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| River Information | The Hutt is a relatively short, steep river with four steep tributary catchments located around the river's headwaters. The river is 62.7-km in length, with a catchment of 648 sq km. | ||||||
| Hutt River
Level Readings |
River Level
4.4m 5.3m 5.95m 6.49m 7.17m 7.6m |
Flood Level
2 yr flood 5 yr flood 10 yr flood 20 yr flood 50 yr flood 100 yr flood |
Cumecs
760 1070 1270 1460 1710 1900 | ||||
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| Worst Case Scenario | 100 year flood event is created by 1900 cumecs. The river bank is designed to take 1900 cumecs. The catchment area is capable of generating 7000 cumecs which means 5100 cumecs could overflow onto the Valley floor. | ||||||
| Flood Depth | Up to 3 metres in some areas on the floodplain (the flat urban area from Upper Hutt to Petone) | ||||||
| Speed Of Water | The 100 year event would create an extreme threat to anyone trying to move in water over knee depth, particularly if the stopbank was breached. | ||||||
| Stopbanks | Designed to contain floods just in excess of the 100 year flood event. The stopbanks are however considered vulnerable at some locations in a 20 year event and restrictions such as bridges also reduce the design capacity. | ||||||
| Stopbank Integrity | 20 year event - Breaching is a possibility.
50 year event - 10 - 20% chance of stopbank failure, high erosion. 100 year event - stopbank failure through bank and foundation is likely, significant flooding of city would result. | ||||||
| Probability | The probability of a 100 year flood within the next 100 years has been calculated to be a 63% chance. | ||||||
| Tides | There is a period of time of two hours before and two hours after high tide where the run-off water cannot escape and will back-fill into low-lying areas. This may affect stormwater outfalls and local streams. | ||||||
| Stormwater Outfalls | Floods with a 1 in 10 chance of occurring will close the flood gates on the stormwater outfalls to the Hutt River. If there has been widespread rain, ponding will spread from the stormwater systems to low-lying areas, which can lead to widespread flood. | ||||||
| Local Streams | Local streams are affected. When the river is high this stops water from streams running into the river, which creates local ponding and flooding in the streams as the water cannot escape. Drains overflow. | ||||||
Warning Systems |
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| River Warning Systems & Timeframe | Hutt River has 2 warning sensors, one at Kaitoke and one at Birchville. They give about 2.5 hours warning for the flood peak. Rainfall recorders in the catchment areas may provide up to an additional 1.5 hours warning.
Wainuiomata River has a trigger alarm system at the Manuka track upstream of the water treatment plant. Alarm is triggered at the level of a two year flood event, sufficient time for the monitoring process to take place. | ||||||
| Weather Warnings | The Met Service produces heavy rain warnings, which are received at the HVEMO. Staff receive and analyse the effect of the rainfall on the Hutt against pre-determined rainfall indicators. This information is passed on to members of the flood warning group. Warnings are received regularly during a period of heavy rain and are continually monitored and analysed. | ||||||
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Headquarters Activation |
When heavy rainfall warnings are received and the river rises to a two year flood event, the Emergency Operations Centre of Hutt City Counciland Upper Hutt City Council is activated. Additional resources such as emergency and essential services are notified of the implications. | ||||||
| Monitoring | The river is monitored by the WRC Hydrology and Rivers staff when the two year flood level is reached. The roads are monitored by Council staff. Monitoring continues until levels subside to a safe level. | ||||||
Sirens |
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| Sirens/Public Warning System | When public safety is an issue, adequate warning will be given to those in the affected areas, generally by sounding the Civil Defence warning sirens. The sirens would sound for a significant period of time, in which case the community should tune to a local radio station. | ||||||
| Public Information | Information and advice by Civil Defence will be broadcast through radio stations. People should listen for official broadcasts. Reports will also be available and updated on this website. | ||||||
Measures |
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| Sandbags | Large supplies of sandbags are readily available throughout Lower Hutt, Wellington and Upper Hutt, for stopbank protection. Response procedures to fill the sandbags are in place using the Rural Fire Authority resources. | ||||||
| Evacuations | Where necessary evacuations will be arranged before the risk from the fast flowing flood water prohibits this action.
As a general rule, the community will be safer staying in their own homes, seeking refuge in their attics. If the stopbank is breached or overtopped, the community will be requested not to try and relocate, because of the danger. | ||||||
General |
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| Priorities | Priorities are set by the Civil Defence Controller and the safety of the community is the major issue. Building and other damage is secondary. | ||||||
| Worst Known Floods | In 1858 and 1898 there was complete inundation of Hutt Valley from hills to hills. In 1939 the flood swept areas north of Boulcott, covering hundreds of acres, isolating houses, blocking roads and destroying bridges. | ||||||
| Population At Risk | 50,000 (includes Upper Hutt). The Hutt is the most densely populated floodplain in NZ. | ||||||
| Damage Estimates | Current damage estimates from the Hutt river floodplain management plan investigation are as follows: | ||||||
| Flow
1900m3/s 2500m3/s 3500m3/s |
AEP
1% 0.1% - |
$ 204 million $ 280 million $ 1,622 million | |||||
| All estimates assume stopbank breaches.
AEP = probability of flood event being equalled or exceeded in any one year. | |||||||