Forest Flooding

Bendigo Farm let  flooded big time
I live in a remote area of a Forest only 10 Km from a bitumen road.
I have had a series of floods in the last few weeks. Something that is very rare in the past.
Last night I had a tidal wave come out of the forest and go through my property about 3 ft deep and 150 M wide.
It shifted cars, engines, building materials, took out 900M of fencing, dumped 1 foot of soil on my shed floors!
Its a mess, a 10,000L water tank was washed 100M and a bonfire  pile I have  been building for 2 years has moved itself about 30M.
Cars are full of water.
I doubt a rabbit has survived that could not body surf.
A tenant had left heaps of things, IBCs, chook sheds, 20L drums, plywood sheets and building materials which caused another blockage between trees, shrubs and fences and washed all over my dam full of fish,  and then pushed another fence over.
It also caused a water level rise that got within 4 inches of one of my homes and scoured the surface rock off 2 driveways I have and ruined the local road surface.
Through magnificent planning and forethought, all motorbikes and sidecars were above the water level.
It has never happened in the 50 years I have been here, but  I will chose to build flood walls and keep potential floodways clear now.
Roads around had 1 foot of water over them, fences and trees are down all over the place.
To get home I had to stop and clear 8 culverts in 6 kilometres so the water level would go down enough to drive through the 12 inches of water left.
We had 92 mm of rain on top of a lot previously.
BUT, I have realised the need to be aware of flood paths and not stack things in them. I will now start a program downstream of unblocking paths that are a result of my activities.
Some of the fences I built to swing away from the water flow and I am hoping when the debris is cleared I can lift them up again. Others that had a longitudinal flow have bee ripped out.
Its not a home that has been damaged, but my pride is my farm layout, the sheds and the toys so its bad enouigh for myself.

Wow. That sounds like a real mess, John. Glad to see that no homes flooded, but I feel bad for all the work that now needs to be done or (worse) redone. At least this means your heatwave has broken for the time being, I hope (it's crossed the Tasman and it's plenty warm here right now).

Wow John, That is shit for you but as you know there are others who got totally washed away.  We can only thank goodness for the small mercies we get during our lives.
Stay well.  As you know, I am only a PM away and can give you a ring if you need to chat.
Cheers
Paul

Thanks Phil and Paul.

Yikes, sorry to hear that John.

Even if the monetary loss isn’t too high, that’s a lot of work. Maybe it’s a chance to play with the layout a little bit?

Ugh, I'm so sorry.  Please let us know if we can help in any way.

Very sorry to read this John, but it sounds like you did a lot to mitigate the damage. I have read about flooding down there, but don’t know a good source to read up on the weather event that caused this. Did a river breach its banks in an unprecedented way or was it an unprecedented rain event? Do you think the bitumen road you mentioned have any exacerbating impact on your site? Flooding like that in a healthy forest seems very uncommon in most places outside of coasts and river deltas (like mangroves or places like the Everglades) or below dams that blow out. Any idea what contributed to the severity of this event? It could be helpful for others in their design and management of properties with similar risks. Of course you have plenty of more pressing work to do, but Thanks, and apologies for my ignorance of what is going on in your part of the world. Best of luck, and I hope these problems can become future solutions for others, and your site.

Thanks Ben,  and others for support. Its not as bad as a house flood, but 20 acres of mess is a 'creative ' sight!
The rainfall was it an unprecedented rain event.
There are no rivers around only creeks, mine is 1 M wide if it gets wet in the area, normally its dry.
I have seen it 150m and I am guesing it must have been 3 ft deep and 300M wide with this flood.
One lesson is to ensure known floodways / paths across the property should be kept clear if possible.
I did have trailers, stacks of timber etc in the path, but I have never seen rainfall like this before in a dry area.
My gauge shows 200mm that day and night.
The official gauge sats 92mm.
A mate suggests we were close to a 'super cell'.
We dont have bitumen roads, they are gravel over the ground and graded from time to time.

That is a lot of water! I am thinking maybe some geographic calculations could help others with their designs to mitigate such events’ destructiveness. Do you have any estimate of how large the water catchment/watershed is above your land? I have used google earth for this in the past.

Is a large portion of that not forested or otherwise prone to a high runoff quotient? Do you think any dams might have been breached above you?

We live in a temperate rainforest area, where we have had 10” (254mm) rain events in a day at our former property in the coastal redwood lowlands (90ft elevation). That was a very hard phenomena to deal with given many times the acreage of deforested land ran onto our 1/2 acre lot. The food forest I designed and installed in Crescent City had to absorb 14acres of hardscape on 1.5 acres. It became necessary to divert that water into designed wetlands that drained over flats into a forested stream.

Our largest event at our new place up at 1750ft elevation on the highest ridgeline inland from the ocean, in what could be called a borderline cloud forest, we have had 17” (431mm) in one 60hr period this year. However, this mountain is made of mostly fractured rock and drains well. We have a pond and swales to absorb the upper 5acres or so above our house, but they have been overflowing most of this rainy winter. I will miss that water during our long dry summer, when fire is a real concern. I hope at the very least this rain during your summer will help mitigate fire concerns!

I am just kind of in awe that someone has had to experience this.

I'm glad that you are okay. It sounds like you almost if not already have experienced both a flood and nearly a rock slide!

I hope you never have another freak flood, I'd love to know what happened upstream of you to send all that water. Makes you wonder!

Well our rain gauge on our weather data system recorded 9.8” (248mm) yesterday (1/13/24)! Pond dam almost breached before I realized debris from fire clearing work had backed up the secondary overflow, which had not been tested since we moved here 4yrs ago. The primary overflow pipe (12” or 300mm) was at capacity, with three rushing torrents feeding the pond. It was a good example of why a round pipe is never as good as an arch/halfpipe with a flat bottom, as the 12” culvert overflow pipe was only running water through 1/3 its volume capacity, and that seems to be the maximum amount of water it can move. It did not clog with debris fortunately, but it was a clear demonstration of how inefficient round pipes are as pond overflows.

Pipes actually run different ways according to the circumstances of the site and situation at the exit end.
Flat bottom culverts are better for low head flows and flat ground.

We are nearly finishing the clean up and in future I will ensure the flood lines are clear of equipment etc and fences can move with debri loads to reduce destruction.