Cottonwood Est-CS191218O’BrienEngineering, Inc.
2340 E. Trinity Mills, Ste 220, Carrollton TX 75006
972.233.2288 Ph ⋅ 972.233.2818 Fx
obrieneng.com
Texas Firm ID # F-3758
SDVOSB | Texas HUB | TxDOT SBE Firm | Since 1987
December 18, 2019
Mr. Kumar Gali, P.E., CFM, D.WRE
Assistant Director of Public Works
City of Coppell
255 Parkway
Coppell, TX 75019
RE: Comments for 125 Cottonwood Drive Lot Improvements, Coppell, Texas
OEI Job number: 347.003
Dear Mr. Gali:
O’Brien Engineering, Inc. is providing hydraulic analysis on behalf of the Holmes Builders for the new
home construction and reshaping of the existing pond at 125 Cottonwood Drive in Coppell, TX. Kimley
Horn provided additional comments on behalf of the City of Coppell, dated November 25, 2019. The
review comments are summarized below in bolded italics and our responses are noted in plain regular
font.
1) The previous comment letter requested the pre-project and post-project HEC-RAS models
to be prepared using 100-year ultimate condition flows from the City’s Storm Water
Management Study by Halff Associates, Inc. dated 1991 (1991 Study). Flows in the
provided HEC-RAS model are inconsistent with the 1991 Study. Per the Cottonwood
Branch Plate 5- 14 in Appendix F of the 1991 Study, we assume the flows should be
consistent with what is shown below. If you believe otherwise, provide justification.
Once the HEC-RAS models are revised, confirm the proposed improvements adhere to the
City of Coppell Floodplain Management Ordinance.
The 50% AC flows have been revised; however, upon review of the HEC-2 results Master
Drainage Study provided to OEI via email on 10/15/2019
(http://weblink.coppelltx.gov/WebLink8/DocView.aspx?id=158093&dbid=1), the flow change
locations listed in the table above are not necessarily representative of the model data provided
for Cottonwood Branch. The image below is extracted from the 1989 Master Drainage Study for
City of Coppell, which OEI received in response to our data request. Also included with this
response is an image of the workmap for Cottonwood Branch included in that study. Above the
cross section located at River Station 8000, all the flows are the same. The first flow change
occurs at River Station 8000. A second flow change occurs at River Station 6710 and that flow
volume continues for the remainder of the creek. According to this information, the OEI model
reflects the correct flow change locations and for the 100-year ultimate flows.
Mr. Kumar Gali, P.E., CFM, D.WRE
December 18, 2019
Page 2 of 2
2) There is a Manning’s “n” value of 0.65 on the left overbank of cross section 8793, which
is incorrect. Revise the HEC-RAS modeling to remove this error.
The 0.65 n-value was revised to 0.065 in the LOB of the pre-project geometry. The n-value of the
ROB in both the pre- and post-project geometry was updated to rebalance the continuity of flow
in the ROB. Report tables have been updated and valley storage loss for both FEMA and City
SMS 100-year flows is reduced to 0.1% or less. The revised HEC-RAS model has been included
with this submittal as well as revised tables.
Sincerely,
O’BRIEN ENGINEERING, INC.
Stephen Bolster, P.E., MS, CFM
Water Resources Task Manager
Attachments
HEC-RAS Model
Tables
Drainage Study Workmap
xc: Mr. Kent Collins, Director of Public Works
Mr. Terry Holmes, The Holmes Builders
Post-Project WSEL -
Pre-Project WSEL
Post-Project Channel
Vel - Pre-Project
Channel Vel
1% AC
WSEL
Channel
Velocity
1% AC
WSEL
Channel
Velocity
1% AC
WSEL
Channel
Velocity
1% AC
WSEL Channel Velocity
Units ft fps ft fps ft fps ft
fps
Column Number ID (1) (2) (3) (4) (5) (6)
(5)-(3) (6)-(4)
10130 482.35 7.51 482.35 7.51 482.35 7.51 0.00 0.00
10120 482.30 7.60 482.30 7.60 482.30 7.60 0.00 0.00
9890 479.99 11.17 479.99 11.17 479.99 11.17 0.00 0.00
Sandy Lake Road (U/S)9590 478.16 9.32 478.85 8.42 478.84 8.42 -0.01 0.00
Sandy Lake Road (D/S)9475 478.01 7.19 478.24 6.95 478.24 6.96 0.00 0.01
9233 477.45 6.44 477.83 5.73 477.82 5.74 -0.01 0.01
Immediately U/S of Pond 8863 - - 475.84 5.72 475.67 5.98 -0.17 0.26
Inside U/S edge of Pond 8836 - - 475.66 6.69 475.54 6.24 -0.12 -0.45
Inside Pond U/S of Transition 8813 - - 475.45 6.42 475.38 5.71 -0.07 -0.71
Inside Pond D/S of Transition 8793 - - 475.38 5.56 475.32 4.83 -0.06 -0.73
Inside D/S edge of Pond 8675 - - 474.77 5.50 474.77 5.50 0.00 0.00
Immediately D/S of Pond 8640 - - 474.56 5.75 474.56 5.75 0.00 0.00
8000 473.27 5.11 473.29 5.09 473.29 5.09 0.00 0.00
6710 470.13 4.87 470.13 4.87 470.13 4.87 0.00 0.00
Table 1: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch FEMA Flows
Description
Cross
Section
Number
Effective Pre-Project Post-Project
Post-Project WSEL -
Pre-Project WSEL
Post-Project Channel
Vel - Pre-Project
Channel Vel
1% AC
WSEL
Channel
Velocity
1% AC
WSEL
Channel
Velocity
1% AC
WSEL
Channel
Velocity
1% AC
WSEL Channel Velocity
Units ft fps ft fps ft fps ft
fps
Column Number ID (1) (2) (3) (4) (5) (6)
(5)-(3) (6)-(4)
10130 482.77 8.45 482.74 8.51 482.74 8.51 0.00 0.00
10120 482.71 8.58 482.67 8.64 482.67 8.64 0.00 0.00
9890 480.93 10.38 481.21 9.61 481.21 9.61 0.00 0.00
Sandy Lake Road (U/S)9590 478.68 10.61 479.59 9.39 479.59 9.39 0.00 0.00
Sandy Lake Road (D/S)9475 478.51 8.25 478.73 8.00 478.73 8.00 0.00 0.00
9233 477.95 6.81 478.31 6.16 478.31 6.17 0.00 0.01
Immediately U/S of Pond 8863 - - 476.24 6.06 476.09 6.31 -0.15 0.25
Inside U/S edge of Pond 8836 - - 476.07 7.16 475.96 6.73 -0.11 -0.43
Inside Pond U/S of Transition 8813 - - 475.85 6.98 475.78 6.32 -0.07 -0.66
Inside Pond D/S of Transition 8793 - - 475.78 5.91 475.73 5.22 -0.05 -0.69
Inside D/S edge of Pond 8675 - - 475.16 5.84 475.16 5.84 0.00 0.00
Immediately D/S of Pond 8640 - - 474.95 6.18 474.95 6.17 0.00 -0.01
8000 473.63 5.41 473.65 5.38 473.65 5.38 0.00 0.00
6710 470.45 5.01 470.45 5.01 470.45 5.01 0.00 0.00
Table 2: Comparison of 1% AC Water Surface Elevations and Channel Velocities on Cottonwood Branch Ultimate SMS Flows
Description
Cross
Section
Number
Effective Pre-Project Post-Project