Boundary

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HEC-RAS Version 3.1 Unsteady Flow
Presented by slides adapted from HEC Unsteady
Flow Course
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Unsteady Flow Documentation

• Technical/theoretical - Chapters 2 and 5 from EM
  1110-2-1416
• http//www.hnd.usace.army.mil - click on
  TECHINFO, then Engineering Publications, then
  Engineering Manuals, scroll for manual
• HEC-RAS Users Manual - Chapter 8 (data input and
  window operations)
• HEC-RAS Application Guide - Chapter 17 (example
  application)

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When to use Unsteady Flow

• Tidal/estuary fluctuation
• Off-channel storage
• Dam breach routing
• Channels with flat slopes
• Levee overtopping
• Hydraulic structures affected by changing
  backwater
• Large amounts of storage behind roads or culverts

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Steady vs. Unsteady

• Difference in handling boundary friction and
  other losses
• Difference in numerical solution algorithm
• Difference in handling non-flow areas
• Difference in flow and boundary condition data
  requirements
• Difference in calibration strategy
• Difference in application strategy

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Unsteady Flow Equations
Momentum Equation
Continuity Equation
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Steady Flow Equations
Energy (momentum) Equation
Continuity Equation
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Distance vs Time Solution Grid

• X distance, feet
• t time, seconds

2,2
1,2
t
1,1
x
2,1
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Finite Difference Term
Q Q (2,2) - Q(1,2)
Q(2,1) - Q(1,1) -----
----------------------- ( 1 - )
--------------------- X
X
X
theta weighting factor 0.5 lt lt 1.0
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Pre-Computation of Hydraulic Properties
Steady Compute exact hydraulic properties at a
section for each trial water surface elevation
from the elevation/station points,
n-values. Unsteady Hydraulic properties are
pre-computed for all possible water surface
elevations at each cross section (hydraulic table)
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Expansion/Contraction Coeffs.

• Not used in the momentum formulation
  (RAS-unsteady)
• Should be in the data, however, for use with
  steady flow analysis

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Data Requirements(Flow and Boundary Conditions)
Steady Discharge (Q) at each cross
section. Unsteady Inflow hydrograph(s) which
are routed by the model.
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Prepare hydrographs (boundary
conditions) Upstream flows Tributary (local
flows) Ungaged/unmodeled flows Downstream
(rating curve?)
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HEC-RAS Main Window
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Entering Geometric Parameters
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Cross Section Table Properties
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Pre-processing Geometry

• For unsteady flow, geometry is pre-processed into
  tables and rating curves
• Cross sections are processed into tables of area,
  conveyance, and storage
• Bridges and culverts are processed into a family
  of rating curves for each structure
• Weirs and gated structures are calculated on the
  fly during unsteady flow calculations
• Pre-processor results can be viewed in graphs and
  tables

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Cross Section Properties Plot
Property Table
RS 138154.4
700
Legend
Conv. Channel
690
Conv. Valley
Conv. Total
680
Storage
Elevation (ft)
670
660
650
0
1000
2000
3000
4000
5000
6000
7000
Conveyance/1000 (cfs) Storage (cu ft)
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Geometry Preprocessor

• What does it do?
• Processes geometric data into a series of
  hydraulic tables and rating curves.
• Why do we use it for unsteady flow?
• Instead of calculating hydraulic variables for
  each cross-section during each iteration, the
  program interpolates the hydraulic variables from
  the tables.

