Hi john what we have to remember is that its not just water ...mixed in is everything that the water has washed down with it...spoil .. silt...rocks boulders tree trunks wheelie bins ...infac the entire contence of the village ....... + the force of the water which is being channelled through tight and narrow streets .....for instance when our thelmere aqueduct burst in walkden Manchester the amount of water was enough to cover the local Tesco's up to smutty mags on the magazine rack (top shelf) and car park but nothing got washed away (in car terms ) they just sat there and only moved slightly.......also when crossing a deep and fast flowing ford you are side on... so you have to 1 or more of 4 things ....1 make sure your water tight and have a rudder and population 2 make sure you can allow water in to give you ballast on the crossing 3 attempt if possible at an angle say 45 deg against the water to allow for drift ...4 don't cross
Hi Mark,
I think your missing the point.
These vehicles were indeed bobbing about like corks, but what caught my attention was the level at which they were FLOATING. It doesn't matter about what was in the water, be it rocks, beds, or even houses. These vehicles were floating and swept away, just because they were floating. The speed of the moving water made it spectacular, but they were NOT being pushed, they were like a raft going over the rapids. These vehicles were floating.
Taking your example, if the area is sheltered and the water rises, then hey presto, the vehicles will float, just like a ship in a dry dock, the ship and the cars will not move (apart from up-wards) In areas like the one you describe, there is not the huge sideways movement that you will have when crossing a ford. Forget my terrible example. Just think back to any disaster where cars try to cross a ford that is too deep. The car floats and gets washed downstream! They do NOT simply float and stay in the same position. They float away and gradually sink.
These cars, vans and 4 x 4's were all floating and the water level was round about the door seal area. In flooded areas the car will just become buoyant and perhaps loose traction. When crossing a ford however, the instant the car looses grip with the road then it becomes a boat and will be washed sideways into perhaps deeper water?
In the areas that just flood then vehicles can attempt to negotiate deeper water without the fear of being washed away.
If you make a slight bow wave then just behind it you develop a trough and that helps keep the wheels on terra firma, BUT..... Once water rises above the level of the floor pan, you are at extreme risk of floating.... unless the vehicle is made of heavy metal, or leaks like a sieve.
Get an empty bucket or washing up bowl and float it in water. Note high high it floats, there is next to nothing under water. A car is a very light steel container and until it leaks, it is extremely buoyant.
Just my non scientific thoughts
John
Once the level of water rises above the level of the floor pan then my advice is to consider fitting a rudder 
or perhaps aborting the crossing?
EVERY LITTLE HELPS!
