Sediment in the Taylor River has risen an average of 1m over the past 10 years. Photo: Supplied / Stuff
A metre's worth of sediment on average has built up in Blenheim's Taylor and Ōpaoa rivers over the past 10 years, prompting a plan to dredge them.
The sediment build-up is a "perfectly natural thing", according to the council's rivers and drainage engineering manager Andy White, but he suggested now was the time to intervene.
White said it was too difficult to determine specifically what had been the cause of the "aggradation" or increase of sediment in the Taylor and Ōpaoa rivers.
"Although it's quite likely to be a mixture of seismic activity, high sediment coming from the upper catchments, and some of the bed roughness that we are now seeing as we restrict our weed cutting," he told the council at a long-term budget meeting on 26 February.
The council had cut back on its weed cutting in recent years due to the whitebait spawning season, following rules in the Proposed Marlborough Environment Plan. A broken weed cutter had also impacted when weeds had been cut, which caused the Taylor River to flood even when there had been no rain.
"It's quite a public issue this one, and drawn quite a lot of conversation from the public," White said.
"It's just a case of do we let it aggrade to a point that we can't deal with it any longer, or do we try to intervene? And that's what we're requesting to do, to try and cover our bases."
In order to address the build-up, the council planned to dredge, or remove, sediment at the Taylor and Ōpaoa rivers, between the Alfred St bridge and Marshall Place over the 2024-25 and 2025-26 financial years.
The budget for this was $150,000 for the 2024-25 year, and $650,000 for the year after, totalling $800,000. This funding was subject to consultation through the council's 2024-2034 long-term plan (LTP).
Operating costs for the two rivers were usually $66,000 for the Taylor River (below Burleigh) and $117,000 for the lower Ōpaoa each financial year.
Weed cutting in Taylor River in May 2023. Photo: Supplied / Stuff
White also recommended that efforts be made over the LTP period to develop a "comprehensive" Taylor and Ōpaoa rivers management plan with council's environmental science and parks and open spaces teams.
He said there had already been some "excellent work" by the council's environmental science team on upper catchment management techniques to reduce the sediment load.
"It looks like there has been some improvements there, for sure, and we suggest that those continue," he said.
"However, the discharge is the main thing, and so it's difficult for us to stop the river from aggrading."
The Taylor River runs through the centre of Blenheim and is popular with walkers and cyclists. Photo: Supplied / Stuff
Meanwhile, a report prepared by White for the meeting said the build-up would begin to affect the council's ability to provide channel capacity for a one-in-100-year flood event if they allowed it to continue beyond the next LTP period.
Wairau-Awatere ward councillor Scott Adams said while the work meant spending money, it was a "key priority" for flood control protection measures for Blenheim.
It also tied in with the Taylor River boardwalk at Riverside Park, which the council was looking to replace with synthetic boards because its wooden boards were rotting, and often underwater.
Marlborough mayor Nadine Taylor agreed and said "the time is now" to address the risk.
The Long Term Plan 2024-2034 would go out for public consultation from 4 April to 6 May and include public meetings and hearings, with dates and locations to be confirmed.
LDR is local body journalism co-funded by RNZ and NZ On Air.
