PART A - HYDRAULIC THEORY AND THE EQUATION OF
CONTINUITY AS IT APPLIES TO OPEN CHANNEL DESIGN
FLUID FLOW TYPES
Any discussion of hydraulics must begin with a study of the types of fluid flow.
It is reasonable to assume that to design an open channel capable of carrying the quantity of water (Q) without
erosion or deposition problems, it is necessary to understand the flow of water in the channel. Fluid flow occurs
in two states: laminar flow or turbulent flow.
Laminar Flow
In laminar flow, the water moves in parallel layers with no cross currents. Osborne Reynolds showed laminar
flow in his classical dye experiment in overland or sheet flow as an example of this type of flow in nature. The
chief advantage of describing the flow as laminar is that the total mathematics theory has been developed to
describe it.
Turbulent Flow
In turbulent flow, the water is mixed thoroughly with velocity fluctuations in all directions. Almost all the flow in
nature is turbulent. A good example of turbulent flow is the flow in a mountain stream. Turbulent flow does not
have a precise mathematical theory describing the flow characteristics. Most of the theory is based upon
empirical equations such as Manning's Equation (see Lesson 4, Part B).
FLOW ASSUMPTIONS
The state of flow is dependent on the strength of the viscosity forces or the thickness of the fluid. (Honey is very
viscous while water is not.) Laminar flow is flow where the viscosity forces predominate and the fluid flows in
smooth paths; for example, honey being poured out of a jar. Turbulent flow occurs when the viscous forces are
relatively weak. It is characterized by individual water particles moving in random patterns while the aggregate
motion represents the forward movement of the fluid. Natural channels contain turbulent flow. (Normally, the
only time laminar flow in water occurs in nature is in thin films called sheet flow where the depth is extremely
small.)
First Assumption
A basic assumption in open channel design is that the flow in the channel is steady. This means the depth is not
varying rapidly with time. Although this assumption is not absolutely true in practice, generally changes in flow
in open channels are slow and the errors introduced by this assumption are very small.
Second Assumption
Another basic assumption is that flow in the channel is uniform. The depth throughout a section of channel of
constant dimension is constant. This assumption is found to be essentially correct for channels of reasonable
slope that are not extremely short. This means the slope of the water surface is the same as the channel bottom.
Flows over spillways or waterfalls are not uniform, while flows in rivers, ditches, and canals are the uniform type.
EN5465
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