Got a project with a floodway elevation at one cross section that’s lower than the 100-year. Other sections are all as expected, with floodway elevation above 100-year. Any ideas on what might be going on?
This occasionally happens when doing floodways. The energy grade is the water surface plus the velocity head.
EG = WSE + Vel^2/2g
WSE = EG – Vel^2/2g
At a constricted cross section, if the velocity goes up enough, the water surface can actually drop even though the energy is higher. If you plot the EG, it should be higher than the 100-year.
Are you using an HEC-2 Model if you are converting from HEC-2 into RAS you most likely will get some that are lower then the 100 year. Also do things like check the channel elevation at that location I have noticed that those can change the WSE quickly if typed in wrong. You should always try and keep it the same if its lower in one section. It’s all dependent on your project however.
From a hydraulic/engineering perspective it is not really a problem. It can be difficult to explain to non-engineers. (For that matter, it can sometimes be difficult to explain to engineers). The key thing is to make sure you understand it well enough to explain/defend it.
I would point out: the drop in water surface can happen at a few cross sections. However, these cross sections will have greater velocity and greater friction losses. Eventually, as you move upstream, the water surface will rise.
If you don’t want the water surface to be below the 100-year answer, you may have to avoid constricting the river at that location.