HomeMy WebLinkAboutVILLAGE ON HORSETOOTH - FDP - FDP160044 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTFort Collins Housing Authority - Village on Horsetooth
ENGINEER’S STATEMENT:
“I hereby certify that this report (plan) for the Preliminary Drainage Design for the Fort Collins
Housing Authority Village on Horsetooth was prepared by me (or under my direct supervision) in
accordance with the provisions of the City of Fort Collins Standards 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.”
______________________________
Charles R. Hager, IV, P.E.
Registered Professional Engineer
State of Colorado No. 37146
Fort Collins Housing Authority - Village on Horsetooth
PRELIMINARY DRAINAGE AND EROSION CONTROL REPORT
FOR
FORT COLLINS HOUSING AUTHORITY
VILLAGE ON HORSETOOTH
JVA, Inc.
25 Old Town Square, Suite 200
Fort Collins, CO 80524
phone: 970-225-9099
JVA Project No. 2595c
DECEMBER 13, 2016
Fort Collins Housing Authority - Village on Horsetooth
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TABLE OF CONTENTS
SITE LOCATION AND DESCRIPTION ......................................................................................... 5
DRAINAGE BASINS AND HISTORIC RUNOFF ............................................................................. 6
HISTORIC DRAINAGE ......................................................................................................... 6
DRAINAGE DESIGN CRITERIA ................................................................................................ 6
HYDROLOGIC METHOD AND DESIGN STORM FREQUENCIES ................................................... 6
HYDRAULIC CRITERIA ........................................................................................................ 6
VARIANCES FROM THE CRITERIA ............................................................................................ 7
DRAINAGE FACILITY DESIGN ................................................................................................. 7
GENERAL CONCEPT ......................................................................................................... 7
PROPOSED DEVELOPED SUBBASINS .................................................................................... 8
PROPOSED DETENTION POND AND LID ............................................................................ 11
HISTORIC/DEVELOPED BASIN COMPARISON ..................................................................... 13
STORMWATER POLLUTION PREVENTION/EROSION CONTROL .................................................. 13
TEMPORARY EROSION CONTROL ..................................................................................... 13
PERMANENT EROSION CONTROL ..................................................................................... 13
STORMWATER MANAGEMENT CONTROLS ............................................................................ 14
SEQUENCE OF CONSTRUCTION ACTIVITIES ............................................................................ 17
CONCLUSIONS .................................................................................................................. 18
REFERENCES ...................................................................................................................... 19
LIST OF TABLES
TABLE 1. HISTORIC BASIN PROPERTIES AND FLOWS ..................................................................... 6
TABLE 2. DEVELOPED BASIN PROPERTIES AND FLOWS COEFFICIENTS ........................................... 11
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APPENDICES
Appendix A – Figures
Appendix B – Hydraulic/Hydrologic Computations
Appendix C – Referenced Documents
Appendix D – Erosion Control Escrow
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SITE LOCATION AND DESCRIPTION
The Village on Horsetooth (site) is being developed by the Fort Collins Housing Authority and is
located in Southeast Quarter of Section 27, Township 7 North, Range 69 West of the 6
th
Principal
Meridian, in Larimer County, Fort Collins, Colorado. Refer to Appendix A, Figure 1, for the site
vicinity map. Horsetooth Road (Horsetooth) to the south, Pleasant Valley and Lake Canal (Canal)
to the east, and residential subdivisions to the north and west border the property. There is open
space and residential housing to the west. The site is located in the Foothills Stormwater Basin.
The Foothill basin drains west to east through open channels and storm sewer that discharges into
the Fossil Creek Reservoir Inlet Ditch. The Foothills Basin Master Plan Water Quality Conceptual
Alternatives Map, provided by the City of Fort Collins, shows that the site is required to provide
flood control and water quality; the map is attached in Appendix C.
The current site is approximately 7.96 acres of partially developed land with an existing house and
horse stable, arena and grazing open space area with a total percent impervious of 16.0%. The site
historically generally drains to the east towards and into the Canal via overland flow with slopes
ranging from 0.2% to 8.0%. The north half of Horsetooth bordering the site drains to the north
onto the site and into the Canal. From the geotechnical report dated July 19, 2016, the soil is
mainly sandy clay and clayey sand with no bedrock encountered in the borings. Ground water was
located 19 to 22 feet below ground surface and did not drastically change several days after
drilling. A soil map created using the USDA Natural Resources Conservation Service Web Soil
Survey identifies the site’s soil as Altvan-Satanta Loams, which is in the hydrologic soil group B
with infiltration rates ranging from 0.6 to 2 inches/hour. Further investigation of the soils
infiltration rate in the locations of the detention pond and low impact development (LID) ponds
will be completed. A summary of the conclusions of the geotechnical report and the web soil
survey results are provided in Appendix C.
According to the FEMA Flood Insurance Rate Map Panel Number 08069C0986G dated
05/02/2012, the site is located outside of the 100-year floodplain area. The map panel is attached
in Appendix C.
