HomeMy WebLinkAboutCHABAD JEWISH CENTER OF NORTHERN COLORADO - BDR - BDR140003 - REPORTS - DRAINAGE REPORT301 N. Howes Street, Suite 100, Fort Collins, CO 80521 970.221.4158 www.northernengineering.com
Drainage Letter Report
Date: May 5, 2015
Project: Chabad Jewish Center Project No. 574-003
Project Development 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 Chabad Jewish Center project. There are two
existing buildings is situated on the site, a single story housing-style structure in the center of the lot and a
single story garage towards the northwest corner. The site currently consists of rooftop, concrete walks
and patios, a concrete parking area, turf, trees, and other vegetation. It presently functions similar to an
‘A’ corner lot, with historic drainage being split towards Westward Drive to the south and towards Shields
Street to the east. The total impervious area of the existing site is approximately 3,056 square-feet. The
existing percent impervious for the entire site is 13.5%. The existing total runoff from the site is 1.1 cfs in
the 100-yr event.
The proposed redevelopment will require demolition of the existing garage along with the parking area to
the northeast. The new facility is planned as 4 phases of construction. For the purpose of this letter, only
Phases 1 and 2 will be considered and permitted. Phases 3 and 4 will require a future application and a
re-evaluation of the conditions and requirements at that time. Minor grading will occur to host the new
facility, including the addition of two bio-swales along the south and east portions of the site to capture
runoff from the proposed facility and parking area. The total impervious area of the proposed
redevelopment (Phases 1 and 2) is roughly 7,774 square-feet. This is an increase of 4,718 SF from the
existing condition. The proposed percent impervious for the entire site also increases slightly to 34.1%.
The proposed total runoff from the site is 3.1 cfs in the 100-yr event.
The site is located in the Old Town Drainage Basin. On-site detention is not required if the increase in
impermeable area is less than 5,000 SF. 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.
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 22,713 sf area of disturbance for the project is less than one acre, a State
Stormwater Discharge Permit is not required 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 will be provided on-
site. Water quality treatment will occur along the south and east sides of the site, where bio-swales will
collect runoff from both the proposed parking lot and the proposed building. This bio-swale 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, the bio-swale will convey major events above ground to an
existing storm inlet near the S Shields St Right-Of-Way.
The City LID requirements specify that a minimum 25% of all new paving shall be treated using permeable
paving technique and a minimum of 50% of new impervious surface area bust be treated by a LID-type
device or technology. In the case of this project, nearly 100% of the runoff passes through either one of
the proposed bio-swales. For this reason, we propose an alternative to minimum 25% permeable paver
concept as all stormwater will result in an equal to or greater than level of LID attainment overall.
To summarize, the proposed grading concept closely matches existing elevations and drainage patterns.
On-site detention is proven to be unnecessary. Stormwater quality has been provided, and meets or
exceeds the city requirements for Low-Impact Development. Therefore, it is my professional opinion that
the Chabad Jewish Center satisfies all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Danny Weber Nicholas W. Haws, PE
Project Engineer Project Manager
enc.
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Chabad Jewish Center
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: D. Weber
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.
E-Hist 13,164 0.30 0.00 0.03 0.03 0.00 0.00 0.24 0.38 0.38 0.48 17.1
S-Hist 9,549 0.22 0.00 0.00 0.01 0.00 0.00 0.21 0.29 0.29 0.36 4.9
Total 22,713 0.52 0.00 0.03 0.04 0.00 0.00 0.45 0.34 0.34 0.43 12.0
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.
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Chabad Jewish Center
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: D. Weber
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.
E-Hist 13,164 0.30 0.00 0.03 0.03 0.00 0.00 0.24 0.38 0.38 0.48 17.1
S-Hist 9,549 0.22 0.00 0.00 0.01 0.00 0.00 0.21 0.29 0.29 0.36 4.9
Total 22,713 0.52 0.00 0.03 0.04 0.00 0.00 0.45 0.34 0.34 0.43 12.0
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.
