11 Multiple Openings in Bridges

7/27/2019 11 Multiple Openings in Bridges

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Multiple Bridge and CulvertOpenings

With HEC-RAS

Objective: To become familiar with

the multiple opening flow options.


2/21

Multiple Openings

Divided Flow Approach

Multiple Opening Approach

Entering Multiple Opening Data

Multiple Opening Output


3/21

Divided Flow Approach

High Ground

1

2

3

4

5

6

7

8

Q

Q

T

T

Stagnation Point

Stagnation Point

Q Q12


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A Multiple Opening

Multiple Opening Test Plan: Multiple Opening planLegend

Ground

Bank Sta

Inef f


5/21

Example of Multiple Opening (see previous slide)

There is a bridge opening over the main stream and aculvert group in the overbank area.

Types of openings: bridges, culvert groups (a group ofculverts is considered to be a single opening), andconveyance areas.

In the above example, the low area on the side wouldnot act hydraulically as a weir. Therefore, instead ofincluding it as part of the bridge, it should be modeledas a separate, conveyance opening.

Up to seven openings can be modeled at a givenlocation, and any combination of bridges and culvertgroups can be used.

A maximum of two conveyance type openings can beused and the conveyance area must be on theextreme left or extreme right side.


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Multiple Opening

0 200 400 600 800 1000510

515

520

525

530

535

540

RS=3.5 Upstream (Multiple Opening)

Station (f t)

Elevation

(ft)

Legend

Ground

Inef f

Bank Sta

Stag Limit

#1

#2

#3


7/21

Plan View of Multiple Opening


8/21

Locating Stagnation Points

Stagnation Pointfloats inside

overlapping arrows.

For low flow,Stagnation Points

should always be

inside of ineffective

area block!400 500 600 700 800

510

515

520

525

530

RS=3.5 Upstream (Multiple Opening)

Station (ft )

Elevation

(ft)

Legend

Ground

Inef f

Bank Sta

Stag Limit

#2#3


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Computational Procedure for Multiple

Opening

1. Program assumes a flow distribution

2. Energy computed for each opening

3. Energies compared for each opening

4. Flows are adjusted and procedure is

repeated

5. Program iterates until convergence or

maximum number of trials


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Multiple Opening Limitations

This is a one-dimensionalmodel!

Energy grade line is assumed constant

upstream and downstream of opening

Stagnation points are not allowed tomigrate past the edge of adjacent

opening.


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Divided Flow Tradeoffs

Energies for each reach/opening canbe different

Energies for each reach/opening

may be too different User must decide how to split the

cross sections.


12/21

Entering Multiple Opening Data

Multiple opening data are entered in thesame manner as any other bridge or

culvert

From the Geometric Data window, pressthe Bridge/Culvert button and select the

appropriate reach

Enter the river station


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Enter Deck and Road Data

200 400 600 800 1000 1200510

515

520

525

530

535

RS=3.5 Upst ream (Multiple Opening)

Station (f t)

Elevation

(ft)

Legend

Ground

Ineff

Bank Sta

Stag Limit

#1#2


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Pier and Abutment Data

200 400 600 800 1000 1200510

515

520

525

530

535


Station (ft )

Elevation

(ft)

Legend

Ground

Inef f

Bank Sta

Stag Limit

#1#2


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Bridge Modeling Approach


16/21

Enter Culvert Data

700 750 800 850 900 950

520

522

524

526

528

530


Station (ft )

Elevation

(ft)

Legend

Ground

Ineff

Bank Sta

Stag Limit

#2#3


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Defining the Openings


18/21

Multiple Opening Output

0 200 400 600 800 1000510

515

520

525

530

535

540

Multiple Opening Test Plan: Multiple Opening plan

Station (f t)

Elevation

(ft)

Legend

EG PF 4

WS PF 4

EG PF 3

WS PF 3

EG PF 2

WS PF 2

EG PF 1

WS PF 1

Ground

Inef f

Bank Sta

.08 .04 .08


19/21

Bridge and Culvert Output


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Conveyance Output


21/21

Multiple Opening Profile Table

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11 Multiple Openings in Bridges