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• Sustainable river crossings in arid regions • |
♦ Streams in arid regions ♦
In nature, there are three types of streams: (1) perennial, (2) intermittent, and (3) ephemeral. Perennial streams always
have flows and the low flows are sustained by permanent baseflow. Intermittent streams have flows only a few months out of the year, with the low flows
being sustained by seasonal baseflow. Ephemeral streams have flows only during and immediately following rainfall.
The location of the groundwater table, above, at, or below
the river bottom, determines whether the flow is perennial, intermittent, or ephemeral, respectively.
The depth to the water table varies across the climatic spectrum, from arid to humid regions. In humid regions the water table is close to the surface, while
in arid regions
it lies at much greater depths.
In general, the drier the climate, the greater the depth to the water table. Therefore, ephemeral streams are common in arid regions.
Erosion and sediment transport peak at mean annual precipitation of 375 mm (15 in), which corresponds to arid regions (200-400 mm/yr).
Thus, streams in arid regions are likely to
carry significant quantities of sediment.
From a geomorphological perspective, when streams carry a lot of sediment,
their aspect ratio (the ratio of width to depth) is large, typically between 50 and 100.
Thus, river crossings in arid regions pose significant design challenges.
The issue is how to provide a crossing in a wide ephemeral river channel effectively and economically (Fig. 1).
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Fig. 1 Washed out road crossing on the Mojave river,
Hellendale, California.
♦ Bridges and fords ♦
Bridges can be of many types, but they must span all or most of the stream width. Not spanning the full stream width in order to save construction costs leads to performance problems. In such cases the stream or river will eventually reclaim its full width, endangering the bridge structure. Excessive flow velocities near bridge piers and abutments is a common cause of local scour, which can lead to bridge failure.
Bridges are usually built when traffic or the road network's importance justifies the cost. Most road bridges are permanent structures and are vulnerable to extreme hydrologic loads. Rivers, particularly in arid regions, can significantly overflow their banks and may change course, defying the bridge's ability to perform its intended function (Fig. 2).
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Fig. 2 Railroad bridge spanning an old channel of the Pirai river,
Santa Cruz de la Sierra, Bolivia.
Fords are an alternative to bridges in cases of low traffic and/or lack of sufficient resources. Fords are engineered to cross the stream without a bridge, when the flow is low or nonexistent. In arid regions, fords are common in places where water flows only a few days out of the year. Thus, fords are an economical way to cope with the problem of river crossing.
The disadvantage of a ford is that it may not be operational a few times during the year, but when this is weighed against the cost of a conventional bridge, the choice is clear. Fords are common in rural areas and in developing countries, where traffic is low and/or resources are usually limited.
♦ The ford-bridge ♦
The ford-bridge concept draws from the strengths of both fords and bridges. Bridges are permanent and costly; fords may be temporary or permanent, but are comparatively inexpensive. Bridges are vulnerable to extreme hydrologic loads, while fords are generally able to resist most loads (Fig. 3). Bridges are always usable, as long as they do not fail; on the other hand, fords may be out of commission a few days per year.
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Fig. 3 Large concrete ford on the Jaguaribe river,
Jaguaribira, Ceara, Brazil.
A ford-bridge is a combination of ford and bridge, featuring most of the advantages and none of the disadvantages. The ford-bridge works like a bridge during dry weather and like a ford during infrequent floods. A ford-bridge in an ephemeral stream looks like a ford, but it has a relatively small bridge designed to pass the mean annual flood.
The ford-bridge can span the entire width of the river. There is no need to constrain the river in any way and there are no bridge piers to scour during floods. Yet, most of the time, usage of the ford-bridge will be similar to that of a small bridge. Essentially, the ford-bridge looks like a ford only when needed; at other times, the focus is on the bridge. Thus, for wide ephemeral streams in arid regions, particularly in developing countries, the ford-bridge concept offers a unique combination of functionality and economy.
♦ Design criteria ♦
All bridge crossings are designed to sustain selected hydrologic loads. These are usually frequency-based floods such as the 50-yr of the 100-yr flood. In a typical bridge design, the flood peak of a given frequency is expressed as flow velocities and pressures and used to calculate the scour at bridge piers. In contrast, a ford crossing has no piers; the only design requirement is the proper anchoring of the slab to minimize the possibility of uplift (Fig. 4). As in the case of a bridge, failure of a ford under extreme hydrologic events remains a possibility.
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Fig. 4 Concrete ford on Arroyo El Descanso,
Primo Tapia, Baja California, Mexico.
The components of ford-bridges should be designed as fords and bridges, respectively. The ford design is pretty much the same as with a conventional ford crossing. The bridge component, however, is much smaller than usual. The bridge design criterion should be the mean annual flood, i.e., the 2-yr flood, interpreted in geomorphology as the channel-forming flood. This contrasts with the 50-yr of 100-yr flood frequency used in conventional bridge design. During typical design floods, the ford distributes the hydraulic loads across the entire river channel width, without any obstructions and thus, it is better able sustain the floods. The bridge deck, lying above the 100-yr flood level, is designed to withstand the hydraulic loads (Fig. 5).
Given the right set of conditions, the ford-bridge affords a practical and useful alternative to a bridge or ford. It is applicable to wide ephemeral streams, particularly in rural areas, or where traffic is light and resources are limited. Its design optimizes the hydrologic loads, using the bridge for floods up to 2-yr frecuency, and the ford for floods up to 100-yr frequency. Thus, the ford-bridge concept is a sustainable economical solution to river crossings in arid regions throughout the world.
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Fig. 5 Schematic drawing of a typical cross section of a ford-bridge.