Requirements for the Contracted-Opening Method

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Detailed Description

This video discusses the general requirements that will be needed to accomplish a contracted-opening indirect discharge measurement. Approach section, contracted section, and necessary high-water mark locations are described along with scenarios where the method will not be successful. 
 

Details

Image Dimensions: 1920 x 1080

Date Taken:

Length: 00:08:14

Location Taken: Austin, TX, US

Video Credits

Jeff East, Megan Poff, Todd Geiger, Joe Capesius, Monica Langhorst, Office of Employee Development
 

Transcript

Hi, my name is Scott Grzyb and I am a Hydrologist at the Texas Water Science Center in Austin, TX. In this video, I will give a brief overview of what a contracted opening indirect discharge measurement is and will discuss some things to consider to ensure proper site selection. For a more in-depth explanation of Width Contraction or contracted opening indirect discharge measurements, please refer to TWRI Book 3 Chapter A4.

 

A contracted opening measurement is possible when there is a contraction of a stream channel, most often a roadway crossing like a bridge, that creates an abrupt drop in water-surface elevation between an approach section and a contracted section. That drop in water surface can then be related to the corresponding change in velocity. By making some substitutions with the continuity equation into the energy equation and then by doing a little bit of simplifying and rearranging, we can arrive at a discharge equation between the two sections.  You might ask, why choose this method when I can just do a slope area?

Well, this indirect method only requires 1 cross section upstream of the constriction. When you have a wide or heavily wooded floodplain, it may be much easier or more efficient to only survey 1 cross section upstream rather than 4 - 5 labor-intensive cross sections throughout a reach.  

 

So, let’s talk about what you should be looking for when you go out to a site? To ensure that you have all the necessary information to compute a contracted opening indirect measurement, you will need the following components:

  1. A suitable contracted section.
  2. A suitable approach section, and
  3. Good quality high-water marks.

First, let’s talk about the contracted section, also known in practice as Section 3:

While there are special cases when various natural features, such as rock constrictions, can be treated as contracted openings, the most common, and the openings I will be referring to in this video are bridge, or roadway contracted openings.

For this type of indirect, the contracted section is acting as our measurement device, so it’s important to make sure it meets the requirements necessary to best compute an accurate peak discharge.  

Here are some simple guidelines to consider when determining if your site is a suitable location.

  1. The channel in the contracted opening should be stable.
    1. If you are dealing with a soft sand channel that tends to scour and fill during large events, it is unlikely we can accurately determine the area in our contracted section at the time of the peak.
  2. The bridge geometry should be reasonably close to one of the four defined types in TWRI book 3 Chapter A4.
    1. Since this method relies on the fact that real world conditions are comparable to those that were extensively studied in a laboratory setting, the bridge geometry at the contracted section will need to be similar to the standard, or modified types specified in the TWRI Manual. If no type matches your bridge, use engineering judgement to select the type to be used.  
    2. This is important because each type defines certain ratios for each type of contraction and their effect on the discharge coefficient.
  3. The fall in water surface should be greater than 0.5 ft between the approach section and the contracted section.
    1. An easy way to measure this is to quickly shoot a high-water mark on both the upstream, and downstream embankments.
    2. If it is less than 0.5 ft, the resulting discharge may be inaccurate.
  4. The fall in water surface should be at least four times the friction loss between the approach section and the contracted section.
    1. An example of this would be a very wide bridge downstream of a heavily wooded approach section. The fall through the bridge would not be as substantial as the head losses in the approach section, therefore, this would not be a suitable location to perform a contracted opening indirect measurement.

Next let’s look at the approach section:

The approach section, also known as Section 1, is represented by a cross section of the natural, un-constricted channel upstream from the beginning of drawdown.  The approach section is generally located one bridge-width denoted as “b” upstream from the contraction and is important for determining characteristics such as whether there is sub-critical flow as it approaches the contracted section.

What does a good approach section look like? There isn’t a whole lot to it. Any straight, unobstructed, natural reach representative of one bridge width upstream will do. Notice I said, “representative of”. That’s because, in a situation where the upstream cross section is too dense to survey, the approach section can be shot just upstream of the contracted opening. We assume this cross section has equal ground elevations and can still treat it as if it were one bridge width upstream. It’s important to mention, we still need to shoot the high-water marks upstream regardless of where the approach cross section is shot.  

If there is one thing to avoid when picking an approach section, it would be to:  

  1.  Avoid a reach that meanders severely.
    1. If the main channel is winding back and forth throughout the flood plain, the energy losses cannot be correctly evaluated.  

 

The final component, and perhaps the most important of any peak discharge indirect, is quality high-water marks. 

High-water marks will be used to determine the water surface elevation at both your approach section and your contracted section.  Without accurate marks, the amount of fall between sections will be inaccurate, ultimately leading to an inaccurate discharge.

For the contracted section, the elevation is determined by surveying high-water marks on the downstream embankment. If the high-water marks on the downstream embankment are insufficient, continue flagging marks downstream until you can determine your section 3 surface water elevation.

When flagging the approach high-water marks, its best to start a reasonable distance upstream of the approach and work your way down to the upstream embankment. This way you can develop a sufficient profile for determining your approach elevation as well as the amount of fall due to roughness along the reach, without being too reliant on one or two high-water marks.  A rule-of-thumb for determining a “reasonable” distance upstream is about 1.5 to 2 times the bridge width “b”.

Like with any indirect measurement, or high-water mark flagging campaign, it is important to recover the high-water marks as soon as possible. Embankments are usually grassy and can rebound quickly following flood events and rain is excellent at washing away seed lines. Always remember, if your high-water marks are poor, it usually means your peak discharge indirect results will be as well.

If your site meets all the requirements mentioned in this video, continue onto the next video on Width Contraction discharge measurements where I will discuss in more detail what geometry and additional measurements around the bridge are needed to complete the computation.  If your site does not meet all the requirements explained in this video, you may want to consider choosing another indirect measurement method.

If you need additional help in the field, don’t hesitate to contact your field office chief, data chief, surface-water specialist, or indirect measurement specialist.