Lake Park-CS 900924 GINN, INC. CONSULTING ENGINEERS
51990
Ms. S shmand, P.E. ~ ~.
City of Coppell
P.O. Box 478
Coppell, Texas 75019
Re: Lake Park Addition - Drainage System (Proposed)
Dear Ms. Daneshmand:
We have reviewed the revised engineering plans for the Lake Park
Addition as per your request of September 20, 1990. With the
exception of the Drainage Plans and the dedication of a larger
utility easement for the sanitary sewer between Lots 7 and 8 of
Block A, it appears that all of our previous comments have been
addressed in a satisfactory manner. For your convenience we have
marked the Final Plat and plans with our comments and are
transmitting them and a set of the previous comments herewith.
Before getting into the drainage issues we feel it prudent to
mention that since an area of wetlands has been located on the
project site, outside of the lake, the developer should be
required to show the location of these wetlands on the plans and
indicate his intentions regarding construction in that area.
After review of the September 10, 1990 letter from the Army Corps
of Engineers, it is clear that a 404 permit will be required
prior to construction which would effect the area of wetlands
found on the project site. We suggest that you require the
developer to satisfactorily address the onsite wetlands (not to
be confused with the lake) before you issue any construction
permit5
Also, under the unusual situation that exists with the
Floodplain/Floodway status of this project, the developer should
be required to provide the City with a letter of indemnification,
in a form acceptable to the City Attorney, which will hold the
City of Coppell harmless from any damages or actions arising from
the developer's or his contractor's activities in the floodplain
associated with this project. I believe that this approach has
been used in the past on a couple of projects.
With regard to the main purpose of this review, the proposed
storm drainage system, we have performed a cursory review of the
system and determined the areas of street flooding which may
occur during the 100 year storm rainfall event. Please be aware
that the drainage plans as submitted, are based on a 25 year
storm and are acceptable under the Subdivision Ordinance No. 185.
17103 Preston Road · Suite 100 · LB 118 · Dallas, Texas 75248 · Phone 214/248-4900
Based on our review of the Drainage Plans, sheets 1,2 and 3 of 3
sheets, we have plotted areas of ponding on Paving Plan sheets
1,2,4 and 5 of 7 sheets, transmitted herewith. The Hydraulic
Gradients we have calculated for the drainage system backwater
elevations, representing the ponding in the streets, are marked
on sheets 2 and 3 of the Drainage Plans transmitted herewith.
The best indication of where ponding may occur during th 100 year
storm event is marked on sheets 1 and 2 of the Site Grading Plans
transmitted herewith. As you can see, the only lots in the
subdivision which would have any accessibility problems during
the 100 year storm are Lots 13, 14, 15, 22 and 23 of Block E on
Briar Cove Drive.
We have performed some cursory calculations based on increasing
the pipe sizes proposed for storm sewer lines "B","C" and "D",
~-hich indicate that the &tree~ flooding may be ~reatiy reduced or
eliminated. In our calculations, we have used a 100 year storm
water surface elevation of 454.16, furnished by Kimley-Horn and
Associates, representing the 100 year storm with ultimate
development discharges for Denton Creek adjacent to the site. We
received this information during our initial review of the
subdivision, prior to the completion of the Albert H. Halff and
Associates study. We have never received or reviewed the Halff
Associates study and therefore have no knowledge of the water
surface elevations they have calculated adjacent to this site.
The revised Hydraulic Gradients, larger pipe sizes, and slopes
for storm sewer lines "B", "C" and "D" are marked on sheets 2 and
3 of the Drainage Plans. Increasing the size of the storm sewer
pipes appears the most economical way of reducing or eliminating
the street flooding during the 100 year storm. Before
proceeding with such a plan, the project design engineer should
be consulted and allowed to verify the effectiveness of the
proposed changes. With the larger diameter pipes we've suggested
there may be some problems with depth of cover or conflict with
other underground utilities.
Please call me if you have further questions.
Sincerely,
John C. Karlsruher, P.E.
Senior Engineer
cc: H. Wayne Ginn, P.E.
File 90441