(SEM VII) THEORY EXAMINATION 2021-22 IRRIGATION AND WATER RESOURCE ENGINEERING

IRRIGATION AND WATER RESOURCE ENGINEERING – KCE 078

B.Tech (Semester VII) – Detailed Answers

SECTION A – Descriptive Answers (2 Marks Each)

a) Probable Maximum Precipitation (PMP)

Probable Maximum Precipitation (PMP) is defined as the greatest depth of precipitation that is physically possible over a given area for a given duration. It is estimated based on meteorological data such as moisture availability, storm characteristics, and atmospheric conditions. PMP is mainly used for the design of large hydraulic structures like dams and spillways where failure can cause catastrophic damage.

b) Water Budget Equation

The water budget equation represents the balance between incoming and outgoing water in a hydrological system. It is expressed as:
P = R + E + ΔS,
where P is precipitation, R is runoff, E is evapotranspiration, and ΔS is change in storage. This equation helps in water resource planning and management.

c) Assumptions of Unit Hydrograph

The main assumptions of a unit hydrograph are:

Rainfall excess is uniformly distributed over the catchment.

Rainfall intensity remains constant during the duration.

The catchment characteristics do not change with time.

The principle of superposition is applicable.

d) Trickle Irrigation System

Trickle irrigation, also known as drip irrigation, is a method in which water is applied slowly and directly to the root zone of plants through emitters. This system minimizes water loss due to evaporation and percolation and improves water use efficiency.

e) Lacey’s Silt Factor

Lacey’s silt factor represents the effect of silt size on the design of regime channels. It is given by
f = 1.76√d,
where d is the mean particle size in millimeters. It is used in Lacey’s equations for channel design.

f) Canal Regulation Works

Canal regulation works are structures constructed to control, regulate, and distribute water in canal systems. These include head regulators, cross regulators, escapes, and outlets to ensure proper irrigation and prevent damage.

g) Silting and Scouring in Canals

Silting refers to the deposition of sediment in canals due to low velocity of flow, while scouring is the erosion of canal bed and banks caused by excessive velocity. Both affect canal efficiency and stability.

h) Objectives of Diversion Headworks

Diversion headworks are constructed across rivers to raise the water level and divert water into canals. Their objectives include irrigation supply, flood control, sediment control, and regulating river flow.

i) Specific Capacity of a Well

Specific capacity of a well is defined as the discharge per unit drawdown of water level in the well. It is an indicator of well performance and aquifer productivity.

j) Specific Yield

Specific yield is the ratio of the volume of water that drains out of a saturated soil or rock under gravity to its total volume. It indicates the water-yielding capacity of an aquifer.

SECTION B – Long Answers (10 Marks Each)

a) Synthetic Unit Hydrograph

A synthetic unit hydrograph is a unit hydrograph derived using empirical relationships rather than actual rainfall-runoff data. It is useful for ungauged catchments. The synthetic unit hydrograph can be derived from a number of unit hydrographs by averaging their ordinates. This method involves selecting representative catchment parameters such as area, slope, and time of concentration and preparing a tabular hydrograph.

b) Definitions

i) Depth Area Duration Curve
It shows the variation of rainfall depth with area for a given storm duration.

ii) Probable Maximum Precipitation
The maximum possible precipitation under extreme meteorological conditions.

iii) Evapotranspiration
The combined loss of water through evaporation from soil and transpiration by plants.

iv) Φ-Index
It is the average rate of infiltration such that rainfall in excess of this rate produces runoff.

c) Water Logging: Problems, Effects and Remedies

Water logging occurs when the groundwater table rises close to the ground surface, saturating the root zone. It reduces soil aeration, decreases crop yield, causes salinity, and damages structures.
Remedial measures include surface drainage, subsurface drainage, canal lining, regulated irrigation, and pumping of groundwater.

d) Design of Trapezoidal Channel Using Lacey’s Theory

Using Lacey’s equations, the wetted perimeter, hydraulic radius, velocity, and channel dimensions are determined based on discharge and silt factor. The design ensures stable regime conditions where neither silting nor scouring occurs.

e) Short Notes

i) Well Shrouding and Well Development
Well shrouding prevents entry of fine particles, while well development improves permeability around the well.

ii) Types of Open Wells
Dug wells, tube wells, and artesian wells.

iii) Infiltration Galleries
Horizontal structures used to collect groundwater near rivers.

iv) Hydraulic Conductivity
It is the ability of soil or rock to transmit water.

SECTION C – Very Long Answers (10 Marks Each)

3(a) Optimum Number of Rain Gauge Stations

The optimum number of rain gauges is determined using the formula:
N = (Cv / ρ)²,
where Cv is coefficient of variation and ρ is allowable error.
Using the given rainfall data, the coefficient of variation is calculated and substituted to determine the required number of gauges for 10% error.

3(b) Infiltration

Infiltration is the process by which water enters the soil surface. Factors affecting infiltration include soil type, moisture content, vegetation, temperature, and rainfall intensity. Measurement methods include double ring infiltrometer and flooding method.

4(a) Methods of Irrigation & Sprinkler System

Methods of irrigation include surface irrigation, subsurface irrigation, sprinkler irrigation, and drip irrigation.
Sprinkler irrigation applies water under pressure through nozzles, simulating rainfall. It ensures uniform distribution and high efficiency.

4(b) Crop Rotation

Crop rotation is the practice of growing different crops in a sequence on the same land. It improves soil fertility, reduces pests, and increases yield. Example: cereal–legume rotation.

5(a) Discharge of Water Course (Kor Period)

The discharge is calculated using kor depth, kor period, culturable command area, and intensity of irrigation. Separate demands for crops A and B are calculated and added to obtain total discharge.

5(b) Regime Channel

A regime channel is one which maintains its shape and slope over time.
Initial regime occurs when channel is adjusting, while final regime is achieved when equilibrium is established as per Lacey’s theory.

6(a) Perennial vs Inundation Canal

Factors for canal alignment include topography, soil condition, command area, and economy.

6(b) Design of Lined Triangular Channel

Using Manning’s equation and given slope, discharge, and roughness coefficient, channel dimensions are calculated to safely carry the required discharge.

7(a) Confined and Unconfined Aquifer

A confined aquifer is bounded by impermeable layers, while an unconfined aquifer has a free water table.
The discharge through a confined aquifer is derived using Darcy’s law.

7(b) Definitions

Aquifer: Water-bearing formation

Aquiclude: Impermeable formation

Aquitard: Semi-permeable layer

Aquifuge: Neither stores nor transmits water

Porosity: Ratio of void volume to total volume