HomeMy WebLinkAboutALPINE BUSINESS PARK - PDP - 2-07 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTEROSION AND SEDIMENTATION CONTROL
Temporary erosion control during site construction will be provided by straw bale barriers,
sediment traps to be located within the detention ponds, and a silt fence as depicted in the
attached Erosion Control Plan. 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. Performance Standards for the property and Effectiveness calculations for the
proposed erosion control measures are included in the appendix.
Permanent erosion control will be achieved via the detention and water quality ponds, designed
as described above to be Extended Detention Basins, a Best Management Practice according to
the USDCM. Landscaped areas will either be seeded with a permanent grass seed mix or
covered with turf sod.
The Association to be created for the proposed project will be responsible for maintaining all the
drainage, detention and landscape features on the site.
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PIPES
Four pipes were sized for the project using the UD-Culvert spreadsheets created by the Urban
Drainage and Flood Control District to convey developed 100-year discharges on the site, or
historic 2-year discharges from the site. This approach was based on the assumption of open
channel flow in these pipes, a reasonable assumption given flow depths in the pipes and pipe
lengths. These spreadsheets are attached in the appendix, and the results are summarized in the
table below.
Pipe ID
Location
Size & Material
Pipe-1
Into the west end of the Northeast Pond
15" ADS
Pipe-2
Into the south end of the Northeast Pond
15" ADS
Pipe-3
From the east end of the Northeast Pond to ex. storm drain
18"RCP
Pipe-4
From the west end of the Southwest Pond to ex. grade
18" RCP
DESIGN FOR FLOODPLAIN CONSIDERATIONS
The site is shown to be currently within the Poudre River and Dry Creek floodplains. As
depicted on the attached drainage plan, the elevation of the base flood is approximately 4968'
(NAVD 1988 datum; 4965' NGVD 1929 datum) in the center of the property. The base flood
elevation is 4968.5' (NAVD 1988 datum; 4965.5' NGVD 1929 datum) at the western edge of the
property. The applicant will submit for a Conditional Letter of Map Amendment (CLOMA)
through the Federal Emergency Management Agency. The CLOMA will note that the proposed
finished floor elevations for both the automobile repair facility and the commercial flex -use
building will be at 4967.8' (NGVD 1929 datum; 4970.8' NAVD 1988 datum). This elevation is
approximately 2.3 feet above the regulatory base flood elevation.
DRAINAGE DESIGN FOR ALPINE STREET
Design of Alpine Street is included with the PDP application for the project site. It is expected
that the full width of Alpine Street will be built out to the west edge of the property (not
including the -sidewalk on the north side). The design of Alpine Street was extended beyond the
property, which roughly coincides to the point at which Alpine turns north into Mason Street,
northward to Pinon Street. A sump will be located in Alpine Street just west of College Ave to
capture runoff adjacent to the property. A 5-foot Type R inlet will be installed at this sump
location, one on either side of the street. The south inlet is discussed above. The north inlet will
replace an existing inlet at the northwest corner of the intersection of Alpine and College. An
additional sump is proposed in Alpine Street north of the property (not to be constructed with
this project). This sump will ultimately connect to proposed regional drainage infrastructure
depicted in the North College Drainage Improvements Design prepared by Ayres Associates.
Property north of the site will drain to the west across the existing Mason Street during the
interim condition, as currently occurs. When Mason Street is built to the ultimate section,
drainage of this north neighbor's property will be ultimately addressed.
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Pond ID
Required detention
Water quality
Total required
volume (ac-ft)
capture volume
pond volume (ac-ft)
(ac-ft)
Northeast
0.1733
0.0321
0.2054
Southwest
0.0238
0.0041
0.0280
Water Quality Pond. Water quality detention for the site is to be provided with Extended
Detention Basins (EDBs), included as a best management practice in Volume 3 of the Urban
Storm Drainage Criteria Manual. Pond volumes, calculated in a spreadsheet in the appendices,
were determined from the following relationship:
WQCv = I.2(Area)(0.91 P —1.I9P + 0.781) / 12
Where:
WQCV = Water Quality Capture Volume, acre-feet
Area = Drainage area contributing to the pond, acres
I = Imperviousness in percent
Approximately 50 percent and 73 percent respectively of the southwest and northeast portions of
the site will be impervious, i.e., under rooftops, concrete or asphalt. The remainder of the site
will be landscaped.
INLETS
Two inlets were proposed as part of the drainage plan. An area inlet will be located in the
landscaped area just north of the commercial flex -use building. This inlet will capture only
runoff from this landscaped area; roof runoff will be routed directly to the pipe downstream of
the inlet. A 24" area inlet, Nyoplast or equivalent, is specified on the attached drainage plan.
Additionally, two Type-R inlets were designed using the Urban Storm Drainage spreadsheet UD-
Inlet, one for either side of Alpine Street. The south portion of Alpine Street to be constructed
adjacent to the property was analyzed in the attached spreadsheet; the north portion will have a
similar drainage basin size and a similar inlet opening. Five-foot openings will be acceptable to
meet both minor and major storm runoff criteria as presented in the City's Storm Drainage
Design Criteria and Construction Standards for a commercial collector road. This south side
inlet will direct adjacent street runoff to the Northeast Pond. The design spreadsheet is attached
in the appendix.
