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HEC-RAS Version 3.1 Unsteady Flow

HEC-RAS Version 3.1 Unsteady Flow. Presented by: slides adapted from HEC Unsteady Flow Course. Unsteady Flow Documentation. Technical/theoretical - Chapters 2 and 5 from EM 1110-2-1416

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HEC-RAS Version 3.1 Unsteady Flow

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  1. HEC-RAS Version 3.1 Unsteady Flow Presented by: slides adapted from HEC Unsteady Flow Course HEC-RAS Version 3.1

  2. 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) HEC-RAS Version 3.1

  3. 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 HEC-RAS Version 3.1

  4. 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 HEC-RAS Version 3.1

  5. Unsteady Flow Equations Momentum Equation: Continuity Equation: HEC-RAS Version 3.1

  6. Steady Flow Equations Energy (momentum) Equation: Continuity Equation: HEC-RAS Version 3.1

  7. X = distance, feet t = time, seconds Distance vs Time Solution Grid 2,2 1,2 t 1,1 x 2,1 HEC-RAS Version 3.1

  8. 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 HEC-RAS Version 3.1

  9. 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) HEC-RAS Version 3.1

  10. Expansion/Contraction Coeffs. • Not used in the momentum formulation (RAS-unsteady) • Should be in the data, however, for use with steady flow analysis HEC-RAS Version 3.1

  11. Data Requirements(Flow and Boundary Conditions) Steady: Discharge (Q) at each cross section. Unsteady: Inflow hydrograph(s) which are routed by the model. HEC-RAS Version 3.1

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

  13. HEC-RAS Main Window HEC-RAS Version 3.1

  14. Entering Geometric Parameters HEC-RAS Version 3.1

  15. Cross Section Table Properties HEC-RAS Version 3.1

  16. 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 HEC-RAS Version 3.1

  17. 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) HEC-RAS Version 3.1

  18. 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. HEC-RAS Version 3.1

  19. Conveyance Calculations • Manning Equation • 1/2 • Q = K Sf • K = Conveyance • Sf = friction/energy slope HEC-RAS Version 3.1

  20. Conveyance Calculations HEC-RAS Version 3.1

  21. Cross Section Example HEC-RAS Version 3.1

  22. Geometry Preprocessor HEC-RAS Version 3.1

  23. Hydraulic Property Plot HEC-RAS Version 3.1

  24. Cross Section Properties Table HEC-RAS Version 3.1

  25. Conveyance Subdivisions HEC-RAS Version 3.1

  26. Conveyance Subdivisions HEC-RAS Version 3.1

  27. Conveyance Subdivisions HEC-RAS Version 3.1

  28. Conveyance Subdivisions HEC-RAS Version 3.1

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

  30. 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 HEC-RAS Version 3.1

  31. Unsteady Flow Data Editor HEC-RAS Version 3.1

  32. Editor shows required external boundaries Boundary Type shows available options Upstream options: Stage Hydrograph Flow Hydrograph Stage & Flow Hydrograph Boundary Conditions HEC-RAS Version 3.1

  33. Downstream Boundary Options: Stage Hydrograph Flow Hydrograph Stage & Flow Hydrograph Rating Curve Normal Depth Boundary Conditions - continued HEC-RAS Version 3.1

  34. Read from DSS Select DSS file Select Pathname Enter in Table Select time interval Select start date/time Enter flow data - or cut & paste Flow Hydrograph HEC-RAS Version 3.1

  35. 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? HEC-RAS Version 3.1

  36. Enter Friction (energy) Slope Program uses Manning’s equation to compute stage Provides semi-dynamic downstream boundary Normal Depth HEC-RAS Version 3.1

  37. 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 Initial Conditions HEC-RAS Version 3.1

  38. File and Options Menus HEC-RAS Version 3.1

  39. 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 HEC-RAS Version 3.1

  40. Output Selection • Unsteady Flow Output • Stage and Flow Hydrographs • Log File Output • Post Processor • Detailed output • Max Stage • Selected Time Intervals HEC-RAS Version 3.1

  41. Stage and Flow Hydrographs User Selected Locations HEC-RAS Version 3.1

  42. 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 HEC-RAS Version 3.1

  43. Stage and Flow Plot Stage HEC-RAS Version 3.1

  44. Unsteady Flow Rating Curve HEC-RAS Version 3.1

  45. 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? HEC-RAS Version 3.1

  46. 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 HEC-RAS Version 3.1

  47. Profile Animation HEC-RAS Version 3.1

  48. 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. HEC-RAS Version 3.1

  49. 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 HEC-RAS Version 3.1

  50. 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 HEC-RAS Version 3.1

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