Cox, S.C.; Ettema, M.H.J.; Chambers, L.A.; Easterbrook-Clarke, L.H.; Stevenson, N.I. 2023 Dunedin groundwater monitoring, spatial observations and forecast conditions under sea-level rise. Lower Hutt, N.Z.: GNS Science. GNS Science report 2023/43. 111 p.; doi: 10.21420/5799-N894
Abstract:
Dunedin City has a large number of assets and critical infrastructure sitting on a low-lying coastal plain that is underlain by a largely unseen and relatively poorly understood hazard. Shallow groundwater in this area limits the unsaturated ground available to store rain and runoff, promotes flooding and creates opportunities for infiltration into stormwater and wastewater networks. Groundwater levels are expected to rise as sea level rises, causing greater frequency of pluvial flooding and/or direct inundation from below once it nears the ground surface. Monitoring network developments in 2019 and 2021 have significantly improved information on Dunedin’s groundwater. Groundwater level, temperature and specific conductance observations at 15 minute intervals have been collated into a time-series database. A wide variety of statistics have been generated for each site, including median, maximum, minimum, 95th and 5th percentiles, mean, standard deviation and range of groundwater levels. Other collated site data include: tidal amplitude, efficiency and phase lag; distance from harbour or sea; groundwater sample pH, electrical conductivity; calculated seawater percentage; and a rainfall response index reflecting the local efficiency of rainfall recharge. This report updates a previous report, published in 2020, to incorporate monitoring and observations over the period 6 March 2019 to 1 May 2023. It is accompanied by derivative ArcGIS data and spatial analysis of these groundwater observations. A series of statistical surfaces have been generated to represent the present-day (2023) water-table elevation and depth to groundwater, the response to rainfall recharge and tidal forcing and the available subsurface storage of rain infiltration. The level of groundwater is influenced by subsurface flow and runoff from the hills in the west and north but will be further encroached from the south and east by sea-level rise in the harbour and ocean in the future. Simple geometric models have been developed using the present shape and position ofthe water table, combined with tidal fluctuations, to forecast the future state of groundwater levels at 10 cm increments of sea-level rise. These geometric models are strongly empirical, with many implicit assumptions and caveats – particularly, that they do not account for groundwater flow and possible changes in water-budget mass balance. Although many variables and controlling processes are simplified into a single parameter, the projected groundwater levels highlight how local variations in the water table shape and slope interact locally with the ground elevation or infrastructure networks. The models depict spatial patterns well, but are relatively conservative and may over-estimate groundwater-related contribution to hazard and how this will evolve over time. Groundwater spatial datasets, such as water-table elevation and rainfall recharge, provide tools from which inundation or flood-vulnerable areas can be identified and other hazards, such as liquefaction susceptibility, modelled. The spatial exposure to the loss of subsurface storage, emergent groundwater and coastal inundation are combined in a summary of negative impacts from these processes as sea levels rise. Hazards associated with groundwater are likely to be gradual and will precede a step-like increase in exposure to coastal inundation. Likewise, groundwater’s contribution to pluvial flooding may well have been experienced in many places prior to the emergence of groundwater. The impact forecast highlights the need for planning to take a holistic multi-hazard long-term view.
