HomeMy WebLinkAboutTHE EXCHANGE - BASIC DEVELOPMENT REVIEW - BDR160019 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTDrainage Letter and LID Report
Date: October 12, 2016
Project: The Exchange Project No. 379-060
Basic Development Review (BDR)
Fort Collins, Colorado
Attn: City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
To whomever it concerns:
This letter serves to address the stormwater impacts of the proposed project known as “The Exchange”.
The project is a redevelopment of the 200 block of North College Avenue. The redevelopment consists
primarily of façade improvements to the existing buildings, along with a small expansion of the southern-
most building. Also, the existing parking lot is being replaced by a pedestrian area with seating areas and
a grassy central lawn.
The total impervious area of the site will be reduced due to the removal of the existing parking area and
the addition of the large grassy area in the center of the site. For this reason, the total runoff was assumed
to also be reduced and, therefore, no stormwater detention is proposed. Although stormwater quantity
detention is not required, stormwater quality will be addressed by permanent Best Management Practices
(BMPs) and Low Impact Development (LID) requirements.
The project will maintain the existing overall drainage patterns for the most part, however modifications to
the interior drainage patterns will be made in order to capture and treat as much of the affected project site
as possible with LID methods. In general, the central grass area will be lowered so that all adjacent areas
of paving can drain to the inlets located at the corners of the grass area. These inlets will direct flows into
an underground detention facility that will provide storage for the water quality capture volume (WQCV).
The vaults will have an open bottom to allow the WQCV to infiltrate into the underlying soils. Flows in
excess of the WQCV will pass over a weir in the downstream inlet and proceed via a storm drain into an
existing area inlet located at the intersection of the alley and Pine Street.
During construction, the Contractor will follow the appropriate and applicable City of Fort Collins standards
for erosion and sediment control. Since the approximately 18,000 sf area of disturbance for the project is
more than 10,000 sf, 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 regulatory floodplains associated with the project.
The City LID requirements specify that either 75% of all impermeable areas are provided LID treatment OR
50% LID treatment by which 25% is treated by permeable pavers. In the case of this project, we will
meet or exceed the 75% treatment of impermeable areas through the use of LID. The treatment will be
provided by an infiltration gallery that will treat 18,319 square-feet of impervious area on the site, which
is 83.1% of the total impervious area within the affected project areas, which exceeds the amount
required by city code. A LID Treatment Exhibit is provided with this report.
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 the city
requirements for Low Impact Development treatment. Therefore, it is my professional opinion that The
Exchange satisfies all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Andy Reese
Project Manager
enc.
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: The Exchange
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%
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
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
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.
A1 0.42 0.00 0.20 0.04 0.07 0.00 0.18 0.73 0.73 0.91 51%
OS1 0.04 0.00 0.01 0.01 0.01 0.00 0.01 0.83 0.83 1.00 59%
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
October 12, 2016
Overland Flow, Time of Concentration:
Project: The Exchange
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: The Exchange
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
Design
Point
Basin(s)
Area, A
(acres)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
C2 C10 C100
Intensity,
i2
(in/hr)
Intensity,
i10
(in/hr)
Intensity,
i100
(in/hr)
Flow,
Q2
(cfs)
Flow,
Q10
(cfs)
Flow,
Q100
(cfs)
Flow,
WQ
(cfs)
A1 A1 0.42 8 8 7 0.73 0.73 0.91 2.46 4.21 8.80 0.8 1.3 3.4 0.38
OS1 OS1 0.04 5 5 5 0.83 0.83 1.00 2.85 4.87 9.95 0.1 0.2 0.4 0.05
DEVELOPED RUNOFF COMPUTATIONS
A. Reese
October 12, 2016
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
Q = C f ( C )( i )( A )
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Composite
% Imperv.
Flow,
Q2
(cfs)
Flow,
Q10
(cfs)
Flow,
Q100
(cfs)
Flow,
WQ
(cfs)
Treatment
Area
(sf)
A1 18,319 0.42 51% 0.75 1.29 3.37 0.38 13,349
OS1 1,786 0.04 59% 0.10 0.16 0.41 0.05 1,583
DEVELOPED RUNOFF SUMMARY
Project Title Date:
Project Number Calcs By:
Basin(S)
0.8
WQCV = Watershed inches of Runoff (inches) 62.00%
a = Runoff Volume Reduction (constant)
i = Total imperviousness Ratio (i = Iwq/100) 0.194 in
A = 0.43 ac
V = 0.0070 ac-ft
V = Water Quality Design Volume (ac-ft)
WQCV = Water Quality Capture Volume (inches)
A = Watershed Area (acres)
The Exchange October 12, 2016
379-060 A. Reese
A1
303 cu. ft.
