Additional HEC-RAS Modeling Features

First CoordinateLeft BankRight BankLast Coordinate

Invert

Downstream Section

Upstream Section

InterpolatedSection

Objective• To allow you to become familiar with some of the

optional capabilities in HEC-RAS

Optional Capabilities• Multiple Plan Analysis• Effective Area Option• Cross Section Interpolation• Graphical Editing Features• Mixed Flow Regime Calculations• Flow Distribution Calculations• Inline Weirs and Gated Spillways

Multiple Plan AnalysisProject File: filename.prj

Plan 1filename.p01

Plan 2filename.p02

Plan 3filename.p013

Steady Flow Datafilename.f01

Geometry Datafilename.g01

Geometry Datafilename.g02

Geometry Datafilename.g03

Run File 1 Run File 2 Run File 3filename.r01 filename.r02 filename.r03

Output File 1 Output File 2 Output File 3filename.o01 filename.o02 filename.o03

Notes on Plans• Modifications can be made to the geometry

and/or flow data, and then saved in separate files.

• Plans are formulated by selecting a particular geometry file and a particular flow file.

• The multiple plan option is useful when, for example, a comparison of existing conditions and future channel modifications are to be analyzed.

Multiple Plan Option• Can be used to perform a design of a specific

geometric feature. • For example, if you are sizing a bridge

opening, a separate geometry file could be developed for a base condition (no bridge), and then separate geometry files could be developed for each possible bridge configuration.

• A plan would consist of a flow file and one of the geometry files. Computations are performed for each plan individually.

Viewing tables and graphs• Once the computations are performed for all the plans,

the user can then view output in a graphical and tabular mode for any single plan or combination of plans.

Ineffective flow areas

KE = V2/2g, where V =Q/A

• Velocity head is associated with flow in downstream direction

• If area in the cross section is not carrying flow it should not be included

• The “ineffective area” option of RAS is used for this

Example:Ineffective Flow Areas at a Bridge

LeLc

typical flowtransition pattern

assumed flow transitionpattern for 1-dimensionalmodeling

3 2 14

Flow

1

ERCR1

BU BD

B b

Four cross sections are needed to describe the active flow area for flow through the bridge opening

Designation of Ineffective Area Related to Type of Bridge Flow

Types of Bridge Flow• Low flow through bridge opening• Pressure flow through bridge opening• Weir flow • Pressure and weir flow• High flows over bridge and approaches

Cross Sections at BridgeLocation of Cross Sections 2 and 3

• Bounding sections for bridge• Define bridge interior for upstream and downstream bridge sections• Must be defined before bridge data are input• Usually have ineffective flow area defined

LeLc

typical flowtransition pattern

assumed flow transitionpattern for 1-dimensionalmodeling

3 2 14

Flow

1

ERCR1

BU BD

B b

Ineffective Flow Areas at Cross Section 2

Ineffective Flow Area Stations and Elevations

Defining Ineffective Area• Permanently block out area• Block out area until water surface reaches a given

elevation• Multiple blocks• Use a levee• Use very high n-value in regions with very low velocity

Normal Blocked Obstructions

Blocked obstructions decrease flow area and add wetted perimeter when the water comes in contact with the obstruction

Multiple Blocked Obstructions

Up to 20 individual blocks can be defined. The left station, right station, and an elevation are entered for each of the blocks.

Ineffective Area Option

Left Ineffective Flow Station

Right Ineffective Flow Station

The area is included in the storage calculations, but it is not included as part of the active flow area. No additional wetted perimeter is added to the active flow area.

Levee Option

Left Levee Station

No water can go to the left of the left levee station or to the right of the right levee station until the levee elevations are exceeded.

Levee Added

Left Levee Station and Elevation

A vertical wall is placed up to the established levee height. Additional wetted perimeter is included when water comes into contact with the levee wall.

Cross Section InterpolationMay Be Needed When: • The change in velocity head is too large• To better model friction losses• To prevent the program from defaulting to critical

depth• To generate a smoother plot of the water surface

profile

Cross Section Interpolation

First CoordinateLeft BankRight BankLast Coordinate

Invert

Downstream Section

Upstream Section

InterpolatedSection

Interpolation within a Reach

Interpolation between 2 Sections

Adding Additional Master Cords

Cross Section Interpolation• CAUTION: Automatic interpolation should not be used

as a replacement for required cross section data!• Interpolation can improve model calculations

(especially for unsteady flow analysis)

Graphical Editing Features

Modified geometry

Mixed Flow CalculationsSteady Flow Calculations

• Subcritical Flow• Supercritical Flow• Mixed Flow

Unsteady Flow Calculations• Subcritical Flow• Supercritical Flow• Mixed Flow

Mixed Flow

0 500 1000 1500 2000 2500 300060

62

64

66

68

70

72

74

76

Mixed Flow Plan: Mixed Flow Plan

Main Channel Distance (ft)

Elev

atio

n (ft

)

Legend

EG PF 1

WS PF 1

Crit PF 1

Ground

Mixed Reach

Specific Force Equation

222

222

111

121 YA

AgΒQYA

AgΒQ

maFonacceleratimassForce

Flow Distribution Option The flow distribution

option allows the user to see in different parts of the cross section:

• The amount of flow• The flow velocity

700 720 740 760 780 800

1804

1806

1808

1810

1812

1814

1816

Critical Creek - Example 1 Plan: Existing Conditions Upstream Boundary - XSec 12

Station (ft)

Ele

vatio

n (ft

)

Legend

EG 100 yr

WS 100 yr

Crit 100 yr

0 ft/s

1 ft/s

2 ft/s

3 ft/s

4 ft/s

5 ft/s

6 ft/s

7 ft/s

8 ft/s

9 ft/s

10 ft/s

11 ft/s

12 ft/s

13 ft/s

Ground

Bank Sta

.1 .04 .1

Setting the Flow Distribution Locations

Plotting Velocity Distribution

Velocity Distribution Plot

700 720 740 760 780 800

1804

1806

1808

1810

1812

1814

1816

Critical Creek - Example 1 Plan: Existing Conditions Upstream Boundary - XSec 12

Station (ft)

Elev

atio

n (ft

)

Legend

EG 100 yr

WS 100 yr

Crit 100 yr

0 ft/s

1 ft/s

2 ft/s

3 ft/s

4 ft/s

5 ft/s

6 ft/s

7 ft/s

8 ft/s

9 ft/s

10 ft/s

11 ft/s

12 ft/s

13 ft/s

Ground

Bank Sta

.1 .04 .1

Flow Distribution Output

CAUTION

• Flow distribution varies vertically and horizontally (all flows are 3-D in nature )

• HEC-RAS is a 1-D model

• Flow distribution is based on area and wetted perimeter of each flow slice (using fewer slices is generally better)

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)

Elev

atio

n (f

t)

Legend

Ground

Ineff

Bank Sta

Sluice and Radial Gates

Sluice Gate

Broad Crested Spillway

Radial Gate

Ogee Spillway Crest

Gate openings can handle both submerged and unsubmerged conditions

Cross Section Layout

FLOWFLOW

1234

EXPANSIONREACH REACH

CONTRACTION

Gated Spillways

Overflow Weir

Inline Weir Editor

Weir/Embankment

Gate Editor

Setting the Gate Opening

RecapWe covered:

• Multiple Plan Analysis• Effective Area Option• Cross Section Interpolation• Flow Regimes• Flow Distribution Calculations• Inline Weirs and Gated Spillways