FM Lecture 1 Gribbin Chapter 06 Open Channel Hydraulics
FM Lecture 1 Gribbin Chapter 06 Open Channel Hydraulics
FM Lecture 1 Gribbin Chapter 06 Open Channel Hydraulics
Objectives
Upon completing this chapter, you should
be able to:
Compute the slope of a channel
Compute the cross-sectional area, wetted
perimeter, and hydraulic radius of a channel
Identify normal depth in a channel
Identify and compute critical depth in a
channel
Fundamental Concepts
Prismatic channel
Maintains constant slope and shape
Essential elements: channel bottom, water surface
and energy grade line (EGL) above water surface
HGL coincides with water surface
Hydraulic grade line (HGL), P/g + z The line that represents the sum of the static pressure and the
elevation heads.
Energy grade line (EGL), P/g + V2/2g + z The line that represents the total head of the fluid.
Dynamic head, V2/2g The difference between the heights of EGL and HGL.
The hydraulic
grade line
(HGL) and the
energy grade
line (EGL) for
free discharge
from a reservoir
through a
horizontal pipe
with a diffuser.
Fundamental Concepts
Slope
Vertical fall divided by horizontal run
Depth of flow D or d or y
Distance from channel bottom to water surface
Cross-sectional area of flow
Area of a cross-section of flowing water (a) see
figure above
Types of Channels
Many different shapes and sizes
Slope, shape or alignment changes:
Nonprismatic
Flow characteristics are affected: chnages
from uniform to varied flow
Normal Depth
Water flow in a uniform channel maintains
constant velocity and constant depth called
Normal depth
So velocity head constant then: EGL parallel
water surface
Flow rate is constant Q
Depends on the slope of the channel i.e. for
greater slopes:
is small
Critical Depth
Specific energy (E=D+v2/2g)
Critical depth
Minimum value of E
Theoretical concept
Depends on channel slope and flow Q
T
g
a = cross-sectional area
T = top width of channel
Q = flow rate
g = acceleration due to gravity
Supercritical flow
Depths less than critical depth
More rapid flow
Critical slope
Channel slope that causes normal depth to
coincide with critical depth
v
F
gD
F = Froude number
v = average velocity
D = flow depth
g = acceleration due to gravity