Floods and Max Water Height

  • UPDATE - The main upgrade is now finished. The site should now be functioning as normal, I will be making tweaks over the weekend, particularly to look of the site. If you notice something is broken or have any comments please let me know. Many thanks Matt (Lithril)

Broch

Full Member
Jan 18, 2009
2,729
1,941
Mid Wales
Are people still taught to walk the route first? Not a great idea when it comes to rivers, but not unachievable either. It used to be a technique taught in the Army and was certainly taught for 'puddles' off road.
Yes, using a stick and ideally more than one person in line or a wedge shape but, again, only if there is no other choice than going through the water.

Generally we tell people if they can't wade it (or consider wading it) then it's not drivable.
 
  • Like
Reactions: santaman2000

petrochemicals

Full Member
Jul 30, 2012
3,488
201
westmidlands
Last edited:

Broch

Full Member
Jan 18, 2009
2,729
1,941
Mid Wales
As I said earlier, it's not so much the 'weight' of water that's the problem but the speed. The force on the side of the car is proportional to the density (1000kg/m3) but it's proportional to the square of the speed x the square area of the body it's pushing against. So, static water exerts no force on the side of the vehicle despite its density. At 1m/s (4.5 miles/hr) it will exert a force of 500N on a surface area of 1m2 (half a ton) at 2m/s it will be 2000N (2 tons) at 3m/s (quite feasible in flood) it will 4500N!

Consequently, relatively shallow water can move a car easily if it's flowing fast.
 

TLM

Settler
Nov 16, 2019
524
167
66
Vantaa, Finland
To be slightly engineering pedantic water has a very substantial static pressure against any immersed surface. In Newtons the calculations are correct but the conversion to tons needs a bit of rethought.
 

Broch

Full Member
Jan 18, 2009
2,729
1,941
Mid Wales
To be slightly engineering pedantic water has a very substantial static pressure against any immersed surface. In Newtons the calculations are correct but the conversion to tons needs a bit of rethought.
Oops, yes, fingers working faster than the brain (I was on my way out), sorry :oops: - 10,000N to 1,000Kg Force so only .05 tons, 0.2 tons and 0.45 tons - as for the static pressure, that is being pedantic for the sake of the discussion but I take your point :)
 

santaman2000

M.A.B (Mad About Bushcraft)
Jan 15, 2011
16,218
804
63
Florida
As I said earlier, it's not so much the 'weight' of water that's the problem but the speed. The force on the side of the car is proportional to the density (1000kg/m3) but it's proportional to the square of the speed x the square area of the body it's pushing against. So, static water exerts no force on the side of the vehicle despite its density. At 1m/s (4.5 miles/hr) it will exert a force of 500N on a surface area of 1m2 (half a ton) at 2m/s it will be 2000N (2 tons) at 3m/s (quite feasible in flood) it will 4500N!

Consequently, relatively shallow water can move a car easily if it's flowing fast.
Well, sorta. Yes it’s true that the total be energy of a current is more dependent on the velocity than the mass. However the mass (weight for any practical consideration) of the water means the vehicle will be buoyed a good bit (completely negating any frictional resistance of the tires in contact with the bottom) Even without that the mass does still play a role in the total energy.
 

Broch

Full Member
Jan 18, 2009
2,729
1,941
Mid Wales
Well, sorta. Yes it’s true that the total be energy of a current is more dependent on the velocity than the mass. However the mass (weight for any practical consideration) of the water means the vehicle will be buoyed a good bit (completely negating any frictional resistance of the tires in contact with the bottom) Even without that the mass does still play a role in the total energy.
Yes, but people keep quoting the 1 ton per cubic metre figure as though it was that 'weight' that was pushing the car. You're right though and normal cars are so low that they get lifted much quicker than a 4x4 with high ground clearance.

However, I have followed a Disco 2 into deep water and the air was trapped under the wheel arches; lifted the whole vehicle and moved it a metre downstream before the air escaped.

The holes in the defender are designed to let the water out :)
 

petrochemicals

Full Member
Jul 30, 2012
3,488
201
westmidlands
As I said earlier, it's not so much the 'weight' of water that's the problem but the speed. The force on the side of the car is proportional to the density (1000kg/m3) but it's proportional to the square of the speed x the square area of the body it's pushing against. So, static water exerts no force on the side of the vehicle despite its density. At 1m/s (4.5 miles/hr) it will exert a force of 500N on a surface area of 1m2 (half a ton) at 2m/s it will be 2000N (2 tons) at 3m/s (quite feasible in flood) it will 4500N!

Consequently, relatively shallow water can move a car easily if it's flowing fast.
Yes, but people keep quoting the 1 ton per cubic metre figure as though it was that 'weight' that was pushing the car. You're right though and normal cars are so low that they get lifted much quicker than a 4x4 with high ground clearance.

However, I have followed a Disco 2 into deep water and the air was trapped under the wheel arches; lifted the whole vehicle and moved it a metre downstream before the air escaped.

The holes in the defender are designed to let the water out :)
This seems to be contradictory broch, lower cars have a lower water line, unless they have specially designed holes to let water in and stop the problem of boyancy
 

MrEd

Native
Feb 18, 2010
1,308
307
Surrey/Sussex
www.thetimechamber.co.uk
So I would not drive into floods in my normal car (Toyota) as the air intake is way down low in the wheel well

But my land Rover 90 I have had it about 2.5 - 3ft deep still water a couple of times when playing off road. I wouldn’t want to make a habit of it though as it burgered a wheel bearing! Technique is the key - bow wave, not to slow or fast and walk the route or probe it first with a stick etc to look for voids and the condition and solidity of the underlying surface.

I wouldn’t want to go in anything like that deep in any remotely flowing water though - to dangerous, not if I could avoid it.

To be honest I avoid flood water - hidden hazards, contaminated with sewage etc, and when my road floods you can’t see the ditches either side which are a good metre deep and could easily drive into it and wreck your motor!

Just avoid it unless you have absolutely no choice, it’s not really worth it.
 

GuestD

Need to contact Admin...
Feb 10, 2019
1,445
685

MrEd

Native
Feb 18, 2010
1,308
307
Surrey/Sussex
www.thetimechamber.co.uk
I may be stating the obvious here, but a lot of people who fit snorkels to Land Rovers omit to check if the differential breathers are affected by fording deep water. This can result in the ingress of water into the differential housing, resulting in rusting from the inside out.


https://www.firstfour.co.uk/defender-differential-breather-kit-3-port.html
Yes. I have breathers from both axles, both gearboxes and timing chest - they run up to the top of the engine bay, join into a common manifold then run into the snorkel ultimately terminating above the roofline.

The way axles etc suffer is they get hot when driving then you plunge them into cold water and the temp differential cause a pressure change which sucks water in past the seals. There are breathers to prevent this as standard but they terminate just above the axles so can easily draw in water.
 
  • Like
Reactions: Nomad64

GuestD

Need to contact Admin...
Feb 10, 2019
1,445
685
Yes. I have breathers from both axles, both gearboxes and timing chest - they run up to the top of the engine bay, join into a common manifold then run into the snorkel ultimately terminating above the roofline.

The way axles etc suffer is they get hot when driving then you plunge them into cold water and the temp differential cause a pressure change which sucks water in past the seals. There are breathers to prevent this as standard but they terminate just above the axles so can easily draw in water.
And on old "88's and 109's" for fording deep water, the trick was a an easily removable fan belt to stop the fan from firing water everywhere.