Flood Control
Unfortunately, flooding is a word that is often associated with disaster, especially when infrastructure and property is damaged.
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Neither urban nor rural people want their houses inundated with flood water.
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Farmers want to maximise grazing generally and minimise stock loss during high rainfall events.
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Hydro electric generators want to store that energy until it is most needed.
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Transport agencies do not want vital roads and rail networks closed because of floods.
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Whole crops can be lost if they are covered with water.
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Flooding is, consistently, New Zealand’s most damaging form of natural disaster.
Most flood damage is the result of poor placement of asset or activity.
Floods generally, are highly predictable. If one builds a house on a flood plain one should not be surprised if one gets flooded!
Often property damage is exacerbated by Regional Councils attempts to constrain floods by building stop banks that do not allow enough area to absorb the amount of water required. Nor is there any allowance for silt entrapment.
The flood control stop banks near Whakatane are classic examples of this as stop banks are built almost directly on top of the existing river banks ensuring that any sediment flows directly to the sea.
New Zealand’s large dams were built for power generation, irrigation and water supply, but they have at times buffered major floods.
Flood plains are a common feature of coastal lowlands and naturally provide an area for entrapment of river born sediments and habitats for freshwater and estuarine fish. Flood plains need to be managed to enable them to carry out these very important roles.
Neither urban nor rural people want their houses inundated with flood water.
Farmers want to maximise grazing generally and minimise stock loss during high rainfall events.
Hydro electric generators want to store that energy until it is most needed.
Transport agencies do not want vital roads and rail networks closed because of floods.
Whole crops can be lost if they are covered with water.
Flooding is, consistently, New Zealand’s most damaging form of natural disaster.
FLOOD CONTROL IS AN IMPERATIVE FOR MODERN SOCIETY
Flooding from Cyclone Bola had a devastating effect on horticulture in the East Coast region. The damaged crops included 3,000 tonnes of grapes, 1,300 tonnes of squash, 7,000 tonnes of sweetcorn, 13,500 tonnes of tomatoes, and several million dollars in vegetables for the local market. Large amounts of horticultural produce were swept into the sea – fishing boats were dredging fruit from the sea floor in their nets for several months after the storm.
On that basis, engineers build stop banks and giant pumping schemes to get flood water off vulnerable land and back into rivers. They also channel-ise many waterways to get rid of water quickly to the coast.
Flood Control is a justifiable activity but it does have ramifications to our aquatic ecosystems.
EELS LOVE FLOOD PLAINS – so should we, if they are non destructive
Flood Plains
The amount of water flowing down streams and rivers is always in a constant state of flux. Water levels rise and fall in accordance with the amount of rainfall in their catchment. During rain events, river flow increases and water levels rise. During dry periods levels fall, and in extreme cases flows stop entirely. A lot of rainfall flows into the ground and flows downward underground, but at a much slower rate than surface water.
These variations in water flows are an inevitable result of variations in rainfall.
Increasingly though, many low flows are being greatly exacerbated by excessive water extraction for irrigation and other use. Sometimes to the point where river flow is stopped entirely. For eels, and especially long finned eels, hi flow events provide a feast of food supply and it is during these events when they feed the most and very rapidly increase body weight and growth.
The most preferred waterway for eel is one that has reliably frequent hi flow events where the waterway spreads into a flood plain greatly increasing food availability. Not surprisingly it is this type of flow regime that provides the most productive fishery and especially so where it is coupled with a pastoral flood plain.
Pastoral flood plains carry very high densities of earthworms and these earthworms are the most desired food for eels. Longfinned eels can stuff 30% of body weight into their stomachs in a single night! Scientists have sometimes had to take a second look at whether an otolith growth ring represented a year or a flood.
The importance of flood plains for eels cannot be overstressed!
Apart from the very important role that floodplains play for resident fish, they are even more important for allowing the deposition of silts and soils that will otherwise be washed into, and lost at sea
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High silt loads into our coastal waters are devastating for coastal marine life. Following such events it is common for beaches to be littered with Dead Sea life such as Crayfish and Paua.
When water rises and spreads out over a larger area in a non destructive situation it slows down. This causes less siltation & retains the water for much longer.
In many regions, riverside areas are reserved as parks, sports fields and parking areas, so flooding will cause minimal damage.
A number of techniques are used in New Zealand to control rivers. Thousands of kilometres of stop banks have been built to keep high river levels within the channels; this has allowed for urban settlement of areas such as Christchurch and the Hutt Valley.
Solution – or problem?
Every flood prone area has its own set of dynamics and, therefore, its own best solution. However, what might be a good solution for one group of stakeholders can, in turn, create a problem for others.
One method of preventing rivers overflowing their stop banks is to lower the river bed by removing gravel. Speeding the passage of flood waters through an area can involve straightening river channels and removing obstructions such as vegetation. Of course, these are pretty radical solutions which hopefully are used only in critical scenarios.
Another method of flood control is damming or diverting flood water until river levels drop.
To protect communities downstream, some river stop banks have floodgates that can be opened to deliberately spill flood water onto less densely settled farmland.
In a number of large low lying flood prone areas large flood pumps are used to move the water back over stop banks into the river course.
EECo strongly advocates that, when pumps are replaced, they are replaced with fish friendly systems.
Flood control systems can be on a very large scale.
Pictured here are the Moutoa flood gates diverting excess water from the Manawatu River into the flood course.
David Young, 'Rivers - Conserving rivers', Te Ara - the Encyclopedia of New Zealand, http://www.TeAra.govt.nz/en/photograph/14709/flood-control-gates (accessed 16 February 2017)
DRIFTWOOD
Floods and especially high tides, bring enormous amounts of post harvest waste wood down from forestry blocks.
This picture shows wall to wall logs and branches in the tidal estuary of a southern North Island River.
These very heavy and awkwardly shaped behemoths gouge out additional damage and siltation as they move to the coast.
Wider riparian margins in plantation forestry would reduce this additional siltation and damage.
By combining a set-back barrier with suitable riparian planting, we:
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Protect the river margins by slowing the river
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protect the farmer’s interest to graze that area when it is not in flood
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Reduce siltation which is creating so much damage in our coastal lakes and lagoons not to mention valuable tourism and inshore fisheries such as paua and rock lobster
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Provide eels with the food foraging opportunities they have evolved to over eons.
Landcare Research have published work specifically looking at the importance of willows vs natives for pastoral riparian margins and flood control. Click Here to read more about this work.