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Final Drainage and Erosion Control Report
Bloom Multi Family
Fort Collins, CO
Prepared for:
Hartford Acquisitions, LLC
4801 Goodman Road
Timnath, CO 80547
June 25, 2024
City of Fort Collins Approved Plans
Approved by:
Date:
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
i C22026 - Bloom Multi Family Drainage Report FDP 3rd Round.docx
June 25, 2024
Mr. Wes Lamarque
City of Fort Collins Engineer
4750 Signal Tree Drive
Timnath, CO 80547
RE: Bloom Multi Family Final Design and Erosion Control Report
Project Number: C22026
Dear Wes,
CivilWorx, LLC is pleased to submit this updated Final Design and Erosion Control Report for the Bloom
Multi Family Development in Fort Collins, Colorado. This report serves to document the Final design plans
for the Bloom Development. The recent modifications include some changes to the local catchment
delineations based on a review of the overall drainage plan for the greater subdivision and minor internal
changes to the site. In general, the proposed plan is not substantially different from the drainage plan
presented with the preliminary plan review.
We understand that review by the City of Fort Collins is to assure general compliance with standardized
criteria contained in City of Fort Collins Storm Drainage Design Criteria and Construction Standards. This
report was prepared in compliance with technical criteria set forth in both the Urban Drainage and Flood
Control District (UDFCD) Urban Storm Drainage Criteria Manual and the City of Fort Collins Storm Drainage
Design Criteria.
If you should have any questions or comments as you review this report, please feel free to contact me at
your convenience.
Respectfully,
Kelly W. Hafner, PE
Senior Project Manager
CivilWorx, LLC
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
ii C22026 - Bloom Multi Family Drainage Report FDP 3rd Round.docx
TABLE OF CONTENTS
PAGE
I. General Location and Description ........................................................................................................................ 1
A. Location ....................................................................................................................................................... 1
B. Description of Property ............................................................................................................................... 1
II. Drainage Basins and Catchments ......................................................................................................................... 2
A. Major Basin Description............................................................................................................................... 2
B. Pre-Project Drainage Patterns ..................................................................................................................... 3
III. Drainage Design Criteria ...................................................................................................................................... 3
A. Regulations .................................................................................................................................................. 3
B. Development Criteria Reference and Constraints ....................................................................................... 3
C. Hydrological Criteria .................................................................................................................................... 3
D. Hydraulic Criteria and Governing Standards ............................................................................................... 3
IV. Drainage Facility Design ....................................................................................................................................... 4
A. General Developed Project Overview .......................................................................................................... 4
V. Sediment/Erosion Control .................................................................................................................................. 12
VI. Conclusions ........................................................................................................................................................ 13
A. Compliance with Standards ....................................................................................................................... 13
B. Drainage Plan ............................................................................................................................................. 13
VII. References ..................................................................................................................................................... 14
VIII. Certification ................................................................................................................................................... 14
List of Figures
Figure 1. Vicinity and Phase Map................................................................................................................................... 1
Figure 2. Lower Cooper Slough Drainage Basin As Shown on the City of Fort Collins Website .................................. 2
Figure 3. Proposed Drainage Plan .................................................................................................................................. 5
List of Tables
Table 1. Existing Catchment Runoff Summary............................................................................................................... 3
Table 2. Developed Catchment Runoff Summary ......................................................................................................... 7
Table 3. Storm Sewer Piping Summary .......................................................................................................................... 8
Table 4. Storm Sewer Hydraulic Grade Line Summary .................................................................................................. 9
Table 5. Storm Sewer Inlet Interception Summary ..................................................................................................... 10
APPENDICES
APPENDIX A Drainage and Erosion Control Plans
APPENDIX B Hydrological Modelling (EPA SWMM)
APPENDIX C LID/Water Quality Calculations
APPENDIX D Referenced Reports
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Fort Collins, CO
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I.General Location and Description
A. Location
The Bloom Development (referred herein as is located north of State Highway 14 (Mulberry
Street) and just west of Interstate 25. The site is located within the southwest half of Section 9, Township
7 North, Range 68 West of the 6th Principal Meridian in the City of Fort Collins, County of Larimer, State of
Colorado. The site is located on the parcel of land # 8709300004 with coordinates of 40.587086, -
105.016334. A vicinity map is shown with Figure 1.
Figure 1. Vicinity and Phase Map
B. Description of Property
Currently, the property is undeveloped but surrounded by developing roads and a larger new subdivision.
There are no structures or significant infrastructure located within the site. The historic use of the
property was agriculture and farming and thus characterized as a tilled field. The site currently has been
overlot graded and largely filled. Below is a detailed summary of the existing conditions:
Ground Cover - The site was being used as cropland and therefore ground conditions can be
characterized as tillage/field with varying seasonal crops.
