Monroe L. Weber-Shirk S chool of Civil and Environmental Engineering HEC-RAS.

Monroe L. Weber-Shirk

School of Civil and Environmental Engineering

HEC-RASHEC-RAS

HEC-RASHEC-RAS

US Army Corps of Engineers Hydrologic Engineering Center

River Analysis System

US Army Corps of Engineers Hydrologic Engineering Center

River Analysis System

LegendWS 10 yr

WS 50 yr

WS 100 yr

Ground

Bank Station www.wrc-hec.usace.army.mil/

Software for Steady-State Water Surface Profiles

HEC-RAS analyzes networks of natural and man-made channels and computes water surface profiles based on steady one-dimensional flow hydraulics.

includes composite channels supercritical-to-subcritical flows multi-waterway bridges culvert options ...

Hydraulic Analysis ComponentsHydraulic Analysis Components

Steady Flow Water Surface Profiles flood plain management flood insurance studies effects of channel modifications

Unsteady Flow Simulation (future) model __________ levee failures

Sediment Transport/Movable Boundary (future) long term trends of scour and deposition maximum scour during large flood events design channel ___________to maintain navigation depths

Steady Flow Water Surface Profiles flood plain management flood insurance studies effects of channel modifications

Unsteady Flow Simulation (future) model __________ levee failures

Sediment Transport/Movable Boundary (future) long term trends of scour and deposition maximum scour during large flood events design channel ___________to maintain navigation depths

storagestorage

contractionscontractions

Steady Flow Water Surface Profiles

Steady Flow Water Surface Profiles

Systems of channels network dendritic single river reach

Subcritical, Supercritical, and Mixed Channel Controls/Obstructions

bridge piers culverts weirs

Systems of channels network dendritic single river reach

Subcritical, Supercritical, and Mixed Channel Controls/Obstructions

bridge piers culverts weirs

branchingbranching

Computational ProcedureComputational Procedure

One-dimensional energy equation (_______ ___)

energy losses friction - Manning Equation contraction/expansion - loss coefficient

Momentum equation hydraulic jumps hydraulics of bridges stream junctions

One-dimensional energy equation (_______ ___)

energy losses friction - Manning Equation contraction/expansion - loss coefficient

Momentum equation hydraulic jumps hydraulics of bridges stream junctions

standard stepstandard step

g

V

g

VCSLhe

22

222

211

f

xSg

VyxS

g

Vy o f

22

2

21

122

Computational ProcedureComputational Procedure

Assume a water surface elevation at the upstream cross section (or downstream cross section if a supercritical profile is being calculated)

Based on the assumed water surface elevation, determine the corresponding total conveyance and velocity head.

Assume a water surface elevation at the upstream cross section (or downstream cross section if a supercritical profile is being calculated)

Based on the assumed water surface elevation, determine the corresponding total conveyance and velocity head.

2/13/21fh ASR

nQ AR

nK h

3/212/1

fKSQ

?

Computational ProcedureComputational Procedure

Compute Sf and solve for losses

Solve the energy equation for the water surface

Compare the computed value of depth with the assumed value and ______ until the values agree within 0.01 feet.

Compute Sf and solve for losses

Solve the energy equation for the water surface

Compare the computed value of depth with the assumed value and ______ until the values agree within 0.01 feet.

g

V

g

VCSLhe

22

222

211

f

xSg

VyxS

g

Vy o f

22

2

21

122

iterateiterate

= he

Data RequirementsData Requirements

Channel description length of reach channel roughness channel cross-section geometry

Boundary conditions Structure geometry

bridges culverts weirs

Channel description length of reach channel roughness channel cross-section geometry

Boundary conditions Structure geometry

bridges culverts weirs

River ReachRiver Reach

Tributary

0.2

0.1

0.0

But t

e Cr.

Upper Reach

10

9.9

9.8

Fa

l l

R i ver

Lower Reach9.7

9.6

9.5

Fa l l Ri ve r

Sutter

River StationsRiver StationsNumeric labels increase upstream

Cross Section DataCross Section Data

x-y coordinates of channel bottom

distance to downstream cross-section

Manning’s n

x-y coordinates of channel bottom

distance to downstream cross-section

Manning’s n

Channel Cross SectionChannel Cross Section

N

iih

i

f ARn

SQi

1

3/21...21 QQQ

...21

fff SSS

2/13/21fh ASR

nQ

Manning n for overbank areas usually higher than for main channelComposite channel calculations...

Channel Section InterpolationChannel Section Interpolation

Water surfaces are calculated at each river station

If water depth changes too much between river stations then the calculations are imprecise

Interpolate between rivers stations of known geometry

Water surfaces are calculated at each river station

If water depth changes too much between river stations then the calculations are imprecise

Interpolate between rivers stations of known geometry

Inline Weir Station Elevation Editor

Inline Weir Station Elevation Editor

Resulting cross sectionWeir Editor

Boundary ConditionsBoundary Conditions

Ways to specify Boundary Conditions Known Water Surface Elevations Critical Depth _______________ Normal Depth _______________ Rating Curve _______________

Boundary Condition Requirements Supercritical Flow ______________ Subcritical Flow ______________ Mixed Flow ______________

Ways to specify Boundary Conditions Known Water Surface Elevations Critical Depth _______________ Normal Depth _______________ Rating Curve _______________

Boundary Condition Requirements Supercritical Flow ______________ Subcritical Flow ______________ Mixed Flow ______________

Mild to Steep TransitionMild to Steep Transition

Uniform flowUniform flow

Control structureControl structure

Upstream depthUpstream depth

Downstream depthDownstream depth

Upstream and DownstreamUpstream and Downstream

Program StructureProgram Structure

Input Output

Channel geometry

Flows and boundary conditions for each profile

Cross Sections

Profiles

Computed Rating Curves

3-D Cross Sections

Tabular Data

Errors

Other Analysis

Scour at bridges

-1020 -1015 -1010 -1005 -1000 -995 -990 -985

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

homework homework plan 2 4/26/99

Main Channel Distance (m)

Ele

vatio

n (m

)

Legend

EG PF#1

WS PF#1

Ground

Change from Mild to Steep SlopeChange from Mild to Steep Slope

M2M2

S2S2

From this plot how can you know if flow is super or sub critical?

Eyc

3

2

Mild slope behind ObstructionMild slope behind Obstruction

0 200 400 600 800 1000

0.0

0.2

0.4

0.6

0.8

1.0

homework homework plan 2 4/21/99

Main Channel Distance (m)

Ele

vatio

n (m

)

Legend

EG PF#1

WS PF#1

Ground

M1M1

HEC-RAS SummaryHEC-RAS Summary

HEC-RAS solves the energy and momentum equations to calculate water surface profiles

Modeling natural rivers is made difficult by the need to obtain and enter the geometric data

Currently restricted to steady-state flow

HEC-RAS solves the energy and momentum equations to calculate water surface profiles

Modeling natural rivers is made difficult by the need to obtain and enter the geometric data

Currently restricted to steady-state flow

Water Surface ProfilesWater Surface Profiles

Broad-crested WeirBroad-crested Weir

3-D Cross Sections 3-D Cross Sectionshomework homework plan 2 4/21/99

Legend

WS PF#1

Ground

Bank Sta

Ground

Boundary Condition EditorBoundary Condition Editor

Known Water Surface Critical Depth Normal Depth Rating Curve

Known Water Surface Critical Depth Normal Depth Rating Curve