Groundwater flooding

Groundwater becomes a hazard when it is close to or above the ground surface for long periods of time as it causes rising damp, saturation of subsurface structures or the ground surface . Groundwater flooding could occur during large and/or prolonged rainfall events, seasonally, daily because of tides, or more permanently as a result rising sea levels.


What we know

Extent of hazard

As this map shows, areas adjacent to the estuary edge north of South New Brighton Park and along Rocking Horse Road in Southshore have the shallowest groundwater at less than one metre below the ground surface. These areas are therefore the most susceptible to current and future flooding from groundwater. Much of the rest of Regeneration Strategy project area may be susceptible from future groundwater flooding because of sea level rise.


Coastal processes influence groundwater

Fluctuations with tides add a twice daily peak in groundwater levels at the coastal margin and close to the tidal reaches of rivers, which can increase the occurrence of dampness and saturation, and slow or not allow for drying out. Rising sea level will raise the groundwater level where influenced by the tide, such as areas close to the Estuary/Ihutai. This could also increase the salinity and have consequences for water quality, vegetation and structures. As the groundwater levels across the Regeneration Strategy project area are already shallow, it is likely that more properties will begin to experience hazardous levels of shallow groundwater with sea level rise.


Likely effects of shallow groundwater

Shallow groundwater may have visible effects such as ponding above the ground surface, or invisible effects such as rising damp and impacts on infrastructure below the ground. These can lead to physical damage, but also affect health and wellbeing, particularly if people are exposed to it over longer periods of time or on a regular basis.


Potential impacts of shallow groundwater on property and households (prepared by Tonkin & Taylor on behalf pf the Christchurch City Council 2018).


Why this is important

As groundwater levels fluctuate over time and to some extent within regular tidal cycles, statistical averages are used to calculate the groundwater level. Guidance from the Netherlands suggests groundwater levels in gardens and parks shallower than 0.5m below ground, and below houses and roads shallower than 0.7m below ground, should be managed.

High groundwater is also likely to exacerbate the effects of flooding in a rainfall event, as there will be less capacity for infiltration into the soils, resulting in increased surface runoff and more pressure on stormwater and secondary drainage pathways.

As much of Christchurch city has shallow groundwater, this is not an isolated problem. However, in the Regeneration Strategy project area the additional influence of tidal fluctuations and future sea level rise create issues of exacerbated hazard risk in areas close to the coast, which will only increase over time.


What we don’t know

While testing of groundwater monitoring wells confirms the influence of coastal water on the groundwater system in the regeneration strategy project area, the timing and scale of climate change and sea level rise effects is not known.


Groundwater becomes a hazard when it is close to or above the ground surface for long periods of time as it causes rising damp, saturation of subsurface structures or the ground surface . Groundwater flooding could occur during large and/or prolonged rainfall events, seasonally, daily because of tides, or more permanently as a result rising sea levels.


What we know

Extent of hazard

As this map shows, areas adjacent to the estuary edge north of South New Brighton Park and along Rocking Horse Road in Southshore have the shallowest groundwater at less than one metre below the ground surface. These areas are therefore the most susceptible to current and future flooding from groundwater. Much of the rest of Regeneration Strategy project area may be susceptible from future groundwater flooding because of sea level rise.


Coastal processes influence groundwater

Fluctuations with tides add a twice daily peak in groundwater levels at the coastal margin and close to the tidal reaches of rivers, which can increase the occurrence of dampness and saturation, and slow or not allow for drying out. Rising sea level will raise the groundwater level where influenced by the tide, such as areas close to the Estuary/Ihutai. This could also increase the salinity and have consequences for water quality, vegetation and structures. As the groundwater levels across the Regeneration Strategy project area are already shallow, it is likely that more properties will begin to experience hazardous levels of shallow groundwater with sea level rise.


Likely effects of shallow groundwater

Shallow groundwater may have visible effects such as ponding above the ground surface, or invisible effects such as rising damp and impacts on infrastructure below the ground. These can lead to physical damage, but also affect health and wellbeing, particularly if people are exposed to it over longer periods of time or on a regular basis.


Potential impacts of shallow groundwater on property and households (prepared by Tonkin & Taylor on behalf pf the Christchurch City Council 2018).


Why this is important

As groundwater levels fluctuate over time and to some extent within regular tidal cycles, statistical averages are used to calculate the groundwater level. Guidance from the Netherlands suggests groundwater levels in gardens and parks shallower than 0.5m below ground, and below houses and roads shallower than 0.7m below ground, should be managed.

High groundwater is also likely to exacerbate the effects of flooding in a rainfall event, as there will be less capacity for infiltration into the soils, resulting in increased surface runoff and more pressure on stormwater and secondary drainage pathways.

As much of Christchurch city has shallow groundwater, this is not an isolated problem. However, in the Regeneration Strategy project area the additional influence of tidal fluctuations and future sea level rise create issues of exacerbated hazard risk in areas close to the coast, which will only increase over time.


What we don’t know

While testing of groundwater monitoring wells confirms the influence of coastal water on the groundwater system in the regeneration strategy project area, the timing and scale of climate change and sea level rise effects is not known.