Welcome to the Hawke's Bay Regional Council webpage on how we monitor flood event phases and the flood protection around the region.
On this page
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Ngaruroro River in flood,
Waitangi Railway Bridge, 24 August 2003 |
The Regional Council has a significant role in flood protection. Our engineers, scientists, technicians and Works Group team work together to provide a comprehensive flood protection programme across the region. Rainfall and water level monitoring provides information to assist flood monitoring and management for Wairoa/Waiau, Esk, Tutaekuri, Ngaruroro, Tukituki, Waipawa, Porangahau catchments.
The aim of flood protection is to reduce the risk of flooding so the impact on the community's livelihood is minimal, the environmental impact (deterioration of water and land resources) is alleviated, and most importantly the health and safety of Hawke's Bay people living on flood plains is protected.
The plains areas of Hawke’s Bay region have been formed over many centuries by the braided rivers bringing soil, silt and gravel down from the mountains. In the past, rivers used to change their channels frequently during major floods, but protection works such as channel training and stopbanks now confine them to set channels. However the threat of a major flood exceeding design capacity is always present.
Regional Council flood engineers run an operational system to manage major flood events on rivers with flood protection works (eg, stopbanks, edge plantings).
This involves five distinct sequential phases. These are:
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|
Flood Event Phase |
Telemetry System Water Level Alarm Settings |
Meaning |
What you will see |
|
1 |
Monitoring |
Alert Level* |
365 days 24/7 operations which relies on telemetry/flood warning system & warnings from Met Service |
Water flowing in the river channel |
|
2 |
Alert |
Green Alarm Level |
Mean Annual Flood (2-3 year event) |
Flood water overflows main channel, on to lower berm areas |
|
3 |
Forecasting - Evaluation |
Orange Alarm Level |
20% Annual Exceedence Probability (5 year event) |
Flood water covering all berm areas to stopbanks |
|
4 |
Flood Phase Activities |
Red Alarm Level |
5% Annual Exceedence Probability (20 year event) N.B. Our stopbanks are designed for a 1% AEP |
Flood water rising against stopbanks but still considerably below the top of the stopbank |
|
5 |
Aftermath |
Progressively reset back to Green or Alert Level |
|
Flood water receeding back to the river channel |
*Alert Level - this provides early warning for Council staff who can give warnings to people leasing river berms and other river users such as gravel extractors that they need to move stock or equipment from flood ways. The setting of Alert Levels for each river is based on local knowledge.
Small rises in river level after rainfall seldom go beyond the monitoring aspect, while floods of a small size usually do not progress beyond the third phase.
When a river reaches Red Alarm levels, staff patrols are set up to monitor stopbanks and other trouble spots. Staff also forecast flood trends using either computer or manual forecasting methods to determine the seriousness of the event and keep Territorial Local Authorities informed of possible implications.
Hawke’s Bay rivers have reached red alarm levels many times in the past, with no serious impacts to the community. That is what the stopbanks are built for - to protect our homes, property and livelihoods from damaging flood waters.
Council maintains 23 flood control schemes across the region - funded through targeted rates levied each year on properties which benefit from the flood protection works. The river control and drainage assets in Hawke’s Bay are worth an estimated $90 million.
Protection works include: 240km of stopbanks, 11 flood detention dams, 267 flood gate structures, 21 pump stations, 570km of drains, 420km of river training and bank protection works. Along these areas there are also 26 river berm areas for public recreation use.
IMPORTANT NOTE:
Stopbanks are vulnerable to erosion from high velocity river water. Live river edge protection in the form of willow trees is the first line of defence to prevent this scouring – the branches and foliage slow the flood water near stopbanks, while the roots hold the soil together. Since 2001 a new pest called willow sawfly has defoliated and killed willows along stretches of the rivers.
Council is carrying out urgent works from 2006-2008 to replace the damaged protection but until work is completed, the risk of failure to the stopbanks has been significantly increased.
Please contact us should you require further information.