Overland Flow, Time of Concentration:
Project: Chabad Jewish Center
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt
= L / 60V
Tc
= Ti
+ Tt
(Equation RO-2)
Velocity (Gutter Flow), V = 20·S
½
Velocity (Swale Flow), V = 15·S
½
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)
Slope,
S
(%)
Velocity,
Rational Method Equation: Project: Chabad Jewish Center
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
E E-Hist 0.30 25.1 25.1 24.1 0.38 0.38 0.48 1.43 2.44 5.09 0.2 0.3 0.7
S S-Hist 0.22 22.4 22.4 21.5 0.29 0.29 0.36 1.53 2.61 5.46 0.1 0.2 0.4
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
EXISTING RUNOFF COMPUTATIONS
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. Weber
May 5, 2015
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
Q = C f ( C )( i )( A )
D:\Projects\574-003\Drainage\Hydrology\574-003_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)
E E-Hist 0.30 0.38 0.38 0.48 25.1 25.1 24.1 0.2 0.3 0.7
S S-Hist 0.22 0.29 0.29 0.36 22.4 22.4 21.5 0.1 0.2 0.4
EXISTING RUNOFF SUMMARY TABLE
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Existing.xlsx\Summary Table
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Chabad Jewish Center
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: D. Weber
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.
E 11,419 0.26 0.00 0.00 0.05 0.00 0.00 0.22 0.37 0.37 0.47 16.0
S 11,295 0.26 0.04 0.04 0.05 0.00 0.00 0.13 0.60 0.60 0.76 47.2
Total 22,714 0.52 0.04 0.04 0.09 0.00 0.00 0.34 0.49 0.49 0.61 31.5
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.
Overland Flow, Time of Concentration:
Project: Chabad Jewish Center
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt
= L / 60V
Tc
= Ti
+ Tt
(Equation RO-2)
Velocity (Gutter Flow), V = 20·S
½
Velocity (Swale Flow), V = 15·S
½
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)
Slope,
S
(%)
Velocity,
Rational Method Equation: Project: Chabad Jewish Center
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
E E 0.26 5.0 5.0 5.0 0.37 0.37 0.47 2.85 4.87 9.95 0.3 0.5 1.2
S S 0.26 5.0 5.0 5.0 0.60 0.60 0.76 2.85 4.87 9.95 0.4 0.8 1.9
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
DEVELOPED RUNOFF COMPUTATIONS
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. Weber
May 5, 2015
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
Q = C f ( C )( i )( A )
D:\Projects\574-003\Drainage\Hydrology\574-003_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)
E E 0.26 0.37 0.37 0.47 5.0 5.0 5.0 0.3 0.5 1.2
S S 0.26 0.60 0.60 0.76 5.0 5.0 5.0 0.4 0.8 1.9
DEVELOPED RUNOFF SUMMARY TABLE
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Proposed.xlsx\Summary Table
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
E E No 0.95 0.95 1.00 40 5.0 1.0 1.0 0.7 0 0.0 0.00 N/A 314 1.0 1.50 3.5 5.0 5.0 5.0
S S No 0.95 0.95 1.00 50 2.3 1.5 1.5 1.0 0 0.0 0.00 N/A 152 1.0 1.50 1.7 5.0 5.0 5.0
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
D. Weber
May 5, 2015
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
C Cf L
Ti
−
=
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Proposed.xlsx\Tc-2-yr_&_100-yr
10-year Cf
= 1.00
May 5, 2015
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Proposed.xlsx\C-Values
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
E E-Hist No 0.15 0.15 0.19 200 1.0 25.1 25.1 24.1 0 0.0 0.00 N/A 0 0.0 0.00 N/A 25.1 25.1 24.1
S S-Hist No 0.15 0.15 0.19 100 0.5 22.4 22.4 21.5 0 0.0 0.00 N/A 0 0.0 0.00 N/A 22.4 22.4 21.5
EXISTING TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
D. Weber
May 5, 2015
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
C Cf L
Ti
−
=
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Existing.xlsx\Tc-2-yr_&_100-yr
10-year Cf
= 1.00
May 5, 2015
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Existing.xlsx\C-Values
10-year Cf
= 1.00
May 5, 2015
D:\Projects\574-003\Drainage\Hydrology\574-003_Rational-Calcs_Existing.xlsx\C-Values