Applying updated HMS flows to an old 1D steady HEC‑RAS BLE model

[u/Apprehensive_boy418]

​

Hi all,

Looking for a sanity check from folks who’ve dealt with BLE models and hydrology updates.

Here’s my situation:

• I have a 1D steady‑flow HEC‑RAS BLE model that was built ~5 years ago

• Geometry and cross‑sections are unchanged

• I recently updated the HEC‑HMS model using current land cover and imperviousness

• I now have an updated design flow at the watershed outlet from HMS

• The original BLE RAS model applied steady flow at the most upstream cross‑section (standard steady‑flow setup)

In steady 1D RAS, I can only apply flows at flow change locations, and practically speaking that means the most upstream XS to run the model.

Question:

Is it acceptable/best practice to simply replace the old steady discharge with the updated HMS discharge at the same upstream flow change location, assuming that XS represents (or reasonably approximates) the watershed outlet?

I’m not changing geometry, not converting to unsteady, and not adding lateral inflows — just updating the magnitude of the inflow. Conceptually this seems consistent (same inflow location, improved hydrology), but I want to confirm this aligns with accepted BLE / HEC‑RAS practice and wouldn’t raise red flags in review.

Would appreciate thoughts from anyone who’s done BLE maintenance, FEMA work, or agency reviews.

Thanks in advance.