EM Chapter 4 · upstream channel by forcing the degradational zone to occur at the structure where...
Transcript of EM Chapter 4 · upstream channel by forcing the degradational zone to occur at the structure where...
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Special Features and Considerations
Chapter 4
Start 9 OCT
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Special Features and Considerations
• Sediment Control Structures• Air Entrainment• Hydraulic Jumps• Open Channel Junctions• Hydraulic Model Studies
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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!
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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
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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
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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