What is Highway drainage?

Highway drainage is the process of extracting and regulating the excess surface water on the pavement by giving the desired slope. The construction of a highway drain helps to divert the water present on the surface, and sub-grade by the means of slope. Highway drainage systems are a necessary part of smooth functioning of traffic, since the highway drainage systems provide proper disposal of unwanted substances from the highways.

Importance of Highway drainage

The excess seepage of surface water increases the groundwater content that hampers the stability of the soil. The reasons to provide a drain and a slope at a highway are listed as below:

• To avoid pavement failure due to the presence of excess moisture in the soil subgrade.
• To reduce the moisture content in pavement materials, such as water-bound macadam.
• The variation in moisture content of clayey soil leads to a considerable increase in the volume of subgrade.
• The absence of drain can lead to the formation of waves and corrugate in the pavement.
• The surface water when comes in contact with bituminous pavement leads to the formation of potholes.
• The excess seepage of groundwater at the shoulder and pavement edges causes deterioration of road structures.
• The lack of proper drainage can further lead to soil erosion near the slope, embankment, and hillside region.

Requirements for Highway drainage

The requirements for building a good drainage design system are as follows:

• The presence of excess groundwater on the road surface from the carriageway and shoulder should be passed through the drain without any seepage to the subgrade soil, and avoid land corrugation.
• The surface water from the adjacent land should be prevented from entering the roadway.
• The maximum level of groundwater table should be kept below the subgrade.
• The drain must be sufficient enough to carry the surface water.
• Seepage of surface water should be drained off by a sub-surface drainage system and highway drainage systems.
• The catchment area near the road structures should be properly connected to the drainage system.
• The curb should not be raised too high above the road surface.
• A soak away must be built, so that the wastewater is piped into the sewer slowly from the surrounding soil.

Components of Highway drainage

Following are the components in highway drainage designs:

• Outlet ditches
• Side ditches
• The culvert
• Road structures
• Underdrains

Outlet ditches

Outlet ditches are those drainage structures that lead the water from the sides of the ditches toward the area of the road. The water from outlet ditches typically discharges to being raceway systems, similar to swash channels and lakes. The outlet ditch is an important part of the highway drainage system but is repeatedly ignored. However, it can produce significant problems to the road over a large area if the outlet is choked.

Side ditches

Side ditches collect surface groundwater and lead it toward outlet ditches. They are of utmost importance when the road is in the cut. However, the side ditches are not always necessary and their need has to be estimated case by case if the road is situated on a high embankment.

The culvert

The culvert is a pipe or box structure generally used as cross rainspouts for ditch relief and to pass water under a road at natural drainage and canal crossings. A culvert is defined as a culvert only if the clear opening is lower than 2 meters; and if it is more than 2 meters, then it is defined as a bridge structure. However, the culvert is defined as a pipe bridge, if the construction is a large pipe with a clear opening of 2-4 m. The shape of a culvert is generally around the pipe, but a culvert can also be pipe bows, structural bows, or boxes. The height and position of the culvert depend on the depth of the side ditch and the level of the drainage system. The culvert is placed at a right angle to the road alignment and can also be placed at an angle depending upon the local circumstances.

Road structures

Vertical road drainage structures include structures similar to sludge courses, special geotextiles, and special asphalt fusions that lead the water down from the road or cut the capillary rise connection from the subsurface to the top portion of the pavement structure. The filter course is the bottommost structural subcaste and is typically spread across the bottom of the excavation. A filter course should always be used in the road structure when the subgrade is frost-susceptible. The filter course is typically separated from the subgrade soil by a geotextile.

Underdrains

The underdrains are drainage structures generally used along roads in wet areas. They are a crucial part of the highway drainage systems. The purpose of these perpendicular drainage structures is to drain or remove the groundwater and keep the silt content dry under the road.

Design of highway drainage

The highway drainage designs are divided into two parts:

Hydrologic analysis

This analysis includes calculation of the expected quantity of water, considering the various factors that affect the run-off, such as rainfall, type of soil, moisture condition, topography, and so on.

