Survey Requirements for Non-Standard Culverts

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Detailed Description

This video discusses how to measure various aspects of non-standard culverts, such as mitered culverts. Using the slope-area method inside a culvert is also discussed. 
 

Details

Image Dimensions: 3264 x 1836

Date Taken:

Length: 00:04:44

Location Taken: Las Vegas, NV, US

Video Credits

Charlie Besteder, Todd Geiger, Megan Poff, Office of Employee Development
 

Transcript

Hi, this is Chris Morris and I’m the surface water specialist at the USGS in Las Vegas, Nevada. I will be talking about non-standard culverts.  In previous videos I discussed surveying the approach high-water marks, the tailwater high-water marks, an approach cross-section, and the general attributes of box, pipe, and pipe arch culverts. Even if you have some aspects of a non-standard culvert, you will still need to measure features described in those videos.  

The information presented in this single-component video can also be found in TWRI 3-A3, Measurement of Peak Discharge at Culverts by Indirect Methods. Not every unique circumstance can be discussed in this video, so if you do have a non-standard culvert, having the TWRI to reference in the field is especially important.

As mentioned in previous videos, good clear notes are needed. With complex culverts or when combined with other methods (say a road overflow) it is easy to forget to measure the beveling or note if you measured the top or bottom of the culverts. There is nothing worse than finding that a critical element was missed a few months ago, and an additional site visit or survey is needed to finish the computation. Even for someone who has surveyed many culverts, I find having a checklist is useful to get everything you need.

Let’s first look at mitered culverts, or those with entrances and exits that are at an angle to match the embankment. The top and bottom of the culvert have different lengths, so where should you measure for the total culvert length? If the culvert entrance was submerged (completely underwater) the length of the culvert from the top of the entrance to the top of the exit should be measured. If the culvert entrance was not submerged, the length of the culvert from where the water surface first intersects with the culvert entrance and where it intersects with the culvert exit should be measured.

The question of the length from the culvert for the approach also arises… should you measure from the top of the culvert entrance or the bottom? This one requires a little math (don’t worry, it is easy!). Take the water surface elevation at the approach and subtract the elevation at the invert, or the bottom of the culvert entrance.  Now divide this number by the culvert diameter. If the result is less than 1, then measure the distance using a tape from the approach to where the water surface first intersects with the culvert entrance.  If the number calculated is greater than 1, then measure the distance using a tape from the approach to the top of the culvert.

Finally, sometimes the first length of a mitered culvert is at a different slope then the rest of the pipe. If so, measure the culvert invert at full pipe, or when the mitering of the pipe stops.

If there are multiple culverts, the distance for the approach upstream should be at least 1-3 times the sum of the culvert widths. Sometimes the culverts may have different slopes, be made of different materials, or have different elevations. All culverts need to be surveyed and measured. Additionally, the separation between each culvert should be measured and noted. For those that are located more than 10% of the diameter or width of the culverts apart, they will be treated as multiple components (as is discussed in another video), however, no additional field work will be needed.

After a flood, material is often found in the culvert itself. More often than not, here in Nevada I find sand fill in the first few feet of the culvert. When this material is loose, it is typically assumed that at the peak itself, the high velocities keep the culvert clean, and the material is deposited on the recession of the peak. The next peak again removes it on the rise. However, at some sites the material is well compacted or naturally cemented and doesn’t appear to change as a result of peaks. The same can be true for debris at the entrance. If it is thought the material was present at the peak, the obstructed area needs to be measured.  For example, if I found gravel well compacted and cemented in a box culvert, I would need survey a cross-section in the culvert (or if that was not possible, dig part of the culvert clear and use a folding ruler to directly measure the depth) to compute an accurate area. Take time in the field to assess if the culvert was clear at the peak or was impacted by debris or fill.

Some culverts actually have natural bottoms. In such situations, a cross-section should be surveyed at both the culvert entrance and exit. An average elevation will be used for the invert.

A final special case is running a slope-area measurement inside of a culvert. Unlike a natural channel, a culvert with a concrete or metal floor is not subject to scour or fill. In a situation when the flow was not conducive to a culvert measurement, this can be a good alternative. However, often high-water marks are hard to find inside the culvert barrel and flow may be wavy or indicative of unsteady flow from high velocities. If a slope-area measurement is attempted, it should be done away from the culvert entrance. More information about slope area measurements can be found in other videos. 

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