DR9604-CS 970128REPLY TO
ATTENTION OF:
DEPARTMENT OF THE ARMY
FORT WORTH DISTRICT, CORPS OF ENGINEERS
P.O. BOX 17300
FORT WORTH, TEXAS 76102-0300
January 28, 1997
Engineering Division
Civil Design Branch
Honorable Tom Morton
Mayor
City of Coppell
P.O. Box 478
Coppell, Texas 75109
Dear Mayor Morton:
We have received several complaints from residents in the city of
Coppell along Denton Creek who expressed a concern about potential flood
damage to their private property.
This concern was partly due to outflow~ from Grapevine Lake.
Another concern was due to blockage and debris in Denton'Creek.
Grapevine Lake experienced an inflow of over 17,000 cubic feet per
second (cfs) on November 25, 1996. This flow was stored and later
released at a rate of 1,800 cfs as downstream conditions improved. This
type of operation helps to protect lives and property downstream of
Grapevine Lake. As the lake level is lowered and the flood storage
space is regained, the reservoir preserves the capability to attenuate
future floods.
Controlled releases up to 2,000 cfs are part of the operational
procedures for Grapevine Lake.
Within the last 15 years, Grapevine Lake has experienced flooding
events which have produced high peak and large volume inflows into the
reservoir. On three occasions, the lake water surface has risen above
the crest of the uncontrolled emergency spillway. In November 1981 and
the spring of 1990, flows of 8,700 and 7,400 cfs, respectively, flowed
through the spillway. Those flows exceeded the controlled releases
observed along Denton Creek in the recent event. When flood releases
must be reduced due to downstream flooding problems, the risk increases
that the spillway will eventually be overtopped and that flows over the
spillway will be greater than the controlled release rate of 2,000 cfs.
Multiple blockages along Denton Creek have caused problems and
concerns. The most severe blockage was a bridge crossing a% Denton
Creek near Sandy Lake Road, which had debris in its openings. The city
of Carrollton personnel removed much of the debris on November 25, 1996.
One log was 5 feet in diameter and about 20 feet long. The city of
Carrollton personnel removed the bridge on December 10, 1996.
The Wellington Place Apartments, located near MacArthur Boulevard
and Sandy Lake Road in Coppell, have their lowest finished floor at
444.65 feet National Geodetic Vertical Datum (NGVD). Floodwater from
Denton Creek blocked the local drain and storm sewer system, which
flooded the parking lot of the apartments. The water level rose to the
engine blocks on the cars in the parking lot. The apartments appear to
have been built lower than the proposed grading and drainage plan
submitted to the city of Coppell in 1985. The apartment manager
indicates that water was less than 1 vertical foot below the lowest
finished floor. Apparently these apartments have flooded several times
in the last 11 years. The 100-year flood on Denton Creek, with a
discharge of approximately 20,000 (cfs) in this reach, can cause
catastrophic flooding of these apartments.
Several residents in the vicinity of Parker Drive complained of
beaver dams, fallen trees, and debris which are constricting the flow in
Denton Creek. They asked us to reduce the releases from Grapevine Lake.
The residents along Parker Drive were concerned about losing
landscaping. They also fear possible damage to foundations.
The Corps of Engineers has no authority to remove trees and debris
from the channel of Denton Creek below Grapevine Lake. While one of the
possible alternatives being considered in the Upper Trinity Feasibility
Study would offer some relief to the existing channel blockage problem,
it would be a long-term solution. A short-term solution would still be
needed. Since the Corps of Engineers cannot help with the short-term
situation, action on the part of the city of Coppell is needed to assure
that controlled releases of 2,000 cfs can flow through the creek channel
without causing flooding along Denton Creek. The city should be even
more concerned about potential blockage from debris during a 100-year
flood which has a peak discharge of approximately 20,000 cfs in the
reach in question. Designs submitted for Section 404 permits for
development along Denton Creek have assumed that the channel would not
be blocked with debris. If it is blocked, floodwater will reach higher
levels than projected in designs, and properties along the creek will
flood.
We trust that this information will be helpful. If we can be of
further assistance, please feel free to call Mr. Paul Rodman at
817/978-3134, extension 1708.
S~ncere~~4~
Deputy District Engineer
of Engineers
Copies Furnished:
Mr. James Witt, City Manager of Coppell
Mr. Ken Griffin, City Engineer of Coppell
Mr. Charles Burger, Reservoir Manager at Grapevine