A 2012 NIWA report, The Effects of Drain Clearing on Water Quality of Receiving Environments, written by Deborah Ballantine and Andrew Hughes for the regional council, found water quality decreased markedly during drain clearing and large amounts of sediment and nutrients, especially phosphorus, were released affecting downstream waterways.
Cleaning also drastically altered the environment within the drain as well as disturbing vegetation on the drain’s bank.
Environment Southland maintains more than 1200km of drains, some of them in the sensitive Waituna catchment. Council’s director of operations and environmental information Warren Tuckey said a range of initiatives, including the recommendations from the NIWA report, had been adopted and ongoing monitoring would show how effective they were.
“Especially in the Waituna catchment, we are reshaping the drains so there is a two-to-one slope to lessen the risks of bank erosion.”
“We’re also doing a trial so that only half of the drain is cleared. We leave 50m and then clear the next 50m and so on. It’s hoped the uncleared part of the drain will filter any sediment and nutrients released into the water during the drain clearing.”
Instead of diggers using a bucket to clear drains, they are using a weed rake.
“It changes the shape of the drain less and doesn’t take out the fish. We also have someone following the digger putting back any fish and eels that are taken out with the digger.”
Drain clearings that used to be left piled on banks are now taken away, or spread on nearby paddocks. The NIWA report found loose sediment left on drain banks often ended up back in the waterway when it rained.
Tuckey said tree planting next to the drains Environment Southland maintains was also progressing as trees shaded the water, helping to stop vegetation growing in the waterways and clogging them.
“The less we have to clear the drains the better. Some we do every three or four years but some we only have to do every 10 years.”
Shading also lowers the water temperature allowing fish and invertebrates to thrive.
Planted riparian strips helped to stop the erosion of the drain’s banks and gave a buffer zone between paddocks and the waterway, helping prevent nutrients and sediments entering it and so lessening the need to clear it so often, he said.
Drain clearing is a permitted activity in artificial and modified Southland waterways but landowners must comply with rules which can be found on the council’s website.
They include that drain clearing cannot be undertaken from June 1 to October 31 in trout habitats as it disturbs spawning and from November 1 to May 31 in tidal areas where there are whitebait.
Bed disturbance and gravel removal must be kept to a minimum and all equipment used must be cleaned before leaving the site to prevent the spread to other waterways of pest plants and animals.
Tuckey said the council was still encouraging landowners to clear drains when needed so water flowed off farmland during times of high rain events.
“However, we would rather farmers used best practice to maintain their drains so they didn’t need to be cleared so often.”
