DR8601-CS 860325March 25, 1986
PIFRC;E-LUNSFORE) ASSOCIATES, INt .
CONSULTING ENGINEERS
Ref. No. 515342.10
CF EP3216
Mr. Ed Powell, P.E.
Coppell City Engineer
City of Coppell
P.O. Box 478
Coppel 1, TX 75011
Subject: Drainage Channel - Parkwood Subdivision
Coppell M.U.D. No. 1
Coppell, Texas
Dear Mr. Powell:
The drainage channel west of Parkwood Section One and Two has
been undergoing a significant amount of deterioration due to
erosion at the downstream end for the last several months, as you
are aware through letters of correspondence from representatives
of General Homes, developer and builder for Sections I, II and
III. As District's Engineer for Coppell M.U.D. No. 1,
Pierce-Lunsford has brought this matter to the attention of
Coppell M.U.D. No. 1 Board members because the situation poses
potential problems for citizens living in the district,
especially immediately adjacent to the channel.
Bids were opened February 26, 1986 for the Parkwood Section Three
utility improvement project. Award of contract is being held up
by Coppell M.U.D. until engineering plans for improvements to
handle the anticipated flows in the channel, at ultimate
development, are executed and approved by City of Coppell and
Coppell M.U.D. officials, and funds are allocated to finance the
improvements. It is General Homes contention that the City of
Coppell has some responsibility for these improvements since the
drainage easement was dedicated to the city along with platting
of the Parkwood I Subdivision and much of the runoff in the
channel comes f~om outside drainage areas outside those owned by
General Homes. This discussion and the items enclosed are
intended to give you a better understanding of the original
design and construction and factors that have led to the current
situation. This will enable you to determine what corrective
actions are appropriate to remedy the erosion problem.
The original channel improvements were constructed under the
Pa rkwood Subdivision, Section I utility improvement project.
The contract between Lanzo Construction Company and Coppel 1
Municipal Utility District No. 1, was executed on September 26,
3109 CARLISLE, SUITE 206
DALLAS, TEXAS 75204
(214) 871-2126
3901 W. PIONEER PARKWAY, SUITE 100
ARLINGTON, TEXAS 76013
(817) 461-5868 Metro
Mr. Ed Powell, P.E.
March 25, 1986
page 2
1983. Threadgil 1-Dowdy Consulting Engineers designed the
construction plans which were approved by the City of Coppell and
Coppell M.U.D. prior to advertisement for bids.
The typical cross-section for the channel includes a 15 foot wide
earthen channel with 3:1 side slopes . An 8 foot concrete pilot
lining was placed in the channel botton at a 0.10% slope starting
at the 3 - span 8'x5' Box Culvert at Parkway Blvd. and ending
approximately 2,000 lineal feet to the north as shown on enclosed
design plan sheets. The pilot channel was required because of
the flat slope of the channel bottom. From that point, the
channel originally continued with the same cross-section but no
concrete lining on a 3.06% grade approximately 255 feet to
connect to an existing ditch that discharged directly to Denton
Creek, approximately 700 feet to the North. These improvements
were completed in April of 1984. Final approval for utility and
drainage channel improvements was made by Coppell M.U.D. No. 1,
The City of Coppell and GINN, Inc. in September of 1984.
The contract for the Parkwood Section II utility improvement
project between Coppell M.U.D. and Regional Utilities Contracting
Corp. was awarded the same month and construction started up in
October of 1984. This project took one year to complete and
throughout its duration, no significant erosion was noticed to
the channel lining or side slopes. It was not until December of
1985 that it became apparent that the drainage channel was
undergoing significant erosion and undermining of about 200 feet
of the north end of the concrete lining and channel side slopes.
A meeting was attended on December 20, 1986 by representatives of
Pierce-Luns ford, General Homes, Wiseco Land Development,
Threadgill-Dowdy to determine the cause and possible solutions to
the problem. Your engineer was asked to be there but could not
se~d a representative. Though an exact cause for the problem
could not be ascertained, the major contributing factors appeared
to be:
1. A large volume' of dirt has been excavated from the area
immediately downstream of the north end of the concrete
channel lining to provide fill dirt for The Park Meadow I
and II subdivisions and possibly other areas to reclaim
these areas and raise grades above the 100-year flood
elevation. The north end of the channel lining was placed
at approximately elevation 452.4. The borrow pit was taken
down to about 438.0. This has created a steep hydraulic
grade and eddy currents where the channel outfalls to the
area of excavation, thereby increasing veloci ties and
contributing to erosion of the channel. The enclosed plans
show how the channel profile has changed in areas of the
Mr. Ed Powell
March 24, 1986
page 3
channel lining, though I do not have survey verification
of the excavation pit grades at this time.
The existing channel cut-bank shows layers of different
soil types including sand, sandy-clay and clay, some of
which appears that it was not natural but was fill
material. In addition, there are no signs that side slopes
were seeded or hydromulched. Volunteer vegetation was too
sparce to protect side slopes.
Runoff has increased significantly from contributing
drainage areas adjacent to and south of the drainage
channel due to commercial and residential growth. It is
not known at this time whether actual flows have increased
beyond those used for the original design.
Because serious erosion was not noticed until recently, it is
thought that the excavation of the borrow pit at the outfall of
the channel was a major contributor to the erosion problem.
Coppell M.U.D. was not given the opportunity to review proposed
gradiag plans for the borrow area, nor to have any input into
design of a lined channel, drop structure or other improvements
to control erosion. A concrete drop structure with baffles was
placed to handle storm water in earthen channel west of Parkwood
I Subdivision when a similar borrow pit was excavated, and that
channel does not appear to be suffering from the type of erosion
as the channel through Parkwood.
With rainfalls normally anticipated this time of year, erosion to
the existiag channel will get more severe and costs for repairs
are sure to escalate. Work on fully lined open channels, box
culverts and off-site drainage improvements are not M.U.D.
sponsored items as are on-site utilities as set forth by the
Texas Water Commission criteria. They are therefore not bid as
part of M.U.D. utility improvement projects. Coppell M.U.D. is,
however, concerned with the overall development of areas within
the district and citizens living in the district. Please review
these enclosure§ and your records so that this situation can be
remedied as soon as possible.
Your help and consideration in resolving this matter is
appreciated. I will be happy to meet with you in your office or
Mr. Ed Powell
Ma rch 2 5
page 4
at the channel site at your earliest convenience
will be beneficial.
Sincerely Yours,
PIERCE-LUNSFORD ASSOCIATES, INC.
Jeffrey M. Hawkins
JMH/DB
CC:
Coppell M.U.D. Board Members
Mr. Tom Wise, Univest
Mr. Wayne Ginn, Ginn, Inc.
Mr. Paul Phy
Mr. George Dudley, Threadgill-Dowdey & Associates
Mr. Skip Hynek, General Homes /
if you feel it