Survey Requirements for Road Overflow Indirect Measurements

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Detailed Description

In this video, you will learn what and where to survey for a road overflow or broad-crested weir indirect measurement. 


Date Taken:

Length: 00:05:49

Location Taken: Henderson, NV, US

Video Credits

Ruby Hurtado, Todd Geiger, Hampton Childres, and the Office of Employee Development


Hi, this is Megan Poff and Iím the Field Office Chief at the USGS in Las Vegas, Nevada.  Iíll be talking briefly about the survey requirements for a road overflow indirect measurement.  This video assumes that you have already flagged the high-water marks through the entire reach; upstream and downstream of the road embankment and are ready to survey them along with the channel geometry and embankment geometry needed to compute peak flow over a road or railroad.  If you need information on flagging high-water marks for a road overflow indirect measurement, refer to the first road overflow video in this series.  These techniques can also be used for flow over 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.  
When you survey for a road overflow indirect measurement, you will need to survey the following items: 1) high-water mark profiles through the entire reach, 2) an approach cross section, and 3) a cross section on the crest of the road.  In addition, youíll also want to record the width of the road and whether the road is gravel or paved.  Donít forget to take pictures during the survey!  
If you havenít done so already, make sure you survey one of the gage reference marks or establish a temporary reference mark and survey that to start the survey.  Now, letís survey the high-water marks.  I like to survey the high-water marks from upstream to downstream on one bank, then move to the other bank and survey from upstream to downstream again.  This approach has helped me when I have many different computations to deal with, because I always know that I started both banks on the upstream side.  However, you can survey the high-water marks in any order youíd like.  Take pictures of some of the more representative high-water marks in the reach to accompany the written descriptions of the area if the flagger didnít already do so for you.  Itís also a good idea to take pictures of the entire reach looking both upstream and downstream.  After you have surveyed the high-water marks and before you survey anything else, take a break and plot the high-water marks in the field.  You can use software like Microsoft Excel to create a quick plot, or you could even import the points into the SAC-GUI or IRIC software and view a high-water mark profile plot.  The point of this exercise is to make sure you can positively identify ponding on the upstream side of the road as well as a drop on the downstream side of the road.  Donít skip this step!  If you donít see ponding on the upstream side of the road or the drop on the downstream side of the road, the computation may not be valid and another indirect measurement method may be needed. 
After you have plotted your high-water marks in the field and identified the ponding on the upstream side of the road and the drop on the downstream side of the road, it is time to survey the two cross sections.  Cross sections can be surveyed in any order, but I prefer to survey mine upstream to downstream, and if possible, I also like to start on the same bank for each cross section.  Locate the approach cross section at a distance of three to four times the depth of the water going over the crown of the road.  For example, if the depth of the water flowing over the crown of the road was 3 ft, youíll need to locate your approach cross section about 12 ft upstream of the upstream side of the road.  When surveying the approach cross section, always start and end at a higher elevation than the high-water marks.  Survey enough points on the cross section to define the cross-sectional area.  For simple cross section shapes, you might need as few as 5 or 10 points, but for complex cross-section shapes, you might need 40 or 50 or more points.  Remember to take pictures of the approach cross section as you survey ñ at minimum youíll want a picture looking from one bank to the other.  Bonus points if you get from left looking right, from right looking left, and a picture of the rod-person holding the rod horizontally at the height of flow.  
Letís talk briefly about troubleshooting.  What happens if there is water that is too deep to wade in the approach cross section?  Be prepared to take a boat across if you have MOCC training.  You could also tether an ADCP with GPS across the approach cross section to measure geo-referenced depths that you could combine into the survey later as long as you can determine a common datum for the different surveying instruments.  Safety first!  Do your best and document your choices in your survey notes. 
Now itís time to survey the cross section on the crown of the road.  Usually, the crown of the road is in the middle near the centerline, but if you canít easily tell where the highest point of the road is located, survey a few quick points to make sure.  Remember to wear reflective safety vests when working on roads and follow the traffic control plan for the site.  Just like the approach cross section, always start and end your cross section on the road at a higher elevation than the high-water marks.  Survey on the highest point of the road using enough points to define the cross-sectional area.  At this point, youíre almost done with the survey.  Remember to close your survey on the gage reference mark or temporary reference mark you established at the beginning of the survey. 
After youíve surveyed the road cross section, take out a tape measure and measure the width of the road itself, parallel to the flow.  Record this value in your notes.  Also make a note of whether the road is composed of gravel or if it is paved.  This is also a good time to take a picture of the road.  If you can do it safely, one of the best pictures you can take is your rod-person standing on the crown of the road holding the rod horizontally approximately at the depth of flow going over the road.  Donít forget your site sketch and cross section sketches!  You should now have enough data to compute the road overflow indirect measurement. 
If you need help in the field, call your supervisor, surface-water specialist, or indirect measurement specialist.