I have an encroachment analysis at a box culvert and cannot get the surcharge less than 1.08. I widen the encroachment and the surcharge increases. The problem is at the downstream cross section of the 4 culvert cross sections. Any suggestions? Also as I narrow the encroachment the surcharge increases also. Getting very frustrated.
The froude number at this cross section is greater than 1. This is the only cross section in the entire model (4 box culverts and a total of 22 cross sections including the culvert routines) where there is supercritical flow. This is the cross section where I cannot get the surcharge to 1 foot or less. Any suggestions? Move this cross section further downstream maybe?
If the energy grade is too high than it may not be possible to get surchage under 1 foot. You probably need to widen the enroachment downstream. High velocities have a higher velocity head. This can cause the water surface to drop even though the energy grade goes up. However, at some point (further upstream) this can cause problems. The image below shows how constricting a cross section can [sometimes] cause the energy grade to go up, the velocity to go up, and the wsel to go down.