2011 ras for_managers_11-22

HEC-RAS for Managers
Presented by
Raymond Walton, Ph.D., P.E,.
D.WRE, F.ASCE
06/04/13
With thanks to
Dr. James L. Martin
and
From NRCS workshop
by Reep, 2004

HEC-RAS for Managers - Lecture
Benefits and Learning Outcomes
 Understand why hydraulic information is
important
 Learn basic hydraulic modeling terminology
 Develop basic understanding of HEC-RAS
and why it is useful
 Learn what is needed to use it
 Become aware of concerns for managers
 Identify resources available to help you use
HEC-RAS
 Develop an appreciation for British Humour
06/04/13

Agenda
Review of basic definitions and terminology
Provide answers for the following questions:
 What is hydraulics?
 Why do we need models?
 What is HEC-RAS?
 What can you do with HEC-RAS?
 What is needed to use HEC-RAS?
 What concerns should I have about the use of
HEC-RAS?
 Where can I go for help with HEC-RAS?
06/04/13

Hydraulic Information - It’s
Important
Necessary for planning,
flood risk reduction,
design, environmental
impact assessment and
mitigation, restoration
06/04/13 1a-4

Hydraulic Parameters
Velocity
Depth
Shear
Width
Area
etc
06/04/13

What is Hydraulics?
 Definition (Webster’s New World Dictionary)
 Hydraulics (hi-dro’liks), n.pl.
 “The branch of physics having to do with the
mechanical properties of water and other liquids and
the application of these properties in engineering”
 The branch of fluid mechanics which
addresses fluid in motion (a.k.a., fluid
dynamics), while hydrostatics addresses fluid
at rest.
06/04/13

Background – Areas of Hydraulics
 Pressure flow (e.g., closed conduits)
 Free surface flow (e.g., rivers and
streams)
 HEC-RAS is primarily devoted to
addressing free surface flow; however, it
does have capabilities to consider culverts
and limited abilities to represent closed
conduits06/04/13

Background – Hydraulic Parameters
 Characteristics that define the properties of
flowing liquid.
 For free surface flow, primary parameters
include:
 Depth
 Flow area
 Velocity
06/04/13
Hydraulic models calculate
these parameters

Background – Types of Flow
 Steady Flow – At any point, depth and
velocity remains constant with respect to
time, such as a constant discharge in a
long straight canal.
06/04/13

06/04/13
Unsteady flow – Depth and/or velocity
changes in magnitude or direction with
respect to time, such as a flood hydrograph
or a curve in a channel.

Uniform Flow
 At a given instance, depth and velocity
remains constant with respect to distance
along a streamline (streamlines must be
straight and velocity is constant, such as in
a canal)
 Slope of the water surface, channel bed
and total head line are all the same.
06/04/13

Non Uniform (varied) Flow
 Examples: flow in a bend or at expansions
or contractions in the flow area.
06/04/13

Basic Open Channel
Flow Concepts
Steady vs. Unsteady
Uniform vs. Varied
Classification of Open Channel Flow
T1,T2
T1
T2

Background – Normal Depth
a.k.a., Steady Uniform Flow
Calculated using Manning’s equation
Q = 1.486/n A R2/3
S0
1/2
Why is this important?
06/04/13

Background – Critical Depth
Flow Regime
Fr = ratio of the inertia force over the gravitational force
gD
V
Fr =
Where:
V = Velocity
D = Hydraulic Depth
g = acceleration of gravity
(1)
Fr < 1 The flow is
Subcritical
Fr = 1 The flow is at
Critical depth
Fr > 1 The flow is
Supercritical

Flow Regime (Cont.)
 Wave propagations
Wave Celerity
gDc =
Flow
Still Water
Subcritical Critical Supercritical
c > V c = V c < V
(2)

Equations for Basic Profile
Calculations
Water surface profiles are computed from one cross section to the next
by solving the Energy equation with an iterative procedure called the
standard step method.
The Energy equation is written as follows:
eh
g
V
yz
g
V
yz +++=++
22
2
11
11
2
22
22
αα
Where: y1, y2 = Water depths cross sections
z1, z2 = Elevation of bed above project datum (e.g., NAVD)
V1, V2 = Average velocities (total discharge/total flow area)
α1, α2 = Velocity weighting coefficients
g = Gravitational acceleration
he = Energy head loss

 Specific Energy is the total of the potential and kinematic energy
g
V
yE
2
2
α
+= (5)
Specific Energy
Where:
E = Specific energy
y = Flow depth
z = Height above
datum
α = Velocity weighting
coefficients
V = Velocity
g = Gravitational
acceleration
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
0 5 10 15 20 25
Specific Energy (ft)
FlowDepth(ft)
y
2g
V2