The proposed site construction includes the demolition of the existing home, horse stables and
arena, gravel roadway and the construction of 9 new multi-unit residential buildings and a
clubhouse building with surrounding asphalt parking areas, playground and grass fields, sidewalks,
LID water quality controls and a detention pond. The site development will increase the overall
percent impervious to 48.4%. The proposed development cannot drain or discharge into the Canal
and must tie into the City of Fort Collins (City) storm sewer system located in Horsetooth or flow
onto Horsetooth, which eventually drains to Fossil Creek Reservoir Inlet Ditch. Storm valley
gutters, catch basins, area inlets, under drains, curb cuts, chases, channels and culverts are located
throughout the site to capture and convey runoff to the LIDs and detention pond.
The main purpose of this study is to analyze the existing and developed drainage patterns in order
to convey stormwater into the Horsetooth storm sewer system without causing adverse effects to
the site and the surrounding and downstream properties. The proposed scope of work will include
new storm piping, valley gutters, area drains, catch basins, curb cuts, and site grading to
accommodate three bioretention ponds, a detention pond, and piping to the existing Horsetooth
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storm system. An outlet structure and pipe with a restrictor plate will be installed in the pond to
control the 100-year storm outflow rate to the calculated 2-year historical runoff rate. WQCV will
be attained in the three bioretention ponds. An emergency spillway is located along the southeast
edge of the pond to direct flow onto Horsetooth if the outlet structure is ever clogged or not
performing correctly.
DRAINAGE BASINS AND HISTORIC RUNOFF
HISTORIC DRAINAGE
The site historically drains to the east towards and into the Pleasant Valley and Lake Canal via
overland flow with slopes ranging from 0.2% to 8.0%. Runoff from the north half of Horsetooth
bordering the site drains north onto the site and into the Canal. The site consists of a gravel
roadway, house, horse stables, arena, and grazing area which have an overall percent impervious
of 16.0%. The peak 2-year and 100-year runoff rates are 2.21 cfs and 28.78 cfs, respectively.
The historic drainage is outlined below. Design flow calculations and the historical drainage map
are provided in Appendix A.
Table 1. Historic Basin Properties and Flows
Drainage
Basin
Design
Point
Area
(acre)
%
Impervious
Q-2
(cfs)
Q-10
(cfs)
Q-100
(cfs)
Runoff
Coefficient
H1 1 7.96 16.0% 2.21 6.75 28.78 0.53
Total 7.96 16.0% 2.21 6.75 28.78 0.53
DRAINAGE DESIGN CRITERIA
HYDROLOGIC METHOD AND DESIGN STORM FREQUENCIES
The drainage for the site was designed to meet or exceed the City of Fort Collin’s Standards and
Specifications, including the Stormwater Criterial Manual which incorporates most of the Urban
Drainage and Flood Control District Criteria Manual (USDCM). The site’s basin percent
impervious values were calculated using impervious values from Table 6-3 in the USDCM. For
the Modified FAA calculations, the values for the 10-year and 100-year event rainfall intensity
duration for 0 to 60 minutes were taken from Table RA-7 in the City of Fort Collins’ Stormwater
Criteria Manual. Equation 5.30 from the USDCM was used to calculate intensities for the 60 to
180 minute time steps. The runoff coefficients were calculated using USDCM’s Table 6-4
equations.
HYDRAULIC CRITERIA
The proposed drainage facility has been designed to comply with the City of Fort Collins
Stormwater Criteria Manual, the Foothills Basin Master Plan design, and UDFCD’s Drainage
Criteria Manual. The Foothills Basin Master Plan requires best management practices to have
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both flood control and water quality. UD-Inlet Version 3.14, Manning’s Equation, Hydraflow,
and/or Flowmaster will be utilized as needed for the inlet and pipe sizing calculations. Final
drainage and pipe network plans are included in the construction documents. As required by the
City’s stormwater criteria manual, the pond volume was calculated using the modified FAA
method and includes the required 100-year runoff volume. The required LID area and WQCV are
being captured and treated separately in the site’s three bioretention ponds. Pond volume, WQCV,
bioretention sizing, and runoff calculations are attached in Appendix B.
VARIANCES FROM THE CRITERIA
As per the City’s comments dated May 9, 2016, the site is required to discharge the 100-year event
at the historical 2-year rate. There is a micropool in the detention basin outlet structure to allow
the bioretention underdrains to tie into the structure without causing standing water in the
underdrain system. There are no forebays in the pond since all concentrated flows entering the
detention pond pass through one of the three bioretention ponds. Riprap aprons will act as velocity
dissipaters for the concentrated runoff entering the bioretention ponds.
DRAINAGE FACILITY DESIGN
GENERAL CONCEPT
As per the Foothills Basin Master Plan design, the site was graded in an attempt to capture all site
runoff to be treated and detained in the three bioretention ponds and the extended detention basin
located along the east side of the site. The detention pond has an outlet structure that ties into the
City’s existing storm sewer system on the south side of Horsetooth. The shallow elevation of this
connection restricts the bottom elevation of the detention pond. A very small portion of the site,
10.2%, was not able to be captured and treated onsite. This offsite flow will either drain onto
Horsetooth Road and follow existing drainage pathways to the east, drain to the east into the Canal,
or drain west into the neighboring detention pond. The open space area in the center of the site
will capture the runoff from the playground, sidewalk, and open space through the uses of area
inlets which tie into the proposed storm system and discharges into LID Pond 3. Area drains
around buildings 1, 3, 5, 6, and 7 and the south parking lot area will tie into the same storm system.
