HomeMy WebLinkAboutCOLLEGE EIGHT THIRTY - PDP - PDP150019 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTDrainage Letter Report
Date: September 9, 2015
Project: College Eight Thirty Project No. 574-006
Fort Collins, Colorado
Attn: Stormwater Staff
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Staff:
This letter serves to address the stormwater impacts of the College Eight Thirty project. The project is a
redevelopment of the Schrader’s Country Store located on the northeast corner of College and Locust
Street. The site currently consists of rooftop, concrete walks, and asphalt parking areas. On-site drainage
currently flows out from the building to the adjacent streets. Once it reaches the streets, water flows south
on College, then is directed east along Locust. The total impervious area of the existing site is
approximately 13,997 square-feet. The existing percent impervious for the entire site is 90.0%.
The proposed redevelopment will remove the existing structures from the site and replace them with a
mixed-use building. In addition to the new building, the parkways will be modified to meet current Land
Use Code requirements, including sidewalks and landscaping that is not currently present or poorly
defined. An area of pavers is also proposed at the northeast corner of the site, which is the only area of
exposed pavement within the property line. Existing drainage patterns will generally be followed. The total
impervious area of the proposed redevelopment is roughly 13,290 square-feet. When the impervious value
of 40% for pavers is considered, the effective impervious area decreases to 11,991 square feet. The
proposed percent impervious for the entire site decreases to 74.9%. An exhibit depicting existing and
proposed impervious areas has been provided.
The site is located in the Old Town Drainage Basin. Historically, on-site detention has not been required
so long as there is no increase in impervious area. Since the redevelopment does not propose an increase
in runoff, detention would not have been required. However, changes to City policy now require that a
project that is being completely redeveloped must provide onsite detention with a 2-yr historic release rate.
However, a variance may be justified if the site has been paying fees for the higher imperviousness.
Verification of the fees paid to date is underway, however the current assumption is that the site has been
paying the higher fees, and will therefore be exempt from the 2-year release and onsite detention
requirements.
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 more than 10,000 sf of the project site will be disturbed, a comprehensive Stormwater Management
Plan will be prepared for this project at Final Design. 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 (Stormwater Planter Boxes) along the west side of the
building that intercepts runoff from rooftop downspouts on the west side. These planter beds 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 Planter Boxes should it
become overburdened. The underdrain will outfall directly into the existing inlet located at Locust Street
and College Avenue. There will also be an area of permeable pavers installed on the northeast corner of
the project site. This area of pavers will treat not only the exposed parking areas, but also the rooftop
drainage by routing runoff into the paver subbase. A subdrain will be provided for the paver subbase that
ties into the same system as the Planter Boxes. The subdrain will have a water quality control structure
that will slow the release of stormwater from the paver subbase and promote infiltration as well as to
increase contact time across the surface area of the subbase.
The City LID requirements specify that no less than 25% of all new paving shall be treated using
permeable paving technique. In the case of this project, 2,165 sf of permeable pavers will be provided,
which is 100% of the total 2,165 sf of new paving, which far exceeds the requirement. The City LID
requirements also specify that no less than 50% of any new impervious areas must be treated using LID
techniques. The combination of treatment provided by the Permeable Pavers and Stormwater Planter will
treat 0.257 sf of impervious area on the site, which is 83.48% of the new impervious areas proposed with
the project, which again far exceeds the amount required by city code. A separate LID Conformance
Report and Exhibit have been provided to document LID compliance.
To summarize, the proposed grading concept closely matches existing elevations and drainage patterns.
Existing site imperviousness has decreased from the existing condition, and a variance to the requirement
to provide onsite detention using a 2-year historic release rate will be requested. Stormwater quality has
been provided, and exceeds the city requirements for Low Impact Development paving and treatment.
Therefore, it is my professional opinion that College Eight Thirty 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: College Eight Thirty
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.
A 13,997 0.32 0.00 0.24 0.08 0.00 0.00 0.00 0.95 0.95 1.00 90.0
TOTAL ONSITE 13,997 0.32 0.00 0.24 0.08 0.00 0.00 0.00 0.95 0.95 1.00 90.0
EXISTING RUNOFF SUMMARY TABLE
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
September 9, 2015
Overland Flow, Time of Concentration:
Project: College Eight Thirty
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
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)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
Rational Method Equation: Project: College Eight Thirty
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
A A 0.32 5.0 5.0 5.0 0.95 0.95 1.00 2.85 4.87 9.95 0.87 1.49 3.20
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)
EXISTING RUNOFF SUMMARY TABLE
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)
A. Reese
September 9, 2015
Q = C f ( C )( i )( A )
D:\Projects\574-006\Drainage\Hydrology\574-006_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)
A A 0.32 0.95 0.95 1.00 5.0 5.0 5.0 0.87 1.49 3.20
EXISTING RUNOFF SUMMARY TABLE
D:\Projects\574-006\Drainage\Hydrology\574-006_Rational-Calcs_Existing.xlsx\Summary Table
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: College Eight Thirty
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.
A 13,997 0.32 0.00 0.02 0.23 0.00 0.05 0.02 0.83 0.83 1.00 74.9
TOTAL ONSITE 13,997 0.32 0.00 0.02 0.23 0.00 0.05 0.02 0.83 0.83 1.00 74.9
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
September 9, 2015
Overland Flow, Time of Concentration:
Project: College Eight Thirty
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
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)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
Rational Method Equation: Project: College Eight Thirty
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
A A 0.32 5.0 5.0 5.0 0.83 0.83 1.00 2.85 4.87 9.95 0.76 1.30 3.20
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)
A. Reese
September 9, 2015
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)
Q = C f ( C )( i )( A )
D:\Projects\574-006\Drainage\Hydrology\574-006_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)
A A 0.32 0.83 0.83 1.00 5.0 5.0 5.0 0.76 1.30 3.20
DEVELOPED RUNOFF SUMMARY TABLE
D:\Projects\574-006\Drainage\Hydrology\574-006_Rational-Calcs_Proposed.xlsx\Summary Table
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
A A No 0.25 0.25 0.31 26 5.0 4.7 4.7 4.4 0 0.0 0.00 N/A 0 0.0 0.00 N/A 5.0 5.0 5.0
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
A. Reese
September 9, 2015
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
-
=
D:\Projects\574-006\Drainage\Hydrology\574-006_Rational-Calcs_Proposed.xlsx\Tc-2-yr_&_100-yr
D:\Projects\574-006\Drainage\Hydrology\574-006_Rational-Calcs_Proposed.xlsx\C-Values
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
A A No 0.25 0.25 0.31 26 5.0 4.7 4.7 4.4 0 0.0 0.00 N/A 0 0.0 0.00 N/A 5.0 5.0 5.0
EXISTING RUNOFF SUMMARY TABLE
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
A. Reese
September 9, 2015
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
-
=
D:\Projects\574-006\Drainage\Hydrology\574-006_Rational-Calcs_Existing.xlsx\Tc-2-yr_&_100-yr
D:\Projects\574-006\Drainage\Hydrology\574-006_Rational-Calcs_Existing.xlsx\C-Values