floodways, spillways and stopbanks
What is a floodway?
A floodway, also called a spillway, is a channel constructed to take a portion of the river flow off the river to take pressure off the main river channel. The floodway then links back up with the river, which then takes the water out to sea.
Floodways are typically dry and are commonly used for grazing during normal river flow conditions, and only operate during flood events when the river is high enough to flow into them. Floodways also typically have stopbanks on one or both sides and sometimes include manually controlled flood gates and/or weir walls (spillways) to let the water enter into the floodway.
Floodways need to have roads crossing them, similar to a ford in a river.
For more information on floodways, you may find the Bay of Plenty Regional Council’s animated video helpful: What is a Floodway and Spillway
A floodway, also called a spillway, is a channel constructed to take a portion of the river flow off the river to take pressure off the main river channel. The floodway then links back up with the river, which then takes the water out to sea.
Floodways are typically dry and are commonly used for grazing during normal river flow conditions, and only operate during flood events when the river is high enough to flow into them. Floodways also typically have stopbanks on one or both sides and sometimes include manually controlled flood gates and/or weir walls (spillways) to let the water enter into the floodway.
Floodways need to have roads crossing them, similar to a ford in a river.
For more information on floodways, you may find the Bay of Plenty Regional Council’s animated video helpful: What is a Floodway and Spillway
What is a stopbank?
A stopbank is a specially designed and maintained mound of earth that helps to limit or prevent the spread of floodwater onto surrounding land. Also known as a levee, dyke, or flood bank, a stopbank is a constructed earthen barrier that runs parallel to a river or floodway to confine flows within the channel. Stopbanks typically need to be designed by specialist engineers and are constructed with appropriate soil.
The Bay of Plenty Regional Council also has a useful animated video: Stopbanks, seepage and bylaws.
A stopbank is a specially designed and maintained mound of earth that helps to limit or prevent the spread of floodwater onto surrounding land. Also known as a levee, dyke, or flood bank, a stopbank is a constructed earthen barrier that runs parallel to a river or floodway to confine flows within the channel. Stopbanks typically need to be designed by specialist engineers and are constructed with appropriate soil.
The Bay of Plenty Regional Council also has a useful animated video: Stopbanks, seepage and bylaws.
What can be done with the land in a floodway?
The land in the floodway may still be able to be grazed if adaptive stock fencing or cattle stops are used along the roads. Alternatively, the grass could be harvested and bailed. The floodways could potentially be used for community recreation or ecological habitat purposes, however buildings, structures, and large trees can’t be in a floodway.
The land in the floodway may still be able to be grazed if adaptive stock fencing or cattle stops are used along the roads. Alternatively, the grass could be harvested and bailed. The floodways could potentially be used for community recreation or ecological habitat purposes, however buildings, structures, and large trees can’t be in a floodway.
How often will the floodway have water in it?
A floodway will be designed to only have water in it for large flood events, which will be very infrequent. With the floodway sill heights in the preferred Option 1C, the floodway is predicted to have water in it statistically once every 10 to 15 years.
A floodway will be designed to only have water in it for large flood events, which will be very infrequent. With the floodway sill heights in the preferred Option 1C, the floodway is predicted to have water in it statistically once every 10 to 15 years.
How will people cross the floodway in a flood?
It is envisaged that an early warning system will be in place and barrier arms will lower along the roads before the floodway entrance. Flashing lights would likely be installed along the sides of the floodway.
People will not be able to cross the floodway once the barrier arms are down, and lights are flashing. It is expected that the maximum duration the floodway would be unsafe to cross would be less than 24 - 28 hours.
It is envisaged that an early warning system will be in place and barrier arms will lower along the roads before the floodway entrance. Flashing lights would likely be installed along the sides of the floodway.
People will not be able to cross the floodway once the barrier arms are down, and lights are flashing. It is expected that the maximum duration the floodway would be unsafe to cross would be less than 24 - 28 hours.