The area between Building 1 and Building 2 will be piped into LID Pond 3. Furthermore, the east
portion of the parking lot and runoff from building 2 will sheet flows into LID Pond 3. The west
side of the site, a portion of the north parking lot, buildings 9 and 10 and areas around those
buildings will drain to the north into LID Pond 1 by sheet flow and piping. LID Pond 1 is
hydraulically connected to LID Pond 2 by a culvert set 1 foot above the bottom of the pond. A
portion of the parking lot to the north and east, building 8 and the area around the building will
drain into LID Pond 2. Therefore, all onsite runoff, besides the detention pond, will be directed to
one of the three LID ponds. The ponds will capture the sites entire required water quality volume
with the assumption that the detention pond is a mostly grass area which will act as a filter for the
basins water quality needs. There are overflows in LID Ponds 2 and 3 to allow for larger storm
events to discharge into the detention pond at peak flow rates. The filter areas correspond with the
required LID area for each pond. The filter area includes 12 inches of growing media, 6 inches of
pee gravel, and at least 10 inches of CDOT Class C filler material with a slotted underdrain pipe
along the bottom. The underdrain system is connected with storm drains and ties into the outlet
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structure’s micropool. The parking lot will act as part of the detention pond for larger storm events,
but will have no greater than one foot of ponding at the deepest point. Vertical curb and gutter
will be installed along Horsetooth road to convey flows east along the road, similar to what is
occurring to the west and east of the site along Horsetooth. The on-site storm system was designed
to convey the 10-year storm event with the 100-year storm event overflowing into the parking lots
and sheet flowing into a LID Pond or ponding until flow can by conveyed to an LID Pond.
PROPOSED DEVELOPED SUBBASINS
The site was delineated into thirty subbasins, A1-A17, B1-B4, and R1-R10. Basins A1-A17 and
R1-R10 will be captured and treated in the bioretention ponds and detention pond. Basin B1-B4
are the offsite basins and will not be have treatment before leaving the site either to Horsetooth
Road, the Canal, or to the western property. Basin A1, A4 and A6 all contain a bioretention pond
for water quality LID treatment. Basins A17 contains the detention pond. All roofs are piped into
the depression areas around the buildings and into the pipe system or are directly piped into the
storm systems and then discharge to a bioretention pond. The developed drainage map is provided
in Appendix A.
SUBBASIN DESCRIPTIONS:
• Subbasin A1 contains the most of the northwest portion of the site. This includes parking
lot, sidewalk and landscaped areas and LID Pond 1. Flow is directed into the parking lot
and flows north into the curb and gutter that directs flow to a curb cut to direct flow into
LID Pond 1. A culvert connects Basin A1 to Basin A2.
• Subbasin A2 contains a section of west parking lot area, sidewalk, and landscaped area.
Flow is directed to the valley gutter sloped to the north and captured in an inlet before
discharging into LID Pond 1. Flows that are greater than the 10-year event will continue
north into LID Pond 1. If the inlet is ever clogged flow will continue north into Basin A1
and still be collect in LID Pond 1.
• Subbasin A3 includes the landscaped area around of Building 9 and a small portion of
sidewalk. Flow is collected in several sumped area drains and piped to LID Pond 1. Flows
for storm events greater than the 10-year event will pond in the sumped area and overflow
to the north into basin A1 and LID Pond 1.
• Subbasin A4 contains half of the north portion of the site and a little of the east portion.
This includes landscaped, parking lot, sidewalk areas, and LID Pond 2. Runoff is directed
across the parking lot to the curb and gutter and a curb cut into the pond. Once the pond is
at capacity, flow discharges into the detention pond.
• Subbasin A5 includes the grass area around Building 8. Flow is collected in several
sumped area drains around building 8 and piped to LID Pond 2. Flows for storm events
greater than the 10-year event will pond in the sumped area and overflow to the north into
basin A4 and LID Pond 2.
• Subbasin A6 contains the east portion of the parking lot, sidewalk, landscaped areas and
LID Pond 3. The parking lot is sloped to drain to the east curb and gutter and discharge
into the pond through curb cuts. An overflow in the pond allows for flows greater than the
WQCV to discharge into the detention pond.
• Subbasin A7 contains a portion of the southeast corner of the site. This includes parking
lot, sidewalk, and landscaped areas. Runoff is directed to the parking lots north curb and
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gutter where it is collected in two sumped inlets and piped to LID Pond 3. Flow greater
than the 10-year event will pond around the inlet and at depths greater than a foot the water
will flow south into the entrance road and onto Horsetooth Road. If the inlet is ever clogged
flow will pond around the inlet and eventually discharge south along the entrance road into
Horsetooth road.
• Subbasin A8 contains a small portion of the south parking lot, landscaped area and
sidewalk. Flow is directed into the parking lots valley gutter, captured in a sumped inlet
and piped into LID Pond 3. If the inlets are clogged or for larger storm events flow will
flow into basin A7.
