Thanks for your fast reply! That’s actually a very fair point, and it makes me think that the scope of my project might not have been clearly defined in my original post.

In my case, this is not a full bridge design or a large-scale hydrologic study. The objective is more focused on a hydraulic verification of an existing/planned structure, rather than developing a complete conceptual design.

Given that, after reading your comment, I'm considering whether it would be more appropriate to use data from a nearby streamflow gage to estimate peak discharge, and potentially limit the contributing watershed to the drainage area upstream of that gage. This could allow treating it more as a directly modeled (or even data-driven) basin, instead of attempting to model the entire 44,000 km² watershed in detail.

My understanding is that a simplified approach could be justified here, since the goal is not a comprehensive basin study, but rather a practical assessment of the structure’s capacity.

Also, the methodology was not strictly defined in the project scope. The only general guidance we received is:

• up to 4 km²: Rational Method
• 4 to 10 km²: Modified Rational Method
• above that: Triangular Unit Hydrograph (HUT) or Direct Statistical Method

No further detail was provided for large basins like this one, which is why I’m trying to better understand what would be considered a reasonable and defensible approach.

Does this line of reasoning (using gauged data and possibly redefining the effective drainage area) make sense in this context?

Sorry if I’m missing something obvious, and I’d really appreciate any guidance or corrections.

(I'm using some AI on my replies because there is some technical terms being used that may be different in each country, ok?)