HEC-RAS Sensitivity to Geometry

[Tigris]

Greetings,

I have a question with regards to Geometry calculations with Hec-Ras and its influence on results.

I am doing an unsteady simulation on a river that is located in France, I use a Discharge Hydrograph as an upstream boundary condition and a normal depth as a downstream boundary condition, in terms of geometry I use the original surveyed river cross sections, I have changed the original sections with trapezoidal uniform section with same slope and I have noticed that same results were obtained in terms of Discharge and WL at the point of calibration, does that mean the river is not sensitive to geometry? or is it Hec-Ras that is not differentiating?

Next question that I have, when using a normal depth as a downstream boundary condition are the full dynamic wave terms of Saint Venant equations neglected?

Appreciate any response to my questions.

[jarvus lunt]

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.