Download - 13 Split Flow and Weirs
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Flood Profile Modeling with Split Flows and Weirs
Objective: To become familiar with RAS split flow and weir options.
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Split Flows
• General Modeling Guidelines
• Flow Around an Island
• Divided Flow Approach
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Weirs
• Inline Spillways and Weirs• Entering Inline Spillway/Weir Data• Lateral Spillways and Weirs• Entering Lateral Spillway/Weir Data• Controlling Inline and Lateral Gate Settings• Inline and Lateral Spillway/Weir Output
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Flow Around an Island
High Ground
1
2
3
4
5
6
7
8
Q
Q
T
T
Stagnation Point
Stagnation Point
Q Q12
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Separate River Reaches
8
West
7
6
5
4
3
2
Sp
r u ce
Cre
ek
1
East
7
6
5
4
3
2
Sp
ruc
e
Lower
Upper
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Splitting the Cross Sections
0 20 40 60 80 100 120 140 16060
65
70
75
80
85
90
Divided Flow Plan: Divided Flow Opening River Mile 3
Station (ft)
Ele
vatio
n (f
t)
Legend
EG PF#1
WS PF#1
Ground
Ineff
Bank Sta
.035 .025 .035
0 20 40 60 80 100 120 140 16060
65
70
75
80
85
90
Divided Flow Plan: Divided Flow Opening River Mile 3
Station (ft)
Ele
vatio
n (f
t)
Legend
EG PF#1
WS PF#1
Ground
Ineff
Bank Sta
.035 .025 .035
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Split Flow Optimization
RAS can optimize flow splits at: • Lateral weirs• Hydraulic connections• Stream junctions
1. Program calculates a water surface profile with flows given in flow data file
2. Using computed profiles new flows are determined and profile is re-run
3. Process continues until flows match
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Turning on the Split Flow Optimizer
From Steady Flow Analysis screen, select Options, Flow Optimizations…
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What is saved by RAS
• Flow data are not changed after optimization• You should input computed flows into the flow
data editor and run the program again to see if the answer changes
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Modeling Weirs in HEC-RAS
Two types of weirs:• Inline weirs• Lateral gated spillways
Can be used to model the following:• Large Dams• Run of the river structures• Drop Structures or natural drops• Lateral storage facilities• Natural overflows to ponding areas• Levee breaching or dambreaks
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Inline Weirs and Gated Spillways
0 200 400 600 800 1000-20
-10
0
10
20
30
Inline Weir and Gated Spillay - Ex 12 Plan: Gated Spillway
Station (ft)
Ele
vatio
n (f
t)
Legend
Ground
Ineff
Bank Sta
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Cross Section Locations
FLOW
1
234
EXPANSIONREACH REACH
CONTRACTION
Gated Spillways
Overflow Weir
FLOW
Inline Weir River Stationing is 2.5
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Cross Section 2
• Cross section 2 is located a short distance downstream from the structure.
• The computed water surface at this cross section will represent the tailwater elevation of the weir and the gated spillways.
• This cross section should not include any of the structure or embankment, but represents the physical shape of the channel just downstream of the structure.
Ineffective Flow Area Stations and Elevations
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Cross Section 3
• Located a short distance upstream of the embankment
• Represents the physical configuration of the upstream channel.
• The structure and the roadway embankment are described by combination of the deck/road embankment data, cross section 3, and the gated spillway data
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Sluice and Radial Gates
Sluice Gate
Broad Crested Spillway
Radial Gate
Ogee Spillway Crest
Up to 10 gate groups can be used at any one river crossing. Each gate group can have up to 25 identical gate openingsEither gate type can be used with an ogee crest shape or a broad crested weir
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Setting the Gate Opening
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Inline Spillways and Weirs
• Radial gates (often called Tainter gates)• Vertical lift gates (Sluice gates)• Ogee or Broad Crested Weir shapes for both gated
spillways and overflow weirs• Gate equations can handle low flow, normal gate
flow (upstream submerged) or fully submerged gate flow (both ends submerged)
• Up to 10 gate groups• Up to 25 identical gates per group.
