Increased rain boosts groundwater levels

Low-lying areas are experiencing drainage issues

Record rainfall over the last year is causing significantly elevated groundwater levels across Kāpiti. Average annual rainfall at the Paraparaumu Aerodrome is 1030mm but 1432mm was recorded between August 2021 and August 2022 — the third highest annual total on record in over 80 years. Similarly, gauges at Te Hapua Wetlands and the Waikanae Water Treatment Plant captured the highest annual rainfall totals of their respective records.

When groundwater levels are high, stormwater can become trapped in low lying areas and sit as ponding for a period of time.

“Groundwater levels rise and fall as the seasons change, and in response to heavy rain,” infrastructure services manager Sean Mallon said. “Here in Kāpiti, our water table will typically rise quickly before draining back out to sea through local waterways but when we have frequent rain events, groundwater levels can remain elevated. The last time groundwater levels were significantly elevated for an extended period was in 2005 and current levels are higher than that event.”

Low-lying areas across the district are experiencing standing water and drainage issues.

“Groundwater inundation is different from stormwater flooding in that it can persist over long periods of time, and can be much more difficult to mitigate. Current groundwater levels are having impacts on council’s land, infrastructure, and work programmes, as well as on private property. Roads and footpaths are degrading at an accelerated rate, pipes are being corroded, stormwater networks have diminished capacity and functionality, septic tanks are being infiltrated, and trees that can’t tolerate extended saturation are dying. High groundwater levels are also preventing or delaying access for maintenance, repairs, renewal works and capital works projects.”

There are no easy fixes but levels will drop given time.

“You can’t pump groundwater in the same way you can pump stormwater — there’s nowhere to pump it to as it will only return to the lowest lying areas. If you experience standing water on your property, please be patient while it recedes. You can call a plumber or drainlayer for more advice. We have a number of actions underway to improve our stormwater network which will help the situation. This includes maintenance on our open channel network, investigations of pipes, renewal and repairs, and our continuing to progress our stormwater capital works programme.”

Mallon warns that surface flooding from groundwater is predicted to become more significant due to both increased rainfall volumes, and sea level rise associated with climate change.

“Kāpiti will continue to be susceptible to fluctuating groundwater levels given our position between mountains and the ocean, the types of natural environments across our district and the natural places where groundwater discharges. We’re going to have to learn to live with more water.”