HR: 0800h
AN: H21B-1020    [Abstracts]
TI: Nonlinear Analysis of Tidal Forcing of Water Level Fluctuations in an Unconfined Permeable Coastal Aquifer, Georgia Coastal Ecosystems LTER
AU: * Brown, R
EM: ryan@brown@ce.gatech.edu
AF: Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332
AU: Ruppel, C
EM: cdr@eas.gatech.edu
AF: Georgia Institute of Technology, 311 Ferst Drive, Atlanta, GA 30332
AU: Schultz, G
EM: gschultz@ara.com
AF: Applied Research Associates, 415 Waterman Rd, South Royalton, VT 05068
AB: The characteristics of tidally-forced water level fluctuations in unconfined permeable aquifers provide insight into physical, chemical, and even biological processes occurring at the margin between open water and marsh-upland systems. It has previously been well-established that permeable aquifers act as low-pass linear filters when upland water table fluctuations are driven by tidal fluctuations in adjacent low-friction estuarine systems. For example, diurnal or semi-diurnal tidal components that dominate in the estuary are attenuated in the upland, leading to water table fluctuations increasingly dominated by the longer period (fortnightly) tidal components with increasing distance from the tidal creek boundary. This analysis, which relies on a linearized solution to the Boussinesq equation, breaks down when the boundary between upland and estuary is nonvertical and/or non-stationary, when there is significant physical heterogeneity (e.g., permeability differences) between upland and adjacent marsh, or when other factors complicate the system. Published studies have adopted various mathematical approximations to deal with these deviations from linearity, but none is fully satisfactory, with some solutions yielding highly non-physical results. This study uses a new, high-quality, 8-week time series of water level fluctuations in shallow ($<$ 6 m) groundwater wells in a permeable upland and narrow, fringing, low permeability marsh and coincident tidal fluctuation data acquired in an adjacent tidal creek (the forcing function) to examine the impact of various nonlinear processes on the propagation of the tidal forcing into the aquifer. The raw time series reveal clear asymmetry to the water level fluctuations, particularly within the permeable upland, indicating that boundary effects or changes in permeability from marsh to upland may affect the character of the water level signals. Although spectral analysis of the data do reveal the expected linear filtering effect of the aquifer, we also observe generation of new harmonics, as predicted by nonlinear tidal forcing theory. To avoid some of the problems with the existing analytical solutions to the nonlinear problem, we develop a nonlinear numerical model that permits us to test the sensitivity of upland water level fluctuations to various processes. The results have implications not only for the physics of groundwater-surface water interaction at the upland-estuary interface, but also for the exchange of chemical species at these interfaces due to tidal forcing.
DE: 4235 Estuarine processes
DE: 1829 Groundwater hydrology
DE: 1255 Tides--ocean (4560)
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting