Modeling Drop Structures in HECRAS Version 3.1

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Modeling Drop Structures in HEC-RAS Version 3.1
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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.

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Overview

• What is a Drop Structure?

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Overview

• Why do we use Drop Structures ?

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Overview
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Overview
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Overview
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Overview
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Overview
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Overview
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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 basinused to dissipate energy
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Overview

• How can we Model a Drop Structure with HEC-RAS ?

• Inline Weir Option

• Using Cross Sections

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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.

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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).

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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 weir
  H Upstream Energy Head

Q CLH3/2
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Inline Weir - Submergence
Submergence is defined as H2/H1
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Inline Weir - Submergence
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Inline Weir - Cross Section Layout
Inline Weir Structure
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Cross Sections
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Model the floor blocks as blocked obstructions.
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Modeling a Drop Structure as an Inline Structure
Under the Geometric Dataclick on Inline
Structure
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Modeling a Drop Structure as an Inline Structure
A series of windows allow for entry of weir
characteristics
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Modeling a Drop Structure with Cross Sections
Cross Sections
Model the floor blocks as blocked obstructions.
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Cross Section Layout for Ogee Shapes Drop
Structure
Cross Sections
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Modeling Baffles in the Stilling Basin
Increase n values at baffle blocks to account
for increased roughness
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Example Using Lab Data

• WES Physical Model Study (WES, 1994)

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Example - Modeled as a Weir
Cross Sections
Inline Weir
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Example - Modeled with Cross Sections Only
Cross Sections
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Example - Comparison of Two Methods
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Example - Comparison Zoomed In
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The End