• Subbasin A9 contains the open space area in the middle of the site. The area also includes
a playground, gravel picnic areas and a sidewalk system. Flow is directed to the center of
the open space area, collected in two sumped area inlets and piped to LID Pond 3. For
storm events greater than the 10-year event runoff will pond in the grass area. There is an
overflow located between buildings 4 and 5 into basin A7 if the inlets ever become clogged
and do not drain.
• Subbasin A10 includes the landscaped area around Building 7. It also includes a picnic
area and some sidewalk. Flow is directed to several sumped area inlets around the building
and piped to LID Pond 3. Flows for storm events greater than the 10-year event will pond
in the sumped area and overflow to the east into basin A6 and LID Pond 3.
• Subbasin A11 includes the landscaped area around Building 6. It also includes a picnic
area and some sidewalk. Flow is directed to several sumped area inlets around the building
and piped to LID Pond 3. Flows for storm events greater than the 10-year event will pond
in the sumped area and overflow to the east into basin A6 and into LID Pond 3 or south
into basin A8.
• Subbasin A12 includes the landscaped the green space to the west of Building 3. A sumped
area inlet collects runoff and the flow is piped to LID Pond 3. Flows for storm events
greater than the 10-year event will pond in the sumped area and overflow to the south onto
Horsetooth road.
• Subbasin A13 includes the landscaped area in front of Building 4. Sumped area inlets
collect runoff and the flow is piped to LID Pond 3. Flows for storm events greater than the
10-year event will pond in the sumped area and overflow to the east into basin A7 or to the
north into basin A9.
• Subbsain A14 includes the landscaped area to the west and north of Building 1. Sumped
area inlets collect runoff and flow is piped to LID Pond 3. Flows for storm events greater
than the 10-year event will pond in the sumped area and overflow to the south onto
Horsetooth road.
• Subbasin A15 includes the green space between Building 1 and Building 2. A sumped area
inlet collects runoff and the flow is piped to LID Pond 1. Flows for storm events greater
than the 10-year event will pond in the sumped area and overflow to the west into basin
A2.
• Subbasin A16 includes the green space area to the west of building 10. Sumped area inlets
collect runoff and the flow is piped to LID Pond 3. Flows for storm events greater than the
10-year event will pond in the sumped area and overflow to the east into basin A6 and LID
Pond 3.
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• Subbasin A17 contains the bottom of the detention pond and the sidewalk along the east
side of the site. The ponds outlet structure is connected to Horsetooth Storm System. An
overflow spillway is located on the south side of the detention pond.
• Subbasin B1 contains the south portion of the site that drains offsite. This includes
landscaped area, sidewalk and the entrance into the property. Runoff will sheet flow across
the landscaped areas and road onto Horsetooth Road and be directed to the east along curb
and gutter.
• Subbasin B2 contains the east side of the detention ponds embankment. Flow from the
embankment will be directed to the east into the Canal. The 100-year peak flow is 1.44
cfs.
• Subbasin B3 contains the north portion of the site that drains offsite. This area exists
because of the detention basins berm. Flow is directed to the north into a swale running
west to east that discharges into the Canal. Minimal flow, 0.65 cfs for the 100-year event,
will enter the Canal.
• Subbasin B4 contains a small portion of the west side of the site that will be directed offsite
and into the neighboring detention basin area. A berm was placed along the west side of
the site keep flow from leaving the site and offsite flows from entering the site. The offsite
flow, 0.12 cfs for the 100-year event, is the result of the berm.
• Roof Subbasin R1’s roof drains will surface discharge to the north into basin A8.
• Roof Subbasin R2’s roof drains will surface discharge to the north into basin A6.
• Roof Subbasin R3’s roof drains will surface discharge to the north into basin A6.
• Roof Subbasin R4’s roof drains will surface discharge to the east and be collected in the
sumped inlets in basin A13.
• Roof Subbasin R5 will be piped into the storm system to the east of the building.
• Roof Subbasin R6’s roof drains will surface discharge and be collected in the sumped inlets
in basin A11.
• Roof Subbasin R7’s roof drains will surface discharge and be collected in the sumped inlets
in basin A10.
• Roof Subbasin R8’s roof drains will surface discharge and be collected in the sumped inlets
in basin A5.
• Roof Subbasin R9’s roof drains will surface discharge and be collected in the sumped inlets
in basin A3.
• Roof Subbasin R10’s roof drains will surface discharge to the west and be collected in the
sumped inlets in basin A16.
