UNINTENDED CONSEQUENCES: GROUNDWATER, CLIMATE CHANGE, URBAN DEVELOPMENT, AND IMPROVING INFRASTRUCTURE

Helen Rutter (Aqualinc Research Ltd), Kolt Johnson (Auckland Council), Simon Cox (GNS Science)

ABSTRACT

Shallow groundwater is groundwater that is relatively close to the surface, typically within a few metres of the ground. Flooding and infrastructure problems can be encountered if high water tables are not properly accounted for in urban planning and engineering design. . Shallow groundwater levels fluctuate considerably with inter-annual and seasonal rainfall recharge as well as higher-frequency variations caused by storm-events, local river floods or tidal cycles. Because the water table is controlled by the coast, low-lying areas potentially affected by sea level change effects on groundwater can extend considerable distances inland^[1].

The hazard comes from groundwater in itself, where it can emerge at the surface and “daylight” but, in turn, also contributes to a number of other hazards, including liquefaction susceptibility and surface flooding, and causes a nuisance in terms of dewatering requirements, roading damage and failure of planting. Understanding the hazard could save councils substantial costs, though monitoring in Christchurch has shown that understanding the spatial and temporal variability of groundwater is complex.

As a consequence of the increased hazard, risks will change dependent on the magnitude of groundwater inundation or elevated levels, the duration of the high levels and frequency with which they occur.

Climate change will affect the groundwater hazard both in terms of the long-term change to levels caused by sea level rise, and the short-term dynamic responses to extreme events. The impact of sea level rise on shallow groundwater systems has been modelled to show that the effects propagate much further inland than the effects of coastal inundation: this provides a higher “base level”. Monitoring from Christchurch has shown that shallow groundwater can respond very rapidly to rainfall recharge: the effects of more extreme events under climate change may cause very rapid response to rainfall to add to the elevation caused by sea level rise.

Urban areas have a high proportion of impermeable surface, with proportionally high stormwater generation. Water landing on such surfaces is diverted to stormwater, and the stormwater either being discharged to streams, stored in retention basins, or channelled to ground soakage. The effects of this can be to concentrate recharge in certain areas, and also to avoid uptake via evapotranspiration by vegetation, effectively increasing recharge to the groundwater system.

In many urban areas, groundwater infiltration into sewers and stormwater systems is an unwanted nuisance, but the effects of “improving” the systems can cause unintended consequences and they are effective groundwater drains, without which groundwater rises and exacerbates the groundwater flooding issues.

One example of the interaction between shallow groundwater, climate change, urban development, and the unintended consequences of improving infrastructure was the January 2023 rainfall in Auckland which resulted in this being the wettest month on record. The impact of the ensuing Cyclone Gabrielle was subsequently exacerbated by the preceding January rainfall, with many areas being in a vulnerable state due to the saturated ground. Groundwater levels rose to the surface in some areas and inundation persisted well beyond the duration of surface flooding, illustrating the compounding effects of extreme events and stormwater disposal to soak pits. In this case, the “nuisance” of groundwater then infiltrating into stormwater pipes was likely to have reduced the severity and duration of the groundwater flooding, an unintended consequence to the positive.

Since the Auckland flooding, there has been increasing talk of dealing with stormwater more effectively through “sponge city” concepts. Sponge cities are urban areas designed to manage and utilise rainwater as a resource, rather than simply trying to get rid of it through drainage systems. The concepts used include various green infrastructure practices, such as green roofs, permeable pavements, rain gardens, parks and grasslands, creeks and lakes, and major drainage projects to allow water to be collected and stored. The approaches generally aim to help absorb and retain rainwater, often allowing increased percolation of water into the ground. However, in order to infiltrate into the ground, there needs to be available capacity.

A question as to whether the sponge city concept, particularly the concept of enhancing infiltration, can be expected to work within our coastal urban areas will depend on whether groundwater is close to the land surface. We would expect that shallow groundwater could limit the effectiveness of certain sponge city features such as sub-surface or above ground storage. It could also potentially cause unintended consequences through enhancing recharge through permeable pavements or sustainable drainage systems, resulting in groundwater levels increasing and contributing to surface flooding.

Ultimately, sponge city concepts need to be optimised to work with the local hydrology and hydrogeology. Without a good understanding of shallow groundwater and how it responds to rainfall events, river flow and tides under different antecedent conditions, the effects of promoting infiltration could have major unintended consequences such as the failure of new infrastructure.

Overall, for dealing with the consequences of climate change, urban development and improving infrastructure in our urban areas, We need to ensure that we have a holistic view of the drivers and cascading effects, in order to understand consequences before we make changes to infrastructure, expand urban areas, and divert stormwater to ground.

[1] Shallow groundwater monitoring: exemplars from Christchurch and Dunedin, New Zealand - New Zealand Geotechnical Society (nzgs.org)