Drain Time
a =
i =
WQCV =
Figure EDB-2 - Water Quality Capture Volume (WQCV), 80th Percentile Runoff Event
0.194
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
WQCV (watershed inches)
Total Imperviousness Ratio (i = Iwq/100)
Water Quality Capture Volume
6 hr
12 hr
24 hr
40 hr
WQCV = a ( 0.91 i 3 - 1 . 19 i 2 + 0 . 78 i )
WQCV = a ( 0.91 i 3 - 1 . 19 i 2 + 0 . 78 i )
V * A
12
WQCV
÷
ø
ö
ç
Vault ID
Total
Required
WQ Volume
(cf)
Flow,
WQ
(cfs)
Chamber
Type
Chamber
Release Rate
(cfs)
Chamber
Volume
(cfs)
Installed
Chamber
Volume
(cfs)
Mimimum
No. of
Chambers
Minimum
Release Rate
(cfs)
Required
Chamber
Volume by
FAA Method
(cf)
Provided
Number of
Chambers
Provided
Chamber
Volume
(cf)
Total
Installed
Chamber
Volume
(cf)
1 303 0.38 SC-740 0.024 45.90 74.90 5 0.12 242 6 275 449
Vault Configuration Summary
Note: "Chamber Volume" refers to the open volume within the vaults. "Installed Chamber Volume" refers to the total volume provided, including the surrounding aggregates.
D:\Projects\379-060\Drainage\LID\379-060_Vault Summary.xlsx
The Exchange
Fort Collins, Colorado
A. Reese Date: October 12, 2016
Pond No.: Vault 1
A1
WQ
0.91
Area (A)= 0.43 acres Quantity Detention 242 ft
3
Max Release Rate = 0.12 cfs
Time Time
Ft.Collins
WQ
Intensity
Q100
Inflow
(Runoff)
Volume
Outflow
(Release) Volume
Storage
Detention
Volume
(mins) (secs) (in/hr) (cfs) (ft
3
) (ft
3
) (ft
3
)
5 300 1.43 0.6 167 36 131
10 600 1.11 0.4 259 72 187
15 900 0.94 0.4 329 108 221
20 1200 0.81 0.3 378 144 234
25 1500 0.72 0.3 420 180 240
30 1800 0.65 0.3 458 216 242
35 2100 0.59 0.2 481 252 229
40 2400 0.54 0.2 502 288 214
45 2700 0.50 0.2 523 324 199
50 3000 0.46 0.2 540 360 180
55 3300 0.44 0.2 562 396 166
60 3600 0.41 0.2 578 432 146
65 3900 0.39 0.2 588 468 120
70 4200 0.37 0.1 600 504 96
75 4500 0.35 0.1 607 540 67
80 4800 0.33 0.1 620 576 44
85 5100 0.32 0.1 629 612 17
90 5400 0.31 0.1 644 648 -4
95 5700 0.29 0.1 647 684 -37
100 6000 0.28 0.1 657 720 -63
105 6300 0.27 0.1 666 756 -90
110 6600 0.26 0.1 671 792 -121
115 6900 0.3 0.1 688 828 -140
120 7200 0.25 0.1 690 864 -174
Vault Volume Calculation | FAA Method
Project:
Project Location:
Calculations By:
Input Variables Results
Design Point
Design Storm Required Detention Volume
Developed "C" =
D:\Projects\379-060\Drainage\LID\379-060_FAA_Vault 1.xlsm\
è
= æ
12 hr
D:\Projects\379-060\Drainage\WatQual\379-060_WQ Volume.xls
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
A1 A1 No 0.25 0.25 0.31 39 1.94% 8.0 8.0 7.4 0 n/a N/A N/A 0 n/a N/A N/A 8 8 7
OS1 OS1 No 0.95 0.25 1.00 21 5.19% 0.7 4.2 0.5 0 n/a N/A N/A 0 n/a N/A N/A 5 5 5
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
A. Reese
October 12, 2016
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
-
=