I am designing a detention basin on a site that has a creek running through it and an off-site 100-year flood elevation determined. The fema maps do not show the floodwaters coming onto our property but according to the elevations they do. I need to model this elevation downstream to see if it will backup the creek on the site and affect the discharge form the detention basin. I have tried placing an initial water surface elevation at a downstream junction, adding a know flow to the stream with the 100 year flood elevation. The program doesn't seem to take any of this information into account on my site. Has anyone came across anything like this? Any information would be greatly appreciated.
Thanks