High and rising
Monday, 27th February, 2012
As the water rises at Menindee, authorities are preparing to deal with what they say could be the biggest floods in 36 years.
About 31,000 megalitres of water are flowing into the Menindee Lakes every day from the floods in the Gwydir and Border rivers valleys in November.
But more floods to the north in January have created a second flood peak, now approaching Bourke.
Once the peak begins to arrive at the lakes in early April, more water might have to be released from the lakes.
The total releases could be the largest since the record Barwon-Darling floods of 1976, according to the NSW Office of Water and the State Water Corporation .
At this stage, they say, it was estimated that the flooding was likely to reach “at least the levels recorded in the 1998 floods” when releases from Menindee Lakes to the Lower Darling reached 46,500 ML per day through Weir 32 (at 7.45m gauge height).
This is about 10 metres at the Menindee town gauge and this level can be expected by around early to mid April, the authorities said.
There will be big flows along the Darling River and through the Talyawalka Creek system, replenishing the many floodplains and lakes, particularly in the Teryawynia area. They were last inundated just 12 months ago.
So far, with the level in Menindee Lakes storage falling from 95 per cent to 87 per cent, about 500 gigalitres of space has been created.
As the river rises downstream of the main weir at Menindee and river levels approach those of the lakes, it becomes more difficult to release water from the lakes and create airspace, said State Water.
In fact, lake outlets have to be closed to prevent ingress to the lakes from the high river.
Water levels in Lake Wetherell were continuing to fall slowly and it was expected that the gates on the main weir may soon be fully removed from the water. This will allow free passage of flood flow and fish passage along the Darling River channel. The gates will then be returned to refill the lakes system to capacity and reduce the size and impact of the flood peak.
Until the flood passes Bourke there remains some uncertainty about what volumes will pass Wilcannia or into the Talyawalka Creek system, as well as in the Lower Darling and the Great Anabranch.
The volume of flow expected to pass into the Great Darling Anabranch from the Lower Darling River will also be large.
State Water said residents along the Darling River, from Bourke to Burtundy, should make preparations for an extended period of flooding, similar to the floods of 1971, 1990 and 1998.
The Darling on Friday was 9.49 metres and steady at Menindee, while in Wilcannia it was 9.64m and falling slowly. At Pooncarie the river was 6.67m and rising.