In 1969, I saw firsthand how easily a flash flood could change lives, and livelihoods.

Ournie, on the Murray River side of Tumbarumba, was my home for four years in the late sixties.

The steep hillsides in the Ournie Valley had streams of running water from springs even before the storm.

The soil was saturated - it had been a wet year.

But on that November evening, six-and-a-half inches (about 170mm) of rain fell in six hours.

On that terrible night, I had strolled onto the verandah at about 11pm.

I thought that the rain was intense, but I was studying for my upcoming university geography exams.

To me, inside the house, it was just heavy rain.

Next morning cockies, kookaburras, magpies, and lambs were silent - only the hushed sound of running water.

Willows were gone. So were the sheep.

The desolation I witnessed that morning was to teach me a geography lesson no textbook could match.

No human life was lost, but some locals returning from Tumbarumba after a meeting had a lucky escape.

When the car turned onto the unmade road leading to the corner of the valley nicknamed Clarkesville, the wooden bridge crossing the Ournie Creek looked to be underwater.

In the pitch black and pouring rain, common sense prevailed and the Clarkesville passenger bedded down at his mate's house. He tried to ring home, but the phone line was dead.

Dawn revealed unbelievable destruction. The Clarkesville bridge was gone, washed into the Murray with thousands of sheep and cattle, trees, and kilometres of fencing.

The concrete bridge on the main River Road was still standing, but the water had carried the concrete flags from each end of the bridge about two kilometres down the river!

This episode taught me that floodwater can do immense damage. When I was looking to buy in Wagga, I checked the 100-year flood map and general overland drainage in the area. It's not always a river rising that floods your house.

The people who bought homes in Sydney suburbs below the Warragamba Dam probably wished that they had checked historic flood maps, too. The problem is that we have been indoctrinated with this idea that we are living in an era of reduced rainfall. The recent floods along the East Coast prove that this is not necessarily so.

The government authority looking at raising Warragamba Dam (or not!) has studied flood patterns from earlier eras. Australia's eastern seaboard is predicted to enter a 30-year flood-dominated cycle, with some floods higher than the 100-year flood line. Other studies suggest 40 or even 50 years of excessive rain.

The largest Hawkesbury flood on record was in 1867, but Aboriginal people described to Governor Gidley King an even bigger flood in about 1780. The 1956 flood washed away construction machinery while Warragamba was being built, but during previous cycles, there were more than a dozen major (greater than 12m) floods.

In January 1972, driving home from holidays in Woolgoolga, I heard about Maitland's flood on the radio. Seeing about-to-be-retired steam engines hauling trains through floodwater is a sight that will never be repeated!

The worst floods I can remember were in 1974. Remember how the road to Lockhart was continually covered with water across the Bullenbung Plain in the 70s and 80s? Remember how the Tarcutta Creek used to cut the Sturt Highway?

If the predicted 30-year flood-dominated cycle proves to be true, we will have floods regularly from now until 2050. It has happened before in my lifetime - the previous "flood-dominated cycle" ran from 1949 until 1990.

Town planners, and perhaps too the buyers of homes in flood-prone areas, believed Professor Tim Flannery's 2007 prediction, "even the rain that falls isn't actually going to fill our dams and river systems". His prediction looked ridiculous after the 2010 and 2012 floods, and more so after the recent Hawkesbury floods.

When the big one comes, let's just hope that our homes are safe.