DR9002-SY 890731HILL DRIVE DRAINAGE STUDY
CITY OF COPPELL. TEXAS
p ELIM HAi Y
Prepared by
DeShazo. Starek & Tang,
July 1989
Inco
TABLE OF CONTENTS
1.0 Introduction ............................................................ 1
2.0 Summary of Data ....................................................... 1
3.0 Analysis ............................................................... u,
q.0 Proposed Solution ....................................................... 8
1.0 Introduction
1.1 Purpose
On March 28, 1989, the Coppell City Council approved funding for' a
study of the drainage system in the 100 block of Hill Drive. The
purpose of this report is to present the findings of that study and
recommendations for consideration by the City.
1.2 Problem Statement
The homeowners in the 100 block of Hill Drive (Exhibit A) have
reported that they experience' flooding of their garages and homes,
several times each year. The water apparently drains toward their
homes from the back or west side of their lots.
2.0 Summary of Data
2.1 Homeowners
Information regarding the nature and extent of the flooding problem
was provided by the homeowners by means of letters, photos and
meetings at their homes.
The observations of each homeowner were generally consistent with
his neighbors. A summary of their input follows.
ao
Several times each year, during heavy rains, the alley behind
their homes flows full and spills over into some driveways and
floods the garages.
b. The water in the gutter of Mockingbird (to the west of Hill Drive)
does not overflow into the alley.
c. The water ponds near the inlet in the alley but does not submerge
the inlet.
d. Water flowing east from Mockingbird in the north gutter of
Cherrybark turns north into the alley.
e. A few times each year, water ponds near the inlet on Hill Drive
between lots 4 and 5, submerging both the inlet and the sidewalk.
Water flowing south in the alley from Sandy Lake Road is not
diverted eastward in the alley between lots lU~ and 15, Much of
it continues south toward the inlet between lots ~ and 5.
Flooding is worst in lots nearest the inlet and in the lot at the
corner of Cherrybark and Hill.
2
2.2. Design Documents
Review of the design drawings for the Woodridge Subdivision, Section
Two provided the following information:
a. Runoff was calculated using the rational method. The following
drainage criteria was used for design:
C = Runoff coefficient = 0.u,5
I = Rainfall intensity = 5.5 inches/hour*
t = Time of concentration = 15 min.
*based on TP 25,' 25-year storm
The drainage area of 0.u,5 acres shown for the inlet in the alley
includes only the 20-ft. alley R.O.W. from lots 2 through lU,.
The runoff for this area was calculated to be 1.11 cfs.
c. The design slope of the alley is 0.50 percent.
d. The design invert (difference in elevation between the centerline
and the edge of the alley pavement) is 5 inches.
It was assumed that all offsite runoff from the 17 acres to the
west is contained in the storm sewer which enters the alley
R.O.W. from the west near lot 10, runs north in the center of
3
the alley, turns east down the centerline of the alley between lots
14 and 15 and connects to the northbound storm sewer in Hill
Drive.
2.3. Field Survey and Reconnaissance
A field survey was performed to determine the actual condition of the
alley. Sections were taken at each driveway, grade break and inlet.
Actual drainage divides were determined by observations where possible
and confirmed by field survey where necessary.
Observations of individual lot grading and landscaping were made in
the field and noted for possible contribution to the flooding problems.
3.0 Analysis
3.1. Design Criteria
The design criteria used for the drainage calculation in the Woodridge
Subdivision are consistent with current City of Coppell standards with
two exceptions:
Current standards call for storm sewers to be designed for the
lO0-yr storm as opposed to the 25-year storm interval used in
the original design of the subject subdivision.
The rainfall intensity curves (based on Technical Paper No. 40)
incorporated in the current City standards are more conservative
than the curves that were used for the design,
The result of these exceptions is that the intensity used in design
(5,5 inches/hr,) is 78% less than the 100-year intensity of 9.8
inches/hr, contained in the current standards.
