HomeMy WebLinkAboutGATEWAY AT HARMONY ROAD PUD, TACO CABANA - PRELIMINARY & FINAL - 1-88H - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT I
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DRAINAGE AND EROSION CONTROL REPORT
FOR
TACO CABANA P.U.D.
THIRD FILING
FORT COLLINS, COLORADO
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PREPARED FOR:
MR. LARRY CLARK
P.I.I.
1318 PARAGON PLACE
FORT COLLINS, CO 80525
PREPARED BY:
WESTEC
2629 REDWING ROAD, SUITE 200
FORT COLLINS, COLORADO 80526
AUGUST 21, 1995
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2629 Redwing Road, Suite 200, Fort Collins, Colorado 80526-2879, Ph. 970 226 3535 Fax 970 226 6475
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Reno Tucson Nevada Arizona Nevada Nevada California Colorado
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WELSH ENGINEERING SCIENCE & rEc IaLoG',. •^.
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III August 21, 1995 WESTEC #95434
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Mr.Glen D. Schlueter
City of Fort Collins
Stormwater Utility Services
al 235 Mathews
Fort Collins, Colorado 80522
RE: TACO CABANA P.U.D.; THIRD FILING - ANAL DRAINAGE AND EROSION
II
CONTROL REPORT ; .
Ill Dear Mr. Schlueter:
III
Transmitted herewith is a copy of the subject report for your review and approval.
IIAll calculations for this report have been made in accordance with criteria established by
the City of Fort Collins.
If you have any questions or require any modifications please call me.
Thank you.
( ;— Sincerely,
WESTEC fry; rs. s..: ,.
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,, Project Engineer `�°0..01,,a„s_•;,' `'':
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. 2629 Redwing Rccd, Suite 200. Fort Collins Colorado 80526-2879, P . 970 226 3535 Fax 970 6475,
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Nevada Arizona Nevada Nevada California Ccic'cJo
DRAINAGE AND EROSION CONTROL REPORT
FOR
TACO CABANA P.U.D.
1." FORT COLLINS, COLORADO
DEVELOPMENT LOCATION AND DESCRIPTION
Location
Taco Cabana P.U.D. will construct a masonry and stucco building to serve as a
restaurant in Lot 3 of The Gateway at Harmony Road P.U.D, Third Filing, with an area
of 1.045 acres. The site is located west of College Avenue between Mason and
4.4 Kensington Streets as shown on Figure 1. The site lies in the southeast quarter of
Section 35, Township 7 North, Range 69 West of the sixth P.M., City of Fort Collins,
County of Larimer, State of Colorado.
Property Description
The site is currently undeveloped but is surrounded streets or existing developments.
s` To the east is College Avenue right-of-way (R.O.W.). A private access road forms the
west margin. To the north of the site is Jiffy Lube. McDonald's Restaurant is to the
south. The site is presently barren and slopes from the northwest to southeast at a
.'. gradient of 1 to 3 percent. Historic drainage from the site was to the south, southeast
to Mail Creek.
DRAINAGE BASIN IDENTIFICATION
• Description
The site is within the McClellands/Mail Creek Basin and drainage is carried to Mail
Creek via a 42 inch storm sewer located in the west R.O.W. of South College Avenue.
The precedent for drainage control in this area was set by the approved drainage report
for The Gateway at Harmony Road P.U.D., prepared by Stewart and Associates in
1989 [11. This plan sets the detention requirement for the property at the difference
between the allowed runoff (0.5 cubic feet per second (cfs) per acre) and 100-year
storm event. The released water shall flow to the 42" storm drainage main via an
existing storm sewer connection as detailed on the drainage study attached to this
report (Drawing 5).
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- TACO CABANA TACO CABANA P.U.D.
_ SITE LOCATION MAP
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DRAINAGE DESIGN CRITERIA
Reaulations
The City of Fort Collins Storm Drainage Design Criteria [2] is being used as the basis
for,this report.
Development Criteria Constraints1111 .
Neither off site detention nor downstream drainage capacity for the 100-year storm
event exists for Lot 3. Therefore, Lot 3 will have to detain stormwater on site and
release at 0.5 cfs per acre for storm events up to the 100-year storm event.
Hydrologic Criteria
The Rational Method was used to predict the peak and cumulative runoff from the
proposed development. Calculations are included in Appendix 1.
Calculations
All calculations for this report have been made in accordance with criteria established
- by the City of Fort Collins.
Variances
A variance is requested to allow stormwater ponding to a depth of 18" in Pond B
during the 100-year storm event. This ponding is necessary to store the volume
difference between the allowed release rate and the 100-year storm event.
toy, A variance is requested to allow for discharge of stormwater from College Avenue and
the contiguous R.O.W. without detention. This area,delineated as Basin C on Drawing
5, is below the elevation of the property and will not drain to the site.
