Pecan Hollow/FP-CS 900413dP Jerry Parch& Consulting Engineers
April 13, 1990
Mr. William Anderson, P.E.
Dan M. Dowdey & Associates
16250 Dallas Parkway
Suite 100
Dallas, Texas 75248-2684
214/931-0694
RE:
JP No. 88009; Request for Drainage Easements;
Parks of Coppell; Coppell, Texas
Dear Mr. Anderson:
On April 5, 1990, our office received a request from you for
three drainage easements in property owned by ABQ Development
Corporation adjacent to the north boundary of the proposed Pecan
Hollow development. In addition to the legal descriptions and
exhibits of each of the easements, you also provided a copy of
the final plat of Pecan Hollow, a drainage area map, and plan and
profile sheets showing the design of each storm drain, Lines A,
B, and C, which will discharge in the requested easement areas.
Our client, ABQ Development Corporation, has asked us to review
plans of your proposed construction on their property and
evaluate impacts on flooding and future channel improvements in
the drainageway. You and I have discussed our concerns in a
telephone conversation yesterday and I also discussed our
concerns with your client, Mr. Doyle Cameron. This letter will
summarize our discussions.
You have informed me that you determined the 100-year flood water
surface elevations in the drainage channel (as shown in your
plans) at the proposed storm drain discharge points by assuming a
hydraulic gradient elevation at the street low point
approximately 1.5 feet below proposed top of curb elevation. The
hydraulic gradient was then calculated in the proposed pipe
downstream to the discharge point at the channel. The ensuing
hydraulic gradient elevation at the discharge point is labelled
as the local WS~00 on the plans.
We are not in agreement with this method~ of calculating the
hydraulic gradient since it disregards concurrent flow in the
channel as well as the channel cross section, slope and
downstream conditions which together will determine the flood
level in the channel. Regarding ABQ property, our concerns for
the 100-year flood levels shown in the drainage channel on your
plans are as follows.
320 ~(/estway Place, Suite 501 · Arlington, Texas 76018 · Metro (817) 467-2418 · (817) 465~)259
Mr. William Anderson, P.E.
JP No. 88009
April 13, 1990
PaGe 2 of 4
At the time ultimate channel improvements are designed, the
City of Coppell or residents 'in Pecan Hollow may insist that
channel improvements yield 100-year flood levels that do not
exceed the levels shown on the Pecan Hollow plans, since
these were the levels used to design the storm sewers in
Pecan Hollow.
Since existing ground levels on proposed lots in Pecan
Hollow abutting the drainage channel are lower than 100-year
flood elevation in the channel, this area presently conveys
a portion of the 100-year flood in the channel. The
proposed fill shown on these lots in your plans will remove
flood conveyance area and force more of the flow onto ABQ
property. Also, without drainage easements on the rear of
lots in Pecan Hollow, residents may construct fences,
vertical walls, or structures up to or along the rear
property line further depleting flood conveyance.
In the design of Heartz Road between Parkway Boulevard and Sandy
Lake Road, our firm had designed interim improvements in the
subject channel from the southwest corner of Parkview Addition to
the proposed culvert in Heartz Road. Sheet Numbers 26 and 27 of
the Heartz Road Plans showing these proposed channel improvements
are enclosed. Adjacent to Pecan Hollow these improvements
Generally consist of lowering the channel bottom approximately
one foot and widening the channel bottom and constructing four to
one sideslopes. The plans for this interim channel were approved
by the City of Coppell.
100-year flood levels shown on the improved channel plans for
Heartz Road are based on a fully developed watershed and the
channel improvements discussed above. These 100-year flood
levels adjacent to Pecan Hollow range from elevation 461.4 at the
southwest corner of the Parkview Addition to elevation 461.7 at
the northwest corner of Pecan Hollow. All proposed improvements
consist of excavation. 100-year flood levels in the drainage
channel shown on the Pecan Hollow Plans you provided range from
elevation 460.0 at the southwest corner of Parkview Addition to
elevation 460.3 near the northwest corner of Pecan ~ollow. These
elevations are approximately 1.4 feet less than the elevations we
determined with proposed channel improvements.
