hec ras training day 2
TRANSCRIPT
Training Course-Day-2Water Surface Profile Modeling Using
HEC-RAS
Aug-2009University of Engineering
and TechnologyLAHORE
Course ContentsDAY-2
Modeling a Bridge
Modeling a Culvert
Modeling Multiple Openings
Practice session / DiscussionsModeling an Inline structure (Weir, Dam etc.)
Modeling Storage Area
Modeling Bridge Scour
Modeling Channel Modifications
Understanding of notes/warnings and errors
Understanding Model Stability
Trouble shooting
Practice session / Discussions
Explanation-Ineffective flow area
•The areas of the cross section that will contain water that is not actively being conveyed (ineffective flow).
•Ineffective flow areas are often used to describe portions of a cross section in which water will pond, but the velocity of that water, in the downstream direction, is close to or equal to zero.
•This water is included in the storage calculations and other wetted cross section parameters, but it is not included as part of the active flow area.
•When using ineffective flow areas, no additional wetted perimeter is added to the active flow area
Bridge Modeling -GeometryRoadway
Low Chord
Deck
PierUpstream Embankment Side Slope
Downstream Embankment Side Slope
Bridge Modeling -Geometry
Bridge Modeling –Flow Types
Low Flow
The flow through the bridge opening is open channel flow
Water surface is not touching the low chord
High Flow
1. Pressure flow
Water surface is touching the low chord (Sluice Flow)
2. Weir Flow
Water overtops the bridge
Bridge deck acts like a Weir
Bridge ModelingEnergy accounts for friction losses and geometry changes through bridge, as well as losses due to flow transition & turbulence. (do not account for pier drag)
Momentum accounts for friction losses and geometry changes through bridge. (requires pier drag coefficient Cd)
FHWA WSPRO Federal Highways Administration method of analyzing bridge. Uses energy equation as well as some empirical attributes. Developed for bridges that constrict widefloodplains with heavily vegetated overbank areas.
Yarnell - Empirical formula developed to model effects of bridge piers. Based on 2600 lab experiments on different pier shapes(Yarnell’s Pier Coefficient, K )
Bridge Modeling
Low Flow Bridge Modeling3 Types of Flow
� Class A Low Flow - Subcritical FlowEnergy, Momentum, Yarnell, and WSPRO
� Class B Low Flow - Flow passes through critical depth
Energy and Momentum
� Class C Low Flow - Supercritical FlowEnergy and Momentum
Bridge Modeling-High Flows
21
- 3 bud 2ZY A CQ ⎥⎦
⎤⎢⎣⎡∗= + g
V2
233α
2gH A CQ =
Pressure (Sluice) flow
Pressure (Orifice) flow
Bridge Modeling-High Flows
Pressure & Weir flow
For high tail water elevations the program will automatically reduce the amount of weir flow to account for submergence on weirs elevations
23
CLHQ =
Locating Cross-Sections Near Bridges
FullyEffectiveFlow
FullyExpandedFlow
Thru Bridge
ExpansionContraction
Locating Cross-Sections Near Bridges
Lc and Le can be determined by field investigation during high flow or can be computed.
LeLc
FullyEffective
Flow
FullyExpanded
Flow
14 3 2
Locating Cross-Sections Near Bridges
1
4
23
Contraction and Expansion Ratios
Bridge Modeling –Expansion Ranges
Contraction and Expansion Ratios
Contraction ExpansionNo Transition 0 0Gradual Transition 0.1 0.3Typical Bridge Transition 0.3 0.5Abrupt Transition 0.6 0.8
Contraction and Expansion Ratios
Expansion Contraction
Cross-Section 4 (furthest US) 0.5 0.4Cross-Section 3 0.5 0.3Cross-Section 2 0.5 0.3Cross-Section 1(furthest DS) 0.3 0.1
Use Cc = 0.4 0.3 0.3 0.1Use Ce = 0.5 0.5 0.5 0.3
Ineffective flow areas
1234
Bridge Data SheetCreated by Clyde Giaquinto NRCS-NY
Bridge Modeling -Geometry
Bridge Modeling -Geometry
Practice SessionBridge Modeling
Culvert Modeling
A culvert is a relatively short length of closed conduit, which connects two open channel segments or bodies of water
In HEC-RAS, Federal Highway Administration (FHWA, 1985) standard equations are used for culvert hydraulics
The definition of culvert geometry is similar to bridge geometry
The layout of cross sections, the use of the ineffective areas, the selection of loss coefficients, and most other aspects of bridge analysis apply to culverts as well
Culvert HydraulicsDepending upon upstream or downstream control, supercritical and mixed flow regimes are calculated
If the control is at downstream side subcritical flow would pass through the culvert from downstream to upstream
When the culvert is at steep slopes, super critical computations are carried out
A hydraulic jump will occur at a location where the specific force of the subcritical flow is larger than specific force of supercritical flow
Culvert Hydraulics
Culvert Hydraulics
Culvert Hydraulics
Culvert data input is almost same as that for bridge input
with some minor specific inputs
Modeling Multiple Openings
Modeling Inline Structures
Modeling Storage Areas
Modeling Bridge Scour
Total Scour = sum of
1) Long term Aggradation/degradation2) General Scour3) Local Scour
Clear Water Scour
Clear-water scour occurs when there is no movement of the bed material in the flow upstream of the crossing
or
the bed material being transported in the upstream reach is transported in suspension through the scour hole at the pier or abutment at less than the capacity of the flow
Live Bed Scour
•Occurs when there is transport of bed material from the upstream reach into the crossing.
Live-bed local scour is cyclic in nature; that is, the scour hole that develops during the rising stage of a flood refills during the falling stage.
Channel Modification Analysis
Trouble Shooting Model
Trouble Shooting Model
Following factors will affect the stability and numerical accuracy of the model
1. Cross section spacing.2. Computation time step.3. Theta weighting factor for numerical solution.4. Solution iterations.5. Solution tolerances.6. Weir and spillway stability factors.7. Weir and spillway submergence factors.
Theta weighing factorsTheta is a weighting applied to the finite difference approximations when solving the unsteady flow equations.
• Theoretically Theta can vary from 0.5 to 1.0.
• A practical limit is from 0.6 to 1.0
• Theta of 1.0 provides the most stability.• Theta of 0.6 provides the most accuracy.• The default in HEC-RAS is 1.0.• Once you have your model developed, reduce
theta towards 0.6, as long as the model stays stable.
Iterations
At each time step derivatives are estimated and the equations are solved. All of the computation nodes are then checked for numerical error. If the error is greater than the allowable tolerances, the program will iterate. The default number of iterations in HEC-RAS is set to 20. Iteration will generally improve the solution. This is especially true when your model has lateral weirs and storage areas.
ToleranceTwo solution tolerances can be set or changed by the user: water surface calculation (0.02 default) and Storage area elevation (0.10 default). The default values should be good for most river systems.
Only change them if you are sure!
Making the tolerances larger can reduce the stability of the solution. Making them smaller can cause the program to go to the maximum number of iterations every time.
Weir and Spillway Stability Factors
The weir and spillway stability factors can range from 1.0 to 3.0.
The default value of 1.0
As you increase the factor you get greater dampening of the flows (which will provide for greater stability), but less accuracy.
Submergence Factors
Can vary from 1.0 to 3.0.
A factor of 1.0 leaves the submergence criteria in its original form.
Using a factor greater than 1.0 causes the program to use larger submergence factors earlier, and makes the submergence curve less steep at high degrees of submergence.
.
Practice Practice and Practice
Last Note
Thank You All !
For Future Reference Please use following contact Information ….
www.bossintl.com
Yasir [email protected]