Townley, L.R., and Barr, A.D. (1993), Coupled water balance modelling of Tom's Dam and the Enterprise Pit, Confidential report to Energy Resources of Australia Ltd, CSIRO Minesite Rehabilitation Research Program, 23pp., September 1993. [Confidential]

In order to flood the Enterprise Pit, it has been proposed that a diversion spillway be constructed adjacent to an existing weir on Pine Creek known as Tom's Dam. This study provides (i) a hydraulic analysis of the diversion spillway and diversion channel, and (ii) estimates of the behaviour of Tom' s Dam and the Enterprise Pit both during and after filling of the Pit.

The hydraulic analysis shows that discharge over the spillway depends on both the level in Tom's Dam and the level in the Enterprise Pit. Even during filling of the Pit, when water levels are much lower than the end of the diversion channel, backwater effects in the diversion channel are sufficient to reduce the discharge over the diversion spillway.

The results of the hydraulic analysis have been used in coupled water balance calculations for Tom's Dam and the Enterprise Pit. The analysis uses streamflow data for 15-minute intervals, which is necessary for a small storage, such as Tom's Dam, with rapidly varying flows. Inflows to Tom's Dam have been approximated by streamflow measurements from a catchment on Copperfield Creek during the period 18 September 1972 to 15 January 1987. Composite daily rainfall records for the township of Pine Creek have been used to estimate rainfall on the surface of the Pit, but evaporation has been approximated by monthly averages.

The coupled analysis shows that if the Pit had started to fill in September 1972, it would have taken about 2 years to fill the Pit. If inflows to Tom's Dam had been 25% lower, it would have taken 3 years. Once full, water levels in the Pit will fluctuate between RL 1197 (or slightly less) and RL 1198.95. The percentage diversion with the proposed diversion spillway is predicted to be greater than 98%. The existing Tom's Dam will rarely be overtopped, whereas the Pit will be overflowing towards a tributary of Copperfield Creek for most of the year in very wet years and for at least a few months in dry years. The outflow from the Pit is sufficient to replace the volume stored in the Pit in somewhere between 4 and 5.5 years.

The proposed diversion structure will meet the objectives of filling the Pit quickly and allowing sufficient throughflow to maintain water quality in the Pit.

A revised Version 1.2 of this report was released in August 1994, and is available for download.