2. Feb 2003HEC-RAS Version 3.11 Slides adapted from HEC Unsteady Flow Course Unsteady Flow Course

3. Feb 2003HEC-RAS Version 3.12 Unsteady Flow Documentation l Technical/theoretical - Chapters 2 and 5 from EM 1110-2-1416 l http://www.hnd.usace.army.mil - click on TECHINFO, then Engineering Publications, then Engineering Manuals, scroll for manual l HEC-RAS User’s Manual - Chapter 8 (data input and window operations) l HEC-RAS Application Guide - Chapter 17 (example application)

4. Feb 2003HEC-RAS Version 3.13 When to use Unsteady Flow l Tidal/estuary fluctuation l Off-channel storage l Dam breach routing l Channels with flat slopes l Levee overtopping l Hydraulic structures affected by changing backwater l Large amounts of storage behind roads or culverts

5. Feb 2003HEC-RAS Version 3.14 Steady vs. Unsteady l Difference in handling boundary friction and other losses l Difference in numerical solution algorithm l Difference in handling non-flow areas l Difference in flow and boundary condition data requirements l Difference in calibration strategy l Difference in application strategy

6. Feb 2003HEC-RAS Version 3.15 Momentum Equation: Continuity Equation: Unsteady Flow Equations

7. Feb 2003HEC-RAS Version 3.16 Energy (momentum) Equation: Continuity Equation: Steady Flow Equations

8. Feb 2003HEC-RAS Version 3.17 Distance vs Time Solution Grid l X = distance, feet l t = time, seconds t x 1,1 2,1 1,2 2,2

9. Feb 2003HEC-RAS Version 3.18 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

10. Feb 2003HEC-RAS Version 3.19 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) Pre-Computation of Hydraulic Properties

11. Feb 2003HEC-RAS Version 3.110 Data Requirements (Flow and Boundary Conditions) Steady: Discharge (Q) at each cross section. Unsteady: Inflow hydrograph(s) which are routed by the model.

12. Feb 2003HEC-RAS Version 3.111 Prepare hydrographs (boundary conditions) Upstream flows Tributary (local flows) Ungaged/unmodeled flows Downstream (rating curve?)

13. Feb 2003HEC-RAS Version 3.112 HEC-RAS Main Window

14. Feb 2003HEC-RAS Version 3.113 Entering Geometric Parameters

15. Feb 2003HEC-RAS Version 3.114 Cross Section Table Properties

16. Feb 2003HEC-RAS Version 3.115 Pre-processing Geometry l For unsteady flow, geometry is pre- processed into tables and rating curves n Cross sections are processed into tables of area, conveyance, and storage n Bridges and culverts are processed into a family of rating curves for each structure n Weirs and gated structures are calculated on the fly during unsteady flow calculations n Pre-processor results can be viewed in graphs and tables

17. Feb 2003HEC-RAS Version 3.116 Cross Section Properties Plot 01000200030004000500060007000 650 660 670 680 690 700 Property Table RS = 138154.4 Conveyance/1000 (cfs) Storage (cu ft) Elevation (ft) Legend Conv. Channel Conv. Valley Conv. Total Storage

18. Feb 2003HEC-RAS Version 3.117 Geometry Preprocessor l What does it do? Processes geometric data into a series of hydraulic tables and rating curves. l 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.

19. Feb 2003HEC-RAS Version 3.118 Conveyance Calculations Manning Equation 1/2 Q = K Sf K = Conveyance Sf = friction/energy slope

20. Feb 2003HEC-RAS Version 3.119 Conveyance Calculations

21. Feb 2003HEC-RAS Version 3.120 Cross Section Example

22. Feb 2003HEC-RAS Version 3.121 Geometry Preprocessor

23. Feb 2003HEC-RAS Version 3.122 Hydraulic Property Plot

24. Feb 2003HEC-RAS Version 3.123 Cross Section Properties Table

25. Feb 2003HEC-RAS Version 3.124 Conveyance Subdivisions

26. Feb 2003HEC-RAS Version 3.125 Conveyance Subdivisions

27. Feb 2003HEC-RAS Version 3.126 Conveyance Subdivisions

28. Feb 2003HEC-RAS Version 3.127 Conveyance Subdivisions

29. Boundary and Initial Conditions Objectives: l Know boundary condition options l Know initial condition requirements l Sources of data for both

