How much is an “acceptable flow”?

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Galloway Irrigation Society member MIKE KELLY explains some of the facts about the Falls Dam and the Galloway irrigation scheme as we continue our Protecting our Water series.

The Manuherikia River is highly valued for aquatic ecosystems, recreation, human water supply, stock water supply, power generation and irrigation. All these water uses are equally important to sustaining life as we know it in the valley.

Fortunately, we have the Falls Dam at the head of the valley and a well-co-ordinated group of irrigators that manage the dam and the river flow through the dry periods. This is no easy feat, and requires considerable communication and co-ordination between the four irrigation schemes to maintain a supply for irrigation and to maintain an “acceptable flow” in the lower river. The balancing act requires a continual assessment of flows in the river, storage in the dam, and the weather forecast.

In most years, the storage in Falls Dam is required to supplement irrigation during the months of February and March. In very dry summers such as 2017-18, the Falls Dam storage was being used in December, and by mid-January the irrigators were on 50% rationing to maintain an acceptable flow in the river.

So just what is an “acceptable” flow? For the last 20 years the river has been unofficially managed with the minimum flow of 900 litres per second at the campground site in Alexandra. Whether this is an acceptable flow for all users of the river is the focus of the minimum flow consultation that the Otago Regional Council (ORC) is currently undertaking. Finding the right balance of water use is the key.

If the “acceptable” minimum flow is too low, the aquatic ecosystem may become degraded. If the acceptable flow is set too high, the local orchards can’t grow as many cherries and the farmers can’t fatten as many lambs. There is an economic impact.

There are other issues to consider along with the “acceptable” flow. The writer last week [Lynne Stewart, “Protecting our Water”] referred to the Galloway irrigation scheme intake in the river. The four irrigation schemes in the valley have intakes from the river and have some form of weir, rock barrier or gravel wing wall to divert water. Again, it’s a balancing act.

If the gravel wing wall is too dominant, then too much water is diverted and flow must be bywashed back to the river. If the gravel wall is too thin, then it is washed away in the first fresh flow of the season.

At the Galloway intake there are two bywash points in the intake canal, and 50% of the water taken is returned to the river. When the photos were taken (in last week’s article) there was bywash returning to the river and the flow of 900 litres per second was maintained at the campground site. The Galloway intake is operated within the conditions of the current resource consent which includes a restriction on the dimensions of the gravel wing wall and maintaining a channel for fish passage. Although the scheme always complies with these conditions, we are aware of the concerns about the intake and looking to improve the design so more water is left in the river.