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Large island
Dear HEC-RAS experts:
I am new at HEC-RAS. I am examining a section of river using an older set of HEC-2 "FEMA" data. In addition, we have measured several additional transects in between the old ones. It just so happens that the study area includes a large island. I have treated the entire river as one reach, just as the original flood study did.
I have noticed that HEC-RAS apparently calculates a single water surface elevation regardless of the "main" channel versus the "secondary" channel. The output is indicating "divided flow" in the island area. In the study area, the "main channel" (I assume you must make a stab at this if flow seems equal between channels) is a longer channel and has a much gentler slope. The secondary channel is steeper.
The initial new transects were projected across the island with an effort to be perpendicular to flow at the channel locations. However, in so doing, the approximate water surface elevations as indicated on a 2-foot-contour topographic map were ignored. My question is this, because of the large difference in slope of the two channels and their divergent trend directions, the resulting transect locations cross the channels at quite different water surface elevations at each channel. That is, one channel water surface elevation may be 2 to 4 feet different from the other channel along the same transect.
In an effort to attempt some calibration of the model, I am not getting good results. Should ALL transects be constructed such that in the case of a large island, they cross the channels at roughly the same channel or water surface elevation? I noticed that this is not the case for some of the old FEMA transects. Do I need to split the reach into two reaches, if so, why?
B. Smith
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