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Table 2. Developed Basin Properties and Flows Coefficients
Drainage
Basin
Design Point
Area
(acre)
%
Impervious
Q-10
(cfs)
Q-100
(cfs)
Runoff
Coeff's
C100
A1 1 0.59 48.9% 1.22 3.32 0.69
A2 2 0.36 83.0% 1.41 3.11 0.86
A3 3 0.15 18.6% 0.20 0.79 0.55
A4 4 0.48 52.7% 1.14 2.99 0.71
A5 5 0.12 3.5% 0.09 0.58 0.47
A6 6 0.84 66.7% 2.59 6.21 0.78
A7 7 0.77 73.0% 2.55 5.90 0.81
A8 8 0.23 89.9% 0.94 2.01 0.89
A9 9 0.87 23.8% 1.00 3.66 0.57
A10 10 0.21 16.2% 0.21 0.91 0.53
A11 11 0.14 11.9% 0.13 0.60 0.51
A12 12 0.10 2.0% 0.06 0.39 0.46
A13 13 0.01 2.0% 0.01 0.04 0.46
A14 14 0.03 2.0% 0.02 0.15 0.46
A15 15 0.06 19.1% 0.06 0.26 0.55
A16 16 0.02 2.0% 0.01 0.06 0.46
A17 17 0.80 11.5% 0.61 2.94 0.51
B1 18 0.34 13.3% 0.32 1.47 0.52
B2 19 0.31 2.0% 0.22 1.44 0.46
B3 20 0.13 9.6% 0.13 0.65 0.50
B4 21 0.03 2.0% 0.02 0.12 0.46
R1 22 0.16 90.0% 0.68 1.45 0.90
R2 23 0.10 90.0% 0.42 0.90 0.90
R3 24 0.16 90.0% 0.69 1.47 0.90
R4 25 0.11 90.0% 0.46 0.98 0.90
R5 26 0.07 90.0% 0.28 0.61 0.90
R6 27 0.10 90.0% 0.42 0.90 0.90
R7 28 0.16 90.0% 0.68 1.45 0.90
R8 29 0.16 90.0% 0.69 1.47 0.90
R9 30 0.16 90.0% 0.68 1.45 0.90
R10 31 0.16 90.0% 0.69 1.47 0.90
Total Site Flow 7.96 48.4% 18.60 49.73 0.69
Total Offsite (B Basins) 0.81 8.0% 0.68 3.67 0.49
Total To LID Pond 1 1.44 63.2% 4.20 10.19 0.76
Total To LID Pond 2 0.77 52.9% 1.91 5.04 0.71
Total To LID Pond 3 4.13 57.6% 11.20 27.95 0.74
Total to Pond 7.15 53.0% 17.92 46.05 0.71
PROPOSED DETENTION POND AND LID
As per the City’s requirement, the 100-year runoff volume is captured in the detention pond and
has an outflow rate equal to the historical 2-year flow rate of 2.21 cfs. This is accomplished
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through the installation of a restrictor plate on the 15 inch RCP outlet pipe. Only a small portion
of the sites area, 10.2%, was not designed to drain onsite into the LID and detention ponds. The
bottom of the detention pond is at an elevation of 5105.25 feet and is 5.25 feet deep with a total
volume of 3.91 acre-ft. The detention pond’s emergency overflow spillway is located on the
southern edge of the pond at an elevation of 5109.50 ft and will discharge to Horsetooth Road if
the outlet structure of the pond is ever clogged or malfunctioning. The spillway will be designed
to be able to discharge the peak 100-year runoff rate of 46.1 cfs with 9 inches of head. For the
100-year storm event, the pond will have a maximum water surface elevation of 5108.86 ft, which
is a pond volume of approximately 1.35 acre-feet. There is a difference of 0.64 ft between the
100-year water surface elevation and spillway elevation because of the tie in elevation along
Horsetooth. The top of pond was designed at 5110.50 ft to allow for 1 foot of freeboard above the
spillway bottom. Access to the pond and outlet can be achieved by entering the pond from parking
lot and driving along the pond bottom. The detention pond will have no water quality controls
except for the grass surface which will provide filtration for the basin runoff flows.
Three bioretention ponds, labeled LID Pond 1, 2, and 3, were designed to capture and treat the
sites required WQCV. The WQCV is a total of 5,073 cf. LID Ponds 1 and 3 have a depth of 1.0
ft and LID Pond 2 has a depth of 6 inches. An overflow weir was designed for both LID Ponds 2
and 3 to discharge into the detention pond at the peak 100-year runoff rates that enter the ponds.
LID Pond 1 and 2 will be connect by a culvert. The LID pond bottoms square footage have been
sized so the ponds volume is equal to or greater than the required WQCV for the basins discharging
to the ponds. The pond bottoms include 12 inches of amended soil, 6 inches of squeegee, and a
minimum of 10 inches of CDOT Type C gravel with an underdrain in the gravel section. Only the
required LID area, the bottom of the ponds, will have the amended soil area. The table below
outlines the water quality LID Pond details are in the Table 3 below. Only 0.81% of the sites
impervious area is not captured and treated in LID Ponds and this is from the offsite basins.
Table 3. Water Quality LID Pond Details
Max
Depth
Required Water
Quality LID
Volume
Provided
Water Quality
LID Volume
Amended
Soil Area
% of WQCV
treated in LID
Pond
(ft) (cf) (cf) (sf)
LID Pond 1 1.00 1,234 1,238 1,238 100%
LID Pond 2 0.50 572 573 1,145 100%
LID Pond 3 1.00 3,267 3,267 3,267 100%
Total 5,073 5,078 5,650 100%
The underdrain pipe will have a very shallow slope and will tie into the outlet structure at a lower
elevation than the outlet pipe creating a micropool in the outlet structure. The head from the higher
elevations of the pond bottoms will force the underdrain to empty into the outlet structure and
overflow into the outlet pipe. There should only be standing water up to the outlet pipes elevation
of 5305.29ft. Therefore, there will only be standing water in the gravel and squeegee portions of
the amended soil areas of the LID Ponds. The detention volume includes the area above the
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overflow elevation for the LID Ponds because the LID ponds are hydraulically connected to the
detention pond.