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Grades In general, the entire site drains to the south. These grades are generally less than 1%,
with a few localized areas that slope at approximately 2%.
Soil Type -According to the Natural Resources Conservation (NRCS) websoil survey and report, the
site consists of Garret loam and Ascalon sandy loam soils of hydrologic soils groups Type B and C.
Soils are assumed type C/D for drainage purposes.
Utilities The site does not have an existing utilities with the exception of storm sewer within
property. Domestic water, sanitary sewers, and storm water utilities have been designed and are
planned to be built with the adjoining roads which are being constructed by others. Natural gas,
telephone, fiber, and electric are planned to be installed in and around the site at a future time.
Storm Drainage / Irrigation Facilities The site does contain storm drainage infrastructure
previously designed for the greater subdivision. No irrigation facilities are present on the site.
Groundwater was observed at depths of 10 to 12 below the general project site (Appendix D).
Note the site has been filled several feet with the initial overlot grading and Pond G was excavated
for the greater subdivision overlot activity. Groundwater is interpolated not be within 2 of sub-
surface drainage facilities designed with this project.
II.Drainage Basins and Catchments
A. Major Basin Description
This site is located within the Cooper Slough drainage basin as most recently described in the Mulberry
Master Drainage Plan (MDP) by Galloway Engineering Firm and dated April 6th, 2021, with the last revision
of the report being dated as June 20, 2023.
Figure 2. Lower Cooper Slough Drainage Basin As Shown on the City of Fort Collins Website
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B. Pre-Project Drainage Patterns
The project site is located within the Cooper Slough Drainage Basin as shown Figure 2 above. The
proposed site area consists of approximately 15.8 acres. Four (4) proposed roads that will be
constructed as part of the Bloom 1st Filing Project and shown in the construction plans by Galloway
engineering surround the site property: Aria way on the west, Greenfields Drive on the east,
International Boulevard to the north, and Donella Drive to the south. The entire site drains to the south
but the site is generally at a very low gradient. The site has localized areas that have grades of
approximately 2%, but most grades are less than 1%. The nearest receiving waters to the site are the
Cooper slough drainage and wetlands (approximately 1,200 feet from the site) that flow into the
Boxelder Creek, which further transports water to the Poudre River. Below is summary of the existing
runoff generated by the site as it exists pre-development.
Table 1. Existing Catchment Runoff Summary
Existing Site Area (ac.) %
Imperv.
Q2
cfs
Q100
cfs
Bloom Property 15.83 2 0.92 19.0
III. Drainage Design Criteria
A. Regulations
The design criteria for this development are the City of Fort Collins Stormwater Drainage Criteria Manual
(FCSDCM) dated March 2018 and the Mile High Flood District Criteria Manuals Volumes 2, and 3 (referred
to herein as MHFDCM).
B. Development Criteria Reference and Constraints
The criteria used as the basis to analyze and design stormwater features of this project were done so
according to the references cited in this report.
C. Hydrological Criteria
In accordance with the City of Fort Collins policy, a minor and major storm are identified as the 2-year and
100-year storms, respectively. These storms have been used as a basis for planning and design. The
proposed developed drainage networks, including the stormwater detention system, are designed using
the USEPA Stormwater Management Model (SWMM version 5.2.4) software. SWMM input parameters
for catchment runoff utilized Table 4.1-1 of the referenced FCSDCM.
Post development runoff for the site will be in accordance with the Mulberry and Greenfields Master
Drainage Plan developed by Galloway Engineering.
D. Hydraulic Criteria and Governing Standards
The developed site will convey runoff to a proposed detention pond on the southeast corner of the site
via a combination of channelized flow, sheet flow, and storm sewer, then release to the downstream
system per the approved Mulberry and Greenfields Master Drainage Report as currently amended by
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Bloom Multi Family Development
Fort Collins, CO
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Galloway Engineering. SWMM modeling was used to estimate the required detention pond volume for
the Bloom site. Hydraulic calculations for all pipes and swales were also completed using EPA-SWMM and
are included in this report. SWMM catchment width parameters are calibrated using the theory of
cascading planes (see Guo reference). This method was also documented in the 2016 SWMM Hydrology
Reference Manual published by the EPA.
IV. Drainage Facility Design
A. General Developed Project Overview
The developed Bloom project site area includes the construction of multi-family homes, parking areas,
landscaped areas and access roads. The Bloom multi-family subdivision project is part of the Cooper
Slough basin whose ultimate outfall is the Boxelder Creek, located south of Colorado Highway 14
(Mulberry Street). Runoff from the Bloom-multifamily site will be captured using a combination of curb &
gutter and storm sewer pipe and conveyed to a proposed onsite detention facility. The proposed
detention pond, designated as Pond G, will also include inflow from adjacent property to the east, west
and north as shown on the Mulberry and Greenfields Master Drainage Plan (as currently amended) by
Galloway Engineering.
