1.3 Regulated river
The volume recognised in the water accounting statement (16,385 ML) represents the volume of water held in the regulated lower Ord River at 30 June 2012.
The regulated river is defined as the lower Ord River, which extends 76 km from Lake Kununurra downstream to the upper tidal limit. Water in the regulated river is recognised as a water asset when the electrical conductivity is less than 1,500 µS/cm. Water in the regulated river above this level is not fit for purpose and as such does not provide a future benefit to the Ord region.
A series of approximately 40 cross sections detailing channel geometry of the lower Ord River was surveyed between 1997 and 2002. The survey information was used to create a hydraulic model of the Ord River between Lake Kununurra and the limit of tidal incursion, 76 km below the Kununurra Diversion Dam (Braimbridge & Malseed, 2007).
The river storage volume is calculated using the model based on a flow rate of 0.074 ML/s as measured at the Tarrara Bar (Station 809339) at the start and end of the 2011–12 year.
Assumptions, limitations, caveats and approximations
The cross sections along the lower Ord River were surveyed for the purposes of making environmental assessments of various parts of the river downstream of Lake Kununurra, not for calculating an accurate volume (e.g. the depths of the river pools were not surveyed).
The cross-sectional profile of the river varies substantially downstream of Lake Kununurra. Approximately 40 cross section profiles were surveyed and the calculated hydraulic model volume is considered only to provide an order of magnitude estimate of the true river volume. It is expected that since the cross sections were surveyed between 1992 and 2002, there will have been changes within the channel. The extent and rate of these changes is unknown.
The method of estimation also assumes that a constant flow rate is occurring along the entire (76 km) length of the lower Ord River for which the calculation was made.
Derived from modelled data. The uncertainty estimate was not quantified.