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Strategically Preparing Open Channel Flow for GATE Civil Engineering

This article on Open Channel Flow has been written by Rohit Sachdeva. He graduated from Delhi College of Engineering (now DTU) in 2012 (a gold medalist in his batch) in Civil Engineering branch. Then he appeared in Civil Engineering (CE) paper in GATE 2017 and secured an All India Rank (AIR) of 93.

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In this blog, I will be discussing how to prepare for one of the important subjects in Civil Engineering: Open Channel Flow. This subject is conceptual and contains application level questions in both theory and numerical parts. Thorough understanding of the subject will be required to attempt these questions. At least one round of revision is necessary for this subject. Spend time in clearing the concepts and their applications. Practicing numerical questions are also important to attempt the questions with right approach in the exam so that the questions can be solved within the time limit.

A rough breakup of questions of various chapters in last 30 years in GATE is as follows:

Sl No Topic No. of Questions
1 mark 2 marks
1 Channel Geometrics 2 3
2 Classification of Open Channels 6 9
3 Specific Energy and Specific Force 6 12
4 Gradually VARIED flow and Rapidly Varied Flow 6 21
5 Surges and Wave Fronts 1 2

An analysis of last 5 years GATE papers reflects that Open Channel Flow carries 4-8 marks.

Time Required for Preparation

14 days (if you have 8-10 months of preparation) with 3-4 hours daily

7 days (if you have 4-5 months of preparation) with 6-7 hours daily

The topic wise preparation strategy is given below, with the most important concepts in bold & italics. Approximate time required for each chapter is mentioned along with it (this can be varied according to individual interests). It is advisable to prepare separate notes for formulae and for working out numerical questions. The textbook(s) to be referred are mentioned in a separate comprehensive blog which you should refer.

1. CHANNEL GEOMETRICS (2 DAYS)

Classification of channels based on their shapes, hydraulic depth and hydraulic radius of each sections depending upon flow in them are included in this chapter.

2. CLASSIFICATION OF FLOWS IN OPEN CHANNELS (2 DAYS)

Classification of flows (steady flow & unsteady flow, uniform & non uniform flow, spatially varied flow), uniform flow formulae (Chezy’s formula, Manning’s formula, Stickler’s formula, Meyerhof’s formula), convergence of open channel, boundary shear stress, Reynold’s, number, Froude’s number and classification (critical, subcritical & supercritical flow), hydraulically efficient channel sections (rectangular, triangular, trapezoidal, circular & combination of trapezoidal-circular sections) are the topics in this chapter. Learn the formulae and prepare a formula sheet for the empirical equations. Practice numerical questions, at least 2 from each topic.

3. SPECIFIC ENERGY AND SPECIFIC FORCE (3 DAYS)

This is one of the more important chapters in open channel flow. Clearly understand the concept of specific energy, specific energy curve, alternate depths, discharge curve; specific force, specific force curve, characteristics of critical flow, conditions of critical flow (rectangular, triangular and parabolic channels), relation between specific energy and critical depth, relation between alternate depths and Froude’s number.

Another important section in this chapter is channel transitions (channel width reduction, channel width expansion, channel bottom rise (hump) & channel bottom drop, for both subcritical and supercritical approaching flow conditions). Understand the concepts to identify the flow downstream of transitions using specific energy curve and discharge curve.

Practice numerical questions from all the topics to getter better understanding of concepts and to get better ideas regarding the approach to solve questions.

4. GRADUALLY VARIED FLOW (GVF) & RAPIDLY VARIED FLOW (RVF) (5 DAYS)

This is the most important chapter in Open Channel Flow. This chapter can be divided into two sections.

Gradually Varied Flow

Assumptions in GVF, differential equation of GVF, modified equations of GVF (using Manning’s & Chezy’s formulae), length of GVF profiles & classification of water slopes of GVF (mild, steep, critical, horizontal & adverse). There will be one sure question on finding the water slopes. Understand each profile thoroughly and the classifications in each profiles based on depths.

Rapidly Varied Flow

Hydraulic jump (stationary jump: steady RVF), components of hydraulic jump, assumptions for analyzing jump, general equations of hydraulic jump (conjugate depths, energy dissipation, power loss), simplified equations for jump in rectangular channel (prepare a formula sheet), classification of jump based on Froude’s number.

Get the concepts cleared and work out maximum numerical questions to get a thorough idea.

5. SURGES & WAVE FRONTS (1 DAY)

Surges are moving hydraulic jumps (unsteady RVF). Topics include positive surge (two cases: surge moving downstream & moving upstream), celerity of waves for both these cases, negative surges, opening & closing of sluice gate (identifying direction of movement of surge-match the following question), combination of surges & control sections. Practice 2-3 numerical questions.

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