Bridges

HEC-RAS FY99

2 Types of Flow @ BridgesLow Flow - Flow where the water surface does not reach the low beamHigh Flow - Flow where the water surface reaches the deck or higherThere are sub-types for both low and high flowOften both types of flow occur in single simulation with different profiles

HEC-RAS FY99

Low Flow

HEC-RAS FY99

Low Flow Bridge Modeling3 Types of FlowClass A Low Flow - SubcriticalClass B Low Flow - Passes through critical depthClass C Low Flow - Supercritical

HEC-RAS FY99

Low Flow Bridge Hydraulics4 methods of modelingEnergy - physically based, accounts for friction losses and geometry changes through bridge, as well as losses due to flow transition & turbulence.Momentum - physically based, accounts for friction losses and geometry changes through bridge.FHWA WSPRO - energy based as well as some empirical attributes. Developed for bridges that constrict wide floodplains with heavily vegetated overbank areas. Subcritical flow only.Yarnell - empirical formula developed to model effects of bridge piers. Subcritical flow only.

HEC-RAS FY99

Low Flow Bridge HydraulicsAppropriate MethodsBridge piers are small obstruction to flow, friction losses predominate - Energy, Momentum, or WSPROPier and friction losses predominate - MomentumFlow passes through critical depth in vicinity of bridge - Energy or MomentumPier losses are dominant - YarnellSupercritical flow without piers - Energy or MomentumSupercritical flow with piers - Momentum

HEC-RAS FY99

Low Flow Bridge ModelingClass A Low Flow - Energy MethodFriction losses are computed as length times average friction slope.Energy losses are empirical coefficient times change in velocity head (expansion and contraction losses).Does not account for pier drag forces.

HEC-RAS FY99

Low Flow Bridge ModelingClass A Low Flow - Momentum MethodFriction losses are external skin friction = wetted perimeter times length times shear stress.Requires entering coefficient of drag for piers, CDCheck Options menu under Bridge & Culvert Data window for momentum equation options. Usually accept program defaults

HEC-RAS FY99

Low Flow Bridge ModelingCD Coefficients for PiersCircular Pier1.20Elongated piers with semi circular ends 1.33Elliptical piers with 2:1 length to width0.60Elliptical piers with 4:1 length to width0.32Elliptical piers with 8:1 length to width0.29Square nose piers2.00Triangular nose with 30 degree angle1.00Triangular nose with 60 degree angle1.39Triangular nose with 90 degree angle1.60Triangular nose with 120 degree angle1.72

HEC-RAS FY99

Low Flow Bridge ModelingClass A Low Flow - Yarnell EquationBased on 2,600 lab experiments on different pier shapesRequires entering pier shape coefficient, KShould only be used where majority of losses are due to piers.

HEC-RAS FY99

Low Flow Bridge ModelingYarnells Pier Coefficient, KSemi-circular nose and tail0.90Twin-cylinder piers with connecting diaphragm 0.95Twin-cylinder piers without diaphragm 1.0590 degree triangular nose and tail1.05Square nose and tail1.25Ten pile trestle bent2.50

HEC-RAS FY99

Low Flow Bridge ModelingClass A Low Flow - WSPROFederal Highway Administrations method of analyzing bridgesUses energy equation in an iterative procedure

HEC-RAS FY99

Low Flow Bridge ModelingClass A Low Flow - SummaryEnergy & Momentum equations are appropriate for most bridgesYarnell should only be used when piers are the major obstacles to flowYarnell cannot be used when there are no piersConservative approach is to select all methods and use highest energy loss

HEC-RAS FY99

High Flow - Pressure

HEC-RAS FY99

High Flow - Pressure

HEC-RAS FY99

High Flow - Pressure & Weir

HEC-RAS FY99

High Flow Bridge ModelingWhen bridge deck is a small obstruction to the flow and not acting like a pressurized orifice, use energy method.When overtopped and tailwater is not submerging flow, use pressure/weir method.When overtopped and highly submerged, use energy method.

HEC-RAS FY99

High Flow - Weir FlowQ = Total flow over the weirC = Coefficient of discharge for weir flow (~2.5 to 3.1 for free flow)L = Effective length of the weirH = Difference between energy elev. upstream and road crestDefault Max Submergence = 0.95 (95%) (see next slide)After max submergence reached, program reverts to energy equation for solution.

HEC-RAS FY99

High Flow - SubmergenceDefault Max Submergence = 0.95 (95%) (see next slide)After max submergence reached, program reverts to the energy equation for the solution.

HEC-RAS FY99

High Flow - Submergence

HEC-RAS FY99

Adding the Bridge

HEC-RAS FY99

Adding the BridgeSelect the Brdg/Culv button from the geometry data window:

HEC-RAS FY99

Adding the BridgeThis brings up the Bridge Culvert Data window:

HEC-RAS FY99

Adding the BridgeFrom the options menu, select Add a Bridge and/or Culvert:

HEC-RAS FY99

Adding the BridgeThis brings up a window asking for the river station of the bridge. It will locate the bridge numerically between cross-sections:

HEC-RAS FY99

Adding the BridgeThis brings up a window with the adjacent upstream and downstream cross-sections plotted:

HEC-RAS FY99

Adding the BridgeNext, select the Deck/Roadway button from the Bridge data window:

