HY8 outlet depths

[geoa]

I'm using HY8 to evaluate fish passage through a culvert where depths are critical to the fish passage performance. HY8 is calculating an 1-s2n profile which is normal depth at the outlet. HY8 also calculates a normal depth and an outlet depth. In this case the outlet depth is less than the normal depth. Why is this outlet depth less than the normal depth? The HY8 help documentation says the outlet depth is based on the s2n profile so shouldn't the outlet depth be equal to the normal depth for an 1-s2n profile?

 

[francesca]

If flow through the pipe is so fast that supercritical flow is achieved, depth will be lower than normal depth. You can switch to a different pipe material (CMP instead of concrete or HDPE) to try to slow the water down.