specific energy diagram

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Chap. 2 Chaudhry - Specific Energy

Energy of open channel flow with respect to the channel bottom

Specific Energy - E

depthofflow-y

y = E

450

y1

y1

v12/2g

v2

2/2gy

2

y2

specific energy : E = y + v2/2g

note: for all E values except criticalconditions there are 2 sets of y and vwhich will satisfy the specific energy

equation

critical conditions: Fr = 1

ONLY AT CRITICAL FLOW IS THERE A SINGLE UNIQUE RELATIONSHIP BETWEENDEPTH AND VELOCITY FOR A GIVEN FLOW

Fr < 1

Fr > 1

SPECIFIC ENERGY DIAGRAMS FOR SPECIFIC FLOWS

water depth y .05 ft .06 ft, 3 ft..:=

specific energy equation: E yv

2

2g+= however v =

q

yso we can write the S.E. equation in terms of

specific flow and depth

E y q,( ) yq2

2 g y2

+:=

specific_energy_diagram_and_discussion.mcd

E:\public_html\specific_energy_diagram_and_discussion.mcd

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0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 30

0.1

0.2

0.3

0.4

0.5

0.61

0.71

0.81

0.91

1.01

q = 10 gal/sec*ft channel width

q = 5 gal/sec*ft channel width

q = 2.5 gal/sec*ft channel width

SPECIFIC ENERGY DIAGRAMS

SPECIFIC ENERGY (FT)

FLOWD

EPTH

(FT)

y

ft

yft

y

ft

E y 10gal

sec ft,

ft

E y 5gal

sec ft,

ft,

E y 2.5gal

sec ft,

ft,

Specific energy diagrams are explored more fully below

What a specific energy diagram says :

1. A given flow can have a large number of specific energy values depending on the flow depth and

velocity. There is one minimum value of specific energy is called the critical specific energy. It is

defined by the red dot on the specific energy curve

2. For any given flow rate there are 2 different sets of depth and velocity which have the

same specific energy.

a. One of the flow depths (Fr < 1) has most of its energy tied up in depth of flow, see

point 2 above

b. the other flow depth (Fr > 1) has most of its energy tied up as velocity head, see point

1 above

3. If the flow depth is changed in a channel the specific energy of that flow will change



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4. For any flow there is a combination of depth and velocity that results in a minimum specific energy. Any

channel feature which causes the specific energy to drop below this value will necessitate a change in the

flow depth and velocity upstream of said feature in order to provide the minimum specific energy at the

feature. Such a feature is referred to as a "choke". A practical example of a choke might be a culvert

under a roadway or a bridge crossing a stream. The significance of a choke is that it may raise the

upstream water surface causing flooding of surrounding land.



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specific energy diagram

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