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DRAINAGE DESIGN
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Lot 3 of The Gateway at Harmony Road P.U.D., Third Filing, will be developed under
� , this drainage report as Taco Cabana P.U.D. Drainage from Lot 3 was evaluated using
Basins A through F as shown on Drawing 5. Drainage from Basins A and E will be
conveyed to the area inlet at Point A in Pond A via overland and gutter flow across the
new parking area. Drainage from Basins B and F will be conveyed to the area inlet at
Point B in Pond B via overland and gutter flow across the new parking. area. Basin C
will flow to the south in the west gutter of College Avenue to an existing 5' curb inlet
at the corner of College and Harmony and the 42" storm main in the west R.O.W. of
College Avenue. Basin D will drain south to the McDonald's parking lot as approved
in the original Gateway at Harmony P.U.D. submittal [11.
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Ponds A and B will detain sufficient water during the 100-year storm to account for
Basins A and B. A 3.5" orifice plate restriction will be placed over the outlet pipes of
Ponds A and B to restrict the discharge. Discharge from Pond A will be conveyed to
the 42" RCP storm drainage pipe via a 129-foot section of 15" RCP as shown on
Drawing 5. Discharge from Pond B will be connected directly to the 42" storm sewer
via 40 feet of 15" RCP to the manhole located in the west R.O.W. of College within
1111
Lot 3. Drainage easements are provided for this pipe section and the inundated areas.
Storm drainage detention will occur in Ponds A and B below the 5034.50' and
5034.00' elevations, respcetively. Emergency spillways will be formed by a crest in
the paving at the east entrance to the south parking area and the southeast entrance
to the Lot 3. Storm flows in excess of the 100-year storm will flow out the east
entrance to College and flow southeast to Mail Creek.
111 A portion of Lot 2 of The Gateway at Harmony Road P.U.D., Third Filing, delineated
as Basin D in the approved Drainage and Erosion Control Report 13], flows to
McDonald's. A small area of this basin (1,700 sq. ft.) will be diverted from
McDonald's to Pond A in Lot 3. This will be replaced by Basin D, located along the
southwest side of Lot 3, (1,000 sq. ft.) which will drain south to the McDonald's
parking lot.
pDetails
Support for the following discussion will be found in the calculations in Appendix 1 and
on the drawings furnished with this report. As stated above,the Rational Method was
11 used for drainage estimates.
The site was evaluated using Basins A through F, as shown on Drawing 5,for analysis
purposes. The weighted runoff coefficients for the developed conditions of Basins A
through F are 0.826, 0.732, 0.789, 0.95, 0.95 and 0.25, respectively. The runoff C-
values were derived with runoff coefficients of 0.95 for impervious surfaces and 0.25
for pervious surfaces.The basin area summary at the beginning of Appendix 1 provides
a breakdown of the surfaces used to estimate the composite runoff coefficients.
1 Basins.A & B
The required detention capacities for Ponds A and B was established based on mass
balances using the post-development runoff coefficient for Basins A and B with the
100-year storm event and the allowable release rate of 0.216 cfs and 0.185 cfs,
respectively. Based on these estimates, the maximum storage required in Detention
Ponds A and B is 3,324 and 2,563 cubic feet (cf), respectively. Tabular
representations of the mass balances are presented in Appendix I.
The theoretical available detention volumes for Ponds A and B is 2,015 and 4,287 cf,
respectively, for a total storage volume of 6,300 cf. This exceeds the required volume
of 5,887 cf by 415 cf. The detention storage will be created by construction of two,
3.5", orifice restrictions over the outlet pipes from Ponds A and B.
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flows that the 100-year storm event will exceed the volume of the
ponds and flow south and east from the detention areas over the change in pavement
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1111 grade at elevation 5034.0 foot elevation located at the southeast entrance to Lot 3.
Excess flows will spill to College Avenue over the entrance drive.
IIDownstream Discharae Flows
Discharge from Ponds A and B will be conveyed to the 42" RCP storm drainage pipe
li via the storm drainage piping system installed under the Gateway at Harmony Road
P.U.D, Third Filling [31. Drainage from Pond A will be conveyed east, northeast to an
} existing manhole via an 8" PVC pipe at 3.6 percent grade. Drainage from Pond B will
be conveyed east, southeast to an existing manhole over the 42" RCP via a 15" RCP
PI pipe at 1.9 percent grade.
Discharge to the pipes will be restricted by the construction of an orifice plate over
each pipe. The orifice openings will be 3.5" in diameter. At maximum head conditions
of 3.5 feet for both Pond A and B, each outlet will discharge 0.602 cfs during the
100-year storm. The total discharge from Ponds A and B of 1.204 will be less than
the allowable of 1.212 for Basins A, B, E and F.
rBasin C
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Due to the small size of Lot 3 and the slope of the lot toward College Avenue,
detention for Basin C is not possible. 'The conditions in this area will not change from
IIIthe existing conditions and thus should not aggravate the situation.
{ Basins D, E & F
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Basins D, E and F are small basins to the west a north of Lot 3. Basin D will flow to
McDonald's, replacing flows in Basin E which will now go to Pond A. Off site Basins
E and F will be routed through the Detention Ponds A and B without detention. The
drainage summary table located in Appendix I provides estimates of the peak, 100-year
storm flows from these basins.
