With the Eppalock reservoir reaching almost 99 per cent of capacity, Goulburn-Murray Water has sought to reassure Rochester residents that a spill from the storage does not necessarily mean a flood threat for the town.
The town, downstream of the storage on the Campaspe River, is on edge following last October’s floods which devastated the town.
G-MW water storage services general manager Martina Cusack said while Lake Eppalock is not currently spilling, when it does spill, this does not mean a flood in the Campaspe River.
“Those residents who have lived in the region for some time will have seen Lake Eppalock spillway activating at least 29 times since it was constructed in 1964. Of these, only six were above the flow at the minor flood level of 21,200 Ml per day,” she said.
The Lake Eppalock embankment walls are nearly 5.2 metres higher than that of the spillway.
“This provides some flood mitigation by temporarily holding water back,” Ms Cusack said.
“What this means in a major event like we had in 2022, is Eppalock mitigates the water levels by ensuring the flows out of the lake are less than the flows in.”
In an online webinar last week, G-MW pointed out that Eppalock reservoir received more inflows in one month of rainfall last year than its entire capacity.
The webinar was organised by G-MW following concerns from people worried that heavy winter rains could result in a repeat of last October’s Rochester flood.
The webinar heard that legislation covering the management of Eppalock required it to be managed for water harvesting, not flood mitigation.
G-MW river operations manager Andrew Shields told the webinar (watched by more than 200 people) that last October the Eppalock inflows totalled 340 Gl, when the capacity of Lake Eppalock was 304 Gl.
“That would have required us to have the storage empty, to accommodate that,” Mr Shields said.
In the early part of October between 100 and 150mm of rain fell in the Eppalock catchment which covers 2124 square kilometres.
The Rochester township was flooded in the ensuing stream flow of the Campaspe River, inundating most houses.
The Department of Energy, the Environment and Climate Action is conducting a technical assessment into how the storage is managed, with a report expected by the end of this year.
Responding to questions about why Eppalock was being held at 98 per cent at the moment, Ms Cusack said the management policies required them to keep a full storage for the bulk entitlement holders, which included Coliban Water, Central HIghlands Water and the environmental water holders.
Some releases could be made through the outlet valve when a rain event is forecast.
The webinar heard that Eppalock, completed in 1964, had three spillways, an outlet valve and no flood gates.
Eppalock is holding 98 per cent of capacity and releasing about 160 Ml/day.
Mr Shields said the Bureau of Meteorology outlook for spring was for lower than average rainfall.
“In July to September the chances of exceeding the median rainfall were on the lower side, but noting in recent times it has been on the wetter side,” he said.
“For August to October the chances of exceeding the median rainfall are diminishing compared to the earlier period.“
Rochester residents have argued that Eppalock should be managed with flood mitigation in mind, by deliberately creating air space in the storage.
“Air space gives storages such as Eppalock a critical flexibility they can never have under current operating systems,” Rochester flood mitigation committee member Tracie Kyne believes.
“Instead of running close to capacity, if it had 10 per cent as a buffer — and if we all knew water was coming down through the catchment — we could keep letting more out and still be able to absorb a significant inflow, instead of seeing it smash through our town again,” she said.
A video of the webinar can be accessed on the G-MW website on the ‘Managing water levels at Lake Eppalock’ page.