Shallow Groundwater Hazard in the Urban Environment

Shallow groundwater under coastal cities may pose a hazard in many different ways. For example, groundwater can contribute to surface water flooding, it can undermine the integrity of pavement sub-grade, and it can drive liquefaction susceptibility in liquefiable soils. With sea level rise and changing climate, the hazard from shallow groundwater will change. However, we currently struggle to properly characterise the spatial and temporal variability of shallow groundwater, let alone understand how the hazard will change under changing future conditions.

The hazard is driven by the pervasive nature of groundwater (it is not constrained to flow within obvious channels) combined with the heterogeneity and anisotropy of the sediments within which it flows. Added to this, it is not visible or easily measured: our only opportunity to measure it is through piezometers drilled into the subsurface. These piezometers were typically manually measured on an irregular or regular basis (weekly or monthly), usually by regional and local councils, in order to provide information on the depth to shallow groundwater.

Following the 2010/2011 Canterbury Earthquake Sequence, EQC installed thousands of shallow test piezometers to inform geotechnical engineers about soil conditions. Subsequently, approximately 1,000 were monitored to inform modelling of the water table. This modelling, together with the detailed geotechnical studies, has led to greater understanding of liquefaction vulnerability.

In 2016, the groundwater network was rationalised to approximately 250 sites and instrumented with transducers which logged groundwater levels and temperature every 10 minutes. Whilst the original 1,000 sites provided exceptional spatial coverage, the more limited, high resolution dataset has proved to be a game changer in understanding the temporal dynamics of the shallow groundwater system. The data has provided insights into the short-term groundwater response to rainfall, tidal variations, river flows and evapotranspiration, and has enabled the generation of tens–of-thousands of surfaces to spatially characterize the temporal variability of groundwater levels and flow (for example, the different responses to storm events in different areas).

The data have highlighted the highly dynamic responses of the groundwater table to various drivers. This has huge potential value in understanding the consequential impacts on antecedent conditions contributing to land flooding, changing liquefaction potential, and effects of sea level rise and climate change. However, trying to unravel the drivers and controls of the groundwater level responses has proved challenging.

The monitoring sets a new standard for urban areas overlying shallow groundwater, where hazards such as liquefaction and flooding are contributed to by changes in groundwater levels that can occur over periods of hours or days.