The effects of beach-slope and near coastal stratigraphy, upon interaction with tidal oscillations, have been the subject of study with regards to impacts on near coastal groundwater level fluctuations. Time-averaged near coastal groundwater levels are of interest for constraining the coastal boundary conditions of regional-scale water resource management models investigating groundwater. The focus of this study is the interaction of a macro-tide with multi-slope beach morphology and a two aquifer beach stratigraphy system and consequent impacts on near coastal groundwater levels. Such coastal morphology and stratigraphy conditions occur at the Pioneer Valley Aquifer in North Queensland, Australia which is used as a field study location.
The Department of Environment and Resource Management (DERM) is actively managing the groundwater resource of the Pioneer Valley Aquifer system with a numerical model. The further understanding and refinement of the coastal boundary condition of this model is a motivating factor for this research. At the commencement of this study, the Pioneer Valley Aquifer combined surface-groundwater model used a density corrected mean sea level fixed head as the coastal boundary condition for the regional scale water resource management model. In combination with other research, this study has helped refined the coastal boundary condition to one which accounts for tidally influenced time average head conditions.
The majority of studies of coastal groundwater resources have explored the hydraulic conditions at coastal and estuarine boundaries with vertical or mono-slope beach morphologies, predominately with single unconfined aquifer coastal stratigraphy, under the influence of micro- or meso-tidal regimes. The Pioneer Valley Aquifer coastline is subject to a macro-tidal spring-neap semi-diurnal fluctuation. This tidal regime has a subsequently different effect on near coastal water-levels.
In this thesis, oceanic controls on the hydrology of coastal aquifers are characterised for the macro-tidal system observed at the Pioneer Valley coastal plain. The introductory Chapter 1, describes the issues at hand, presents a literature review and defines the scope and objectives of this research. Chapter 2 (as published in Australian Journal of Earth Sciences) presents tidally influenced time averaged water-levels that are acquired for two field sites and compared to numerical estimates. Measurements for beach-slope and the results from auguring and percussion drilling to inform coastal stratigraphy are presented in this chapter. The results indicated that local geological conditions, beach morphology and characteristics of tidal forcing control the behaviour of nearshore groundwater within the system. The field data collected in the Pioneer Valley Aquifer are not readily represented by derived analytical models.
Chapter 3 focuses on the effects and implications of beach-slope and slope breaks on near coastal groundwater elevations in the presence of a macro-tidal oscillation. The numerical modelling in this chapter examines dual-slope and tri-slope beaches and the inherent horizontal and vertical positions of their slope breaks. The results show the impacts of slope break position on exit point and seepage face generation and the consequent impacts on groundwater levels adjacent to the shoreline for an unconfined aquifer.
Chapter 4 examines the impacts of a confining layer in relation to overlying beach morphology on confined aquifer head levels. A four domain numerical model is used to quantify the effects of varying near coastal stratigraphy geometries on confined aquifer water-levels. The results are in agreement with existing studies, and further include observations with regards to the interaction between the confining layer geometry with overlying beach morphology. This interaction is established as a controlling factor for confined aquifer water-levels.
Both Chapters 3 and 4 examine the implications of Spring-Neap Fluctuations in relation to beach morphology and coastal stratigraphy geometries. These investigations inform Chapter 5 which qualitatively examines near coastal groundwater levels using a simplified regional conceptualisation of the DERM Pioneer Valley Aquifer model using a steady-state MODFLOW model. The closing Chapter 6, summaries and concludes this thesis with recommendations for extension of results and further research.