Whispering Hills-SY 890714 PRELIMINARY ENGINEERING STUDY AND REPORT
FOR
DRAINAGE PROBLEMS ALONG FIELDCREST LOOP AND WHISPERING
HILLS DRIVE IN THE WHISPERING HILLS SUBDIVISION IN THE
CITY OF COPPELL, TEXAS.
JULY 14, 1989
TABLE OF CONTENTS
I. GENERAL
II. DATA COLLECTION/FIELD INVESTIGATION
III. EXISTING CONDITIONS
IV. PRELIMINARY DESIGN
V. COST ESTIMATES
VI. SUMMARY
VII. RECOMMENDATIONS
VIII. APPENDIX A
· r. GENERAL
This Preliminary Engineering Study and Report will provide an
evaluation of the storm drainage problems along Fieldcrest Loop
and Whispering Hills Drive located in the Whispering Hills
subdivision of the City of Coppell.
This report will evaluate the existing system's capacity to
handle the storm water run-off, provide analysis and make
recommendations for improvements, along with preliminary cost
estimates for engineering and construction.
D~TA COLLECTION/FIELD INVESTIGATION
The City of Coppell provided "as-built" development plans, which
included street paving plans and profiles, water, sanitary, and
storm sewer plans and profiles of the Whispering Hills
Subdivision. Individual lot grading or site plans were not
available.
Additionally, a cursory field investigation was made to determine
the existing conditions and to identify possible causes for the
problems. No field surveys were made nor were any detailed
measurements taken.
Page 1
III. EXISTING CONDITIONS
Whispering Hills Drive is a 38' wide concrete street with no curb
and gutter, situated within a 50' right-of-way having 10'
drainage easements on each side. Fieldcrest Loop and Fieldcrest
Circle have 28' wide concrete pavement located within a 40'
right-of-way and 10' drainage easements on each side. Both
streets, constructed without curb and gutter, were designed with
a 5" crown to slope to the drainage ditches/swales on either side
of the street, with the ditches/swales being parallel to the
street, conveying stormwater run-off to inlets or culverts.
The drainage areas are shown in Appendix A, along with typical
sections for the streets and ditches as they were designed.
Drainage criteria used in the original design indicates the
ditches and culverts were designed for 100-year storm frequency,
utilizing Technical Paper 25. Total flow for the drainage area
was calculated to be approximately 17 c.f.s. The depth of the
ditches ranged from 6" deep at the upper end to 3' deep at the
lower end, with 3:1 side slopes. The capacity of the ditches, as
designed, is adequate for the anticipated flows.
During heavy rains, the ditches/swales, as they currently exist,
in combination with the storm drainage system do not appear to
adequately handle the run-off. At times of low flow, the
ditches/swales are too flat to handle the overland flows thus
allowing ponding of water to occur, creating a potential safety
and health hazard.
Page 2
The major problem appears to occur beginning at the area near 219
Fieldcrest Loop, going in an easterly direction towards the curve
in the road, then northerly to the 18" RCP culvert located under
the road, tying into the drainage ditch, situated between 128
and 124 Fieldcrest Loop. Most of the reported flooding problems
seem to exist in the curved portion of Fieldcrest Loop on either
side of the existing street.
IV. EVaLUaTION OF "~S-BUILT" CONDITIONS
The following is an evaluation of the entire site under the
existing conditions, some of which may, or may not, be
contributing factors to the problems that are occurring.
1. Drainage inlets at the intersection of Fieldcrest Loop
(northernmost entry) and Whispering Hills have smaller openings
than present standards dictate. Flowlines of the openings are
located higher than existing surrounding ground, in some places,
thereby preventing overland flow from entering the inlets at the
lower stormwater flows.
2. The existing ditches/swales in yards do not allow positive
flow of surface run-off due to a number of factors such as: the
ditches/swales are filled in with debris, grass clippings, fill
material etc.; PVC culverts placed under walks and drives are
undersized, and in some instances the culverts placed
incorrectly, thereby preventing positive drainage.
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3. The culverts under the walks and drives have debris, silt
and trash in them which prevents the maximum allowable flows to
be maintained.
4. Small PVC drain pipes, which are open on one end, but
clogged on the other end, contributes to ponding water.
5. On the upper end of the culvert under Fieldcrest Loop, just
east of Fieldcrest Circle, debris, grass clippings, silt, trash,
etc. appears to prevent uniform flow into the culvert and
ultimately downstream. The culvert under the road appears to be
in good condition for conveyance of water that does enter the
system.
6. At 136 Fieldcrest, the ditch/swale has been filled in, but
on homes either side of #136, the ditch/swale exists. The pipe
may have been inadequately sized to handle any appreciable flow,
laid on too flat of a grade, or even on a reverse slope, thereby
creating backup, ponding, etc. on both ends.
7. The drive and 12" CMP culvert located between 203 Fieldcrest
and 207 Fieldcrest appear to be higher than adjoining ditch
flowline, thereby preventing positive drainage.
8. From 223 Fieldcrest, east, to 207 Fieldcrest, the swale,
which is supposed to convey overland run-off, is
indistinguishable creating a condition where it is unable to
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convey the amount of run-off it may have been designed to carry.
9. The ditch along the north side of the south entry of
Fieldcrest Drive, immediately east of intersection with
Whispering Hills is unkempt, with tall grass, weeds, debris, etc.
in the ditch/swale. Grass clippings are being dumped into this
ditch/swale causing it to become clogged and preventing it from
draining properly.
10. The culvert under Whispering Hills, south of the southern
entry to Fieldcrest Loop, appears to be functioning inadequately.
