing zone development, basic wind events and more severe storm events may have more influence and lasting effect on the morphology of the mixing zone.