20170311 Ver 1_Re splitting the base flow of a stream_20140821

Wanucha, Dave
From:Buncick, Marella <[e-mail]>
Sent:Thursday, August 21, 2014 3:45 PM
To:Chambers, Marla J
Cc:Van Der Wiele, Cynthia ([e-mail]); Johnson, Alan; [e-mail]
Subject:Re: splitting the base flow of a stream
Hi Marla,

I can't remember the specific crossing on I-3802 but I always think it's a bad idea to split the stream, mostly because of
the potential for damming with large woody debris at the culvert entrance and the decreased hydraulic
conveyance/increased velocity at the outlet during higher flows. I think that even when the modeling tells the
hydrologist there is enough total area to pass a stream at some discharge that is only 1/2 the equation. From the stream's
perspective---habitat, stability, function--- there are still concerns with how that stream and the culvert interact in
funneling the flow and increasing velocities at the 2 or 3 outlet points. I don't know that this is demonstrable even with
monitoring except maybe over long perionds of time but I also don't know that anyone has looked. There is reluctance
to replace culverts rather than extend them so I imagine there are cases we just have to live with but I would strongly
advocate for not intentionally doing more of this with new culverts.

We've had conversations in the past about designing different size barrels in the same culvert to address this concern
and I guess at some sizes or depths of fill it becomes structurally impossible. I would be interested to know if there are
studies or known thresholds and if there are examples.

my nickle...marella

On Thu, Aug 21, 2014 at 11:37 AM, Chambers, Marla J <[e-mail]> wrote:
I want to make sure I’m not off-base or overly concerned about this and that we are on the same page. At a
concurrence meeting last week, I 3802A, we discussed a 3-barrel culvert that was using 2 barrels for the base flow,
effectively splitting the base flow of the stream. I argued against such practice, as it does not attempt to carry the
stream’s natural dimension, pattern and profile through the culvert, disrupts the natural flow pattern and can lead to
destabilized stream channel up or downstream of the culvert. I’d say it’s also hard to predict what will happen during
drought conditions – will the stream continue to use two barrels fairly equally, making aquatic life passage more
unlikely, or will the reduced flow concentrate mainly to one barrel? No way to tell.
I’m now looking at a bridge replacement project in Catawba County (B-5150) that is proposing the same – putting a
20’ wide stream in a 3-barrel culvert, each barrel 10’ wide, and the stream will use 2 barrels for base flow. (See
https://connect.ncdot.gov/resources/Environmental/ link to the application, if you want more details.) According to the
CE it has a drainage area of 4.2 sq. mi., but the bridge to culvert check list I was sent says 3.64 sq. mi.. I had thought
that NCDOT had gotten away from that practice, for the most part, especially after all the Rosgen training a few years
back. I expressed my concern for splitting the stream on this project and am in communication with Carla Dagnino
about it and the practice in general. I’ll be sending her my concerns and possible alternatives for her to discuss with
the hydraulics folks, which may or may not have an influence on other projects.
To me, this is not avoidance or minimization. We try hard to keep bridge bents out of the stream flow to maintain
proper natural flow and reduce debris jams, etc. We often see how stream channels are negatively affected from mid-
channel structures, such as islands forming and banks eroding due to the altered flow around the island, etc.
Please let me know your thoughts on this. Thanks,
Marla
1

Marla J. Chambers
Western NCDOT Coordinator
North Carolina Wildlife Resources Commission
206 Charter Street
Albemarle, NC 28001
Office: 704-982-9181
Work cell: 704-984-1070
[e-mail]
www.ncwildlife.org

Get NC Wildlife Update -- news including season dates, bag limits, legislative updates and more -- delivered to your
Inbox from the N.C. Wildlife Resources Commission.
_______________________________________________________________________________
Email correspondence to and from this sender is subject to the N.C. Public Records Law and may be disclosed to third
parties.

--
Marella Buncick
USFWS
160 Zillicoa St
Asheville, NC 28801
[phone] ext 237
fax [phone]
2