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Why do flood waters do so much damage?


old man emu
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Sitting in the house listening to the incessant rain, and watching the TV news reports of rushing rivers, I begin to wonder - Why do rivers run? And after that - Why do rivers running at the flood damage the land?

 

In the twelve hours to 9:00 am Sunday 21st March, Camden Airport weather station recorded 90 mm of rainfall. That equates to 90 litres of water per square metre of flat surface. Most of that rainwater is going to enter the Nepean/Hawkesbury system and eventually reach the sea.

 

Camden Airport is 75 metres AMSL. With reference to MSL, the 90 litres of rain has a Potential Energy.

PE - mgh

Where m = mass

g = acceleration due to gravity 9.8 m/s/s

h = change in elevation

Therefore, that 90 litres of rainwater, having a weight (mg) of 90 kilograms, has a Potential Energy of 90 x 75 x 9.8 = 66,150 Joules sitting on the ground at the airport. (Let's assume that it has no energy left that came from its height in the cloud.)

 

Ignoring losses of energy due to friction within the fluid and change in momentum due to collisions with massive objects (riverbanks and bridges) all the potential energy will be converted to kinetic energy at MSL. This can be expressed this way due to the Law of Conservation of Energy

mgh  = 1/2 mv^2 

 

We can also work out the theoretical final velocity of the water when it reaches MSL

Dividing by "m" on both sides gh = 1/2 v^2

Multiplying by "1/2" on both sides 2gh = v^2

Taking the sqr rt on both sides sqr rt (2gh) = v

Putting in the numbers sqr rt (2 x 9.8 . 75) = v 

v = sqr rt 1470  v = 38.3 m/s

38.3 m/s = 138 kph

 

Now try to imagine the amount of water that is going to enter that river system by considering the catchment area of the Hawkesbury/Nepean system. Mind-boggling, isn't ?

 

 

 

 

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We are getting a bit of it in Queensland, but not as bad as NSW. From yesterday and last night I measured 5" which is the biggest single event fall for a few years now. It's not unusual to get big falls here, it's just that in the last few years it's been spread out into smaller lots.

 

Fluid dynamics is an interesting subject although a bit above my pay grade. So do the water molecules just bulldoze the soil in it's path or does capillary action have an influence as well. As in the clay colloids sticking to the water.

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We are still waiting for rain, we have had little bits, just enough to get the grass green and so far less than 60mm for the mont and less than 200 for the year to date. my dam is dry or as near as matters, but we have had about 15mm today. Just praying for some of that deluge.

Roads and railways have been built across flood plains, which are seldom wet, but when they do run the water goes over the road and increases speed as it falls on the downstream side. Result is massive erosion on the downstream side and I have seen railway lines suspended in space where the ballast and embankment have all gone downstream. Having big rocks around makes it worse as the water speeds up as it is deflected around the rocks, resulting in big gullies.

It seems that most of the older people who understood water have retired or been pushed aside and I don't think the younger generation know what really happens. I am sure that the TV and radio weather experts have no idea. They keep coming up with North Qld having massive rainfall which will cause floods  and they are talking of 30 to 100mm, which up here is just about enough to settle the dust.

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The year I left Carnarvon the Gascoyne flooded big from rain in the east and north. The water flowing back to the empty river after the flood washed away roads, pipes and everything else that had been dug up previously. I took pictures of power poles supported by wires, with a column of compacted dirt under them  from being rammed in.

at one stage they had a giant pump spitting water over the highway to get it back to the riverside of the mound. At one point it was over 100 km wide but shallow.

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Another aspect of the topic: not the quantity of water, but the quality.

Years ago I waded across the flooded Warrah Creek. Despite the water being almost knee-deep, I could see my toes.

There being no cultivation or over-grazing upstream, the water carried very little eroded matter. A very rare event.

 

Most floods today carry masses of soil, plant debris, dead animals, rubbish, etc. That gets dumped in someone’s lounge-room. No wonder people toss out flood-damaged household stuff.

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When I was young, Maitland flooded often and I enquired of the Boss whether  we should buy flood affected cars and repair them. It's not practical generally as the damage from mud and rapid corrosion if you don't strip it quickly is very destructive. That is for the mechanical parts. Instruments upholstery  electrics  etc are worse. Nev

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23 hours ago, facthunter said:

When I was young, Maitland flooded often and I enquired of the Boss whether  we should buy flood affected cars and repair them...

Cars are a whole different ball game, Nev. I knew a bloke who had to sell his near-new car for scrap after a moronic wedding prank by some of his “mates”. They hid a large fish up under his dash. When he got back from their fly-away honeymoon his car reeked and nothing could be done to remove the stench.

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Our cat was a low class rescued feral cat. But He had better manners. Driving down from Queensland, we had him cramped up in one of those cat carriers, on the back seat. It was perched on top of a pile of possession. Day one, he was quiet. About lunchtime day two,  he started meowing and wailing loudly. My darling wife took pity on him and opened the cat carrier door. I'm doing 110kph along the highway with a large,  distressed semiferal cat suddenly on the loose in the cab! But all was not lost. Sparky stretched, calmly came forward to sit on wife's knee whilst looking intently out the window. Then he went back and got into his cat carrier box. Seems he simply wanted to stretch his legs and see where we were. Never disgraced himself in the car.

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2 hours ago, nomadpete said:

Our cat was a low class rescued feral cat. But He had better manners. Driving down from Queensland, we had him cramped up in one of those cat carriers, on the back seat. It was perched on top of a pile of possession. Day one, he was quiet. About lunchtime day two,  he started meowing and wailing loudly. My darling wife took pity on him and opened the cat carrier door. I'm doing 110kph along the highway with a large,  distressed semiferal cat suddenly on the loose in the cab! But all was not lost. Sparky stretched, calmly came forward to sit on wife's knee whilst looking intently out the window. Then he went back and got into his cat carrier box. Seems he simply wanted to stretch his legs and see where we were. Never disgraced himself in the car.

Good old Sparky. Is he named after Clark 'Sparky' Griswold?

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I have spent four decades repairing my little part of the world and often see where cats have killed my wildlife. 

I suspect most are tame domestic cats “let out for the night”, yet I have never seen one fitted with a bell or any of the wild-life-protection devices available.

 

On my patch they are fair game.

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6 minutes ago, Old Koreelah said:

I have spent four decades repairing my little part of the world and often see where cats have killed my wildlife. 

I suspect most are tame domestic cats “let out for the night”, yet I have never seen one fitted with a bell or any of the wild-life-protection devices available.

 

On my patch they are fair game.

Old Koreelah, I must admit I like cats but would never own one for that reason, that they are bad for the wildlife. When it comes to the crunch, I like the wildlife more.

Edited by willedoo
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8 hours ago, Old Koreelah said:

I have spent four decades repairing my little part of the world and often see where cats have killed my wildlife. 

I suspect most are tame domestic cats “let out for the night”, yet I have never seen one fitted with a bell or any of the wild-life-protection devices available.

 

On my patch they are fair game.

Bells do nothing.  Our last cat (we're never getting another) had a bell, arthritis and 19 years of age, yet still killed birds.  Since old Nellie shuffled off this mortal coil, the little birds around the place are amazing.  I've had blue wrens land on my legs while sitting still on the deck.

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