A highway drainage system controls water both above and beneath the roadway to maintain the integrity of the road. A drainage system intercepts and diverts water from the road surface and allows it to safely dissipate. Without the proper drainage system, any road surface would become flooded and would eventually fail as the supporting sub-grade layers are washed out from under it.
How it Works
When a rain storm occurs, some of the water remains on the surface of the soil while the remainder of it seeps down until it reaches the water table. Removal of water from a roadway and adjacent land is referred to as surface drainage while removal of water from the soil and the sub-grade of a road are referred to as the sub-drainage system. Excess moisture trapped within the subgrade can create a significant amount of instability. If the problem is not corrected, the pavement will fail from sub-grade erosion.
Asphalt pavements that are in continual contact with water will eventually be stripped of their bituminous element, leading to potholes, loosening of the surface layer and general degradation of the road itself. In places where seasonal temperatures drop below 32 degrees, the freeze-thaw cycle can result in further cracking and erosion of the road.
Planning and Execution
Because the installation of a road has an impact on the drainage process of an area, it is extremely important to compensate for water flow when planning a road. Excess water within the sub-grade of a road will weaken the roadway material and could lead to premature failure. As a general rule, the natural drainage systems of an area should not be altered.
In order to remain effective, a drainage system must meet the following criteria:
- It must follow the natural drainage patterns of the area as closely as possible.
- It must transport both surface and subsurface water away from the pavement and allow it to drain in a manner that avoids build-up of water in vulnerable areas to prevent erosion.
Surface water is collected in drains running along the side of the road and is rerouted to the nearest water course. If a water course needs to cross the roadway, a culvert or a small bridge may be installed. If the width of the perpendicular water course is less than six meters, a culvert may be used. Waterways greater than six meters require small bridges to span them.
Subsurface water is managed by lowering the water table with a subsurface drainage system using pipes, controlling the rise of capillaries and controlling the amount of seepage allowed to permeate the area.