Following from the information presented above, our concerns and
recommendations to resolve these issues are summarized below.
Mr. William Anderson, P.E.
JP No. 88009
April 13, 1990
Page 3 of 4
Any improvements adjacent to the subject drainage channel
should not increase water surface elevations higher than
elevations shown on the Heartz Road improvement plans.
Therefore, this 100-year flood level can be used for
extending drainage improvements into and setting minimum
finished floor levels in adjacent property. This water
surface elevation should be shown on any plans of
improvements adjacent to the channel in order to promote an
understanding and establish guidelines for future
development adjacent to the channel among residents, city
officials and developers.
Please provide hydraulic models which demonstrate that
proposed filling in Pecan Hollow will not increase 100-year
flood levels above the elevations shown for the improved
channel in the Heartz Road improvement plans. Proposed
ground levels used in portions of cross sections through
Pecan Hollow in your model will be used to establish fill
limits in the rear of lots abutting the channel.
Since the south bank of the ultimate channel improvements
will extend into the rear of the lots backing onto the
channel in Pecan Hollow, a drainage easement sufficient in
width to include the flood carrying area on these lots
should be dedicated to the City of Coppell. This will give
the City authority over any construction in the ehsement
area and help preserve the flood conveyance.
The designed flowline (bottom) elevation of the proposed
storm sewers at the drainage channel matches the existing
channel bottom elevation. The channel bottom elevation in
the improved interim channel designed for Heartz Road
improvements is lower than the existing channel bottom.
Since the proposed storm sewers are substantial structures,
the pipe bottom elevation should match the bottom elevation
of th~ proposed improved channel. The existing channel
should be deepened to accept these storm sewers. This will
eliminate the need for lowering the storm sewer discharge
structures when the channel is improved.
Permanent headwalls should be constructed at the discharge
points of proposed storm sewers to retard erosion. No
headwalls are proposed in the plans provided to us.
Additionally, flow exiting bines B and C should be directed
downstream as shown on the drainage area map provided to us.
The plans indicate flow exiting Storm Sewers B and C will be
perpendicular to the flow in the channel.
Mr. William Anderson, P.E.
JP No. 88009
April 13, 1990
Page 4 of 4
We have requested Dowdey Engineers realign proposed Beechwood
Drive (in Pecan Hollow) at the intersection with ABQ's east
property line to conform to the curve in the drainage channel and
to permit us to plat 120 foot deep lots backing up to the channel
next to Pecan Hollow. I provided you our requested adjusted
alignment of Beechwood Lane by telephone facsimile on March 12,
1990.
On April 3, 1990, you sent me a possible alignment adjustment
that would require small radius reverse curves in Beechwood Lane
when the street is extended westward. Approximately two lots
between Beechwood Lane and the channel would have approximately
only 100 feet of depth.
ABQ is very concerned about the lot depth that will be available
north of Beechwood Lane due to the proximity of the channel and
the filling that will be required due to existing low elevations
in this area. On behalf of ABQ, we still request adjustment of
the alignment of Beechwood Lane as shown in my March 12, 1990
facsimile to you.
With resolution of the issues presented above, approval of the
requested easements can be recommended by our firm. However, by
copy of this letter, we advise ABQ to consider other possible
impacts on marketing, liability, financial, etc., as well as
requirements due to previous agreements that have been made
associated with this property, if any.
If you have any questions, please call.
Sincerely, //~
errParch'e ' P.E.
JP:ch
Enclosures: Sheet Nos. 26 & 27 - Heartz Road Plans
cc:
Ms. Mary Hicks/ABQ Development Corporation
Mr. Gary Sieb/City of Coppell
Mr. Alan Ratliff/City of Coppell
Mr. Wayne Ginn, P.E./Ginn, Inc.
Mr. Jim Schell/Law, Snakard & Gambill