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Storms and Floods
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| The Ngaruroro River in August 2003 – high velocity river water is kept in the centre channel away from stopbanks by willow edge protection plantings. (Photo: Hawke’s Bay Regional Council) |
The most common natural hazard in Hawke's Bay are floods - a severe storm or flood happens every 10 years on average.
On this page
A storm is a disturbance of the atmosphere causing strong winds, rain, hail, thunder or snow. Cyclones are large storms that often come to Hawke's Bay and are big wheels of wind and heavy rain (they always turn clockwise, compared to hurricanes in the northern hemisphere which are the same but turn anti-clockwise). Winds of more than 300 km per hour can uproot trees, tear roofs from buildings, push over cars and make huge waves at sea. The heavy rain causes slips and floods.
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 very bad storm, rivers could breach stopbanks (go over the top or burst through) and the flood waters may go through homes, shops and schools, and damage roads, land and crops.
Previous impacts in Hawke's Bay
 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 Table B below). 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. ^Top^
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| 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) |
Some memorable storms in Hawke's Bay
| 1867 | 25 May - 4 June | A large flood in Hawke’s Bay, which according to the local Maori, there was no flood to compare with it in the previous forty years. Rainfall in Napier was 380 mm in four days.The Tukituki, Ngaruroro and Tutaekuri all overflowed their banks at several locations, causing extensive flooding. | | 1893 | 4 December | Heavy rain cause flooding in the Waipawa River, with the highest levels ever known. The Tutaekuri and Ngaruroro Rivers broke their banks with resulting widespread damage. | | 1897 | 17 April | 356 mm of rain fell in Napier over four days. The Ngaruroro River broke its banks between Roy’s Hill and Fernhill and menaced Hastings. It also broke its banks south of Roy’s Hill and flowed along a very old course. The Tutaekuri River broke its banks and joined with floodwaters from the Ngaruroro River to flood Clive and Napier. | | 1917 | 13 June | Flooding estimated to be bigger than that of 1897 and nearly as bad as the 1867 flood, caused widespread damage in Napier. 187 mm fell in 36 hours. At Morere, 522 mm fell in four days, of which 319 mm fell in 24 hours. | | 1924 | 11-12 March | Rainfall at Rissington was 510 mm in 10 hours with 230 mm falling in 2.75 hours. At Eskdale, 419 mm was recorded in nine hours. | | 1936 | 1 February | A cyclonic storm resulted in extensive flooding throughout Hawke’s Bay. In Napier 101 mm fell in 24 hours. | | 1938 | 23-25 April | Esk Valley Floods. Severe flooding was widespread after three days of heavy rain, with exceptional falls in some areas. In three days, 610 mm fell at Tutira, and a staggering 1,000 mm at Puketitiri (with 390 mm in one day). | | 1941 | 4 May | Very heavy rain fell on central and southern Hawke’s Bay. At Porangahau 406 mm fell in 24 hours, and the Porangahau River rose 14.3 m above normal causing extensive flooding. | | 1948 | 13-14 May | In the Wairoa River catchment 307 mm fell in three days at Onepoto and 260 mm at Tuai in the same period. The Wairoa River rose to a record height and flooded buildings in the Wairoa township. | | 1953 | 27-28 January | Exceptionally heavy rainfall over the Wanstead, Elsthorpe and Maraetotara area. In the Mangarouhi Valley 349 mm was registered in 24 hours, with the bulk of the fall occurring over six hours. | | 1974 | 15 June | Flooding in Napier from 157 mm of rain in 24 hours. | | 1980 | 28 December | Rainfall at Whanawhana was recorded at 157 mm in 48 hours. The Ngaruroro River breached the stopbank at Twyford resulting in serious flooding. | | 1988 | 7-10 March | Cylcone Bola was the most significant event in New Zealand since Cyclone Alison his the South Island in 1975. Bola caused considerable damage in the Gisborne and Wairoa districts. The highest total rainfall for the three day period was 635mm recorded at Pukeorapa. | | 1997 | 2-3 June | Wairoa District Storm declared a Civil Defence Emergency at 1900, terminated at 2100 next day. About 166 people evacuated at Nuhaka after flooding and power failure associated with storm. DRC appointed for 14 days. | | 2001 | 9 December | A chain of thunderstorms formed up the eastern coast of New Zealand, which resulted in downpours in Hawke's Bay. In Napier and Hastings areas 50mm of rain fell in the hour before noon - close to the average for the entire month. It caused millions of dollars of damage from water and surface flooding and damaging some roofs and shop stock . It was cited as a 1/100-year rainfall event for Napier and Hastings city areas. | | 2002 | 10 January | An electrical storm formed near Waipukurau in the evening, travelled north and resulted in 77mm of rain in 90 minutes in Hastings and 70mm in Napier. The storm turned streets into rivers, damaged footpaths and properties, caused powercuts and flooded shops as stormwater systems were unable to cope with the second 1/100 year downpour in a month. | | 2004 | 15 February | Southern Hawke's Bay was hit with southerly winds and heavy rain. In 24 hours starting from around noon on Sunday, 15,228.5 mm fell at Shag Rock, and 197 mm fell at Wallingford. The Tukituki River reached a 5-year level. Surface flooding occured in Otane, Waipawa, Waipukurau, and Takapau. Porangahau area was worst hit, with roads, the cemetery, business and houses flooded, and around 6 families evacuated. | | 2004 | 18 October | A thunderstorm dumped several days worth of rain on Napier in just a few hours – described as a rainbomb producing a 1/50 year event – the rain quickly filled up drains, and then roads which are constructed to be below the level of properties designed to act as large drains. However, the rain was so intense (up to 180mm of rain recorded in a few hours in the epi-centre of Tamatea/Greenmeadows) the water then also flooded numerous properties, with 8 homes being flooded and firms in the Onekawa industrial area estimating losses in the millions. | | 2007 | 17-19 July | High intensity rainfall caused localised flooding in Flaxmere, Puketapu, and Maraekakaho and sections of the Ruataniwha Plains on 17 July. Widespread rainfall continued at a lesser extent on 18 & 19 July over most of HB and the Tararua District. The main rivers in HB all experienced at least a 5-year return period water level. Two schools were evacuated and five homes in Maraekakaho evacuated due to flooding. Household water supplies in the area were contaminated and 30+ farms were affected with silt, damaged fences, broken culverts and scattered debris. | | 2010 | 7 July | A deep low pushed a strong moist southeast flow over HB producing long periods of heavy rainfall. This led to rivers rising, surface flooding, slips and hazardous driving conditions. 