INTRODUCTION TO HEC-RAS

HEC- RAS stands for Hydrologic Engineering Center’s River Analysis System

One dimensional analysis of :1. Steady flow2. Unsteady flow3. Sediment transport (mobile bed, as opposed to rigid

bed)4. Water temperature modeling

By U.S. Army Corps of Engineers

ABOUT THE U.S. ARMY CORPS OF ENGINEERS

• Under the Department of Defense

• Doing engineering, design and construction managements : dam, canals and flood protection

• Established since 1775 (about 240 years)

HEC-RAS(THE ENGINE FOR STEADY FLOW AND UNSTEADY FLOW)

Steady flow

• Energy equation or momentum equation are used

• Energy equation (refer previous notes)

• Momentum equation

2 21 2

1 1 2 22 2 eV Vz h z h h

g g

2 1 force acting on control surfaceQV PA QV PA

Application of momentum equation on flow where energy is not conserved

Unsteady flow

• Continuity and momentum equation for unsteady flow are used

The continuity equation for unsteady flow

0A Qt x

MOMENTUM EQUATION

Momentum equation (unsteady flow)

Neglecting Reynold shear stress,

sin Reynold's stressess byQ uQ gA gAt x x gR

2

2u

so f

yQ uQ gA gA S St x x

Derivation skipped

The continuity equation for unsteady flow

0A Qt x

so f

so f

so f

yQ VQ gA gA S St x x

Q VAVA V VA ygA gA S St x t

yV VV g g S St x x

ST. VENANT’S EQUATION(SHALLOW WATER EQUATION) Flood waves propagation

which are adequately represented by this model is called the Dynamic Wave.

Continuity equation

0A Qt x

Momentum equation

so f

yV VV g g S St x x

So this model is also known as the Dynamic Wave model

Shallow water equation• Horizontal length scale >> vertical length scale, • Therefore, vertical pressure gradient is almost

hydrostatic

KINEMATIC WAVE THEORY

- It is one of the many approximations from dynamic wave

- Some insignificant terms in dynamic wave equations are neglected

Momentum equation

so f

yV VV g g S St x x

Insignificant due to long and flat wave approximation in kinematic wave theory

o fS SBalance between gravitational and frction forces ! uniform flow

KINEMATIC WAVE THEORY

Continuity equation 0 constantQ Qx

1/6

2/3 1/2

Uniform flow equations :

1) Chezy's equation 11) Manning's equation :

1Therefore, =

o

o

Q C RS

C Rn

Q R S An

• REQUIRED INFORMATION

• GEOMETRY DATA – CHANNEL DIMENSION ETC

• FLOW DATA – DISCHARGE

• BOUNDARY CONDITIONS (B.C.)

• DOWNSTREAM FLOW DEPTH, UPSTREAM FLOW DEPTH.

• DEPENDING ON FLOW TYPE (SUPER OR SUB CRITICAL), THE REQUIREMENT OF B.C ARE DIFFERENT

• SUBCRITICAL FLOW = DOWNSTREAM FLOW CONDITION IS REQUIRED

• SUPERCRITICAL FLOW = UPSTREAM AND DOWNSTREAM FLOW CONDITION ARE REQUIRED

• OTHER INFORMATION – EXISTENCE OF STRUCTURES : EMBANKMENT, BRIGDE PIER, JUNCTION ETC

HEC- RAS : SIMULATION OF STEADY FLOW

HEC- RAS : SIMULATION OF STEADY FLOW

HEC-RAS MAIN WINDOW

Geometric data

Perform steady flow cal. (RUN) View cross section, profiles etc

Flow data

Open example Critical Creek – Example 1

Geometric data

Step 1 : Check geometric data

You will see this window showing the Geometric data

Load other geometric data

Step 1 : Check geometric data

You will see this window showing new Geometric data

Reach length

upstream

downstream

Cross section data

You will see this window showing the cross section data

Step 1 : Check geometric dataSummary of geometric data (can edit here as well)

Step 1 : Check geometric data LOB = left over bank, ROB= right over bank

LOB ROB

Main channel(btwn two red dots)

Step 1 : Check geometric data

Save geometry data !

Step 2 : Check flow data

HEC-RAS MAIN WINDOW

Flow data

Step 2 : Check flow data – Boundary condition

For this example, normal depth with So=0.01 is set for downstream B.C.

Step 2 : Check flow data – Boundary condition

Save Flow data !

Step 2 : Check flow data

Step 3 : Run SIMULATION

Geometry data file

Flow data file

The flow type in the regime

Give an ID

RUN

See Options

Set critical depth always calculated

Step 3 : Run SIMULATION

RUN STATUS

Computation messages !!

Step 4 : Flow Output Review

HEC-RAS MAIN WINDOW

View profiles

Flow depth is close or equal to critical depth from station 8 to 12

View summary errors, warning and notes

RS 12 1. Velocity head has change more than 0.5m 2. Energy loss was greater than 0.3m between current and previous section3. Reason of 1. and 2. are inter-related and are due to drastic change in velocity. Since velocity is influenced by

cross section. For example, between two sections there is a expansion/contraction of channel section area, but only two sets of data on each sections are given (not enough to represent the expansion / contraction of the channel section) inadequate cross-section data !. Other reasons might be : cross section data inputted wrongly.

RS 11 1. Energy equation could not be balanced – program could not calculate enough energy losses to achieved

subcritical flow upstream. Therefore, program choses to use critical depth (default in the program), and continued the calculation

1. Divided flow computed for his cross-section – flow occurs at two or more separate portion in the channel

There is flow in this portion too !!

Step 5 : Modify and fix cross section ! –

- Maybe cross sections information not adequate - Add more cross sections

Before modification of cross section

Step 5 : Modify and fix cross section ! –

- To add more cross sections for all reaches Interpolation feature

You can set other values

Step 5 : Modify and fix cross section ! –- To add more cross sections for all reaches Interpolation feature

Press this to interpolate

Before addition of cross sections After addition of cross sections

Correct interpolation between RS 11 and RS 10

Use this feature

Delete one interpolation

Re-interpolate

Step 6 : Change simulation conditions

- In previous simulation, some subcritical depth could not be calculated, the program use the critical when this occurs.

- Therefore, there might be possibility of mixed flow (subcritcal and supercritical in the reaches)

- So, we change the simulation to mixed flow type

- But, we need to add in boundary conditions to do mixed flow type simulation !

Upstream BC is set to Normal Depth with S=0.01m

Step 7 : Re-run with new data and simulation conditions

Step 8 : Examine results

View of profile output table

Water surface < Critical depth(supercritical flow)

Froude number <1 ??!

average velocity = / , flow areaaverage depth at that section

e

e

e e e

e

VFrgy

V Q A Ay

HEC-RAS MAIN WINDOW

View Detail output

Hydraulic jump

12

11

10

Hydraulic jump ?Supercritical to subcritical