Low water crossing
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A low water crossing (also known as an Irish bridge, 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, particularly in developing countries or in semi-arid areas with rare high-volume rain.
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[edit] Construction
The simplest form of low water crossing is a ford. A ford permits vehicular traffic to cross a waterway with wet wheels. The term "low water crossing" implies that the crossing is usually dry, while "ford" implies that the crossing is usually wet.
A simple low-water crossing can be constructed with culverts. Culverts (often concrete pipes) are used to carry the water in a stream keeping the crossing surface dry for most of the year. High flows, i.e. spring runoff or flash floods, flow over the top of the crossing, as the culverts are not large enough to carry these flood-type runoff events.
A more elaborate low-water bridge will usually be an engineered concrete structure. There are thousands of such structures in 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.
[edit] Navigation
A low-water bridge renders the waterway non-navigable. In almost all cases this is not a practical concern, since the waterway would be non-navigable except during flood conditions anyway.
A low-water bridge is sometimes called a submersible bridge, but this is a misnomer. A true submersible bridge is used on navigable waterways and is actively lowered into the water.
[edit] Safety
The concept behind fords and low water crossings is that they are safe to use in normal conditions. The obvious corollary is that they are not safe to use in high-water conditions. Unfortunately, many lives are lost each year when people attempt to cross a ford or a low-water bridge when the water level is higher than the safe level.
[edit] Single lane bridge
Another way of reducing the cost of a lightly used bridge is to make it single lane even though most of the roadway on either side has two lanes.
