Stream bed mods to change flood zone designation

[DPAJR]

I am dealing with a ravine that will experience runoff during a 100 year flood.

Can installing a culvert and adding fill over the top be a justification for changing the flood zone designation?

I am interested in hearing from people who have experience with FEMA in cases like this

Thanks

 

[cvg]

you can study any watercourse and request a LOMR which might change the flood zone designation. Are you planning to put a storm drain down the entire ravine? Is the floodplain currently mapped?

http://www.fema.gov/plan/prevent/floodplain/nfipkeywords/lomr.shtm

 

[DPAJR]

Expensive as it sounds a storm drain down the entire ravine is the plan. This is very pricey real estate with a spectacular view and only 18 miles from a ski area.

We already requested an LOMR using a log-Pearson Type III analysis which was based on 52 years worth of daily rainfall data. A spreadsheet of over 300 pages. It was rejected because it was daily rainfall not every 10 or 15 minutes.

So the question is has anyone had any experience getting an LOMR based on proposed stream bed modifications? Or do you have to get the LOMR before you can get permission to do the streambed modifications?

 

[beej67]

francesca is one of the resident rockstars when it comes to flood plain modeling, pay attention to whatever she says.

I've done some LOMR stuff before, where streams were mismapped and we adjusted the maps based on better survey data. It's not extremely difficult, but pleasing your FEMA reviewer can be challenging.

Typically if you're planning on filling the floodplain, you get a CLOMR (conditional letter of map revision) based on your design before you do the construction, then do the construction, then the map gets changed. That varies some depending on whether you're in floodplain or floodway, which zone, and what your local flood plain manager wants you to do.

Curious - If FEMA isn't accepting your historical data, is there any reason you couldn't do a more traditional stochastic hydrology analysis?



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

http://www.campbellcivil.com

 

[cvg]

agree, a CLOMR is the route to take. however, you need to have somebody with prior FEMA experience to do this, it is not a simple task.

 

[froude]

I have done a project similar to yours (in North Texas) where we enclosed long reaches of a stream in a box culvert. The existing stream was a Zone AE with Floodway.

Our case was unique in the sense that portions of the stream were going to remain active and convey local drainage, but the majority of the stream was being put underground.

Because the floodplain and floodway for much of the stream was going to "disappear," in effect, we had to get a CLOMR approved first. As beej67 implied, it took FOREVER to get the reviewer to buy off on the changes.

What is the current zoning of the stream you are working on?

What programs do you plan on using to model the changes?

 

[DPAJR]

Thanks for all the good comments.

I think you are right on the money about the CLOMR-F being the best approach.

My project is similar to froude's except most of the stream will stay above ground and only a small portion will go underground.

You are right about FEMA being hard to convince. We are approching year two of FEMA submittals. Every time we get a rejection we get a whole new set of questions or requirements.

The project area is about 8 acres. The tributary area is over 600. On the previous rejection (two rejections ago) they required us to include the area below the project even though the flow volumes will not change and the lower areas already have modifications that are way more than adequate to handle any flow predicted by the 100 year storm.

It is a floodway that so far has never been known to flood.

I am using HEC-HMS to calculate the runoff and some basic equations for pipe flow to size the culvert.

Thanks
dpajr

 

[froude]

Unfortunately, it seems that different case reviewers at FEMA have different requirements at times.

I ended up with 5 reaches (A,B,C,D,E downstream to upstream, where B and D were open channel sections). To avoid messing with EPA-SWMM, I ended up using a combination of HEC-RAS and StormCAD to create a "continuous" model. I used rating curves to represent the transitions from open channel to box system. Of course, there are so many ways to do this.

The box culvert basically contained the 100-year floodplain. The 0.2% chance floodplain ended up being a sticking point with FEMA. Ultimately, the overland portion of the 500-year storm was mapped using the 500-year flows, less the box capacity. We ended up removing the floodway delineation through our study reach.

Don't know if this helps, but I hope it does!

 

[beej67]

Echo the "forever" statements. My first major experience with FEMA wasn't even a CLOMR, it was a LOMR based on more accurate data, just a simple map revision because the FIRM didn't follow the surveyed creek and we wanted to move it to follow the actual creek and actual topo. The only HEC-RAS change was the removal of a small wooden foot bridge. Took us two years.

Lessons learned:

1) Never ever do a LOMR on a fixed fee.
2) Educate your clients on what you're getting yourself into.

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

http://www.campbellcivil.com

 

[DPAJR]

beej67,

All the world's philosphers could not have come up with better advice than that.

Thanks goodness I didn't do it for a fixed fee.

Thanks,
dpajr

froude,

500 year flood yikes

What set of codes governs cleanouts and other structural details, bedding etc of putting in a culvert? I am guessing AASHTO but that is just a guess. Also was there a specific advantage of a box culvert as opposed to round? Was it just flow size?

Thanks,
dpajr

 

[beej67]

There's typically a break point in cost between box and round, where boxes become the more affordable alternate. (and at higher flows, the only alternate)

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

http://www.campbellcivil.com

 

[froude]

Your state transportation departments design guidelines may be a good start for those details. In our case, a lot of our client cities have their own details for embedment, cleanouts, spacing, etc.

Use of box culverts was dictated by very high flowrates. The lower end of our system was basically a double barrel box that you could probably drive a car through in each box.

 

[DPAJR]

Thanks for all the good answers. Your latest made me realize I should probably size the culvert on this property as if someday the drainage areas above will also have culverts and fill and therefore higher veolcity flows than they do now.

dpajr