JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B. Tech III Year WATER RESOURCES ENGINEERING-I Previous Question Paper (March - 2021 - R13/R15)
Answer any five questions. All questions carry equal marks.
Main
Describe the different methods of recording of rainfall.
Answer
Rainfall is recorded using two primary types of instruments: non-recording and recording rain gauges.
Non-recording rain gauges: The Symons' rain gauge is the standard type used in India. It consists of a cylindrical vessel with a funnel and a receiver bottle. The rainfall is measured manually at fixed intervals (usually 8:30 AM) using a graduated measuring cylinder. It provides the total depth of rainfall over the preceding 24 hours but does not provide information on the intensity or duration of the storm.
Recording rain gauges: These instruments provide a continuous plot of rainfall against time, allowing for the determination of intensity and duration. Common types include:
- Tipping Bucket Type: Consists of two buckets that tip when a specific amount of rain (e.g., 0.25 mm) is collected, triggering an electrical pulse recorded on a chart.
- Weighing Bucket Type: Measures the weight of the rain collected in a bucket over time and records it on a clockwork-driven drum.
- Natural Syphon Type: Uses a float chamber where the rising water level moves a pen on a rotating drum. When the chamber fills, a syphon mechanism empties it rapidly, resetting the pen to zero.
Write the three conditions which are important for setting a raingauge. A catchment area has seven raingauge stations. In a year the annual rainfall recorded by the gauges are as follows:
| Station | P | Q | R | S | T | U | V |
|---|---|---|---|---|---|---|---|
| Rainfall (cm) | 130.0 | 142.1 | 118.2 | 108.5 | 165.2 | 102.1 | 146.9 |
For a 5% error in the estimation of the mean rainfall, calculate the minimum number of additional stations required to be established in the catchment.
Answer
The three conditions for setting a rain gauge are: 1) The site should be on level ground and not on a slope or a roof. 2) The gauge must be located in an open area, away from obstacles like trees or buildings that could obstruct rainfall; the distance to any object should be at least twice the height of that object. 3) The site must be fenced to protect the instrument from animals and human interference.
To calculate the additional stations required:
- Calculate the mean rainfall (P_avg): (130.0 + 142.1 + 118.2 + 108.5 + 165.2 + 102.1 + 146.9) / 7 = 130.43 cm.
- Calculate the standard deviation (σ): σ = sqrt[Σ(Pi - P_avg)^2 / (n - 1)].
Σ(Pi - P_avg)^2 = (130.0-130.43)^2 + ... + (146.9-130.43)^2 = 3381.79.
σ = sqrt(3381.79 / 6) = 23.74 cm. - Calculate the coefficient of variation (Cv): Cv = (σ / P_avg) * 100 = (23.74 / 130.43) * 100 = 18.20%.
- Calculate the optimal number of stations (N): N = (Cv / ε)^2 = (18.20 / 5)^2 = 13.25.
- Rounding up, the total number of stations required is 14.
- Additional stations required = 14 - 7 = 7.
Discuss the factors that affect the evaporation from a water body. Explain the Analytical methods of Evaporation estimation.
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In a 140-min storm the following rates of rainfall were observed in successive 20-min intervals: 6.0, 6.0, 18.0, 13.0, 2.0, 2.0 and 12.0 mm/h. Assuming the φ- index value as 3.0 mm/h and an initial loss of 0.8 mm, determine the total rainfall, net runoff and W-index for the storm.
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What is a unit hydrograph? List the assumptions involved in the unit hydrograph.
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The ordinate of a 6-h unit hydrograph are given.
| Time (h) | 0 | 6 | 12 | 18 | 24 | 30 | 36 | 42 | 48 | 54 | 60 | 66 |
| 6-h UH ordinate (m3/s) | 0 | 20 | 60 | 150 | 120 | 90 | 66 | 50 | 32 | 20 | 10 | 0 |
If two storms, each of 1-cm rainfall excess and 6-h duration occur in succession, calculate the resulting hydrograph of flow. Assume base flow to be uniform at 10 m3/s.
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Develop the equation relating the steady state discharge from a well in an unconfined aquifer and depths of water table at two known positions from the well. State clearly all the assumptions involved in your derivation.
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A 30-cm well completely penetrates an unconfined aquifer of saturated depth 40 m. After a long period of pumping at a steady rate of 1500 litre per minute, the drawdown in two observation wells 25 m and 75 m from the pumping well were found to be 3.5 m and 2.0 m respectively. Determine the transmissivity of the aquifer. What is the drawdown at the pumping well?
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Write short notes on:
- a) Sodium-Absorption-Ratio
- b) Boron concentration in irrigation water
- c) Surface irrigation
- d) Limitations of sprinkler irrigation.
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What is meant by ‘Duty’ and ‘Delta’ of canal water? Derive a relationship between duty and delta for a given base period.
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After how many days will the farmer supply the water to soil in order to ensure sufficient irrigation of the given crop, if:
Field capacity of the soil = 28%
Permanent wilting point = 13%
Dry density of soil = 1.3 gm/cc
Effective depth of root zone = 70 cm
Daily consumptive use of water for the given crop = 12 mm.
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Design a channel using Lacey’s theory to carry a discharge of 50 cumecs. Assume silt factor as 1.1.
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What is meant by regime? Differentiate between regime in natural and artificial channels.
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Explain the salient features of a current meter. Describe briefly the procedure of using a current meter for measuring velocity in a stream.
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Describe the factors affecting SCS curve number.
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