Round Mountain Red - Low Water Crossing

A low-water crossing (also known as an Irish bridge or Irish Crossing, causeway in Australia, low-level crossing or low-water bridge) provides a bridge when water flow is low. Under high-flow conditions, water runs over the roadway and precludes vehicular traffic. This approach is cheaper than building a bridge to raise the level of the road above the highest flood stage of a river.

The simplest type of low water crossing is called an unvented ford or drift. This type of low water crossing is used mainly for shallow waterways or dry streambeds. 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.

To cross somewhat deeper waterways, a type of low water crossing called a vented ford can be constructed with culverts. 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. During periods of high-water flow (e.g., spring runoff or flash floods), water will flow over the top of the crossing, as the culverts are not large enough to carry these flood-type runoff events.