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Radial Gates
HEBETE HBTWgCQ 2
TH
Z sp
U
D
Z
ZB
Where: H = ZU - Zsp
C = Discharge Coefficient, typically 0.6 – 0.8
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Submerged Radial Gates
When the downstream tailwater increases to the point at which the gate is no longer flowing freely (downstream submergence is causing a greater upstream headwater for a given flow), the program switches to the Submerged Form of the equation:
HEBETE HBTWgCQ )(2
Where: H = ZU - ZD
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Fully Submerged Gate Flow
Orifice Equation
gHCAQ 2
Where: H = ZU – ZD
C = Discharge coefficient, typically 0.8
A = Area of gate opening
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Sluice Gates
BZ
Z
ZD
U
sp
gHBWCQ 2
Where: H = Upstream energy head (ZU – Zsp)C = Coefficient of discharge (0.5 to 0.7)W = WidthB = Vertical opening
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Submerged Sluice Gates
HgBWCQ 2
Where: H = ZU - ZD
gHCAQ 2
Transitions to fully submerged orifice flow at 0.8 submergence:
Submergence begins when the tailwater depth above the spillway divided by the headwater energy above the spillway, is greater than 0.67
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Low Flow Through Gates
Z sp
U
D
Z
Z
H
2/3HLCQ
Upstream water surface is equal to or less than the top of the gate opening
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Uncontrolled Over-Flow Weirs
• Can represent emergency spillway or flow over entire embankment.
• Uses standard weir equation• Can have Ogee or Broad Crested weir shape• Weir flow submergence is calculated
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Entering Inline Spillway/Weir Data
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Weir and Embankment Profile
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Gated Spillway Data
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Lateral Spillways and Weirs
• Radial and sluice gates• Uncontrolled overflow weir• Same hydraulic equations for gates as with
inline spillways• Lateral weir can handle a sloping water
surface as well as irregular weir profile.• Includes culvert flap gates (flow limited to one
direction)
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Cross Section Layout
5.3
5.2
5.1
MainChannel
LateralWeir
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Lateral Spillway and Weir Hydraulics
ChannelInvert
Main ChannelBank Elevation
OverflowWeir Gated
SpillwaysWaterSurface
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Lateral Weir Calculations
dQ
X1 X2
Yws = awsX + Cws
Yw = awX + Cw
dX
))()((5
2 2/5111
2/5121
121
CxaCxaa
CQ xx
See notes on next slide for definition of the constants
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Lateral Weir Calculation Equation
• The equation for a sloping line representing the water surface and the weir segment are shown on previous slide
• Constants aws and aw represent the slope of the water
surface and the weir segment, respectively
• Variables Cws and Cw are constants representing the
initial elevations • The standard weir equation assumes that the weir is
parallel with the water surface. The above general equation is derived for a sloping weir and water surface by integrating the standard weir equation
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Entering Lateral Spillway/Weir Data
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Lateral Weir Embankment Data
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Lateral Weir Gate Data
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Lateral Weir Culvert Data
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Lateral Diversion Rating Curve
Use for modeling irregular structures
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Controlling Inline and Lateral Gates
For steady flow simulations specify number of gates open and gate opening height
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Controlling Inline and Lateral Gates
For unsteady flow simulations the following gate controls are available from the HEC-RAS unsteady flow data editor:
• Time Series of Gate Openings
• Elevation Controlled Gates
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Time Series of Gate Openings
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Elevation Controlled Gates
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Inline and Lateral Spillway/Weir Output
The following types of output are available for inline and lateral spillways/weirs:
• Stage and Flow Hydrograph Plots
• Time Series Tables
• Profile Plots
• Cross Section Plots (Inline Spillways/weirs only)
• Detailed Output Tables
• Profile Summary Tables
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Example Profile Plot With a Lateral Spillway/Weir
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Cross Section Plot of Inline Spillway/Weir
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Detailed Output Table For a Lateral Weir
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Profile Output Table for Lateral Weir