HomeMy WebLinkAboutDrainage Reports - 04/11/2016Drainage Compliance Letter
Date: April 5, 2016
Project: Value Plastics – Minor Amendment Project No. 207-015
Fort Collins, Colorado
Attn: Mr. Dan Mogen
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
Dear Dan:
This letter serves to summarize the stormwater impacts of the above referenced project. The project is
located at the northwest corner of Danfield Court and South Timberline Road. The project will consist of
both Lots 1 & 2, Value Plastic PUD, City of Fort Collins, County of Larimer, State of Colorado.
Lot 2 currently consists of a large office building, previously occupied by Value Plastics. It also consists of
a drive aisle, associated walkways and a limited amount of parking area. Lot 1 is currently a vacant lot
and consists of only native vegetation. These two lots were originaly designed by Water, Waste and Land,
Inc. and approved on March 2, 1995. The originally plan divided the project into three design basins
(Basins 1-3). At the time of the analysis, Basin 1 consisted of an existing building and was designed to
include future parking area. The overall percent imperviousness calculated for this basin was 56.5
percent. Within the original design, Basin 1 was calculated to discharge at a runoff rate of 12.2 cfs during
the 100-year runoff event. This basin will be affected slightly by a decrease in contributing area. Basin 2
included most of Lot 1 and was designed to include a future building located on Lot 1 with associated
parking area and drive. Basin 2 also includes a portion of the main access drive to the west of the Value
Plastics building and a parking area to the north this building. The overall percent imperviousness of this
basin was calculated to be 66.5 percent. Within the original design, Basin 2 was calculated to discharge
at a runoff rate of 14.9 cfs during the 100-year runoff event. Basin 3 includes the entire Value Plastics
building, a portion of the access drive to the southwest of the building, and the landscaping surrounding
the building to the south and east. The basin was designed to include area for future parking and was
designed with an overall percent imperviousness of 86.8 percent. Within the original design, Basin 3 was
calculated to discharge at a runoff rate of 15.1 cfs during the 100-year runoff event. Please see the select
sheets from the original report attached within Appendix C.
The minor amendment is proposing to construct two parking areas within the overall P.U.D. project limits.
One parking area is proposed within Basin 2 and will be constructed with TRUEGRID Technology. The
surfacing material for this parking lot is planned to be a fine aggregate. The ¾-inch washed aggregate is
proposed to be installed on top of a larger granular section for maximum drainage within the subgrade.
The TRUEGRID system is proposed to be installed with an underdrain system. Without including the
anticipated improvements within Basin 2, the current percent imperviousness of this basin is 32 percent.
With the proposed improvements, the overall percent imperviousness increases to 41.0 percent, which is
well below the previously designed 66.5 percent approved within the original study. This increased
percent imperviousness results in a runoff rate of 9.35 cfs during the 100-year event. The other parking
area is proposed within Basin 3 and will also be constructed with TRUEGRID Technology. The surfacing
material for this parking lot is planned to be grass seed. The grass seed and planting material is proposed
to be installed on top of a larger granular section for maximum drainage within the subgrade. The
TRUEGRID system is proposed to be installed with an underdrain system. Without including the
anticipated improvements within Basin 3, the current percent imperviousness is 38 percent. With the
proposed improvements, the overall percent imperviousness within Basin 3 increases to 41.4 percent,
which is well below the previously designed 86.8 percent approved within the original study. This
increase in percent imperviousness results in a runoff rate of 9.94 cfs during the 100-year event. All
runoff rates were calculated utilizing the current rainfall intensities. Refer to the attached documents for all
supporting calculations.
With TRUEGRID being a new technology within this region, it has been agreed to use this site as a test
project to understand if this is a viable option within this Arid Highplain Climate Region. If this system
fails, these two parking areas will need to be reconstructed to incorporate all current LID Standards while
draining properly within the constraints of the originally approved discharge rates. To look at future
options, both basins were analyzed to represent the TRUEGRID system being removed and replaced by
asphalt. With the proposed improvements within Basin 2 being converted to asphalt surfacing, the overall
percent imperviousness increases to 55.0 percent, which is still below the previously designed 66.5
percent approved within the original study. This increase in percent imperviousness results in a runoff rate
of 12.39 cfs during the 100-year event. With the proposed improvements within Basin 3 being converted
to asphalt surfacing, the overall percent imperviousness increases to 45.0 percent, which is still below the
previously designed 86.8 percent approved within the original study. This increase in percent
imperviousness results in a runoff rate of 10.46 cfs during the 100-year event. All runoff rates were
calculated utilizing the current rainfall intensities. Please see below for a comparison table summarizing
the details from each scenario.