250-300mm of rain was recorded in Mahia and SH 2 from Napier to Wairoa closed due to major slips. Some coastal communities were also affected by sea swells. | Case Study - Hakowhai Hakowhai in the Dartmoor Valley is an example of a community in an area, which experienced severe flooding from the Tutaekuri River and decided to do something about the problem. Renewed awareness of flood vulnerability of the Dartmoor Valley became evident during the mid-1980’s, after a relatively flood-free period. The July 1985 flood caused considerable damage and exceeded expectations of magnitude. This increased interest in flood protection works. In 1988 the land flooded twice, firstly in March and then in September. Both floods were devastating to the local community. The community recognised its risk and put forward an application to erect a stopbank and to fund its own works. There were many stages in the consultative process that the Hakowhai Community went through in order to achieve flood protection. 1. The Hakowhai Community recognised their flood problem. 2. The problem was quantified. The flood risk was calculated together with an economic analysis. This showed the benefits of flood protection works. 3. Alternative flood protection solutions were quantified. 4. A solution was chosen and the design standard was for a 1 in 100 year flood. The community funded the scheme through a Government subsidy and a loan arranged by the Catchment Board. The loan was repaid by a special rating district, and the Hawke’s Bay Regional Council maintains the scheme. The scheme took four summers to complete, but the local benefits have already exceeded expectations through the increase in land value capability and production. ^Top^
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| Dartmoor residents formed a committee that campaigned for a stopbank to keep their land free from flooding (Photo HB Today 2001) |
Future floods
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.
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.
Stopbanks are also vulnerable to erosion from high velocity river water, and normally rely on live river edge protection in the form of willow trees to prevent scouring. Willow sawfly, which strips willows of their vegetation, was first found in NZ in 1997 and in Hawke’s Bay during Easter 2000. In the summer of 2000/2001 some river protection willows in Hastings were stripped of their vegetation by willow sawfly. Damage as continued since to such an extent that large numbers of willow trees are now dead or dying & their root holding capacity severely reduced. ^Top^
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| Willow sawfly lava eating willow vegetation |
Although Council is carrying out project works to mitigate the affects of sawfly over the worse affected areas on the Heretaunga Plains, until work is completed, the risk of failure of stopbanks has been significantly increased.
With no live vegetation to protect the river bank edge, modelling indicates scour and erosion of the berm will occur with just a moderate flood and eventually would threaten some stopbanks within a matter of hours.
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| The Maraekakaho community was flooded in July 2007 |
What to 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.
►Determine your risk.
►Get your household ready.
►Make an evacuation plan.
►Keep an ‘in case of flood’ to-do list.
If a flood is imminent:
- Listen to a battery-operated radio for the latest information.
- Be ready to act quickly. Floods and flash floods can happen quickly and without warning. Be ready to act immediately and keep your previously assembled Getaway Kit is near.
- Be prepared to evacuate.
- Follow the instructions and advice of emergency services and civil defence emergency management authorities. Local authorities are the most informed about affected areas you should avoid.
- Bring outdoor belongings, such as patio furniture, indoors.
- Where possible, move pets inside or to a safe place, and move livestock to higher ground.
- If flooding is expected, consider using sand bags to keep water away from your home. It normally takes two people about one hour to fill and place 100 sandbags, giving you a wall 0.3m high and 6m long. Make sure you have enough sand, or plastic bags, shovels, strong helpers, and time to place them properly. Don't overfill bags, typically no more than 5 shovel fulls per bag. Place bags on their side and stagger them to build your wall.
- Move valuable household possessions to the upper floors or to safe ground if time permits. Raising this equipment will prevent damage.
- If your home is in a flood-prone area, check your emergency water supplies and if needed fill plastic bottles with clean water for drinking. Water may become contaminated or water service may be interrupted. An undamaged water cylinder may be your best source of fresh water after a flood.
- Turn off utilities if told to do so by authorities. Authorities may ask you to turn off water or electricity supply to prevent damage to your home or within the community.
- Unplug small appliances. Small appliances may be affected by electrical power surges. Unplugging them reduces potential damage.
- Never try to walk, swim, or drive through flood water. Many flood fatalities are caused by people attempting to drive through water, or people playing in high water. If it is moving swiftly, even water 15 centimeters deep can sweep you off your feet.
Learn more about what you should do before a flood strikes, and when a flood threatens at the National Civil Defence Website.
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