Natural hazards and emergencies

Flooding

Flooding is Auckland’s most common natural hazard. Most flooding occurs over a short period and affects relatively localised areas. Flooding is dependent on several factors including rainfall intensity and duration, soil conditions, local river levels and the physical characteristics of the catchment.


Flood conditions

The heaviest rainfall occurs during warm moist north or north-easterly wind flows, depressions from the north or northwest and slow-moving anti-cyclones to the east. These flows produce about 60 per cent of the annual rainfall. In contrast, south-westerly flows produce showery weather, especially in winter. Stormy westerlies also produce rainfall and may be accompanied by thunderstorms and sometimes tornadoes.

Localised falls can often occur during mid-afternoon in summer when sea breezes from the main east and west coasts sweep inland and converge over Auckland city sometimes producing thunderstorms. Heavy rainfall can also occur in moist and unstable northeast airflows.

Areas prone to flooding may include low-lying flood plains with streams or rivers, valley floors of steep river catchments susceptible to intense rainfall and ex-tropical storms, and low-lying areas near sea-level and the coast.

The conditions that lead to flooding in Auckland are often dependant on whether the catchments are predominantly rural or urban. Ground conditions influence flooding in rural catchments as run-off to streams and rivers occurs much faster if soils are already saturated. As much of Auckland’s urban catchments are concrete-lined or modified, flooding generally occurs from prolonged or sustained rainfall.

 


Recent flood events

The following are some of the recent Auckland flood events:

Auckland City flood: 1997

Heavy rain flooded a number of homes in Auckland’s western and southern suburbs during the morning of 24 May, and caused chaos on the roads. On average, the Fire Service recieves 15 emergency calls between midnight Friday and 1pm on Saturday. This time it got 230 calls. The meteorological maps for the event show a classic 'blocking' pattern. A broad ridge of high pressure extended from a high in the south Tasman Sea to another high east of New Zealand, cradling a shallow low in the central Tasman.

Pukekohe flood: 1999

In January 1999, 145mm of rain fell in six hours causing flooding in Pukekohe.

Floodwaters rose to 1.5m in some houses and resulted in:

  • flooding of residential homes and consequent evacuation of a number of Pukekohe residents
  • many roads being made impassable
  • sediment deposition causing extensive damage to land and buildings
  • water supply being contaminated due to the infiltration of sewer overflows causing a potential health risk to local communities. This continued for weeks.

The Leigh flood: 2001

On 29-30 May 2001, 132mm of rain fell in 24 hours over Leigh In an hour on 30 May, 109.4mm of rain fell, creating a new one-hour rainfall record for New Zealand. This event caused significant damage in the township and surrounding farmland. One family lost about 200 sheep and fences were flattened. Houses were flooded to a depth of well over one metre. On two properties, cars parked outside were swept some distance. A nearby footbridge across the Kohuroa Stream broke in two and there were large amounts of debris. For several nights, local fishermen didn’t put to sea because of the risks of running into whole trees just off-shore. The Rodney District Council estimated it caused $700,000 worth of damage mainly to the Leigh and Mangawhai districts.

Tropical Cyclone Wilma flood: 2011

On 28-29 January 2011, Tropical Cyclone Wilma passed across the northeast side of North Island. This was the first recorded storm to hit New Zealand with a ‘tropical cyclone’ status. Rainfall associated with the event reached 300mm in 24 hours in parts of northeast Auckland. This heavy rain, along with saturated ground from another storm event a few days before, led to flooding of properties and other infrastructure, particularly in low-lying rural areas near the east coast. The heavy rainfall also triggered several landslides in the region.  


 

Sandbagging your property
 

  • It is the responsibility of the home owner/occupier to protect their property from flooding.
  • During low level flooding, sandbags placed in the right locations around your home can reduce the impact of flooding.
  • Sandbags are not stored pre-filled as they begin to rot when damp. Keep sandbags dry and separate from sand until needed.
  • Sandbags require time and effort to fill and place, therefore they generally need to be filled and placed in advance of an event rather than in the middle of an event.
  • Alternative Solution
    Sheets of PVC are a practical way to waterproof around doors provided they are fixed in place adequately.


1. Where do I get sandbags?

Sand bags can be purchased from:

  • Gubba, Located in Albany sells unfilled bags for $1.00 each.
  • Sand is available from any garden centre.
    Sand bags can be made out of feed bags, hesian bags or plastic (polypropylene) sacks. If you have only the plastic open weave type bag, put one inside the other.  Don’t use kitchen rubbish bags as they will slip around in the water and may split.


2. How do I fill sandbags?

  • Use sand or sandy soil to fill bags. Dirt is not recommended.
  • Only fill sandbags two-thirds full
  • Do not tie or seal the sandbag
  • Take care when filling and lifting the sandbag, to avoid injury.


3. How to I lay sandbags?

  • Lay sandbags like brickwork. Stagger rows so that the joins to not line up.
  • Start at one end and work to the other end.
  • Ensure the unfilled part of the bag is covered by the next bag.
  • Tuck flap under bag at the end of the row.
  •  If the sandbag wall is going to be more than five bags high, you will need to lay two rows wide.


4. Where do I place the sandbags?

  • Place a small sand bag wall across doorways (for houses on a concrete pad). The number of layers will depend on the expected flood height. It is generally around two layers.
  •  If available plastic sheeting may be used under sandbags to reduce the seepage.
  • Make sure that you have at least one doorway that you can use to access or exit the building.
  • Cover drainage holes in the home e.g. toilets, showers, sinks to stop the back flow of water.


5. How do I dispose of sandbags?

  • Gloves should be worn when handling wet sandbags as they can contain chemicals, waste and diseases.
  • Sand and sandbags that have been in contact with floodwater need to be thrown away. They can be taken to a local transfer station. 

Watch this clip on YouTube for a sandbagging explanation.

 

6. Why is council not proactively sandbagging an area known to flood?

Maintenance crews employed by council are focused on maintaining the function and safety of key infrastructure such as roads, stormwater, water and sewerage.

Their priority is keeping critical services available for emergency services and the wider public.

Available resources are limited in times of crisis, crew will be prioritised to where they provide the greatest effect

 


 

More information

Visit the Metservice website for the latest information on weather in New Zealand and the NIWA website for general information on flooding in New Zealand.

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