Basic Open Channel
Flow Concepts
 Critical depth (Yc) is the depth associated with the minimum specific
energy
 Dependent on cross section geometry and flow.
Specific Energy – Critical Depth
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
0 5 10 15 20 25
Specific Energy (ft)
WaterSurface(ft)
Supercritical Flow
Subcritical Flow
Yc
SEmin
y
2g
V2

Gradually Varied Flow (GVF)
 Where changes in cross section take place
very gradually with distance along the
channel, acceleration effects are negligible
 Changes include geometry, obstructions
and balance between resisting forces and
force of gravity tending to accelerate the
flow
 HEC-RAS calculates water surface profiles
under GVF conditions06/04/13

Rapidly Varied Flow (RVF)
 Examples: Waterfalls, hydraulic jumps and
steep channels (greater than 10 percent
slope)
 HEC-RAS can not directly represent RVF
06/04/13

Basic Open Channel
Flow Concepts
Gradually Varied vs. Rapidly Varied

Hydraulic Computer
Models: Why do we care?
Direct Measurements:
hard, expensive,
dangerous
Analytical: predictive
06/04/13

Background
One, Two and Three Dimensional Flow
06/04/13
X direction
Ydirection
Z
direction

Background – 1 Dimensional Flow
Changes in flow velocity are restricted to only
one direction (X).
Models readily available
Well understood
Commonly used
06/04/13
X
Direction

Changes in flow velocity are restricted to only
two directions (X, Y)
Better answers?
Great visualization tool
Accounts for momentum transfer
Use in confluences/complex areas
06/04/13

Two-Dimensional Modeling
X
Direction
YDirection

Changes in flow velocity occur in three
directions (X,Y,Z)
Data for calibration?
When necessary?
Sophisticated
Computer intensive
06/04/13

Background – What is a model?
A mathematical representation
(approximation) of physical processes.
Useful for
 Characterization of existing conditions
 Evaluation of proposed conditions
 Estimation/mitigation of potential impacts
06/04/13

Background – Modeling
Wisdom
06/04/13
“All models are wrong. Some are useful”
George E.P. Box, UCL Mathematician
and Statistician (Un. Wisconsin)
“Everything should be made as simple as
possible, but not simpler ” Albert Einstein
"For every complex problem there is
a simple solution, and it is wrong."
H.L. Mencken

Background – More Modeling
Wisdom
06/04/13
“The model is never wrong. The modeler
often is”
Ray Walton, UCL Mathematician
and (part-time) Statistician (Un. Florida)

What is HEC-RAS?
US Army Corps of
Engineers’
Hydraulic Engineering
Center
River Analysis System
06/04/13

The HEC-RAS Modeling System
1D River Hydraulics
Graphical User Interface
Steady & Unsteady Flow
Bridges, Culverts, Dams,
weirs, levees, gates, etc…
Data storage/management
Graphics, Tabular Output &
Reporting
GeoRas – ArcGIS
More coming………
06/04/13

History of HEC-RAS Development
In the beginning, there was HEC-2 and UNET and
HEC-6, and ……
1D Steady Flow Analysis
 FY 1992 - 1999
 Produced Steady flow versions of HEC-RAS (Beta 1&2,
Versions 1.0 - 1.2, 2.0 – 2.2)
1D Unsteady Modeling for River Analysis
 FY 2000 – 2005
 Versions 3.0 – 3.1.3
1D Steady and Unsteady Hydraulics. Sediment
Transport and Water Quality Modeling, Version 4.0,
March of 2008
 Now up to Version 4.1.0 (2010)
1D/2D/(3D?) Steady and Unsteady Hydraulics and
dynamic groundwater, Version 5.0, stay tuned!!!06/04/13

HEC-RAS is software that allows the
user to perform one dimensional
steady and unsteady flow river
hydraulics calculations.
- Public domain and freely distributed
by the Corps of Engineers
- Support available from private
vendors
go to:
www.hec.usace.army.mil/software/hec-ras/
06/04/13 1-36

What is HEC-RAS?
Graphical User Interface (GUI) – Allows
efficient input of data and evaluation of output
Data Storage and Management – Provides
ability to create, store and combine data files
containing hydrologic, geometric, and
hydraulic design data into unique
combinations
06/04/13

What Units Can HEC-RAS
Use?
SI units (meters, seconds, etc.)
Units from less-developed countries (e.g.,
“U.S. Customary”)
06/04/13

RAS Project Schematic
•Existing
•Proposed
•Existing 10yr,
100yr, & 500yr
•10% Increase
•Existing
•Existing +10% Q
•Proposed
•Proposed +10%

