The South Dunedin coastal aquifer & effect of sea level fluctuations

The South Dunedin urban area is low-lying having been reclaimed from coastal dunes, marshes and intertidal deposits since European settlement. The water table lies close to the surface, typically 0.3 m to 0.7 m under the urban area. A coastal aquifer comprising mostly sands, estuarine muds and alluvial gravels makes up the basin infilling between the Dunedin mainland and the Otago Peninsula. The coastal aquifer is exposed to the sea at the Harbour Basin and Ocean Beach (St Kilda Beach, Middle Beach and St Clair Beach). Recent drilling investigations have characterised a sandy aquifer in hydraulic communication with the sea, including tidal fluctuations of the water table in proximity to the ocean. It is inferred that any long-term rise in the mean level of the sea would translate into a rise in the water table. Thus, groundwater modelling was used in this investigation to assess the magnitude of sea level rise effects on South Dunedin from groundwater causes. Scenarios or projections of future sea level rise encompassing the 21st Century were used in formulation model scenarios. The following bullet points outline the results of computer modelling: • Groundwater computer modelling suggests that even the mildest continuation of the measured rate of sea level rise would manifest in groundwater ponding of a few tens of centimetres in the parts of the South Dunedin urban area. • Based on the LiDAR land surface data and the groundwater model-derived water table surface, the greater potential for groundwater ponding at the surface is within the Bayview Road – Hargest Crescent – Forbury Park area