Ord
1.3 Regulated river

Supporting Information

The volume recognised in the water accounting statement (70,444 ML) represents the volume of water held in the regulated lower Ord River at 30 June 2011.

The volume at 30 June 2011 was approximately four times greater than that observed on 30 June 2010 (15,555 ML). The greater volume can be attributed to wet conditions in the Ord region during the 2010–11 year.

The regulated river is defined as the lower Ord River, downstream of the Kununurra Diversion Dam to the upper tidal limit, 76 km downstream. 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.

Quantification Approach

Data Source

Western Australian Department of Water, Water Resource Assessment unit.

Provided by

Western Australian Department of Water.

Method

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 the Kununurra Diversion Dam 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 614 m3/s as measured by the gauging station at Tarrara Bar (No. 809339) at the start and end of the 2010–11 year.

Assumptions, Limitations, Caveats and Approximations

Storage–volume curves represent specifically surveyed parts of the water storage, and may not reflect the storage–volume relationship across the entire storage.

Water storages are subject to sedimentation and other physical changes over time, which in turn affects the accuracy of the storage–volume curves.

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 the Kununurra Diversion Dam, not for calculating an accurate volume. For example, the depths of the river pools were not surveyed.

The cross-sectional profile of the river varies substantially downstream of the Kununurra Diversion Dam. 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, 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.

Uncertainty Information

Derived from modelled data. The uncertainty estimate was not quantified.

Comparative year

Comparative information

The following table compares the value for the 2009–10 year reported in this account with the corresponding value reported in the 2010 Account.

Comparative information from the 2010 Account for line item 1.3

2011 Account line item

2010 Account line item

Value reported in the 2010 Account (ML)

Value reported in the 2011 Account (ML)

Additional information

1.3 Regulated river

1.3 Regulated channel

15,555

15,555

Change in line item name