HomeMy WebLinkAboutDrainage Reports - 11/04/2020JJ
Addendum to Final Drainage and Erosion Control Report
Waterglen Solar Array
Fort Collins, Colorado
October 5, 2020
Prepared for:
Sandbox Solar
430 N College Ave, Suite 435
Fort Collins, Colorado 80524
Prepared by:
301 N. Howes Street Suite 100
Fort Collins, Colorado 80521
Phone: 970.221.4158
www.northernengineering.com
Project Number: 1628-001
City of Fort Collins Approved Plans
Approved by:
Date:
Wes Lamarque
11/4/2020
Drainage Addendum To
Final Drainage and Erosion Control Report for Waterglen P.U.D.
Date: October 5, 2020
Project: Waterglen Solar Array Project No. 1628-001
Preliminary Development Plan (PDP)
Fort Collins, Colorado
Attn: City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
To whom it may concern:
This letter serves to address the stormwater impacts of
. The site is located in northeast Fort Collins near East Vine Drive and Interstate I-25. More
specifically, the site is bounded to the north and west by the Larimar-Weld Canal and to the south and
east by the Waterglen P.U.D.
Engineering for that project. The proposed solar array site is approximately 37,569 square feet (0.74
acres) in area. In addition to the solar arrays, the project includes an emergency access road.
Existing Conditions
The site is within open space for the Waterglen PUD. The site is adjacent to the Larimer-Weld Canal and
the existing gravel access road along the south side of the canal. The existing topography of the proposed
site area slopes to the east at approximately 1% to 4%. The onsite soils are Nunn clay loam and
Hydraulic Soil Group C, per the USGS soils survey maps. The existing vegetation within the site area is
native grass at about 80% ground cover. The existing canal access road is gravel, and the outer edge of
the access is native grass and weeds at about 40% ground cover.
Original Drainage Patterns
The original JR Engineering drainage design included , which encompasses the proposed solar
array site, within Basin E-8. Basin E-8 is 7.19 acres and generally drains from the north to the south.
The basin drains to t designated as Design Point 34. The box culverts were
sized for the 100-year flow of 601.9 cfs. Drainage flows from Basin E-8 continue to the south to Pond C
of the Waterglen P.U.D development. Site runoff will then discharge from Pond C into the Coopers Slough
drainage. A Drainage Exhibit, and excerpts from the original report, is provided at the back of this report
for reference.
Proposed Drainage Patterns
The Waterglen Solar Array project has proposed no change to any drainage patterns or surface slopes.
Site Imperviousness Changes
A percent impervious exhibit was developed to calculate and compare the existing vs proposed site area
conditions. Using this approach, we find the proposed site conditions have a negligible increase on the
Drainage Report October 5, 2020
Sand Box Solar Page 2 of 3
composite percent impervious within Basin E-8.
improvements, is 2,445 square feet.
Runoff
The approved JR Engineering report showed a 100-yr flow of 601.9 cfs at Design Point 34. According to
the JR Engineering report, the 601.9 cfs includes overflow from the Larimer Weld Canal. The impervious
06 cfs at Design Point 34. The additional
0.06 cfs results in a negligible rise in both Swale T-T and the box culvert at Design Point 34. Due to the
minimal change, the existing storm drainage facilities will continue to operate as they did prior to the
project .
Detention
The proposed solar array and emergency access road results in a negligible increase in the depth of Pond
C. According to the approved JR Engineering report, the total depth of the 100-Year Storm Event in Pond
C is 3.5 feet. The additional 0.06 cfs from the project will result in a total depth of the 100-Year Storm
Event in Pond C of 3.5 feet.
Water Quality & LID
is between the eastern edge of
the solar array project and Design Point 34. The existing coverage consists of well-established native
grasses with over 90% coverage. This large, stabilized area will provide water quality treatment as runoff
passes over the vegetation. The project will not disturb the existing vegetation outside of the solar
envelopes, security fence, and driveway platform.
Erosion and Sediment Control
During construction, the Contractor will follow the City of Fort Collins standards for erosion and sediment
control. The total disturbance for the solar array project includes 32,695 square-feet of native topsoil and
seeding, plus 4,858 square-feet CDOT class 5 or 6 road base for the emergency access road
improvements. Since more than 10,000 square feet will be disturbed as a part of this project, an erosion
control plan and escrow has been provided. Since the project area is less than an acre, a comprehensive
Stormwater Management Plan is not required. Post construction water quality and erosion control will be
achieved by a fully established final landscape to stabilize the site and existing downstream stormwater
facilities. All areas disturbed during construction will receive permanent hardscape or landscape.
