Blackberry Farm-SY130404O'Brien ninerin, Inc.
Hydraulics Hydrnlogy Civil Engineering
April 4, 2013
Mr. Ken Griffin, PE
Director of Engineering and Public Works
City of Coppell
255 Parkway Blvd.
P.O. Box 9478
Coppell, TX 75019
RE: Hydraulic Analysis of Denton Creek for The Holmes Builders, City of Coppell,
Dallas County, TX
OEI Job # 347.002
Dear Ken:
This letter includes the hydraulic evaluations for Denton Creek in the vicinity of the
proposed Denton Creek Estates Development. The Holmes Builders (Owner) proposes
to subdivide a 54.6 -acre tract of land in Coppell and Carrollton, Texas and develop as
single - family residential lots. The subject property is located north of Sandy Lake Road,
west of the Elm Fork of the Trinity River (Elm Fork), and approximately 2000 feet east of
MacArthur Boulevard. Denton Creek flows through the subject tract and drains into Elm
Fork approximately 5,400 feet downstream.
Objective
The primary objective of the study is to evaluate the floodplain hydraulics of Denton
Creek in the vicinity of the project site for the purpose of determining a layout for the
proposed development, which will not result in an adverse condition in the floodplain.
Reports summarizing the analysis of the impacts of the proposed development to the
Elm Fork floodplain are provided in separate studies developed for requesting a Federal
Emergency Management Agency (FEMA) Letter of Map Change (LOMC) and for
requesting a Corridor Development Certificate (CDC) within the North Central Texas
Trinity River Corridor.
Hydrologic Analysis
No Hydrology was developed for this project. The hydraulic model for this project
evaluation uses the peak flood flow rates based on ultimate development conditions
obtained from the City -Wide Storm Water Management Study (SWMS) by Half
Associates, Inc. (January 1991). Table 1 provides a comparison between the ultimate
development and FEMA Flood Insurance Study (FIS) peak flood flow rates.
14900 Landmark Blvd., Ste 530, Dallas. Texas 75254
972.233.2288 Ph 972.233.2818 Fx
OBrienEng.com
Texas Firm ID # F -3758
VOSB
Mr. Ken Griffin, PE
4/4/2013
Page 2of5
Table 1
Comparison of Denton Creek
100 -Year Peak Flood Flow Rates
1991 Coppell City -Wide
Drainage
1984 Flood
Storm Water
Area
Insurance Study
Management Study
Location
(sq -mile)
(cfs)
(cfs)
At confluence with Elm Fork
23.88
17,800
19,100
Below Cottonwood Branch
20.84
21,300
22,468
Above Cottonwood Branch
12.10
19,400
16,681
Hydraulic Analysis
The hydraulic model for Denton Creek, referenced in the SWMS, was developed by
Kimley -Horn and Associates (KHA) in HEC -2 format. The model uses flood flow rates
developed in the KHA study and listed in Table 1 above. The downstream -most cross
section is located approximately 18,000 feet upstream of the confluence of Denton
Creek with Elm Fork. This 18,000 -foot reach was not modeled in either the FIS or the
SWMS and is located within the Elm Fork floodplain: The subject property is located
within this reach between river stations 5,300 and 9,000. Based on the FIS flood profile
of;Denton Creek, the Elm Fork flood profile is more dominant than the Denton Creek
flood profile through this 18,000 -foot reach (see Table 3 below). For this study, a
hydraulic model of Denton Creek was developed through the subject tract to determine
channel velocities and whether or not the proposed development would cause an
adverse condition within the floodplain.
The attached Exhibit 1, Cross Section Location and Floodplain Map, indicates the
location of the Denton Creek model cross sections. . Topography indicates that
floodplain flow in the left overbank (east side of the Denton Creek Channel) between
sections 12834 and 11070 would be diverted towards the east away from the subject
tract. A lateral weir was used in the model to estimate the amount of flow that splits
from Denton Creek. Flow optimization was selected in the model with the assumption
that any flow leaving the channel through the lateral weir would leave the system and
contribute to the Elm Fork flooding to the east. The estimated remaining ultimate
conditions channel flow through the subject tract in Denton Creek would be
approximately 3,630 cfs. A second lateral weir was used to simulate a similar condition
between sections 7617 and 6468. Below section 6468 the remaining flow in Denton
Creek determined using the HEC -RAS flow optimization would be approximately 3,290
cfs.
