# Presentation: 656621

## 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.

- HEC-RAS Version 3.1

- March 2003

**Notes:**
Objective:  To know the function of drop structures and the HEC-RAS approaches to model them.  The limitations of the HEC-RAS software will be emphasized.

## Overview

- What is a Drop Structure?

- HEC-RAS Version 3.1

- March 2003

**Notes:**
A drop structure is a designed drop in elevation of the stream invert.  A drop structure can be as simple as some well placed rock, or as complex as a concrete drop with a stilling basin and baffles.  

## Overview

- Why do we use Drop Structures ?

- HEC-RAS Version 3.1

- March 2003

**Notes:**
Drop structures are used to dissipate energy.  They are often used in steep channels to prevent scour from occurring.  Drop structures are also used to prevent “Head Cutting”.  Head Cutting is the process by which a scour hole continues to migrate upstream.  A drop structure is often used to stop the migration of the scour.

## Overview

- HEC-RAS Version 3.1

- March 2003

**Notes:**
This is an example of a simple drop structure in which rock was placed in the stream and cement was used to hold the rock together.  This drop structure is only a few feet in height. 

## Overview

- HEC-RAS Version 3.1

- March 2003

**Notes:**
This is an example of a detailed drop structure on the Santa Ana river in Orange County California.   This structure has a ogee shaped drop, a stilling basin, and baffles for energy dissipation.  

In this picture there is a temporary barrier upstream of the drop structure.  This is tube that is filled with water to act as a temporary dam.  Notice the lateral gates at the top of the picture.  The temporary dam is used to capture flow, while the lateral gates are used to divert the water for groundwater recharge purposes.

## Overview

- HEC-RAS Version 3.1

- March 2003

**Notes:**
This photo was take looking upstream at a drop structure in the Santa Ana River, located in Orange County California.  This drop structure provides about an 8 foot drop in elevation.  Notice that there are wing walls on the sides of the drop.  Since the channel is trapezoidal, the wing walls were used to force the flow into the stilling basin where the baffles are located.  

## Overview

- HEC-RAS Version 3.1

- March 2003

**Notes:**
This photo was take looking upstream at a drop structure in the Santa Ana River, located in Orange County California.  This drop structure provides about an 8 foot drop in elevation.  Notice that there are wing walls on the sides of the drop.  Since the channel is trapezoidal, the wing walls were used to force the flow into the stilling basin where the baffles are located.  

## Overview

- HEC-RAS Version 3.1

- March 2003

**Notes:**
This photo was take looking upstream at a drop structure in the Santa Ana River, located in Orange County California.  This drop structure provides about an 8 foot drop in elevation.  Notice that there are wing walls on the sides of the drop.  Since the channel is trapezoidal, the wing walls were used to force the flow into the stilling basin where the baffles are located.  

## Overview

- HEC-RAS Version 3.1

- March 2003

**Notes:**
This photo was take looking upstream at a drop structure in the Santa Ana River, located in Orange County California.  This drop structure provides about an 8 foot drop in elevation.  Notice that there are wing walls on the sides of the drop.  Since the channel is trapezoidal, the wing walls were used to force the flow into the stilling basin where the baffles are located.  

## Components of Drop Structure

- yc

- yb

- 3 or 4
-    yb

- yb = 0.715 yc

- Rectangular channel

- stilling basin:used to dissipate energy

- subcritical

- subcritical

- Control Section
- Energy Dissipation Section
- Adjacent Protection

- HEC-RAS Version 3.1

- March 2003

**Notes:**
As subcritical flow approaches a vertical drop, as shown above, the flow will pass through critical depth a short distance upstream of the drop.  For a rectangular channel, it has been found that the flow passes through critical depth at a distance of about 3 to 4 times the depth at the top of the drop, yb.  

Below the drop, a stilling basin is used to dissipate energy.  The flow transitions from supercritical flow to subcritical flow inside of the stilling basin.  This transition is very turbulent, and is called a hydraulic jump.  

Drop structures may or may not have baffles inside of the stilling basin for additional energy dissipation.  

## Overview

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

- Inline Weir Option

- Using Cross Sections

- HEC-RAS Version 3.1

- March 2003

**Notes:**
Drop structures can be modeled with HEC-RAS by using the inline weir option or  a series of cross sections.  If you are just interested in getting the water surface upstream and downstream of the drop structure, then the inline weir option would probably be the most appropriate.  However, if you want to compute a more detailed profile upstream of and through the drop, then you will need to model it as a series of cross sections.