Information on the proposed Bloom site detention pond (Pond G) outlet structure listed the peak outflow
is limited to 13.5 cfs and requires a minimum of 4.8 AF of storage to work with the overall drainage master
plan. It is noted that the Galloway drainage modelling for the Bloom site and Pond G assumed 100%
impervious coverage based on the catchment parameters within the referenced report (Appendix D). The
proposed impervious area which considerably less than the full impervious coverage originally assumed
by the drainage master plan. Modifications to the drainage plan from previous submittals include the
addition of building roof drain collection and a more detailed design for the LID chamber system. The
changes in the local catchment reflect the additional collection of roof drainage which is brought into the
site internal drainage system and directed to the LID system.
Overall Drainage Plan
The overall drainage plan is to convey runoff to the detention facility on the southeast corner of the site
prior to discharging via a proposed outlet structure and storm pipe. Following is an analysis of the site
drainage concepts and routing as well as a summary of all of the proposed on site drainage Catchments.
All flow summaries at design points, including conveyance facilities, have been designed in detail as part
of the EPA SWMM model.
Catchment 01:
The 01 Catchment has an area of 2.29 acres and generally flows to the southeast. Runoff is conveyed via
street flow and channelized curb/gutter/pan flow to a Type C inlet located at the southeastern corner of
the Catchment. From here it is carried through Storm Line A into the proposed-on site detention pond.
Street capacities, inlet sizing, and storm sewer sizing were all completed using EPA-SWMM.
Catchment 02:
The 02 Catchment has an area of 1.77 acres and generally flows to the southeast. Runoff is conveyed via
street flow and channelized curb/gutter/pan flow to a Type C inlet located at the southern side of the
Catchment. From here it is carried through Storm Line C & B and A respectively, after which it drains into
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Fort Collins, CO
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the proposed-on site detention pond. Street capacities, inlet sizing, and storm sewer sizing were all
completed using EPA-SWMM.
Figure 3. Proposed Drainage Plan
Catchment 03:
The 03 Catchment has an area of 2.18 acres, is located at the central eastern portion of the site and
generally flows to the south. Runoff is conveyed via street flow and channelized curb/gutter/pan flow to
a Type R inlet located at the southern portion of the Catchment. From here it is carried through Storm
Lines C, B, and A respectively, after which it drains into the proposed-on site detention pond. Street
capacities, inlet sizing, and storm sewer sizing were all completed using EPA-SWMM.
Catchment 04:
The 04 Catchment has an area of 1.65 acres, is located at the central western portion of the site and
generally flows to the south. Runoff is conveyed via street flow and channelized curb/gutter/pan flow to
a Type C inlet located at the south side of the Catchment. From here it is carried through Storm Line A,
after which it drains into the proposed-on site detention pond. Street capacities, inlet sizing, and storm
sewer sizing were all completed using EPA-SWMM.
Catchment 05:
The 05 Catchment has an area of 1.03 acres, is in the central portion of the site and generally flows to the
south. Runoff is conveyed via street flow and channelized curb/gutter/pan flow to a Type R inlet located
at the southern portion of the Catchment. From here it is carried through Storm Lines B and A
respectively, after which it drains into the proposed-on site detention pond. Street capacities, inlet sizing,
and storm sewer sizing were all completed using EPA-SWMM.
Catchment 06:
The 06 Catchment has an area of 0.31 acres, is in the central portion of the site and generally flows to the
south. Runoff is conveyed via street flow and channelized curb/gutter/pan flow to a sidewalk chase/curb
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Fort Collins, CO
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cut where water then travels further south via a grass swale and into the proposed-on site detention
pond. Street capacities, inlet sizing, and storm sewer sizing were all completed using EPA-SWMM.
Catchment 07:
The 07 Catchment has an area of 2.01 acres, is located at the southwestern portion of the site and flows
to the center of itself where a proposed low point and Type R inlet is located. Runoff is conveyed via
street flow and channelized curb/gutter/pan flow to the Type R inlet. From here it is carried north and
east through Storm Lines A, after which it drains into the proposed-on site detention pond. During
to Pond A to control the HGL in this location for major storm events. Street capacities, inlet sizing, and
storm sewer sizing were all assessed using the post-project EPA-SWMM model.
Catchment 08:
The 08 Catchment has an area of 2.39 acres and is located at the southwestern portion of the site. This
Catchment is where the proposed detention pond for the site lies, and water eventually leaves the
detention pond via an outlet control structure located at the southwest corner of the Catchment. Overall
pond volume requirements were calculated with EPA-SWMM in conjunction with the Mile High Flood
District Drainage Criteria.