30. Feb 2003HEC-RAS Version 3.129 Unsteady Flow Data l External Boundaries required n Upstream and Downstream ends of the river n Typically flow or stage hydrograph upstream n Typically rating or “normal depth” downstream l Internal Boundaries can be added n Add flow within the river system n Define gate operation l Initial Conditions - at the start of simulation

31. Feb 2003HEC-RAS Version 3.130 Unsteady Flow Data Editor

32. Feb 2003HEC-RAS Version 3.131 Boundary Conditions l Editor shows required external boundaries l Boundary Type shows available options l Upstream options: n Stage Hydrograph n Flow Hydrograph n Stage & Flow Hydrograph

33. Feb 2003HEC-RAS Version 3.132 Boundary Conditions - continued l Downstream Boundary Options: n Stage Hydrograph n Flow Hydrograph n Stage & Flow Hydrograph n Rating Curve n Normal Depth

34. Feb 2003HEC-RAS Version 3.133 Flow Hydrograph l Read from DSS n Select DSS file n Select Pathname l Enter in Table n Select time interval n Select start date/time n Enter flow data - or cut & paste

35. Feb 2003HEC-RAS Version 3.134 Sources of Time-Series Data l Historic Records (USGS) n Stage Hydrographs n Flow Hydrographs l Computed Synthetic Floods l Rainfall-runoff modeling l Peak Discharge with assumed time distribution l Others?

36. Feb 2003HEC-RAS Version 3.135 Normal Depth l Enter Friction (energy) Slope l Program uses Manning’s equation to compute stage l Provides semi-dynamic downstream boundary

37. Feb 2003HEC-RAS Version 3.136 Initial Conditions l Requires an initial flow for all reaches l Restart file can be read from DSS l Enter steady-flow at upstream boundary l Can add a flow- change location l Pool elevation for storage areas

38. Feb 2003HEC-RAS Version 3.137 File and Options Menus

39. Feb 2003HEC-RAS Version 3.138 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

40. Feb 2003HEC-RAS Version 3.139 Output Selection l Unsteady Flow Output n Stage and Flow Hydrographs n Log File Output l Post Processor n Detailed output –Max Stage –Selected Time Intervals

41. Feb 2003HEC-RAS Version 3.140 Stage and Flow Hydrographs User Selected Locations

42. Feb 2003HEC-RAS Version 3.141 Viewing Unsteady Flow Results l All of the output that was available for steady flow computations is available for unsteady flow (cross sections, profile, and 3D plots and tables). l Stage and flow hydrographs l Time series tables l Animation of cross section, profile and 3- dimensional graphic

43. Feb 2003HEC-RAS Version 3.142 Stage and Flow Plot Stage

44. Feb 2003HEC-RAS Version 3.143 Unsteady Flow Rating Curve

45. Feb 2003HEC-RAS Version 3.144 Log File Output l can be generated during computations l information about progression of simulation l can make a large, large file n are you sure you want to open it?

46. Feb 2003HEC-RAS Version 3.145 Post Processor l Can be run after the unsteady simulation is completed l Provides profiles for the maximum stage and at regular intervals l All regular graphics and tables can be used to view the post process results l Graphics can “animate” the simulation

47. Feb 2003HEC-RAS Version 3.146 Profile Animation

48. Feb 2003HEC-RAS Version 3.147 Accuracy/Stability/Sensitivity Objective l For students to have a better understanding of model accuracy, stability, and sensitivity. l To become familiar with the available parameters within HEC-RAS that will allow you to develop a stable and accurate model. l To learn how to detect, find, and fix model stability problems.

49. Feb 2003HEC-RAS Version 3.148 Overview l Model Accuracy l Model Stability l Factors Affecting Accuracy and Stability n Cross section spacing n Computational time step selection n Practical delta t, hydrograph rise time / 20 l Common Stability Problems l Detecting Stability Problems l Model Sensitivity

50. Feb 2003HEC-RAS Version 3.149 Model Accuracy l Accuracy can be defined as the degree of closeness of the numerical solution to the true solution. l Accuracy depends upon the following: n Assumptions and limitations of the model (i.e. one dimensional model, subcritical flow only for unsteady flow) n Accuracy of the geometric Data (cross sections, Manning’s n values, bridges, culverts, etc…) n Accuracy of the flow data and boundary conditions n Numerical Accuracy of the solution scheme

51. Feb 2003HEC-RAS Version 3.150 Numerical Accuracy l 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. l Finite difference solutions are approximate. l An exact solution of the equations is not feasible for complex river systems, so HEC- RAS uses a finite difference scheme.

