We came across this website recently and found some good information on culvert design and fish passage. There is some pretty technical stuff as well, but we really enjoyed the video section--particularly "Y'all Jump". It's got a homemade feel with music and lyrics provided by USFS staff!
According to the website: "This software is intended to assist engineers, hydrologists, and fish
biologists in the evaluation and design of culverts for fish passage. It
is free and available for download".
We hope you can find it useful--or just entertaining!