HomeMy WebLinkAboutHUMAN BEAN AT SPRING CREEK - FDP - 37-06/A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTAPPENDIX
• SITE HYDROLOGY
• WATER QUALITY POND
• OPEN CHANNELS
• PIPES
• EROSION CONTROL EFFECTIVENESS CALCULATIONS
• REFERENCE MATERIAL
EROSION AND SEDMENTATION CONTROL
Temporary erosion control during site construction will be provided by straw bale barriers, a
sediment trap to be located within the water quality pond, vehicle tracking control at the access
to College Ave., and a silt fence as depicted in the attached Erosion Control Plan. Performance
Standards for the property and Effectiveness calculations for the proposed erosion control
measures are included in the Appendix. Any disturbed soil to remain bare for more than 30 days
will be seeded with a temporary seed mix of either warm season or cool season grass, depending
on the month of the year. Two acceptable alternative seed types for this temporary vegetation
are:
Seedine Season
Jan -Feb
Do not seed
Mar — May 15
Annual ryegrass, drilled at 20 lbs seed per acre
May 16 — July
Hybrid sudan grass, drilled at 15 lbs seed per acre
Aug — Sep
Annual ryegrass, drilled at 20 lbs seed per acre
Oct — Dec
Do not seed
The contractor will be required to install and maintain all erosion control features on the site and
replace any damaged elements during the construction period.
Permanent erosion control will be achieved via the water quality pond. Landscaped areas will
either be seeded with a permanent grass seed mix or covered with turf sod.
The property owner will be responsible for maintaining all the permanent drainage and landscape
features of the site.
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DRAINAGE PLAN/HYDRAULIC REPORT
The drainage plan for the proposed project is attached as a full-size drawing in the map pocket.
The plan includes construction of curb cuts to convey site and off -site runoff to the proposed
water quality pond, and HDPE pipes to convey storm water runoff from the pond to the Spring
Creek channel. Each type of hydraulic structure to be installed for site drainage is discussed
below.
WATER QUALITY DETENTION
Water quality will be provided for the Human Bean site through an Extended Detention Basin
(EDB), a recognized Best Management Practice in the Urban Storm Drainage Criteria Manual
Volume 3. The volume of the pond was determined based on 40-hour drain time and an 80
percent imperviousness for the site. The pond will treat a runoff depth of 0.33" over the 0.37-
acre project. Calculations for pond size and outlet structure design are included in the Appendix,
and outlet structure details are depicted in the Utility Plans. Storm water runoff will enter the
pond via a curb cut and riprap rundown. The riprap rundown will be grounted due to the slope of
the pond's sides, and will be comprised of Type VL (d5o = 6 inches) angular riprap.
OPEN CHANNELS
Two curb cuts will be installed to allow storm water runoff to be conveyed through the site, one
at each design point listed above. The curb cuts were designed using Mannings' equation,
assuming normal flow conditions, with a conservative n-value of 0.020. Both curb cuts were
assumed to have a 1.0 % longitudinal slope though the cut. A spreadsheet presenting these
calculations is attached in the Appendix.
PH'ES
Two high density polyethylene (HDPE) pipes will be installed to convey storm water runoff
from the water quality pond to Spring Creek. The pipes will have capacity to convey the 100-
year developed discharge from both the Human Bean and the Vision Eyeland properties (Basins
A and OS-1, respectively). The pipes were sized under non -pressure flow (as open channels)
using the UD-Culvert spreadsheets created by the Urban Drainage and Flood Control District.
These spreadsheets are attached in the Appendix, and the results are summarized in the table
below.
Pipe ID
Location
Size, Circular
Design
Pipe Capacity,
HDPE
Discharge, cfs
cfs
Pipe-1
Conveys flow from the pond east
18"
6.1
7.5
to MHl, slope = 0.6%
Pipe-2
Conveys flow from MH1 to
18"
6.1
21.1
Spring Creek, slope = 6%
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I
RUNOFF
Methodolgy. Runoff at the project was evaluated for the 2-year, 5-year, 10-year and 100-year
storm events using the rational method. The rational method relates discharge to the product of
three terms: a runoff coefficient appropriate for site conditions and the design storm, rainfall
intensity at the time of concentration, and basin area. Topography at the site was based on. 1-foot
contour intervals generated from site survey. Specific guidance provided by the City's Storm
Drainage Design Criteria and Construction Standards was followed in the estimation of the time
of concentration. Intensity was determined from the equations summarized above.