3.2. Design
Two design elements appear to have contributed to the drainage
problem:
The offsite drainage from the west is not all contained in the
storm sewer that enters the subject alley near lot 10. Except
for the front yards, the lots along the east side of Mockingbird
Lane drain to the east into the subject alley, thereby increasing
the actual drainage area to the alley and, consequently, to the
inlet, by more than five times.
The design slope for the alley (0.5 percent) is the minimum
usually recommended for concrete pavement and is difficult to
control during construction.
5
3.3. Construction
Comparison of the field survey results and the design drawings
revealed apparent construction deviations which have contributed to
the flooding problem. These differences and their practical effects
are summarized as follows:
The slope of the alley centerline ranges from 0.3 percent to 0.8
percent rather than the constant 0.5 percent shown on the
drawings. Areas near lots q and 20 are on adverse (or uphill)
grades. These irregularities in slope have significantly reduced
the hydraulic capacity of the alley at critical locations and,
therefore, allow water to' spill over and flow down drivewayS.
(See Table A).
The invert of the alley section ranges from 1.5 inches to 5 inches.
In sections where the invert is less than the design value of 5
inches, the hydraulic capacity is further reduced complicating the
problem identified in "a" above.
The Iow point (invert) of the alley section is 6 inches to the east
of the alley centerline. The design drawings show the invert on
the centerline. This deviation further complicates the problem
identified in "a" above,
6
The drainage divides (high points} shown at lots 2 and 14 on
the drawings are not distinct enough to prevent the flow from
cresting and flowing toward the inlet in the alley.
3.4. Summary
The fact.ors contributing to the flooding problem are summarized as
follows:
a. The actual area that drains to the alley is more than 5 times the
design drainage area.
b. In some sections, the channel capacity as constructed is
significantly less than the design capacity.
~-c. The downstream storm sewer system does not appear to have
sufficient reserve capacity to accommodate the excess runoff
generated by the actual drainage area.
· ~ COMPUTATI~ SHEET
SUBJECT
p~P__
,JOE ,,, ~ I I~ PAGE
MADE BY ~ DATE
CHECKED BY __ DATE
of
5 Z+14,~ O. fig -o.~3 g,o 12,14
Proposed Solution
In addressing the factors contributing to the flooding as summarized
in Section 3.4., two factors were considered: reducing the drainage
area to the alley and increasing the alley capacity. Following is a
summary of our proposed solution. The proposed improvements are
illustrated in Exhibit A.
Reconstruct the alley at its intersection with Cherrybark to prevent
the flow in the gutter of Cherrybark from spilling over into the
alley.
Place inlets in the alley near Jots 7, 10 and lq. Connect these inlets
to the northbound storm sewer in the alley that carries the offsite
drainage from the west. The flow will thus be diverted into the
system for which it was designed.
Increase the alley capacity from lot no. 2 to lot no. 6 by
reconstructing the east half of the alley so that the east edge is
raised.
The resulting alley capacity should be adequate for 100-year storms
(See Table B).
8
By diverting approximately two-thirds of the current alley flow to the
north, the flooding of Hill Drive should be mitigated.
The probable cost for the proposed construction would be approxi-
mately $32,000. The costs are summarized in Table C.
COMPUTATI~'N SHEET
SUBJECT, JOB ~*_ PAGE__of
MADE BY DATE
CHECKED BY DATE
~e41.B e, 3~ - o.oo4 3,5 ~.~
· o~ 0,40 -o,~V 4.7 I. II
....... ~ '. ..... ~1~ '
~ ; 2;.5'1: ~ ~'~-~'
TABLE C
ENGINEERIS SUMMARY OF PROBABLE COST
9/18/89
UNIT
DESCRIPTION UNIT QUANTITY PRICE AMOUNT
Grate Inlet Each
18" RCP L.F.
6" Concrete Pavement S.Y.
Remove Concrete Pavement S.Y.
Sawcut L.F.
SUBTOTAL
25~ Contingency
3 $8,000
860 $35
330 $85
330 $5
400 $1.50
$6,000
$9,100
$8,850
$1~650
$600
$85,600
$6~400
$38,000
TOTAL~ ESTIMATED COSTS
10