The hydrologic analysis is calculated using a rational formula,

Q = C*i*A_d

Where,

• Q = run-off (m³/sec)
• C = highway runoff coefficient
• i = intensity of rainfall (mm²/sec)
• A_d = Drainage area in 1000 m²

The value of C varies depending upon the type of pavement:

• C = 0.8 to 0.9 (Bituminous and cement concrete pavements)
• C = 0.35 to 0.7 (Gravel and water bound macadam pavements)
• C = 0.4 to 0.65 (Impervious soil)
• C = 0.3 to 0.55 (soil covered with turf)
• C = 0.05 to 0.3 (Pervious soil)

Hydraulic design

The side drain and culvert are designed using the principle of flow through open channels. The highway runoff quantity Q is calculated by the formula,

Q = A*V

Q = runoff quantity of water (m³/sec)

A = Cross sectional area (m²)

V = Velocity of flow (m/sec)

Manning’s Formula is used to determine the velocity of flow,

V = *R^2/3* S^1/2

where,

• n = Manning’s roughness coefficient
• R = Hydraulic radius
• S = Longitudinal slope
• The value of m varies depending upon the type of soil:
• n = 0.02 (earth with heavy vegetation or grass)
• n = 0.05 to 0.1 (lined channels)
• n = 0.013 (well finished concrete)
• n = 0.04(rough rubble)

FHWA (Federal Highway Administration)

The FHWA is the body that provides control over the construction and protection of the country’s roadways, islands, and culverts. It is responsible for highway drain and Highway Drainage Systems. FHWA performs inspection and gives specialized support to state and verified agencies to improve and optimize safety, mobility, and to encourage discoveries.

Road drainage system

A system designed for roads for proper collection and disposal of surface as well as sub-surface water, is called road drainage.

Importance of road drainage

To maintain the steadiness and continuity of the road, its face and foundation must be kept as dry as feasible. This practice is feasible with the proper working of the entire road drainage system. Also, good drainage of the road helps to maintain trace transportation with minimum cost. When the substances on the highway going into the drain, it clears pathways which leads to fewer accidents. The substances from the main drain must drain into water bodies after being treated preferably to avoid unnecessary toxicity in them.

Context and Applications

The highway drainage system is of a great importance in the construction of pavements for structural engineers in highway design. Many research papers in civil engineering have been written on the risk management of roadways to prevent the drainage. Also, in Geomechanics, how soil is the most significant ingredient in the construction of the road is studied. The convex structure provided on the surface of the road in the transverse direction for water to drain off is known as cross fall or cross slope. The cross fall or cross slope is helpful during heavy rains to avoid flooding on the streets.

It is of a great importance in both undergraduate and postgraduate courses such as,

• Bachelors of Technology in Transportation Engineering
• Masters of Technology in Transportation Engineering

Practice Problems

1. What is the process of extracting and regulating the excess surface water on the pavement coming from the sub-base known as?

1. Highway engineering
2. Highway maintenance
3. Highway drainage
4. None of these

Answer: Option c

Explanation: The process of extracting and regulating the excess surface water on the pavement coming from the sub-base is called highway drainage.

2. Where is the seepage flow present?

1. Surface drainage
2. Subsurface drainage
3. Camber
4. Cross slope

Answer: Option b

Explanation: The seepage flow is present in the subsurface drainage, which is excavated from the soil.

3. For proper collection and disposal of surface as well as sub-surface water of the road, which system is designed?

1. Road engineering
2. Road maintenance
3. Road drainage
4. None of these

Answer: Option c

Explanation: For proper collection and disposal of surface as well as sub-surface water of the road, the Road Drainage system is designed.

4. Which of the following is used to calculate the velocity of Hydraulic design?

1. Manning’s formula
2. Chezy’s formula
3. Bazin’s formula
4. All of the above

Answer: Option c

Explanation: The velocity of in Hydraulic design is calculated using Manning’s Formula.

5. What is the full form of FHWA?

1. Federal Highway Administration
2. Federal Highway Action
3. Both a and b
4. None of these

Answer: Option a

Explanation: Full form of FHWA is Federal Highway Administration.

Related Concepts

• The sub-grade
• Bituminous Pavement
• The culvert (Hydraulic Conduit)
• Water Bound Macadam