Groundwater Resurgence

Groundwater resurgence refers to the rising of groundwater levels in response to changes in the hydrological system. Groundwater resurgence occurs when water from underground aquifers rises to the surface. This can be triggered by heavy rainfall, or changes in the water table level. In some cases, groundwater resurgence can be beneficial, replenishing surface water sources and supporting ecosystems. However, excessive resurgence can lead to flooding, causing damage to properties, infrastructure, and posing risks to human safety.

In parts of the district, particularly Mandeville and Ohoka, high groundwater has led to groundwater resurgence flows (springs or surface discharge of groundwater). The graph below shows rainfall totals and long term groundwater levels from an Environment Canterbury shallow groundwater monitoring bore, near the intersection of Tram Road and Chapmans Boundary Road, approximately 10km upgradient of Mandeville. It can be seen from this graph that the groundwater level measured in 2014, 2017 and 2022 were at some of the highest levels on record, with the record dating back to the late 1970s.

While the levels shown in the graph are still several metres below ground level, the depth to groundwater decreases further east. When the groundwater level in the well is at 10m below ground level, the water table in Mandeville is generally at or above the ground surface. During times when the water table is at these high levels, groundwater emerges, ponds and flows via overland flow paths throughout these areas. This groundwater resurgence flow can be constant for many months.

Click to enlarge chart.

Moreover, if groundwater resurgence coincides with heavy rainfall events or seasonal variations, the combined effect can exacerbate flooding, further stressing existing drainage systems and causing potential damage to properties and infrastructure. This can happen in areas with inadequate drainage systems or regions where the natural topography is not suited to handle increased water levels.

Groundwater significantly impacts existing drainage systems. When groundwater levels rise, it reduces the capacity of drainage systems as it restricts the flow of surface water. This can lead to increased flooding in low-lying areas. To address this, any drainage improvements need to be designed with adequate capacity to handle both surface water and rising groundwater levels.