Chap 1 Civil Channel Flow Stver Online
Chap 1 Civil Channel Flow Stver Online
Chap 1 Civil Channel Flow Stver Online
CHAP 1 INTRODUCTION
1. CONCEPTS
Channel flows are flows that take place in natural or artificial channels under the influence of
gravitational forces, and are subjected to atmospheric pressure with a free surface.
A channel with either varying cross-section or varying bottom slope is called a non-
prismatic channel. Examples are stream, river channels.
Figure 10 Rigid Rock Canal Figure 11 Rigid concrete Canal Figure 12 Non-Rigid Earth Canal
An open channel having a bottom slope greater than 1 in 10 is called a channel of large slope
(Chow, 1959). Man-made channels fall in this category- artificial channels like drops and chutes
have far more than 1 in 10.
(V) There is a clear contrast between full bore pipe and free flow channel hydraulics:
(VI) The similarity of the energy concepts in both pressure and gravity flows.
• The longitudinal profiles are identical in pressurized and channel flows. The free
surface may be defined by the hydraulic gradient, total energy and specific energy with
reference to the datum and channel bed respectively.
• The changes in (y) in an open channel follow the pattern of pressure (p) changes in
pipe flow.
The fundamental types of flow have been discussed in the last semester(s). [Read: laminar,
turbulent, ideal, real rotational, non-rotational etc]. However, it is appropriate here to expand those
descriptions as applied to open channel hydraulics. Recalling that flow may be steady or unsteady,
uniform or non-uniform, rotational or irrotational, gradually varied or rapidly varied, laminar
[lamina open channel flow is very rare in practice] or turbulent. The major classifications applied
to open channels are presented in the following flow diagram and the accompanying short notes.
Time as the Criterion: Flow in an open channel is said to be steady if the flow parameters such as
depth (d), cross-sectional area (A) of flow, velocity (v) and discharge (Q) do not change or if it
can be assumed to be constant during the time interval under consideration.
𝑸 = 𝑽𝟏 𝑨𝟏 = 𝑽𝟐 𝑨𝟐 ------[𝟏].
Where the subscripts designate different channel sections. Equation [1] is obviously invalid if the
discharge of a steady flow is non-uniform along the channel, that is, where water runs in or out
along the course of flow. This type of flow is, known as spatially varied or discontinuous flow, is
found in road-side gutters, side channel spillways, the effluent channels around sewage treatment
tanks.
Space as the criterion: Open channel is said to be uniform if the depth and velocity of flow are
𝛿𝑑 𝛿𝑣 𝜹𝑺 𝛿𝐴
the same at every section of the channel [𝛿𝑋 = 0 or 𝛿𝑋 = 0, 𝜹𝒙𝒐 = 𝟎 and 𝛿𝑋 = 0 ]. In addition, in
uniform flow, the energy line, water surface and channel bottom are parallel, i.e. the slopes are
equal. This is normally true in prismatic (constant geometry and slope) channels.(𝑺𝒇 = 𝑺𝒘 = 𝑺𝟎 = 𝑺)
This is the type of flow in which the depth is constant, both with time and distance thus
𝜹𝒅 𝜹𝒅
| 𝜹𝒕 | = 𝟎, | 𝜹𝒍 | = 𝟎.
𝟏−𝟏 𝟎−𝑳
Steady uniform flow constitutes the fundamental type of flow in an open channel in which the
gravity forces are in equilibrium with the resistance forces.
This is a type of flow in which the depth varies with time, but not distance.
𝜹𝒅 𝜹𝒅
| | ≠ 𝟎, | | =𝟎
𝜹𝒕 𝟏−𝟏 𝜹𝒍 𝟎−𝑳
Varied Flow
This is the type of flow in which the depth of flow varies with both time and distance [unsteady-
non-uniform flow is very rare]. Varied flow may be further classified as either rapidly (abrupt) or
gradually (smooth) varied. The flow is rapidly varied if the depth changes abruptly over a
comparatively short distance; otherwise, it is gradually varied. A gradually varied flow produces
draw-down and backwater effects.
In laminar flow the fluid particles will appear to move in definite smooth paths or streamlines. It
is known to occur in shallow overland or sheet conditions. Turbulent flow is the most common
type occurring in open channel facilities and is the type considered for most hydraulic procedures.