First Time HEC-RAS User - Please HELP! 1

[cfj104]

Hello,

I am attempting to model the limits of the 100-year floodplain for a creek site I am working on using the HEC-RAS software. I have very limited surface water hydrology experience so any help is appreciated. If there is another software tool that I should use, please let me know.

I was wondering what design information do I need before starting this program. I only need to identify the limits in a small section of the creek (fortunately there are no flow restrictions in the area of concern).

I have already surveyed the creek and 7 cross sections along the area of interest. Velocity readings and creek depth measurements have been collected on a bi-weekly basis for about 3 months now. Is there more information needed?

According to the FEMA maps, this is a Zone A area with no BFE given. There is also no USGS monitoring gauge for this stream.

Thank You!

 

[RWF7437]

First, get copies of the published Flood Plain maps. These should be available at City Hall or the County Engineers Office. Also, they can be downloaded from FEMA's website ( FEMA.gov). They will show you what the Flood Plain is and where.

If all you want to do is map the Flood Plain you are done.

If you want to establish the Base Flood Elevation (BFE) You will also need a copy of the Flood Study Report. Also available locally. You need to estimate the 10, 50, 100 and 500 year flows based on the the same methods used to create the Flood Study. Then using your estimated flow, setup a steady state model in HEC-RAS. Your flow measurements are NOT the flows you need but they may allow you to estimate the Manning's "n" values to use in your model. You will probably find the examples in the HEC-RAS documentation and in the Hydraulic Reference Manual ( not the User's Manual) to be the most helpful.

good luck

 

[francesca]

Unfortunately your seven cross-sections won't come close to being enough, but fortunately if you have a good survey of the area you can extract your cross-sections off that because 100-year flows extend a long way beyond the banks and most of the flow is on the overbanks.

Your depth calculation performed by HEC-RAS takes a few cross-sections to settle from the assumed starting depth downstream (usually normal depth), so you will need to extend your model safely beyond your project limits. When it comes to cross-sections it's a case of "the more the merrier." Not only do you need cross-sections at every change in channel geometry (widening, narrowing, branching, etc.) but you also need them close enough to keep the change in water elevation small. You also need to extend the cross-sections further up the overbank than you can possibly imagine necessary because it's a huge PITA to go back and extend them after entering them into HEC-RAS.

You need a sound method of calculating your 100-year flow (regression equations on a large basin, SCS CN/HEC-1 on a smaller basin) and need to consider the worst-case roughness condition on the overbanks. (More foliage equals rougher equals slower flow equals smaller velocity head equals higher water surface elevation.)

The Hydraulic Reference Manual is your friend in this. Good luck!

 

[RWF7437]

"I have already surveyed the creek and 7 cross sections along the area of interest."

As Francesca points out, this may not be enough X-secs. Area of interest extends both above and below the stream section your client "owns". And the flood plain extents, if known, may be quite a bit wider than you imagine. This all depends on the actual facts of the case, as Perry Mason would say, and requires some judgment and experimentation on your part. If there are experienced HEC-RAS users in your area, seek them out for advice and guidance.

Although the stream may not be gaged, there may be nearby, similar streams which are and will help you get a better estimate of probably flows.

good luck

 

[cfj104]

Thank you for all the input. I kinda figured this was not going to be an easy task! I have sent out a company wide e-mail and have yet to find anyone with direct experience within my company...

Fortunately, the creek is surrounded by steep slopes and I know that the water has never crested the northern bank (based on an employee who has been at the plant for 30+ years) so I have an idea where the model should end up.

My first try was to overlay the FEMA map on my drawing, unfortunately the creek does not match up and the flood elevations on each side of the creek vary by as much as 15 feet.

Our survey of the area was done to 2' contour intervals, so I am able to get a detailed view of the site and stream.

I will look into getting any flood studies that may have been done locally as well.

Thank you!

 

[RWF7437]

"Fortunately, the creek is surrounded by steep slopes and I know that the water has never crested the northern bank (based on an employee who has been at the plant for 30+ years) so I have an idea where the model should end up."

Good clue. It will help you in your analysis BUT, FEMA will not accept anecdotal evidence alone. Your 30 year employee's story is much less useful than is the fact that the slopes are steep.

"My first try was to overlay the FEMA map on my drawing, unfortunately the creek does not match up and the flood elevations on each side of the creek vary by as much as 15 feet."

This not at all unusual. Many flood maps are drawn at a scale of 1"=2000' and can be overlaid on USGS Quadrangle maps. Some of these have contour intervals of 40' and even 80' on rural areas. Do the best you can with the overlay and remember that some things, like the creek itself, may have changed since the flood map was drawn while other things, like section corners, road intersections, ridge lines, etc. have likely NOT changed.

Also, don't overlook the obvious. Go to the site and look for evidence of past flooding. High water marks, evidence of erosion, changes in vegetation, stains on building and bridges, exposed rock, water worn rock, abrupt changes in slope, fallen trees undercut by fast moving water are just a few.

Don't wear tassel loafers and good luck

 

[queque]

FEMA has the HEC-RAS input and output files available for you to use. You can request them from the Regional FEMA office. You are probably in an area without detailed mapping. The FEMA study that developed the inflows for the HEC-RAS model is also available. Usually, it is a HEC-1 data set, also available.
Your floodplain administrator has a copy of the HEC-1 data and HEC-RAS data.
The HEC-1 data provides the basin characteristics used to calculate the 100 year discharge for a portion of the area. Take the discharge, divide it by the corresponding area, and that provides a guide (ratio Q/A)
to estimate the discharge for your area of interest. Calculate the drainage area, multiply it by the ratio, and that provides a quick estimate of the 100 year Q.
This is a basic crude method to provide the delineation boundaries of your stream.
Then refine the discharge with actual Hydrologic parameters.

 

[cfj104]

I have a question for RWF7437, you said that the flow rates I have collected are not the ones I am supposed to use.

Are the numbers I collected valid for anything? and What are the correct flow values or how do I find them?

Thank You.

 

[RWF7437]

If you want to establish the Base Flood Elevation (BFE) You will also need a copy of the Flood Study Report. Also available locally. You need to estimate the 10, 50, 100 and 500 year flows based on the the same methods used to create the Flood Study. Then using your estimated flow, setup a steady state model in HEC-RAS. Your flow measurements are NOT the flows you need but they may allow you to estimate the Manning's "n" values to use in your model. You will probably find the examples in the HEC-RAS documentation and in the Hydraulic Reference Manual ( not the User's Manual) to be the most helpful.

The Flood Study will tell you what flows were used on previous studies of your stream or on nearby, similar streams. They are usually based on regional regression equations but you must get the Flood Study and see what they atr in your case.

 

[cfj104]

Great, thank you for the clarification.

I have been trying to get the flood study report so hopefully I can get that and have some good starting information. I also made a call to the DOT since they are going to be replacing a bridge over the creek and they are going to get me their information. Hopefully I can get this information and it won't be too painful!

Thanks for all the advice and info!

 

[RWF7437]

cfj104,

You got lucky ! Now, make friends with the people at DOT. They may do your job for you if you treat them right. Even if they don't, they can teach you how to do your job for your section ( reach) of the creek.

good luck