Why do flood waters do so much damage?

[old man emu]

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 ?

 

 

 

 

[willedoo]

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.

[onetrack]

The simple problem is the water catchment area concentrates all the runoff into smaller areas - the creeks, then the rivers. You can get absolutely monstrous volumes of water from just one big thunderstorm.

 

I built a 26km haul road for the Gascoyne Trading Company (a local trucking company) in 1986, between Kanowna and the Paddington Mill, North of Kalgoorlie townsite.

A company had found a large open-cut gold deposit at the old Kanowna Gold Field and had arranged to have it toll-treated at the Paddington Mill. 

There was a substantial volume of ore to be hauled, it was going to keep a fleet of road trains going for at least 18 mths, and there was a likelihood of more ore to be found.

 

Gascoyne Trading got the haulage contract, and were using road train doubles to haul the ore. I built the road from scratch, from virgin bush, and it was just built to a good unsealed gravel road standard.

While I was doing the initial survey of the project, I realised the countryside all sloped Northwards across the route, and there were already a couple of wide shallow creeks crossing the route.

I did some quick calculations on the catchment area, and a potential thunderstorm rainfall event of 100mm over a couple of hours (as is common in the W.A. Goldfields).

 

It didn't take long to calculate runoff volumes that were huge. Another factor was, the country was all heavy red loam and clay, and it didn't soak up water rapidly, so the runoff level in relation to rainfall, was high.

I was going to have to make allowance for some serious levels of rainfall runoff crossing the haul road.

 

So, I installed a couple of flood crossings in the region of the creeks, that were about a metre deep and about 150 metres wide.

There was no allowance for culverts in the job costing, so it simply a case of installing floodways, and letting the big volumes of water cross the road above road level at a shallow depth, so it didn't scour the compacted gravel surface out.

The road couldn't be used for a couple of days after rain, anyway, so the system would work just fine, and all the water would have run off, by the time the rest of the area dried out.

 

The boss of Gascoynes came out for road inspection as I was finishing up, and was amazed at the huge floodways. He said, "What are they there for? Are you expecting a Biblical flood?"

I said, "When we get a big thunderstorm, you'll see why they're there! There's a huge catchment area feeding those floodways, and I fully expect they will run a banker after a big storm."

 

Sure enough, on a Monday night about 9:00PM, around 2 months after the haul road was finished, a huge thunderstorm swept through the area.

The official rainfall figures in Kalgoorlie showed over 50mm in a couple of hours, I reckon the rainfall in the region of the haul road was over 100mm, judging by the amount of wash.

 

I went out to the road later on the Monday, and drove along it in my Holden ute, to the floodways. They were running around 800-900mm deep for their full width!

The boss of Gascoynes turned up shortly after, and he was doubly amazed at the volumes of water going through the floodways. Within 24 hrs, the flow had reduced to a trickle, and there was very little road surface damage.

The road trains were back on the road on the Wednesday, with no problems. It was a real eye-opener for the Gascoynes boss, who obviously had little experience with the results of heavy rainfall events.

As someone with over 20 yrs of experience of water harvesting, flood control, earthworks, and road building, I knew exactly what kind of conditions could arise from short periods of heavy rainfall.

 

I really am amazed at how much floodplain areas are allowed to be built on in the Eastern States. It seems like planning authorities don't keep any "old hands" around, to remind them of the many previous disastrous flood events, and the lessons that should've been set in stone, from those events.

 

 

[pmccarthy]

Several open pits in that area have been inundated by sheet flooding. It’s ok if you can escape, but the EMU decline off the pit floor drowned five men.

[Yenn]

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.

[ClintonB]

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.

[facthunter]

Droughts and flooding rains.  That's Australia. Nev

[Old Koreelah]

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.

[facthunter]

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

[spacesailor]

My local ' Toongabbie creek ' was reasonable clear in the floods,

BUT

Today  it,s muddy brown, but quite shallow !.

Pollution from hosing every thing down perhaps.

spacesailor

[facthunter]

Did you attend St Mary's High School ,Spacey?  Nev

[spacesailor]

No 

BUT

I was married in St,Mary,s church, Derby UK.

Only did woodwork at St Mary,s school.

spacesailor

[Old Koreelah]

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.

[facthunter]

I have no difficulty believing that one.  Nev

[Marty_d]

I had a cat in a carrier in one car - damn thing sprayed sideways through the cage and soaked the back seat.  It took a sh*tload of cleaning to get that goddamn smell out.

[nomadpete]

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.

[nomadpete]

PS,

 

No floodwaters were harmed in my cat's tale.

 

Sorry for drifting the thread, I forgot what the topic was.

[willedoo]

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?

[facthunter]

The  best cat I ever had was found as an absolute feral. Probably a fluke as some ferals  I have encountered are Crazy Wild and ugly. Cats I'm speaking of. Dogs are fine but you are getting loyalty on the cheap with a dog.   Nev

[willedoo]

They say dogs have masters and cats have servants.

[facthunter]

You have to come to some arrangement with your cat. You just tell your dog to "sit" and give him a pat. Too easy. Nev

[Old Koreelah]

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.

[willedoo]

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 March 30, 2021 by willedoo

[Marty_d]

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.

[Old Koreelah]

7 minutes ago, Marty_d said:

Bells do nothing...

Depends how heavy they are...