DR9001-CS 890517 GINN, INC.
May 17, 1989
Mr. Russell Doyle, P.E.
city Engineer
city of Coppell
P.O. Box 478
Coppell, Texas 75019
Re: Denton Tap Road Improvements
CONSULTING ENGINEERS
from Sandy Lake Road to Denton
Creek - Handwritten Memorandum dated 5-3-89.
Dear Mr. Doyle:
This letter is in response to your Handwritten Memorandum dated
May 3, 1989, copy attached. During the recent rains resulting
from heavy thunderstorms our resident project representative
observed the lane flooding situation in the vicinity of the storm
water inlets along the west curb line at project stations 176+50
and 177+80. Based on the field reports we have received and your
verbal and written inquiries, we have been studying the situation
in search of both long and short term solutions to the lane
flooding recently experienced. We will address the specific
issues raised in your Memorandum, and then cover the critical
elements of the potential solutions identified.
As you are aware, intense rainfalls are usually associated with
the frequent heavy thunderstorms typically experienced in this
locality at this time of year. Street flooding occurs both
during the rainfall and until the resulting significant run-off
subsides. This lane flooding cannot practically be eliminated in
most cases. Typically, as in the city of Coppell, the enclosed
storm sewer systems are designed to handle the stormwater run-off
from a 100-year storm and the on-grade ink, ets are designed to
remove stormwater run-off generated from a(g~-f~st6~ from the
gutters. During more intense storms th~-~Le~C~apacity is
temporarily exceeded and storm water is allowed to pond in the
designated traffic lanes for the duration of the event which
produces stormwater run-off in excess of the design capacity of
the inlets.
The subject roadway has been designed in accordance with the City
of Coppell Standard Construction Details, and as such has a
standard cross slope of 1/4" per foot in the area of concern.
The longitudinal slope (gutter slope) is essentially 0.5%. It
should be noted that a grade change occurs at station 176+50, the
location of the more southerly of the two inlets in question.
This particular inlet was located at the grade change in order to
remove as much gutter flow as possible before it reached the
lower gutter capacity of the paving section immediately downslope
of the grade change.
17103 Preston Road · Suite 100 · kB 118 · Dallas. Texas 75248 · Phone 214/248-4900
The current problem appears to be related to the fact that
portions of the adjacent properties which were not originally
included in the drainage area for this project may have been
recently graded in such a fashion that they now contribute run-
off to this project. Additionally it appears that the run-off
flow is concentrated in a couple of areas, rather than being a
sheet flow condition for which the storm drainage system was
originally designed. Compounding the above, the slopes behind
the property line of the adjacent property cause the run-off to
flow further out into the roadway than originally anticipated.
The long term solution to this problem is related to the fact
that the adjacent property in the area of concern is currently
undeveloped. In an~t~cipat~on of the ultimate development of the
p~operty~ a ~t~-= sewcr~la~eral stubout (lateral D-13a) hasten
p__ar~o.vided from e~ m~ Denton Tap Road ~v.m ~_ewer syste~"as ~
rt of the original project design, to provide for the developed
storm water run-off ~r~m ~h~ ~jacent pr~er~v. ±n or, er ~o
~ccompiisn =nis"long term solution, the adjacent property owners
must design and build an onsite extension of the existing storm
drainage system as a part of the development of their properties.
This morning we were notified of the closure of Denton Tap Road
between Sandy lake Road and Parkway Blvd. as a result of the
recurrence of the lane flooding due to the heavy thunderstorms
experienced last night and this morning. After traveling to the
site this morning and observing the lane flooding situation, we
directed the Contractor to create openings at the rear of the
inlet at station 176+50 and grade the slope immediately behind
the inlet to the bottom of the openings. We also had the
Contractor deposit excavated materials in such a way as to
collect the run-off at the property line and funnel it into the
back opening of the inlet. The work and street cleanup was
completed by 2:30 P.M. this afternoon and the road opened to
traffic at that time.
If the above method of temporarily controlling the run-off
discharges from the adjacent property is successful we may have a
concrete riprap flume constructed at the rear of the inlet as a
temporary measure until the property develops.
Please call me if you have any questions.
Sincerely,
John C. Karlsruher, P.E.
Project Engineer
attachment
cc:
H. Wayne Ginn, P.E.
Randy Fleming
File 379 (A:DENFLOOD)