HISTORIC/DEVELOPED BASIN COMPARISON
The site contains one historical basin that drains to the east into the Pleasant Valley and Lake
Canal. The developed site still drains primarily to the east, but instead of draining into the Canal,
the runoff is collected and discharged into the Horsetooth storm sewer system. A very small
portion of the site ,10.2%, drains offsite. This includes the east and north sides of detention ponds
embankment, 0.45 acres and 2.09 cfs, which will drain into the Canal. There is also a portion of
the site, 0.34 acres and 1.47 cfs, which will drain south onto Horsetooth and west from the site into
the neighboring sites detention area, 0.03 acres and 0.12 cfs. The existing site has a peak 100-year
runoff rate of 28.78 cfs. As required by the City, the detention pond has a discharge rate of 2.21
cfs into the Horsetooth system, which is equivalent to the historical 2-year peak runoff rate. In
the event that the detention ponds outlet pipe is clogged, the pond’s overflow spillway will safely
convey flow onto Horsetooth at a peak rate of 46.1 cfs. The site’s three LID ponds will treat 100%
of the onsite required WQCV, with only 0.81% of the impervious area draining offsite. The LID
ponds will have amended soils with an underdrain system to promote infiltration.
STORMWATER POLLUTION PREVENTION/EROSION CONTROL
TEMPORARY EROSION CONTROL
A temporary erosion control plan is to be implemented for the site during construction of the
proposed 10 building multi-family residential development. Temporary erosion control measures
include, but are not limited to, slope and swale protection provided by the use of erosion control
wattles/sediment control logs, inlet protection, vehicle tracking control at access points to the site.
All temporary erosion control measures are to be removed after they are deemed unnecessary.
Refer to the Erosion Control Plan and Details for the Village on Horsetooth for additional detailed
information.
PERMANENT EROSION CONTROL
Chapter 2 of the Urban Drainage and Flood Control District’s Urban Storm Drainage Criteria
Manual Volume 3 (USDCM) provides guidelines for the selection of appropriate permanent
structural BMPs for a site that is to be developed or redeveloped. The Village on Horsetooth is
best characterized as a “conventional redevelopment” with over 1 acre of impervious area on the
project site. The BMP decision tree for such sites is provided in Figure 2-2 of the USDCM. As
previously stated, the site and surrounding vicinity is comprised of hydrologic soil group Type B
soils, per the NRCS soil survey, with a depth to bedrock of greater than 5 feet. The existing percent
vegetative density for the property is approximately 75%. The rainfall erodibility is K=0.28 for
Type B soil and will cause rilling and gulling over longer distances and steeper slopes. Perimeter
control will be the primary means of minimizing the extent of gulling.
The site is located in the Foothills Stormwater Basin. The Foothill basin drains west to east through
open channels and storm sewer that ultimately discharges into the Fossil Creek Reservoir Inlet
Ditch. There are no irrigation canals passing through the site; however, the Pleasant Valley and
Lake Canal flows south to the east of the site. No stormwater will be permitted to discharge into
the canal. An earthen berm adjacent to the canal and to the east of the proposed detention pond
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will prevent onsite developed runoff from entering the canal, see grading plans. The drainage from
the detention pond will discharge to the City storm sewer system located on the south side of
Horsetooth Road.
Permanent erosion control measures include, but are not limited to, the constructed detention
ponds, riprap pads placed for culvert and flared end section outlet protection, seeding and mulch
placed to enable and established vegetative growth, etc. Long-term maintenance of these erosion
control measures shall be the responsibility of the owner of the property. A general erosion control
plan has been provided in the utility plans on sheet CE1.0. An erosion control escrow calculation
is provided in Appendix D.
STORMWATER MANAGEMENT CONTROLS
The Stormwater Management Plan (SWMP) administrator is to be determined.
According to the CDPHE Stormwater Fact Sheet – Construction, construction activities produce
many different kinds of pollutants which may cause stormwater contamination problems. The main
pollutant of concern at construction sites is sediment. Grading activities remove grass, rocks,
pavement, and other protective ground covers, resulting in the exposure of underlying soil to the
elements. The soil is then easily picked up by wind and/or washed away by rain or snowmelt.
In addition, construction activities often require the use of toxic or hazardous materials such as
fuel, fertilizers, pesticides, and herbicides, and building materials such as asphalt, sealants, and
concrete, which may also pollute stormwater. These materials can be harmful to humans, plants,
and aquatic life.
Other pollution sources that could be present include: runoff from vehicle washing, vehicle
maintenance and/or fueling, There are no known existing non-stormwater components of
discharge (i.e., springs, landscape irrigation return flow, etc.), or any anticipated during
construction.
