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RAS uses whatever geometry that is entered. So either your results are not that sensitive to geometry at that location, or the trapezoidal approximation has a similar enough conveyance that it gets the same result.
In order to solve a single, simple reach using the Saint Venant equation, you need two, known, external boundaries. These are usually upstream flow and downstream water surface. Instead of a known water surface, normal depth creates a known relationship between flow and the water surface. I don't see in what sense this would cause any of the terms to be neglected. Or maybe I don't understand what you are asking.
You might browse through the Hydraulic Reference manual if you want to learn more about how RAS does its computations.
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