OPINION | Change of land use causes extensive washout

Photo: Eddie Spearing

A number of recent media articles have highlighted impacts of stormwater runoff from large urban developments.  Runoff from Northlake and Hikuwai has discoloured the Clutha River and sediment from Meadowstone Alpha Series has overflowed into Bullock Creek and Lake Wanaka.  Current rules appear inadequate to control or prevent future runoff events, with weak resource consent conditions and minor penalties for failure to comply.

The Northlake/Hikuwai runoff into the Clutha River is dwarfed by rural runoff events at Reko’s Bluff, upstream of Luggate Bridge, which have resulted in gullies of up to 30m wide and 20m deep being gouged in the riverbanks and damaging Newcastle Track.  One of these large scale washouts initially extended almost half way across the Clutha.

A geotechnical report on the Reko’s Bluff washout was prepared in August for Otago Regional Council. The supplementary follow up report dated 28 September states: “In summary, the geological evidence indicates that the gullies are the result of runoff water from heavy rainfall events flowing down ancient shallow river channels in the farmland above.  This spills over the terrace edge generating highly erosive gravel/water slurries termed debris flows. These have flowed down the gullies building large alluvial fans at the base. It is concluded that the change in farm vegetation from tussock grassland to pasture has resulted in increased runoff from heavy rainfall events, resulting in four major gully erosion events in the 4½ years since the change of land use”.

The erosion events arose following the conversion of terrace vegetation from tussock grassland to pasture in 2014.  The current landowners installed irrigators in 2018, which have the potential to exacerbate erosion.

Development, whether urban or rural, inevitably alters the landscape.  When trees or tussocks are removed, there is no longer anything to slow the water down or remove sediment.  In the case of Reko’s Bluff, the former tussock grassland displaced by pasture acted as a sponge to retain rainfall, producing less runoff than pasture grasses. The long term impacts of the huge Reko’s Bluff washouts on the Clutha River ecosystem are unknown.

The Upper Clutha Lakes Trust is working with the community to develop an enduring integrated catchment management plan (ICMP) for the Upper Clutha catchments.  The ICMP brings together a range of stakeholders to better understand these types of issues and protect long-term water quality by limiting negative impacts caused by factors such as the growth being experienced in agriculture, tourism and urban development.

More information on the Trust and the ICMP scope can be found on the Upper Clutha Lakes Trust website www.uppercluthalakestrust.org.


Read: Bike track damaged, washout enters river



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