1. All Disturbed and Stored Soil:
The total disturbed area will be approximately 8.3 acres. Soil stockpiles should be kept
below the locally mandated maximum height of 10 feet and surrounded with sediment
control devices. If the storage of these soils occurs over a period greater than 30 days,
then consider temporarily seeding the soil to minimize soil loss to wind and water
erosion. No phasing of grading activities is proposed. Silt fence around the perimeter
of the property with prevent sediment and disturbed soils from being carried offsite.
Inlet protection will be used on all inlets to prevent sediment and debris from entering
the storm drain system.
2. Vehicle Tracking of Sediments:
The majority of the existing site is unpaved. A temporary stabilized stone pad (Vehicle
Tracking Control pad, VTC) will be provided at points of ingress and egress onto the
unpaved area. The VTC is designed to reduce the amount of mud transported onto the
roads by construction traffic. If the site is extraordinarily muddy, then consider
designating an area to wash construction vehicles before they leave the site and enter
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the public right-of-way. A BMP such as this is a good way to avoid the costs associated
with street cleaning or accidentally discharging large amounts of sediment into other
storm drain systems. Street sweeping will be conducted as necessary to remove
sediment tracked on to pavement and adjacent streets.
When in doubt, choose methods that prevent pollution rather than ones that force
clean-up at the downstream end just before the stormwater enters the receiving
waters. In other words, the most efficient construction site BMPs are those that
prevent erosion from happening.
3. Management of Contaminated Soils:
There are no known contaminated soils on site. If contaminated soils are found, the
contractor shall report to the SWMP administrator and engineer to determine
appropriate removal and/or mitigation measures.
4. Loading and Unloading Operations:
Loading/unloading activities that may result in sediment tracking or spills will occur
within the paved area of the site or a temporary gravel staging area will be provided
in the area shown on the plans.
5. Outdoor Storage Activities:
Materials storage and waste disposal will occur within a contained and secure area
with the use of berms and silt fence around the storage areas and dumpsters for waste
disposal. Coordinate location with SWMP administrator.
6. Vehicle and Equipment Maintenance and Fueling:
Materials should be stored and handled in covered areas to prevent contact with
stormwater, and chemicals should be stored within berms or secondary containment
devices to prevent leaks and spills from entering stormwater runoff.
Spill Management
In general, the spill prevention and response plan must be kept on site at all times
during construction and should include the following:
a) Notification procedures to be used in the event of an accident. At the very
least, the SWMP Administrator should be notified. Depending on the nature
of the spill and the material involved, the Colorado Department of Health,
downstream water users, or other agencies may also need to be notified.
b) Instructions for clean-up procedures.
c) Provisions for absorbents are to be made available for use in fuel areas, and
for containers to be available for used absorbents.
d) Procedures for properly washing out concrete trucks and other equipment in a
manner and location so that the materials and wash water cannot discharge
from the site and never into a storm drain system or stream.
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7. Significant Dust or Particulate Generating Processes:
Disturbed areas not yet ready to be seeded, landscaped, paved, or otherwise stabilized
shall be watered, or ripped as necessary to preclude visible dust emissions. Contractor
shall make every effort to minimize particulate emissions and dust generation from
vehicles performing the work (including but not limited to regular maintenance of
construction equipment). Contractor to follow local and state dust ordinances at all
times.
8. Routine Maintenance Activates Involving Fertilizer, Pesticides, Detergents, Fuels,
Solvents, Oils, Etc:
Materials should be stored and handled in covered areas to prevent contact with
stormwater, and chemicals should be stored within berms or secondary containment
devices to prevent leaks and spills from entering stormwater runoff.
9. On-site Waste Management Practices (Waste Piles, Liquid Wastes, Dumpsters, etc):
Materials storage, waste disposal, and vehicle maintenance and fueling can occur
within an enclosed and secure area. The area can be enclosed with temporary fencing
and accessed through a gate with a lock.
10. Concrete Truck/Equipment Washing:
A concrete washout facility will be provided onsite. Properly wash out concrete trucks
and other equipment in a manner and location (a minimum of 50 feet away from any
inlet structures) so that the materials and wash water cannot discharge from the site,
and never into a storm drain system or stream.
11. Dedicated Asphalt and Concrete Batch Plants:
There will be no dedicated batch plant on site.
12. Non-Industrial Waste Source Such as Workers Trash And Portable Toilets:
Garbage cans shall be located at break areas and portable toilets shall be provided if
onsite bathroom facilities cannot be used. Locate dumpsters and portable toilets away
from drainage courses (minimum 50 feet from any inlet structures) and stake down
toilets to prevent tipping.
13. Other Areas or Procedures where Potential Spills can Occur:
Records of spills, leaks, or overflows that result in the discharge of pollutants must be
documented and maintained. Record all spills that are responded to, even if they do
not result in a discharge of pollutants. Information that should be recorded for all
occurrences includes the time and date, weather conditions, reasons for the spill, etc.
Some spills may need to be reported to immediately. Specifically, a release of any
chemical, oil, petroleum product, sewage, etc., which may enter waters of the State of
Colorado (which include surface water, ground water and dry gullies or storm sewers
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leading to surface water) must be reported. More guidance is available on the web at
www.colorado.gov. The Division’s toll-free 24-hour number for environmental
hazards and chemical spills and releases is 1-877-518-5608.