The floodplain and channel topography for the subject area were obtained from on -the-
ground field data surveyed by Kadleck & Associates mapped to one -foot contour
interval topography. The topography for limited portions of the floodplain outside of the
SAOEI Current Jobs \347.002 Denton Creek Estates - Coppell -The Holmes Builders \Reports \City Letter Report \4 -5 ?013
Submittal \347.002 Denton Creek Letter Report -rev 2013- 04- 01.doc
Mr. Ken Griffin, PE
4/4/2013
Page 3 of 5
project site was augmented with the North Central Texas Council of Governments
(NCTCOG) 2007 aerial photography mapped to 2 -foot contour intervals. The
topography for portions of the Denton Creek channel was augmented with topography
developed for the Trinity River Corridor Mapping from a 1991 aerial survey mapped to
2 -foot contour intervals. The hydraulic analysis was conducted using the U.S. Army
Corps of Engineers (USAGE) water surface profiles computer program, HEC -RAS
(version 4.1). Channel and overbank reach lengths were digitally measured using
ESRI's ArcView software. Manning's roughness coefficients were verified through field
reconnaissance with reference to Guide for Selecting Manning's Roughness
Coefficients for Natural Channels and Flood Plains Water - Supply Paper 2339, by the
United States Geological Survey, 1989 and, Open - Channel Hydraulics by Ven Te
Chow.
The Pre - Project model was used as a platform to develop the Post - Project model. The
Post - Project model of Denton Creek represents floodplain modifications, which would
result from the proposed development, which includes placement of fill in the right
overbank due to the proposed development. The proposed floodplain modifications
begin near cross section number 9141 and continue through cross section 5599.
Floodplain modifications include floodplain fill to elevate the residential building pads
above the base flood elevations according to City of Coppell ordinance and excavated
basins to compensate for loss in valley storage to satisfy the Trinity River Corridor
Development regional criteria.
Sub - divided lots are proposed on both sides of Denton Creek. Access to the eastern
lots is proposed by constructing a bridge over Denton Creek as indicated on the
attached grading and bridge plans. The bridge will be 36 -foot wide with a span of 66-
feet with the low chord of the bridge beams elevated 2 -feet above the ultimate 1 % AC
floodplain elevation per city ordinance. The channel beneath the bridge (Exhibit 3 —
Cross Sections Plotted Upstream and Downstream Face of Bridge) would transition to
the natural section within about 25 of the upstream and downstream faces of the bridge.
Results and Conclusions
Our evaluation indicates that the proposed project would not cause an increase in the
Denton Creek ultimate 1 % AC flood water surface elevations or in the FIS 1 % AC flood
water surface elevation, nor would it create or increase erosive velocities. The ultimate
1% AC flood channel velocities on Denton Creek would be a maximum of 2.9 feet per
second (fps) through the subject tract.
A summary tabulation .of Ultimate 1% AC flood water surface elevations and velocities
for various conditions on Denton Creek is provided in Table 3. The workmap map with
cross section locations is included as Exhibit 1 and the Proposed Grading Plans are
included as Exhibit 2. Digital copies of the hydraulic models are included on a CD
(attached).