Basin ID
Original Percent
Impervious
Original
Design Q
Existing Percent
Impervious
Proposed Percent
Impervious
(w/ Paver Section)
Proposed Design Q
(w/ Paver Section)
Percent Impervious
(Converted to Asphalt)
Proposed Design Q
(Converted to Asphalt)
1 56.5% 12.2 cfs 51.1% 53.5% 11.7 cfs 53.5% 11.7 cfs
2 66.8% 14.9 cfs 32.3% 41.0% 9.4 cfs 55.0% 12.4 cfs
3 86.8% 15.1 cfs 38.4% 41.4% 9.9 cfs 45.0% 10.5 cfs
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Cody N. Snowdon
Project Engineer
Nicholas W. Haws, PE, LEED AP
Colorado Registration No. 39767
04.05.16
APPENDIX A
HISTORIC & PROPOSED HYDROLOGY CALCULATIONS
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Value Plastics
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By:
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
(s.f.)
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.
Basin 1 90169 2.070 0.565 0.250 0.298 0.000 0.000 0.957 0.58 0.58 0.72 51.1%
Basin 2 90605 2.080 0.375 0.286 0.043 0.000 0.000 1.376 0.42 0.42 0.53 32.3%
Basin 3 99317 2.280 0.144 0.112 0.700 0.000 0.000 1.324 0.49 0.49 0.61 38.4%
TOTAL 280091 6.430 1.084 0.018 1.041 0.000 0.000 4.287 0.42 0.42 0.52 31.7%
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.
HISTORIC % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
C. Snowdon
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Value Plastics
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By:
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
(s.f.)
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.
Basin 1 86115 1.977 0.565 0.250 0.298 0.000 0.000 0.864 0.60 0.60 0.75 53.5%
Basin 2 94659 2.173 0.375 0.303 0.043 0.507 0.000 0.944 0.50 0.50 0.62 41.0%
Basin 3 99317 2.280 0.144 0.127 0.700 0.137 0.000 1.172 0.51 0.51 0.64 41.4%
OVERALL TOTAL 280091 6.430 1.084 0.680 1.041 0.644 0.000 2.981 0.53 0.53 0.67 45.0%
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 28, 2015
Overland Flow, Time of Concentration:
Project: Value Plastics
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = T
i + 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,
V
(ft/s)
Rational Method Equation: Project: Value Plastics
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
basin 1 Basin 1 1.98 12 12 10 0.60 0.60 0.75 2.09 3.57 7.88 2.48 4.23 11.68
basin 2 Basin 2 2.17 13 13 13 0.50 0.50 0.62 1.98 3.39 6.92 2.14 3.66 9.35
basin 3 Basin 3 2.28 14 14 14 0.51 0.51 0.64 1.95 3.34 6.82 2.27 3.90 9.94
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
DEVELOPED RUNOFF COMPUTATIONS
C100 Notes
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)
C. Snowdon
October 28, 2015
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
Q = C f ( C )( i )( A )
Page 4 of 5 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs.xlsx\Direct-Runoff
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
C2 C100
2-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q100
(cfs)
basin 1 Basin 1 1.98 0.60 0.75 12 10 2.48 11.68
basin 2 Basin 2 2.17 0.50 0.62 13 13 2.14 9.35
basin 3 Basin 3 2.28 0.51 0.64 14 14 2.27 9.94
Page 5 of 5 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs.xlsx\SUMMARY-TABLE
Required Minimum Impervious Area to be Treated
(=50% of new impervious
area)
New Impervious Area 30838 sq. ft.
Impervious Area Treated by LID Treatment Method #1
(TRUEGRID Gravel Pavers Northern Parking Lot) 22089 sq. ft.
Impervious Area Treated by LID Treatment Method #2
(TRUEGRID Grass Pavers Northern Parking Lot) 5971 sq. ft.
Total Impervious Area Treated 28060 sq. ft.
Percent Impervious Area Treated 91.0%
Required Minimum Area of Porous Pavement
(=25% of new
pavement area)
New Pavement Area 30838 sq. ft.
