HomeMy WebLinkAboutFAIRVIEW SHOPPING CENTER, FILING FOUR - FDP - FDP120007 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTADDRESS:
200 S. College Ave. Suite 10
Fort Collins, CO 80524
PHONE: 970.221.4158
FAX: 970.221.4159
WEBSITE:
www.northernengineering.com
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D:\Projects\688-001\Drainage\Reports\Drainage Report\688-001_Drainage-Letter_2012-02-28.docx
Drainage Letter Report
Date: February 28, 2012
Project: Fairview Shopping Center Filing Four Project No. 688-001
Replat of Lot 1, Fairview Shopping Center
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 above referenced multi-family redevelopment.
The existing building is situated towards the western portion of the site. The site currently consists of
rooftop, concrete walks and patio, asphalt parking areas, some turf, trees, and other vegetation. It
presently functions similar to a ‘B’ lot, with historic drainage being split towards Elizabeth Street to the
north and towards an area inlet to the south. The landscaped area adjacent to the western property line
has a low point such that it too drains primarily to the south. Flows then reach an area inlet located on
the C.B. & Pott’s Restaurant and Brewery Property. Historic drainage patterns will generally be preserved.
The total impervious area of the existing site is approximately 22,611 square-feet.
The proposed redevelopment will completely remove all existing above-grade structures and adjacent
hardscape. The new project will consist of a larger apartment building, as well as a modified parking area
and bioretention rain garden. Concrete pedestrian connections will be made from the units north to the
Elizabeth Street sidewalk. The total impervious area of the proposed redevelopment is roughly 22,688
square-feet.
The existing curb cut and concrete apron to Elizabeth Street will be slightly reconfigured to meet current
code requirements. The area of pavement draining north across the Elizabeth sidewalk will be
approximately 450 square-feet, which is less than the maximum 750 square-feet allowed by LCUASS.
The site is located in the Old Town master drainage basin, and therefore, on-site detention is not required
so long as the increase in impervious area is less than 5,000 square-feet. The total increase in impervious
area proposed with this development is approximately 77 square-feet (22,688 – 22,611). A detailed
breakdown of the existing and proposed impervious areas is attached to this letter.
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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.
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.
This portion of the Old Town basin does not receive regional water quality treatment; therefore, on-site
water quality mitigation is required. Much of the existing landscaped buffers on the west side and north
side of the building will be preserved as permeable areas to promote infiltration and treat surface runoff
prior to entering the adjacent gutters and inlets. However, the primary water quality treatment method to
be employed is the construction of an on-site bioretention rain garden. Water quality computations have
been performed for this facility utilizing the UDFCD’s UD-BMP Version 3.01 “Rain Garden (RG)” Design
Procedure Form. A copy of the RG worksheet is attached to this letter.
The proposed grading concept closely matches existing elevations and drainage patterns, and only varies in
an effort to improve the existing condition from a floodplain protection and water quality standpoint. On-
site detention is proven to be unnecessary. Stormwater quality has been provided to bring the historic site
into compliance in the redeveloped condition. Therefore, it is my professional opinion that the new
apartment building proposed with Fairview Shopping Center Filing Four at 1409 W. Elizabeth Street
satisfies all applicable stormwater criteria.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Nicholas W. Haws, PE
enc.
cc: Dave Derbes, LEED AP – Brinkman Construction
02.28.12
Fairview Shopping Center Filing Four
Preliminary Drainage Letter Report
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Fairview Shopping Center
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
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
H1 29,010 0.67 0.38 0.02 0.11 0.00 0.15 0.80 0.80 0.99 75.9
TOTAL 29,010 0.67 0.38 0.02 0.11 0.00 0.15 0.80 0.80 0.99 75.89
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.
10-year Cf = 1.00
February 28, 2012
D:\Projects\688-001\Drainage\Hydrology\688-001_Rational-Calcs_Existing.xlsx\C-Values
Fairview Shopping Center Filing Four
Preliminary Drainage Letter Report
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Fairview Shopping Center
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
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(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 1,349 0.03 0.00 0.00 0.00 0.00 0.03 0.25 0.25 0.31 0.0
B 8,731 0.20 0.00 0.00 0.20 0.00 0.00 0.95 0.95 1.00 90.0
C 4,743 0.11 0.03 0.03 0.00 0.00 0.04 0.66 0.66 0.83 56.5
D 3,284 0.08 0.04 0.02 0.00 0.00 0.01 0.85 0.85 1.00 82.3
E 10,903 0.25 0.14 0.05 0.00 0.00 0.06 0.79 0.79 0.98 74.8
TOTAL 29,010 0.67 0.22 0.10 0.20 0.00 0.15 0.80 0.80 1.00 73.73
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
Fairview Shopping Center Filing Four
Preliminary Drainage Letter Report
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia = 75.8 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.758
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.24 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area) Area = 29,309 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 = 0.30 in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER = 419.4 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 = in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = ft / ft
(Use "0" if rain garden has vertical walls)
Design Procedure Form: Rain Garden (RG)
Andy Reese
Northern Engineering
February 28, 2012
Fairview Shopping Center Filing Four
Fort Collins, Colorado
C) Mimimum Flat Surface Area AMin = sq ft
D) Actual Flat Surface Area AActual = sq ft
E) Area at Design Depth (Top Surface Area) ATop = sq ft
F) Rain Garden Total Volume VT= cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
City of Fort Collins Standard Planting 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 = 0.5 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 = 419 cu ft
iii) Orifice Diameter, 3/8" Minimum DO = 0.63 in
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
688-001_UD-BMP_v3_01.xls, RG 2/28/2012, 3:34 PM
Fairview Shopping Center Filing Four
Preliminary Drainage Letter Report
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
NO SPRINKLER HEADS ON THE FLAT SURFACE
A) Will the rain garden be irrigated?
Notes:
February 28, 2012
Fairview Shopping Center Filing Four
Fort Collins, Colorado
Design Procedure Form: Rain Garden (RG)
Andy Reese
Northern Engineering
Choose One
Choose One
Choose One
Sheet Flow- No Energy Dissipation Required
Concentrated Flow- Energy Dissipation Provided
Plantings
Seed (Plan for frequent weed control)
Sand Grown or Other High Infiltration Sod
Choose One
YES
NO
YES
NO
688-001_UD-BMP_v3_01.xls, RG 2/28/2012, 3:34 PM
February 28, 2012
D:\Projects\688-001\Drainage\Hydrology\688-001_Rational-Calcs.xlsx\C-Values