Imagine an Olympic swimming pool filling up in 40 seconds. That’s how fast the water pulsed through Mulloon Creek – twice during one week in February 2020. After the driest nine months on record and after bushfires within the upper half of the catchment, finally the heavens opened on the 9th February dumping possibly 200mm at the top end of the catchment.
In ‘normal’ times, the flow from such an event would have taken at least two days to make its way down to the lower end of the Mulloon catchment. This time however, the flow came down in a turbulent torrent of ash, mud and debris soup.
After such a long period of drought, the creek bed was all but dry. On the up side – perfect conditions for building leaky weirs into the creek, of which we had recently completed 18. On the down side – there was no established vegetation to armour the structures and in any way dissipate a flow the likes of which Mulloon catchment had rarely seen before.
Structure PM14 during construction at the Palerang property
We held our breath as a peak flow of 23 cubic metres per second surged through the areas in which we had been working as recently as two days before. Within 2 hours, Mulloon Creek went from bone dry to fully charged.
Pulse of water flowing over structure PM14
Peak flow over same structure
How did everything hold up? As it turned out – very well! Designed to work as an interconnected system, once full the cushioning effect of the ponds brilliantly protected the structures above them.
This was the WOW factor in action (Water On Water). There was some minor scouring due to the sheer force of the water against dry and still barren ground but in all a great result in the face of yet another extreme event.
If that wasn’t enough for the time being, another test to the system came only five days later when an even higher peak flow came down as a result of an intense storm over the fire ground at the top of the catchment. This time it was a genuine chocolate milkshake – a tragic sight really. Though impossible for the new system to ameliorate such a turbid flow, still many tonnes of silt was trapped within the ponds – which would otherwise have ended up in Sydney’s already contaminated water supply.
Silt build up on a MS3(2) structure on Mulloon Farm South
After these two events, we can feel confident now that further pulses will be well dissipated by the renewed chain of ponds system. From here on in the resilience of the system will rebuild at an accelerating rate as the vegetation will now begin to establish on the freshly hydrated and soiled banks.