Tutorial HEC Ras Puentes

Bridges

2 Types of Flow @ Bridges • Low Flow - Flow where the water surface does not reach the low beam • High Flow - Flow where the water surface reaches the deck or higher • There are sub-types for both low and high flow • Often both types of flow occur in single simulation with different profiles

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Low Flow

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Low Flow Bridge Modeling 3 Types of Flow  



Class A Low Flow - Subcritical Class B Low Flow - Passes through critical depth Class C Low Flow - Supercritical

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Low Flow Bridge Hydraulics 4 methods of modeling  Energy - 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. May, 1999

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Low Flow Bridge Hydraulics Appropriate Methods  Bridge piers are small obstruction to flow, friction losses predominate - Energy, Momentum, or WSPRO  Pier and friction losses predominate - Momentum  Flow passes through critical depth in vicinity of bridge - Energy or Momentum  Pier losses are dominant - Yarnell  Supercritical flow without piers - Energy or Momentum  Supercritical flow with piers - Momentum

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Low Flow Bridge Modeling Class A Low Flow - Energy Method • Friction 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.

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Low Flow Bridge Modeling Class A Low Flow - Momentum Method • Friction losses are external skin friction = wetted perimeter times length times shear stress. • Requires entering coefficient of drag for piers, CD • Check “Options” menu under “Bridge & Culvert Data” window for momentum equation options. Usually accept program defaults

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Low Flow Bridge Modeling CD Coefficients for Piers Circular Pier Elongated piers with semi circular ends Elliptical piers with 2:1 length to width Elliptical piers with 4:1 length to width Elliptical piers with 8:1 length to width Square nose piers Triangular nose with 30 degree angle Triangular nose with 60 degree angle Triangular nose with 90 degree angle Triangular nose with 120 degree angle May, 1999

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1.20 1.33 0.60 0.32 0.29 2.00 1.00 1.39 1.60 1.72 9 of 56

Low Flow Bridge Modeling Class A Low Flow - Yarnell Equation • Based on 2,600 lab experiments on different pier shapes • Requires entering pier shape coefficient, K • Should only be used where majority of losses are due to piers.

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Low Flow Bridge Modeling Yarnell’s Pier Coefficient, K Semi-circular nose and tail Twin-cylinder piers with connecting diaphragm Twin-cylinder piers without diaphragm 90 degree triangular nose and tail Square nose and tail Ten pile trestle bent May, 1999

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0.90 0.95 1.05 1.05 1.25 2.50 11 of 56

Low Flow Bridge Modeling Class A Low Flow - WSPRO • Federal Highway Administrations method of analyzing bridges • Uses energy equation in an iterative procedure

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Low Flow Bridge Modeling Class A Low Flow - Summary • Energy & Momentum equations are appropriate for most bridges • Yarnell should only be used when piers are the major obstacles to flow • Yarnell cannot be used when there are no piers • Conservative approach is to select all methods and use highest energy loss

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High Flow - Pressure

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High Flow - Pressure

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High Flow - Pressure & Weir

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High Flow Bridge Modeling • When 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.

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High Flow - Weir Flow 3

Q  CLH 2 Q = Total flow over the weir C = Coefficient of discharge for weir flow (~2.5 to 3.1 for free flow) L = Effective length of the weir H = Difference between energy elev. upstream and road crest Default Max Submergence = 0.95 (95%) (see next slide) After max submergence reached, program reverts to energy equation for solution. May, 1999

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High Flow - Submergence Qsubmerged  Q free  Reduction Factor H2 Submergenc e  H1

Default Max Submergence = 0.95 (95%) (see next slide) After max submergence reached, program reverts to the energy equation for the solution. May, 1999

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High Flow - Submergence

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Adding the Bridge

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Adding the Bridge Select the Brdg/Culv button from the geometry data window:

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Adding the Bridge This brings up the Bridge Culvert Data window:

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Adding the Bridge From the options menu, select “Add a Bridge and/or Culvert”:

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Adding the Bridge This brings up a window asking for the river station of the bridge. It will locate the bridge numerically between crosssections:

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

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

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

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Adding the Bridge

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Adding the Bridge Enter 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:

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Adding the Bridge Use the “Copy Up to Down” button to repeat the stationhigh chord-low chord data from the upstream to the downstream side, if applicable:

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Adding the Bridge After selecting OK from the bridge deck window, it replots the U.S. and D.S. xsections showing the bridge deck:

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

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

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Adding the Bridge The pier is then shown graphically on the plot:

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

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Adding the Bridge This 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:

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Adding the Bridge

See following graph Orifice Coef. between 0.7 and 0.9

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High Flow - Orifice Discharge

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Adding the Bridge There are several other options in the options menu from the bridge data window …

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Adding the Bridge … as well as the view menu:

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Adding the Bridge The geometric data schematic is also updated automatically:

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Locating Cross-Sections Near Bridges

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Cross-Sections Near Bridges

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Cross-Sections Near Bridges

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Cross-Sections Near Bridges

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Cross-Sections Near Bridges

 Fc2  5   1.8 10 Q ER  0.421 0.485  Fc1  May, 1999

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Cross-Sections Near Bridges

 Fc2   nob  Qob     1.86  CR  1.4  0.333   0.19  Q   Fc1   nc  2

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0.5

Cross-Sections Near Bridges Rule of Thumb: ER = 2:1

CR = 1:1

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Bridge Cross Sections

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Bridge Cross Sections

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Bridge Cross Sections Example Computation of Le and Lc 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

This assumes ER=2 and CR=1

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Expansion & Contraction Coefficients

Contraction Expansion No Transition 0 0 Gradual Transition 0.1 0.3 Typical Bridge Transition 0.3 0.5 Abrupt Transition 0.6 0.8

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Bridge Modeling Expansion and Contraction Coefficients at the 4 cross-sections for a bridge (example) Expansion Contraction Cross-Section 4 (furthest US) Cross-Section 3 Cross-Section 2 Cross-Section 1(furthest DS)

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0.5 0.5 0.5 0.3

0.3 0.3 0.3 0.1

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Ineffective Flows

At XS’s 2 & 3

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The End

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