OPEN CHANNELS
Four curb cuts (two in series in Basin B-2 near the site's entrance off Alpine) will allow surface
runoff to flow to the Northeast Pond. Each was designed as an open channel, using Mannings'
equation, assuming normal flow conditions, with an n-value of 0.020. All four curb cuts will
need to be two feet wide in order to allow the flow depth not to exceed the curb height. A
spreadsheet is attached with the appendix materials.
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Design Point
Hvdraulic Feature
Contributing Basin(s)
1
Type R Inlet
A
2
Area Inlet, ADS pipe
B-1
3
Two curb cuts
B-2
4
Curb cut
B-3
5
Curb cut, ADS pipe
B-4
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 a two detention ponds to control flood releases from the site,
with water quality capture volume included in the ponds to treat development runoff prior to
release. Curb cuts will convey storm water runoff from the parking lot to the Northeast Pond, an
area inlet and pipe will convey runoff from the north half of the commercial flex -use building to
the Northeast Pond, two Type R inlets will be installed in Alpine Street, and a pipe will convey
runoff from the drive aisle south of the redeveloped automobile service building to the Northeast
Pond. Each type of hydraulic structure is discussed below.
DETENTION AND WATER QUALITY PONDS
Detention Ponds. Storm water runoff will exit the developed property at two locations: 1) from
the Northeast Pond through an outlet structure and pipe to connect to an existing storm drain in
North College Avenue; and 2) from the Southwest Pond through an outlet structure and pipe to
existing grade at the property boundary. The City of Fort Collins Storm Drainage Design
Criteria and Construction Standards requires 100-year developed flows to be detained and
released at 2-year historic rates. For the project site, the combined historic 2-year discharge
from the three historic basins was estimated to be 2.0 cfs: 1.4 cfs to the west from Basin H-1, 0.5
cfs to the east from Basin H-2, and 0.1 cfs to the east from Basin H-3.
The site has excess available area for detention storage in its northeast corner; along its west
boundary the site is best developed with the proposed commercial flex -use building. Thus, the
total allowable site 2-year historic release rate was re -allocated to take advantage of the northeast
corner's available space. The Northeast Pond was designed to release at 1.7 cfs, and the
Southwest Pond at 0.3 cfs. It should be noted that the 100-year historic release from the
combination of Basins H-2 and H-3 is estimated at 2.5 cfs; thus, the property's combined
releases will be equal to the aggregate historic 2-year rate, and the release to College Ave will
not exceed the 100-year historic rate.
The required detention volumes for these release rates were calculated using the FAA method, as
presented in the spreadsheets in the appendix, and summarized in the table below. The total
volume includes water quality capture volume, defined as discussed in the next section.
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i EX. CARQUEST
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FIGURE I ALPINE BUSINESS PARK
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DATE: Jan 24, 2007
3 HISTORIC DRAINAGE BASINS
Cia
Associates
PROJECT NO.: 5 1. 101 1
DRAWN BY: KB
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 a site survey (NGVD 1929 datum). 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, gravel, roofs,
and paved areas were 0.25, 0.50, 0.95, and 0.95, respectively. Composite runoff calculations
were performed for the project site to determine the percent imperviousness for each basin and
for the site as a whole, 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.
Site Runoff Coefficients
Basin
% Impervious
2-year to 10-
year storms
100-year storm
H-1
58
0.64
0.79
H-2
64
0.69
0.86
H-3
40
0.50
0.63
A
80
0.81
1.00
B
72
0.77
0.96
C
1 45
1 0.60
0.75
Basins and Design Points. The project site, for analysis of development hydrology, was divided
into three historic basins (depicted in Figure 3) and three main developed drainage basins. The
table below summarizes the basin locations and areas.
Basin ID
Area
(acres)
Location
H-1
1.16
West'/ - drains to west
H-2
0.24
East % - drains to College Ave.
H-3
0.23
Alpine Street — drains to west
A
0.25
South half of Alpine, drains to NE Pond
B
0.86
East % of site, drains to NE Pond
C
0.20
Southwest % of site, drains to SW Pond _
Five design points were used in the hydraulic design below. These design points and the basins
that contribute to the points are summarized in the following table.
Page 6
112312007 .
the site from previous development, and an asphalt parking lot lies east of the existing Quick
Lube building. A gravel surface extends between the Quick Lube and the existing shed, to the
west of the shed, and around much of the western portion of the site. The existing Alpine Street
also has a gravel surface. Area -weighted calculations indicate that the site is currently 64%
impervious in the vicinity of the Quick Lube, and 58% impervious over its western portion.
