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D:\Projects\970-001\Drainage\Reports\Drainage Report\970-001_Drainage-Letter.docx
Drainage Letter Report
Date: March 5, 2014
Project: Plum & Shields Greek Housing Project No. 970-001
Project Development Plan/Final Plan
Fort Collins, Colorado
Attn: Mr. Wes Lamarque
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Wes:
This letter serves to address the stormwater impacts of the Plum & Shields Greek Housing project. The
existing building is situated towards the northern portion of the site. The site currently consists of rooftop,
concrete walks, asphalt parking areas, some turf, trees, and other vegetation. It presently functions similar
to a ‘B’ lot, with historic drainage being split towards Plum Street to the north and towards Shields Street
to the east. The total impervious area of the existing site is approximately 12,325 square-feet. The
existing percent impervious for the entire site is 65.1%. The existing total runoff from the site is 3.7 cfs in
the 100-yr event.
The proposed redevelopment will leave the existing building footprint unchanged, aside from the removal
of a small addition on the west side of the building, while expanding the parking area south of the building.
Portions of existing paving will be modified, and a paver area will be added at the northwest corner of the
new parking area. While very minor grading changes will occur, existing drainage patterns will be
maintained. The total impervious area of the proposed redevelopment is roughly 13,108 square-feet. This
is a slight increase from the existing condition. The proposed percent impervious for the entire site also
increases slightly to 68.5%. However, due to the addition of pavers and the removal of some concrete
paving along the north side of the building, total runoff from the site remains unchanged at32.7 cfs in the
100-yr event.
The site is located in the Old Town Drainage Basin. On-site detention is not required so long as there is no
increase in runoff. Since the redevelopment does not propose an increase in runoff, detention storage is
not required. Calculations using the rational method to determine the existing and proposed runoff are
attached to this letter, as is a drainage exhibit.
There are no regulatory floodplains associated with the project.
Page 2 of 2
D:\Projects\970-001\Drainage\Reports\Drainage Report\970-001_Drainage-Letter.docx
Although stormwater quantity detention is not required, stormwater quality mitigation will be addressed by
both temporary and permanent Best Management Practices (BMPs). During construction, the Contractor
will follow the appropriate and applicable City of Fort Collins standards for erosion and sediment control.
Since the approximately 11,707 sf area of disturbance for the project is less than one acre, a
comprehensive Stormwater Management Plan will not be prepared for this project. Post construction
water quality and erosion control will be achieved by a fully established and stabilized site. All areas
disturbed during construction will receive permanent hardscape, landscape, or building structure.
There are no existing stormwater BMP’s currently on-site. In order to bring the site into compliance with
the Low Impact Development Standards adopted by the city, water quality mitigation and permeable
pavers will be provided on-site. Water quality treatment will occur through a variety of methods. The first
method is the use of a raised bio-retention area (Infiltration Garden) along the south side of the existing
building that intercepts runoff from rooftop downspouts. This planter bed will have an engineered section
designed to remove particulates and contaminants from the runoff. While most minor events are expected
to infiltrate into the ground, a subdrain is provided to drain the Infiltration Garden should it become
overburdened. The underdrain will outfall directly into Shields. In addition to treating the roof runoff, a
paver area is being provided in 3 of the new parking stalls, which will allow minor rain events to infiltrate
into the soil rather than leaving the site. The final method that is being utilized on the north side of the
building is to discharge roof drains into landscaped areas.
The City LID requirements specify that no less than 25% of all new paving shall be treated using an LID
technique. In the case of this project, 375 sf of permeable pavers will be provided, which is 35% of the
total 1066 sf of new paving. The City LID requirements also specify that no less than 50% of any new
impervious areas must be treated using LID techniques. As mentioned above, this project is proposing
375 sf of permeable pavers, which is 35% of the new paving. This does not meet the city LID
requirement. However, in addition to treating some of the new paving, the project proposes to treat 1,909
sf of existing roof area that was previously untreated. Combining the two, the project will treat a total of
2,284 sf of impervious area, which is 214% of the new impervious areas proposed with this project.
To summarize, the proposed grading concept very closely matches existing elevations and drainage
patterns. On-site detention is proven to be unnecessary. Stormwater quality has been provided, and
exceeds the city requirements for Low Impact Development. Therefore, it is my professional opinion that
the Plum & Shields Greek Housing satisfies all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Andrew Reese Nicholas W. Haws, PE
Project Engineer Project Manager
enc.
