Natural hazards

Flooding

In the Auckland region, most flooding events are of short duration and affect relatively localised areas. Artificial drainage systems consisting of the primary piped system together with the overland flowpath are designed to cope with a 1:100 year a flood event (1 chance in 100 of occurring in any given year). This may equate to different rainfall intensities depending on the catchment size.

The Leigh flood event - 2001

On 29-30 May 2001, 132 mm of rain fell in 24 hours over the Leigh township, located an hour north of Auckland, New Zealand's largest city. Between 1:40-2:40am on 30 May, 109.4 mm of rain fell, creating a new 1 hour rainfall record for anywhere in New Zealand.

Analysis of the event has confirmed it was an exceptional event with a return period far in excess of 100 years.

The Damage

This event caused significant damage in Leigh township and surrounding farmland.

One family lost about 200 sheep and fences were flattened. Houses were flooded to a depth of well over 1 m. On two properties cars parked outside were moved by flood waters rushing over the ground. In one case a car broke through a ranch slider and then bounced around the living room while the terrified family watched from upstairs. Slips were also common on adjacent farmland. A nearby footbridge, stretching across the Kohuroa Stream broke in two. Large amounts of debris were deposited. For several days, whole trees placed just off-shore prevented local fishermen putting to sea at night for fear of running into them.

The Rodney District Council estimated $700,000 worth of damage mainly to the Leigh and Mangawhai districts.

The Pukekohe Flood Event - 1999

In January 1999, 145 mm of rain fell in 6 hrs causing flooding in Pukekohe, 40 mins south of Auckland, New Zealand's largest city. During this flood event, flood waters rose 1.5 m in some houses and resulted in:

• flooding of residential homes and consequent evacuation of a number of Pukekohe residents. Evacuation required assistance from emergency services and created stress for residents
• made many roads impassable
• extensive damage to land and buildings from sediment deposition
• contaminated water supply due to the infiltration of sewer overflows represented a potential health risk to local communities. This was a concern for weeks after the event.