Townley, L.R., and Barr, A.D. (1994), Flow characteristics of the Enterprise Lake system, Stage I, Prepared for Energy Resources of Australia Ltd, CSIRO Minesite Rehabilitation Research Program, 27pp., October. [Confidential]

In order to flood the Enterprise Pit, a diversion spillway has been constructed in Tom's Dam to divert flow from Pine Creek into the Enterprise Pit. An earlier study by Townley and Barr [1993] analysed the coupled behaviour of Tom's Dam and the Enterprise Pit, assuming that outflow would be allowed from the Pit to Copperfield Creek, This study provides (i) a hydraulic analysis of the diversion spillway and diversion channel, allowing flows in both directions, and (ii) estimates of the behaviour of Tom's Dam and the Enterprise Pit, assuming that flow must return to Tom's Dam and Pine Creek as return flow in the diversion channel.

The hydraulic analysis shows that discharge over the spillway depends on the levels in both Tom's Dam and the Enterprise Pit. Backwater effects in the diversion channel are sufficient to reduce the discharge over the diversion spillway towards the Pit, and resistance also affects the return flow.

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 even without outflow to Copperfield Creek, there will be return flow to Pine Creek, capable of flushing the Enterprise Pit. If no structural modifications are made to Tom's Dam, the residence time in the Pit is predicted to be 47.4 years, more than 10 times larger than with outflow to Copperfield Creek. The residence time can be reduced to less than 20 years by cutting a "notch" in the crest of Tom's Dam. Every combination of length and crest elevation of such a notch result in a different residence time. The residence time can also be reduced to about 18 years by reopening two 0.5m diameter culverts through the wall of Tom's Dam.

All results are based on observed data during the wettest 14 years on record, so residence times will be even longer in drier periods.