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Road (Highway) Drainage: Surface drainage system, types, methods & diagram

What is highway (Road) Drainage?

The term drainage is defined as the interception and removal of water from, over, and under an area. Hence the highway drainage the process of the removal of excess water from the road surface and also from the road subgrade.

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 operations in 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 rainwater 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.

In this post, the problems related to the highway drainage consisting of surface drainage and sub-surface drainage will be discussed.  


Following are the main four sources of water which enter the road structure.

(1) Capillary action of water: The water which is existing in the soil rises due to the capillary action and enters the subgrade portion of the road structure.

(2) Floods: Due to heavy rains, there is overflowing of culverts and bridges along the road and thus the water covers the road surface at the time of such floods.

(3) Rain water falling on road surface: The part of rain water which directly falls on the road surface may percolate through the body of the road structure.

(4) Rain water from surrounding area: The rain water accumulating on the surrounding area finds its way to the subgrade of the road structure.

Defects due to Improper Highway (Road) drainage

The water should be prevented from reaching the road structure wherever possible or attempt should be made to remove it quickly from the road surface by laying a well-designed drainage system. The improper highway drainage leads to the deterioration of the highways in the form of the following defects:

  • It allows the washing out of highway portions and causes excessive erosion leading to the formation of gullies along the roadsides or road embankments.
  • It causes considerable damage to the shoulders and pavement edge due to the presence of excess water.
  • It causes the failure of bituminous pavements due to stripping of bitumen from aggregates like loosening or detachment of some of the bituminous pavement layers and formation of pot holes.
  • It is the prime cause of failures in rigid pavements due to mud pumping by the presence of water in fine subgrade soil.
  • It leads to the failure of earth slopes because excess moisture causes increase in weight and thus the stress is also increased which ultimately reduces the strength of the soil mass.
  • It makes to the formation of waves and corrugations in flexible pavements.
  • It leads to the freezing action due to moisture held in the soil and results in heaving with consequent breaking up or shattering of road surface or pavement.
  • It makes the road surface soft due to submergence in water for a long time and because of the heavy movement of traffic, the road surface cracks or settles down and the patches formed on it, develops an ugly and undesirable surface.
  • It makes the road surface soft especially when constructed of the soil itself or of porous materials such as sand clay, gravel, broken stone, etc.
  •  It softens the subgrade soil and decreases its supporting power or bearing capacity.


Following are the essential requirements of a good highway drainage system:

(1) Adjoining land: The surface water from the adjoining land should be prevented from entering the roadway.

(2) Camber: The road surface should be provided with suitable camber so as to drain off quickly the water that falls on it without allowing the water to percolate.

(3) Cross-drainage works: The design of cross-drainage works should be such that the overflowing of water on the road surface does not occur at the time of the highest flood.

(4) Gradient: The roads passing through zones of heavy rainfall should be provided with minimum gradient even if it is not theoretically required.

 (5) Highest flood level: The carriageway should be provided least 600 mm higher than the highest flood level (H.F.L.) of the surrounding area.

(6) 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.

 (7) Side drains:  it is necessary to construct sufficiently wide and deep side drains with suitable longitudinal slope to carry away all the water that accumulates to some drainage structure The water level in these drains should remain at all times below the subgrade level

(8) Underground sources of water: All springs and underground sources of water should be tapped and the water should be drained off by the sub-surface drainage system.

(9) Water-logged areas: It is necessary to take special precautions in case of water-logged areas especially if detrimental salts are present or if flooding is likely to occur.

(10) Water table: The sub-surface drainage system should aim at keeping the highest level of ground water table well below the level of subgrade preferably by at least 1.20 m.

Surface Drainage System

The main object of surface drainage is to remove rain water from the carriageway as rapidly as possible so that traffic may move more safely and efficiently.

The provision of surface drainage is thought of at the initial stage of location or alignment of the highway. In fact, the surface drainage forms one of the essential consideration for the location of a highway and it is seen that all the streams flow away from the highway. Thus the drainage problem is reduced to tackling the water which falls within the roadway boundary only.

For the purpose of collecting the surface water, the longitudinal side drains or ditches are laid and the water is then disposed-off to the nearest stream, valley or water course. The side drains are usually V-shaped or trapezoidal, the capacity of trapezoidal shape being greater. They should be preferably lined, if possible, with rubble masonry work which may either be laid dry or joints filled with grout.

Read More: subsurface drainage system

(1) Side drains for road in embankment:

For roads in embankment, the side drains are provided on one or both sides of the road beyond the shoulder, as shown in fig. 2. The side drains are constructed at a minimum distance of about 2 m from the edge of embankment so that the water flowing in the drains does not enter the earthwork.

