The design of the road drainage system is a major intervention in watershed management.
If a road is equipped with a (proper) drainage system, the water along the roads will collect and be removed from the road body. This will protect the road surface, especially in case of dirt roads, and make the water available for productive use. Typically, the raison-d’être of road drainage systems is to preserve the roads and prevent runoff from interfering with road operations. However, at the same time they are large water-harvesting systems and should be managed accordingly. When not well designed and managed, road drainage commonly causes uncontrolled flooding and erosion that affects the road body, neighbouring land, and the environment. It is also a missed opportunity in terms of water harvesting for productive purposes.
The runoff is preferably “given back” to the land through water harvesting and the diversion of water to farmland, spread over rangeland, or used for forest development. This is to keep the hydrological connectivity of the road network (i.e., the connections between the roads and the streams in the watershed) from becoming very high and water being rapidly drained from the watersheds. This would cause amplified flood peaks and give water less time to infiltrate and for aquifers to be recharged. Rather than road drains connected to the streams in the watershed, it is better to have road drainage water run into vegetation bunds, farm fields, or pastures. This would also reduce sediment deposition in the streams.
An initial requirement is that a road drainage system be in place that is able to redirect peak runoff volumes, but also to make it available for reuse. The presence of a good drainage system can prevent road scour or erosion along road drains and waterways, provided that drains are properly aligned or alternatively protected with erosion-control measures. Especially when the road material is highly erodible, it is important to have road drainage that will protect the road from water running on it (Figure 3.4). If not, this will create rills and scour tracks on the road, and the fine grades in the road body will be washed out. The road itself may increasingly develop into a natural gully (see pictures from Taijkistan, above).
When in place, the road-drainage system can consist of: (a) cut-off drains (that shield the road from uphill runoff); (b) side drains that channel the water along the road; (c) culverts, pipes, and bridges that take water across the road body; and (d) mitered (turn-out) drains that divert water to the land adjacent to the road. The road template itself is also part of the road drainage.
Road runoff can also be influenced by shaping the road surface and having it tilted downhill or having it crown shaped, guiding water to the side of the road from the middle section. Figure 3.5 shows several such road templates. However, depending on the road material, there is a risk that such purposely shaped road surfaces may disappear under the impact of traffic and rainfall.
In the case of unpaved roads, water bars and rolling drainage dips that remove water directly from the road surface to the adjacent land are also important. Another important measure, especially for unpaved roads without well-developed drainage systems, is the infiltration bunds running parallel to the road. These are stone lines that slow the road runoff and help it infiltrate (see Chapter 9).
Road drainage systems offer a number of opportunities to make a beneficial contribution to watershed management, as listed below:
Table 3.3: Recommended practices for road drainage systems and water harvesting
|Systematically collect and divert runoff||The road drainage system is a mechanism to effectively collect and divert all water that is gathered around the road. If adequately designed, it can help to “harvest” a large part of the runoff from the catchment uphill of the road and avoid waterlogging upstream of the road. The capacity of the road drainage system should be sufficient to remove the peak runoff in time.|
|Disposing runoff in areas where it is used beneficially||Where the water from the road is disposed of is important. The road drainage water should not be disposed of in areas where it creates damage or where it serves no useful purpose, but should be led to agricultural areas, tree plantations, rangeland, recharge areas, or ponds. It is also important that road drainage water (including the sediment and other particles it carries) is not directly discharged into streams where it will create turbidity and cause stream sedimentation.|
|Having an adequate number of outlets from the drainage system||It is important to have adequate outlets from the road drainage system in order to distribute the water over a wide area rather than having it become too voluminous to handle from a limited number of outlet points.|
In a few cases, however, on steeper and erodible slopes where gullies develop below each cross-drainage structure, it is suggested that runoff be concentrated at fewer points that lead to well-reinforced waterways. This helps to optimize resources in waterway stabilization.
|Reduce the hydrological connectivity of the road network||To keep hydrological connectivity low, it is important that the runoff collected from road drainage be diverted/distributed to vegetative bunds, pasture, or farmland and not allowed to quickly discharge in local streams and drains. This prevents the rapid buildup of flood peaks after rainfall in the watershed.|
|Harvesting sediment from road drainage systems||Road drainage systems can also be used to collect sediment. This requires the side-drain slope to be broken with drop structures such as scour checks. Sediment will be deposited in the flat sections. This can be collected and used as building material, particularly in the vicinity of towns. Sand harvesting from road drains can be an important job opportunity.|
As part of designing drainage systems, decisions on the number and location of culverts and other cross-drainage structures along a road body have an important effect on the opportunities to collect water, retain moisture, and control erosion and sedimentation. Cross-drainage structures are commonly placed in line with existing natural drainage paths such as gullies and small streams. They allow water from the upper catchment to pass underneath roads and connect directly to the downstream portion of the catchment. Because of cost, the number of cross-drainage structures is often minimized. These cross drains then commonly constitute bottlenecks, whereby naturally distributed runoff water is concentrated at a few points.
Culverts are important elements in road water management. They can be major sources of damage where they discharge water in an uncontrolled manner, but they can also guide the runoff from the catchment to places where it is used beneficially. Culverts, including their bed-sill and some of the ancillary structures, can also help control erosion.
There are a number of important recommended practices:
- Culverts should be placed on drainage lines. As obvious as it sounds, culverts are often incorrectly located in the road design and construction process.
- There should be an adequate number of culverts. As discussed earlier, if the number of cross-drainage points along a road is limited and natural drainage patterns are distorted, highly erosive runoff and flooding are likely to occur by concentrating runoff in a limited number of culverts. This should be considered.
- Culverts should be installed close to productive land, storage ponds, and recharge areas and, when necessary, equipped with diversion canals to take the runoff water to benefitting areas and structures such as ponds or infiltration trenches.
- Proper design and protection measures on culverts upstream are required (Berhe 2018) to guide water with controlled velocity through the culvert. These may comprise:
- Widened intake channels to slow the runoff passing through the culvert; and
- Protection of the intake channels with riprap or vegetation to avoid erosion.
- Proper design and protection measures on culverts downstream are required, depending on the soils and slope immediately downstream, such as:
- V-shaped floodwater spreaders (see picture);
- Check dams and (stepped) drop structures;
- Riprap protection; and
- Diversion channels to take runoff to benefitting areas or storage/recharge structures.
- Proper design of the culvert is required, particularly in steep, erodible streams. The lower sill of the culvert can be raised and help to stabilize the stream and prevent further scouring out of the stream.