The Wyangala Counterpoint Group says the proposed larger Wyangala Dam would have filled and spilled in early October, 2021.
The group says the wet weather event in September generated a very large volume of rainfall runoff in the Upper Lachlan catchment that the proposed larger dam would not hold back.
"The volume of inflows to Wyangala Dam would have filled the proposed additional 650 GL by early October," the group said.
"Dam releases would still have occurred at the same time the increased tributary inflows from the Belubula River, Boorowa River and other swollen water sources below the dam entered the Lachlan River upstream of Forbes.
"Up to 50% of the river flows causing flooding at Forbes have come from tributaries downstream of the dam," the group said.
Wyangala Counterpoint Group has analysed inflows to Wyangala Dam since it first filled in August (last year) and inflows to the Lachlan River from tributaries below the dam.
They say this analysis is based on real time inflow reporting and river gauge readings available on the WaterNSW website.
"An additional problem within the Lachlan River is the 150 km long sediment slug that has raised the riverbed causing it to break banks at much lower flow levels," the group said.
The low-level bridge at Cowra is now flooded by a flow of 4,700 ML when 20 years ago it took twice as much water (a flow of 9,600 ML) to cut access.
Wyangala Counterpoint Group is encouraging thorough examination of the mixed messages and competing reasons for raising the dam wall to ensure an informed debate.
"The proposed raising of the dam wall would permanently flood a further 10 m height of prime agricultural land and critically endangered woodlands and over 80 roads in the upper catchment in addition to capturing important flows required to protect the nationally listed wetlands in the mid and lower Lachlan," the group said.
"Importantly, the dam will not be able to perform the competing purpose of flood mitigation while also guaranteeing increased water security for downstream water users. Increased water security requires a full dam while increased flood mitigation requires low levels in the dam.
"Other solutions are needed to solve problems on the downstream floodplain. A larger dam is not the solution, especially during this flood event," they said.
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