HomeMy WebLinkAboutDrainage Reports - 11/05/1993STORMWATER REVIEW COPY
WEIDMANN ENGINEERING AREA CODE 303 /r` `'•:. 18 ASH COURT #6
CONSULTING ENGINEERS FinW Approved �r� n6-a885 ` '; LONGMONT, CGLORADO 80501
VICTOR H. WEIOMANN. P E..B L.S. PRINCIPAL I z /y3 \Jy
August 19, 1993 Revised October 28, 1993
3
SL)lot^:
Chris and Sandy Garcia
4470 South Lem=_y Avenue, #211
Fort Collins, Colorado 80525
Dear Chris and Sandy Garcia:
Drainage and Erosion Control Report
7400 South Lemay Avenue
Lot 2, Emerson Acres Subdivision
Sec. 18, T.6 N., R.68 W., 61,P.M.
City of Fort Collins
Larimer County, Colorado
This letter and the attached copy of a plan drawing (1111 x 1711)
dated 93-07-19, revised 93-08-18, is my report of a drainage and erosion
control study on the above described site. This report supersedes my
Drainage Report of July 19, 1993. Emerson Acres Subdivision is considered
a special condition (exception) regarding standard provisions under City
of Fort Collins development and building regulations and this report will
not contain all the usual details for such submittals.
The basic purpose of this report is to show the proposed improve-
ments to (on) Lot 2, to develope a final composite Rational Method coeff-
icient 11C11 for Lot 2, and to set out a procedure for soil erosion control
during and after construction.
The land lies in the newly developing.southeast portion of Fort
Collins. The subdivision sits at the northeast corner of the intersection
of Windsor Road (County Road 32) and Lemay Avenue (County Road 13). Lot
2 is generally rectangular in shape with it's long axis running east -west
and contains approximately 5 acres. It is bounded on the west by Lemay
Avenue and on the east by Stanton Creek.
Stanton Creek flows northerly to Fossil Creek and is part of the
Fossil Creek drainage basin. A tributary to Stanton Creek flowing from
west to east lies along the border between Emerson Acres Subdivision Lots
3 and 4 south of Lot 2. In accordance with the, "Fossil Creek Master
Drainageway Planning Study", no detention is required (nor is any considered)
for Lot 2. The 100 year event water surface elevation in this area is
considered to be about 4930. (City records show elevation 4928.29.)
An east -west ridge crosses Lemay Avenue about 1,000 feet north
of the north -south center of Lot 2. Drainage runoff along Lemay Avenue
remains .on each side of the road flowing south passing the center of Lot
Chris and Sandy Garcia Weidmann Engineering
Drainage and Erosion Control Report Page 2 of 4
Lot 2, Emerson Acres August 19, 1993
Rev. Oct. 28, 1993
2 and continuing about another 400 feet to the easterly flowing tributary
to Stanton Creek mentioned earlier in this report. A roadside ditch
carries the Lemay waters from the street center to the top of the east
bank of the roadside ditch which is about 35 feet east of the road center.
Runoff on the'lot itself is primarily in a west to east direction from
the east side of Lemay Avenue to Stanton Creek. Some waters in their
easterly travel can be expected to flow back and forth across the Lot 2
north and south lines. Some waters flow south toward the tributary onto
Lot 3 from the high point on the west end of Lot 2 and this condition
will not change following construction.
The -land appears to have been used as agricultural dry land pas-
ture. The soil surface is considered loam.and lies fallow with a fairly
uniform stan� of dry land grass cover. The City of Fort Collins Stormwater
Utility clasjfies the site as moderate regarding wind and rainfall erosion
susceptibility. The surface slope down to the east runs mostly around 3.5%.
The proposed improvements are a single family residence set back
about 150 feet from the Lemay Avenue right-of-way and about in the north -
south center of the lot. Improvements will include the house, a temporary
culvert in.the Lemay Avenue east roadside ditch, a gravel drive, a con-
crete pad and a concrete apron, and lawn surrounding the house. The remain-
der of the lot will be left fallow. The lawn surrounding the house is
expected to be about 175' x 135' with an 85' x 100' easterly extension
on it's southerly end to accomodate view and drainage from the walk out
portion of the basement. The east limits of construction are expected to
be about 200' west of the west 100 year floodplain limit of Stanton Creek.
