HEC-RAS, flood encroachment makes the 100 year elevation go down?
HEC-RAS, flood encroachment makes the 100 year elevation go down?
(OP)
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.
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.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com





RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Did you change your Manning's values? Are the x-sections identical other than the encroachment?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Data entry error I suppose. I'll look at it again Sunday with a fresh mind.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Now you know why we are getting more 100 year floods than ever!
Mike McCann
MMC Engineering
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Francesca?
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
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)
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
I can't see why it would go down in F then up in E.
RE: HEC-RAS, flood encroachment makes the 100 year elevation go down?
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com