Hec Ras Critical Depth 1

[mj406]

When hec ras cannot balance the energy equation with a subcritical answer between two cross sections, it defaults to the critical depth and carries on upstream with the step calcs. When I preview the cross section and profile, the wsel is shown at critical depth, however what's puzzling me is that the profile summary table shows a froude number not equal to 1, something lower like 0.8. Shouldn't the froude number be 1 if the section is at critical depth?

 

[cvg]

are you running sub-critical, super-critical or mixed flow?

 

[Ryb01]

Are you looking at a section of reach in the close vacinity of a culvert or bridge?

 

[mj406]

Well, I'm running sub critical on a 4.5 mile reach with 100+ XS's. Several cross sections are defaulting to critical depth, slope max is around 1%, flow area seems pretty consistant. I've tried interpolating cross sections, and that works in some cases, other cases the interpolated sections become critical. My topo source is from Lidar, I'm wondering if the terrain is too complex for Hec ras (250 points, up to 1000' in length).

 

[cvg]

1% slope is fairly steep for a channel, you could have critical or supercritical flow. run in mixed flow mode and see what happens

 

[mj406]

Ran it in mixed and several sections are at critical and supercritical. After taking a closer look, the invert slopes on most of these are 1 - 1.5%. The hec reference manual claims that froude number calculation for irregular channels is not accurate, so that may explain the critical sections with froude numbers of .6 to .8. Thanks for the help!

 

[SMIAH]

I have some doubts about that.

Critical Depth should be equal to a Fr = 1.
Or very close (not 0.6).

 

[mj406]

I agree that it should! I'm using 4.0 and just reviewed the release notes for 4.1, in things they've fixed, they say the alpha coefficiet was applied to the velocity for computing froude #.

 

[nenik]

MJ-Did you ever find a solution to this problem. I am having the same issue. My model is defaulting to critical depth and I am getting Froude #=0.65-0.8

 

[francesca]

When HEC-RAS cannot find a solution it defaults to critical depth. Look at your cross-sections; is there a hinge point at which the flow area expands rapidly? If so, can you block some of it out as ineffective? Remember that areas with <3 ft depth AND <0.5 fps velocity are not conveying significant enough flow and can be marked ineffective without significantly altering the model results. HEC-RAS is a 1-dimensional calculation and cannot determine if an available flow area is truly effective or not.