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					HEC-RAS Version 3.1
           Unsteady Flow



                 Presented by:

            slides adapted from HEC
Feb 2003      Unsteady Flow Course
                   HEC-RAS Version 3.1   1
    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 User’s Manual - Chapter 8 (data
    input and window operations)
   HEC-RAS Application Guide - Chapter 17
    (example application)
Feb 2003            HEC-RAS Version 3.1             2
      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
Feb 2003            HEC-RAS Version 3.1         3
           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

Feb 2003             HEC-RAS Version 3.1           4
           Unsteady Flow Equations


     Momentum Equation:

           Q  ( αQ2/A)
                           gA( h  S o  S f )  0
           t       x          x
     Continuity Equation:

                       Q + A = 0
                       x t
Feb 2003                 HEC-RAS Version 3.1           5
             Steady Flow Equations

           Energy (momentum) Equation:

              ( αQ2/A)
                        + gA( h - So + S f ) = 0
                  x          x
            Continuity Equation:

                           Q = VA


Feb 2003                 HEC-RAS Version 3.1        6
    Distance vs Time Solution Grid

   X = distance, feet
   t = time, seconds                   1,2       2,2



                                    t




                                    1,1       x   2,1

Feb 2003             HEC-RAS Version 3.1            7
           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 <          < 1.0



Feb 2003               HEC-RAS Version 3.1                       8
    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)
Feb 2003               HEC-RAS Version 3.1          9
   Expansion/Contraction Coeffs.

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




Feb 2003          HEC-RAS Version 3.1       10
           Data Requirements
       (Flow and Boundary Conditions)



  Steady: Discharge (Q) at each
  cross section.
  Unsteady: Inflow hydrograph(s)
  which are routed by the model.


Feb 2003         HEC-RAS Version 3.1    11
            Prepare hydrographs
           (boundary conditions)
           Upstream flows
           Tributary (local flows)
           Ungaged/unmodeled flows
           Downstream (rating curve?)


Feb 2003              HEC-RAS Version 3.1   12
           HEC-RAS Main Window




Feb 2003         HEC-RAS Version 3.1   13
  Entering Geometric Parameters




Feb 2003     HEC-RAS Version 3.1   14
   Cross Section Table Properties




Feb 2003      HEC-RAS Version 3.1   15
           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
Feb 2003            HEC-RAS Version 3.1          16
                   Cross Section Properties Plot
                                             Property Table
                                              RS = 138154.4
                 700                                                                     Legend

                                                                                    Conv. Channel
                 690
                                                                                        Conv. Valley
                                                                                        Conv. Total
Elevation (ft)




                 680
                                                                                          Storage


                 670


                 660


                 650
                       0   1000   2000      3000      4000       5000     6000   7000
                                  Conveyance/1000 (cfs) Storage (cu ft)

    Feb 2003                                HEC-RAS Version 3.1                                       17
           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.

Feb 2003             HEC-RAS Version 3.1             18
           Conveyance Calculations

              Manning Equation

                                 1/2
                 Q = K Sf

                 K = Conveyance
                 Sf = friction/energy slope
Feb 2003           HEC-RAS Version 3.1        19
           Conveyance Calculations


           n1               n2            nch                 n3

           A1 P1          A2 P2          A ch P ch         A3 P3


                K lob = K 1 + K 2                         K rob = K 3

                                           K ch



Feb 2003                            HEC-RAS Version 3.1                 20
           Cross Section Example




Feb 2003        HEC-RAS Version 3.1   21
           Geometry Preprocessor




Feb 2003         HEC-RAS Version 3.1   22
           Hydraulic Property Plot




Feb 2003         HEC-RAS Version 3.1   23
   Cross Section Properties Table




Feb 2003      HEC-RAS Version 3.1   24
           Conveyance Subdivisions




Feb 2003         HEC-RAS Version 3.1   25
           Conveyance Subdivisions




Feb 2003         HEC-RAS Version 3.1   26
           Conveyance Subdivisions




Feb 2003         HEC-RAS Version 3.1   27
           Conveyance Subdivisions




Feb 2003         HEC-RAS Version 3.1   28
Boundary and
Initial Conditions

   Objectives:
    Know boundary condition options
    Know initial condition requirements
    Sources of data for both
               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
    Feb 2003           HEC-RAS Version 3.1        30
Unsteady Flow Data Editor