Runoff Coefficients. Runoff coefficients for application in the rational method were determined
for the site using tables provided in the City of Fort Collins Storm Drainage Design Criteria and
Construction Standards (May 1984, rev. April 1999). These coefficients are a function of the
soil type and corresponding hydrologic soil group for the site and the percent of site
imperviousness due to the expected presence of parking and drive areas, sidewalks, and rooftops.
Runoff coefficients for frequent storms used for this analysis for landscaped areas, roofs, and
paved areas were 0.25, 0.95, and 0.95, respectively. The proposed site will be 80 percent
impervious; the remaining 20 percent will be landscaped. Composite runoff calculations were
performed for the project site to determine the percent imperviousness for each basin, as depicted
in the Appendix. The table below summarizes the developed runoff coefficients for the site for
the 2-year, 5-year, 10-year, and 100-year storms.
Oita Rnnnff f neffirienta
% Impervious
2-year
5- ear
10- ear
0-year
Site, weighted
80
1 0.81
10.81
0.81
1.00
Off -site, weighted
94
1 0.93
1 0.93
0.93
1.00
Basins and Design Points. The project site, for analysis of development hydrology, was divided
into two drainage basins: the off -site basin and the project site basin, totaling 0.655 acres. A
small landscaped portion of the developed site (a strip of land a few feet wide) will drain south to
Spring Creek College. The table below summarizes the basin locations and areas.
Basin ID
Area
acres
Location
A
0.369
1 Human Bean site
OS-1
0.286
1 East half of Vision E eland property, and south half of building
Two design points was used in the hydraulic design below, as summarized in the following table.
Design Point
Hydraulic Feature
Contributing Basin(s)
1
Curb cut in parking island
OS-1
2
Curb cut to pond, south curb
OS-1 + A
k:
0.
L.
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211512007
Most of the cite is currently an asphalt parking surface. Existing vegetation includes a grass
berm with shrubs and trees next to College Avenue, and a few trees in the southeast corner of the
site.
DRAINAGE ANALYSIS
The site was analyzed to determine the depth of rainfall for storms of varying frequency and the
predicted rate of discharge associated with these storm events, both under historic and proposed
or developed conditions. The methods used are detailed below, and all calculations are included
in the Appendix to this report.
RAINFALL
Rainfall depths for various storm frequencies and durations were calculated using City of Fort
Collins rainfall design criteria and the intensity -duration -frequency relationships that correspond
to these criteria. A set of IDF curves is included in the Appendix materials.
These IDF relationships can be approximated mathematically with an equation of the form:
I = K /(10 + tc)x
Where: I = intensity in inches/hour
K = a constant which is a function of the specific IDF curve
t, = time of concentration in minutes, and
x = an exponent which is a function of the specific IDF curve
The variables K and x can be determined for any set of known IDF curves, and were calculated
for the 2, year, 5-year, 10-year and 100-year storms. The IDF equation for the 2-year storm in
Fort Collins, for example, is determined to be:
I = 23.7 /(10 + tc)0.791
The corresponding IDF equation for each of these four storm events is summarized in the
following table:
Storm Event
K
x
2-year
23.7
0.791
5-year
30.1
0.766
10-year
40.6
0.793
100- ear
82.9
0.793
These relationships allow the easy determination of rainfall intensity for these four storm events
for any basin's calculated time of concentration.
