(SEM VII) THEORY EXAMINATION 2018-19 OPEN CHANNEL FLOW

SECTION A

(Attempt all questions in brief – 2 × 10 = 20 marks)

(a) Varied Flow with Neat Sketch

Varied flow in an open channel is the flow in which the depth of water changes along the length of the channel. This variation may occur due to changes in slope, channel geometry, or discharge. Varied flow can be gradual or rapid. In gradual varied flow, the depth changes slowly, while in rapidly varied flow, the depth changes suddenly, such as in hydraulic jumps.

(b) Prismatic Channels

A prismatic channel is an open channel in which the cross-sectional shape, size, bed slope, and roughness remain constant along its length. Examples include straight canals and rectangular channels. Such channels simplify flow analysis and are commonly used in hydraulic design.

(c) Velocity Distribution in Open Channel Flow

Velocity in an open channel is not uniform throughout the cross-section. Due to friction at the bed and sides, velocity is minimum near the boundaries and maximum near the free surface slightly below the water surface. This variation is influenced by channel roughness and flow conditions.

(d) Difference between GVF and RVF

Gradually Varied Flow (GVF) is a flow where depth changes slowly over a long distance, allowing hydrostatic pressure distribution. Rapidly Varied Flow (RVF) occurs when depth changes abruptly over a short distance, such as in weirs or hydraulic jumps, and pressure distribution is non-hydrostatic.

 SECTION B

(Attempt any ONE – 10 marks)

(a) Critical Flow and Minimum Wetted Perimeter in Rectangular Channel

Critical flow occurs when the specific energy of flow is minimum for a given discharge. In a rectangular channel, when flow is critical and the wetted perimeter is minimum, both conditions occur simultaneously only if the channel width is equal to twice the depth of flow. This relationship shows that hydraulic efficiency and energy efficiency can coincide under specific geometric conditions.

(b) Gradually Varied Flow Numerical Concept

In this problem, water flows through a rectangular channel whose bed level rises in the direction of flow. Given discharge, channel width, Manning’s roughness coefficient, and initial depth, the rate of change of depth with distance is calculated using the GVF equation. The problem demonstrates how channel slope and resistance affect flow depth variation along the channel length.

SECTION C

(Attempt any ONE – 10 marks)

(a) Most Economical Depth of Flow in Circular Channel

The most economical depth of flow is the condition at which discharge is maximum for a given channel size. In a circular channel, this condition is achieved when the wetted perimeter is minimum and the hydraulic radius is maximum. The derivation shows that for maximum discharge and maximum velocity, the depth of flow is a fixed fraction of the channel diameter. This concept is important for efficient channel design.

(b) Maximum Discharge and Velocity Conditions

For maximum discharge in an open channel, the cross-section must provide the largest hydraulic radius. Similarly, maximum velocity occurs when resistance is minimized. These conditions help in designing channels that carry maximum flow with minimum material and energy loss.