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Home Environment Flood hazard management Types of flooding

Types of flooding

The Kāpiti Coast’s stunning landscape, with its interconnected waterways and wetlands weaving their way between the Tararua Ranges out to the Tasman Sea, is one of our district’s defining features. However, this beautiful environment also makes Kāpiti particularly vulnerable to flooding. With most residents living in low-lying areas, understanding the types of flooding and their causes is crucial for building resilience and protecting our community.

Flooding occurs when water inundates normally dry land. It can arise from various sources, such as rivers, heavy rainfall, groundwater, or coastal tides. Each type of flooding presents unique challenges, shaped by our environment and the increasing impacts of climate change.

This page explores the different types of flooding affecting Kāpiti, how they occur, and the steps we can take to adapt and reduce risk. We can better prepare for and respond to these natural events by being informed.

Factors that contribute to flooding: 

River and stream flooding 

River and stream flooding occurs when large volumes of water, caused by heavy rain, come down a river system. The volume of water can mean water flows out of riverbeds, spilling across floodplains, and overtopping or breaching stopbanks or structures.  

Surface flooding 

Surface flooding occurs when a river or stream bursts its banks or the Council’s stormwater network is overwhelmed because of sustained or intense rainfall. Urban areas have a lot of concrete or hard surfaces. These surfaces stop rainwater from soaking into the soil, which is channelled into stormwater systems such as pipes and open drains. We get surface flooding when the rain falls faster than the stormwater system can manage. These floods usually happen very quickly and don’t last very long, but they can block roads and damage buildings, including homes. 

Groundwater flooding 

Groundwater flooding occurs when water seeps from the ground and creates surface flooding issues. It is much slower to occur than surface or coastal flooding—it usually happens days, weeks, or even months after heavy or prolonged rainfall and can last weeks or even months. 

Coastal flooding 

Coastal flooding is caused by extreme tidal conditions, including high tides and storm surges. There is a tidal and groundwater influence on surface flooding where it interacts with high groundwater levels and/or an incoming tide.   

Impacts of extreme weather events 

Across the country, large scale storms are becoming more frequent and severe - threatening people, property, and public infrastructure. In the last two decades there have been more than 100 state of emergencies declared in New Zealand, such as Cyclone Gabrielle in February 2023 that caused $14 billion in damage to northern and eastern parts of the North Island.  

But that number doesn’t account for undeclared emergencies such as: 

  • the severe weather event in May 2015, which saw 143.6mm of rain fall in our district within 24 hours – almost double the monthly average. This resulted in significant flooding, slips, disruption to schools and transport links, and homes being evacuated.  
  • recent record rainfall contributed to significantly elevated groundwater levels causing stormwater to be trapped in low lying areas and sit as ponding.  
  • and in August 2024, parts of the district experienced almost 30mm of rainfall between 8–10am in a single day. The Paraparaumu monthly average for August is 71mm. A MetService spokesperson said 3-4mm an hour was usually considered to be ‘heavy’ rain.  

Climate change predictions for Kāpiti indicate the amount of rain that falls during extreme rain events will continue to increase. To adapt to climate change, we need to anticipate and plan for these kinds of impacts. 

Managing flood risk 

We can’t stop floods; however, we can manage the risk through a combination of measures, including: 

 

Updated 18 December 2024