I know you have covered the perils and techniques of driving in the wet and in floods, but what single factor would you put at the top of the list when judging whether or not a flood crossing is safe?
Before and above anything else, judge how fast the water is flowing. Because if you get that wrong you could be in life-threatening peril. The depth of the water is important, of course, as is the condition of the road. But if you get those wrong the consequence is you stall or get stuck.
If the crossing is safe (eg less than 15cm deep and relatively calm) that will be obvious. If it is lethal (eg roaring waves going like a galloping horse and with solid things floating in them) that will be obvious. It’s the in-between conditions that demand judgment. And it is the possibility of being swept away that must be considered because the car will be swung around and float downstream like a boat (if you are lucky) and be turned upside down (if you’re not).
While most skilfully driven cars (principally involving high revs and low speed) can cross a distance of calm water 30cm deep without undue hazard, very fast-flowing water only half that depth can impose a sideways force sufficient to throw you offline or carry you away.
In doubtful cases, consider whether this is the start of a flash flood (that could get worse very suddenly) or the end of a flash that is abating.
Wait a few minutes and watch. Consider the likely road surface under the water and the surrounding terrain.
If you deem the risk essential but quite high, undo your safety belt and hold the doors open both upstream and downstream to let the water through to reduce possible side drag and add traction weight in the foot wells, get the engine running at 3000 revs or more, engage first gear, power through at a low speed and, once you are past the point of no return, make advance resolve to keep-moving with determination. See you on the other side.
