FLOOD/STORM EVENT

RISK

Much of the settled Hawke’s Bay region is low lying and built on river flood plains. This brings the risk of flooding, which is our most common natural hazard - a severe storm or flood happens every 10 years on average. Major storms affect wide areas and can be accompanied by strong winds, heavy rain or snowfall, thunder, lightning, and rough seas. They can cause damage to property and infrastructure, affect crops and livestock, disrupt essential services and cause coastal inundation.

Rivers normally flood every winter when a storm brings more rainwater than can soak into the soil. When floods threaten communities they become a hazard. In Hawke's Bay stopbanks have been built alongside many of the rivers to hold in the extra flood water. However in a severe storm, rivers could breach stopbanks and the flood waters may go through farms, homes, shops, schools and damage roads and other infrastructure.

Examination of the rainfall and flood records shows that there have been long periods without major floods and other periods where they have been more frequent. Despite the long breaks, the potential for flooding is high and any large flood would have a major effect on the regional community, environment and economic infrastructure.

flood
The Heretaunga Plains in flood in 1935 before stopbanks had been built by the Hawke’s Bay Rivers Board and Hawke’s Bay Catchment Board. This photograph highlights how horticultural land would regularly be flooded if we weren’t protected by stopbanks. (Photo: Hawke’s Bay Regional Council)

There have been significant flood protection systems completed on the Heretaunga Plains and the Ruataniwha Plains. Flood protection works designed to contain a 1% annual exceedance probability (AEP) flood). These works have significantly reduced the effect of small to medium sized floods, but a large flood could overwhelm the works and have a devastating effect. Such a flood, which exceeds the design capacity of the flood protection system, is called a Super Design Flood. Flooding from localised downpours in urban areas can also overwhelm drainage systems, so events below the AEP can still be costly.

With climate change, rainfall patterns in the Hawke’s Bay will change over the next century; winters are predicted to become drier, but overall flood risk is not expected to decrease as single events may be more intense.

The Hawke’s Bay CDEM is the lead agency for managing any natural hazard event, like a flood that affects the people of Hawke’s Bay.

HISTORY
By national standards Hawke’s Bay’s rivers are not large with the exception of the Wairoa and Mohaka Rivers. However, many major damage causing floods have been recorded. Individual floods have ranged in size from cyclonic storms that have caused regional flooding to localised downpours that have affected minor catchments or caused drainage systems to overflow.

Since 1867 there have been numerous major storms resulting in severe flooding in Hawke’s Bay (see the Table below). You can learn more about these events on the NIWA historical weather events database. There are a number of common factors in most flood events in Hawke’s Bay.

  1. Heavy prolonged easterly or southeasterly rains.
  2. The greatest percentage variation of rainfall in New Zealand, with hot dry spells followed by heavy rains, resulting in high runoff.
  3. Local soils are comparatively shallow and have limited capacity to absorb large amounts of rainfall. They are frequently underlain by impermeable sub strata, especially mudstone.
  4. Short, steep catchments resulting in rapid runoff. This is aggravated by highly erodible soils.
  5. The lack of lake storage with few ponding areas.
  6. Three major rivers on the Heretaunga Plains, the Ngaruroro, Tukituki and the Tutaekuri, discharge into the sea within about 5 km of each other.
  7. Many major river headwaters are in areas where the rainfall averages are much higher than the plains. The annual rainfall for the ranges is 3500 mm a year compared to 800 mm for the Napier/Hastings area.

flood history

 

flood jul2007The Maraekakaho community was flooded in July 2007

WHAT CAN YOU DO?
Before a flood strikes find out about the worst flood in your locality and how high it rose. Calculate where such a flood would reach in your home. Know how to reach the nearest high ground.

Check the weather forecast regularly as severe weather watches and warning are issued by the MetService and available through radio and television, by email alerts and at www.metservice.com

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