Geometry
 Cross Section
 Spacing
 Structures
 Manning’s n
Flows
 Calibration Data
06/04/13

Weirs or Dams
Treated as in-line
structure
Can be modeled
using elevation-
control or time-control
Can be controlled with
scripts
06/04/13

Gates
06/04/13
Variety of gate types
Can be modeled
using elevation-
control or time-control
Can be controlled with
scripts

GIS Interface
Interface with GIS software for efficient
data input and output display and
evaluation - Identify areas of flood
inundation
HEC-GeoRAS
More GIS tools
06/04/13

Hydraulic Design Functions
 Bridge scour
 Uniform Flow
 Stable channel
design
 Sediment transport
capacity
 Sediment Impact
Assessment
(SIAM)
06/04/13

Sediment Transport and Budgets
06/04/13

Water Quality (including temperature)
06/04/13

Graphical
Tabular
Profile
Section
06/04/13

From NRCS workshop by Reep, 2004
06/04/13

From NRCS workshop by Reep,
2004
06/04/13

What concerns should Managers
have about HEC-RAS?
1-Dimensional Model
 Appropriate application of
model?
Recognize and
Understand Limitations
 Flow is gradually varied with
distance
 Dominant flows in x direction
 Applicable to channel slopes
less than 1:10H
06/04/13

Know what it can do – HEC-RAS
Capabilities
Bridge Hydraulics -extensive
Culverts (9 types)
Multiple Opening (bridges &
culverts)
Inline Structures – gates & weirs
Lat. Structures – gates, levees,
weirs, culverts, and rating curves
Storage/ponding areas
Hydraulic connections between
storage areas
Pump Stations
Floating ice
Levees
Extensive data import and export
GIS Connections
Steady flow profiles
Unsteady flow simulations
FEMA floodway encroachments
Split flow optimization
Sediment Transport Capacity
and Bridge Scour
Dam and Levee Breaching
Navigation Dam Operations
Channel Modifications
Mixed Flow regime
Extensive Calibration Features
Geometric Features: Analysis Features:
06/04/13

Water Surface Profile Plots
Cross Sections
Rating curves
Stage and flow hydrographs
Generalized profile plot of any
variable (I.e velocity)
3D view of river system
Graphical Animations
250+ output variables at every
cross section per profile
Detailed output tables for XS
and all structures
Summary output tables
User define output
Extensive Manuals
 User’s Manual
 Hydraulic Reference Manual
 Applications Guide
Online Help System
Example Data Sets
Graphical Output: Tabular Output:
Documentation:
06/04/13
Know what it can do – HEC-RAS
Capabilities (continued)

have about HEC-RAS?
(continued)
Input Data
 Remember: Garbage In, Garbage Out (GIGO)
 Need enough to get the job done (spacing)
 Data can be expensive to collect
 Look for existing data sources
 Pay attention to datums
 LiDAR data can be inaccurate
 Use model to guide data collection
06/04/13

have about HEC-RAS? (continued)
Calibration/Validation
 What is calibration?
 When is it “calibrated”?
 What is validation?
 What if data for calibration/validation are
unavailable?
06/04/13

Rigid bed assumption
 Hydraulic calculations are all made with a
rigid bed assumption
 Hydraulic roughness can change with
bedform (dunes, gravel bars, debris)
 Bedforms can change with sediment input
and temperature (seasonality)
 Sediment transport / Unstable channels
 Lateral channel migration not considered
06/04/13

Sensitivity of results
 Conduct a sensitivity analysis/risk
assessment
 Look at accuracy of involved input data
 Topography/geometry
 Hydrology
 Hydraulic roughness
 Structures/Modeling techniques
 Look at accuracy of output application
 Mapping accuracy
06/04/13

Any idiot can use it
Need qualified users
 Qualifications, Training and
Experience
Provide QA/QC
 Appropriate application
 Input, Output
06/04/13

What is so great about HEC-RAS?
Public Domain/Free/Widely used
Supported by Corps of Engineers
Continuing Improvement
 2-D capabilities being developed
 Expected by end of 2012
Graphical Abilities
Good Documentation
Training Available
06/04/13

HEC-RAS - Documentation
 User’s Manual: provides an introduction and overview of
the modeling system, installation instructions, how to get
started, a simple example, detailed descriptions of each of the
major modeling components, and how to view graphical and
tabular output
 Hydraulic Reference Manual: describes the theory
and data requirements for hydraulic calculations
 Applications Guide: contains a series of examples that
demonstrate various aspects of HEC-RAS.
06/04/13

The HEC-RAS software is an
engineering tool.
Not a replacement for sound
engineering.
06/04/13

2011 ras for_managers_11-22

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