SA0Ft Current Jobs \347.002 Denton Creek Estates- Coppell -The Holmes Builders \Reports \City Letter Report \4 -5 ?013
Subnutis1\347.002 Denton Creek Letter Report -rev 2013 -04- 01.doc
Mr. Ken Griffin, PE
4/4/2013
Page 4of5
Table 2
Comparison of Ultimate 1% AC Water Surface Elevations
Location
Denton
Creek
Cross
Section
Denton
Creek
(ft)
Elm Fork
Interpolated
(ft)
Difference
Near U/S Limit of Property
9467
447.0
448.3
1.3
Water
Surface
Elevation
(Ft)
9141
446.8
448.2-
1.4
8947
446.7
448.2
1.5
(3) - (1)
8838
446.7
448.1
1.4
4.45
8588
446.5
448.1
1.6
8269
446.5
448.0
1.5
2.93
7617
446.4
448.1
1.7
447.25
7020
446.3
447.5
1.2
0.00
6468
445.9
446.7
0.8
446.92
5992
445.7
446.9
1.2
Near U/S Limit of Property
5599
445.5
446.7
1.2
-0.02
5279
445.3 1
446.4
1.1
Table 3
Summary of Ultimate 1 % Annual Chance Water Surface Elevations and Velocities
Location
Cross
Section
Pre- Pro'ect
Model
Post= Pro'ect
Model
Difference
Water
Surface
Elevation
(Ft)
Velocity
Channel
(fps)
Water
Surface
Elevation
(Ft)
Velocity
Channel
(fps)
Water
Surface
Elevation
(Ft)
Velocity
Channel
(fps)
Column Number ID
(1)
(2)
(3)
(4)
(3) - (1)
(4) - (2)
11574
448.83
4.45
448.82
4.43
-0.01
-0.02
10823
447.62
2.93
447.62
2.93
0.00
0.00
10277
447.25
2.27
447.25
2.27
0.00
0.00
9467
446.93
2.02
446.92
2.02
0.00
0.00
Near Upstream Limit of Property
9141
446.81
1.90
446.79
2.07
-0.02
0.17
Upstream of Proposed Bride
8947
446.73
2.17
446.73
2.06
0.00
-0.11
Downstream of Proposed Bride
8838
446.69
2.20
446.62
2.11
-0.07
-0.09
8588
446.62
2.20
446.53
2.81
-0.09
0.61
8269
446.51
1.87
446.46
1.62
-0.05
-0.25
7617
446.46
1.18
446.45
0.74
-0.01
-0.34
7020
446.28
2.82
446.28
2.87
-0.00
0.05
6468
445.96
2.76
445.94
2.81
-0.02
0.05
5992
445.69
2.76
445.67
2.75
-0.02
-0.01
Near Downstream Limit of Property
5599
445.51
2.63
445.50
2.63
-0.01
0.00
5279
445.30
2.90
445.28
2.90
0.02
0.00
SAOE1 Current Jobs \347.002 Denton Creek Estates- Coppell =The Holmes Builders\Repoits \City Letter Reporl \4 -5 -2013
SubmitNtl \347.002 Denton Creek Letter Report -rev 2013 -04 -Ol .doc
Mr. Ken Griffin, PE
4/4/2013
Page 5 of. 5
We trust that the above details and enclosed data will provide the City of Coppell with
sufficient information to review the project; we ask that the City of Coppell approve the
study. We appreciate your time and assistance in the matter. If you have questions or
require further information, please feel free to contact us. We may be reached at (972)
233 -2288.
Sincerely,
O'BRIEN ENGINEERING, INC
TBPE FIRM No. F -3758
t
Jim O'Brien, PE, CFM
President
attachments Exhibit 1 — Cross Section Location and Floodplain Map
Exhibit 2 — Grading Plans prepared by Kadleck and Associates
Exhibit 3 — Cross Section Plots Upstream & Downstream Face of Bridge
2010 Preliminary FIS Flood Profile for Denton Creek
CD — Digital copies of the hydraulic models
xc: Mr. Terry Holmes, The Holmes Builders
S:\0E1 Current Jobs \347.002 Denton Creek Eslates- Coppell ,ne Holmes Builden\Keports\City Letter Report \4 -5 3013
Sit bniit(a1 347.002 Denton Creel: Letter Report -rev 2013- 04- 01.doc
4iM QVlEX
55861
attachments Exhibit 1 — Cross Section Location and Floodplain Map
Exhibit 2 — Grading Plans prepared by Kadleck and Associates
Exhibit 3 — Cross Section Plots Upstream & Downstream Face of Bridge
2010 Preliminary FIS Flood Profile for Denton Creek
CD — Digital copies of the hydraulic models
xc: Mr. Terry Holmes, The Holmes Builders
S:\0E1 Current Jobs \347.002 Denton Creek Eslates- Coppell ,ne Holmes Builden\Keports\City Letter Report \4 -5 3013
Sit bniit(a1 347.002 Denton Creel: Letter Report -rev 2013- 04- 01.doc
FILE:\\obden9erverthsharel Data \OEI Border Templates\BSrz11 Templale.mvtl
Legend
Cross Section % % ; z�'
i
Stream Centerline
Proposed Lot Layout
Intermediate Contour,
Index _Contour
0 400 800 1,600
Fee — ^V
d �
9/
i I
/uu
+ 1!I� III cy
r
„ +9
V—
�lJ �I I`
S EXHIBIT _
B
O , BrienEngineering, Inc.
