History Of Flooding

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History Of Flooding

The Gwydir Valley has a history of flooding that brings both devastation and life to the region.  While every flood is different in magnitude, duration and size they can be compared to help be prepared and predict possible behaviours.

Below is a key comparison of the big floods and these are generally ones that are regarded as 1:100 year flood events, our key events.

When a flood warning is issued, we often compare preficted heights to these key events which are largely reported by the Bureau of Meteorology in meters.

River Height in meters

Gauging Location 2016 2012 2011 1976 1955
Gwydir at Gravesend 8.739 15.668 13.266 16.01 17.34
Gwydir at Pally 7.668 10.518 10.105 9.237 N/A
Gwydir at Yarraman 6.95 7.512 7.326 7.5 N/A
Mehi at Moree 4.172 10.68 10.206 10.59

River peak flow rate in megaltires per day

Gauging Location 2016 2012 2011 1976
Gwydir at Gravesend 58,058 341,849 181,547 264,662
Gwydir at Pally 56,006 169,186 141,701 145,838
Gwydir at Yarraman 47,100 141,136 78,017 N/A
Mehi at Moree 5,769 55,778 58,710 N/A

To understand the magnitude of these key event, the 2011 and 2012 floods were part of wide-spread significant and extended flooding in the north west.  The first in November/December 2011 and then again later in January/February 2012.  During this time 3,450 GL of total system flows distributed across the valley and floodplains.  The Gwydir event was not isolated and 8,600GL was gauged at Bourke as many valley flooded simultaneously and Menindee Lakes filled and were surged as they were over capacity.  

We include 2016 in these records, as this event is a moderate flood level that created substantial flooding in the watercourse, along the lower Gwydir and Gingham area due to operational constraints within the river.  The volume of this flood was a third of that in 2011 and 2012 with the majority flowing towards the watercourse and wetlands.  Rivers and streams in surrounding valleys also flowed but flooding was limited in those areas.  River flows provided for water along the Barwon Darling delivered via water sharing plan rules and Menindee Lakes almost filled at 91% capacity.

The 2016 flood is important to understand the limitations in managing flows in the Gwydir Valley when the flow rate is higher than 20,000ML/day upstream of the Tarelaroi Regulator.  For safe operation of the river infrastructure, WaterNSW are required to remove of regulating infrastructure and allow flows to naturally pass.  This often results in a lowering of hydraulic pressure at the regulator and limits the management of water by the river operator until flow rates decline.  The result, there is limited ability for WaterNSW to direct this water down the Mehi River and flows remain within the Gwydir River system and naturally flows towards the watercourse area west of Moree. 

The current flood warning in the Gwydir was issued via the Bureau of Meterology on 17 December 2020, is similar to the 2016 flood, although a shorter, sharper event.  A comparison of the predication and actual is presented below.

River prediction and actual December 2020

Gauging Location Height prediction Actual peak height Actual peak flow rate
Gwydir at Gravesend NA 7.2 40,000
Gwydir at Pally 7 5.7 33,159
Gwydir at Yarraman 6.5 5.4 21,432
Mehi at Moree 5.5 3.1 3,244

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