Alexander Ct-CS080909
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Ken Griffin - RE: Floodplain
From: "Dan Buhman" <dan.buhman@buhwater.com>
To:
"'Ken Griffin'" <kgriffin@ci.coppell.tx.us>
Date:
9/9/2008 2:08 PM
Subject:
RE: Floodplain
Ken,
Just wanted to follow up on the clarification below and see if you had any questions or wanted to discuss
further?
Thanks,
Dan Buhman, P.E.
Buhman Associates, LLC
Mobile: 214-864-1555
dan.buhman@buhwater.com
From: Dan Buhman [mailto:dan.buhman@buhwater.com]
Sent: Tuesday, September 02, 2008 12:07 PM
To: 'Ken Griffin'
Subject: RE: Floodplain
Ken,
Sorry I wasn’t clear in the letter. Let me try to clarify here.
The ‘n’ value in the original flood study was 0.08, so the fence will not cause any adverse impact because it will
not raise the ‘n’ value above what it is already. When I modeled the n=0.09 versus n=0.08, I was trying to do a
sensitivity analysis to raising the ‘n’ value; it showed very little (0.01) to 0.00 change.
Because there is no change to the ‘n’ value (because of the heavy riparian vegetation on the bank), I felt that
adding cross sections would not be beneficial and would not produce any additional accuracy to the study. I
completely agree with your requirements when the ‘n’ value is increased but felt it was not needed here
because of the existing conditions.
Let me know your thoughts and I will modify the letter to clarify.
Thanks,
Dan Buhman, P.E.
file://C:\Documents and Settings\radloo\Local Settings\Temp\XPgrpwise\48C68344City_o...2/22/2010
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Buhman Associates, LLC
Mobile: 214-864-1555
dan.buhman@buhwater.com
From: Ken Griffin [mailto:kgriffin@ci.coppell.tx.us]
Sent: Tuesday, September 02, 2008 11:26 AM
To: Dan Buhman
Subject: RE: Floodplain
Dan
Not sure I followed what you proposed. It looks like you are comparing the fence encroachment to a
run that changes the 'n' value from .08 to .09. The comparison I'm looking for is comparing the fence
encroachment to the existing study, using whatever 'n' value that was used in the existing study. If the
existing study used .035, then you should compare that to the fence encroachment 'n' of .08.
In the other situations that I have allowed this approach, I have required that the original flood study be
used as a base line. Then, the consultants have added new cross-sections to accurately depict the
existing field conditions in the area to be fenced. Then, a new study was run to compare the existing
study to the new study, which includes new cross-sections and a new 'n' value for their side of the creek
of .08. The information submitted to my office includes the original study, the revised study and a
table comparing the differences throughout the entire reach of the original study.
Based on what you have submitted, I can not approve any encroachment into the floodplain.
Ken,
Thanks for your consideration of this request for a waiver for construction of a security fence within the
Grapevine Creek floodplain. Please see the attached PDF file containing the letter report and appendices.
If you have any questions, please feel free to contact me any time.
Thanks,
Dan Buhman, P.E.
Buhman Associates, LLC
Mobile: 214-864-1555
dan.buhman@buhwater.com
From: Ken Griffin [mailto:kgriffin@ci.coppell.tx.us]
Sent: Wednesday, August 13, 2008 8:31 AM
To: Dan Buhman
Subject: Re: Floodplain
file://C:\Documents and Settings\radloo\Local Settings\Temp\XPgrpwise\48C68344City_o...2/22/2010
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The "n" value that has been used for studies to determine the impact of wrought iron fences in the
floodplain is 0.08, in lieu of 0.035.
Ken Griffin, P. E.
Director of Engineering & Public Works
972-304-3686
kgriffin@ci.coppell.tx.us
Ken Griffin, P. E.
Director of Engineering & Public Works
972-304-3680
kgriffin@ci.coppell.tx.us
>>> "Dan Buhman" <dan.buhman@buhwater.com> 8/12/2008 5:31 PM >>>
Ken,
We spoke a few weeks ago about some landowners that want to build a fence in the floodplain at the
NW corner of Denton Tap and Bethel. I have received the model from NathanMaierand am readyto
start looking at the impact of the fence. You had said that you could send me the Manning's'n'
values that you recommend to show that impact. Can you send me your criteria? I'll put it in the
model and send you a short memo summarizing the findings.
Thanks,
Dan Buhman, P.E.
Buhman Associates, LLC
211 Cherrybark Drive
Coppell, TX 75019
Mobile: 214-864-1555
dan.buhman@buhwater.com
file://C:\Documents and Settings\radloo\Local Settings\Temp\XPgrpwise\48C68344City_o...2/22/2010