Surface drainage is the removal of water from the surface of the land. The water may be from excess rainfall, over irrigation, ,losses from conveyance channel and storage systems or seepage from areas at higher elevations. Flat or level land having impermeable sub –soil with shallow topsoil frequently requires surface drainage. Open drains are advantageous for removing large volume of either surface or sub-surface water.
Disadvantages:
1. They cut away farm area into fragments and interfere with movement of machinery.
2. They require frequent cleaning.
3. They harbour and distribute obnoxious weeds.
Surface Drainage Comprises:
1. On-farm Field Drainage System ( Rainfed Drains) :
Consisting of graded channels that collect the excess water from fields.
2. Intermediate Drains ( Collection or Carrier Drains):
Which are link drains between various field drains and sub-main drains or main drains.
3. Main Drains:
They are either excavated or natural drains collecting water from link drains or directly from field drains.
4. Interceptor Drains:
Also called seepage drains are located at foot hills to intercept seepage from hill sides/ earthen reservoirs, canals and intercept surface or subsurface flow.
5. Land Grading and Smoothening:
To produce a level or gently sloping land surface including a number of cultivated fields which many times are irregular , consisting of randomly distributed elevated areas and depressions. It results in ponding of water, inefficient irrigation, interrupted surface drainage, difficulties in mechanized field operations resulting in reduction in crop yield. The construction of the drain is always started at the outlet and progresses upstream.
Hydraulic Design of Open Drains:
The design discharge capacity of the channel could be estimated from the value of the drainage coefficient adopting the curve no. method. Normally the design value of the water face in the ditch is kept about 30 cm below the surface of the fields to be drained.
Ditch Cross Section:
Open ditches for drainage are designed with trapezoidal cross-section. The size of the ditch will vary with the velocity and quantity of water to be removed. The manning’s formula is used for design.
For the ditches with bottom width of 1.2m or less the value of Manning’s is assumed as 0.04 and for larger ditches the usually assumed value is 0.035.
The depth of the channel at all points along it should be sufficient to adequately drain the area. Usually serving as outlet foe title lines must be of depth 1.2 m or more. A minimum of 30 cm between the tile flow line and the normal water flow in the channel should be provided.
Channel Side Slopes:
Channel side slopes are determined by the soil texture and stability. Recommended side slope are 1:1 for silt loam and 2:1 for loam and 3:1 for loose sandy soils.
Channel Grades:
Channel grades are determined by local topographic and soil conditions. The grade of the ditch must follow slope of ground surface. Normally 0.1 to 0.2% gradient provided.
Permissible Velocities:
Permissible velocities in open ditch drains for different soil textural groups are –
Types of Soils | Max velocity m /sec |
Sand and Sandy loam | 0.75 |
Silt Loam | 0.90 |
Sandy clay loam | 1.00 |
Clay loam | 1.20 |
Stiff clay | 1.50 |
After the channel grade, depth and side slopes are selected, the bottom width can be computed for a given discharge.
For most efficient cross-section , and minimum volume of excavation the bottom width is determined by,
b= 2d tan tita /2
Where, b=bottom width
d= depth
tita= side slope angle
