The following article will guide you about how to design and construct open ditches for drainage of agricultural lands.
Design Discharge for Open Ditches:
The rate at which the open drains have to remove water from an area depends upon-
(1) Rainfall,
(2) Size of the drainage area,
(3) Characteristics of the drainage area, and
(4) Nature of the crops grown including the degree of protection needed for them from flooding.
In watersheds with average slopes of lands more than 1 to 2 per cent, the peak rate of runoff to be handled is calculated by the runoff estimation methods. In flat areas (slopes less than 1 per cent) consideration has to be given to the time interval for removing a certain volume of excess surface water occurring with a certain probability. The runoff for open ditch design may be expressed as drainage coefficient.
Drainage Coefficient:
The concept of drainage coefficient is used in the design of drainage systems for agricultural lands. The drainage coefficient is defined as the depth of water in cm (or inches) to be removed in 24 hour period from the entire drainage area. It is also expressed as the flow rate per unit area (cubic m per second per sq. km).
The drainage coefficient is decided such that no appreciable damage is caused to the crops grown in the area. The intensity of rain and its duration are inversely proportional to the time allowed for removal of water (which depends upon the kind of crop grown).
In deciding the drainage coefficient of an area past experience with similar soils, climatic conditions and crops is useful. For open ditches for small agricultural areas the value ranges from 0.6 to 2.5 cm and in extreme cases upto 10 cms.
Example 1:
A watershed of 1500 hectares is discharging through a drain at an average rate of 2.5 m3/s. Calculate the drainage coefficient. If the drainage coefficient is 3 cm, what would be discharge through the drain?
Solution: 
Some of the methods for determining the design capacity of open ditches for drainage are as follows:
1. The Cypress Creek Formula:
This formula in English units is given by-
This formula is entirely empirical and has been derived upon a large number of observations carried out in U.S.A. The value of Q is not the same as peak discharge and it may so happen that its value will be much less than the peak discharges as certain amount of flooding is allowed before the water is drained.
For using the formula in a particular region, the values recommended for that region should be known. For example – the value of C is related to the rainfall excess (Re) by the following relationship for the south eastern part of U.S.A.
The rainfall excess is expressed in inches and is determined from the maximum 24 hour storm.
2. The Boston Society Formula:
The surface drain design given by the Boston Society of Civil Engineers’ is –
Where, Q is the peak discharge in cusecs, C is a coefficient and A is the peak catchment area in sq, miles. Uppal and Sehgal (1965) gave values of C (Table 17.2) for adoption in Eq. 17.3. The values proposed by them are mainly applicable for the catchments in different parts of Punjab and Haryana States in India.
In order to economise the construction of drainage system, sometimes the drains are designed for a discharge varying from 1/4 to 1/12 of the calculated peak discharge. The practical consideration is to clear the heavy storms from the fields with standing crops within seven days.
The drainage systems on Sarda Canal Project in Uttar Pradesh (India) were designed for a capacity of 0.11 cu.ms–1 km–2 of catchment, whereas the drainage systems in Punjab were designed for a capacity of only 0.04 cu.m s–1 km–2 of catchment.
3. Simplified Hydrologic Procedure:
The procedure given by Raadsma and Schulze (1974) consists in analyzing the rainfall data and estimating the number of hours required to remove the excess water using the information about crop tolerance.
The rainfall data are analysed for duration-frequency. As drainage is to be planned taking the crop into consideration, the rainfall duration-frequency for the particular crop season is considered.
The rainfall excess is calculated and an allowance is made for channel storage. Knowing the drainage coefficient i.e., the depth of water to be removed in 24 hours, the capacity of the drainage system needed can be known. The following example illustrates the method. The data assumed is hypothetical.
In the above example, storage of 10 mm is uniformly assumed in the surface channels. The information given in Table 17.3 is plotted as shown in Fig. 17.6. In addition, drainage capacities are plotted.
These graphs can be used to determine the drainage capacity or for a given drainage capacity, the number of hours required to remove the excess surface water. Referring to Fig. 17.6, for example with a drainage capacity of 30 mm/24 hours, a rainfall excess occurring once in 5 years can be drained in about 40 hours.
The method is useful when a single crop is involved and its drainage coefficient is known. When more crops are involved the calculations are to be done for each crop separately.
Pai and Hukkeri (1979) recommend that the drains should be designed for 1 day average of 3-day maximum rainfall of 5-year return period with a runoff percentage, calculated after taking into account infiltration rate, evaporation and permissible storage in water in the fields or by rainfall-runoff observations. If no dependable data are available, the runoff percentage for different types of soils and the intensity of vegetation as given in Table 17.4 are assumed.
The period of disposal of rainfall depends upon type of crops and stage of their growth. Values recommended by them are given in Table 17.5.
