HEC-RAS, flood encroachment makes the 100 year elevation go down? 1

[beej67]

I've got a HEC-RAS file with some hand-entered sections in it, extrapolated from a survey. If I go into two sections and narrow the floodplain by showing an encroachment, the flood elevation goes down, not up like it should.

These are my sections, from upstream to downstream. Encroachment is at sections D and E:

F: flood goes down a tenth
E: flood goes up three tenths
D: flood goes down .05
C: flood goes down .01
B: no change
A: no change

Why would F go down a tenth? For that matter, why would any of them? I'm guessing I made an error somewhere, but I'm not sure where to look. The net effect of the project is to fill in about a foot and a half of flood plain at E and D, approx 75 feet from the banks of the creek. Flows are 650cfs-ish.



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[cvg]

does it go supercritical?

 

[beej67]

No, that's the thing. I could buy it if there was a hydraulic jump moving around or something, but all the Froude numbers are 0.75 or below.

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[cvg]

try interpolating more cross sections on both models, see if that smooths things out

 

[francesca]

Doesn't have to go supercritical for the velocity head to increase and lower the WSE. But typically upstream of the encroachment will get an increase in WSE because of the constriction.

Did you change your Manning's values? Are the x-sections identical other than the encroachment?

 

[beej67]

Mannings are the same, and the cross sections are the same except for the encroachment. That's the weird part. There's no real reason for the velocity in the channel to increase.

Data entry error I suppose. I'll look at it again Sunday with a fresh mind.



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[francesca]

Q = VA, if A decreases, either V increases or Q decreases.

 

[beej67]

Obviously. And the decrease in conveyance due to flood plain fill does two things typically in a steady state model, it increases V, and it pushes the WSEL up to make more conveyance. (A) What doesn't make sense is the WSEL going down on its own due to a restriction, unless there's a supercritical transition.

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[francesca]

I have to disagree with you. The WSE routinely goes down through a bridge constriction without passing through supercritical.

 

[msquared48]

You've really discovered something here...

Now you know why we are getting more 100 year floods than ever!

Mike McCann
MMC Engineering

 

[gbam]

Break out and dust off the old specific engergy diagrams. and look and a rectangle channel section to see what happens during an encroachment. If memory serves me correctly, the results for subcritical regime were counter intuitive as in the contraction lowering the water surface and an expansion making it increase. All due to velocity head moving faster than the depth. Please correct me if I am wrong but that is what I recall from Hydraulics way back when.

 

[beej67]

I may have to dust mine off as well, but I believe that's the behavior for the supercritical flow regime, not the subcritical.

Francesca?




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[francesca]

All I'm saying is that in about 50 bridge models I did last year, the WSE was often lower than "natural" through the bridge and the Froude #s were well in the subcritical range. We were dealing with designs criteria that allowed up to 1' backwater (with purchase of flood easements). It's a simple product of the energy equation that the WSE drops with increasing velocity and the energy equation doesn't change with flow regime IIRC. (I may not have looked at a bona fide hydraulics text in over a decade, even if I have quoted Chow.)

 

[cvg]

any reduction in the wetted perimeter could cause this

 

[JALeng]

I'm agree with Francesca, negative increases or surcharge may occur in the floodway computations due to constrictions that increase the velocity in the point of analysis. Similarly, extremely short elevation increase can be taken as unacceptable. The following conditions may create negative surcharge values:

1. The floodway is too narrow compared to the natural top width. This condition is relates to a smaller area and hight velocity. The floodway should be widened or revised with data collected on field.

2. If the energy grade line for the floodway profile is equal to or lower than the natural profile you may have these conditions: The floodway is nearly as wide as natural top width at the cross section downstream. Try narrowing the nonoptimized floodway at the downstream cross section. Based on reference added below HecRas computes WSEL based on the gradually varied flow assumption. Adding cross section may be needed if the following condition are present: the velocity head difference between the two cross section is more than 0.5 ft; the conveyance ratio is less than 0.7 or more than 1.4; the depth is less than 0.9 or more than 1.1; the top width ratio is less than 0.5 or more than 2.0; the distance between the two cros sections is more than 500 feet; and|or the discharge from one overbank area shifted to the other other overbank area between two cross sections.

3. The channel bank stations are not located at the natural banks beyond which relatively flat overbank exists.

Furthermore, you may verify changes in key variables (top width, conveyance, velocity, and friction slope) from cross section to cross section.

Are several references that explain this issue: Floodplain Modeling Using HEC RAS, explain negative and excessive surcharge values. FEMA Guidelines 2002 (

http://www.fema.gov/library/viewRecord.do?id=3352)

 

[SMIAH]

As cvg pointed out, it could be a "model" issue though.

I can't see why it would go down in F then up in E.

 

[beej67]

Thanks JAL. Pink star inbound.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

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