OS (offsite)_Catchments
OS 1: Catchment OS-1 is 0.46 acres, located at the Northwestern portion of the site and drains offsite to
the west via sheet flow and curb and gutter flow. All of the OS-1 flows will be captured in the proposed
Bloom Filing 1 storm sewer infrastructure and directed via storm pipe to detention features planned with
Bloom Filing 1 detention facilities.
OS 2: Catchment OS-2 is 0.22 acres, located at the Northwestern portion of the site and drains offsite to
the west via sheet flow and curb and gutter flow. All of the OS-2 flows will be captured in the proposed
Bloom Filing 1 storm sewer infrastructure and directed via storm pipe and inlets to the existing-off site
detention pond located on the north side of International Boulevard.
OS 3: Catchment OS-3 is 0.58 acres, located at the Northeastern portion of the site and drains offsite to
the north and east via sheet flow and curb and gutter flow. All of the OS-2 flows will be captured in the
proposed Bloom Filing 1 storm sewer infrastructure and directed via storm pipe and inlets to an offsite
detention pond to the east of the site.
OS 4: Catchment OS-4 is 0.51 acres, located at the western portion of the site and drains offsite to the
west via sheet flow and curb and gutter flow. OS-4 flows will be captured in the proposed Bloom Filing 1
storm sewer infrastructure and directed via storm pipe and inlets to the off-site detention facilities
associated with Bloom Filing 1.
OS 5: Catchment OS-5 is 0.40 acres, located at the southwestern portion of the site and drains to the west
via sheet flow and curb and gutter flow. OS-5 flows will be captured in the proposed Bloom Filing 1 storm
sewer infrastructure and directed via storm pipe and inlets to the off-site detention facilities associated
with Bloom Filing 1.
OS 6-9: Catchments OS-6 through OS-9 comprise perimeter and local street catchments that are not part
of the site but are included on the initial drainage plan as designs collaboration continues with the
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Galloway project. These catchments comprise 1.21 acres and are made up of areas that will be flowing
to the proposed detention pond that are not on the Bloom-multifamily property and is not included in the
pond WQ calculations provided in Appendix B. These Catchments include areas that surrounds the site
located within City of Fort Collins Right-of-Way and includes mostly from back of curb and towards the
property line primarily perimeter landscaping, walks, and drive areas. The surrounding street drainage
system was designed by Galloway.
Drainage Catchment Summaries
The following is a summary of the Catchment runoff characteristics:
Table 2. Developed Catchment Runoff Summary
Catchment Area
(ac.)
%
Imperv.
C2 Q2
cfs
C100 Q100
cfs
C-01 2.296 83% 0.757 2.7 0.926 16.0
C-02 1.769 83% 0.759 2.3 0.927 12.9
C-03 2.181 87% 0.793 2.7 0.94 15.7
C-04 1.646 88% 0.803 2.4 0.944 12.8
C-05 1.030 71% 0.656 1.5 0.889 7.6
C-06 0.314 46% 0.445 0.4 0.809 2.1
C-07 2.105 94% 0.854 2.7 0.963 15.6
C-08 2.387 23% 0.135 0.8 0.713 14.9
C-OS-1 0.464 52% 0.487 0.5 0.821 2.9
C-OS-2 0.224 21% 0.206 0.1 0.699 1.0
C-OS-3 0.579 22% 0.215 0.3 0.703 2.7
C-OS-4 0.506 55% 0.519 0.7 0.837 3.5
C-OS-5 0.400 27% 0.265 0.3 0.733 2.2
C-OS-6 0.275 40% 0.374 0.2 0.767 1.3
C-OS-7 0.303 23% 0.222 0.2 0.702 1.3
C-OS-8 0.164 33% 0.317 0.1 0.752 0.9
C-OS-9 0.464 24% 0.227 0.3 0.685 1.6
Note the infiltration parameters prescribed by the FCSDCM represent fully saturated underlying soil
moisture conditions and thus produce very high runoff rates and volumetric yields.
Storm Sewers
The proposed drainage system for the Bloom Multifamily development was modeled in EPA-SWMM to
determine pipe sizing and flow routing. The SWMM models for the 100-year and 2-year storms are
included in Appendix B for reference.
Storm Line A This system drains the western portion of the site to the proposed WQCV
chambers and detention pond. This storm sewer connects to the other storm sewer
networks proposed at the site at STMH A-1-1. This system contains connected Type C
inlets and manholes working in series. Building roof drains are connected to this system.
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Storm Line B This system collects drainage from the central and southern portions of
the site via a type R inlet and receives flows from Storm Line C. This line is connection to
the LID chambers.