HEC-RAS FY99

Adding the BridgeNotice the weir coefficient and max submergence window are showing the default values:

HEC-RAS FY99

Adding the Bridge

HEC-RAS FY99

Adding the BridgeEnter the data for the required values. Note that the U.S. and D.S. sideslopes are for cosmetic purposes only unless using the WSPRO method for low flow:

HEC-RAS FY99

Adding the BridgeUse the Copy Up to Down button to repeat the station-high chord-low chord data from the upstream to the downstream side, if applicable:

HEC-RAS FY99

Adding the BridgeAfter selecting OK from the bridge deck window, it replots the U.S. and D.S. x-sections showing the bridge deck:

HEC-RAS FY99

Adding the BridgePiers can be added by selecting the Pier button from the bridge data window:

HEC-RAS FY99

Adding the BridgeThis brings up the Pier Data Editor where you can add the data for the pier(s) :

HEC-RAS FY99

Adding the BridgeThe pier is then shown graphically on the plot:

HEC-RAS FY99

Adding the BridgeThe modeling options, including low flow and high flow modeling methods, are available by selecting the Bridge Modeling Approach button:

HEC-RAS FY99

Adding the BridgeThis brings up Bridge Modeling Approach Editor window. Notice the option to compute each type of low flow method and the option to select which one you use:

HEC-RAS FY99

Adding the BridgeSee following graphOrifice Coef. - between 0.7 and 0.9

HEC-RAS FY99

High Flow - Orifice Discharge

HEC-RAS FY99

Adding the BridgeThere are several other options in the options menu from the bridge data window

HEC-RAS FY99

Adding the Bridge as well as the view menu:

HEC-RAS FY99

Adding the BridgeThe geometric data schematic is also updated automatically:

HEC-RAS FY99

Locating Cross-Sections Near Bridges

HEC-RAS FY99

Cross-Sections Near Bridges

HEC-RAS FY99

Cross-Sections Near Bridges

HEC-RAS FY99

Cross-Sections Near Bridges

HEC-RAS FY99

Cross-Sections Near Bridges

HEC-RAS FY99

Cross-Sections Near Bridges

HEC-RAS FY99

Cross-Sections Near BridgesRule of Thumb:ER = 2:1CR = 1:1

HEC-RAS FY99

Bridge Cross Sections

HEC-RAS FY99

Bridge Cross Sections

HEC-RAS FY99

Bridge Cross SectionsExample Computation of Le and LcThis assumes ER=2 and CR=1

HEC-RAS FY99

Example Computation of Le and Lc

North Dakota FY98

HEC-RAS 2.0

Given:Fully expanded flow top width at Cross Section 1 = 300 feet

Fully expanded flow top width at Cross Section 4 = 250 feet

Distance from Point B to Point C (bridge opening width) = 40 feet

Find:Recommended locations of Cross Sections 1 and 4

Le = 2 * (300 40) / 2 = 260 feet downstream of bridge

Lc = 1 * (250 40) / 2 = 105 feet upstream of bridge

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Expansion & Contraction Coefficients

HEC-RAS FY99

Sheet1

ContractionExpansion

No Transition00

Gradual Transition0.10.3

Typical Bridge Transition0.30.5

Abrupt Transition0.60.8

Sheet2

Sheet3

Bridge ModelingExpansion and Contraction Coefficientsat the 4 cross-sections for a bridge (example) Expansion ContractionCross-Section 4 (furthest US) 0.50.3Cross-Section 3 0.50.3Cross-Section 2 0.50.3Cross-Section 1(furthest DS) 0.30.1

HEC-RAS FY99

Ineffective FlowsAt XSs 2 & 3

HEC-RAS FY99

The End

HEC-RAS FY99

BridgesThis module will cover bridges and how they are input into HEC-RAS.HEC-RAS FY99BridgesThese are not that important to know - except you must specify analysis methods for each.HEC-RAS FY99BridgesAgain, not that important. You will see in the output from time to time.HEC-RAS FY99BridgesAgain, not that important. You will see in the output from time to time.HEC-RAS FY99BridgesAgain, not that important. You will see in the output from time to time.HEC-RAS FY99BridgesIf piers are large part of obstruction to flow, this method is probably not appropriate. Also appropriate for supercritical flow.HEC-RAS FY99BridgesMomentum is best for when piers and friction losses are predominant.HEC-RAS FY99BridgesTaken from Table 5.2 of Hydraulic Reference Manual - page 5-13HEC-RAS FY99BridgesAppropriate for Class A low flow only. Should be used only when piers are the predominant obstruction ( such as a RR trestle)HEC-RAS FY99BridgesTaken from Table 5.3 of Hydraulic Reference Manual (page 5-14)HEC-RAS FY99BridgesThis is applicable for sub-critical flow only. Should not probably be used where piers are large obstruction to flow.HEC-RAS FY99BridgesAlso note that WSPRO and Yarnell are only applicable for sub-critical flows. If piers are involved, Momentum and Yarnell approaches are best. If both piers and friction are about equal, then momentum is best.HEC-RAS FY99BridgesHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesBest to enter after main cross sections are in (4, 3, 2, and 1). Automatically brings up bounding cross sections based on station of bridgeHEC-RAS FY99BridgesRefer to same handout for locations of x-sections 2, 3, and 4HEC-RAS FY99BridgesHEC-RAS FY99