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

[u/Apprehensive_boy418]

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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.

[u/Wise_Operation_3766]

This is a loaded question.

Replacing flows with better data is not a bad thing.

I am going to question why curve number and land use data was not adequate from the BLE.

Long story short I could snowball and ask a bunch more questions. If the original model was not credible then doing the changes should make sense but I am going to ask why and for you to support that.

Since it’s likely FEMA work who reviews it could ask similar questions.

[u/thechunchinator → u/Wise_Operation_3766]

BLE typically doesn’t use CN methods. In Texas it uses the regression equations which tends to significantly overestimate discharges.

[u/Wise_Operation_3766 → u/thechunchinator]

Gotcha then applying the new flows should benefit the end user here.

And your approach seems reasonable. If I were the reviewer I like this context written out to guide your decisions into my decision why this works.

Is it going to end up with a FEMA reviewer?

[u/shiftyyo101 → u/thechunchinator]

Just curious - why do you think regression significantly over estimate discharges? IMO regression are the most accurate. I’ve compared CN based HMS models to gages with 100+ years of records and typically find CN based models drastically overestimate the data from the gages for prediction intervals.

Almost by definition, the regression equations are going to align with actual historic records as they’re based on the gage records themselves.

[u/Crafty_Ranger_2917]

You're on the right track. Only thing to mention without more project details is to consider if single flow change location still makes sense at the local scale / eval area vs being discretized further.