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low water crossing
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. Low water crossing can be dangerous when over flooded.
==Construction==
The low water crossing was developed from the traditional 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, e.g. 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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