For this report for drainage purposes the area considered is all
of Lot 2 plus 50 feet of the right-of-way of Lemay Avenue along the lot
frontage. For erosion purposes only those areas proposed to be disturbed
on the lot and within the Lemay right-of-way are considered.
The front center of the house crosses contour (elevation) 4946,
the top of the.foundation is considered at 4949.51 the main finished
floor assumed to be 4950.5, and the basement floor elevation is assumed
to be 4941.5. From the walk out basement door the ground is proposed to
be graded down to the east at a slope of 2% to intersect (daylight) the
existing ground slope of approximately 3.5%. Minimum finished grade slopes
around the house are assumed to be 2% and maximum grade slopes at 4:1.
The following components are used to determine the composite "C":
Concrete, asphalt, roof 8,066 s.f. Cc.? C = 0.95
Gravel 5,138 s.f. Ca C = 0.50
Lawn, heavy soil, 2-7% 26,087 s.f. r C = 0.25
Fallow (pasture grass) 1979511 s.f. CE C = 0.20
This results in composite !'C" for Lot 2 = 0.238
The City intends to improve Lemay Avenue by widening and adding
Chris and Sandy Garcia
Drainage and Erosion Control.Report
Lot 2, Emerson Acres
Weidmann Engineering
Page 3 of 4
August 19, 1993
Rev. Oct. 28, 1993
curb, gutter, and storm sewer as needed. A temporary culvert is necessary
in the east roadside ditch under the drive at this time. A 12" culvert
with crojecting ends will be more than adequate to carry the runoff from
a s,_,o year event. The City's minimum size of 15" is proposed.
The total area proposed to be disturbed (concrete, house, gravel,
lawn) is 35,132 square feet which is less than one acre and thereby ex-
cluded from specific wind erosion control measure by Section 8.1.1.1.2,
Design Criteria, of the City of Fort Collins, "Storm Drainage Design Cri-
teria_ and Construction Standards", revised March 1991. Provisions for
rainfall erosion control shall be more than adequate to control any wind
erosion within the standard tolerance.
For construction it is assumed that work will be limited to the
access drive plus the area outlined as lawn. It is expected that all to
within this area be stripped, stockpiled, and brought back as surface
to achieve finished grade. The 175' x 135' portion of lawn is expected to
be for construction activities and material storage. The 85' x 100' exten-
sion is considered to be constructed and finished all as part of the init-
ial grading procedure.
It is not expected that any sheet flow carrying dislodged soil
particles will carry them offsite to the creek. Rivulets may. Rainfall
erosion control from the 175' x 135' construction area is proposed to be
as on an as needed basis. Whether initially caused by natural gradients
or tire tracks any rivulets observed to be carrying sediment beyond the
175' x 135' limits shall be filtered by the structural control method of
installing staked bales of hay or straw across.the rivulet. A supply of
these bales is expected to be kept on site during the construction phase.
This method is preferred over a specific structural plan that might not
fit the construction schedule as it developes. Correction measures are
proposed to be taken whether such silt carrying waters are moving on to
undisturbed fallow areas or the 85' x 100' lawn area.
The 85' x 100' lawn extension to the east is proposed to be
brought to subgrade during initial.grading and then brought to finished
grade with topsoil, seeded, and covered with.an.anchored mat or blanket.
(Hay or straw mulch may be substituted for the mat or blanket.) The seed
is proposed to be a sod forming dryland grass mix, with preference to nat-
ive species, placed at a double rate over City minimum recommendations
in two passes of the drill (if one can be used) 901 from each other in
direction of travel (N.E. to S.W. and N.W. to S.E.).
The 85' x 100' piece has an area of 0.20 acres, a slope of 2%,
and a .length of 100' along the slope in an east -west direction. The piece
has a Performance Standard (PS) of 78.4% for construction and 92.2% for
after construction. The use of the mat or blanket results in an Effective-
ness of Erosion Control Plan (EFF) of 90% which exceeds the required 78.4%.
Chris and Sandy Garcia Weidmann Engineering
Drainage and Erosion Control Report Page 4 of 4
Lot 2, Emerson Acres August 19, 1993
Rev..Oct. 28, 1993
(Mulch would be more effective than the mat or blanket.) While the final
sod would result in an EFF = 99% the initial grass even if only having
about 505a' cover would meet the required EFF = 92.2%.
Thank you.
Sincerely,
WEIDINANN ENGINEERING
Victor H. Weidmann
Att=ch• Site Layout Exhibit (1) 0;1"'rr
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