ST0301C-CS120831
Rhonda Adloo
From:Caleb Thornhill <bct@freese.com>
Sent:Friday, August 31, 2012 2:09 PM
To:Keith Marvin
Cc:Thomas Caffarel; Mike Wayts; Tricia Hatley; CPL03243
Subject:Bethel Road - Floodplain analysis
Attachments:TribG-1GrapevineCreekFloodplains_withXS.PDF
Keith,
The attached floodplain map is in response to our last conversation. I’ve had the sections added as you indicated. As far
as your other questions, here is a brief response to each:
1.How much detainage is needed upstream to make this area of the channel work?
a.The existing gabion channel north of Bethel Road has a capacity of 500 CFS based on cross
sections. 1700 CFS of upstream detainage is required to keep the flows in the existing gabion channel
with no freeboard.
2.What would be the detainage policy/procedures if DFW detains on their property?
a.If DFW does detain, typically, DFW would have to detain to the City of Coppell’s drainage
standards. However, this would require coordination and approval from DFW.
3.How far downstream are improvements required to make this section of the channel work?
a.We modeled channel improvements downstream of Coppell (500’ D/S) to the extents of our model. It
was not enough to get the flows back in the channel. 1100’ D/S of Coppell you get into residential area
confining the channel. Based on the flows and limited channel section in these areas, the flows will not
be contained within the channel. If downstream improvements were used in combination with
detention, it may be a possible solution. However, it would require extensive modeling effort and a cost
benefit analysis to determine an alternative that works best for the City.
4.What can be done to make land adjacent developable?
a.Based on your current drainage design standards, all structures must be one foot above BFE. However,
no improvements will be allowed that adversely impact the WSEL’s. Any fill brought in to raise the
properties will adversely impact surrounding properties and would not be allowed. A combination of
upstream detention noted above and possible channel improvements would likely be required to
reclaim this property without causing negative impacts on their neighbors. This would be a project by
itself to model and develop the proposed improvement alternative.
5.Provide map with sections turned on.
a.I’ve attached the revised map.
If you need additional information or have any questions, please let me know. We can come by to visit more on the
channel concerns if you’d like. Just let me know how you would like to proceed.
B. Caleb Thornhill P.E.
Project Manager
Freese and Nichols, Inc.
Main: 214-217-2200
Direct: 214-217-2282
Fax: 214-920-2565
Email: bct@freese.com
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