14 L.mm.n aiwe sm.vo F417 r.e. rsza�
n
�� �`1� r �:� — Y - \✓ �t % / 1 � Tun Repl�M PnOlneInO Fl�im FJ]]9 BrbnEn0.mm
5 _ 4 .. �' /J THE HOLMES BUILDERS
CROSS SECTION LOCATION
1S�Ih� AND FLOODPLAIN MAP
���y�''t \s O ~ L = %% J c;' DALLAS COUNTY TEXAS
r" DATE: 04 104/13 OEIJOB #: 347.002
ENO
Ld
Ld ------ ---
LLJ
Ld
.x
ix NINA
LOCK F h
A/1
.T . �If—
P.
-S —IT IS
m ME PRR w I.".
FEWIPF WDM ME
=12
MATCH LINE SEE SHEET 3
It it
YZ),
LEGEND
---------
AV,
-
le,
ENO
Ld
Ld ------ ---
LLJ
Ld
.x
ix NINA
LOCK F h
A/1
.T . �If—
P.
-S —IT IS
m ME PRR w I.".
FEWIPF WDM ME
=12
MATCH UNE — SEE SHEET 2
_ \♦♦
— ; I i
LEGEND
FF-
it J
Q
LLJ
En
LLJ
in
Z
– `
MOCK c / \' \` ,- �`• ♦ r' � aLaw c I
_ tx
— ,
ir -- - - - - -- ^�- s �•"�
s� �ioac c rwoenr !1!
1po
er s.,.r Id. nee° ..a lap° �� '90 ♦ ♦� Ms Ixtoaar a� GRADING PLAN
m s�w b
BLACKBERRY FARM nMtt
ltle Roe4 M el°. oI �bs,ee 1v p.Fcl. 6 L Lr�n Kalerll vE ♦T1'J
err�e.ur nm rye � er lu.r Rs,c o.� wa, mta
El.sllm arms COPPELL, DENTON COUNTY, 7EXAS
1. \�\
°u°°ri.°' er ut— d 1/1 Y.T iee el °,. ns°ret �LEOC s ASSOCI TES rr ®° as.n arr rwc veoLCr
Ise py.eF slLr
O.slls aw:m - � t..«.�... s „gym nan n°a wcm,a t•.w• �oaa
Denton Creek
Estates -rev 2013 -04
-01 Plan: 1) PostPr66ftArchBr 7:14:42
PM 2) Pre - Project 7:14:58 PM
Geom: Post
- Project with 66ft Arch Bridge Flow: Denton Creek
- combined
River
= Denton Creek Reach = Reach 1 RS = 8893
BR
I
I
Legend
- -_• -• � - - - -_ ,- �
�
F -�; S 1% Ultimate - PostPr66ftArchBr
WS FIS P0stPr66ftArchBr
47
!
PostPr66ftArchBr
- I
- - -�- PostPr66ftArchBr
i ; - - - --� -_-
.,;. ------ F------
-;- - _ +__._ -� � Ground - PostPr66ftArchBr
- -i -- Ineff - PostPr66ftArchBr
_T •��
Bank Sta - PostPr66ftArchBr
46 0
I
I
' t I
I �
1 -•
C
O
.5
i t I
I
I
45 0
w
i
it
I
44
i
:
I
43
0 500
1000
1500 2000
2500
Station (ft)
1 in Horiz. = 400 ft 1 in Vert. = 10 Oft
I
I
i
it
I
i
:
I
47(
46(
.2
CU
> 45(
.2
ul
M, I
43
Denton Creek Estates-rev 2013-04-01 Plan: 1) PostPr66ftArchBr 7:14:42 PM 2) Pre-Project 7:14:58 PM
Geom: Post-Project with 66ft Arch Bridge Flow: Denton Creek - combined
River = Denton Creek Reach = Reach 1 RS = 8893 BR
IJ �uu 1 uuu I buu ZUUU' 2500
Station (ft)
i in MONZ. = 4VU TT i in very. = I U TE
Legend
- - - - - - - - - - - - - - - -
- - - - -
- - - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - - - - - -
- - - - - - 7 - - - - - - - -
WS 1% Ultimate - PostPr66ftArchBr
--------- — ----- ----------
- --- ---- ....
WS FIS - PostPr66ftArchBr
----- ----
... . ..
