Valley R Center-CS 890421DUNAWAY ASSOCIATES, INC.
TO:
VIA:
FILE
Job No. 8804702
James R. Dunaway
3oseph P. Caddel
Tom Huffhines
FROM:
REFERENCE:
Steve K. Lemke
lO0-Year Storm
Valley Ranch Center
Coppell, Texas
Il",', Il
DATE: 04/21/89
MEMORANDUM
Several times during the past few weeks I have spoken to the following people concerning
the potential of the lO0-year storm to inundate 95% of the front parking lot of Tom Thumb
Page and Phase II retail area (see Attached Drainage Area Map):
Pat Donovan
Scott Morway
Bill Cook
Rick Walker
Kevin Peiffer
Tom Huffhines
Dunning Development
Dunning Development
Herman & Cook
Urban Architecture
Ginn, Inc. (Consulting engineer to City of Coppell)
Dunaway Associates, Inc.
Mr. Peiffer has been in contact with other city officials, Bud Beene with
Dallas County, and Ginn engineers responsible for Belt Line Road design.
This memo is written in an effort to document these discussions and to further explain
the expected reaction of subject site to the lO0-year storm.
As explained to me by Kevin Pefffer, the Valley Ranch Center (V.R.C.) site improvements
designed by Albert Halff Associates were approved under the old city drainage policy.
Belt Line Road improvements were designed by Ginn, Inc. using guidelines set forth in the
current city drainage policy. These existing approved V.R.C. site plans forced special
consideration on the Belt Line Road design not ordinarily taken.
The proposed elevation of Belt Line Road (adjacent to V.R.C.) is 6.7 feet higher than
existing pavement surface and within 0.8 feet of proposed site finished floor elevations.
The existing bar ditch drainage system is being replaced with an enclosed concrete box
system (see grading plsn for system route).
During the 100-year storm (and some more frequent storms i.e. 75-year storm, 50-year
storm), this enclosed system is flowing under pressure. This is due to the combination of
a high 100-year water surface elevations in Grapevine Creek and the relatively low
elevation of the Belt Line Road subsurface drainage system (necessarily low to drain
upstream watersheds).
ENGINEERS · PLANNERS" SURVEYORS · 4100 INTERNATIONAl. PLAZA * TOWER I! · SUITE 636* FORT WORTH, TEXAS 76109 · 817 731-0636
Page Two
Memo To File
Job No. 8804702
04/21/89
The V.R.C. drainage system ties into the Belt Line Road drainage system at only one
point. The Belt Line Road hydraulic grade line elevation at this point is 447.5. This
pressure elevation is higher than 80% of the V.R.C. parking lot that drains to this
point. Therefore, this provides a place where the Belt Line Road system can relieve some
pressure resulting in stormwater backing out of inlet and spreading into parking lot.
The expected behavior of this backwater is as follows:
o
The peak runoff of onsite water(V.R.C, parking lot) exits property and enters Belt
Line Road system approximately 15 minutes before the adjacent Belt Line Road system
is carrying its peak runoff from upstream sources. In other words, inlet A-2
essentially dries the parking lot of its major stormwater before the Belt Line Road
system forces other water back into the parking lot.
The surcharge calculations for this backwater (see storm drain profile sheet) assume
water backing from system to parking lot is completely unrestricted and the
elevation of water in parking lot rises at the same rate as the hydraulic grade line
(HGL) rises in Belt Line Road system. However, it is expected that the HGL in Belt
Line Road will peak and be on its way back down before the water surface elevation
in the parking lot equals the Belt Line Road peak HGL elevation. Said calculations
are therefore considered conservative.
I contacted Bill Cook on two occasions to make him aware of this situation. Me said he
understood this site may have some problems like this and would himself document what I
explained to him.
I discussed the following alternatives/solutions to this problem with Pat Donovan and
Scott Morway.
O
Raise the Entire Site. Since the site is severly restricted on the north, south, and
west sides by unchangeable grades and has a minor grade restriction on the east
side, any raising of the site beyond what is proposed by current grading plan will
require exposed grads beams, retaining walls, or a combination of both on south side
of buildings and south property line. Another undesirable solution to this alternate
would be to provide an unused "buffer zone" along south property line to take up
excess grade.
~ipe the Front Parkin~ Lot Storm Water to the South, Under Building, and Direct.~y
into Grapevine Creek. This option would also require filling the site more than
currently proposed to prevent Grapevine Creek flood water from backing into parking
lot. In addition, this option would take special review and approval by Flood
Control District, City, and County which may not fall within time-frame of project.
Page Three
Memo To File
Job No. 8804702
04121/89
Pat and Scott directed me to leave the design as currently proposed (see attached grading
SKL/rel
cc: Pat Donovan
Scott Morway
Bill Cook
Rick Walker
~ell Doyle
Kevin Peiffer
Bud Beene