HEC RAS Output troubleshooting 3

[mbherlihy]

I am modeling a river, adjacent to property we are developing. I have flow data from a nearby USGS rain guage, obtained from FEMA's FIS study. I have 5 x field run cross sections (survey crew using a river boat) to get bottom of channel elevations. Depth of river is approx 4-6'. When I model the cross sections, my 100 year water surface is about 6-7 feet lower than the FEMA flood study elevation. Why is this?....could it be that I need to add the exisiting water surface to the model?.... I am new to HEC RAS. I'm pretty confident of my input stead state flows. I am also pretty confident of the shape of the cross sections, and the roughness manning's n for the bottom and sides of the river..... can anyone help?

 

[RWF7437]

Here are a few suggestions and questions:

"I have flow data from a nearby USGS rain guage, obtained from FEMA's FIS study. "

If you have the flood study ( the report, not just the FIRmaps) check to make sure you are using the same flows as were used in the study.

If you can, get the HEC-II or HEC-RAS data from FEMA's contractor. This may cost a little money but will allow you to see if you can duplicate FEMA's work.

Plot your field surveyed cross sections. Make sure they don't cross each other. Especially, make sure they extend far enough to the right and left overbank areas to include ALL of the floodplain.

Check areas both upstream and downstream from your project boundaries . Are there bridges, culverts, weirs or other structures present ? If so, these may control the starting water surface elevations for your model.

What did you use for a starting water surface elevation ?

Obvious, but make sure your cross sections are based on the same datum as the Flood Study. If possible, use the same bench marks as were used for the study. These may be found in the FIS report.

If you imported HEC-II data into HEC-RAS, make sure you got all the culverts and bridges. These can be lost when importing.

Make sure you are using the same Manning's "n" values as were used in the FEMA study. Even if you don't agree with FEMA's numbers, you need to make sure your work and FEMA's agree.

Also obvious, but check to be certain there are no major changes in the flood plain since the FEMA Srudy was done. Some of these studies are decades old.


Finally, why are you doing this ?

good luck

 

[cvg]

another possibility is that the river cross sections have changed since the FEMA study was done, due to scouring, dredging, channel widening, degradation. Also possibly a bridge was widened downstream which reduces the starting water surface elevation

 

[francesca]

How far out onto the overbanks do your cross-sections go? Are you confident of your Manning's on your overbanks? I expect most of the flow to be on the overbanks in a 100-year event.

Another red flag for me is only 5 x-sections. HEC-RAS needs at least 2-3 additional x-sections to let the WSE calculation settle. The recommendation is to use Normal Depth for the downstream WSE, but as you're concerned about being so much lower than the FEMA study, use the FEMA WSE for your downstream WSE. If it's "wrong" for your geomerty/flow scenario then HEC-RAS will settle to a "right" WSE and you can then question why your WSE is different.

Typically it would be a different flow condition or that your x-sections don't go far enough out/Manning's is underestimated and if your flow condition is different, it doesn't mean that yours is wrong. However, one rain gauge for a stream flowing 4-6 ft deep tells me your one rain gauge is not representative of your entire basin.

 

[francesca]

Oh, and what do the HEC-RAS error/warning messages say? More x-sections? If the WSE drops half a foot or more from one to the next, HEC-RAS wants one in between. You can interpolate x-sections if necessary. Also make sure that you have ineffective flow areas defined if your channel bottom has deep pools.

 

[mbherlihy]

Some really good input...I appreciate all of it. To reply to many of the questions.

Reason I'm doing this: We are filling a little bit in the flood plain. We are attempting to show little to no rise in 100 year wse elevation, thus allowing us to fill in the flood plan. (someone please comment on this is not req'd, etc)

1. Single reach length is apprx 1400 feet. (i have 5 x cross sections)
2. I double checked FIS study flow data with model.
3. I checked that i have no 'crossing cross sections'.
4. I get no error messages.
5. There is not any upstream bridges, weirs, peirs.... there is however, a large lake dam approx 3 miles upstream.
6. There is also a US highway bridge about 2 miles downstream.

Stupid question, but checking datum = ? My vertical topography shots apprear to be relatively close to the elevations found in the FEMA references.

Question, how do I set my water surface elation?....i don't know where this is found.

