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Using HEC-RAS –Data Requirements

Using HEC-RAS –Data Requirements. Geometric Data Requirements for Water Surface Profile Calculations. Example Schematic. Where do we need cross sections?. At changes in discharge - stream confluences At changes in slope  At changes in shape At changes in roughness

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Using HEC-RAS –Data Requirements

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  1. Using HEC-RAS –Data Requirements Geometric Data Requirements for Water Surface Profile Calculations

  2. Example Schematic

  3. Where do we need cross sections? • At changes in discharge - stream confluences • At changes in slope  • At changes in shape • At changes in roughness • At control points - Critical Depth • At levees, bridges, culverts, and other features

  4. Study Limit Determination • Evaluate upstream and downstream impacts of alternatives. • Establish limits for data collection and analysis • Upstream Limits - Upper study boundary plus additional distance where profile resulting from a structure-caused energy loss converges with the existing condition profile. • Downstream Limits - Start sufficiently far downstream to assure accurate results at the lower limit of the study reach.

  5. Study Limit Determination

  6. Upstream Reach Length Determination Lu = Distance upstream HD = Avg. hydraulic depth for Q100 HL = Headloss at the structure for Q100 S = Average reach slope Some guidance from HEC

  7. Downstream Reach Length Determination Some guidance from HEC Ld = Distance downstream HD = Avg. hydraulic depth for Q100 S = Average reach slope

  8. Surveyed Cross Section Data Approximation to actual channel

  9. Limits of a Floodplain

  10. Limits of a Floodplain (1 XS)

  11. Limits of a Floodplain (2 XS)

  12. Ineffective Area Limits of a Floodplain (3 XS)

  13. Ineffective Area Limits of a Floodplain (4 XS)

  14. Ineffective Area Limits of a Floodplain

  15. Cross Sections on Floodplain

  16. Ineffective Area Cross Sections on Floodplain

  17. Reach Lengths Reach lengths represent the distance between adjacent cross sections. They represent the flow path length for the center of mass of the fluid in each subsection.

  18. Energy Loss Coefficients The program utilizes loss coefficients to evaluate energy losses: • Manning's n values for friction loss • Contraction and expansion coefficients to evaluate transition (shock) losses.

  19. Manning’s n Value Factors affecting the n value: • -Surface Roughness • - Geometry Irregularity • - Land Use - Vegetation • - Degree of Meandering • - Scour and Deposition • - Flow Obstructions • - Bed Forms

  20. Manning’s n Value Sources of Information for Estimating Manning’s n • Published Documents (text books, USGS docs • Field Observations • Formulas for n-value (Jarrett, Limerinos, etc.) Calibration to Observed Data • Need high water and flow data

  21. Contraction and Expansion Coefficients Some typical values

  22. Ineffective Flow Areas Left Ineffective Flow Station Right Ineffective Flow Station

  23. Levee Option Left Levee Station

  24. Levee Added Left Levee Station and Elevation

  25. Normal Blocked Obstructions

  26. Multiple Blocked Obstructions

  27. Stream Junction Stream junctions are defined as locations where two or more streams come together or split apart. Junction data consists of reach lengths across the junction (and tributary angles, if the momentum equation is selected). Reach lengths across the junction are entered in the Junction Data Editor

  28. Steady Flow Data Options • Insert a Known Water Surface • Enter and Additional Energy Loss • Enter Observed Water Surfaces for Comparison • A flow is required at every cross section (but you need to enter values only 1) at the upstream boundary 2) where the flow changes)

  29. Froude number

  30. Flow Regime Types • Critical Flow Fr = 1 • Subcritical Flow Fr < 1 • Supercritical Flow Fr > 1

  31. Flow Transitions • Establishment of Uniform Flow • What is Uniform Flow? • Chezy Equation: • Manning’s Equation: Note: C is not dimensionless

  32. Required Boundary Conditions • Subcritical Flow Regime Downstream only • Supercritical Flow Regime Upstream only • Mixed Flow Regime Both upstream and downstream

  33. Types of External Boundary Conditions • Known water surface elevation • Critical depth • Normal depth (also called slope area - based on Manning's equation) • Rating curve

  34. Discharge Information • Tributary inflows • Local inflow • Lateral weir flows • Account for hydrograph attenuation

  35. Steady Flow Data Options • Specify a change in energy • Specify a change in water surface • Insert a known water surface • Enter an additional energy loss • Enter observed water surfaces for comparison

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