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VICINITY MAP
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OVERALL DRAINAGE CONSIDERATIONS
PROJECT DESCRIPTION
The Human Bean at Spring Creek PDP is a proposed 0.37-acre commercial development to be
located on the east side of South College Avenue adjacent to the existing Vision Eyeland clinic
at 1820 S. College Ave. Figure 1 is a vicinity map for the project. The property, which is
rectangular in shape, is situated just north of Spring Creek. The proposed project will include a
drive -through coffee shop with drive aisles, and with on -site parking also provided. Figure 2 is a
reduced version of the site plan. The project will be built in one construction phase.
MAJOR BASIN
The site is located within Fort Collins' Spring Creek Basin, which discharges into the Cache La
Poudre River approximately 3 miles east of the site.
EXISTING HYDRAULIC FACILITIES AND FLOOD HAZARD AREAS
The southwest corner of the site is located within a flood hazard area. The adjacent mapped
flood hazard area for Spring Creek is depicted on FIRM Panel 08069C0987F, Community
number 080102 (see attached reference material). The portion of the property affected by the
floodplain is located within the flood fringe, and not within the floodway. An approved
Floodplain Use Permit will be required prior to the start of any site improvements in the
floodplain, including landscape improvements.
No existing irrigation facility is located on or immediately adjacent to the project.
An existing area drain and 8" storm drain pipe capture storm water runoff and convey it to
Spring Creek. A riprap apron has been placed at the outlet of this pipe and is observed to be
stable.
EXISTING TOPOGRAPHY, SOILS AND VEGETATION
The site currently has a general north to south slope, with the area drain mentioned above located
in a sump approximately 30 feet north of the property line. Off -site runoff flows to the property
from the adjacent Vision Eyeland property. The northwest comer of the Vision Eyeland
property is extremely flat, and topographic survey indicates that it will not drain to the subject
property.
According to the Soil Survey of Larimer County, prepared by the Soil Conservation Service
(SCS, now the Natural Resources Conservation Service or NRCS), the Fort Collins loam
underlies the proposed project. The Hydrologic Soil Group for this soil is Group B,
corresponding to relatively slow runoff or relatively rapid percolation rates.
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ENGINEER CERTIFICATION OF DRAINAGE REPORT
I hereby certify that this report and attached plans for the drainage design of the Human Bean at
Spring Creek PDP was prepared by me or under my direct supervision for the owners thereof and
meets or exceeds the criteria in the City of Fort Collins Storm Drainage Design Criteria and
Construction Standards.
REG/ST l��i,,r Date
Og' °°°.,° F4
: AZ °° t
9' V,7:&�q
Registered Pro essional Engineer
W 26325 �
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x State of Colorado No.
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1
TABLE OF CONTENTS
DRAINAGE AND EROSION CONTROL REPORT.................................................................................................. 1
Tableof Contents.......................................................................................................................................................... 2
Engineer Certification of Drainage Report ................................................................................................................... 3
Overall Drainage Considerations.............................................................................................................................. 4
ProjectDescription............................................................................................................................................... 4
MajorBasin.......................................................................................................................................................... 4
Existing Hydraulic Facilities and Flood Hazard Areas......................................................................................... 4
Existing Topography, Soils and Vegetation......................................................................................................... 4
DrainageAnalysis..................................................................................................................................................... 5
Rainfall................................................................................................................................................................. 5
Runoff................................................................................................................................................................... 6
Drainage Plan/Hydraulic Report ............................................................................................................................... 7
WaterQuality Detention....................................................................................................................................... 7
OpenChannels...................................................................................................................................................... 7
Pipes...................................................................................................................................................................... 7
Erosion and Sedimentation Control.......................................................................................................................... 8
Appendix .......................
.............................. 9
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211512007
CCU
Associates
DRAINAGE AND EROSION CONTROL REPORT
For:
HUMAN BEAN AT SPRING CREEK
CITY OF FORT COLLINS, COLORADO
ON BEHALF OF APPLICANT: PIAZZA, LLC
QED ASSOCIATES
204 WALNUT STREET, SUITE C
FORT COLLINS, CO 80524
FEBRUARY 169 2007
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1 204 walnut street, suite c, fort collins, co 80524 9 office: 970.416.0600 9 fax: 970.416-0601