Test: Irrigation Engineering - Civil Engineering (CE) MCQ

Distribution Efficiency:
The water distribution efficiency represents the extent to which the water has penetrated to a uniform depth, throughout the field. When the water has penetrated uniformly throughout the field, the deviation from the mean depth is zero and the water distribution efficiency is 1.0.
Water Distribution Efficiency(η_d)

Where,
d = Average of the absolute values of deviations from the mean
D = Mean depth of water stored during irrigation
Calculation:
Given that,
Mean deviation from mean = 0.1 cm
Mean depth of water = 1.5 cm
∴ Water Distribution Efficiency(η_d)

η_d = 93.33%
η_d ≈ 93% 

Bligh’s Creep Theory:

• Bligh assumed that the percolating water follows the outline of the base of the structure which is in contact with the subsoil.
• The length of the path traversed by the percolating water is called the length of creep or creep length.
• He further assumed that the head loss per unit length of creep (i.e. H/L) which is called hydraulic gradient is constant throughout the percolating passage i.e. Loss of head is proportional to length of the creep.

Hydraulic Gradient (i) = 
The reciprocal of hydraulic gradient (i.e. L/H) is known as Bligh’s Coefficient of creep.
Additional Information
Percolation Coefficient:
It is defined as the ratio of coefficient of permeability to porosity of soil.

Where,
K = co-efficient of Permeability
η = Porosity.

Integrated Weed Management (IWM)

• It is a long-term management approach to managing weeds through certain techniques. These techniques are a combination of biological, physical, chemical, mechanical, and cultural practices. Overall, these help to improve crop production, revenue, and environmental condition. Before managing weeds, we should know the definition and impact of weeds on various components like the environment, economy, and other parts of society.
• Weeds are defined as unwanted plants in crop fields which grow along with the main crop. They are strong and dominating competitors for crops.

The proliferation of weeds

• Their copious seed production, quick population establishment, and adaptation characteristics make them tough competitors and help in their survival.
• It can submerge during the flood occur.
• Weeds can be controlled in many ways. Weed management includes land preparation, water management, hand weeding, hand hoeing, crop rotation, and herbicides.
• Land preparation helps in the removal of seeds and uprooting of weeds before sowing seeds of the main crop. Hand weeding is done manually which is very tiresome and time-consuming.
• Nowadays, weedicides and herbicides are most commonly used and are effective ways of weeding. Chemicals like MCPA and 2,4-D are some commonly used weedicides. They can be applied at three stages like before and after crop planting and after weed emergence. But these chemicals can cause health hazards and environmental issues.

Retaining wall:

• A retaining wall or retaining structure is used for maintaining the ground surfaces at different elevations on either side of it.
• Whenever embankments are involved in construction, retaining walls usually necessary.

Types of retaining wall:

• Depending upon the mechanisms used to carry the earth's pressure, These are classified into the following types.
  • Gravity retaining wall.
  • Cantilever retaining wall.
  • Butters wall.

Cantilever retaining wall:

• It is the most common type of retaining wall and its height ranges up to 10-25 feet (3 to 8m).
• Counterfort retaining walls are economical for height over about 6 m.
• A cantilever retaining wall resists the earth pressure horizontal & another, by the cantilever bending action.

Additional Information
Gravity retaining wall:

• It is not used for heights of more than 3.0 m.
• In it, the resistance to the earth's pressure is generated by the weight of the structure.

The plantation crop refers to those crops which are cultivated on an extensive scale in an area. In this type of agriculture, single crop is raised on a large area.
These crops include tea, coffee, rubber, cocoa, coconut, oil palm and cashew etc.
Other type of Crop categories in India:

• Food Crops: Wheat, Maze, Rice, Millet, Pulses.
• Cash Crops: Sugar cane, Tobacco, Jute, Oilseeds.
• Horticulture Crops: Fruits and Vegetables.

• Crop Period: The time period that elapsed for the instant of the sowing of the crop to the instant of its harvesting is called crop period.
• Base Period: The time between the first watering of a crop at the time of its sowing to its last watering before the harvesting is called the base period or the base of the crop.
• Crop period is slightly more than the base period, but for all practical consideration, they are taken as one and the same thing, and generally expressed in days.

Important Point:

• Duty: It is the number of hectares of land irrigated for full growth of a given crop by a supply of 1 cumec of water continuously during the entire base period of that crop.
• Delta: The total water depth required by a crop to attain its full maturity in its base period.
• The relation between Duty (D), Delta (Δ) and Base period (B) is given as:

For an irrigation land:

SC = Saturation capacity, FC = Field capacity, OMC = Optimum moisture content, PWP = Permanent welting point and UWP = Ultimate welting point

• Equivalent depth of water held at field capacity (x) = S × d × F_c
• Equivalent depth of water held at PWP (x’) = S × d × (PWP)
• Available moisture/storage capacity of soil (y) = S × d × (F_c - PWP)
• Readily available moisture content (d_w) = S × d × (F_c - OMC)

Given:
Field capacity (F. C) = 25 % = 0.25
Permanent willing point (PWP) = 15 % = 0.15
Depth of the root zone (d) = 80 cm
Dry Unit weight of soil (γ_d) = 1.5 g/cc

Calculations:

Hence, the storage capacity of the soil is 12 cm.

• The minimum furrow grade for effective surface drainage is 0.05%. This ensures that water flows away efficiently, preventing pooling and saturation.
• A slope below this may not provide adequate drainage, risking waterlogging and crop damage.
• While 0.5% is the maximum recommended slope to prevent soil erosion, the ideal furrow slope helps maintain soil integrity and promotes healthy crop growth.

Temporary spurs or bunds are temporary structures constructed every year after floods. A bund is a structure made to project flow from a riverbank into a stream or river with the aim of deflecting the flow away from the side of the river on which the groyne is built.

Important Points:

• Bunds are temporary in nature whereas Weirs and barrages are permanent in nature.
• In a weir, the water overflows the weir, but in a dam, the water overflows through a special place called a spillway. Weirs have traditionally been used to create mill ponds.
• A barrage is a weir that has adjustable gates installed over top of it, to allow different water surface heights at different times.

Based on Alignment Canals are classified into 3 categories. These are:
1. Ridge Canal, 2. Contour Canal and 3.  Side Slope Canal
Their characteristics are given below: