HomeMy WebLinkAboutDrainage Reports - 12/21/2017 (3)December 15, 2017
Dan Mogen
City of Fort Collins Stormwater
700 Wood Street
Fort Collins, CO 80522-0580
RE: First Replat of Prospect Riverside Subdivision (Riverside Storage)
Dear Dan,
This letter is to summarize the proposed changes at Riverside Storage made necessary due to some
construction issues that have arisen and based on our site meetings on July 29th and December 14,
2017.
I have attached a revised grading plan sheet 4 and detail sheets 4A and 9 to show the proposed rain
garden in the southeast portion of the site and the associated details. This rain garden is proposed
to replace the grass swale along the south boundary of the site. The required calculations are
attached to this letter.
Sheet 4 also shows the addition of a 24” dry well near the existing outlet structure. The dry well is
being added to drain the bottom 8 inches of the detention pond. The dry well will extend to an
infiltrating gravely layer at a depth of 14’ to prevent standing water in the bottom of the pond. This
solution is proposed to be an interim solution while an easement is worked out with the adjacent
property owner to the north. This interim solution and easement are required due to the outlet
structure being constructed lower than designed.
The outlet structure outfall elevation will also need to be modified. I have attached a detail plan
sheet 4A which shows the detail of the interim outlet structure modification and the detail of the
dry well. The interim modification of the outlet structure and the addition of the dry well will
eliminate the need to remove and reconstruct the detention pond outlet structure.
700 Automation Drive, Unit I
Windsor, CO 80550
Phone: 970-686-6939
Fax: 970-686-1188
City of Fort Collins Approved Plans
Approved by:
Date:
Dan Mogen
12/21/2017
The retaining wall between the north most building and the Stow-Away property will be removed
and replaced with a concrete wall at the correct elevations.
I appreciate your time and consideration in reviewing this revision. Please call if you have any
questions.
Sincerely,
Patricia Kroetch, P.E.
STAGE - STORAGE TABLE
(100-YEAR)
LOCATION: First Replat of Prospect Riverside Subdivision
PROJECT NO: 264-10
COMPUTATIONS BY: PPK
SUBMITTED BY: North Star Design, Inc.
DATE: 7/8/2015
100 yr Detention Volume Required = 0.342
Water Quality Volume Required = 0.076
Total Volume Required= 0.418
DESIGN POND VOLUME
Surface Incremental Total
Stage Area Storage Storage
(ft)(ft2)(ac-ft) (ac-ft)
4934.30 0
4935.0 7,850 0.042 0.04
WQCV 4935.30 9,528 0.060 0.102
4936.0 13,442 0.242 0.284
100 WSEL 4936.36 19,395 0.134 0.418
4937.0 30,070 0.487 0.770
AS CONSTRUCTED POND VOLUME
Surface Incremental Total
Stage Area Storage Storage
(ft)(ft2)(ac-ft) (ac-ft)
4934.02 0
4935.0 6,956 0.052 0.05
WQCV 4935.27 8,823 0.050 0.102
4936.0 13,769 0.233 0.286
4936.2 16,068 0.068 0.354
100 WSEL 4936.36 18,726 0.134 0.419
4936.4 19,277 0.151 0.437
POND VOLUME with Rain Garden (Revised 10/10/17)
Surface Incremental Total
Stage Area Storage Storage
(ft)(ft2)(ac-ft) (ac-ft)
4934.02 0
4935.0 6,956 0.052 0.05
WQCV 4935.27 8,693 0.049 0.101
4936.0 13,294 0.229 0.281
4936.2 15,648 0.066 0.347
100 WSEL 4936.38 18,628 0.139 0.419
4936.4 18,909 0.147 0.428
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia =74.0 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100)i = 0.740
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 = 5,952 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV =117 cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER =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 =6 in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft
(Use "0" if rain garden has vertical walls)
C) Mimimum Flat Surface Area AMin =88 sq ft
D) Actual Flat Surface Area AActual =180 sq ft
E) Area at Design Depth (Top Surface Area)ATop =340 sq ft
F) Rain Garden Total Volume VT=130 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
12" 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)
Patricia Kroetch
North Star Design Inc
October 31, 2017
Riverside Storage
Basin 2
UD-BMP (Version 3.06, November 2016)
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
Rain Garden.xlsm, RG 10/31/2017, 11:26 AM
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?
PROVIDE A 30 MIL (MIN) PVC LINER WITH CDOT CLASS B
GEOTEXTILE ABOVE IT. USE THE SAME GEOTEXTILE BELOW
THE LINER IF THE SUBGRADE IS ANGULAR
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)
Patricia Kroetch
North Star Design Inc
October 13, 2017
Riverside Storage
Basin 2
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
Rain Garden.xlsm, RG 10/13/2017, 2:09 PM
0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00 9.00
35.03
35.23
35.43
35.63
35.83
36.03
36.23
36.43
36.63
36.83
HGL
EGL
Capacity (cfs)