HIGHWAY  DRAINAGE SYSTEM-ALL YOU NEED TO KNOW

The system which is designed for road for proper collection, removal and disposal of surface as well as sub-surface water is called as road drainage. To maintain the stability and durability of the road, the road surface and its foundation should be kept as dry as possible.

This practice is only possible with proper and working road drainage system. Moreover a good drainage of road helps to maintain the highway transportation with minimal cost.

Highway drainage consists of removing or controlling surface water and subsurface water away from the road surface and the subgrade supporting it. Part of the rain water flows on the ground or road surface, while the other part percolates into the ground and reaches the ground water table, raising its level. The subgrade soil above the ground water table may raise through the soil pores due to the phenomenon of capillarity.

The construction of highways and urban roads generally requires the installation of some type of drainage system. It is also observed at the design of highway drainage systems is comparatively simple and the drainage works are relatively inexpensive.

However, highway drainage forms one of the most important operationsin connection with the construction of a road because it depends on the subsequent maintenance of the road.

As far as the highway drainage works are concerned, they may be grouped into four main heads as follows:

• Interception of surface waters which would flow across the road or along with it or would flood it.
• Surface drainage of rain  water from the road and its margin.
• Interception of seepage water.
• Under-drainage of the roadbed and its crust.

The groups (i) and (ii) are known as surface drainage and the groups (iii) and (iv) are termed as sub-surface drainage. It will also be necessary to provide suitable drainage structures in the form of culverts, bridges, and causeways at places where the watercourses or streams, or rivers cross the highway. These are special structures and they are covered up under a separate subject of engineering known as Bridge Engineering.

Types of Road Drainage :

1. Surface Drainage

It is the road drainage in which the surface water of the road is collected and disposed within right-of-way of the road.

Surface drainage is basically designed to prevent the flow of surface water to the shoulders, sub-grade or any other layer of the road surface.

Basic Functions of Surface Drainage :

• The maintain the road surface completely dry from water.
• To collect the drained off water from the road surface.
• Increase the stability of the road or highway.
• By using the gravitational force, carrying of collected water into nearby stream or river or nallah.

2. Sub-surface Drainage of Road

Sub-surface drainage is the system in which the sub-soil water from underside of road pavement is collected and removed efficiently. It is generally adopted to control the moisture content of road sub-grade.

If the moisture content in the sub-soil increases, it can weaken the road structure. Hence it becomes more important  to control the moisture of road sub-grade.

Factors increasing the sub-soil moisture content are as follows :

• Increase in ground water table.
• Water seepage from adjoining areas.
• Surface water percolation through joints and cracks.
• Rise of the moisture above ground water table which is caused by the capillary action.

Thus longitudinal drains, cross drain and impervious bituminous layer are provided to prevent the rise in sub-soil moisture or sub-surface moisture and its specially provided to control the capillary rise.

Requirements of a Good Highway Drainage System:

A good highway drainage system should fulfill the following requirements:

1. The surface water on the carriageway and shoulder should be drained off effectively as soon as possible.
2. The surface water from the adjoining land should be prevented from entering into the roadway.
3. The groundwater table should be maintained well below the bottom surface of the subgrade soil.
4. The capillary rise & seepage water should be controlled effectively.
5. The longitudinal drains & cross drains should have sufficient capacity to carry the collected water.
6. The longitudinal drains & cross drains should have sufficient bed slope for gravity flow.
7. The flow of water across the road surface & in the drains should not cause erosion.
8. Complex & costly cross drainage structures should be avoided as much as possible.
9. Intercepting drains ,Where the topography of the area is such that the water flows towards the roadway itself, it becomes essential to construct intercepting drains parallel to the ad, but outside the road limit, to intercept water before it reaches the road.

Need and Importance of Highway Drainage:

The continued presence of water on the road surface weakens the pavement causing pot holes and ruts; similarly, the presence of water in the subgrade reduces its bearing power and load dispersion capacity. Loss of subgrade support leads to the failure of the road pavement under traffic loads. Hence efficient drainage is an imperative need.

Lack of drainage or inadequate drainage has been the primary reason for the failure of highway pavements. The importance of drainage in the successful maintenance and operation of highways is reflected in the adage- “There are just three factors necessary for a good road: drainage, drainage and more drainage.” Road builders of ancient era like the Romans and Aryans themselves understood the importance of drainage; pioneers of the modern era like MacAdam, Telford and Tresaguet also appreciated the need of good drainage for the success of a roadway.

In fact, constructing an efficient drainage system for the road is considered to be a cheaper, yet effective method to enhance its life than the current practice of the designing pavements for soaked subgrade conditions, which leads to the formation of thicker road sections.