Gateway BP (1.1)-CS 900131ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS · SCIENTISTS · SURVEYORS
DALLAS · ARLINGTON · CHICAGO · FORT WORTH
8616 NORTHWEST PLAZA DRIVE · DALLAS, TEXAS 75225
2141739 0094
January 31, 1990
AVO 10564
City of Coppell
255 Parkway
Coppell, Texas 75019
Attention: Mr. Ru~ell~O~o~le, P.E.
City/~tgineer
Re: Storm Drainage Criteria
for Gateway Business Park.
Dear M~. Doyle:
This letter is in response to my recent conversation with Ms. Shohre Daneshmand
of your office. The City of Coppell Storm Drainage Criteria currently states that the
criteria will generally conform to the "Storm Drainage Design Manual" Published by the
City of Dallas and that the computed rainfall intensities should be based on Technical
Paper 40.
The City of Dallas design criteria utilizes Hydro-35 for durations of 0 to 60 minutes
and TP-25 for durations greater than 60 minutes as shown on the attached Rainfall
Intensity Curve Page 2, Section IV Appendix, City of Dallas Public Works Department
Storm Drainage Design Manual Rev. Spring 1988. Also please find enclosed a comparison
of the Rainfall Intensities for Hydro 35, TP-25 and TP-40 (compared in cumulative inches).
We are requesting that the Hydro 35 criteria be utilized for durations of 0 to 60 minutes.
We are not requesting the use of TP-25 for durations greater than 1 hour and would
prefer the use of TP-40 for durations greater than 1 hour. I am enclosing a copy of the
Hydro 35 study and as you will see this study was performed to supplement the TP-40
study for durations of less than 1 hour.
TRANSPORTATION WATER RESOURCES LAND DEVELOPMENT MUNICIPAL ENVIRONMENTAL STRUCTURAL
SURVEYING GLOBAL POSITIONING SYSTEM (G PS) REMOTE SENSING AND MAPPING
LANDSCAPE ARCHITECTURE PLANNING
ALBERT H. HALFF ASSOCIATES, INC.
ENGINEERS · SCIENTISTS · SURVEYORS
This request was approved in theory for this project. The previous construction
plans for Gateway Business Park, Phase I approved May 5, 1988, reflect the modification
to the City criteria that was approved in the fall of 1987 for this development. At that
time city staff approved the use of a 0.7 runoff coefficient rather than the use of Hydro
35 intensities. The reason was that for times of concentrations between 0 and 30 minutes
the 0.7 times TP-40 intensities approximated 0.9 times Hydro 35 intensities.
As we discussed today by phone, we are proceeding with the storm sewer design
based on a runoff coefficient of 0.9 and utilizing Hydro 35 Rainfall Intensities for duration
of less than 1 hour per the City of Dallas Storm Drainage Design Manual.
Thank your for your time and consideration in this matter. I will be contacting Ms.
Daneshmand this week to set up a meeting on this project for early next week.
Very truly yours,
A! ~gERT H. HALFF ASSOCIATES, INC.
Patrick L. Acker, P.E.
CC-'
Mr. Wayland Threadgill
Santa Fe Pacific Realty Corporation
Ms. Shohre Daneshmand
City of Coppell
SHORT DURATION ($ TO 60 MINUTES)
PRECIPITATION DATA (INCFP~S) FROM U.S. WEATHER BUREAU
PUBLICATIONS TP-25, TP-40, HYDRO-35
DALLAS AREA
STORM RETURN T.P.-25 T.P.-40 HYDRO-35
DURATION PERIOD (1955)** (1961) (1977)
(MINUTES) (YEARS)
5
10
15
3O
60
2 .49 .57 .49
5 .56 .74 .58
10 .63 .85 .64
25 .69 1.00 .74
50 .76 1.06 .81
100 .83 1.23 .89
2 .77 .87 .82
5 .88 1.14 .96
10 .96 1.32 1.07
25 1.08 1.54 1.23
50 1.22 1.64 1.35
100 1.32 1.89 1.48
2 .99 1.10 1.05
5 1.18 1.44 1.23
10 1.26 1.66 1.37
25 1.40 1.94 1.57
50 1.58 2.07 1.73
100 1.75 2.39 1.88
2 1.47 1.53 1.47
5 1.77 2.00 1.80
10 1.82 2.31 2.04
25 2.20 2.70 2.38
50 2.40 2.87 2.65
100 2.60 3.32 2.91
2 1.92 1.89 1.90
5 2.39 2.52 2.39
10 2.73 2.89 2.73
25 3.20 3.40 3.21
50 3.40 3.84 3.59
100 3.80 4.25 3.97
TP-25 NUMBERS REFI.~.CT NATIONAL AVERAGE ADJUSTMENT
FACTORS IN ORDER TO COMPARE THE ANNUAL SERIES DATA
(TP-25) WITH THE PARTIAL DURATION SERIES DATA (TP-40 AND
HYDRO-35)