Storm Line C This system collects drainage from the eastern central and northern of the
site using 2 Type C inlets and conveys it through Storm Lines B then to the A system
respectively and into the proposed LID system and then to the detention pond. Building
roof drains are connected to this system.
Storm Line D This system collects drainage from the southwesterly portion of the site
ern central and northern of the site using manholes and a Type R inlet and conveys to the
Storm Lines A system respectively and into the proposed LID system. Building roof drains
are connected to this system.
Storm Line D-1 This system is an overflow outlet for major storm ponding in catchment
7. A higher stage outlet pipe is connected to a type R inlet to allow major storm surcharge
in the storm sewer network to reach Pond G.
Table 3 summarizes the pipe flows for the storm sewer systems for the project.
Table 3. Storm Sewer Piping Summary
Link Element Size
(in)
V2
fps
Q2
cfs
V100
fps
Q100
cfs
Upstream
Node
Downstream
Node
P-A-1 18 3.7 1.2 3.9 3.8 J-SDI-A-1-5 J-SDI-A-1-4
P-A-2 18 2.6 1.8 3.7 6.5 J-SDI-A-1-4 J-SDMH A-1-3
P-A-3 18 4.1 6.4 4.7 8.3 J-SDMH A-1-3 J-SDI A-1-2
P-A-4 18 5.2 9.2 7.9 13.9 J-SDI A-1-2 J-SDMH A1-1
P-A-5 24 3.9 10.2 6.3 19.7 J-SDMH A-4 J-SDMH A1-1
P-A-6 24 1.3 4.1 6.2 19.6 J-SDMH A-4 J-SDMH A-3-IN
P-A-7 24 2.9 4.2 6.4 20.2 J-SDMH A-3-OUT J-SDMH A-2
P-A-8 24 3.3 4.2 6.4 20.2 J-SDMH A-2 POND-G
P-B-1 18 1.4 1.3 4.0 7.1 J-SDI B-1 J-SDMH A1-1
P-C-A 18 1.7 1.0 1.8 3.1 J-SDI C-3 J-SDI-C-3-1
P-C-B 18 2.4 2.2 3.4 5.9 J-SDI-C-3-1 J-SDMH C-1
P-C-C 18 2.2 2.4 3.2 5.6 J-SDI-C-1 J-SDMH C-1
P-C-D 18 4.0 4.6 4.1 6.4 J-SDMH C-1 J-SDMH A-1-3
P-D-1-A 18 1.0 1.7 2.0 3.0 J-SDMH_D.1.1 J-SDMH A-4
P-D-1-B 18 3.0 1.7 3.9 3.0 J-SDMH_D.1.1 J-SDMH D-1.2
P-D-1-C 18 2.1 1.3 2.6 2.1 J-SDMH D-1.3 J-SDMH D-1.2
P-D-1-D 18 0.9 1.2 1.2 2.0 J-SDMH D-1.3 J-SDMH D1.5
P-D-1-E 18 0.4 0.3 0.5 0.5 J-SDMH D1.6 J-SDMH D1.5
P-D-1-F 18 4.1 2.6 4.0 5.9 J-SDI-D-1.5.1 J-SDMH D1.5
P-D-2-A 18 0.5 0.5 0.7 1.1 J-SDMH_D.2.1 J-SDMH D-1.2
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Table 3. Storm Sewer Piping Summary
Link Element Size
(in)
V2
fps
Q2
cfs
V100
fps
Q100
cfs
Upstream
Node
Downstream
Node
P-D-2-B 18 0.3 0.2 0.6 0.6 J-SDMH_D.2.2 J-SDMH_D.2.1
P-D-2-C 8 0.0 0.0 0.6 0.2 POC_RD-D-2.2 J-SDMH_D.2.2
P-D-2-D 8 0.1 0.0 0.7 0.2 POC_RD-D-2.1 J-SDMH_D.2.1
P-E-1-N 24 3.5 8.5 4.1 12.0 J-SDMH A-4 J-SDZ-CHAMB_N
P-E-1-S 24 4.2 3.5 4.7 7.8 J-SDMH D1.5 J-SDZ-CHAMB_S
P-E-CHAMB chamber 0.2 4.1 0.4 5.3 J-SDZ-CHAMB_S J-SDZ-CHAMB_N
P-E-UD 6 1.8 0.4 2.9 0.6 J-SDZ-CHAMB_N J-SDMH A-3-IN
P-F-1 18 1.9 0.8 3.2 5.7 J-SDI-D-1.5.1 POND-G
P-Z-POND-OUT 18 2.7 1.0 2.7 1.0 POND-G OUT-POND
Table 4 summarizes the hydraulics for of the proposed drainage plan for the major and minor storm
events.
