scour depth for culvert footings 1

[dskurnik]

I am placing a 9 ft wide, 22.5 ft long bottomless arch box culvert in a creek. There is no firm data for 100 year peak volume. The dimensions are set by fish passage requirements, rather than hydraulic. The capacity is more than 3X compared to upstream culverts on the same creek.

The design calls for concrete footings placed below the streamgrade. Since this is a small project, the cost of concrete is cheaper than detailed geotechnical analysis. Is there a rule of thumb to estimate the necessary depth for the bottom of the footings to ensure that it is below the scour depth?

 

[RWF7437]

I don't know any "rule of thumb" for this but you may be able to estimate the scour depth using HEC-RAS. There are also many riprap sizing charts floating around, based on Corps of Engineers data. Because, as you suggest, the waterway opening is based in fish passage requirements I would expect design velocities to be low, even for large storm events. You do need to calculate the probable flow in cfs ( not volume in cf) to do any sensible design. Choose a probability you feel comfortable with and calculate the confidence level for that probability.

Just curious: why is an Electrical Engineer designing culverts ?

good luck

 

[cvg]

estimate the normal high flow of the creek by going to a natural channel section, surveying the section, obtain the channel slope and then visually identify the top of bank / high water mark. Using Mannings equation for normal depth you could then estimate the mannings n value and then back calculate the peak discharge which corresponds to the high water mark that you observe. Use this in your scour analysis and then use FHWA procedures for scour analysis. Of course, the cost of this detailed hydrologic analysis may also be more than the cost of concrete. You might also just call your department of water resources / flood control department (the one that may be issuing your environmental or floodplain use permits, you are getting a permit aren't you) and see if they can give you a rule of thumb. Absent any rules, use at least 4 feet deep.

 

[dskurnik]

CVG: Thanks for the tip. The four foot number was close to my guess of 3 feet. If I go another foot down, I will spend about $300 more in concrete, but that is cheap insurance. BTW, I am getting permits, but the only ones needed for this project are a county grading permit, and a hydraulic project approval from the Dept of Fish and Wildlife. I am not confident of their ability to make an accurate judgement regarding scour.

RFW7437: Yes, I am an EE, and completely unqualified to design this job, but it is on my own property. I have managed to cobble together the required information by asking lots of questions (many of them really dumb). It's been fun learning about civil engineering and hydraulic projects. Its a good break from making chips.

Thanks to both of you for your help!

 

[Schopsy]

It is your property now, but will it be in 5 years? 20?

 

[RWF7437]

What are the possible consequences of failure of this culvert to you AND to your neighbors ?