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Flood protection methods

Having a good warning system, and understanding how our rivers behave while in flood, can save lives, property and money. Environment Bay of Plenty has teams of people who endeavor to keep the people of the Bay of Plenty as safe as possible during times of flood. But how do we do that?

Flood predictionFlood prediction

Information comes in from the metservice when there is a warning of coming heavy rainfall, this data in conjunction with rainfall and river flow data is used to predict the response a river to the weather system and if, where and when flooding could occur. This allows us to give advanced warning to farmers and the public when necessary. The system is not 100% accurate but has proven to be a valuable tool in past floods. As time goes by and we have more flood information to work with our flood prediction capability will become even more accurate.

Flood monitoring

Bay of Plenty Regional Council collects data from a range of strategic locations throughout the Bay of Plenty. These data recorders collect information such as river and tide level, and rainfall depth. Data is transmitted it back to Environment Bay of Plenty via the radio or phone network. We are able to use the data to monitor how much rain is falling at strategic locations, and how much water is flowing in the rivers. This data is also fed into our flood prediction models.

View live monitoring

Stopbanks and floodwalls

Stopbanks

Stopbanks (also known as levees) are banks of earth constructed along the edges of rivers or streams to stop water from flooding nearby land when the waterway is in flood. Stopbanks make it possible for the river to hold greater amounts of waters within their banks without damaging neighbouring land.
In past times stopbanks were often made from river silts dug from the river, or from soil available from land nearby. This has caused problems in some locations because the material used to build the bank has been of a poor quality and prone to erosion.stopbank
Now when new banks are constructed, or old banks upgraded, the soil used to make the banks is tested to check its quality and suitability. If suitable soil cannot be found nearby soil will need to be trucked in, but it will mean stronger more stable banks, and better flood protection.
To check that a stopbank is providing enough flood protection a computer model of the river or stream system is created. Different amount of water can be put into the model to see how much the banks can hold, and if there are any low spots that need to be topped up.

Floodwalls

Floodwalls are used in locations where there is not enough land space to construct a stopbank. Most flood walls are made out of concrete or steel. The Bay of Plenty has a number of floodwalls, most of which are found in urban areas.

Riverbank Protection

There are a number of techniques used by Environment Bay of Plenty to prevent the erosion of our river banks. By protecting the banks in vulnerable locations it is possible to protect the land directly behind the banks from washing away during periods of high river flows. This also helps to keep the river within its current channel boundary.

Rock ProtectionRockwork

Rocks can be placed on the banks of a river to create greater stability and erosion protection. The rocks need to be carefully chosen and placed to ensure they will not be washed away when the river is in flood. The correct rock size and weight are important. Working rock is a protection option that has minimal negative effects on the environment, as rock is naturally found in this environment, and if the rock should be washed away in a large flood there are no negative consequences to the downstream river environment.

Willow Plantings

Willow trees are easy to plant and they rapidly establish themselves in the river environment. Their root system helps to hold banks together, and they collect river silts and debris that builds up banks.
Willows need to be maintained and kept to a manageable size as they can become too big in only a few years. If willows wash away in high river flows they may begin growing in unmonitored locations, this can be a problem but is not harmful and can be easily fixed.

Dams

Dams are usually placed on major New Zealand rivers in order to generate power. In times of flood dams can also be used to help control the amount of water flowing down a river.
If a dam has a lake or water storage area behind it this can be used to store some of the flood water when the river flow is highest. The stored water can be released slowly after the flood has passed and river flows are not as high. Dam owners and river managers need to work together. If we know there is a flood coming the dam can be lowered before the flood arrives, this way it can store greater amounts of the flood water when needed. This may mean higher river flows for longer, both before and after the peak ofMatahina Dam the flood, but it can help reduce the amount of land flooded downstream and increase public safety.

The Bay of Plenty has a number of significant dams

Flood gates and tidal gates

Flood gatesFlood gate 1

These are culverts that have flaps on their ends, these flaps allow the water to go one way but stop it going the other. The flap is pushed open and close by the water as it pushes against it. We often use these on culverts at the end of drains that go through a stopbank and into a river. We want the drain water to flow into river when the river is lower than the drain, but if the river is high and in flood we don't want the flood waters to go through the culvert from the river and flood the farm land behind the stopbank. Remember water will always flow downhill unless being pushed or pumped. Have a look for a flood gated culvert the next time you are walking along a stopbanked waterway.

Tide Gates

These work very similarly to flood gates, except they will be put at the end of a waterway that flows into a river where the river is affected by the tides. When the tide is out and the river level is low the tide gates will open allowing the waterway to flow into the river, but when the tide is in the gates will be held shut by the water level in the river, stopping the water from flowing up the waterway.

Public Awareness

A community that is aware of the risk of flooding to their community they are more likely to be prepared, mentally, emotionally and physically. Being prepared helps us to cope when flooding occurs and respond appropriately, and as a result the impacts on the community are reduced.
The Bay fo Plenty has a number of locations that experience a significant flood risk, the Regional Council has various way it promotes public awareness of these, including media releases, liason with the public over local issues, the civil defence website as well as this website.

Ways you can become more aware