Table 4. Storm Sewer Hydraulic Grade Line Summary
Node Element D2
ft
HGL2
ft
Q2
cfs
D100
ft
HGL100
ft
Q100
cfs
Storm Sewer Structures
J-SDI A-1-2 2.8 4935.9 9.2 6.6 4939.7 14.8
J-SDI B-1 2.3 4935.7 1.3 5.4 4938.8 10.5
J-SDI C-3 0.3 4938.6 1.0 2.5 4940.8 3.1
J-SDI-A-1-4 0.7 4936.1 1.8 5.4 4940.8 6.5
J-SDI-A-1-5 0.3 4938.7 1.2 2.7 4941.2 3.8
J-SDI-C-1 0.9 4937.0 2.4 3.9 4940.0 5.6
J-SDI-C-3-1 0.9 4936.9 2.3 4.6 4940.6 5.9
J-SDI-D-1.5.1 2.2 4935.7 2.6 4.3 4937.8 11.0
J-SDMH A1-1 3.3 4935.7 10.3 5.7 4938.2 19.8
J-SDMH A-1-3 1.2 4936.1 6.4 5.6 4940.4 8.5
J-SDMH A-2 1.0 4932.3 4.2 3.3 4934.6 20.2
J-SDMH A-3-IN 3.8 4935.6 4.3 4.7 4936.5 20.2
J-SDMH A-3-OUT 0.9 4932.7 4.3 4.4 4936.2 20.2
J-SDMH A-4 3.8 4935.7 10.2 5.8 4937.7 19.7
J-SDMH C-1 1.0 4936.8 4.6 4.6 4940.4 6.6
J-SDMH D-1.2 2.5 4935.7 1.7 4.6 4937.7 3.0
J-SDMH D-1.3 2.9 4935.7 1.3 5.0 4937.7 2.1
J-SDMH D1.5 3.4 4935.7 3.6 5.4 4937.7 7.9
J-SDMH D1.6 2.9 4935.7 0.1 4.9 4937.7 0.3
J-SDZ-CHAMB_N 3.7 4935.7 8.8 5.7 4937.7 12.3
J-SDZ-CHAMB_S 3.4 4935.7 7.5 5.4 4937.7 12.4
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Table 4. Storm Sewer Hydraulic Grade Line Summary
Node Element D2
ft
HGL2
ft
Q2
cfs
D100
ft
HGL100
ft
Q100
cfs
J-STMH-D-1.1 3.2 4935.7 1.7 5.2 4937.7 3.0
Roadway Flowline/Curb at Storm Sewer Inlets
RD-SDI A1-2 0.2 4939.2 2.9 0.74 4939.74 20.0
RD-SDI-A-1-4 0.1 4941.2 1.4 0.27 4941.4 11.6
RD-SDI-A-1-5 0.2 4942.7 2.7 0.3 4942.9 15.8
RD-SDI-B-1 0.1 4938.3 1.4 0.7 4938.9 44.3
RD-SDI-C-1 0.2 4939.3 2.5 0.8 4939.9 27.6
RD-SDI-C-3-1 0.2 4940.6 3.8 0.4 4940.8 24.2
RD-SDI-C-3A 0.1 4942.2 2.3 0.3 4942.4 12.8
RD-SDI-D-1.5.1 0.2 4937.2 2.6 1.0 4938.0 11.9
RD-SOUTHPARKING* 0.2 4937.2 2.7 0.9 4938.0 15.6
* Note this is the parking lot in catchment 7 modeled for ponding potential.
Street Capacity
The proposed drainage system for the development was modeled in SWMM with depths captured at
the storm sewer inlet locations. This included internal surface conveyance pathways. The internal
gutter system capacity is presented as adequate for the conveyance of runoff to the perspective outfalls.
Inlets
The proposed drainage system was modeled in EPA-SWMM. Inlet sizing and intercepted flows were
modeled using the SWMM analysis, with corresponding flowline depths associated with inlets listed in
Table 4 from the assessment.
Table 5 below summarizes intercepted flow with the proposed inlets for the minor and major storms:
Table 5. Storm Sewer Inlet Interception Summary
Inlet Name Inlet Type/Size Q2
cfs
Q100
cfs
SDI A1-2 2.8 8.5
SDI B-1 1.3 10.5
SDI C-3 Type C 1.0 3.1
SDI-A-1-4 Type C 0.7 2.7
SDI-A-1-5 Type C 1.2 3.8
SDI-C-1 2.4 5.6
SDI-C-3-1 Type C 1.3 3.6
SDI-D-1.5.1 2.6 11.0
Detention Pond Summary
The onsite detention pond (G) was modeled in the system noting the pond volumes and release rates
were completed with the overall subdivision drainage assessment (Appendix D). The Bloom site detention
pond G also includes off-site flows and therefore, final calculations were taken from the Bloom Filing 1
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
11
Amendment Drainage Modifications report by Galloway Engineering for the overall development.
