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Modeling Drop Structures in HEC-RAS Version 3.1

Modeling Drop Structures in HEC-RAS Version 3.1. Modeling Drop Structures. Overview Modeling a Drop Structure as an Inline Structure (Weir). Modeling a drop structure with cross-sections through the drop. Example using Lab Data. Overview. What is a Drop Structure?. Overview.

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Modeling Drop Structures in HEC-RAS Version 3.1

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  1. Modeling Drop Structures in HEC-RAS Version 3.1

  2. Modeling Drop Structures • Overview • Modeling a Drop Structure as an Inline Structure (Weir). • Modeling a drop structure with cross-sections through the drop. • Example using Lab Data. HEC-RAS Version 3.1

  3. Overview • What is a Drop Structure? HEC-RAS Version 3.1

  4. Overview • Why do we use Drop Structures ? HEC-RAS Version 3.1

  5. Overview HEC-RAS Version 3.1

  6. Overview HEC-RAS Version 3.1

  7. Overview HEC-RAS Version 3.1

  8. Overview HEC-RAS Version 3.1

  9. Overview HEC-RAS Version 3.1

  10. Overview HEC-RAS Version 3.1

  11. Components of Drop Structure • Control Section • Energy Dissipation Section • Adjacent Protection 3 or 4 yb Rectangular channel yb = 0.715 yc subcritical yc yb subcritical stilling basin:used to dissipate energy HEC-RAS Version 3.1

  12. Overview • How can we Model a Drop Structure with HEC-RAS ? • Inline Weir Option • Using Cross Sections HEC-RAS Version 3.1

  13. Overview • Which is more appropriate? • Inline Weir Option - probably better if just interested in elevations upstream and downstream of structure. • Using Cross Sections - better if interested in profile through the structure. HEC-RAS Version 3.1

  14. Cross-Section Locations When placing cross-sections near and through a drop structure, they need to be placed where the water surface and velocity are changing rapidly (this applies when using an inline weir also). HEC-RAS Version 3.1

  15. Modeling a Drop Structure as an Inline Weir • The standard weir equation is used: • where: C = 2.6 - 4.0 (dependent on shape) L = Length of weirH = Upstream Energy Head Q = CLH3/2 HEC-RAS Version 3.1

  16. Inline Weir - Submergence Submergence is defined as H2/H1 HEC-RAS Version 3.1

  17. Inline Weir - Submergence HEC-RAS Version 3.1

  18. Inline Weir - Cross Section Layout Inline Weir Structure 6 Cross Sections 5 3 4 1 2 Model the floor blocks as blocked obstructions. HEC-RAS Version 3.1

  19. Modeling a Drop Structure as an Inline Structure Under the “Geometric Data”click on “Inline Structure” HEC-RAS Version 3.1

  20. Modeling a Drop Structure as an Inline Structure A series of windows allow for entry of weir characteristics HEC-RAS Version 3.1

  21. Modeling a Drop Structure with Cross Sections Cross Sections Model the floor blocks as blocked obstructions. HEC-RAS Version 3.1

  22. Cross Section Layout for Ogee Shapes Drop Structure Cross Sections HEC-RAS Version 3.1

  23. Modeling Baffles in the Stilling Basin Increase ‘n’ values at baffle blocks to account for increased roughness HEC-RAS Version 3.1

  24. Example Using Lab Data • WES Physical Model Study (WES, 1994) HEC-RAS Version 3.1

  25. Example - Modeled as a Weir Cross Sections Inline Weir HEC-RAS Version 3.1

  26. Example - Modeled with Cross Sections Only Cross Sections HEC-RAS Version 3.1

  27. Example - Comparison of Two Methods HEC-RAS Version 3.1

  28. Example - Comparison Zoomed In HEC-RAS Version 3.1

  29. The End

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