The area to the east of Whispering Hills, where the lots of
Whispering Hills Addition back up to Shadowridge I, has a
utility easement along the back lot lines of each subdivision,
where stormwater run-off naturally flows. It appears fences have
been built back to back in these locations. This condition may
prevent adequate positive drainage flows into the culvert.
11. There is a drainage easement west of Whispering Hills that
has a relatively flat slope that needs to be cleared of debris,
grass, etc. to properly function.
V. PRELIMINARY DESIGN
Using the latest design criteria as determined by the city of
Coppell, trying to minimize disruption to home owners in the area
and allowing for the most economical and feasible solution, the
Page 5
following alternatives are proposed to alleviate some of the
problems.
Drainage criteria utilized in evaluations of alternate solutions
are 100-year storm frequency, and Technical Paper 40. Total flow
for the area is calculated to be approximately 30 c.f.s.
ALTERNATE 1. Remove all existing undersized culverts under
drives and walks, as required, and replace them with minimum 12"
diameter RCP culvert, up to an 18" diameter RCP culvert. Re-
grade the existing ditches/swales to provide positive flow. The
depth of the ditches would vary from 6" at the upper end to
approximately 2.75' at the lower end. Average depth required to
handle the anticipated flow would be 2.0', with 3:1 side
slopes. Remove and replace concrete walks and drives as required.
Construction of this alternative should handle the calculated
run-off stated above and should minimize any future damage due to
flooding. This is the most cost effective alternative and least
disruptive to homeowners.
This alternative represents an expenditure of approximately
$3,000.00 per lot adjacent to the proposed improvements.
ALTERNATE 2. Provide an underground drainage system where
ditches/swales presently exist, utilizing a combination of inlets
and storm drain pipes, and minor re-grading of existing yards.
Page 6
This alternative presents a problem in that some homes are
situated with the finished floor elevation below the street
pavement and consequently a swale in the front yard of some of
the homes will still have to be maintained. Additionally, a
continuous underground drain system requires access for the
overland flows which still occur from the street and from the
houses. As the lots are presently sited and graded, the existing
swales are the natural drainage courses. This would mean that
the construction of storm inlets, or grate inlets, in the front
yards, would have to be utilized.
This alternative would provide the same degree of protection from
localized flooding as Alternate 1. Disruption to existing lawns
and homeowners would be more extensive.
This alternative represents an expenditure of approximately
$7,500.00 per lot adjacent to the proposed improvements.
AL?ER~ATE 3. The most drastic and costly alternative would be
to totally reconstruct the existing street to present city
standards with curb and gutter, inlets and a storm drainage
system.
This alternative represents an expenditure of approximately
$13,500.00 per lot adjacent to the proposed improvements.
Page 7
COST ESTIMATES
Cost estimates provided herein are for construction of alternates
as outlined above. Costs do not include construction inspection
or testing. These are estimates only, based upon information
developed in this study, and the latest unit prices available to
us. They are not to be construed as "bid prices". Estimates
were developed from unit prices of recent projects in the City of
Coppell. Due to nature of the work, prices could vary by as much
as 15%-20% in either direction.
Costs include, but are not limited to: re-grading of
ditches/swales; installation of storm piping; inlet
modifications, if required; connections to existing culverts or
inlets; saw and remove, and replace concrete drives and walks
where applicable; water meter relocation; water valve relocation;
sprinkler repair; landscape replacement; solid sod replacement;
storm sewer pipes; water line relocation; and, all appurtenances
for a complete project.
Page 8
ALTERNATE NO. I
SURVEYING .............................. $ 2,250.00
CONSTRUCTION COSTS ..................... $ 40,000.00
CONTINGENCIES (15%) .................... $ 6,000.00
ENGINEERING ............................ $ 8,000.00
INSPECTION/TESTING ..................... $ 5,000.00
TOTAL ................................ $ 61,250.00
ALTERNATE NO. 2
SURVEYING .............................. $ 3,000.00
CONSTRUCTION COSTS ..................... $ 80,000.00
CONTINGENCIES (15%) .................... $ 12,000.00
ENGINEERING ............................ $ 16,000.00
INSPECTION/TESTING ..................... $ 9,000.00
TOTAL ................................ $120,000.00
ALTERN&TE NO. 3
SURVEYING .............................. $ 5,300.00
CONSTRUCTION COSTS ..................... $305,000.00
CONTINGENCIES (15%) .................... $ 45,700.00
ENGINEERING ............................ $ 28,000.00
INSPECTION/TESTING ..................... $ 21,000.00
TOTAL ................................ $405,000.00
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SUMMARY
Since field investigations were made during "dry" weather
conditions and may not have revealed all problems, either actual
or potential, a homeowner's survey should be made to determine
exact extent of damages due to flooding. The survey should
ascertain potential or actual problems that exist at the present,
and are contributory to the current problem. This survey would
be in addition to field investigations and would be most helpful
in the final design stage so as to maximize the utilization of
the proposed recommendations. A sample form for the survey is
included in Appendix A.
VIII. RECOMMENDATIONS
Due to limitations caused by existing street layout and type of
construction, existence of homes on lots, existing yard grading,
landscaping, etc., in our opinion, Alternate 1 would provide a
adequate measure of safety against flooding of property during
most minor storms, with the least amount of disruption to the
homeowners, and at the most economical cost.
Should a major storm, or storms of greater duration or intensity
occur, it must be recognized and emphasized that the storm water
run-off system proposed cannot be expected to prevent
flooding, inconvenience and minor property damage.
Page 10
It must also be understood that providing protection against the
worst storm of record does not guarantee that a greater run-off
event will not occur during the useful life of the property.
It is our recommendation that Alternate 1 be constructed when
funds become available.
Page 11
APPENDIX A
Page 12
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