Arawhata River sediment trap fully functioning

Sediment trap on the Arawhata River in Levin on Hokio Beach Rd.

Questions about the effectiveness of new sediment traps designed to protect Lake Horowhenua have surfaced online, however the regional council says they are working as they are supposed to.

The traps have been constructed at the Arawhata Stream, and are designed to stop sediment washing into the badly polluted and controversial lake.

A drone video circulating on social media shows large amounts of brown-coloured water washing straight through the traps and into the lake after a heavy rainfall event, but Horizons Regional Council freshwater and partnerships manager Logan Brown said the trap was operating as it was designed.

He said he had seen the footage.

"The fact that the image starts with muddy water being in the trap shows that the flood event was large enough to start spilling some of the water into the trap but the flood wasn't large enough to divert water around the entire trap," he said.

He said the sediment trap was designed to operate in large flood events.

"[It] operates when water levels in the Arawhata Stream get high enough that the culvert cannot take all of the flow, and this excess flow is diverted into the sediment trap," he said.

"It will not and wasn't designed to divert all of the flows around the sediment trap."

Brown said the trap was part of a wider programme to reduce sediment and its design focused on coarse sediment during large storm events.

This meant it does not always operate during rainfall events and may not change the colour of the water, which was caused by fine sediment.

"The measure of success over time will be the amount of sediment accumulated in the trap, and subsequently removed from[it], rather than deposited into the lake," he said.
He said that Horizons was currently investigating if modifications could be made to the trap to make it operate more frequently.

"Work also continues with the horticulture sector in reducing the sediment lost from land in the first place and for the sediment trap to be the final polishing to the water as it comes down the Arawhata Stream during high flows."

Modelling carried out by NIWA during early stages of the project estimated that the trap would reduce the annual load of sediment entering Lake Horowhenua from the Arawhata Stream by more than 50 per cent, equivalent to reducing the yearly sediment inputs from all of the inflowing streams by approximately 25 per cent and the annual load of phosphorus from these by 30 per cent.

Horizons communication advisor Cara Hesselin said a paper was being presented to the regional council next week, which would include more scientific data about the behaviour of sediment in the area, especially in terms of highlighting how the trap has the biggest impact during large flood events.