Mapping the floodway

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    I completed a regular run and used those files, along with changing the CONT.DAT IFLOODWAY from 0 to 1 and ENCROACH from 0 to 1.00 and reran the model. But where do I get the floodway results? I see nothing in the output files, plotting the inundation limits is the same as it was for the regular run, and I see nothing in MAPPER menus that gives the floodplain results. Furthermore, I have searched the 2009 and Pro manuals, tutorials, workshops, and forum and find nothing on floodways.

    I’m puzzled. How do I get the floodway limits?


    As a follow up to the previous post, I have compared the MAXWSELEV.OUT between the floodplain run and floodway runs. With the ENCROACH at 1.0 there was very little change. With ENCROACH at 1.8 the maximum change in water surface elevation was plus 0.46′ and minus (yes, minus) 1.12′. With ENCROACH set to 2.5 in an attempt to get up to a foot increase, the maximum increase was less at 0.46′ and the decrease was more at 2.15′. The only other time I used FLO-2D for floodway analysis I recorded similar issues.

    Given the narrative about the approach FLO-2D uses to calculate the floodway water surface elevation, I wondered if there must be a channel throughout to first convey the water before spill occurs after the FLOODWAY.OUT elevation plus ENCROACH depth is reached. In each project where I attempted using FLO-2D for floodway analysis there were segments without a defined channel. However, both models start at the top with channels. With the current model, there is channel from the top to 3800 feet downstream before partial flow goes into a culvert and the rest overland around through an underpass and back to a channel again, but even in the channel areas, even at the top where the water starts and computations should be good, they are not.

    What am I missing here?

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