Design of Open Ditches:
The open ditches for drainage should satisfy the following conditions:
1. Adequate capacity to carry the design discharge.
2. The velocity of flow should neither cause scouring nor silting in the channel.
3. The side slopes should be stable.
Channel Cross Section:
For drainage ditches, the most convenient cross-section for excavation and maintenance is the trapezoidal section.
The design of the ditch section is done using Manning’s formula as done for irrigation channels. In case of drainage ditches, it may not be possible to use the most efficient hydraulic section. This is because the side slopes have to be stable from the soil point of view.
The depth of the ditch cannot also be fixed as deeper depths are to be adopted to get the desired grade. If the open ditch is to serve as an outlet for any subsurface drainage system, again deeper depths are to be adopted.
Ditch side slopes should be stable and the stability depends upon soil texture. Side slopes tend to become unstable because of the pressure of the water entering the sides of the ditches.
Sandy soils because of lack of cohesiveness are relatively unstable than clayey soils. Stabilization of the side slopes by compacting the surface by mechanical means can be done but this is costly and also the effect may not last long. Suggested side slopes for narrow and deep ditches for different soils are given in Table, 17.6. These are to be used as a general guide.
The grade given to the ditches is controlled by natural topographic conditions. The grade should be such that the velocity of flow in the channel should neither cause silting nor scouring of the bed and the banks.
The grade should also permit the outflow of the drainage water into a natural outlet wherever such outlet is available. The drainage ditches as they flow only intermittently are likely to be infested with weeds.
These weeds retard flow and as such it is generally recommended to adopt slightly higher velocities than the soils can withstand from scouring action.
Table 17.7 serves as a guide for selecting the safe velocities for ditches in different soils. While there are no standard recommendations on the minimum velocities, generally velocities of the order of .6 to .9 m per second are found to be non-silting.
As in the design of grassed waterways and irrigation channels, in case of drainage ditches also a free board is provided to the designed depth. A 20 per cent free board to the designed depth is generally enough. This will help as a safety factor for discharge as well as for sediment accumulation.
Alignment:
Drainage ditches serving individual fields could be straight but drainage ditches conveying water over longer distances will have bends. Where change in direction is necessary, a gradual curve for the ditch is to be adopted to prevent erosion of sides.
The radius of curvature (R) is expressed by the degree of the curve (D), which is defined as the angle subtended at the centre of a circle by a 30 m chord (Fig. 17.8).
The recommended range for drainage ditches is from 4 degrees for large capacity ditches or for channels with steep side slopes to 20 degrees for ditches with small capacity or with relatively flat sides slopes.
If the gradual curvature does not eliminate erosion, the velocity is sometimes reduced by increasing the width or side slopes or bank protection measures are used along the curvature.
Example 2:
Design an open ditch to drain 550 hectares of land having a drainage coefficient of 2.5 cm. The soil is silt loam and maximum permissible slope of the channel bed is 0.1 per cent.
Construction of Open Ditches:
The methods adopted for construction of open ditches depend mainly on the size of the drains and the availability of equipment. Small field ditches are constructed using manual labour.
A wide variety of mechanical equipment is used for excavation of the ditches.
Some of these equipment are:
(1) Drain ploughs,
(2) Crane shovels, and
(3) Scrapers.
The drain plough is a V-shaped tool drawn using a crawler tractor. This is used for constructing smaller size field drains.
The crane shovel consists of a lower travel unit which can be either wheel type or crawler type. Mounted on the lower travel assembly, it has an upper revolving frame. Different forms of end attachments are mounted on the top of the upper revolving frame.
While there are several types of front end attachments, the three common-types used for excavating open drains are:
(1) Clam shell
(2) Dragline, and
(3) Backhoe.
The clam shell bucket has two halves hinged at the top. The buckets can be opened using the hinge and when drawn together a bowl shape is formed. The clam shell is useful for digging in relatively soft soils and also under watertable.
Excavation is achieved by draping the bucket into the soil with the buckets open. The weight of the buckets help in digging into the soil and also soil gets collected into the buckets while closing them using the closing line.
The drag lines are common equipment used for excavating open drains. The dragging action is used to load the bucket and hence the name drags line. To excavate the soil, the bucket is thrown away from the machine and using the drag cable, the bucket is drawn towards the machine.
The excavated soil can be deposited on either side of the ditch or on both sides. Because of the long boom the excavated soil can be deposited at certain distance from the machine. The dragline can be used for excavating soils under watertable also.
The back hoe to some extent incorporates the features of the dragline. For excavating the soil, the dipper stick is lowered and using the cable pulled or dragged towards the machine. After the dipper is filled, it is raised using the hoist cable and again using the dipper stick the soil is deposited at the required place.
As compared to the clam shell and the drag line, the back hoe can be used to dig harder soils as the weight of the boom is also used to force the bucket into the soil. More precision in digging is also possible with the back hoe as compared to the drag line.
The characteristics of operation of the different excavating machinery and other details are available in publications on earth moving machinery.