We have ordererd the FEMA sflood study. (to verify flows, mannings n, see date of study and cross reference to the building of the bridge (downstream) or the dam (upstream).

 

[RWF7437]

"Reason I'm doing this: We are filling a little bit in the flood plain. We are attempting to show little to no rise in 100 year wse elevation, thus allowing us to fill in the flood plan. (someone please comment on this is not req'd, etc)"

If you are only filling in the flood plain fringe you probably do NOT need to do any study. It is only if you are filling in the floodway that you must demonstrate "no rise" in the water surface. Check your local flood plain ordinance or other controlling regulations and talk to the local authorities to be sure you are doing what is necessary.

good luck

 

[mbherlihy]

RWF7437, I've heard of what you are refering to, could you or anyone else tell me where to find this reference to Floodway vs. Floodplain, and the effects, or requirements when you would like to fill in these areas.... what to do, vs. do not need to do, unless local ordinances apply Thanks.

 

[RWF7437]

Here's a link to a file which explains, in part, some of the terms and requirements of FEMA's regulations. I'll send a more detailed example in a separate post if you wish.

We can provide some better answers if we knew some other things about your project:

Where is your project; latitude and longitude?
What flows are you using and where did you get them?
Is there an esrablished Base Flood Elevation ?
Is there a floodway shown in your FEMA Maps ?

good luck

 

[RWF7437]

http://www.fema.gov/plan/prevent/floodplain/nfipkeywords/floodway.shtm#0

 

[RWF7437]

More than you ever wanted to know..........attached.

 

[RWF7437]

http://www.pdhonline.org/courses/h110/h110content.pdf

 

[francesca]

There is also a US highway bridge about 2 miles downstream.

Probably this makes no difference; unless the bottom slope is very flat and the bridge opening is tiny with big fill abutments. There's no harm in contacting the DOT and seeing if they have a flood study on record for the bridge. It would at least give you something else to compare to other than the FEMA study.

 

[mbherlihy]

Where is your project; latitude and longitude?
What flows are you using and where did you get them?
Is there an esrablished Base Flood Elevation ?
Is there a floodway shown in your FEMA Maps ?

West Point, Georgia (Chattahoochee River)
32 deg 52' 28.0714 Lat
85 deg 10' 55.9816 Long

Flows:
100 year = 79,000 cfs (from FIS Flood Study,... table 1 summary of discharges)

Established Base Flood Elevations:
576 (see attached)

No floodway shown, ..... from what i can tell.

By the way, when i run the HEC RAS model, the 100 year flows through my cross sections, I get a flood elevation of about 562....wondering why its not near 576...

 

[RWF7437]

Thank you MBHerlihy for the additional information.

The FIRM tells us that a detailed study of the River has been done by FEMA.

The FIRM says that the Base Flood Elevation near both the North and South Boundaries of your site is 576.0 feet (NGVD).

This suggests to me the you do NOT need to re-do the Flood Study unless you have reason to disagree with this elevation.

OR, you might re-do the study if you wish to or are required to establish a floodway.

Before you do all this work, it would be in your interest to talk to the City of West Point and Troup County and ask them if you can fill in the area your map indicates. If they say yes, your work is done.

If they say no, then you would have to study this reach of the River in detail,using HEC-RAS, to establish the floodway.

Doing such a study would require you to use FEMA's data and to extend your cross sections in the east-west direction AND to add cross sections both upstream and downstream. This would be expensive and time consuming and take 6 months to a year, at least. If you do undertake such a task it would be very helpful to get all 3 of the HEC-RAS Manuals. It would also be helpful to get an example of a similar study. It would also be helpful to get some training in HEC-RAS to get comfortable with the program.

Can your client afford this ?
Is this small fill worth this ?
Can you modify your plan to avoid filling ?


good luck.

 

[RWF7437]

"It would also be helpful to get some training in HEC-RAS to get comfortable with the program."

F.Y.I.,
there is an excellent, but expensive, ASCE course scheduled for Feb. 18-20, 2009 in Atlanta. If you expect to do many such HEC-RAS studies this will be well worth the time and expense because it will save you much work later and give you useful contacts to experts on the program.

Email: seminars@asce.org for more information.

Also note that you need the adjacent FIRM to see ALL of the floodplain. FEMA's practice of cutting mapping off at political boundaries makes no sense but that's the way they do it.

good luck