Area of Paver Section #1 22089 sq. ft.
Run-on area for Paver Section #1 (up to 3:1 is permited) 8597 sq. ft.
Area of Paver Section #2 5971 sq. ft.
Run-on area for Paver Section #2 (up to 3:1 is permited) 16729 sq. ft.
Impervious Area Treated 28060 sq. ft.
Total Area Treated 56164 sq. ft.
Percent Impervious Area Treated 91.0%
Percent Total Area Treated 50.0%
LID Table
APPENDIX B
HYDROLOGY CALCULATIONS WITH ASPHALT REPLACIMENT
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Value Plastics
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By:
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
(s.f.)
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.
Basin 1 86115 1.977 0.565 0.250 0.298 0.000 0.000 0.864 0.60 0.60 0.75 53.5%
Basin 2 94659 2.173 0.882 0.303 0.043 0.000 0.000 0.944 0.60 0.60 0.75 55.0%
Basin 3 99317 2.280 0.281 0.127 0.700 0.000 0.000 1.172 0.54 0.54 0.67 45.0%
OVERALL TOTAL 280091 6.430 1.728 0.680 1.041 0.000 0.000 2.981 0.58 0.58 0.72 51.0%
DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
(Parking Lots Converted to Asphalt)
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: Value Plastics
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt = L / 60V
Tc = T
i + 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,
V
(ft/s)
Rational Method Equation: Project: Value Plastics
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
basin 1 Basin 1 1.98 12 12 10 0.60 0.60 0.75 2.09 3.57 7.88 2.48 4.23 11.68
basin 2 Basin 2 2.17 13 13 11 0.60 0.60 0.75 1.98 3.39 7.57 2.59 4.44 12.39
basin 3 Basin 3 2.28 14 14 14 0.54 0.54 0.67 1.95 3.34 6.82 2.40 4.10 10.46
DEVELOPED RUNOFF COMPUTATIONS
(Parking Lots Converted to Asphalt)
C100 Notes
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)
C. Snowdon
October 30, 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 )
Page 3 of 4 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs - Asphalt.xlsx\Direct-Runoff
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
C2 C100
2-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q100
(cfs)
basin 1 Basin 1 1.98 0.60 0.75 12 10 2.48 11.68
basin 2 Basin 2 2.17 0.60 0.75 13 11 2.59 12.39
basin 3 Basin 3 2.28 0.54 0.67 14 14 2.40 10.46
Page 4 of 4 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs - Asphalt.xlsx\SUMMARY-TABLE
Basin ID
Original Percent
Impervious
Original
Design Q
Existing Percent
Impervious
Proposed Percent
Impervious
(w/ Paver Section)
Proposed Design Q (w/
Paver Section)
Percent Impervious
(Converted to Asphalt)
Proposed Design Q
(Converted to Asphalt)
1 56.5% 12.2 cfs 51.1% 53.5% 11.7 cfs 53.5% 11.7 cfs
2 66.8% 14.9 cfs 32.3% 41.0% 9.4 cfs 55.0% 12.4 cfs
3 86.8% 15.1 cfs 38.4% 41.4% 9.9 cfs 45.0% 10.5 cfs
APPENDIX C
SELECTED SHEETS FROM ORIGINAL DRAINAGE REPORT
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
This unofficial copy was downloaded on Oct-12-2015 from the City of Fort Collins Public Records Website: http://citydocs.fcgov.com
For additional information or an official copy, please contact City of Fort Collins Utilities 700 Wood Street Fort Collins, CO 80524 USA
M
CONTROL
IRR
CONTROL
IRR
UD UD UD
UD
UD UD
1
1.98 AC
3
2.28 AC
2
2.17 AC
Q100
= 0.45 CFS
DESIGN POINT A
DESIGN POINT B
EXISTING
BUILDING
EXISTING
BUILDING
EXISTING STORM
OUTLET PIPE
ORIGINAL
BASIN LINE
PROPOSED
BASIN LINE
EXISTING
AREA DRAIN
PROPOSED GRAVEL
PAVER PARKING LOT
PROPOSED GRASS PAVER
PARKING LOT
S.TIMBERLINE ROAD
EASTBROOK DRIVE
DANFIELD COURT
No. Revisions:
By: Date:
REVIEWED BY:
N. Haws
DESIGNED BY:
DRAWN BY:
SCALE:
DATE:
4.1.16
PROJECT:
207-015
Sheet
Of 9 Sheets
VALUE PLASTICS - MINOR AMENDMENT
DRAWING FILENAME: D:rojects\207-015\Dwg\Drng\207-015_DRNG.dwg LAYOUT NAME: DR1 DATE: Apr 04, 2016 - 7:09am CAD OPERATOR: Cody
LIST OF XREFS: [207-015_xEXST] [NES-xborder] [207-015_xPgrad] [207-015_xPutil] [207-015_xTOPO]
E N G I N E E R I N G
N O R T H E RN
301 North Howes Street, Suite 100
Fort Collins, Colorado 80521
www.northernengineering.com
Phone: 970.221.4158
3325 S. TIMBERLINE ROAD
04.01.16
CALL 2 BUSINESS DAYS IN ADVANCE BEFORE YOU DIG, GRADE, OR
EXCAVATE FOR THE MARKING OF UNDERGROUND MEMBER
UTILITIES.
