Stream erosion main cause of harmful sediment in waterways

Tonnes of sediment smothering the beds of Auckland’s estuaries and harbours every year could be significantly reduced if stream erosion is addressed, according to the Auckland Regional Council.

ARC Stormwater Action Team leader Earl Shaver says international studies have identified stream channel erosion as the primary source of sediment entering waterways from developed areas, and local studies indicate Auckland is no exception to this problem.

“Stream erosion is caused by huge volumes of stormwater washing into streams and gouging them out, carrying the resulting sediment downstream into estuaries such as the Whau, and Waitemata and Manukau harbours,” Mr Shaver says.

“As the region grows so does the area of paved and impervious surfaces that rain can’t soak into,” he says. “Instead what happens is the amount of stormwater running off the land and into Auckland’s streams increases, and streams just can’t handle the volumes of water rushing into them, so their banks and beds start to erode away.”

ARC Environmental Management Committee Chair Cr Dianne Glenn says the best way to stop this sediment degrading Auckland’s waterways is to take a new approach to development.

“All over the world a Low Impact Design approach is being taken to development and intensification of urban areas, and it’s gradually starting to catch on in Auckland,” Cr Glenn says. “LID involves using development practices that have a reduced impact on our sensitive waterways, and I thank those developers who are already working with us, meeting our environmental and design requirements to achieve better outcomes overall.”

Sediment from stream erosion smothers bottom dwelling animals such as shellfish, and promotes the growth of mangroves by depositing sediment in shallow, low energy areas where mangroves thrive. Over time sedimentation can also change once sandy-bottomed beds into mudflats, and bury natural features of streams such as stream pools and rock-beds.

Low Impact Design practices that reduce volumes of stormwater include the use of roof gardens, roof tanks, planting of vegetation, and limiting the total amount of paved surfaces on a site so rain can soak into the ground.

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