Amendment 1, excerpts in Appendix D, required a minimum storage volume of 4.8 Acre-ft of storage. The
grading design shown on the submitted plans for this facility includes an active storage of 5.1 AF with 1
foot of freeboard below spillway. The proposed design exceeds the required volume for the overall
drainage master plan.
WQCV Overall Subdivision
The Bloom site detention pond G also includes provisions for WQCV for the greater subdivision. Final
calculations for the facilities previously designed were taken from the Bloom Filing 1 Amendment
Drainage Modifications report by Galloway Engineering for the overall development. Amendment 1,
excerpts in Appendix D, required a minimum WQCV 0.78 Acre-ft of storage. The grading design for Pond
G shown on the submitted plans for this facility includes an active WQCV of 1.1 AF. The proposed design
exceeds the required volume for the overall drainage master plan.
Spillway Design
The proposed detention system is a cut-in or depressed system with limited embankment.
broad-crested weir outlet is in the southern part of the east side of the proposed detention facility.
Emergency overflow will be directed towards Greenfield Drive and south towards the Donella Drive
intersection.
Water Quality and LID Design
The site will incorporate water quality capture volume into the proposed detention pond for the
settlement and deposition of particles and contaminants. Sizing and calculations for this water quality
volume and outlet release structure can be found in Appendix C. For additional Low-Impact Development
water quality control, an underground stormwater chamber system is proposed with the site that will
treat on-site runoff prior to discharge into the proposed-on site detention pond. The criterion for this
approach requires one of the following options:
1) 50% of the newly added or modified impervious area must be treated by LID techniques and 25%
of new paved (vehicle use) areas must be pervious.
2) 75% of all newly added or modified impervious area must be treated by LID techniques.
For this site Option 2 is utilized. The below table outlines the LID treated area calculations.
SF Acre % of
Total
Total Site Area 692,610 15.900 100.0%
Measured Impervious 469,190 10.771 67.7%
Measured Landscaped 223,420 5.312 32.3%
New Impervious
Required to LID 368,213 8.453 75%
Actual to LID Feature 410,665 9.428 75.1%
Required LID Volume
Min. Required Volume 16,650 CF
Provided Volume 17,548 CF
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
12
The required LID volume calculations assumed 8.453 acres of impervious tributary area. The proposed
design utilized the ADS StormTech design software for preliminary sizing and general layout
recommendations. The proposed system is a StormTech MC-3500 system and is slightly modified and
oversized to fit the site drainage plan. Appendix C contains the vendor recommended design and details
for the proposed system. The found groundwater in this area was between 10 and greater than 12
grade (prior to development). The base of the proposed LID chambers is above 4932 with the geotech
estimate of groundwater below 4926 with a differential of >6 .
The proposed system was modelled in SWMM to evaluate the drain time to reach the target 40-hour drain
It was noted that the 6-inch underdrain recommended for the project is
installed per the 2nd round review. A 3.4 -3 is proposed to meet the LID requirements for
the site. The LID controls are included with the modelling and impacts on the HGLs for the minor and
major storm.
Mapping of the LID calculations along with calculations for the water quality chamber system can be found
in Appendix C.
V. Sediment/Erosion Control
In addition to the information provided below, A separate Storm Water Management Plan (SWMP) was
prepared for this site in accordance with the Stormwater Discharge Permit for Colorado Department of
Public Health and Environment and the City of Fort Collins criteria. This document is not included with
this report but is part of the submittal. The SWMP has more detailed information on the sediment and
erosion control items for this project. For reference, the erosion control plan sheets are provided in
Appendix A.
LID SYSTEM PERFORMANCE
Elapsed Time (hours)
6050403020100
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Node J-SDMH A-3-IN Depth (ft)
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
13
VI. Conclusions
A. Compliance with Standards
Storm drainage calculations have followed the guidelines provided by the City of Fort Collins Stormwater
Criteria Manual and the Mile High Flood District Manuals Volumes 2, and 3.
B. Drainage Plan
The drainage system is not anticipated to have issues associated with conveyance, collection or detention
based on the work to date in the subdivision planning. No negative impacts are expected due to this
development as related to the existing downstream drainage networks or subdivisions. The proposed
development is proposed at less impervious than originally assumed for the greater subdivision. Pond 5,
as designed with this system meets or exceeds the required volume required by the referenced Galloway
drainage plan.
The Final drainage plan for the Bloom multifamily development is submitted as being both viable and
sound in terms of drainage planning in both the short and long-term for the City of Fort Collins.
