Lake Awoonga's saddle dam eroded but still intact
THREE weeks ago, few people had heard of Lake Awoonga's saddle dam.
That began to change on the Australia Day weekend.
If it had rained hard for another day or two, Saddle Dam 3 would have become very famous, very quickly.
Saddle Dam 3 performed as it was designed to during the floods two weeks ago. It suffered a large amount of erosion, but remained intact.
Had the pressure behind the dam become more extreme, the saddle dam was designed to collapse entirely allowing a significant, rapid flow of water from the lake and lessening pressure on the main Awoonga Dam.
If that had happened, the Boyne River, already at its highest ever level, would have flooded even higher.
The Gladstone Area Water Board invited The Observer to examine the saddle dam on Thursday and explained its design.
To the untrained eye, it looks like a natural embankment with a basic road over the top.
However, as water board chief executive Jim Grayson and dam engineer Terry Ward explained, the saddle dam is one of the most carefully planned parts of the lake's design.
As the water rose on Australia Day weekend, surging 8.3m over the main dam's spillway and flooding the Boyne River, Saddle Dam 3 had just 400mm of water rushing over it.
That was enough to cause massive erosion on the opposite side of the saddle dam, as per the design.
Had the intensity of the rushing water increased, or continued longer, the saddle dam was designed to collapse entirely and allow water to flow out rapidly at a depth of 3-5m across a 250m stretch.
That flow would have followed an overflow path before joining the Boyne River a few kilometres downstream of Awoonga's main spillway.
"While there was some erosion to Saddle Dam 3, ultimately it was not sufficient to cut through the embankment," a water board spokesman said.
"Had the erosion continued, it would have resulted in rapid increases in localised flooding in Benaraby and Pikes Creek and South Trees Areas."
Saddle Dam 3
- Saddle Dam 3 is about 2km west of the main Awoonga Dam.
- Its purpose is to erode entirely during an extreme flood, releasing water and lessening pressure on the main Awoonga Dam.