Surveying Contracted-Opening Indirect Measurements

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Detailed Description

This video goes into detail about surveying high-water marks, various aspects of bridges, and cross sections for the contracted-opening method. 


Date Taken:

Length: 00:09:01

Location Taken: Austin, TX, US

Video Credits

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


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

For a contracted opening indirect, there are many individual measurements that contribute to the final computed discharge. Some of the data are used to define channel geometry while others are used to derive an adjustment factor, or Discharge Coefficient, from empirical charts created in a laboratory setting. Like with any indirect measurement, careful documentation is very important for both the computation and subsequent review. In an effort to keep the process efficient, I will list the requirements in an order that I find to be ideal when working in the field.

First, start with high water marks. High water marks are always a good starting place for any indirect measurement because without reliable high-water marks, a peak discharge measurement may not even be possible. If available, begin your survey by tying into a local datum so that recorded sensor readings can be used as reference while you survey. As discussed in the previous video on contracted opening indirect measurements, shoot some high-water marks on the upstream, and downstream embankments to verify that you have the 0.5 ft of fall required. If the fall is 0.5 ft or greater, continue shooting the rest of the high-water marks on the downstream embankment. If the high-water marks are poorly defined on the downstream embankment, it may be necessary to continue downstream until a reliable elevation is determined for section 3. Finally, shoot the high-water marks throughout the approach upstream of the contracted section. It is important to remember, that the high-water marks will be the sole representation of both the section 1 and section 3 surface water elevations, so take your time and survey plenty of marks.  

Once you have shot the high-water marks and determined they are sufficient for a contracted opening measurement, you may begin measuring the channel geometry. It does not matter whether you start at the bridge, or the approach section, as long they are tied to the same reference datum.  

The approach cross section is normally surveyed one bridge width upstream and is used to represent the natural, unobstructed flow in the channel. One bridge width was adopted because it is usually far enough upstream that you are no longer in the drawdown zone.  While surveying, be sure to make a site sketch, take pictures, and describe the roughness throughout the section just as you would with a slope-area indirect measurement.

After the approach section is finished, you can move on to the contracted section. The contracted section is located at the smallest cross section between the abutments on a line parallel to the contraction. If the bridge is skewed to the flow, Section 3 is still parallel to the contraction even though the minimum section may be perpendicular to the abutments. The most common location for the contracted section is between the abutments at the downstream side of the bridge, but it doesn’t hurt to break out a steel tape and double check.


For a contracted opening, since the bridge will be acting as our measuring device, it is necessary to record complete details of the bridge geometry including all the following where applicable:

Wingwall angles and lengths – Measure the angle of the wingwalls using a protractor and measure the lengths using a steel tape. The easiest way to determine the angle is to align the straight portion of the protractor with the face of the bridge at the intersection of the wingwall and the bridge. You can now read the wingwall angle directly.

Lengths of the abutments – Each bridge type defines a unique measurement location for length based the type of bridge entrance you are working with. The most common length measurement, denoted L, is the distance from the intersection of the section 1 elevation with the upstream embankment, to the contracted section, or section 3. To make sure you are measuring the correct length, refer to the TWRI manual based on your bridge type.  

The positions of the embankments and abutments are important for determining what additional adjustment factors are necessary for your type of bridge. It may also be important when starting computations because additional information may be described that otherwise wouldn’t be.  An easy way to record this information is to use your total station to shoot points at both the top and bottom of the embankments and abutments.  Additionally, make sure to note any skew or irregularities in the abutments.

Side slopes are needed to determine adjustment factors for the discharge coefficient. Unlike what you may remember from math class, these slopes are measured as the ratio of Run over Rise. You can measure these by using a level of known length held horizontally out from the abutment, this is your “run”.  Next measure down with a folding rule to get your “rise” elevation. The ratio of the run over the rise, is your side slope. If the side slopes are different from the inside of the bridge to the embankment, Measure both so that later, an average can be taken as the final side slope.

Radius of curvature – This is the radius of curvature at the entrance of the bridge abutments. This measurement is only used for a type 1 bridge with vertical embankments and abutments. To measure, lay a small hand level or another straight edge flat on one side of the abutment and a folding ruler flat on the other side. In between, there will be a gap from the 90-degree corner created by the two tools and the rounded corner of the abutment. Then measure the distance on the folding ruler from the hand level to where the abutment wall is again parallel with the folding ruler.  Repeat this step by switching the positions of the straight edge and the folding ruler.  The average of the two values is the radius of the entrance rounding.

Elevation of the roadway – This information is useful for ruling out the possibility that the flood overtopped the roadway. If it is determined that the flood caused flow over the road, it will be necessary to treat this measurement as a “combination site” which is explained further in either the TWRI book 3 chapter A4, on page 33, or in the multiple components video available in this series. 

Top width of the embankment – The top width of the embankment is necessary for an accurate plan view and elevation view.  This can be measured either by a steel tape, or in the event of heavy traffic, can be shot using your total station or differential leveling equipment.

Details of the piers or piles – Complete information about the piers and piles including the width, height, shape, and quantity is necessary so that you can accurately define the area submerged in the cross section. These occupied spaces will be subtracted from the overall area when determining the conveyance for the contracted cross section, section 3. The pier and pile dimensions are also necessary for determining the wetted perimeter when computing the hydraulic radius, R.

Finally, elevations of the bottom of the girders or beams spanning the contraction might need to be shot. This information is useful for determining whether the flood was touching or backing up, against the girders or beams. You can shoot these elevations using the reflectorless setting on your total station, if so equipped. If the bridge is low enough, you may be able to use the inverted rod technique by flipping your digital rod upside down and reading the negative elevation returned. Finally, if all else fails, you can shoot the bridge seat at both ends of the bridge and assume a straight-line elevation between the two points. If you think this was a possibility during the flood, make sure to document the heights accordingly.

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