Water Surface Profile appears lower when compared to FEMA model

[Goverdhan Reddy]

Hi

I am modeling a small creek to submit to FEMA for a LOMR application. When compared to the effective model from FEMA my model is showing lower water surface elevations(approximately 1 - 1.5ft), I used the same manning's coefficients and flow data as in the effective model.

Can any one tell me what might be the cause for the difference.

[Canada]

Same sections geometry?
Same boundary conditions?
Same flow (that's a given)?
Same contraction/Expansion Coefficients?

[pinkpig]

and same software version?

Originally Posted by Canada

Same sections geometry?
Same boundary conditions?
Same flow (that's a given)?
Same contraction/Expansion Coefficients?

[CTCoker]

Same datum?

[Goverdhan Reddy]

@ Canada

I used the same boundary conditions, flow data, and contraction expansion coefficients as in the FEMA effective model, but regarding the section geometry I used the surveyed, and LIDAR data and I extended cross-sections a bit wide than in the FEMA model.

@ pinkpig

the effective model is in HEC-2 and my model is in HEC-RAS 4.0 Beta.

@ CTCoker

Yes, I am using the same datum.

and one more thing I would like to specify here, at the first 5 down stream cross sections the WS elevations are matching with the FEMA model but as I go upstream the remaining 8 cross sections show the difference, the WS profile is some what flat as I go upstream from C/S-6 to C/S-13 in my model where as in the FEMA model its ascending gradually.

Thank you all for your response

[Canada]

HEC-RAS 4.0 Beta ?

[Goverdhan Reddy]

@ Canada

HEC RAS 4.0 Beta, its the version of HEC RAS software I am using.

Thanks
Goverdhan Reddy

[gauss2069]

I am currently working on a project requiring LOMR-F. My project so happens to be on a confluence of a studied Creek (with existing FIS) and a tributary (not studied). After careful analysis of the current topo to the existing FIS sections (surveyed in 1953 by the Corp of engineers and SCS) there is a staggering 3 foot differential in flow lines within the 1000 ft length (old topo higher than the current topo). I corrected the existing FIS to HEC-RAS and I have 1-2.5 ft in WSEL within the confluence.) The datum is the same after careful review of the existing and on-site topo I am using. I have to document this in the report because I strongly believe there has been either massive eroding or they dredged that area (56 years differential).
This is my humble opinion.

[elcidce]

Originally Posted by Goverdhan Reddy

@ Canada

I used the same boundary conditions, flow data, and contraction expansion coefficients as in the FEMA effective model, but regarding the section geometry I used the surveyed, and LIDAR data and I extended cross-sections a bit wide than in the FEMA model.

@ pinkpig

the effective model is in HEC-2 and my model is in HEC-RAS 4.0 Beta.
@ CTCoker

Yes, I am using the same datum.

and one more thing I would like to specify here, at the first 5 down stream cross sections the WS elevations are matching with the FEMA model but as I go upstream the remaining 8 cross sections show the difference, the WS profile is some what flat as I go upstream from C/S-6 to C/S-13 in my model where as in the FEMA model its ascending gradually.

Thank you all for your response

You need to adjust the settings in HEC_RAS to duplicate HEC-2 results. This is listed in the users guide.