Survey Requirements for Critical Depth Indirect Measurements

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Detailed Description

This video describes the high-water marks needed along with cross-section locations for critical depth indirect measurements. Critical depth computations are those performed when flow goes through a critical flow regime at a drop, such as at a waterfall. 


Date Taken:

Length: 00:05:15

Location Taken: Las Vegas, NV, US

Video Credits

Office of Employee Development, Terry Kenney, Megan Poff, Todd Geiger, Jonathan Yldefonso, Puerto Rico USGS office


Hi, this is Chris Morris and I’m the surface water specialist at the USGS in Las Vegas, Nevada. I will be talking about critical depth indirect measurements. A critical depth measurement is based on a flow through a fairly uniform channel going through a critical flow regime at a fall. The information presented in this single-component video can also be found in the chapter Indirect methods in hydrometry, by Barnes and Davidian, in the 1978 book, Hydrometry published John Wiley and Sons, pages. 149-204.

There are two key elements that are needed for a critical depth computation. Most important, is a sharp break or drop in the channel slope causing the flow to go through critical depth. I like to think of this as a waterfall or water spilling over a dam with no submergence. If the flow did not go through critical the method cannot be applied as the computed discharge will be artificially too large. Secondly, the approach needs to be relatively uniform with a mild slope.

Given these limitations, when visiting a site after a flood, before any surveying I like to try and visualize how the site would appear during the flood. Maybe there is a bedrock ledge now forming a good fall, but would it have been drowned out at the peak and therefore the drop was submerged? The channel upstream looks uniform now, but did it reach into a wide, densely vegetated floodplain at the peak?

If it looks good, let’s proceed.

First, you want to flag and survey the high-water marks. I start flagging two times the flood channel width upstream of the critical section. High water marks can be surveyed in any direction, although I find it less confusing in the office to start from the upstream end on both sides. The proper techniques for identifying, flagging, and surveying high-water marks are presented in other videos.

Survey the high-water marks from the approach, past the drop to a distance of one flood channel width downstream of the critical section. You are surveying the downstream side to document the tailwater conditions below the critical section. It is possible that there are few to no high-water marks at the critical section itself but do your best. During the computation, you will actually be “solving” for the water surface elevation at the critical section, so it is helpful to have an idea of the elevation by having some high-water marks available. Also, be aware of the safety of surveying near the drop. You may have to wait till low flow before such an area can be safely accessed.

Once the high-water marks have been surveyed, take a break and plot the high-water marks in the field. It can be tempting to skip this step, but if there is submergence and the flow does not go through critical, evidenced by a noticeable drop in water surface elevation and shallowing of the flow, the critical depth method cannot be used to obtain a true solution. You can use software like Excel, SAC-GUI, or IRIC to create a high-water mark profile plot to verify the mild slope then the steep drop. At this point, while it is still fresh in my mind, I like to start my site sketch and make any important notes about the high water marks. Site sketches are discussed in more detail in another video.

On to the cross-sections. First, you will survey an approach cross-section. This cross-section should be at least one flood channel width upstream from the critical section. Place your cross-section in the middle of the high water marks you surveyed since you will be using the high-water marks to determine the water surface of this approach cross-section. The cross-section should be perpendicular to the flood flow. For simple cross-sections, you may only need a 5-10 points, however, for complex ones you may need many more. You want to characterize the changes in the approach section geometry, so you can compute an accurate cross-sectional area. Make sure your first and last cross-section points are above the water surface during the peak. If you have really good marks, this may be obvious, but when marks are poor, it is always better to go a little higher just to make sure you end up be above the final water surface. Finally, don’t forgot to take pictures of the approach to help estimate the roughness coefficient. A picture of the rod person holding the rod horizontally at the height of the water surface from both sides is quite useful.

At this point, while it is still fresh in my mind, I like to continue my site sketch and make any important notes about the approach cross-section. I will also estimate a roughness value for the approach. This topic is discussed in more detail in another video.

Next, you will survey the cross-section at the critical section. You will probably not be able to survey exactly where the flow went critical—it is very difficult to know exactly where this is, however, you want to be as close as possible to the drop off. Again, safety concerns are very important. Depending on the depth and the velocity, you may have to wait until low flow. If it is never shallow enough to wade, a tethered ADCP could be used in the cross-section, however, it would have to be combined with the existing survey at the same datum.

The procedure is the same as for the approach cross-section, try to characterize the area and make sure points above the water surface are included. Pictures of the rod person holding the rod horizontally at the height of the water surface from both upstream and downstream of the drop are very helpful to illustrate the critical section.

Again, just like with the approach, continue the site sketch, make any important notes, and estimate a roughness value for this cross-section.

At this point, you are done with the survey. By planning ahead, it is often possible to shoot everything from one setup.

If you need help in the field, call your supervisor, surface-water specialist, or indirect measurement specialist.