FS8601-CS 870921 1
mr-
1177"7--
2 31987
P AWA •Wnkelmann
& Assoc iates, Inc.
September 21, 1987
1
Ms. Maryetta Cunningham
Natural Hazards Program Specialist
Federal Emergency Management Agency - Region VI
Federal Center
800 North Loop 288
Denton, Texas 76201 -3698
1 re: Improvements to Andrew Brown Community Park
Denton Creek Floodplain - City of Coppell, Texas
Dear Ms. Cunningham:
The City of Coppell desires to make improvements to its existing Andrew
Brown Community Park, which is located in the common floodplain of Denton
Creek and Cottonwood Branch, just west of Denton Tap Road. Improvements
will involve minor cut and fill operations (within 3 ft above or below
existing grade) for proposed playing fields. Concession buildings may also
be placed, which will be floodproofed or considered as enclosures.
The proposed actions are partially within the designated regulatory
floodway of Denton Creek. However, the impacts of the proposed project
have been analyzed and found to have no effect upon 100 -yr water surface
elevations or floodway widths. The following sections briefly describe the
1 proposed project and its ramifications. Maps of the vicinity and the
specific site details are also provided for clarification.
1
BACKGROUND & STUDY APPROACH
The existing conditions HEC -2 encroachment model for Denton Creek is based
on the currently effective Flood Insurance Study (FIS) computer model.
Field surveyed cross sections have been added, based on data supplied by
Ginn, Inc. Section extensions were supplemented by aerial topographic
information with an original scale of 1 in = 200 ft, and are plotted in
Appendix 3. The proposed park improvements are located in the right
overbank area of Denton Creek, from stream station 327 +25 to stream station
I 342+60. In the existing conditions scenario, the Manning's "n" value
assigned for the right overbank through the project reach length was 0.055.
12660 COIT ROAD, SUITE 200 • DALLAS, TEXAS 75251 •214/661 -9406
Ms. Cunningham, 9/21/87, Page 2 of 3
1
The existing conditions HEC -2 encroachment model of Appendix 1 is based on
the principle of equal conveyance reduction in both overbanks, based upon
the Method 1 and Method 4 options of the HEC -2 model. The results indicate
floodway widths through the project site ranging from 110 ft at stream
station 327 +25 to 968 ft at stream station 342+60. It should be noted from
this computer model that the 100 -yr discharges vary between the natural
profile and the encroached profile. This is due to the common floodplain
with Cottonwood Branch that occurs in the natural
profile for
In the encroached profile representation, the floodway Denton
termination
U procedure of the HEC -2 model is based on the placement of vertical walls on
each side of the channel such as to optimize the target rise in water
surface elevation. Thus, the channel sections of the two streams must be
I separated, and the discharge conveyed by each determined. This has been
accomplished by way of a split flow analysis such that the energy grade
line elevations of each stream are approximately equal, with their
combined flow being equal to the total base flood discharge.
1 1 EVALUATION
The proposed conditions model for Denton Creek is included as Appendix 2 of
this report. The geometry of the sections has been modified to account for
the proposed park improvements in the right overbank of Denton Creek. These
modifications are shown on the cross sections plotted in Appendix 3. The
Manning's "n" value has been lowered from 0.055 to 0.050 to account for the
more manicured condition of the playing fields in the right overbank. As
with the existing conditions model, the discharges vary between the natural
and encroached profiles due to the functioning of the split flow action
between Denton Creek and Cottonwood Branch.
Comparison of the floodway widths and encroached water surface elevations
presented in Appendices 1 and 2 indicate no adverse impacts of the proposed
park improvements. Floodway widths in the proposed conditions model are
less than or equal to those of the existing conditions model. The
encroached base flood water surface elevations also compare in a similar
fashion.
1
1
1
1
1
Ms. Cunningham, 9/21/87, Page 3 of 3
CONCLUSIONS & RECOMMENDATIONS
1 The proposed improvements to the Andrew Brown Community Park within lthin the
City of Coppell, and
partially within the regulatory floodway of Denton
Creek, present no adverse impacts upon the floodway conditions.
Regulations of the 44 CFR Parts 59, et.seq, as administered by the FEMA
state that, "Once the floodway has been adopted, encroachments, including
fill, new construction, substantial improvements, and other development
within the adopted floodway that would result in any increase in flood
levels within the community during the occurrence of the base flood
discharge is prohibited." Conditions and Criteria for Floodway Revisions,
(FEMA, August, 1984). Documentation is hereby supplied illustrating that
no increase in flood levels during the occurrence of the base flood
discharges will result from implementation of the proposed improvements.
To the best of our knowledge, the attached data should suffice for FEMA
review of the proposed project. If additional data are required, or if you
have any questions, please advise.
Sincerely,
PAWA- Winkelmann & Associates, Inc.
c
c, N. , _
Carl V. Anderson, PE
Vice President
attachments: Appendix 1 - extended FIS HEC -2 encroachment model
Appendix 2 - proposed conditions encroachment model
Appendix 3 - cross sections
Appendix 4 - maps of project area
enclosure: grading plan
cc: John Karlsruher, PE - City Engineer with Ginn, Inc.
Shohre Daneshmand - City of Coppell
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LEGEND
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/ FLOODPLAIN MANAGEMEM
/........../-"■-
• _..._ / 1„, ,/. Existing
— - .........---K og
Zwilirr!;;ZZ,N0.. Date : June I 9 87
TEXAS _ Approved by : •et
Proposed ----'------....–
A.... ANDERSON ENGINEERS INC.
Engineering Hydrologists
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