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Modeling Drop
Structures in HEC-RAS
Version 3.1
March 2003 HEC-RAS Version 3.1 Slide 2 of 27
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.
March 2003 HEC-RAS Version 3.1 Slide 3 of 27
Overview
What is a Drop Structure?
March 2003 HEC-RAS Version 3.1 Slide 4 of 27
Overview
Why do we use Drop Structures ?
March 2003 HEC-RAS Version 3.1 Slide 5 of 27
Overview
March 2003 HEC-RAS Version 3.1 Slide 6 of 27
Overview
March 2003 HEC-RAS Version 3.1 Slide 7 of 27
Overview
March 2003 HEC-RAS Version 3.1 Slide 8 of 27
Overview
March 2003 HEC-RAS Version 3.1 Slide 9 of 27
Overview
March 2003 HEC-RAS Version 3.1 Slide 10 of 27
Overview
March 2003 HEC-RAS Version 3.1 Slide 11 of 27
Components of Drop Structure
yc yb
3 or 4
yb yb = 0.715 ycRectangular channel
stilling basin:used to dissipate energy
subcritical
subcritical
•Control Section•Energy Dissipation Section•Adjacent Protection
March 2003 HEC-RAS Version 3.1 Slide 12 of 27
Overview
How can we Model a Drop Structure with HEC-RAS ?
Inline Weir Option
Using Cross Sections
March 2003 HEC-RAS Version 3.1 Slide 13 of 27
Overview
Which is more appropriate?
Using Cross Sections - better if interested in profile through the structure.
Inline Weir Option - probably better if just interested in elevations upstream and downstream of structure.
March 2003 HEC-RAS Version 3.1 Slide 14 of 27
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).
March 2003 HEC-RAS Version 3.1 Slide 15 of 27
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
March 2003 HEC-RAS Version 3.1 Slide 16 of 27
Inline Weir - Submergence
Submergence is defined as H2/H1
March 2003 HEC-RAS Version 3.1 Slide 17 of 27
Inline Weir - Submergence
March 2003 HEC-RAS Version 3.1 Slide 18 of 27
Inline Weir - Cross Section Layout
12
345
6 Inline Weir Structure
Cross Sections
Model the floor blocks as blocked obstructions.
March 2003 HEC-RAS Version 3.1 Slide 19 of 27
Modeling a Drop Structure as an Inline Structure
Under the “Geometric Data”click on “Inline Structure”
March 2003 HEC-RAS Version 3.1 Slide 20 of 27
Modeling a Drop Structure as an Inline Structure
A series of windows allow for entry of weir characteristics
March 2003 HEC-RAS Version 3.1 Slide 21 of 27
Modeling a Drop Structure with Cross Sections
Cross Sections
Model the floor blocks as blocked obstructions.
March 2003 HEC-RAS Version 3.1 Slide 22 of 27
Cross Section Layout for Ogee Shapes Drop Structure
Cross Sections
March 2003 HEC-RAS Version 3.1 Slide 23 of 27
Modeling Baffles in the Stilling Basin
Increase ‘n’ values at baffle blocks to account for increased roughness
March 2003 HEC-RAS Version 3.1 Slide 24 of 27
Example Using Lab Data
WES Physical Model Study (WES, 1994)
March 2003 HEC-RAS Version 3.1 Slide 25 of 27
Example - Modeled as a Weir
Cross Sections
Inline Weir
March 2003 HEC-RAS Version 3.1 Slide 26 of 27
Example - Modeled with Cross Sections Only
Cross Sections
March 2003 HEC-RAS Version 3.1 Slide 27 of 27
Example - Comparison of Two Methods
March 2003 HEC-RAS Version 3.1 Slide 28 of 27
Example - Comparison Zoomed In
The End