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+t,)s
Where: I = intensity in inches/hour
K = a constant which is a function of the specific IDF curve
tc = 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 is determined to be:
I = 23.7 /(10 + t,) 0.711
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- ear
40.6
0.793
100-year
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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FIGURE ALPINE BUSINESS PARK DATE: Jan 24, 2007
CCi) PROJECT NO.: 51-101
2 SITE PLAN
Associates DRAWN BY: KB
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FIGURE ALPINE BUSINESS PARK ` DATE: Jan 24, 2007
CC� PROJECT NO.: 51-101
VICINITY MAP
Associates DRAWN BY: KB
OVERALL DRAINAGE CONSIDERATIONS
PROJECT DESCRIPTION
The Alpine Business Park PDP is a proposed 1.4-acre commercial project to be located on the
west side of North College Avenue just south of Alpine Street. Figure 1 is a vicinity map for the
project. The property is immediately east of the Union Pacific Railroad and just north and west
of the existing Human Bean drive -through coffee shop, and currently houses the Quick Lube
automotive repair shop, and a house and storage shed. North of the property, across Alpine
Street, is an existing CarQuest auto parts store. The house and shed on the property will be
removed, and the existing Quick Lube facility will be replaced with a new automotive repair
shop that will be similar in design and function. Additionally, a commercial flex -use building
will be constructed on the west portion of the property. Figure 2 is a reduced version of the site
plan. The project will include construction of Alpine Street adjacent to the property.
MAJOR BASIN
The site drains southwest toward the Cache La Poudre River along the Union Pacific right-of-
way and across the Josh Ames Ditch. The property is located close enough to the Dry Creek
drainage to be impacted by the Dry Creek floodplain.
EXISTING HYDRAULIC FACILITIES AND FLOOD HAZARD AREAS
The site is located within a flood hazard area, as depicted on FIRM Panel 08010200977F for the
Cache La Poudre River and Dry Creek in the City of Fort Collins. According to this map, two
cross sections for the Poudre River model cross the project property; the base flood elevations
for these cross sections are depicted in the Drainage Plan attached with the appendices.
The Josh Ames Ditch is located approximately 300 feet south of the property.
EXISTING TOPOGRAPHY, SOILS AND VEGETATION
The property slopes from its northeast corner to the southwest, with an average slope of
approximately 0.7 percent. Stormwater runoff from the property flows parallel to the railroad
tracks toward the Poudre River. Off -site runoff to the property is minimal. The property to the
north of the site currently drains poorly; it is flat and sloped slightly to the southwest.
Stormwater runoff from the north is directed south toward the project property and westward
toward the railroad ROW.
According to the Soil Survey of Larimer County, prepared by the Soil Conservation Service
(SCS, now the Natural Resources Conservation Service or NRCS), the Nunn clay loam underlies
the proposed project. The Hydrologic Soil Group for the Nunn clay loam is Group C,
corresponding to relatively rapid runoff or relatively slow percolation rates.
Existing vegetation includes a little grass around the existing house and trees in the northern
portion of the property. A number of concrete and asphalt pads still cover the western portion of
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112312007
UxTi-IINVIP.R CERTIFICATION OF DRAINAGE REPORT
I plans for design of the
pine
I hereby certify that this report andunder attached d rect supervision for ethe owners thereof and meets Business
Park PDP was prepared by me or Y
or exceeds the criteria in the City of FortCollins Storm Drainage Design Criteria an
Construction Standards.
`\\\pOlp0 )o RE Er;ur���/q
p15T
�•ooc•,
• 9,
Date
r :(.Glut
Registered Pro essional Engineer
State of Colo do No. 2L3Z�
Page 3
112312007
TABLE OF CONTENTS
DRAINAGE AND EROSION CONTROL REPORT
Tableof Contents..........................................................................................................................................................
2
Engineer Certification of Drainage Report ...................................................................................................................
3
OverallDrainage Considerations..............................................................................................................................
4
ProjectDescription...............................................................................................................................................
4
MajorBasin..........................................................................................................................................................
4
Existing Hydraulic Facilities and Flood Hazard Areas.........................................................................................
4
ExistingTopography, Soils and Vegetation.........................................................................................................
4
DrainageAnalysis...................................................................................................................................
.................. 5
Rainfall.....................:...........................................................................................................................................
5
Runoff...................................................................................................................................................................
6
DrainagePlan/Hydraulic Report ......................... !.....................................................................................................
7
Detentionand Water Quality Ponds.....................................................................................................................
7
Inlets.....................................................................................................................................................................
8
OpenChannels......................................................................................................................................................
8
Pipes......................................................................................................................................................................
9
Design for Floodplain Considerations..................................................................................................................
9
DrainageDesign for Alpine Street........................................................................................................................
9
Erosionand Sedimentation Control........................................................................................................................
10
Appendix....................................................................................................................................................................
1 I
Page 2
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CAI
Associates
DRAINAGE AND EROSION CONTROL REPORT
For:
ALPINE BUSINESS PARK PDP
CITY OF FORT COLLINS, COLORADO
ON BEHALF OF APPLICANT: GARA, LLC
m
QED ASSOCIATES
204 WALNUT STREET, SUITE C
FORT COLLINS, CO 80524
JANUARY 24, 2007
204 walnut street, suite c, fort collins, co 80524 • office: 970.416.0600 • fax: 970.416-0601