Stream Confluence – Under the Floodplain

[SeanWatt]

Hi, I’ve got two watercourses coming together, one flowing into the floodplain of the other (large wetland extents). I’m working on the cross-section locations, and trying to lay things out without crossing multiple streams, or cross-sections. Just upstream of where the confluence is, I’ve also got a roadway crossing the main watercourse, further restricting my options for cross-sections on the smaller watercourse.

I’m wondering which option people generally prefer, building lateral structures to “confine” the flow, or just letting the cross-sections end and having RAS assume a vertical wall?

Most of the floodplain volume of the main watercourse is a result of backwater from a downstream crossing, though the smaller watercourse can contribute a surprising amount of flow at times. The previous modeling for these creeks was done in two pieces, one model for the main watercourse, one model for the smaller watercourse, where it had the boundary condition of the floodplain calculated by the for the main watercourse. I’m working on an update, and would really just prefer to run it all as one piece.

Thanks,

Sean

[Chris G.]

If youre working in unsteady flow, the amount of volume that leaves a cross section can have a significant impact on the attenuation of your flood wave, and you should account for it. Letting RAS use the vertical wall of water does not properly account for volume going into storage. Whether you extend your cross sections to high ground to contain all of the flow (and storage volume), or you use a lateral structure to account for volume leaving depends on the behavior of your storage volume and how connected it is to the main channel flow. i.e. if the water surface elevations between cross section and storage are significantly different, use a lateral structure.

[Author]

Posts