Park, Peter-CS 970204 CONSULTING ENGINEERS, INC.
Mr. Kenneth M. Griffin, P.E.
Assistant CiW Manager / CiW Engineer
CiW of Coppell
255 Parkway Boulevard
Coppell, Texas 75019
Subject: Revisions and Additional Data Requests
18.9 acre Sandy Lake Road Property
Dear Mr. Griffin:
We have addressed the issues raised during our meeting with you on January 23, 1997
regarding the project cited above. These issues relate to desired revisions to the Grading
Plan and to additional data pertaining to hydraulic analyses for Denton Creek. These items
are discussed individually below.
Grading Plan Revisions
The development located on the opposite side of Sandy Lake Road from our project site
intends to discharge storm water to the existing draw through the central portion of the
property. According to the plans we reviewed in your office, 2 - 30 inch diameter storm
sewer pipes will discharge into the draw just north of Sandy Lake Road. Contrary to the
preliminary plans for the Sandy Lake Road expansion which we received from Dallas
County, this storm sewer outlet will be permanent. We have therefore made provision to
connect to and extend these pipes through our site to empty into Denton Creek on the north
side of the property. A revised Grading Plan illustrating this addition is enclosed. We would
like Dallas County to consider connecting the storm sewer system from the southern
development into the system proposed for Sandy Lake Road. If you so desire, we can pose
this question to Dallas County. Please let us know how you would like to proceed in this
regard. If connection to the Sandy Lake Road system is not feasible, we will install the pipes
as shown at the time of fill placement for the project site.
Concern was also expressed that the cut slope in the southeastern portion of the property
begins too close to the proposed right-of-way such that transition to the proposed grade will
be steeper than a 4:1 slope. This cut slope has been moved about 10 feet further north into
the property to provide for the slope transition to the proposed right-of-way grade. Other
cut slopes have been adjusted such that the storage volume is preserved. These
modifications are shown on the provided Grading Plan.
Three NorthPark / 8800 N. Central Expwy. / Suite 300 / Dallas, Texas 75231 / (214) 739-4741
Mr. Griffin
2..'4.."97
Page 2
You mentioned some revisions that may be necessary to the planned site layout as
previously submitted. The previous plan was intended only to give a general idea for the
planned single family residential land use. Details of the site plan will be coordinated with
your office to ensure City of Coppell (City) requirements are met. This will be pursued once
the floodplain reclamation permitting has been accomplished
Denton Creek Hydraulic Analyses
Denton Creek is analyzed for the conditions where the project area is not controlled by
backwater from the Elm Fork of the Trinity River. A total of 10 cross sections were derived
from the Corps of Engineers (CoE) aerial topographic mapping assembled for the Elm Fork
Feasibility Study. The sections are shown on the enclosed Cross Section Location Map. In
addition, two sections are repeated at locations 10 feet downstream and 10 feet upstream of
the subject property to provide transition to the area to be modified. Approximations are
necessary for the channel of Denton Creek since the channel bottom is not portrayed on
the aerial topographic mapping. The flow line elevations at the section locations are
determined from the stream bed profile within the Flood Insurance Study report. Side
slopes of 2:1 are then assumed for the channel up to the banks determined from the aerial
topography.
The primary purpose of these hydraulic analyses is for comparison of water surface
elevations along Denton Creek for existing and proposed floodplain conditions as related to
reclamation for the Sandy Lake Road Property. Effective flow limits for these sections and
other hydraulic parameters are subject to engineering judgment. Thus, the results of these
preliminary analyses may differ significantly from the results of other studies or reference
materials.
Denton Creek hydraulics are analyzed for four different discharges. A discharge of 2000 cfs
is used as a typical release rate from Grapevine Lake upstream, as per the CoE. The
ultimate development 2-year and 100-year discharges for Denton Creek are determined
from the City-Wi¢le Storm Water Management Stucly by Albert H. Halff Associates, Inc.
