Survey Requirements - Road Overflow/Broad-Crested Weir

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Detailed Description

This video describes how and where to flag high water marks for a road overflow or broad-crested weir indirect measurement of peak discharge. 


Date Taken:

Length: 00:04:42

Location Taken: Las Vegas, NV, US

Video Credits

Todd Geiger, Office of Employee Development


Hi, this is Megan Poff and I’m the Field Office Chief at the USGS in Las Vegas, Nevada.  I’ll be talking briefly about the survey requirements for a road overflow indirect measurement.  These techniques can also be used for flow over structures that behave like broad crested weirs or dams.  Keep in mind that this video will only cover the single-component road overflow type of indirect measurement.  If you have an indirect measurement that will have multiple components such as flow going over a road and through a culvert or bridge opening at the same time, please refer to the multiple component video as well.  The information presented in this single-component video can also be found in TWRI 3-A5, Measurement of Peak Discharge at Dams by Indirect Methods. 

So - you have come across a site where water flowed over the road.  What do you do?  How do you know if you have the correct conditions to survey and compute a road overflow indirect measurement?  As with all indirect measurements, you first will need to look at your high-water marks. Do you have high-water marks on both banks upstream and downstream from the road?  If the answer is yes, at this point, you should take a step back and do a quick reality check before flagging your high-water marks.  The road has to have acted as a control for a road overflow indirect measurement, and one way to determine that the road was the control is to see if there was ponding on the upstream side.  Scan the high-water marks – are they mostly at a similar elevation upstream from the road?  On the downstream side, does it look like there is a drop?  If you can answer yes to both of those questions, proceed with flagging the high-water marks.

For your upstream high-water marks, you’ll need to cover enough distance upstream from the road to determine ponding and also reliably determine the water surface elevation at the approach cross section that you’ll need to survey later.  The approach cross section is always located at a distance three to four times the depth of water flowing over the crown of the road. As an example, if the depth of the water flowing over the crown of the road was 3 ft, you’ll need to locate your approach cross section about 12 ft upstream of the upstream side of the road.  You’ll want to flag high-water marks starting further upstream than the approach cross section so you can see what the water surface profile is coming into your approach cross section.  For our example, you could start flagging high-water marks about 30 ft upstream, but you might want to go farther depending on the quality of the marks and the site conditions.  There may be sites where you can’t find high-water marks at the cross section, so you’ll want to go further upstream.  As you flag high-water marks, make sure you note the vertical uncertainty of each mark.  This can be written in a notebook, recorded in electronic field forms, or written directly on the flagging itself. 

Likely, any high-water marks that existed on the road surface itself will have already been destroyed by rain or vehicle traffic.  However, if you are lucky enough to be able to find high-water marks on the road, flag them with a marker or neat spray paint marks.  Remember to wear reflective safety vests when working on roads and follow the traffic control plan for the site.  Also, take a look at the road and make sure you can positively identify the crest of the road and especially that it exists in the first place.

Now let’s talk about the downstream high-water marks.  Make sure there is a drop in the elevations of the high-water marks on the downstream side of the road compared to the upstream side of the road.  If you don’t see a drop in the elevations of the high-water marks, the road overflow indirect measurement method may not be valid.  No tailwater drop in elevation suggests that the embankment, or road, was submerged during the event.  When you flag high-water marks on the downstream side, you only need to cover enough distance to define the tailwater conditions and determine if there was any submergence on your road.  Submergence occurs when you have backwater conditions.  While you may see a large drop in the elevation of the high-water marks on the downstream side in comparison with those on the upstream side, never assume that there was no submergence on the road.  Always flag high-water marks on the downstream side of the road.  With any indirect measurement, more data usually comes in handy, but missing data can significantly downgrade the quality of the computation. 

What about troubleshooting?  Let’s say you only are able to locate high-water marks on one bank.  While this kind of situation isn’t ideal, you can still proceed.  Simply flag the high-water marks that you can find.  What if you can’t visually determine if ponding occurred upstream from the road?  You have two options.  Either flag the high-water marks anyway and see how the data looks after the survey, or find a different section and use a different indirect measurement technique. 

For information on surveying a road overflow indirect measurement, refer to the next road overflow video in this series.  If you need help in the field, call your supervisor, surface-water specialist, or indirect measurement specialist.