Erosion control in roads

 

Erosion control in roads

The control of water and soil is basic to road.

Types of erosion

a.      Wind erosion

b.     Rain erosion

Factors controlling wind erosion

a.      Wind velocity

b.     Particle size of the material exposed to wind

c.      Moisture content of soil

d.     Vegetative cover

Factor controlling rain erosion

a.      Potential of flowing water

b.     Physical features, topography, gradient, embankment, slope

c.      Soil erodibility governed by clay content, particle size distribution, compaction, permeability, shear strength parameters.

Erosion control system

a.      Traditional/conventional method

b.     Agronomic Method (simple vegetative turfing, mulching of slope)

c.      Non-agronomic method (proper roadside drainage, soil cement stabilization, provision of apron)

d.     Reinforced vegetation (geo-textiles, synthetic jutes)

e.      Geotextiles

Labor-based construction method means utilization of optimum mix of labor and equipment in the delivery of public infrastructure wherever this is found to be technically, economically and socially viable.

Use of local resources

Promotion of social protection issues

In practice soil erosion problems related to road works are categorized into three areas based on the peculiarities of their erosion problems and the required mitigation measures thereof.

1.     Upper Catchment

Runoff from upper catchment areas often leads to excessive wash away or erosion in the road reserve damaging drainage structure. Main factors contributing to problems are:

a.      High run off rate caused by naturally low water retention capacity in the catchment areas due to poor vegetation cover and/or impervious soils and hilly terrain.

b.     Excessive water runoff because of poor land management practices resulting in low water retention capacity in cultivated fields and grazing land and from built up areas (roofs, footpaths and cattle tracks etc.)

c.      Runoff from cultivated land, grazing areas, foot path and cattle track etc adjacent to road carries sediment and transport it towards the road resulting in the siltation of road drainage structure.

d.     Lack of appropriate soil and water conservation measures.

Responsibilities for control

In case of excessive runoff and silt transport towards the road, land owners are generally required to carry out soil and water conservation activities on their land.

In some cases, road agencies construct secondary drains (cut off drain/catch drain) on the upper catchment areas to reduce the amount of water and silt flowing into road to safely channel these to an appropriate location.

2.     Road Reserve

Typical soil erosion problems on the road servitude include

a.      Scouring/gullying in side drain

b.     Scouring in culvert inlets and outlets

c.      Gullying on culvert outlets

d.     Scouring of bridges wing walls and abutments

e.      Siltation of culvert and drains

f.      Slope failure on embankment

g.     Failure of surface layer and underlying layers of pavement

Responsibilities for control

Road agencies and local authorities are responsible for the soil erosion control measures needed on the road under their jurisdiction.

3.     Lower catchment

a.      Gullying of culvert outfall

b.     Flooding and silt deposition causing damage to crops and property

c.      Embankment failure

d.     Entry of water into subgrade

Responsibilities for control

Initial construction of water ways with the required erosion control measures (scour checks, check dams and slope stabilization) is the responsibility of road authorities.

Grass planting in off-carriage way drains and on embankments including waterways to be responsibility of land owner.

 

Soil erosion control related to road works

Identify erosion prone areas such as high rainfall areas, hilly areas with unstable slope, deforested areas and areas with easily erodible soil types.

A.    Gully Prevention

Potential erosion problems can be determined using historical weather data, topographic and geographic maps, land use pattern, visual inspection and or through community informants. The working principle for gully management is therefore “arrest before it is established and control it when it exists”

In all works on upper catchment, road servitude and lower catchment area aim at preventing new gully formation.

Within road reserve

Active gullies, threatening to destroy the road, must first of all stabilized to prevent further damage. In addition, reclamation work is often needed in gullied road side drain to bring the road back to its original standards.

In lower catchment- Control, stabilizing

Principles for gully control

There are basically two ways for gully control.

a.      Reduce runoff volumes entering the gully

b.     Reduce erosive power of the water flow by installing scour check/check dam/drop structure which create steps that dissipate energy of the water flow allowing vegetation to establish and stabilize the gully.

Types of gullies

V- shaped

U-shaped

Gullies are established gradually and if not treated in time can grow out of hand engulfing large areas of land.

Selection of gully control measures

Types	Suggested Measures
A.    V-shaped gullies	Install check dams And plant grass in trapped sediments
B.    U-shaped gullies	No much need, since they do not manifest active erosion and usually stabilize on their own. Plant vegetative
C.    In gully heads	Dam construction, sloping and protecting channel, construction of dropping structures

Method of estimating water-runoff rates

1.     Rational Method

     Q =  (CIA)/360

c----surface runoff coeff,  A----area,  I------------rainfall intensity (mm/hr)

2.     Modified rational method

3.     Pikes formula

Q = C * A (0.5)

 

4.     Cook’s method

 

A.    Primary Drainage Structure

1.     Road Surface Camber

2.     Side drain

3.     Mitre drain

4.     Catch drain

5.     Infiltration ditch

6.     Soakaway pond

7.     Culverts Waterway

Mitre drain leads the water out of the side drains and safely disperse it on adjoining land.

 

B.    Secondary soil erosion control components

a.      Growing vegetation

b.     Hydroseeding, geotextiles

c.      Stone pitching, stone lining

d.     Scour checks (raised) **reducing water flow velocity by creating small water fall

e.      Scour checks at level

f.      Check dam

g.     Gully head control structure

h.     Gabion structure

Gully head control can be achieved in 3 ways

a.      Dam construction downstream of the gully head

b.     Sloping and protection of gully head

c.      Construction of fall structures