## 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.

- HEC-RAS Version 3.1

- March 2003

**Notes:**
Drop structures can be modeled with HEC-RAS by using the inline weir option or  a series of cross sections.  If you are just interested in getting the water surface upstream and downstream of the drop structure, then the inline weir option would probably be the most appropriate.  However, if you want to compute a more detailed profile upstream of and through the drop, then you will need to model it as a series of cross sections.

## 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

- March 2003

**Notes:**
Drop structures can be modeled with HEC-RAS by using the inline weir option or  a series of cross sections.  If you are just interested in getting the water surface upstream and downstream of the drop structure, then the inline weir option would probably be the most appropriate.  However, if you want to compute a more detailed profile upstream of and through the drop, then you will need to model it as a series of cross sections.

## 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

- HEC-RAS Version 3.1

- March 2003

**Notes:**
When modeling a drop structure with the inline weir option, the standard weir equation is used to compute the upstream energy head for the given flow.  The user is required to enter the type of weir (broad crested or Ogee shape), a weir coefficient, and a series of station and elevation points describing the weir cross section.

Weir coefficients for drop structures can range from 2.6 - 4.0, depending upon the weir shape, approach channel, and discharge level.  

## Inline Weir - Submergence

**Submergence is defined as H****2****/H****1**

- HEC-RAS Version 3.1

- March 2003

**Notes:**
The inline weir option in HEC-RAS accounts for reduced flow over the weir due to submergence.  This is accomplished by multiplying the weir coefficient by a submergence reduction factor.  As shown above, the tailwater does not effect the weir until it is greater than 76 percent submerged.

Submergence is defined as the depth of water above the minimum weir elevation on the downstream side divided by the height of the energy gradeline above the minimum weir elevation on the upstream side.  Submergence corrections are also based on the shape of the weir.  The curve shown above is for a broad crested weir.  Another curve, similar to this one, is used for an Ogee shaped weir.

## Inline Weir - Submergence

- HEC-RAS Version 3.1

- March 2003

**Notes:**
The inline weir option in HEC-RAS accounts for reduced flow over the weir due to submergence.  This is accomplished by multiplying the weir coefficient by a submergence reduction factor.  As shown above, the tailwater does not effect the weir until it is greater than 76 percent submerged.

Submergence is defined as the depth of water above the minimum weir elevation on the downstream side divided by the height of the energy gradeline above the minimum weir elevation on the upstream side.  Submergence corrections are also based on the shape of the weir.  The curve shown above is for a broad crested weir.  Another curve, similar to this one, is used for an Ogee shaped weir.

## Inline Weir - Cross Section Layout

- 1

- 2

- 3

- 4

- 5

- 6

- Inline Weir Structure

- Cross Sections

**Model the floor blocks as blocked obstructions.**

- HEC-RAS Version 3.1

- March 2003

**Notes:**
When modeling a drop structure with the inline weir option, the user must place cross sections in the appropriate places to get an adequate estimate of the tailwater and headwater elevations.  

Downstream of the weir, cross sections should be placed just below the stilling basin (section 2), just inside of the stilling basin at the downstream end (section 3), and at the point in the stilling basin that would represent the tailwater location for the weir (section 5).  Additional cross sections should be added to represent baffles if they exist (sections 5 and 6).

Upstream of the weir, the first cross section should be located a short distance away from the drop (section 6) .  This cross section location represents the point at which the water surface begins to dip and accelerate before going over the weir.  

The program computes an upstream energy head from the weir equation, puts that energy into the upstream cross section (section 6), and then computes a water surface that corresponds to that energy for the given flow rate.

## Modeling a Drop Structure as an Inline Structure

- Under the “Geometric Data”click on “Inline Structure”

- HEC-RAS Version 3.1

- March 2003

## Modeling a Drop Structure as an Inline Structure

- A series of windows allow for entry of weir characteristics

- HEC-RAS Version 3.1

- March 2003

## Modeling a Drop Structure with Cross Sections

- Cross Sections

**Model the floor blocks as blocked obstructions.**

- HEC-RAS Version 3.1

- March 2003

**Notes:**
When modeling a drop structure as a series of cross sections, the most important thing is to have enough cross sections at the correct locations.  Cross sections need to be closely spaced where the water surface and velocity is changing rapidly (i.e. just upstream and downstream of the drop).  

For a vertical drop, as the one shown above, the downstream cross section locations are the same as if modeling the structure with the inline weir option.

However, the upstream cross sections should start right at the top of the weir.  The second cross section should be a very short distance away from the drop (say about 5 feet or so).  As you proceed upstream, the cross section spacing should be increased with each new cross section, until you get back to normal cross section spacing for the river.