How will people evacuate if the floodway has water in it?
People will not be able to cross a floodway when it has water in it, or when the barrier arms are down, and lights are flashing. Planning is underway to develop an emergency management plan for the community with measures put in place to ensure people have access to safe shelter and supplies until the flood waters reside. This could include the use of community resilience hubs or emergency and resilience pods, like EPODs.
People will not be able to cross a floodway when it has water in it, or when the barrier arms are down, and lights are flashing. Planning is underway to develop an emergency management plan for the community with measures put in place to ensure people have access to safe shelter and supplies until the flood waters reside. This could include the use of community resilience hubs or emergency and resilience pods, like EPODs.
How will I access the western end of Ruataniwha Road and Waihirere Road if there is a floodway?
As mentioned, people will not be able to cross a floodway when it has water in it. When there is no water, people will be able to use the road to cross the floodway similar to a ford in a river.
As mentioned, people will not be able to cross a floodway when it has water in it. When there is no water, people will be able to use the road to cross the floodway similar to a ford in a river.
How can we ensure the integrity of the floodway?
The flood mitigation solution will be modelled extensively and designed to help protect Wairoa from a 1-in-100-year flooding event (noting that in Wairoa, Cyclone Gabrielle did not reach this level of flooding).
It should be noted that with any flood scheme there is always a risk of a large event exceeding the level of mitigation, and no solution will provide absolute, foolproof flood protection.
The flood mitigation solution will be modelled extensively and designed to help protect Wairoa from a 1-in-100-year flooding event (noting that in Wairoa, Cyclone Gabrielle did not reach this level of flooding).
It should be noted that with any flood scheme there is always a risk of a large event exceeding the level of mitigation, and no solution will provide absolute, foolproof flood protection.
How will the floodway be managed?
Once constructed, the operational and asset management of any flood mitigation solution in Wairoa, including a floodway, will be the responsibility of the Hawke’s Bay Regional Council.
Once constructed, the operational and asset management of any flood mitigation solution in Wairoa, including a floodway, will be the responsibility of the Hawke’s Bay Regional Council.
What will the regular maintenance be and who will do this?
Like all flood mitigation assets, annual maintenance will be required on an ongoing basis and could include site monitoring and maintenance such as fencing, weed control, mowing and possibly grass harvesting. As mentioned, this regular maintenance and ongoing management will be the responsibility of the Hawke’s Bay Regional Council.
A plan will also be developed for the operational phase of the floodway, in times that it will operate. This will include triggers for closing the roads that cross it and the clearing of debris and repairs after it has operated. This plan will be developed in conjunction with Wairoa District Council, mana whenua and the local community.
Like all flood mitigation assets, annual maintenance will be required on an ongoing basis and could include site monitoring and maintenance such as fencing, weed control, mowing and possibly grass harvesting. As mentioned, this regular maintenance and ongoing management will be the responsibility of the Hawke’s Bay Regional Council.
A plan will also be developed for the operational phase of the floodway, in times that it will operate. This will include triggers for closing the roads that cross it and the clearing of debris and repairs after it has operated. This plan will be developed in conjunction with Wairoa District Council, mana whenua and the local community.
What safety measures / risk management plan will be in place to mitigate these if not properly managed or if conditions become extreme?
The operational plan to be developed will contain a high health and safety component. As mentioned, this plan will be developed in conjunction with Wairoa District Council, mana whenua and the local community.
The operational plan to be developed will contain a high health and safety component. As mentioned, this plan will be developed in conjunction with Wairoa District Council, mana whenua and the local community.
Can we also see a plan and picture of what the spillway will look like?
There is a ‘flyover’ animation (video) available on our Video Resources page.
Please note, this animation is indicative only, noting that further refinements to Option 1C have been made since the animations were first developed.
There is a ‘flyover’ animation (video) available on our Video Resources page.
Please note, this animation is indicative only, noting that further refinements to Option 1C have been made since the animations were first developed.
