INTRODUCTION TO HEC-RAS - USMredac.eng.usm.my/EAH/document/Lecture 9 (PUAY) Introduction to...
Transcript of INTRODUCTION TO HEC-RAS - USMredac.eng.usm.my/EAH/document/Lecture 9 (PUAY) Introduction to...
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