River in the sky, flooding on the ground
Yesterday was World Water Day, which, according to the website, is a day to celebrate water and reflect on the value of water in our lives.
It's safe to say that very few people on the NSW Mid North Coast or the Hawkesbury-Nepean Valley were reflecting on the value of water yesterday. With every waterway on the Mid North Coast under flood and river levels in Western Sydney rising up to fourteen metres, residents had more pressing matters to attend to.
As the rain has fallen across the NSW coast, so to have the weather records, with a number of weather stations recording figures not seen since records began in 1889.
What's driving all this rain is a bog standard trough, the kind we'd see a few times each year. However, in this instance many factors have combined to make an ordinary event something else entirely.
For the last few months there's been a deficit of atmospheric moisture in the Central Pacific with high pressure systems driving it towards the Coral Sea. There it has gathered, contributing to above average tropical disturbances over the summer.
During the middle of last week a high pressure system developed in the Tasman Sea driving easterly tradewinds from the Coral Sea towards the East Coast. This isn't unsual, it's a classic late summer/early autumn pattern, and it would often bring steady rainfall, however as the atmosphere in the Coral Sea has above average moisture, so to do the winds blowing from there.
The result was an 'atmospheric river': a cloud band that transports moisture from one area to another. In this instance, from north of New Zealand to Australia's East Coast where it fell like a waterfall to Earth.
Significant rain began falling along the northern NSW coast last Thursday, and by Friday some rivers - real rivers - began to breech their banks.
Veteran shaper Mitchell Rae, who works out of a factory near Urunga on the Mid North Coast, went down to Macksville on Friday. "It wasn't looking good," said Rae. "The river [Nambucca River] had broken its banks and was flooding houses, there was debris floating down the river, and it was still rising. Even then it felt like was going to get worse."
Rae was right, the 'ordinary' coastal trough was extraordinarily slow moving. It sat in a quasi-stationary position from Friday to Monday, all the while picking up moisture from the humid Coral Sea atmosphere and aiming it like a firehose at the East Coast.
Though the figures haven't come in yet, it's likely that some parts of the Mid North Coast received greater daily rainfall figures last February. However, that was just two or three days of rain following twelve months of drought with the soil dry down to the substrate. This time around, we've had above-average rainfall this summer and the soil is already soaked. "The water has got nowhere to go," says Rae, "so the rivers are just going up and up."
"I'm counting my blessings," adds Rae. "I lost power for a while, but it's come back on, and the new highway, which is raised, means I didn't get cut off. I'm lucky, some people around here are doing it very tough indeed."
"I'd say...up here at least, it's as bad as the 1974 storm when we had to get rescued in a boat," says Rae referring to the infamous May '74 storm that also created huge damaging waves. The difference is the current system isn't an East Coast Low, arguably it isn't even a storm, in its first phase it was simply a very slow-moving coastal trough.
As of today, the lower atmosphere is saturated right up to the 500hPa level, some 5km above us, and we're now seeing a secondary infeed of moisture from the north-west that will further prolong the rain event. Satellite images (see below) show a distinct cloud band stretching down from the north-west - another atmospheric river.
The two 'rivers' are combining to bring further rainfall to the coastal margins, and also to inland New South Wales. Yesterday, areas of central Australia received flooding falls.
Today, a low pressure system is forming at the junction of the cloud bands, which will soon move offshore and further south. When that happens, the clouds will dissipate, the atmospheric rivers will stop flowing, and the weather event will ostensibly be over.
Of course, it will take up to a week for all that water to move down through the river systems and out to sea, and that brings us to the next issue: how the coast will be affected.
Helicopter footage of the flooding shows an untold volume of sediment-laden water heading for the coast. Whether it be rain scouring away topsoil or a swollen river doing the same thing on a grander scale, the result is the same: fluvial sand heading for the rivermouths and out to sea.
Thinking that perhaps the banks and bars may get a welcome top up, we called Associate Professor Ian Goodwin who's spent most of his adult life surfing and studying the NSW East Coast, and now works for Climalab, a a climate science brokerage.
"It doesn't work that way," said Goodwin, quickly hosing down our hopes. "Right now, the hydraulic energy [flowing through the rivermouths] is so strong that the sediment load is being jetted many kilometres out to sea."
"We did a study a few years ago," continues Goodwin, "and we found that fluvial mud - that fine river sand - is transported out to around the inner shelf, or the middle shelf, and is dropped in about 40m of water. That's its fate."
Once there, the fluvial sand is largely beyond the reach of longshore drift or wave action, though it can, according to Goodwin, "very slowly work it's way towards shore, however we're talking over a timescale of decades."
As for the banks and bars, well, there's good news and bad.
The high volume of water currently flowing out the East Coast rivermouths will "blast" the bars aside. Between wave action and tidal flow, each rivermouth finds its own equilibrium. That is, a water depth that balances the various forces. However, events like this cause an extended surging outflow and the result is a deeper bar.
The good news for surfers, however, is that unlike the fluvial sand flowing out the rivers and out to sea, the marine sand is usually just pushed to the margins.
"That sand will be sitting at the edge of the ebb tide deltas," says Goodwin referring to offshore bars. "Then, once we get small, constructive swell again, the sand will either migrate onto adjacent beaches or into the rivermouths forming swash bars." Swash bars being the scientific name for triangle-shaped banks offshore from a waterway.
"And that," according to Goodwin, "can happen surprisingly fast. Sometimes the sand will move into place in just a few weeks."
Being a surfer himself, Goodwin appreciates the process. "Those swash bars, they're the classic wave set ups."
Heading into autumn, the East Coast - at least anywhere near a significant estuary - is being primed for swell delivery. Perhaps there'll be a way to celebrate all this water after all.
// CRAIG BROKENSHA and STU NETTLE