(January 1991). These are determined to be 7432 cfs for the 2-year and 23,083 cfs for the
I00-year. A regression analysis is performed using these two values in order to estimate a
10-year ultimate development discharge of 13,871 cfs. Manning's "n" values are also
determined from the Halff Study as 0.045 for the overbanks and 0.040 for the channel.
For the four discharges mentioned above, two different starting conditions are considered.
A normal depth assumption is used as a condition for each of the discharges. Secondly,
conditions on the Elm Fork are assumed to dictate the starting condition for Denton Creek.
For the lake release, the 2-year ultimate and 10-year ultimate discharges the normal water
level for the Elm Fork is used as a starting condition. This is equal to elevation 431.0, which
is the crest elevation for the Carrollton Dam located a short distance downstream on the Elm
Fork. For the 100-year ultimate discharge on Denton Creek, the 10-year ultimate water
surface elevation on the Elm Fork is used as a starting condition. This elevation is
determined from the CoE HEC-2 Feasibility Model.
Mr. Griffin
2."4.-'97
Page $
The scenarios described above are evaluated using the HEC-2 computer program for both
existing and proposed floodplain conditions. The only differences between the existing and
proposed conditions are the modifications in the right overbank for development of the
Sandy Lake Road Property. For all four discharges, the difference in water surface elevation
at the downstream end of the subject property due to starting condition is negligible, as
shown in the output tables of the included HEC-2 models. Thus, further discussions and
comparisons address only the starting condition dictated by the Elm Fork.
A comparison table is included which illustrates the water surface elevations at the cross
section locations for the four discharges analyzed. Some increases of proposed over existing
are noted at the downstream end of the project site, at sections 23+70 and 26+I0. For all
other locations, the proposed yields water surface elevations less than or equal to the
existing conditions. No increases in water surface elevation occur upstream of the Sandy
Lake Road Property due to the proposed modifications for any of the discharges analyzed
herein.
Summary
Modifications have been made to the Grading Plan based on the concerns expressed in our
previous meeting. Hydraulic analyses have been performed for Denton Creek to assess the
impact of the project development. Results indicate development of the Sandy Lake Road
Property will have negligible impact on the water surface elevations of Denton Creek. The
project has previously been illustrated to have negligible impact on the water surface
elevations of the Elm Fork of the Trinity River.
We hope this fulfills all of the City's requirements for this project and that City approval will
be quickly forthcoming. If you have any questions or would like to discuss these results
further, please let us know.
Sincerely,
NATHAN D. MAIER
CONSULTING ENGINEERS, INC.
Mark D. Walter, P.E.
enclosures
COMPARISON OF DENTON CREEK WATER SURFACE ELEVATIONS
18.9 acre Sandy Lake Road Property
Lake Release 2-Yr Ultimate 10-Yr Ultimate 100-Yr Ultimate
Section Exist I Prop Exist Prop Exist Prop Exist Prop Remarks
13+30 435.70 435.70 440.44 440.44 441.57 441.57 443.57 443.57
23+60 437.38 437.38 442.16 442.16 444.02 444.02 445.16 445.16
23+70 437.39 437.50 442.19 443.04 444.06 444.39 445.18 445.33 Dnstrm End of Property
26+ 10 437.63 437.62 443.21 443.20 444.47 444.49 445.38 445.42
31 +40 437.75 437.67 443.43 443.23 444.74 444.57 445.65 445.52
34+20 437.98 437.79 443.61 443.40 444.83 444.69 445.74 445.64
38+90 438.26 437.52 443.86 443.32, 445.03 444.72 445.94 445.71
44+20 439.24 438.80 444.12 443.86 445.28 445.06 446.23 446.07 Upstrm End of Property
44+30 439.25 438.81 444.13 443.88 445.29 445.08 446.25 446.09
47 + 80 439.61 439.23 443.97 443.88 446.19 446.19 446.97 446.96