DR8902-CS 900925 ENGINEERS * SCIENTISTS · SURVEYORS
DALLAS * ARLIN(iTON · (]ItICAG() * FORT W()RTII
8616 NORTHWEST PLAZA DRIVE · DALLAS, TEXAS 75225
September 25, 1990
P 1008.90.3430
City of Coppell
732 DeForest Lane
Coppell, Texas 75019
Att: Shohre Daneshmand
Dear Ms. Daneshmand:
At your request, we have investigated a timber retaining wall recently constructed on
the west bank of Stream G6 approximately 150 feet upstream (north) of Bethel School
Road.
The Draft City-Wide Storm Water Management Study proposes erosion and flood
control improvements in this area. These recommended improvements, shown on Exhibit
Ah will correct erosion occurring in this reach, eliminate the overtopping of Bethel School
Road during a 1.00-year flood event, and remove 3 homes from the 100-year flood limits.
On June 26, 1990, Thomas Guillory and I visited the site. The purpose of our visit
was to visually confirm the feasibility of the proposed solution to flooding and erosion
problems in the area. While there, we noticed a landscape timber retaining wall under
construction in the above mentioned location. Based on that field trip, photographs, and
drainage plans for this subdivision, it appears that the retaining wall is constructed within the
City's 40 foot drainage easement and may, in fact, impede flood waters. In the absence of
as built wall plans referencing the wall location to property corners, this could be confirmed
by a lot survey with ties to the wall. This information would be required to evaluate the
exact flooding and erosion impacts of the wall.
The improvements recommended in the Draft City-Wide Study consist of culvert
enlargement at Bethel School Road and channel enlargement both upstream and
downstream of Bethel School Road. Downstream of Bethel School Road, a grass-lined
earthen channel with a 20-foot bottom width and 3 to 1 sideslopes is recommended.
Upstream, a 12 foot width concrete pilot channel with grouted rock rip-rap side slopes is
recommended for 400 feet.including channel transitions. A concrete retaining wall will be
required for a short (50 feet) distance along the east bank. Field conditions as determined
by final design surveys may vary slightly and minor adjustments in dimensions and elevations
should be made in conjunction with preparation of construction documents for the culvert,
1RANSPORTA"IION · WATER RESOURCES * LAND DEVE[ OI~Mt NI i',4tlld¢;IPAI
SURVEYING G[ OBAL POSI1'IONING SYSIEM (G P S ) RE MOIE SLNSIIJG ArID
LANDSCAPE ARCHITECTURE * Pt ANNIHG
ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS · SCIENTISTS · SURVEYORS
·
City of Coppell
September 25, 1990
Page 2
channel, and wall. The estimated probable construction cost of these recommended
improvements is $152,000.
The proposed improvements did not consider the newly-constructed timber retaining
wall. This wall will probably be demolished as a part of the improvements. In general,
timber retaining walls should not be allowed in areas subject to inundation and swiftly
flowing flood waters because of their history of failure. The resulting floating debris can
even damage downstream structures.
I hope this information is useful. Please do not hesitate to call if I can answer any
questions or provide additional services.
Yours very truly,
ALBERT H. HALFF ASSOCIATES, INC.
Walter E. Skipwitht, P.E.
Vice President
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