(SEM VII) THEORY EXAMINATION 2017-18 WATER RESOURCES ENGINEERING

WATER RESOURCES ENGINEERING

B.Tech VII Semester – Theory Examination (2017–18)

SECTION A

(Attempt all questions – Answer in brief but descriptive form)

a) Hydrologic Cycle

The hydrologic cycle is a continuous natural process by which water circulates between the earth’s surface, atmosphere, and subsurface. It begins with the evaporation of water from oceans, rivers, lakes, and other water bodies due to solar energy. The evaporated water vapor rises into the atmosphere, cools, and condenses to form clouds. When the condensed moisture becomes heavy, it falls back to the earth as precipitation in the form of rain, snow, or hail. A part of this precipitation flows over the land surface as runoff, another part infiltrates into the soil to recharge groundwater, and the remaining portion returns to the atmosphere through evaporation and transpiration, thus completing the cycle.

b) Transpiration and Factors Affecting It

Transpiration is the process by which plants lose water in the form of water vapor through small openings called stomata present on the leaves. It plays an important role in the hydrologic cycle by transferring soil moisture back into the atmosphere. The rate of transpiration is affected by several factors such as temperature, humidity, wind velocity, availability of soil moisture, type of vegetation, and stage of plant growth. Higher temperature and wind speed generally increase transpiration, whereas high humidity reduces it.

c) Standard Project Flood (SPF)

Standard Project Flood is a flood estimate used in the design of hydraulic structures such as dams and spillways. It represents the flood that may be expected from the most severe combination of meteorological and hydrological conditions considered reasonably possible in a region. SPF is less severe than the Probable Maximum Flood (PMF) and is commonly adopted for medium-sized hydraulic structures where complete safety against extreme floods is not economically feasible.

d) Phreatic Line and Its Use

The phreatic line is an imaginary line that represents the upper surface of seepage flow through an earth dam or embankment. It separates the saturated and unsaturated zones within the structure. The position of the phreatic line is important in the design of earth dams because it helps in determining seepage quantities, pore water pressure distribution, and overall stability of the structure.

e) Crop Rotation

Crop rotation is an agricultural practice in which different crops are grown in a planned sequence on the same land over different seasons. This practice helps in maintaining soil fertility, improving soil structure, controlling pests and diseases, and optimizing water use. Crop rotation also reduces the excessive depletion of nutrients caused by growing the same crop repeatedly.

f) Economic Viability of Lining of Canals

Lining of canals is considered economically viable when the benefits obtained from reduced seepage losses exceed the cost of lining. Canal lining reduces water losses, prevents water logging, improves flow efficiency, and reduces maintenance costs. Although the initial construction cost is high, long-term savings in water, increased agricultural productivity, and prevention of land degradation justify canal lining in areas facing water scarcity or seepage-related problems.

g) Principles of Regulation of Canal System

The regulation of a canal system aims to ensure that the required quantity of water is delivered to the command area efficiently and equitably. It involves controlling discharge at various points to match irrigation demand, minimizing losses, maintaining proper flow depth, and ensuring reliable water supply. Proper regulation improves water distribution efficiency and prevents wastage.

h) Well Loss and Well Efficiency

Well loss refers to the additional head loss that occurs due to turbulence and friction near the well during pumping. It includes losses caused by improper screen design, clogging, and high pumping rates. Well efficiency is defined as the ratio of theoretical drawdown to actual drawdown in the well. A higher well efficiency indicates better performance and lower energy losses during pumping.

i) Water Logging

Water logging is a condition in which the water table rises close to or above the ground surface, resulting in saturation of the root zone. This condition restricts air circulation in soil, reduces crop yield, and may render agricultural land unproductive. Water logging commonly occurs due to excessive irrigation, poor drainage, and seepage from canals.

j) Dupuit’s Theory

Dupuit’s theory is used in groundwater flow analysis, particularly for steady radial flow to wells. The theory assumes that flow lines are horizontal and hydraulic gradient is equal to the slope of the water table. Although simplified, Dupuit’s theory provides reasonably accurate results for unconfined aquifers and is widely used in well design calculations.

SECTION B

(Attempt any three – Long descriptive answers)

2(a) S-Hydrograph and Its Derivation

An S-hydrograph, also known as summation hydrograph, represents the cumulative effect of continuous effective rainfall of constant intensity over a catchment. It is derived by successively adding unit hydrographs lagged by their duration. The S-hydrograph is particularly useful in converting a unit hydrograph of one duration into another duration. To derive a unit hydrograph from an S-hydrograph, two S-hydrographs separated by the desired duration are subtracted, and the resulting hydrograph is scaled appropriately.

2(b) Optimum Number of Rain Gauges

The optimum number of rain gauges in a basin is determined to ensure accurate estimation of average rainfall within an acceptable error limit. The calculation involves analyzing the variability of rainfall data obtained from existing gauges and applying statistical methods. Increasing the number of gauges reduces the percentage error and improves rainfall estimation accuracy, which is crucial for hydrological studies and water resource planning.

2(c) Design of Irrigation Channel by Kennedy’s Theory

Kennedy’s theory is based on the concept of critical velocity required to prevent silting and scouring in an alluvial channel. According to the theory, the channel dimensions are designed such that the actual velocity of flow equals the critical velocity. Parameters such as discharge, channel slope, Manning’s coefficient, and width-to-depth ratio are used to determine the final dimensions of the channel ensuring stable flow conditions.

2(d) Pumps Used for Tube Wells

Various types of pumps are used for tube wells depending on depth of water table, discharge requirement, and power availability. These include centrifugal pumps, turbine pumps, submersible pumps, and jet pumps. Each type has its own advantages and limitations in terms of efficiency, installation cost, maintenance, and suitability for different depths and operating conditions.

2(e) River Training Works

River training works are constructed to guide and control river flow in order to protect banks, improve navigation, prevent flooding, and safeguard hydraulic structures. These works include embankments, spurs, guide banks, and levees. Proper river training helps in stabilizing river channels and reducing erosion-related damages.

SECTION C

(Attempt any one)

Infiltration Capacity and Infiltration Indices

Infiltration capacity is the maximum rate at which soil can absorb water under given conditions. It depends on factors such as soil type, moisture content, vegetation cover, and land use. Infiltration indices such as φ-index and W-index are used to estimate effective rainfall by accounting for infiltration losses during storm events.

Rainfall Intensity and Intensity-Duration Curves

Rainfall intensity is defined as the rate of rainfall per unit time. Intensity-duration curves are prepared using historical rainfall data and represent the relationship between rainfall intensity and duration. These curves are widely used in the design of drainage systems, culverts, and stormwater management structures.

SECTION C (Numerical/Conceptual)

Consumptive Use of Water

Consumptive use of water refers to the total quantity of water used by crops for growth and metabolism, including transpiration and evaporation from soil. It differs from evapotranspiration in that consumptive use also includes minor water losses that are not recoverable for reuse.

SECTION C

Methods of Irrigation

Different irrigation methods such as surface irrigation, sprinkler irrigation, and drip irrigation are adopted based on soil type, crop requirement, and water availability. Each method has its own advantages and limitations in terms of water use efficiency, cost, and suitability.

Regulation of Canal System

Canal regulation involves controlling the flow of water through distributaries and outlets to match irrigation demand. Various methods such as upstream control, downstream control, and combined control are used to ensure equitable water distribution and efficient canal operation.

Open Wells vs Tube Wells

Open wells are shallow and suitable for low discharge requirements, whereas tube wells are deep wells capable of supplying large quantities of water. Tube wells are more efficient, require less land area, and provide reliable water supply compared to open wells.