The experts' advise not to drive or wade into flood waters at all, especially if you can't see the bottom. Water over a road, no matter how deep, can hide washed out pavement. As little as six inches of moving water is enough to float a small car and carry it away. That's enough force to punch a car or light truck off a flooded road if the water's up to door level. Mudslides also can become death traps.
Rain falling onto a hillside can lead to it giving way and tumbling into roads or houses.
An on-line graphic and text explains how these form. Alaska Australia Brit.