Planning road alignments for adequate drainage and water management
Much damage to unpaved roads can be prevented. The main requirement is to avoid water running along the unpaved road surface at high speed, taking out fine material and then small stones and gravel, causing a large part of the road material to be removed. Requirements for reducing the speed of runoff include an alternation of slopes, the use of bends where runoff can evacuate the road surface, and the presence of a basic road drainage system.
The angle and length of the road’s slope are very important in reducing the risk of water-related road degradation (Zeedyk 2006). These two factors determine the velocity of the water running along the road surface and with it the scouring effect. A number of factors are at work:
When the velocity of the runoff on the unpaved road surface doubles, the volume of sediment that can be moved increases fourfold.
The size of particles that can be transported increases eightfold when the velocity is doubled.
Velocity increases as water depth on the road surface increases, because the relative surface tension decreases.
As flow velocity increases, sheer force “plucks” larger particles from the road surface.
Much of the eroded material will be deposited downstream, clogging drains and covering fields. Part of this sediment will travel further downstream, reducing the capacity of downstream storage. In some cases, however, the nuisance can be turned into an asset with sand being harvested as a business opportunity (see Box 9.1).
Ideally, the grade of an unpaved road should range from 4 percent to 10 percent, with frequent grade reversals. Natural drainage is assured if the road reverses grade every 60 m to 100 m (Zeedyk 2006). Water naturally exits the roadway at every grade reversal. According to Napper (2008), the most important dos and don’ts are:
Try to reverse the slope of the road and avoid long uniform stretches.
Where possible, use crowned and/or out-sloped/tilted road templates to drain the water immediately to the side of the road, although such templates are sensitive to wear and tear.
Use rolling drainage dips and water bars to remove water from road surface (see Section 9.3.2) at designated places where the water can be used productively.
Locate drainage features at greater spacing on soils that are fine-grained and erosive, or cover the most vulnerable sections with at least 10 cm of coarse aggregate.
Ensure a well-vegetated buffer zone or row of stones (infiltration bund) at the edge of the road to disperse flow, reduce runoff velocity, and collect sediment from road runoff (Section 9.3.3).
Avoid long road sections that are entrenched with no opportunity for water to be evacuated to either side of the road. For the same reason, avoid degradation of the unpaved roads over the years that causes the road to sink and become a drain.
Where there is considerable subsurface flow, provide side drains to collect this flow and reuse it. Alternatively, provide permeable fills and French mattresses close to saturated water pockets.
Alternatively, provide full road-drainage systems in such places with side drains and cross culverts. Such well-developed drainage systems will not only protect the road but will also help to more systematically collect and harvest water from around the road.
Maintain a vegetative cover around the roads to increase roughness and reduce erosion. Where vegetation is removed (by road construction), sedimentation may increase sevenfold.
To keep drains self-cleaning, ditches and the road surface need to have a slope equal to or greater than the contributing source of sediment. The faster the water, the more sediment it can transport at an increasing rate.
On very steep hill slopes (>35 percent), develop internal drains guiding water away from steep cut slopes and intercepting subsurface flows to keep them from causing severe erosion on the downward fill slope.
Take care with stream-road crossings to ensure that the diversion of the stream to the road at times of high flow is controlled.
Care is required to connect the drainage from the unpaved roads to land where it can be used productively. This can be for farming, in particular for building up soil moisture ahead of planting or for direct use during the growing season. In this case, the use of farm trenches is preferred: the runoff is guided to the farm and directly irrigates the root zone. This is preferred over using runoff on the land, where it may damage the crop.
Farm trenches in Kenya: road drainage water serving the root zone
Box 9.2. Connecting drainage cuts to farm trenches in Mozambique
Sanjas in Mozambique
In the absence of a developed road drainage system along most roads in Mozambique, it is common to have drainage cuts on the sides of paved and unpaved roads. These cuts serve to remove water from the road surface and are usually made by a road grader during construction or maintenance. The drainage cuts, also called sanjas, are often made close together, typically 100 m or less apart. At present, these drainage cuts end in the road reserve and do not serve farmland, although they could be used in this way.
To help direct water to farmland for productive use, sanjas can be made longer. By connecting the sanjas through trenches to farmland, more moisture will be available for crops, and water will be carried directly to plant roots. This should enable a 20 percent increase in yields and make it possible to grow other, more profitable crops. The practice of using road cuts to water farmland is common, as seen in Kenya.
Below are recommended practices in making trenches that connect drainage cuts to farms:
In consultation with the farm owner, extend the drainage trench beyond the center of the farm. If the land is terraced, the trench should follow the terraces.
Make the trench approximately 50 cm deep.
The ideal width of a trench is around 40 cm: a relatively narrow and deep trench will preserve most moisture. Some crops, like bananas, require wider trenches.
The trench should ideally be rectangular.
Make sure the water from the road flows naturally to the land: choose the inlet at a position that makes best use of the slope.
Make gentle curves in the trench to minimize erosion.
Make sure the trench is not too steep: i.e., less than 3o. If the trench is too steep, it will erode easily.
The main trench can branch into side trenches. The drainage trench can feed a farm pond (discussed in the next chapter). In that case, it is recommended that a silt trap be installed before the pond entrance to catch the sediment in the trench water.
To strengthen trenches and prevent collapse, plant grasses and small trees on the banks.
If there is heavy rain, it is recommended that the entrance to the fields be closed with an earthen heap to prevent overflow of the road to the agricultural land.