CALL UTILITY NOTIFICATION CENTER OF
COLORADO
Know what's below.
Call before you dig.
R
LEGEND:
4953
PROPOSED CONTOUR 93
PROPOSED STORM SEWER
PROPOSED SWALE
EXISTING CONTOUR
PROPOSED PROPERTY BOUNDARY CURB & GUTTER
PROPOSED INLET
A
DESIGN POINT
FLOW ARROW
DRAINAGE BASIN LABEL
BASIN
DESIGNATION
BASIN
AREA (AC)
DRAINAGE BASIN BOUNDARY
PROPOSED SWALE SECTION
1 1
B2
1.45 AC
NORTH
( IN FEET )
0
1 INCH = 40 FEET
40 40 80 120
DR1
DRAINAGE PLAN
C. Snowden
L. Seeber
1"=40'
FOR DRAINAGE REVIEW ONLY
NOT FOR CONSTRUCTION
NOTES:
Date
Date
Date
Date
Date
Date
APPROVED:
CHECKED BY:
CHECKED BY:
CHECKED BY:
CHECKED BY:
CHECKED BY:
City Engineer
Water & Wastewater Utility
Stormwater Utility
Parks & Recreation
Traffic Engineer
Environmental Planner
City of Fort Collins, Colorado
UTILITY PLAN APPROVAL
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
basin 1 Basin 1 No 0.60 0.60 0.75 300 1.50% 14.1 14.1 9.9 N/A N/A N/A N/A N/A N/A 12 12 10
basin 2 Basin 2 No 0.60 0.60 0.75 330 2.30% 12.8 12.8 8.9 218 1.00% 2.00 1.8 N/A N/A N/A 13 13 11
basin 3 Basin 3 No 0.54 0.54 0.67 235 1.10% 15.6 15.6 11.8 N/A N/A N/A 483 3.25% 2.70 3.0 14 14 14
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
(Parking Lots Converted to Asphalt)
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
C. Snowdon
October 30, 2015
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
-
=
Page 2 of 4 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs - Asphalt.xlsx\Tc-10-yr_&_100-yr
October 30, 2015
**Soil Classification of site is Sandy Loam**
C. Snowdon
Page 1 of 4 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs - Asphalt.xlsx\C-Values
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
basin 1 Basin 1 No 0.60 0.60 0.75 300 1.50% 14.1 14.1 9.9 N/A N/A N/A N/A N/A N/A 12 12 10
basin 2 Basin 2 No 0.50 0.50 0.62 330 2.30% 15.5 15.5 12.3 218 1.00% 2.00 1.8 N/A N/A N/A 13 13 13
basin 3 Basin 3 No 0.51 0.51 0.64 235 1.10% 16.3 16.3 12.8 N/A N/A N/A 483 3.25% 2.70 3.0 14 14 14
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter Flow Swale Flow
Design
Point
Basin
Overland Flow
C. Snowdon
October 28, 2015
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Ti C Cf L
-
=
Page 3 of 5 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs.xlsx\Tc-10-yr_&_100-yr
**Soil Classification of site is Sandy Loam**
C. Snowdon
Page 2 of 5 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs.xlsx\C-Values
October 28, 2015
**Soil Classification of site is Sandy Loam**
10-year Cf = 1.00
Page 1 of 5 D:\Projects\207-015\Drainage\Hydrology\207-015_Rational-Calcs.xlsx\Historic C-Values