'Rock armour' helps streams

Bay of Plenty Regional Council Land Management Officer Robin Pieper planting grasses along the Ngongotaha Stream. Photo / Bay of Plenty Regional Council.

Hundreds of tonnes of rocks help repair erosion damage - plus new survey guards against floods.

High priority works to improve stream health and repair erosion damage are progressing on several streams in the Rotorua area.

Bay of Plenty Regional Council has been stabilising stream banks with rock armouring, re-establishing trout habitats and has planted more than 700 native plants to help regenerate the Ngongotaha Stream after a severe weather event caused significant erosion along its banks last April.

The erosion of the soil results in sediment entering waterways, one of the four main contaminants that impact on water quality.

Bay of Plenty Regional Council Area Engineer Kerry Smith says his team has also recently finished repairs on the Utuhina and Puarenga streambanks with the placement of 700 tonnes of rock armouring. It is expected they would use 600 tonnes to regenerate the Ngongotahā Stream too.

Rock armouring strengthens and stabilises local stream banks which can be fragile due to the light pumice and soft soils present naturally. Grass and native plants will be added once the rock armouring sites have settled.

As part of this work, the council has been collaborating with Rotorua Lakes Council on stream erosion repairs, including identifying opportunities to repair significant erosion sites on stream banks impacting properties, reserves, walk and cycle ways.

Smith says other work under way by the council includes gathering cross-section survey data on the Ngongotahā Stream. Due to be completed this month, the survey will inform future flood modelling work and the development of options for engineered flood control mitigation.

"The cross-sectional survey involves measuring stream capacity and recording width, depth and bank height at 70 sites along the stream," he says. "That information can be used to model the impacts of different sized rainfall events on the catchment and help identify any places where stream banks are vulnerable under different scenarios.

"It will provide information that councils and communities can use to explore options for mitigating flood risk from the Ngongotahā Stream."

Smith is asking locals who use the Ngongotahā Stream over summer to take care to avoid tripping on survey pegs and to please leave any they find in place.

Rotorua Lakes Council Sports, Recreation and Environment Manager Rob Pitkethley says it had been really positive working with the regional council to deliver a more coordinated and streamlined approach to stream bank damage besides waterways in the district.

"During this year our Open Spaces team has helped to identify issues like erosion and, by working with the team at regional council, we have had more of an opportunity to enhance our natural environment, improve adjoining recreational spaces and connect with the neighbouring communities," he says.

"Erosion control, tree removal and plant management or restorative planting is something both councils can work on together to improve outcomes for our community."

This work also contributes to improved water quality, as less sediment makes its way into waterways, often why rivers and streams can look muddy or brown in colour.