## Cross Section Layout for Ogee Shapes Drop Structure

- Cross Sections

- HEC-RAS Version 3.1

- March 2003

**Notes:**
If the drop structure is an Ogee shaped drop, then you may want to place several cross sections along the drop at very short intervals (2-5 feet).  This will allow the program to compute the transition from subcritical to supercritical flow over the weir.  The program will have to be run in a mixed flow mode in order to compute this transition, as well as computing the hydraulic jump that will occur inside of the stilling basin.

Additionally, it should be noted that when modeling a drop structure in this manner, the energy equation assumes that the water maintains a hydrostatic pressure distribution.  This assumption begins to break down for slopes greater than 10 percent.  Therefore the results at the cross sections on the face of the drop will have some error.  In general the program will show a lower water surface than what might actually occur on the face of the drop.

## Modeling Baffles in the Stilling Basin

**Increase ‘n’ values at baffle blocks to account for increased roughness**

- HEC-RAS Version 3.1

- March 2003

**Notes:**
To model the baffles inside of the stilling basin, the user can use the blocked obstruction option from the cross section editor.  This option allows the user to have up to 20 blocks placed inside of the cross section.  The user is required to enter a left station, right station, and elevation for each block.  If there are more than 20 baffles at a particular cross section, then combine some of them into single blocked obstructions.

Additionally, the user should increase the Manning’s n values at the cross sections that have baffles.  The magnitude of the Manning’s n values will depend upon the height of the baffles as well as the depth of flow in the stilling basin.  

## Example Using Lab Data

- WES Physical Model Study (WES, 1994)

- HEC-RAS Version 3.1

- March 2003

**Notes:**
To demonstrate the two methods of modeling a drop structure, a comparison was made between a physical model study and two HEC-RAS models of the drop structure.  During the design phase of improvements to the Santa Ana river, the Waterways Experiment Station (WES) was contracted to study the drop structures and make recommendations.  The results of this study were reported in General Design for Replacement of or Modifications to the Lower Santa Ana River Drop Structures, Orange County, California (Technical Report HL-94-4, April 1994, USACE).  Over 50 different designs were tested in 1:25 scale flume models and 1:40 scale full width models.  The designs evaluated existing structures, modifying original structures and replacing them with entirely new designs.  The drop structure design used in the Santa Ana River is similar to one referred to as Type 10 in the report.  

Two HEC-RAS models were developed to model the Type 10 drop structure and the model results were compared to the flume results. The total reach in the model was 350 feet, 150 upstream of the crest of the drop structure and 200 feet below the crest.

## Example - Modeled as a Weir

- Cross Sections

- Inline Weir

- HEC-RAS Version 3.1

- March 2003

**Notes:**
As shown from the graphic above, the inline weir option does a good job at estimating the upstream energy and water surface elevation.  However, it does not show the water surface passing through critical depth, going supercritical, and then going through a hydraulic jump.  When you use the inline weir option you do not get any detailed information about the water surface as it passes over the weir.  You only get information at the two cross sections that bound the inline weir.

## Example - Modeled with Cross Sections Only

- Cross Sections

- HEC-RAS Version 3.1

- March 2003

**Notes:**
The same structure was also modeled with cross sections only, as shown above. Some differences occur right at the crest and through the hydraulic jump.  The differences at the crest are due to the fact that the energy equation will always show the flow passing through critical depth at the top of the crest.  Whereas, in the field it has been shown that the flow passes through critical depth at a distance upstream of 3-4 times critical depth.  However, as shown above, a short distance upstream of the crest the HEC-RAS program converges to the same depth as the observed data.  Correctly obtaining the maximum upstream water surface in the most important part of modeling the drop structure.  
Downstream of the drop, the flow is supercritical and then goes through a hydraulic jump.  The flume data shows the jump occurring over a distance of 50 to 60 feet with a lot of turbulence.  The HEC-RAS model cannot predict how long of a distance it will take for the jump to occur, but it can predict where the jump will begin.  The HEC-RAS model will always show the jump occurring between two adjacent cross sections.  The HEC-RAS model shows the higher water surface inside of the stilling basin and then going down below the stilling basin.  The model shows all of this as a fairly smooth transition, whereas it is actually a turbulent transition with the water surface bouncing up and down.  In general, the results from the HEC-RAS model are very good at predicting the stages upstream, inside, and downstream of the drop structure.

## Example - Comparison of Two Methods

- HEC-RAS Version 3.1

- March 2003

**Notes:**
The figure above shows a comparison of both methods against the observed data.  Both methods are very good at predicting the upstream water surface away from the drop structure.  Neither method is perfect right at the drop structure, but they are both adequate.  

## Example - Comparison Zoomed In

- HEC-RAS Version 3.1

- March 2003

**Notes:**
Shown above is a zoomed in view of the results from the two methods compared to the observed data.

## The End