SEQUENCE OF CONSTRUCTION ACTIVITIES
Construction Start Date (This is the day you expect to begin disturbing soils, including grubbing,
stockpiling, excavating, demolition, and grading activities): January 2017
Final Stabilization Date (this is when the site is finally stabilized. This means that all disturbed
areas have been either built on, paved, or a uniform vegetative cover reaching over 70% of the
initial vegetative density has been established.) Even if you are doing one part of the project, the
estimated completion date must be for the overall project: June 2018
Construction Sequencing: (Detailed construction sequencing to be determined by the contractor)
1. Installation of temporary erosion and sediment control measures.
This includes all sediment control logs, silt fencing, vehicle tracking pad, and inlet
protection where land disturbing activities will occur. Implementations of individual
BMPs is to coincide with construction to minimize the exposure of unprotected areas. Inlet
protection will be installed as the storm sewers are constructed. Structural BMPs that do
not become part of the permanent stormwater management plan are to be removed as the
paving, landscaping, and other permanent groundcover installations are completed.
2. Sequence of all land disturbing activity.
Overlot grading in preparation for pouring of new sidewalks and building foundations will
occur early in the construction schedule. Additional land disturbance will occur around the
site for utility installation and the construction of the new driveway approach and sidewalk.
3. Drainage facility construction.
Construction of the detention pond, storm inlets and piping will occur early in the schedule.
4. Sediment basins, temporary channel stabilization.
Sediment control logs or erosion control blankets will be installed on the banks of the
vegetated detention pond and swale on the east side of the site.
5. Seeding
All disturbed areas will be seeded upon completing of finish grading. Soil amendment will
be required for the LID basin water quality treatment areas, see utility plans. Refer to the
landscape plans for detailed planting and seeding schedule. Temporary seed mix to be as
follows:
SPECIES COMMON NAME VARIETY LBS/ACRE
AGROPYRON SMITHI WESTERN WHEATGRASS ARRIBA 8.0
ARRHENATHERUM ELATES TALL OATGRASS 3.0
LOLIUM PERENNE PERENNIAL RYEGRASS PENNFINE 2.0
6. Mulching
Spread straw mulch uniformly over seeded areas at a rate of 90 lbs per 1000 square feet of
ground. No more than 25% of the ground should be visible. Crimp or pinch mulch into
soil 2-4 inches by using either a mulch anchoring tool, a farm disk operating on the contour
of the slope, or by cleating with bulldozer tracks operating up and down the slopes (to
prevent tracks from forming gullies).
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7. Required maintenance activities (e.g. after-storm checks of all BMPs, etc.).
All temporary sediment control BMPs shall be inspected after each rain event to ensure
proper protection and to determine if cleaning is necessary. Final cleaning of all storm
facilities shall be provided upon final stabilization of site.
CONCLUSIONS
The goal of the Drainage and Erosion Control Report for the Fort Collins Housing Authority
Village on Horsetooth is to describe appropriate controls and measures to improve water quality
by reducing pollutants in stormwater discharges and ensure compliance with the requirements of
the City of Fort Collins Erosion Control and Stormwater Standards and the UDFCD.
The proposed drainage system presented in this report is designed to convey the 100-year peak
stormwater runoff through the site to the proposed detention pond. Through calculations and
review of the proposed storm drain system, there appears to be adequate capacity to convey the
proposed flows. This report also proposes best management practices for erosion control,
permanent landscaping, riprap, and on-site construction facilities. The proposed permanent
structural BMP’s on site, including the vegetated swales, water quality LID basins, and detention
areas, capture the majority of the runoff discharged from the site and improve storm water
quality by facilitating sedimentation and filtering while limiting erosion through slowed-release
of runoff.
Temporary BMP’s to be employed during the construction of the proposed improvements
include silt fence, sediment control logs, and rock socks at the downstream limits of disturbance,
slope protection on the vegetated swales and LID basins, a concrete washout facility on site,
street sweeping practices to eliminate tracked sediment, and construction of a temporary
sedimentation basin in the proposed detention basin.
These measures, along with the other standard Best Management Practices described in this
report, will serve to reduce pollutants in the storm water runoff and satisfy the requirements of
the City of Fort Collins Erosion Control Standards.
It can therefore be concluded that development of the detention pond complies with all of the
stormwater jurisdictional criteria and will not adversely affect the existing streets, storm drain
system and/or detention/water quality facilities.
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REFERENCES
1. Urban Storm Drainage Criteria Manual (Volumes 1, 2, and 3), Urban Drainage and Flood
Control District, June 2001, January 2016.
2. City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, December 2013.
3. “Geotechnical Investigation FCHA Village At Horsetooth 1506 West Horsetooth Road
Fort Collins, Colorado”, CTL Thompson Incorporated, Dated July 19, 2016.
4. City of Fort Collins Foothills Basin Master Plan, Retrieved from http://www.fcgov.com/
utilities/what-we-do/stormwater/drainage-basins/foothills-basin on August 8, 2016.