Fargo-Moorhead Troll Bridge during 2023 Spring Flood

A low-water crossing (also known as an Irish bridge or Irish Crossing, causeway in Australia, low-level crossing or low-water bridge) is a low-elevation

roadway A carriageway (British English) or roadway (North American English) is a width of road on which a vehicle is not restricted by any physical barriers or separation to move laterally. A carriageway generally consists of a number of traffic lane ...

traversing over a

waterbody A body of water or waterbody is any significant accumulation of water on the surface of Earth or another planet. The term most often refers to oceans, seas, and lakes, but it includes smaller pools of water such as ponds, wetlands, or more rare ...

that stays dry above the water when the flow is low, but is designed to get submerged under high-flow conditions such as

flood A flood is an overflow of water (list of non-water floods, or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are of significant con ...

s. This type of crossing is much cheaper to build than a high

bridge A bridge is a structure built to Span (engineering), span a physical obstacle (such as a body of water, valley, road, or railway) without blocking the path underneath. It is constructed for the purpose of providing passage over the obstacle, whi ...

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

bank A bank is a financial institution that accepts Deposit account, deposits from the public and creates a demand deposit while simultaneously making loans. Lending activities can be directly performed by the bank or indirectly through capital m ...

s to build a more elevated bridge and thus costs more), particularly in

developing countries A developing country is a sovereign state with a less-developed Secondary sector of the economy, industrial base and a lower Human Development Index (HDI) relative to developed countries. However, this definition is not universally agreed upon. ...

. Low-water crossings are essentially elevated floodways, functioning like

causeway A causeway is a track, road or railway on the upper point of an embankment across "a low, or wet place, or piece of water". It can be constructed of earth, masonry, wood, or concrete. One of the earliest known wooden causeways is the Sweet T ...

s under normal conditions and serving as limited fords during high- discharge conditions. They are generally considered unsafe for traffic during floods due to the potentially fast and turbulent currents flowing over the road surface, and can be dangerous even after the flood has receded because

silt Silt is granular material of a size between sand and clay and composed mostly of broken grains of quartz. Silt may occur as a soil (often mixed with sand or clay) or as sediment mixed in suspension (chemistry), suspension with water. Silt usually ...

mud Mud (, or Middle Dutch) is loam, silt or clay mixed with water. Mud is usually formed after rainfall or near water sources. Ancient mud deposits hardened over geological time to form sedimentary rock such as shale or mudstone (generally cal ...

and

debris Debris (, ) is rubble, wreckage, ruins, litter and discarded waste, garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can ref ...

left on the road by the water can reduce traction and cause skidding risks for both

vehicle A vehicle () is a machine designed for self-propulsion, usually to transport people, cargo, or both. The term "vehicle" typically refers to land vehicles such as human-powered land vehicle, human-powered vehicles (e.g. bicycles, tricycles, velo ...

s and

pedestrian A pedestrian is a person traveling on foot, by wheelchair or with other mobility aids. Streets and roads often have a designated footpath for pedestrian traffic, called the '' sidewalk'' in North American English, the ''pavement'' in British En ...

s. In Japan, a Konnyaku bridge is a type of low-water crossing.

Construction

The low-water crossing was developed from the traditional ford. A ford permits vehicular traffic to cross a waterway with wet wheels. In some countries the term “low-water crossing” implies that the crossing is usually dry, while “ford” implies that the crossing is usually wet. 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

culvert A culvert is a structure that channels water past an obstacle or to a subterranean waterway. Typically embedded so as to be surrounded by soil, a culvert may be made from a pipe (fluid conveyance), pipe, reinforced concrete or other materia ...

s. 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. A more elaborate ''low-water bridge'' will usually be an engineered concrete structure. 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.

Navigation

A low-water bridge renders the waterway non-

navigable A body of water, such as a river, canal or lake, is navigable if it is deep, wide and calm enough for a water vessel (e.g. boats) to pass safely. Navigability is also referred to in the broader context of a body of water having sufficient under ...

. 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 A submersible bridge is a type of movable bridge that lowers the bridge deck below the water level to permit waterborne traffic to use the waterway. This differs from a lift bridge or table bridge, which operate by raising the roadway. Two sub ...

is used on navigable waterways and is actively lowered into the water.

Safety

The concept behind low-water crossings is that they are convenient and safe to use in normal conditions. Once the water level rises to the point where it crosses the bridge surface, the bridge is generally unsafe to use due to high current. The force of the moving water may be strong enough to physically push the vehicle off the bridge: the higher the percentage of the vehicle in the water, the more likely the water's force will take the vehicle off the bridge and send it downriver with substantial damage. 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. As people will typically try to then get out of the vehicle, they may step into water currents that cause them to fall or be pulled down into the water. As the current during floods is often quite strong, it may sweep them downriver and carry them into debris, possibly causing injury or death. In developed countries this will usually be indicated by warning signs. An additional hazard is that the bridge surface may become completely obscured by the water, making it relatively easy to fall off the bridge surface into the deeper and more hazardous water on either side. As a consequence, the line of the submerged bridge is often marked with poles or other structures to indicate its course to unwary travelers and emergency users even when submerged. 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. One attempted solution/deterrent is the Stupid motorist law in the American state of

Arizona Arizona is a U.S. state, state in the Southwestern United States, Southwestern region of the United States, sharing the Four Corners region of the western United States with Colorado, New Mexico, and Utah. It also borders Nevada to the nort ...

. Drivers who become stranded on flooded low-water bridges are charged with the cost incurred by emergency services to come to their rescue. File:Flood buried bridge, Ashida River 20180706.jpg, Flooded Low-water crossing File:Flood buried bridge.jpg, Bridge after the flood (No Entry) File:Morgan Ford Low Water Bridge 2015.JPG, The former single-lane low-water bridge over the

Shenandoah River The Shenandoah River is the principal tributary of the Potomac River, long with two River fork, forks approximately long each,U.S. Geological Survey. National Hydrography Dataset high-resolution flowline dataThe National Map accessed August ...

Virginia Virginia, officially the Commonwealth of Virginia, is a U.S. state, state in the Southeastern United States, Southeastern and Mid-Atlantic (United States), Mid-Atlantic regions of the United States between the East Coast of the United States ...

. This bridge was replaced in 2019.

Fish passage at low-water crossings

Since 1980, the ecological impact of road crossings on natural streams and rivers has been recognised. Baffles may be installed along the culvert to provide some fish-friendly alternative. But baffles can reduce drastically the culvert discharge capacity for a given afflux, thus increasing substantially the total cost of the culvert structure to achieve the same design discharge and afflux, or increasing the risk of road flooding in an existing structure.

References

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External links

List of Fords and Irish Bridges in the UK
Bridges Road infrastructure River crossings Bridges by structural type