Low-water crossing

This type of crossing is much cheaper to build than a high bridge that keeps the road surface consistently above the highest water level, and is usually deployed in semi-arid areas where high-volume rainfall is rare and the existing channel is shallow (which requires extra ramping on the banks to build a more elevated bridge and thus costs more), particularly in developing countries.

It consists of an improved surface of gravel, gravel-filled geotextile grid, paving blocks, or concrete slabs positioned to match the slope and elevation of the adjacent waterway and floodplain.

The size of the culverts (often concrete pipes) is usually selected to allow the water to flow below the roadway and provide a dry crossing surface for most of the year.

There are thousands of such structures in the arid climates of the western United States; some of them accommodate four-lane city streets or highways.

Typically, a low-water bridge that accommodates a high daily volume of vehicular traffic will be underwater only a few days per decade.

An additional risk for trying to cross a bridge under water, especially when more than a couple of feet deep, is the possibility of the vehicle's engine stalling.

Despite the obvious dangers and warnings given there are still a significant number of emergencies and even deaths attributable to the unwary use of low-water bridges during flood conditions.