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More than 15 fallen trees and log jams have been cleared from Matakana and Glen Eden Rivers, all thanks to tenacious volunteers, community spirit and Auckland Council support.
The weather-related issue was discovered in 2020 when locals living near the banks of the rivers told Friends of Awa Matakanakana (FOAM) about the growing number of dams being formed by trees and debris washing downstream.
FOAM’s vice-chair Martin Evans says there are numerous problems and risks associated with leaving log jams in rivers.
“One main issue is log jams cause the rivers to move sideways and erode the riverbanks which, in turn, releases a lot of sediment into the river,” Evans says.
“Some large sections of the river were lost and washed downstream, eventually reaching Sandspit and the sea.
“Log jams can continue to grow and become large debris dams. One was more than eight metres high and about 20 metres wide – catching more slash, trees and debris, which blocked the water flow, causing localised flooding.”
Evans says there is also a risk the dams will burst when they can no longer hold the water that builds up, and the dam becomes unstable.
“Then you get a tsunami-type wave of water and logs that crash through anything downstream, causing significant damage.”
He says the log jams in the Matakana and Glen Eden Rivers continued to build up due to ongoing high rainfall and runoff events that FOAM measures and monitors.
About every two to four years, the Matakana River flow reaches about 40 cubic metres per second (cumecs), which causes the river to overflow.
High flows are not unusual, and several storms have recorded more than three times this figure. In 1938, a flood was reported to be over 600mm above the Leigh Road/Matakana Valley Road roundabout.
One cubic metre of water equals one thousand litres and weighs one tonne so there are a lot of tonnes coming down the river, at speed.
“The Auckland Anniversary rain and other storms made the land unstable; we all saw the slips, and trees that couldn’t hold on in the subsequent Cyclone Gabrielle winds. Many trees and branches ended up in the river flood plain.
“Log jams have happened before and were cleared by locals, and we need to be aware we regularly get much larger storm events than those last year.”
Residents and FOAM volunteers advocated for the removal of the log jams and when Restore Rodney East chair Tim Armitage met council’s Storm Recovery Liaison (North), he connected the team with FOAM.
Members of the community and FOAM liaised with council, which allocated budget to help clear out the catchment.
Log jam locations were identified, contractors booked and when fine weather arrived, the job was finished last month.
Many private landowners supported the project by allowing access through their land.
“The difficulty was heavy machinery was needed in some places and FOAM is grateful to landowners who accepted a bit of mess to make the rivers less vulnerable to future flooding and erosion,” Evans says.
Cleared logs were left on private land, out of the flood plains, and some people are making the most of the resource that was hauled out of the river – some of it tōtara. Without community support, the work would not have happened.
Council’s project manager Mark Thomson said it was an extremely challenging job.
“But the stress of all the operational difficulties were overcome by the warmth, knowledge, generosity and hospitality of the local community,” he added.
FOAM is keen to work with partners on developing a strategy for dealing with ongoing log debris in the flood plains, consequential log jams and erosion reduction to safely manage the river catchments.
In the meantime, FOAM is investigating increased riparian planting for damaged riverbanks and hopes people are enjoying being able to paddle up the awa again.