Road side drain for embankment filling road
Fig2. Road side drain for embankment filling road

These drains are also helpful in arresting the rain water falling on the adjacent land parallel to the road and it is thus not allowed to reach the embankment. The water flowing in the side drain can then be suitably disposed off without causing any harm to the roadway.

(2) Side drains for road in cutting:

For roads in cutting, the side drains are provided on either side of the formation, as shown in fig. 3. These drains are carefully designed and it is to be seen that they do not overflow under any circumstances and making the roadway submerged in water.

Road side drain for road in cutting area
Fig.3. Road side drain for road in cutting area

The open deep side drains may prove to be dangerous and unsightly especially in cases where there is a restriction of space. In such circumstances, the covered drains or pipe drains or ditches filed properly with suitable materials like coarse sand and gravel may be provided

(3) Design of side drains:  

Under normal conditions, it may not be necessary to go into the detailed design of computing the Capacity of side drains. But it is absolutely necessary to ensure that the slope of the drain in the longitudinal direction is sufficient to drain the water quickly so as to ensure an effective arrangement.

However the design of side drains is primarily based on the following two considerations:

(i )Hydrologic analysis:

 The hydrology is the study of how water moves over the surface of the earth and the laws and geographical distribution of the movement It includes the study of the quantity and rate of run off from precipitation, the magnitude of floods and the frequency of occurrence of these phenomena. The most critical problem in drainage design will be the estimation of peak rates of storm run-off from watersheds adjacent to the highway

(ii ) Hydraulic analysis:

The hydraulics is the branch of engineering mechanics which deals with the flow of liquids. As applied to highway drainage, it is primarily concerned with how to provide efficient and safe transport of water so as to avoid danger to property, highway structures or highway vehicles.

The cross-sectional area of side drain is then obtained by the application of the following formula:

Q = A V


Q = Design run-off in m/sec

A = Cross-sectional area of side drain in m2

V = Allowable velocity of flow in m/sec.

 It should be seen that the allowable velocity of flow in the side drain is neither too high so as to cause erasion nor too low to encourage silting.


The surface drainage of city or urban roads is radically different than that of the rural highways. In urban roads, the land width is limited and the open drains cannot be provided as they are unsightly, occupy more space, and serve as a source of danger to the traffic. The underground drains or sewers are, therefore, an essential requirement of street construction and are not so often necessary on rural highways.

Following are the two commonly adopted ways of providing drainage to the urban roads:

(1) Catch basins

(2) Inlets.

1. Catch basins:

A catch basin is a structure in the form of a chamber which is provided along the sewer line to admit clear rain water free from silt, grit debris, etc into the combined sewer Is provision on the sewer line serves the following two objects.

  • It prevents the entry of silt, grit, debris, etc contained in the rain water,
  • It prevents the escape of sewer gas.

Fig. 4 shows the details of a typical catch basin. It consists of a basin or chamber constructed of walls. At the bottom of the basin, space is provided for the accumulation of impurities. At the top of the basin, cover with perforations is fixed and at the pavement edge or curb, openings are kept to admit rainwater into the basin. A hood is provided to prevent the escape of sewer gas which may find its way through the sewer line.

Catch Basin for road drain system
Fig.4. Catch Basin for road drain system

The catch basin provides a temporary storage of impurities contained in rain water. Hence, it demands periodical cleaning. Otherwise the organic matter decomposes and gives out bad smell. It also then forms a breeding place for mosquitoes and causes annoyance to persons passing or living nearby.

2. Inlets:

An inlet is an opening through which stormwater is admitted and conveyed to the stormwater sewer. The inlets are located or placed by the sides of roads at a distance of about 30 m to 60 m. The inlets are so located that storm water is collected in a short period and there is no flooding or accumulation of a huge quantity of storm water on the roads. The inlets are connected to nearby manholes by pipeline.

An inlet is simply a concrete box. It may have gratings or openings in vertical direction or in horizontal direction. The former is known as vertical inlet or curb inlet and the latter is known as

horizontal inlet. The details of vertical inlet and horizontal inlet are shown respectively in fig. 5 and fig. 6. below

Vertical Inlet
Fig.5. Vertical Inlet
Horizontal inlet
Fig.6. Horizontal inlet

The inlet leads the storm water directly to the sewer and hence, its design should be made in such a way that the least opportunity is given for stormwater to stop. The openings of the inlet therefore should be of such pattern that the chances of clogging are brought down to the minimum possible extent.

The objects that cause most of the troubles at the openings of an inlet are sticks, waste papers, leaves, etc. The cleanliness of footpaths and streets is, therefore, most essential for the successful and efficient functioning of the inlets.

Subsurface drainage:

You can read subsurface drainage system In other post here [Subsurface drainage]

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