Floodplains
This project site is within the floodplain in zone AE on FEMA FIRM map 08069C0982F dated
12/19/2006 and updated by LOMR 17-08-1354P effective 2/21/2019. A floodplain use permit will be
required for the project. An as-built survey is required to document the required flood elevation protection
is satisfied.
The solar array is located outside the 100-year floodway. The solar array is also outside the drainage
conveyance channels and detention pond limits outlined in the Final Drainage & Erosion Control Report -
Waterglen PUD, by JR Engineering, dated 11/6/1998. There is to be no grading, construction activity or
storage of materials proposed in the floodway. All equipment is a minimum of 1-foot above the base flood
-construction elevation certificate must be approved before the solar array is
operational.
Drainage Report October 5, 2020
Sand Box Solar Page 2 of 3
Project Summary and Conclusions
The proposed Waterglen Solar Array site maintains the original drainage paths and results in a negligible
increase to downstream improvements. Because the increased flows are not significant no additional
drainage improvements are required for this solar project. The existing drainage improvements provided by
the original Final Drainage & Erosion Control Report - Waterglen PUD, BY JR Engineering, dated
11/6/1998 will not need any modifications or increase in detention. Therefore, Waterglen Solar Array will
not have any negative impact on the downstream drainage infrastructure.
Please do not hesitate to contact me if you have questions or require additional information.
Sincerely,
Northern Engineering Services, Inc.
Frederick S. Wegert, PE
Project Engineer
10/5/2020
ATTACHMENT 1
PREVIOUS REPORT FINAL DRAINAGE AND EROSION CONTROL REPORT WATERGLEN, P.U.D.
By JR Engineering, Ltd. Dated August 11, 1997 (revised November 6, 1998)
ATTACHMENT 2
DRAINAGE CALCULATIONS & DRAINAGE EXHIBITS
Project:
Calc. By:
Date:
Description Surface Area
(ft2)
Percent
Impervious
Impervious
Area ( ft2)
Solar Array - 0%-
Concrete - 100% -
Gravel - 50% -
Greenbelt/Landscaping 39,870 0%-
Total 39,870 -
0.0%
Description Surface Area
(ft2)
Percent
Impervious
Impervious
Area ( ft2)
Solar Array 34,149 0%-
Concrete 16 100% 16
Gravel 5,705 50%2,853
Greenbelt/Landscaping - 0%-
Total 39,870 2,869
7.2%
Description Surface Area
(ft2)
Percent
Impervious
Impervious
Area ( ft2)
Existing 0 50% -
Developed 5,721 50% 2,869
2,869
Description C2 C10 I2 in/hr I10 in/hr Q2 (cfs)Q100 (cfs)
Existing 0.50 0.63 1.95 6.82 - -
Developed 0.50 0.63 1.95 6.82 0.06 0.28
0.06 0.28
Impervious Area & Flowrate
Waterglen Solar Array
F. Wegert
May 6, 2020
Weighted Percent Impervious
Weighted Percent Impervious
Change in Flowrate (cfs)
Flowrate from Impervious Area
Developed Impervious Areas
Existing Impervious Areas
Change in Impervious Area
Net Change in Impervious Area
Existing Q2
(cfs)
Depth at
Culvert (ft)
Design Q2
(cfs)
Depth at
Culvert (ft)
Existing Q2
(cfs)
Depth at
Culvert (ft)
Design Q2
(cfs)
Depth at
Culvert (ft)
3.90 0.11 3.96 0.11 601.9 3.00 602.18 3.00
3.90 0.19 3.96 0.19 601.9 1.38 602.18 1.38
601.9 3.50 41,758 602.18 3.51
Water Surface
Area (ft2)
Design Q100
(cfs)
Depth of
Pond (ft)
Q100 w/ Road
(cfs)
Depth of
Pond (ft)
Pond C
100-Year Storm Event
2 3 ft x 10 ft Box Culvert at DP 34
Hydraulic Impacts to Waterglen Subdivison
Swale T-T Upstream from DP 34
Existing
2-Year Storm Event 100-Year Storm Event
Q2 (cfs)
Depth in
Swale(ft)Q2 (cfs)
Depth in
Swale (ft)
Existing Q2
(cfs)
Depth in
Swale(ft)
Design Q2
(cfs)
Depth in
Swale (ft)
Existing Developed Existing Developed
Developed Existing Developed
2-Year Storm Event 100-Year Storm Event