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Conveyance Calculations

• Manning Equation
• 1/2
• Q K Sf
• K Conveyance
• Sf friction/energy slope

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Conveyance Calculations
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Cross Section Example
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Geometry Preprocessor
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Hydraulic Property Plot
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Cross Section Properties Table
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Conveyance Subdivisions
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Conveyance Subdivisions
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Conveyance Subdivisions
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Conveyance Subdivisions
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Boundary andInitial Conditions

• Objectives
• Know boundary condition options
• Know initial condition requirements
• Sources of data for both

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Unsteady Flow Data

• External Boundaries required
• Upstream and Downstream ends of the river
• Typically flow or stage hydrograph upstream
• Typically rating or normal depth downstream
• Internal Boundaries can be added
• Add flow within the river system
• Define gate operation
• Initial Conditions - at the start of simulation

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Unsteady Flow Data Editor
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Boundary Conditions

• Editor shows required external boundaries
• Boundary Type shows available options
• Upstream options
• Stage Hydrograph
• Flow Hydrograph
• Stage Flow Hydrograph

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Boundary Conditions - continued

• Downstream Boundary Options
• Stage Hydrograph
• Flow Hydrograph
• Stage Flow Hydrograph
• Rating Curve
• Normal Depth

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Flow Hydrograph

• Read from DSS
• Select DSS file
• Select Pathname
• Enter in Table
• Select time interval
• Select start date/time
• Enter flow data - or cut paste

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Sources of Time-Series Data

• Historic Records (USGS)
• Stage Hydrographs
• Flow Hydrographs
• Computed Synthetic Floods
• Rainfall-runoff modeling
• Peak Discharge with assumed time distribution
• Others?

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Normal Depth

• Enter Friction (energy) Slope
• Program uses Mannings equation to compute stage
• Provides semi-dynamic downstream boundary

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Initial Conditions

• Requires an initial flow for all reaches
• Restart file can be read from DSS
• Enter steady-flow at upstream boundary
• Can add a flow-change location
• Pool elevation for storage areas

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File and Options Menus
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Unsteady Flow Simulation Simulation Manager
1. Define a Plan
2. Select which programs to run
3. Enter a starting and ending date and time
4. Set the computation settings
5. Press the Compute button
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Output Selection

• Unsteady Flow Output
• Stage and Flow Hydrographs
• Log File Output
• Post Processor
• Detailed output
• Max Stage
• Selected Time Intervals

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Stage and Flow Hydrographs User Selected Locations
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Viewing Unsteady Flow Results

• All of the output that was available for steady
  flow computations is available for unsteady flow
  (cross sections, profile, and 3D plots and
  tables).
• Stage and flow hydrographs
• Time series tables
• Animation of cross section, profile and
  3-dimensional graphic

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Stage and Flow Plot
Stage
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Unsteady Flow Rating Curve
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Log File Output

• can be generated during computations
• information about progression of simulation
• can make a large, large file
• are you sure you want to open it?

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Post Processor

• Can be run after the unsteady simulation is
  completed
• Provides profiles for the maximum stage and at
  regular intervals
• All regular graphics and tables can be used to
  view the post process results
• Graphics can animate the simulation

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Profile Animation
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Accuracy/Stability/SensitivityObjective

• For students to have a better understanding of
  model accuracy, stability, and sensitivity.
• To become familiar with the available parameters
  within HEC-RAS that will allow you to develop a
  stable and accurate model.
• To learn how to detect, find, and fix model
  stability problems.

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Overview

• Model Accuracy
• Model Stability
• Factors Affecting Accuracy and Stability
• Cross section spacing
• Computational time step selection
• Practical delta t, hydrograph rise time / 20
• Common Stability Problems
• Detecting Stability Problems
• Model Sensitivity

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Model Accuracy

• Accuracy can be defined as the degree of
  closeness of the numerical solution to the true
  solution.
• Accuracy depends upon the following
• Assumptions and limitations of the model (i.e.
  one dimensional model, subcritical flow only for
  unsteady flow)
• Accuracy of the geometric Data (cross sections,
  Mannings n values, bridges, culverts, etc)
• Accuracy of the flow data and boundary conditions
• Numerical Accuracy of the solution scheme

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Numerical Accuracy

• If we assume that the 1-dimensional unsteady flow
  equations are a true representation of flow
  moving through a river system, then only an
  analytical solution of these equations will yield
  an exact solution.
• Finite difference solutions are approximate.
• An exact solution of the equations is not
  feasible for complex river systems, so HEC-RAS
  uses a finite difference scheme.

