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Hydraulic Engineering

CEWB

CHAPTER 1INTRODUCTION

Dr. Mohd Hafiz bin Zawawi

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What is Hydraulics?

• Hydraulic may be defined as the sciencethat deals with the mechanical behavior of

water at rest or in motion.

• The mechanical behavior may entailcomputing forces and energy.

• The science of fluid at rest is called

hydrostatics• The science of moving fluid is called

hydrodynamics

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Distinction 

• Fluid Mechanics – is the study of fluids

itself under all condition. The fluids maybe

gaseous or liquid.

• Hydraulic – Primarily deals with water. A

hydraulic approach is more empirical and

emphasis on finding solution of practical

value practically

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• Hydrology – Study of space and time and

frequency characteristics of the quality and

quality of water on earth.

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Classification of Hydraulics

• Source of water

 – Surface water, vadose zone, saturated geological

formation and ice glaciers

• Type of conduit

 – Open channel, pipe flows and watershed

hydraulics

• Properties of water

 – Water quality and water quantity

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•

Type of Environment – Agriculture, coastal, wetland, lake and river

• Land use

 –

Agriculture, urban, forest and rural• Solution Technique

 – Mathematical and statistical

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Types And Classification Of Flow

• Closed conduit flow (Pipe flow) – It does not

have any free surface. If there is free surface

in water flow in pipes, therefore it is classified

as open channel flow.

• Open channel flow – It has a free surface.

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Types Of Open Channel Flow

NATURAL channels describe all naturally occurring watercoursesincluding rivers of all sizes, streams and small gullies. Underground

rivers and streams having a free surface are also considered natural

channels

ARTIFICIAL  channels are water conveyance structures built or developedby human means. Such channels include pipes and other conduits, sewer,

drained ditches, canals, spillways, gutters, flumes, etc. They are usuallyconstructed in regular cross-section shape.

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NOOR ALIZA AHMAD 9

Properties of Open Channels

Regardless of whether an open channel is a culvert, a

drainage ditch, or a natural river; there are certain geometricproperties which are useful in describing and analyzing open

channel flow. The figures Shown below illustrate these

geometric properties. Note that these properties will assume

unique values for each individual cross-section.

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Open channel flow may be classified in a variety of ways;

however, the following types are the most widely used.

1) Time as the Criterion

(Steady and Unsteady Flow)

2) Space or Distance as the Criterion

(Uniform and Varied Flow)

Flow Types

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1) Time criterion

Using time as a condition, flow in an open channel may be classified

as either Steady or Unsteady

Steady Flow refers to the case where the depth and velocity  at a

particular point in a channel are constant over time. In steady flow,the depth and velocity may vary from point to point along a channel

reach; however, in order for the flow to be steady, the depth and

velocity at any single point must be fixed over all time.

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2) Space or Distance criterion

Using space or distance as a condition, flow in an open

channel may be classified as either Uniform or Varied.

Uniform Flow

Refers to the flow characteristic where the streamlines are parallel

to one another.

Since the water surface itself is a streamline, then the depth and

velocity of flow is the same everywhere along a channel reach or

length.

In uniform flow, the channel bottom, the Energy Grade Line, and

the Hydraulic Grade Line are parallel along the entire channel

length.

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Varied Flow

Refers to the condition where the flow depth can change

along the channel reach. Varied flow can be further

subdivided into Gradually Varied or Rapidly Varied flow.

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  Varied flow is for flows with the depth of flow thatchanges along the length of the channel.

Varied flow can be further classified into :

i) Gradually varied flow (GVF) – where the depthchanges gradually over a comparatively long distance.

ii) Rapidly varied flow (RVF) where the depth changesabruptly over a comparatively short distance.

iii) Spatially varied flow or discontinuous flow where

some flow is added to or abstracted from the flowsystem. Example would be like flows at roadsidegutters and feeding channels in irrigation systems.

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Types Of Flow In Open Channel

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Steady flowUnsteady flow

Uniform flow Varied flow

Rapid varied flowGradually varied

Flow

Classification of Open Channel Flows

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  The state is governed by:

a) The effect of viscosity

b) The effects of gravity

State of flow

The effect of viscosity relative to inertia would causethe flow to be in either one of the following states :

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State of flow

ii) Turbulent flow :• Viscous forces are weak relative to the inertial

forces• Water particles move in irregular paths but still

represent the forward motion of the entire

stream• Reynolds number , Re > 1000

iii) Transitional flow• The flow state between laminar and turbulent

flows 500 < Re < 1000

i) Laminar flow :Viscous forces are so strong relative to the inertial forces.

Water particles appear to move in streamlines

Reynolds number Re < 500

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State of flowb) The effects of gravity

The effect of viscosity relative to inertia would cause the flow to be in eitherone of the following states:

i) Subcritical flow :

• Gravity force is more pronounced

• The flow has a low velocity and often tranquil and streaming

• Froude Number , F 1

iii) Critical flow :

• A state of flow between subcritical and supercritical flow

•

Froude number, F = 1

 gy

  

F =

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Froude number, Fr, is a dimensionless value thatdescribes different flow regimes of open channel

flow. The Froude number is a ratio of inertial and

gravitational forces.

· Gravity (numerator) - moves water downhill

· Inertia (denominator) - reflects its willingness

Froude Number

http://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Open_Channel_Flow.htmhttp://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Open_Channel_Flow.htmhttp://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Open_Channel_Flow.htmhttp://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Open_Channel_Flow.htmhttp://www.fsl.orst.edu/geowater/FX3/help/8_Hydraulic_Reference/Open_Channel_Flow.htm

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  The combined effects of viscosity and gravity wouldresult in either one of the following regimes of flow is

governed by:

1. Subcritical-laminar flow, F < 1 and Re < 500

2. Supercritical –

laminar flow, F > 1 and Re < 500

3. Supercritical Turbulent flow, F > 1 and Re >1000

4. Subcritical –Turbulent flow, F 1000

Regimes of Flow

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Froude Number for Open Channel Flow

i t

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picture

figure 1

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figure 1

Hydraulic jump

Note that there is  head loss in an hydraulic jump

jump

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 jump

Hydraulic jump on Rattan Creek, TX. July 2, 2002

steady jump: 4.5 9.0, rough

wavy surface downstream