Riverview Estates-CS 910909'he City With A Beautiful Future
September 9, 1991
FIL COPY
P.O. Box 478
Coppell, Texas 75019
214-462-0022
Mr. C. Michael Daniel, P.E.
Nathan D. Maier Consulting Engineers, Inc.
Three Northpark
8800 N. Central Expressway, Suite 300
Dallas, Texas 75231
I~E: Riverview Estates/Storm Drainage Design Criteria
Dear Mr. Daniel=
This letter is in response to your request for clarification of the
criteria to be applied to the determination of the starting
hydraulic gradient for the storm sewer outfall of the Riverview
Estates Addition to the City of Coppell. This outfall is proposed
to discharge into the waters of the Elm Fork of the Trinity River.
As you are aware, the City of Coppell's subdivision ordinance
references the "Design Manual for Storm Drainage facilities",
published by the City of Dallas, as a general guide for design
criteria for storm drainage. The current criteria by Dallas is
that the 100-year water surface elevation of a creek, stream or
other open channel is to be calculated for the time of peak
discharge in the same storm. That water surface elevation shall be
used as the starting hydraulic gradient for the storm sewer system.
The City of Dallas updated drainage design manual (not yet adopted)
revises that criteria to allow for consideration of a coincident
peak determined from an approved flood hydrograph.
Both methods outlined above are open to a wide scale of
interpretation and are subject to a case-by-case determination.
For the Riverview Estates Addition (and for all subdivisions whose
storm sewers discharge into the waters of the Elm Fork), it is
recommended that the beginning hydraulic gradient be the 100-year
water surface elevation of Elm Fork, unless engineer documentation
is provided by the developer which confirms justification for a
lower water surface elevation not to be below the 10-year design
frequency. This information should be of sufficient detail to
allow for a determination to be made based on accepted engineering
practices. It is also recommended that the streets, alleys, or
alleys, or finished lots be at a grade as required by the
floodplain management ordinance #87390.
It is our intent to work with you on determining a reasonable
beginning hydraulic gradient for this project based upon sound
engineering practice and judgment. We look forward to working with
you on the project. If you have any questions, please do not
hesitate to call me or Gary Hendricks at (214) 363-1016.
Sincerely, ~ ~ /
Acting City Engineer
cc:
Gary Sieb, Director of Planning and Community Services
Gary Hendricks, P.E., Plan Review Engineer
msd/gch/bhj
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