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Model Stability

• An unstable numerical model is one for which
  certain types of numerical errors grow to the
  extent at which the solution begins to oscillate,
  or the errors become so large that the
  computations can not continue.

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Factors Affecting Model Stability and Numerical
Accuracy

• Cross Section Spacing
• Computation time step
• Theta weighting factor
• Solution iterations
• Solution tolerances

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Calculation Options and Tolerances
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Cross Section Spacing

• Cross sections should be placed at
  representative locations to describe the changes
  in geometry.
• Additional cross sections should be added at
  locations where changes occur in discharge,
  slope, velocity, and roughness.
• Cross sections must also be added at levees,
  bridges, culverts, and other structures.

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Cross Section Spacing - Slope

• Bed slope plays an important role in cross
  section spacing.
• Steeper slopes require more cross sections
• Streams flowing at high velocities may require
  cross sections on the order of 100 feet or less.
• Larger uniform rivers with flat slopes may only
  require cross sections on the order of 1000 ft or
  more.

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Cross Section Spacing - How do you know if you
have enough XS

• Use the HEC-RAS cross section interpolation.
• Make a new plan and run the model.
• Compare the before and after.
• If no significant difference, then OK!

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Theta Weighting Factor

• Theta is a weighting applied to the finite
  difference approximations when solving the
  unsteady flow equations.
• Theoretically Theta can vary from 0.5 to 1.0.
  However a practical limit is from 0.6 to 1.0
• Theta of 1.0 provides the most stability. Theta
  of 0.6 provides the most accuracy.
• The default in HEC-RAS is 1.0. Once you have
  your model developed, reduce theta towards 0.6,
  as long as the model stays stable.

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Common Stability Problems

• Too large of a time step.
• Not enough cross sections
• Model goes to critical depth RAS is limited to
  subcritcal flow for unsteady flow simulations
• Bad downstream boundary condition (i.e. rating
  curve or slope for normal depth)
• Bad cross section properties, commonly caused by
  levee options, ineffective flow areas, Mannings
  n values, etc..

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Common Stability Problems - Continued

• Cross section properties that do not go high
  enough, or are way to high (curves are spread to
  far apart).
• Bad bridge/culvert family of rating curves.
• Wide and flat lateral weirs/spillways send to
  much flow over a given time step.
• Gated spillways that are opened or closed to
  fast.

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Detecting Stability Problems

• How do you know you have a stability problem?
• Program completely blows up during run
• Program goes to maximum number of iterations for
  several time steps in a row.
• Program has oscillations in the computed stage
  and flow hydrographs

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Detecting Stability Problems - Continued

• What do you do when this happens?
• Note the simulation time when the program either
  blew up or first started to oscillate.
• Turn on the Detailed Output for Debugging
  option and re-run the program.
• View the text file that contains the detailed log
  output of the computations. Locate the
  simulation output at the simulation time when the
  solution first started to go bad.
• Find the river station locations that did not
  meet the solution tolerances. Then check the
  data in this general area.

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Turning on Detailed Output for Debugging
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Viewing Detailed Log Output
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Model Sensitivity

• Numerical sensitivity
• Computation time step try a smaller value to
  see if the output changes significantly.
• Theta start at 1.0, after you have a working
  model then try to reduce it towards 0.6.
• Weir/Spillway stability factors if you are
  using stability factors, try to reduce them to
  the lowest value you can get away with.
• Weir/Spillway exponential decay factors in
  general I would leave them alone, they will not
  effect the sensitivity of the output much.

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Model Sensitivity - Continued

• Physical Parameter Sensitivity
• Mannings n Values What if the true n values
  were 10 higher or Lower?
• Cross Section Spacing Test by interpolating
• Cross Section Storage What if there is really
  more or less storage in the cross sections (I.e.
  ineffective flow areas, etc)
• Weir/Spillway coefficients For lateral
  weirs/spillways the coefficient selected can have
  a great impact on the results.
• Bridge/Culvert Parameters normally only effect
  the locally computed stages, unless it is a flat
  area in which the bridge causes great backwater.

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The End