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WIND AND RAINFALL EROSION EVALUATION
Site Soils
The Larimer County Soil Conservation Service Soils Report identifies the soils as
belonging to the Nunn Clay Loam, which is described as having medium rainfall
erodibility. The rainfall erosion season as detailed in the City of Fort Collins Erosion
Control Reference Manual [2] (the Manual) is between May 1 and October 31. The
natural drainage of the site is from the northwest to east where runoff is discharged
to College Avenue. For the most part Lot 3 is barren of vegetation.
The Manual details the site as having moderate wind erodibility characteristics and
states the predominant wind direction is from the west-northwest in this area. The
wind erosion season as detailed in the Manual is between November 1st and May
31st.
1111 Construction at the site is estimated to begin in March 1996 and continue through July
1996, with the majority of the structural and grading work completed by May 1996.
This schedule•places the project within the wind and rain erosion seasons.
Performance Standard
The estimated "During Construction" and "After Construction" Performance Standard
for the site are 76.8 and 90.4 percent,respectively. For the purpose of estimating the
site's Performance Standard the entire site was evaluated as one basin using the final
development contours.
The Performance Standard for the site was estimated using the methods detailed in the
Manual and the estimated average slope length and slope for the basin. Flow lengths
and slopes developed for calculation of the "During Construction" Performance
Standard are presented on Standard Form A in Appendix II. The Performance Standard
was taken from Table 5.1 of the Manual. ;The "After Construction" Performance
Standard is calculated by dividing the "During Construction" Performance Standard by
0.85.
Practice Factor Measures
Sediment control practice measures for the site will consist of the construction of a
straw bale dike, straw bale filter inlet and silt fencing. The location of these features
are shown on Drawing 5.
Practice factor measures for the site after construction will be dismantled and,
therefore, a weighted P - Factor of 1.0 was used to calculate the effectiveness. The
calculations for the weighted P - Factor for both during and after construction cases
are presented on Standard Form B in Appendix II.
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Cover Factor Measures
II Cover factor measures for the site during construction will consist of the building pad
and the existing paving of the adjacent access roads. The building and existing paved
areas were assigned a C - Factor value of 0.01.
II • Cover factor measures for the site after construction is completed will consist of
implementation of the landscaping plan. The revegetated areas were assigned C -
Factor values of 0.01 as selected from Table 5.2 [2]. Cover factor values for the
II paved areas were not changed from those used during construction conditions. The
calculations for the weighted C - Factor for both during and after construction are
presented on Standard Form B in Appendix II.
II "Durina Construction" Effectiveness Evaluation
U The Effectiveness of the erosion control measures at the site during construction was
calculated as 78.1 percent and exceeds the "During Construction" Performance
Standard value of 76.8 percent. Calculations of the basin effectiveness value for the
site are presented on Standard Form B in Appendix II.
I II
i "After Construction" Effectiveness Evaluation
IIIThe Effectiveness of the erosion control measures at the site after construction was
calculated as 99.0 percent and exceeds the "After Construction" Performance
II Standard value of 90.4 percent. Calculations of the basin effectiveness value for the
site are presented on Standard Form B in Appendix II.
IWIND EROSION ANALYSIS
The surface soils at the site are rated as moderately wind erodible. Construction at the
II site will occur during the windy season of November to May. The site, with a total
disturbed area of less than 1 acre, falls under the exclusion from wind control
requirements. Although the site is excluded from the requirements, the following site
mifeatures will act to control wind erosion:
• The silt fencing along the,east margin.
IIIa The new curb and gutters to be placed across the site.
• The existing vertical curb, landscaping and buildings along the north and south
margins.
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11111 INSTALLATION SCHEDULE
111 Construction of Taco Cabana is proposed to start in March 1996 and to continue
during the spring with the floors, grading and paving completed by June 1996. The
approximate date of implementation of the erosion control measure during construction
are shown on the Erosion Control Chart in Appendix II. As shown on the chart, the
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silt fence will be constructed prior to overlot grading. Permanent landscaping will be
1111 placed in the late spring. The silt fence will be left in place until landscaping is
complete.
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EROSION CONTROL SECURITY
The erosion control security requirement for the site was estimated based on the
specification of Section 2.0 of The Manual. The cost of the erosion control measures
at the site as detailed above were estimated to cost $800 with a security requirement
of $1,210. The estimated cost to revegetate the disturbed areas with a dryland grass
mix according to the City's 1993 mulch and revegetation bid was $1,300 with a
security requirement of $1,663. The larger amount is required prior to construction.
Both cost estimates are detailed in Appendix II.
•
REFERENCES
1. Stewart & Associates, INC., Final Drainage_Report, The Gateway at Harmony Road
P.U.D., May 1, 1989.
2. City of Fort Collins, Colorado, Storm Drainage Desian Criteria and Construction
Standards, May 1984, Revised January 1991 .
2. WESTEC, Fort Collins, Colorado, Drainage.and Erosion Control Report For the Gateway
' at Harmony Road P.U.D., Third Filling, July 19, 1995.
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