1

Special Features and Considerations

Chapter 4

Start 9 OCT

2

Special Features and Considerations

• Sediment Control Structures• Air Entrainment• Hydraulic Jumps• Open Channel Junctions• Hydraulic Model Studies

3

Sediment Control Structures

• Stabilizers• Drop Structures• Debris Basins and Check Dams

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Two types of grade control structures:

A Bed Control Structure is designed to provide a hard point in the streambed that is capable of resisting the erosive forces of the degradational zone (analogous to locally increasing the size of the bed material in Lane’s relationship).

A Hydraulic Control Structure is designed to function by reducing the energy slope along the degradational zone to the point that the stream is no longer capable of scouring the bed.

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The purpose of a Bed Control Structure is to maintain status quo of the upstream channel by forcing the degradational zone to occur at the structure where non-erodible materials will prevent scour of the bed.

riprap

flow headcut

launched riprap local scour

bed degradation

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Stabilizers

• Rock, sheet piling and concrete sills• May/may not provide hydraulic

control• Design guidance given in Plates 44

through 47

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Stabilizers

11Plate 44

• Plate 44– Grouted stone stabilizer– No hydraulic control– Rock extent varies with

potential scour depth

12Plate 45

• Plate 45– Sheet piling design

for channels with Vavg < 14 fps

– Dimensions permit adjustment for geometry

– Does not address structural stability

13Plate 46

• Plate 46– Used to size

derrick stone– Function of

structure submergence

– Look at model rock sizes!

14Plate 47

• Plate 47– Design discharge

coefficients • Sill submergence

(T)• Critical depth (dc)

– Limit of T/dc > 0.8

15Plate 47

2

2 22HQE

A C g=

3/ 2Q CBH=3/ 22 2

3Q Cd gBH=

Discharge Coefficients

0.611 0.075w

HCdH

= +

3.2 4.0C = −

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Procedure

• With know discharge and Plate 47, determine energy head, H, at a location 5dc upstream of structure

17Plate 47

• Plate 47– With known Q and

submergence, determine EH

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Procedure

• With know discharge and Plate 47, determine energy head, H, at a location 5dc upstream of structure

• Plate 46 and H permit derrick stone size to be estimated

19Plate 46

• Plate 46– Used to size

derrick stone– Function of

structure submergence

– Look at model rock sizes!

20

Procedure

• With know discharge and Plate 47, determine energy head, H, at a location 5dc upstream of structure

• Plate 46 is used with calculated H to determine required derrick stone size

• Curves in Plates 29 and 30 to size stone in all non-critical areas

21Plate 29

22Plate 30

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Hydraulic Control Structures

The purpose of a Hydraulic Control Structure is to reduce the upstream energy slope which will render the degradational zone inactive. These structures are situated downstream of the degradational zone, creating a backwater situation with both reduced velocities and scouring potential.

S1 S2

L

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Drop Structures

• Control channel gradient• Provide for abrupt changes through a

vertical drop• Two designs

– Weir or flow notch– Stilling basin

• Stability achieved through proper spacing

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Drop Structures

• Discharge computed by weir equation3/ 2Q CBH=

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Drop Structures

• C = 3.0• Iterative procedure to balance weir

height and width• Structure dimensions given in Plate 48

3/ 2Q CBH=• Discharge computed by weir equation

28Plate 48

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Drop Structure

30

Drop Structure

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Debris and Check Dams

• Constructed in headwaters to trap and contain sediment

• Rule of thumb for storage– 100,000 yd3 per square mile (upper canyons of

LA area)– 62 acre-ft per mi2

• Operate to keep capacity to > 75%• Plate 49 shows basic layout

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Air Entrainment

• Considered in supercritical channel design

• May cause flow bulking and therefore increased wall heights

• Typically not with Fr < 1.6• Plate 50 gives criteria

33Plate 50

• Plate 50– Permits aerated

depth to be determined as a function of assumed Froude number

– Extrapolated for Fr<5

– Watch best fit curve

34

Hydraulic Jumps

• Momentum used to calculate conjugate depths

• Energy equation used to determine losses• For rectangular channels, Plate 51 gives:

– Depth– Length– Energy loss

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Channel Junctions

• Appendix E gives theoretical analysis• Provides initial guidance on:

– Wave effects (Plate 53)– Height criteria (Plate 54)– Plan view layout (Plate 55)– Confluence design (Plates 56, 57)– Side drainage inlets (Plate 58)

• good write up of design procedure• Keep in mind for CE614

– Box and pipe culvert inlets

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Hydraulic Model Studies

• Brief background• Introduces the language• Some other time….

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Methods for Predicting n Values for the Manning

Equation

Chapter 5