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
14
VII. References
1. City of Fort Collins Stormwater Criteria Manual, As adopted by the City Council of the City of
Fort Collins, as referenced in Section 26-500 of the Code of the City of Fort Collins, Revised
December 2018.
2. Mile High Floor District Drainage Criteria Manuals 2 and 3, Urban Drainage and Flood Control
District, Denver, Colorado, November 2010 with some sections revised January 2021.
3. Storm Water Management Model, Reference Manual Volume I, Hydrology, U.S.
Environmental Protection Agency, January 2016.
4. Storm Water Management Model, Reference Manual Volume II, Hydraulics, U.S.
Environmental Protection Agency, May 2017.
5. Mulberry Master Drainage Report, City of Fort Collins Galloway, revised October 13th, 2021.
VIII. Certification
"I hereby attest that this report for the Final drainage design of the Bloom Multi-Family was prepared by
me, or under my direct supervision, in general accordance with the provisions of the City of Fort Collins
Storm Drainage Design Criteria for the responsible parties thereof. I understand that the City of Fort
Collins does not and shall not assume liability for drainage facilities designed by others.
<Sealed with Final Approval>
_____________________________________
Kelly W. Hafner
Registered Professional Engineer
State of Colorado No. 40968
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
A
APPENDIX A
DRAINAGE AND EROSION CONTROL PLAN MAPS
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
B
APPENDIX B
SWMM Calibration Parameters
Inlet Calibration Parameters
SWMM Model Overview
100-Year SWMM Model Status Report
2-Year SWMM Model Status Report
Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
C
APPENDIX C
WATER QUALITY AND LID CALCULATIONS & MAP
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Final Drainage and Erosion Control Report
Bloom Multi Family Development
Fort Collins, CO
D
APPENDIX D
Detention Pond Volume excerpt
Geotechnical Borings
Memorandum
HFH22 - Memorandum F1 Amendment.docx
Page 1 of 2
To: Wes Lamarque, PE
City of Fort Collins
From: Matthew Pepin, PE
Galloway, Inc.
Date: 06/20/2023
Re: Bloom Filing 1 Amendment - Drainage Modifications
To Mr. Lamarque,
Bloom Filing 1 required revisions that would include a modified pond, an added pond, and
modifications to associated storm infrastructure. This memorandum is to help provide
information on the changes from the approved Bloom F1 Final Drainage Report for the stated
Amendment.
Pond Modification / Addition
Pond G was initially approved to provide 9.5 ac-ft of volume and release at a rate of 17.6 cfs.
This pond is being split into two separate ponds to allow for increased development within the
piece of land where Pond G initially was planned. Pond G will now be reduced to a volume of
4.8 ac-ft and releasing at 13.5 cfs. A second pond, Pond G1, will be located on the western
part of the site. Pond G1 will provide 5.1 ac-ft of volume and release at 4.4 cfs. This would
increase the amount of total flow existing both ponds from the original by 0.2 cfs. In order to
maintain a maximum of 22.3 cfs releasing into the southern part of the Cooper Slough outfall,
Pond I’s flow will be restricted from 0.35 cfs to 0.15 cfs.
Storm Infrastructure
The associated basin that were tributary to the effected modification storm infrastructure are
labeled as A-G1, A-G2,…A-G6. Inlet calculations were added for these basins as well. An
updated StormCAD output for the effected storm lines has been included a part of this memo.
Tailwater conditions were set based on the 100-year WSEL for the major storm event and a
free outfall condition in the minor event. Note: there is HGL surcharging from SD Inlet L1.5
and SD Inlet L1. The HGL at this location is .27 higher than the RIM at the MH of SD MH
L1.4. This equates to 3.24 inches and would remain within the ROW, not impacting the lots.
Storm Infrastructure incorporating Bloom F2
In order to take into account LID’s within Bloom F2, there is another modification to the storm
pipe infrastructure. A concern of too much flow entering the LID in the major storm events
would cause damage needed to be mitigated. To prevent this from happening, a manhole
would split flow from outfalling into the LID and by passing downstream to outfall into the EDB
(Pond E). This flow split would have flow entering into the LID at a maximum of 15.5 cfs in the
major storm event. The flow split is achieved by having the smaller 15” pipe sit lower than the
larger pipe, restricting flow for the minor event to only pass through the 15” pipe. The flow
rate allowing for all flow in the minor storm event to enter the LID at a flow rate of 12.02 cfs.
Bloom Filing 1
06/20/2023
HFH22 - Memorandum F1 Amendment.docx
Galloway & Company, Inc. Page 2 of 2
CONCLUSION
The modifications to the site will not change the overall design intent that was originally
approved with Bloom F1 Drainage Report.
Sincerely,
GALLOWAY
Matthew Pepin, PE
Civil Project Engineer
MatthewPepin@Gallowayus.com 01/02/2024