Feb 2003    HEC-RAS Version 3.1   31
    Boundary Conditions
   Editor shows required
    external boundaries
   Boundary Type shows
    available options
   Upstream options:
        Stage Hydrograph
        Flow Hydrograph
        Stage & Flow
         Hydrograph

    Feb 2003                HEC-RAS Version 3.1   32
Boundary Conditions - continued




   Downstream Boundary Options:
          Stage Hydrograph
          Flow Hydrograph
          Stage & Flow Hydrograph
          Rating Curve
          Normal Depth
Feb 2003                HEC-RAS Version 3.1   33
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

Feb 2003   HEC-RAS Version 3.1                       34
     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?
Feb 2003          HEC-RAS Version 3.1   35
Normal Depth

                    Enter Friction (energy)
                     Slope
                    Program uses
                     Manning’s equation to
                     compute stage
                    Provides semi-dynamic
                     downstream boundary



Feb 2003   HEC-RAS Version 3.1             36
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
Feb 2003     HEC-RAS Version 3.1                37
File and Options Menus




Feb 2003    HEC-RAS Version 3.1   38
                 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

      Feb 2003            HEC-RAS Version 3.1   39
                  Output Selection

   Unsteady Flow Output
     Stage  and Flow
      Hydrographs
     Log File Output

   Post Processor
     Detailed   output
        – Max Stage
        – Selected Time Intervals
    Feb 2003              HEC-RAS Version 3.1   40
     Stage and Flow Hydrographs
       User Selected Locations




Feb 2003      HEC-RAS Version 3.1   41
     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
    Feb 2003      HEC-RAS Version 3.1       42
           Stage and Flow Plot




                                 Stage




Feb 2003         HEC-RAS Version 3.1     43
     Unsteady Flow Rating Curve




Feb 2003      HEC-RAS Version 3.1   44
                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?




Feb 2003              HEC-RAS Version 3.1   45
            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

Feb 2003        HEC-RAS Version 3.1     46
           Profile Animation




Feb 2003        HEC-RAS Version 3.1   47
        Accuracy/Stability/Sensitivity
                 Objective

 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.

    Feb 2003       HEC-RAS Version 3.1      48
                     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
Feb 2003               HEC-RAS Version 3.1           49
                  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, Manning’s
      n values, bridges, culverts, etc…)
     Accuracy of the flow data and boundary conditions
     Numerical Accuracy of the solution scheme
    Feb 2003             HEC-RAS Version 3.1              50
               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.
    Feb 2003        HEC-RAS Version 3.1     51
             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.



Feb 2003           HEC-RAS Version 3.1      52
Feb 2003   HEC-RAS Version 3.1   53
    Factors Affecting Model Stability
        and Numerical Accuracy
 Cross Section Spacing
 Computation time step
 Theta weighting factor
 Solution iterations
 Solution tolerances




    Feb 2003      HEC-RAS Version 3.1   54
           Calculation Options and
                 Tolerances




Feb 2003          HEC-RAS Version 3.1   55
       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.
Feb 2003          HEC-RAS Version 3.1       56
    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.
Feb 2003            HEC-RAS Version 3.1          57
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!
 Feb 2003           HEC-RAS Version 3.1       58
               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.
    Feb 2003            HEC-RAS Version 3.1           59
          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, Manning’s n values, etc..
    Feb 2003       HEC-RAS Version 3.1      60
        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.
    Feb 2003      HEC-RAS Version 3.1      61
     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


Feb 2003            HEC-RAS Version 3.1             62
        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.
    Feb 2003              HEC-RAS Version 3.1                63
  Turning on Detailed Output for
           Debugging




Feb 2003     HEC-RAS Version 3.1   64
    Viewing Detailed Log Output




Feb 2003      HEC-RAS Version 3.1   65
               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.
    Feb 2003           HEC-RAS Version 3.1            66
        Model Sensitivity - Continued
   Physical Parameter Sensitivity:
     Manning’s 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.
    Feb 2003             HEC-RAS Version 3.1               67
The End

				
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