This slide contents very basic knowledge about open channel flow. It contains flow classifications based on the effect of viscosity and gravity.

1. Chapter-1
Course Code : CE-421
Course Title : Water Resources Engineering-II
Romana Saila
Assistant Professor
Department of Civil Engineering

2. Classification of Open Channel Flow
• Steady and Unsteady flow (Criterion-Time): du/dt=dh/dt=dQ/dt=0 ; fixed space (x)
• Uniform and Varied flow (Criterion-Space): du/dx=dh/dx=dQ/dx=0; fixed time (t)
For Uniform flow (UF) channel bottom, water surface and enery line slope are parallel to each other.
Uniform flow can be steady only . Unsteady uniform flow is practically impossible.
• Varied or Non-uniform flow is further classified into :
Gradually Varied flow (GVF)
Rapidly Varied flow (RVF)
Spatially varied flow.

3. Classification of Open Channel Flow

4. Watch the video using the link below
• https://www.youtube.com/watch?v=gT2BL8OQXo0
State of Flow
Effect of Viscosity (Inertia vs viscosity): expressed by Reynolds no Re
Re=UR/ν
U= Flow velocity
R= Hydraulic radius
ν= Kinematic viscosity
Re<500; Laminar flow (Viscous force is prominent)
500 <Re < 12500; Transitional Flow
Re> 12500; Turbulent flow (Inertial force is prominent)

5. State of Flow
• Effect of Gravity (Inertia vs Gravity ) : Expressed as Froude no Fr
Fr= U/√gD
U= Flow velocity
g= gravitational acceleration
D= Hydraulic depth
Fr =1 ; Critical flow
Fr <1 ; subcritical flow
Fr >1 ; supercritical flow
* Froude no of an open channel flow varies over a wide range covering both
subcritical and supercritical flows and the state of open channel flow is primarily
governed by Froude no .

6. State of Flow
Combined effect of Viscosity and Gravity :
Re<500, Fr<1 ; Subcritical Laminar flow
Re<500, Fr >1 ; Supercritical Laminar flow
Re > 12500, Fr <1 ; Subcritical Turbulent flow
Re > 12500, Fr >1; Supercritical Turbulent flow
Subcritical laminar and Supercritical laminar are very rare to get.
*Flow in most rivers and canals are subcritical turbulent. Flow in the feet of drops or spillways is
supercritical turbulent.