HomeMy WebLinkAboutDrainage Reports - 04/25/2018April 24, 2018
4045 St. Cloud Dr, Ste 180
Loveland, CO 80538
970.622.2095
Mr. Dan Mogen
Stormwater Engineering and Development Review Division
City of Fort Collins Utilities
700 Wood Street
Fort Collins, Colorado 80521
RE: Drainage Addendum Lightfield Enterprises, Inc.
2600 Midpoint Drive
Fort Collins, Colorado 80525
CES Project No. 2461.00
Dear Mr. Mogen:
This letter serves as an addendum to the approved Drainage Report for Lightfield Enterprises, completed
by Quality Engineering on January 2015. This addendum addresses the replacement of approximately
1,500 ft 2 (0.03 acres) of permeable pavers with concrete, and how the site will continue to meet the City
of Fort Collins LID requirements. The site is located in the southeast quarter (SE ¼) of the northwest
quarter (NW ¼) of Section 20, Township 7 North, Range 68 West of the 6th P.M. in Larimer County,
Colorado. The property address is 2600 Midpoint Drive, Fort Collins, Colorado, 80525.
The permeable pavers are being replaced by concrete with the intention being to not disturb the existing
paver subgrade/underdrain, which will be abandoned in place. It appears that the roofdrains were never
connected to the subgrade per the original plans, and roof runoff will sheet flow across the concrete
maintaining the existing drainage patterns. Per the attached calculations and information, the change
from pavers to concrete for this area will have negligible impact on impervious area and detention
requirements for the site and water quality/LID requirements are being met with the existing rain
garden.
The current City of Fort Collins LID requirements are as follows:
a) 50% of the newly added or modified impervious area must be treated by LID techniques and 25% of
new paved areas must be pervious or
b) 75% of all newly added or modified impervious area must be treated by LID techniques.
The Lightfield Enterprises site exceeds option b), 100% of all newly added or modified impervious area
is treated by the on-site rain garden. The rain garden is sized to treat the entire site, including the
additional runoff created by replacing the permeable pavers with concrete. The previous approved
Drainage Report for Lightfield Enterprises was calculated correctly, however it included a rain garden
calculation for the wrong site due to a technical issue. Please see the attachments for the correct rain
garden calculation and all relevant calculations for the replacement of permeable pavers with concrete.
City of Fort Collins Approved Plans
Approved by:
Date:
Dan Mogen
04/25/2018
P a g e |2
If you have any question or concerns, please contact Cody Geisendorfer or Ramsey Pickard at
(970) 622-2095.
Sincerely,
COFFEY ENGINEERING & SURVEYING, LLC Reviewed by,
Ramsey Pickard, EI Cody Geisendorfer, PE
Civil Designer I Project Manager
Attachment Rational Method Drainage Calculations
Effective Imperviousness Adjustment for Level 2 MDCIA
Rain Garden Calculation
Modified FAA Method Calculation
Basin Area Calculation
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, I a Ia =54.5 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100)i = 0.545
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.18 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area) Area = 37,500 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV =cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 =0.43 in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER =547.4 cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER =cu ft
(Only if a different WQCV Design Volume is desired)
2. Basin Geometry
A) WQCV Depth (12-inch maximum)DWQCV =5 in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 0.00 ft / ft
(Use "0" if rain garden has vertical walls)
C) Mimimum Flat Surface Area AMin =365 sq ft
D) Actual Flat Surface Area AActual =1970 sq ft
E) Area at Design Depth (Top Surface Area)ATop =2445 sq ft
F) Rain Garden Total Volume VT=861 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
4. Underdrain System
A) Are underdrains provided?
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y =N/A ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 =N/A cu ft
iii) Orifice Diameter, 3/8" Minimum DO =N/A in
Design Procedure Form: Rain Garden (RG)
RJP
Coffey Engineering
March 30, 2018
Vogel Concrete 2600 Midpoint Dr.
FORT COLLINS, COLORADO
UD-BMP_RAIN GARDEN.xlsm, RG 3/30/2018, 1:54 PM
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
6. Inlet / Outlet Control
A) Inlet Control
7. Vegetation
8. Irrigation
A) Will the rain garden be irrigated?
Notes:
Design Procedure Form: Rain Garden (RG)
RJP
Coffey Engineering
March 30, 2018
Vogel Concrete 2600 Midpoint Dr.
FORT COLLINS, COLORADO
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
UD-BMP_RAIN GARDEN.xlsm, RG 3/30/2018, 1:54 PM
3.15' MAX
2.34' MAX
DRAINAGE & L.I.D. PLAN
LIGHTFIELD ENTERPRISES, INC
2600 MIDPOINT DRIVE
FORT COLLINS, CO