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Shields & Plum Greek Housing
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: A. Reese
Asphalt ……....……………...……….....…...……………….………………………………….0.95 . 100 Date:
Concrete …….......……………….….……….………………..….………………………………… 0.95 90
Gravel ……….…………………….….…………………………..……………………………….0.50 . 40
Roofs …….…….………………..……………….…………………………………………….0..95 90
Concrete Pavers…………………………...………………..…………………………………………….0.40 . 22
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0 2-year Cf
= 1.00 100-year Cf = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Concrete
Pavers
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
N 7,785 0.18 0.02 0.05 0.03 0.00 0.00 0.08 0.64 0.64 0.81 52.1
S 10,186 0.23 0.11 0.02 0.05 0.00 0.00 0.05 0.80 0.80 0.99 75.0
Total 17,971 0.41 0.14 0.07 0.07 0.00 0.00 0.13 0.73 0.73 0.91 65.1
EXISTING COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
10-year Cf = 1.00
Overland Flow, Time of Concentration:
Project: Shields & Plum Greek Housing
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = Ti + Tt (Equation RO-2)
EXISTING TIME OF CONCENTRATION COMPUTATIONS
A. Reese
March 5, 2014
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
Tc = Ti + Tt (Equation RO-2)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
Rational Method Equation: Project: Shields & Plum Greek Housing
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
EXISTING RUNOFF COMPUTATIONS
A. Reese
March 5, 2014
Q C f C i A
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
N N 0.18 5.0 5.0 5.0 0.64 0.64 0.81 2.85 4.87 9.95 0.3 0.6 1.4
S S 0.23 5.2 5.2 5.1 0.80 0.80 0.99 2.85 4.87 9.95 0.5 0.9 2.3
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
C100
Design
Point
Flow,
Q100
(cfs)
Flow,
Q2
(cfs)
10-yr
Tc
(min)
2-yr
Tc
(min)
C2
Flow,
Q10
(cfs)
Intensity,
i100
(in/hr)
Basin(s)
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Existing.xlsx\Direct-Runoff
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
C2 C10 C100
2-yr Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q10
(cfs)
Q100
(cfs)
N N 0.18 0.64 0.64 0.81 5.0 5.0 5.0 0.3 0.6 1.4
S S 0.23 0.80 0.80 0.99 5.2 5.2 5.1 0.5 0.9 2.3
EXISTING RUNOFF SUMMARY TABLE
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Existing.xlsx\Summary Table
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Shields & Plum Greek Housing
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: A. Reese
Asphalt ……....……………...……….....…...……………….………………………………….0.95 . 100 Date:
Concrete …….......……………….….……….………………..….………………………………… 0.95 90
Gravel ……….…………………….….…………………………..……………………………….0.50 . 40
Roofs …….…….………………..……………….…………………………………………….0..95 90
Concrete Pavers…………………………...………………..…………………………………………….0.40 . 22
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0 2-year Cf
= 1.00 100-year Cf = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Concrete
Pavers
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
N 7,843 0.18 0.03 0.05 0.03 0.00 0.00 0.08 0.60 0.60 0.75 52.5
S 10,128 0.23 0.11 0.04 0.04 0.00 0.01 0.03 0.83 0.83 1.00 81.0
Total 17,971 0.41 0.14 0.09 0.07 0.00 0.01 0.11 0.73 0.73 0.91 68.5
DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
10-year Cf = 1.00
Overland Flow, Time of Concentration:
Project: Shields & Plum Greek Housing
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = Ti + Tt (Equation RO-2)
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
A. Reese
March 5, 2014
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
Tc = Ti + Tt (Equation RO-2)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
Rational Method Equation: Project: Shields & Plum Greek Housing
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
DEVELOPED RUNOFF COMPUTATIONS
A. Reese
March 5, 2014
Q C f C i A
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
N N 0.18 5.0 5.0 5.0 0.60 0.60 0.75 2.85 4.87 9.95 0.3 0.5 1.4
S S 0.23 5.2 5.2 5.1 0.83 0.83 1.00 2.85 4.87 9.95 0.5 0.9 2.3
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
C100
Design
Point
Flow,
Q100
(cfs)
Flow,
Q2
(cfs)
10-yr
Tc
(min)
2-yr
Tc
(min)
C2
Flow,
Q10
(cfs)
Intensity,
i100
(in/hr)
Basin(s)
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Proposed.xlsx\Direct-Runoff
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
C2 C10 C100
2-yr Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q10
(cfs)
Q100
(cfs)
N N 0.18 0.60 0.60 0.75 5.0 5.0 5.0 0.3 0.5 1.4
S S 0.23 0.83 0.83 1.00 5.2 5.2 5.1 0.5 0.9 2.3
DEVELOPED RUNOFF SUMMARY TABLE
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Proposed.xlsx\Summary Table
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
N N No 0.95 0.95 1.00 45 2.8 1.3 1.3 0.9 194 0.6 1.55 2.1 0 0.0 0.00 N/A 5.0 5.0 5.0
S S No 0.15 0.15 0.19 5 1.0 3.8 3.8 3.7 184 1.2 2.17 1.4 0 0.0 0.00 N/A 5.2 5.2 5.1
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow Time of Concentration
(Equation RO-4)
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Proposed.xlsx\Tc-2-yr_&_100-yr
March 5, 2014
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Proposed.xlsx\C-Values
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
N N No 0.95 0.95 1.00 45 2.8 1.3 1.3 0.9 194 0.6 1.55 2.1 0 0.0 0.00 N/A 5.0 5.0 5.0
S S No 0.15 0.15 0.19 5 1.0 3.8 3.8 3.7 184 1.2 2.17 1.4 0 0.0 0.00 N/A 5.2 5.2 5.1
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow Time of Concentration
(Equation RO-4)
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Existing.xlsx\Tc-2-yr_&_100-yr
March 5, 2014
D:\Projects\970-001\Drainage\Hydrology\970-001_Rational-Calcs_Existing.xlsx\C-Values