WaterNSW will meet with Moree local government representatives at the end of the month to discuss operations on the lower Gwydir River, in light of forecasts suggesting a wetter than average summer.
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Weir structures operated by WaterNSW on the Lower Gwydir River are lifted in times of flows in excess of 30 gigalitres per day (GL/day) to avoid damage. The Bureau of Meteorology (BoM) is forecasting an active QLD cyclone season through summer, a scenario that may impact local rivers.
In relation to the operation of WaterNSW weir gates and gates on other river structures in the Gwydir Valley the following principles are observed in times of high, unregulated flows:
Tareelaroi weir: If river flows are forecast to be above 30GL for greater than 24 hours, Tareelaroi and the Mehi Regulator are used to replicate as closely as possible, the distribution of flood flows that occurred before these structures were constructed. Under these high flow conditions the Tareelaroi weir will be lifted clear of the water and the Mehi regulator is fully open. The Tareelaroi weir is lowered only when flows recede to approximately 20 to 25GL. These are manual operations undertaken by site staff so exact timing may vary slightly from each event.
At these higher flow rates the distribution of flows between the Gwydir and Mehi will return to the natural proportions, with the bulk of water in the main Gwydir river channel, to terminate at wetland areas in the Lower Gwydir and Gingham watercourse.
Under routine operating conditions and with Tareelaroi weir at 100 per cent capacity, flows diverted to the Mehi can be as high as approximately 3.6GL/day while WaterNSW work approval limits regulated discharge to 5.8GL/day. However inflows to the Mehi are highly dependent on the river level or weir pool volume and as such flow rates may drop below the specified flow rates when the weir gates are removed from service, and will increase with higher unregulated Gwydir River flows.
Mehi regulator: If river flows are forecast to be above 15GL for greater than 24 hours, Mehi regulator will be opened fully. It may be also opened fully below these flows rates depending on demand and distribution requirements.
Boolooroo weir: The gates are generally lifted clear of the water at flow rates of above 10 to 12GL/day. At these rates flows down the Carole Creek are not influenced by the weir and any flows are naturally occurring. Under normal regulated conditions WaterNSW work approval limits discharge into the Carole of only 2.2GL/day. Tyrel regulator: The gates are generally opened to two metres when flow rates are forecast to be above 2GL/day and lifted to the upper limit when flow forecast are above 5GL/day.
Mallowa Creek regulator: It opened whenever there are flows forecast that exceed the Mehi River channel capacity. These breakouts occur when river flows reach between 1.5-2GL/day, depending on location.
