Gateway BP (1.1)-CS 900223~b~ru~ary 23, 1990
Mr. Pat Acker
Albert H. Halff Associate,
8616 Northwest Plaza Drive
Dallas, Texas 75225
:~ P.O. Box 478
Future
Inc.
RE: Storm Drainage Criteria- Hydro 35
TO & Circulate otf
RRD
LD
'SD
BDC
BCB
~ FILE
exas 75019
)022
Dear Pat:
This is in response to your inquiry of January 31, 1990, regarding
use of the N.O.A.A. Hydro - 35 data for durations from 0 to 60
minutes in place of technical paper 40 rainfall data. We have
consulted Ron Morrison on this matter as well as had several
meetings in house.
Our floodplain ordinance specifically requests the use of TP 40.
Since the rainfall amounts from TP 40 are generally higher than
those of Hydro 35 this adds an additional factor of safety to our
designs.
In order to revise our ordinance (or provide a variance)to allow
the use of Hydro 35 data, we must investigate the impact as it
relates to our other ordinances and design criteria. While we
recognize that the Hydro 35 data represents updated information
from N.O.A.A. and as we understand is used by other jurisdictions
in the metroplex, we still must be careful about making changes to
long standing established criteria, especially with the historical
record of flooding in Coppell and the metroplex.
Due to our small staff limitations, we haven't the time to properly
investigate this matter for a rational and sufficiently responsible
manner. Therefore, we must ask you the requestor to furnish the
necessary information in a reduce and condensed form for review
and recommendation to the City Council, etc.
Please provide at least the following:
A survey of at least six cities (if available) in the
metroplex that accepts this criteria, provide copies of their
requirements, documenting when approved etc. Provide their
experience and success.
Page 2 of 2
Provide a comparative evaluation between the two methods
providing not only the rainfalls as you have already
submitted, but also provide a schedule or list of varying size
drainage basins (e.g. 1/4, 1/2, 3/4, 1, 2, 3, 4, 5, & 6 acres)
with related flows and pipe or culvert sizes at minimum and
maximum slopes.
When we have this kind of information, then we can evaluate and
make recommendations, etc. As you know, while we are sensitive to
your client's needs for pending development, we can only respond
in this manner to make proper, sufficient, and responsible
decisions for the City of Coppell.
We appreciate your continued cooperation on this matter and look
forward to you providing the information, as that will permit a
quick, efficient, and proper decision on this request. Thank you.
~e~;g~/ P.E.
RRD / bb
xc:
Shohre Daneshmand, P.E., Civil Engineer
Don Davis, Consulting Engineers
Ron Morrison, Kimley-Horn Associates
Wayland Threadgill, Santa Fe Pacific Realty Corp.