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.