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Design of surface drains

Field drains for a surface drainage system have a different shape from field drains for subsurface drainage. Those for surface drainage have to allow farm equipment to cross them and should be easy to maintain with manual labour or ordinary mowers. Surface runoff reaches the field drains by flow through row furrows or by sheet flow. In the transition zone between drain and field, flow velocities should not induce erosion. Field drains are thus shallow and have flat side slopes. Simple field drains are V-shaped. Their dimensions are determined by the construction equipment, maintenance needs, and their cross ability by farm equipment. Side slopes should not be steeper. Nevertheless, long field drains under conditions of high rainfall intensities, especially where field runoff from both sides accumulates in the drain, may require a transport capacity greater than that of a simple V-shaped channel. Without increasing the drain depth too much, its capacity can be enlarged by constructing a flat bottom, thereby creating a shallow trapezoidal shape.

All field drains should be graded towards the collector drain with grades between 0.1 and 0.3%. Open collector drains collect water from field drains and transport it to the main drainage system. In contrast to the field drain, the cross-section of collector drains should be designed to meet the required discharge capacity. Besides the discharge capacity, the design should take into consideration that, in some cases, surface runoff from adjacent fields also flows directly into the field drains, which then require a gentler side slope. When designing the system, maintenance requirements must be considered. Attention must also be given to the transition between the field drains and the collector drains, because differences in depth might cause erosion at those places. For low discharges, pipes are a suitable means of protecting the transition. For higher discharges, open drop structures are recommended. Permissible values for average velocity of flow to avoid scouring may be adopted from Table 3.

Table 3. Maximum allowable velocities in channels for different soil textures.

SI. No.

Soil texture

Max. Allowable velocity (m/sec)

1.

Very light silty sand

0.30

2.

Light loose sand

0.50

3.

Coarse sand

0.75

4.

Sandy and sandy loan

0.75

5.

Silty loan

0.90

6.

Firm clay loan

1.00

7.

Stiff clay or stiff gravelly soil

1.50

8.

Coarse gravel

1.50

9.

Shale, hardpan, soft rock etc.

1.80

10.

Hard cemented conglomerates

2.50

Slightly higher velocities are allowed if water contains colloidal silt. If the land slopes are steeper to create scouring velocity, the same has to be reduced by a gentle slope of the drain through provision of suitable drops/falls in the channels.

 
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