HEC-RAS Flow Regime

[DrainageEnthusiast]

If the existing or proposed channel I'm modeling (with steady flow condition) has a mix of mild and steep ground slopes, which flow regime shall I use in order to predict the possible water depths along the channel?

Subcritical
Supercritical
Mixed

I tried to compare the output results of each but it seems very little difference.

 

[gbam]

Great Question. It depends on the bed material and resulting bedforms. If your bed is rigid then I would run the mixed regime; ie.: roadway pavement or concrete channel type. Now if you have a sand bed channel, there are a number of white papers and studies showing that a sand bed channel will not allow flow to transform to supercritical regime and will default to critical when not subcritical; otherwise one needs to understand the bedforms being developed for your design flow. Hope that helps.

 

[DrainageEnthusiast]

@gbam

That's interesting! So you're saying I should stick with subcritical than mixed flow regime? As you can see in the image I have attached, the channel bed is made up of existing ground/soil, covered with very thin loose gravel that were washed away. We're proposing to fill the bed with gravel (not loose) all throughout the channel. I used Manning's coefficient of 0.040 for the channel bed in the HEC-RAS model.

Also, I noticed this on the program warnings:

Warning:
During the standard step iterations, when the assumed water surface was set equal to critical depth, the calculated water surface came back below critical depth. This indicates that there is not a valid subcritical answer. The program defaulted to critical depth.

This warning has nothing to do with the bed material, right?

Is sand bed the only material that will not allow flow to transform to supercritical regime? or all flexible type of cover will default to critical when not subcritical?

Thank you for taking the time to respond.

 

[gbam]

Sorry for the delay, the studies I have reviewed have been all sand bed channels. I suggest you look at the potential bed forms that will form in your storm event. Maybe research bed forms. I still use my text book from college: Sediment Transport Theory and Practice by: Chih Ted Yang. Regarding your messages, when you run a model in one regime it will not cross over the critical boundary and default to critical for either regime. The model can be run in mixed but you need to be aware of movable bed material... Also confirm your roughness, i suggest you use Chow as a guide if the agency does not have guidelines. Hope this helps, this gets pretty complex at times and sometimes you can justify a specific direction (i.e. higher velocity or depths based on design constraints.)

 

[DrainageEnthusiast]

@gbam
Thank you very much