PostPr66ftArchBr
- - --- - --------
----- -------
PostPr66ftArchBr
Ground - PostPr66ftArchBr
------------
----- ------ ------ - ------ ------
----- ------
------ - --- ------
- +-- --- +----- --- -- -- - -- --
__J ------------ - - - - - -
- - - - - - - - - - - - - - - - - -
- - - - -- 4 - - - - - - - - - - -
- - - - - - - - - 4 - - - - - - - - - - - - - - - - -
Ineff - PostM61tArchBr
------
------ - - - - - - -------------------
------ ------ -----------
'r
T,
Bank Sta - PostPr66ftArchBr
--- --------------
--------- -------
-------- T ------------------
T.
...............
------ ---- ----- - ------------------------------------
- ......
-- - -------------
1 I
12,
i
------
i
I ----- --------
-----
i
4 1'!
- ----- --------- -----
-------
----- ----
------------
---- --- ------------ ------------
----------- -------
---------- - ----------
------ 4 ------ 4 ------------------
------------
- ---------- ------
---- 1-
--_------------------
------- ----- ------ -----------
----------------_-------
--
_. «...___ --- ------- - -------
...
---- ---- --------- -----
------- -------- ------------
--------------
----------------- ------_----------------
--------- ----- - -
IJ �uu 1 uuu I buu ZUUU' 2500
Station (ft)
i in MONZ. = 4VU TT i in very. = I U TE
47
4
.2
76
> 451
w
44
43
Denton Creek Estates-rev 2013-04-01 Plan: 1) PostPr66ftArchBr 7:14:42 PM 2) Pre-Project 7:14:58 PM
Geom: Post-Project with 66ft Arch Bridge Flow: Denton Creek- combined
River = Denton Creek Reach = Reach I RS = 8838 8838
UUV I UUU I Ovu zuuu 2500
Station (ft)
1 in Horiz- = 400 ft 1 in Vert. = 10 ff
Legend
WS 1% Ultimate - Pre-Project
WS FIS - Pre-Project
WS 1% Ultimate - PostPr66ftArchBr
WS FIS - PostPr66ftArchBr
Ground - Pre-Project
•
Bank Sta - Pre-Project
PostPr66ftArchBr
Ground - PostPr66ftArchBr
Ineff - PostPr66ftArchBr
Bank Sta - PostPr66ftArchBr
- - - - - - - - - - -
- - - - - -
•
- -- -- - - -
- - - -- - - - - - - -
-- - - -----
- - - - - -- - - - - - - - - -
- - - - - - - - - - - - - - - - - - - - - - -
•
- -
------------- ------- -
------ - --- ------ - ----- - r ------
- - - ---
•
---------
- ---- - - - - - - -- t -------
--------- - - ---- --
- - - - - -- - - --
- - - - - - -------
•
------ - - - - -- - - - - - -
- - - - - - - ---- - - - - -
- - - - - 4 -- - - - -
-- --------
------
------------- -
----- ---- ---
T T
i ------ i ------ i ------_--_---
------ -----------
I ------ -----
-- T----------- ------ --------
- - --
- ----- ------- - - --- -------
-------- --
----------------------_-
--- -
-- -------------------------
---- ----------------
---
------
--------- - - - - - - - - - - - - - - -
- - - - - - - - - - - - - - -
-
- - - - - - -
--
---------------
- — - -
- - - - - -
------
- - - —
--- ---- 1'.
- - - - - - - _ I ---
- -_ - - - -
- - — - — - - - - - --- -
- - -
------
----
- -----------
- -- ....... _.-I ------------
--------------- -----I-----_- ---
-- -- - ----------------
- ------- -------------
- --
-------- - - ------- ---------
------------
------
----- ------------- --------
------- ----- -----
--- - ------- - _- - --- -
----------- 7--i --------
----------- ---------
- - -----------
-------------- ------ ------ 7 -----
- - = -- ---------- -------------
--- -- -- ----- r
........ :___.___' -------
----- 4 ------ ----- ------
----_------------
------ ---------------- 4 ----------
---------------
- - - - -- i -- - - - --
- - - - - - - - - - - - - - -
--- - -- - - - - - -
- --
UUV I UUU I Ovu zuuu 2500
Station (ft)
1 in Horiz- = 400 ft 1 in Vert. = 10 ff
Legend
WS 1% Ultimate - Pre-Project
WS FIS - Pre-Project
WS 1% Ultimate - PostPr66ftArchBr
WS FIS - PostPr66ftArchBr
Ground - Pre-Project
•
Bank Sta - Pre-Project
PostPr66ftArchBr
Ground - PostPr66ftArchBr
Ineff - PostPr66ftArchBr
Bank Sta - PostPr66ftArchBr