52. Feb 2003HEC-RAS Version 3.151 Model Stability l 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.

53. Feb 2003HEC-RAS Version 3.152

54. Feb 2003HEC-RAS Version 3.153 Factors Affecting Model Stability and Numerical Accuracy l Cross Section Spacing l Computation time step l Theta weighting factor l Solution iterations l Solution tolerances

55. Feb 2003HEC-RAS Version 3.154 Calculation Options and Tolerances

56. Feb 2003HEC-RAS Version 3.155 Cross Section Spacing l Cross sections should be placed at representative locations to describe the changes in geometry. l Additional cross sections should be added at locations where changes occur in discharge, slope, velocity, and roughness. l Cross sections must also be added at levees, bridges, culverts, and other structures.

57. Feb 2003HEC-RAS Version 3.156 Cross Section Spacing - Slope l Bed slope plays an important role in cross section spacing. n Steeper slopes require more cross sections n Streams flowing at high velocities may require cross sections on the order of 100 feet or less. n Larger uniform rivers with flat slopes may only require cross sections on the order of 1000 ft or more.

58. Feb 2003HEC-RAS Version 3.157 Cross Section Spacing - How do you know if you have enough XS: l Use the HEC-RAS cross section interpolation. l Make a new plan and run the model. l Compare the before and after. l If no significant difference, then OK!

59. Feb 2003HEC-RAS Version 3.158 Theta Weighting Factor l Theta is a weighting applied to the finite difference approximations when solving the unsteady flow equations. l Theoretically Theta can vary from 0.5 to 1.0. However a practical limit is from 0.6 to 1.0 l Theta of 1.0 provides the most stability. Theta of 0.6 provides the most accuracy. l 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.

60. Feb 2003HEC-RAS Version 3.159 Common Stability Problems l Too large of a time step. l Not enough cross sections l Model goes to critical depth – RAS is limited to subcritcal flow for unsteady flow simulations l Bad downstream boundary condition (i.e. rating curve or slope for normal depth) l Bad cross section properties, commonly caused by: levee options, ineffective flow areas, Manning’s n values, etc..

61. Feb 2003HEC-RAS Version 3.160 Common Stability Problems - Continued l Cross section properties that do not go high enough, or are way to high (curves are spread to far apart). l Bad bridge/culvert family of rating curves. l Wide and flat lateral weirs/spillways – send to much flow over a given time step. l Gated spillways that are opened or closed to fast.

62. Feb 2003HEC-RAS Version 3.161 Detecting Stability Problems l How do you know you have a stability problem? n Program completely blows up during run n Program goes to maximum number of iterations for several time steps in a row. n Program has oscillations in the computed stage and flow hydrographs

63. Feb 2003HEC-RAS Version 3.162 Detecting Stability Problems - Continued l What do you do when this happens? n Note the simulation time when the program either blew up or first started to oscillate. n Turn on the “Detailed Output for Debugging” option and re-run the program. n 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. n Find the river station locations that did not meet the solution tolerances. Then check the data in this general area.

64. Feb 2003HEC-RAS Version 3.163 Turning on Detailed Output for Debugging

65. Feb 2003HEC-RAS Version 3.164 Viewing Detailed Log Output

66. Feb 2003HEC-RAS Version 3.165 Model Sensitivity l Numerical sensitivity: n Computation time step – try a smaller value to see if the output changes significantly. n Theta – start at 1.0, after you have a working model then try to reduce it towards 0.6. n Weir/Spillway stability factors – if you are using stability factors, try to reduce them to the lowest value you can get away with. n Weir/Spillway exponential decay factors – in general I would leave them alone, they will not effect the sensitivity of the output much.

67. Feb 2003HEC-RAS Version 3.166 Model Sensitivity - Continued l Physical Parameter Sensitivity: n Manning’s n Values – What if the true n values were 10% higher or Lower? n Cross Section Spacing – Test by interpolating n Cross Section Storage – What if there is really more or less storage in the cross sections (I.e. ineffective flow areas, etc…) n Weir/Spillway coefficients – For lateral weirs/spillways the coefficient selected can have a great impact on the results. n Bridge/Culvert Parameters – normally only effect the locally computed stages, unless it is a flat area in which the bridge causes great backwater.

68. The End