Tuesday, June 24, 2014

Critical Depth Primer

Then check the box next to “Critical Always Calculated.

1. Thank you very much.

2. Thank you very much for this post!

3. bahnisikha.das513@gmail.comMarch 21, 2016 at 11:17 AM

can u please tell where do I find the details of how critical depth is calculated by parabolic method in steady flow analysis

1. The hydraulic reference manual is a good place to start.

4. Thanks Prof. Goodell. Just completed a project using RAS. Hydraulic is nice to study.

5. Thank You... But have being looking for solution to any "Incomplete data error" when running a steady flow analysis in subcritical flow regime... the errors includes:
* Station data contains a wall with zero width, found the elevation goes up and then down (or down and then up) without moving over. Please remove wall (take out the middle point) or add some width to the wall. At point(s): 2,3,4,5,6,7,8,...26.
* No manning n data or friction heights K set
* Right bank station must be greater than left bank
* Main Channel length is less than or equal to zero, HEC-RAS requires a positive reach length.
*Right over bank length is less than or equal to zero, HEC- RAS requires a positive reach.
NB: Am working with two rivers that met at a junction and continues flowing.

PLS AM NEW TO HEC-RAS, but i need this to do my Masters Thesis. THANKS

6. Regarding The hydraulic reference manual (v4.1. page2-13), which value of Froude number in Cross Section Output Table is for critical depth, Fr=1 or Fr=0.94?

1. FR=1. FR = 0.94 is just a quick threshold check for when to determine critical depth by more accurate methods (specific energy method).

7. Great blog. I work a lot with FEMA flood studies where a "no-rise" condition means no rise to the 0.00' comparing pre- and post-project modeling when determining if a conditional letter of map revision (CLOMR) is required. Frequently the post project model will have an identical cross section as the pre-project but will have a different critical depth and this can show as a 0.01 or greater rise which of course is not allowed if trying to achieve a no-rise. The models are all run in subcritical flow mode (a FEMA requirement). I have set both models to parabolic and also multiple methods but although I get different answers the rise still occurs. Adjusting computation tolerances courser does not help with matching the critical depth and adjusting finer appears to make the differences greater. Other identical pre- and post-project cross sections flowing at critical depth match elevations very well. Any thoughts on what is causing this would really be helpful. Thank you.

1. Not sure what would cause that other than some difference between your cross sections. You might try copying the cross section from one geometry to the other geometry just to double check there is nothing different.

8. Chris, my bad, I found the top of bank was set differently between the two models. I do see this a lot and usually can find a tiny error between the cross sections and correcting that solved the problem.

1. Glad you found it!

9. Hi chris, its morgan from Nigeria... i just succeeded in running an unsteady flow simulation..using my dam breach hydrograph output data as initial boundary condition..(flow hydro graph...but my water level in the profile was constant even the energy line was also constant...How do i manipulate my way through the energy line and water level..although the discharge Q was reducing from upstream downstream because of the dam breach over topping flood..PLEASE HELP ASAP...

1. Sounds like maybe you have a downstream boundary or control in your model that is keeping the water high, level, and slow. Could be an error too. Check your htab parameters and make sure they cover the full range of ws elevations computed in the simulation. Especially the bridge and culvert htabs. Also make sure your computational interval is small enough. For a dam breach model, it should typically be 30 seconds or less.

10. Chris, do you know the basis for why HEC-RAS (and hydraulic modeling in general) defaults to the flow profile with the higher specific force value at a particular cross-section? Why can't the lower specific force value be just as correct? I've tried searching the HEC-RAS manual and other references but haven't had any luck finding an explanation for why the higher specific force governs.

1. You might try reading the Specific Force section in Chow's "Open Channel Hydraulics". That's Chapter 3-7 in my version of Chow. It might help explain.

11. Hi Chris- I am running a Hec-Ras model which I am having a divided flow computed for my cross sections and at one of my sections, just downstream of a box culvert my flow hits super critical and the divided flow is not computed. Can you think of any reason or way to fix this? The program wants to push the entire flow back through the channel rather than use the storage as defined outside of the banks. Any help as quick as possible would be greatly appreciated.

Chris