HomeMy WebLinkAboutCHERRY STREET COTTAGES - PDP - PDP160041 - SUBMITTAL DOCUMENTS - ROUND 2 - DRAINAGE REPORTCherry Street Cottages
(previously referred to as “Pierce Subdivision”)
Cherry Street and Lyons Street
Project Development Plan
Preliminary Drainage and Erosion Control Report
prepared for
Mr. Evan Gilmartin
December 5, 2016
Revised: June 5, 2017
Sorensen Engineering & Construction, LLC
Civil/Environmental Engineering
1901 Bear Court
Fort Collins, Colorado 80525
(970) 590-1579
PaulS51@comcast.net
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IX. Preliminary Drainage and Erosion Control Report
TABLE OF CONTENTS
A. GENERAL LOCATION AND DESCRIPTION ................................................................... 3
1. Location ......................................................................................................................... 3
2. Property Description ...................................................................................................... 5
3. Floodplain Submittal Requirements ............................................................................... 7
B. DRAINAGE BASINS AND SUB-BASINS ..................................................................... 11
1. Major Basin Description ............................................................................................... 11
2. Sub-Basin Description .................................................................................................. 12
C. DRAINAGE DESIGN CRITERIA .................................................................................. 12
1. Optional Provisions ...................................................................................................... 12
2. Impervious Area ........................................................................................................... 12
3. Development Criteria Reference and Constraints ......................................................... 13
4. Hydrologic Criteria ....................................................................................................... 14
5. Hydraulic Criteria ......................................................................................................... 14
6. Floodplain Regulatory Compliance ............................................................................... 14
7. Modifications of Criteria .............................................................................................. 14
D. DRAINAGE FACILITY DESIGN .................................................................................. 14
1. General Concept .......................................................................................................... 14
2. Specific Details ............................................................................................................. 15
E. CONCLUSIONS ........................................................................................................ 16
1. Compliance with Standards .......................................................................................... 16
2. Drainage Concept ......................................................................................................... 17
REFERENCES .............................................................................................................. 18
Appendix A: Hydrologic Computations ....................................................................... 19
Appendix B: Hydraulic Computations ......................................................................... 21
Appendix C: (reserved for future use) ........................................................................ 23
Appendix D: FEMA FIRM Map .................................................................................... 24
Appendix E: Erosion Control Report ........................................................................... 25
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LIST OF FIGURES
Figure 1. Vicinity Map .................................................................................................. 3
Figure 2. City Flood Risk Map ....................................................................................... 4
Figure 3. Property Aerial Photograph ........................................................................... 5
Figure 4. West Vine Drainage Floodplain Map .............................................................. 8
Figure 5. Fort Collins Master Plan Basin Recommended Improvements ..................... 11
LIST OF TABLES
Table 1. BFE, RFPE, FF, and HVAC Elevations ................................................................ 9
Table 2. Detention Pond Design Summary ................................................................. 15
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A. GENERAL LOCATION AND DESCRIPTION
1. Location
a. Vicinity Map
Figure 1. Vicinity Map
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b. The Cherry Street Cottages project site is located in the Northeast quarter of Section 10,
Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort Collins, County
of Larimer, State of Colorado.
c. Cherry Street Cottages is bordered to the north by an existing residential property; to the
east by single-family homes (Hanna Farm Neighborhood); to the west by Lyons Street and
single family homes (Mountain View Heights); and to the south by Cherry Street and the
Putnam School.
d. The General Location Map presented below identifies drainage flow entering and leaving
the property, and is based on the City of Fort Collins West Vine Drainage Flood Risk Map. As
shown, and discussed in detail below, a small channel of the West Vine 100-year floodway
and floodplain exists along the south and east sides of the development.
Figure 2. City Flood Risk Map
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e. The Putnam School is present to the south across Cherry Street. Open property containing
one single-family home is located to the north. Historic residential properties are present to
the east and west, across Lyons Street.
2. Property Description
a. The Cherry Street Cottages property is comprised of approximately 1.01 acres.
b. As shown in the site aerial photograph (Figure 3), there are no existing improvements on
the property, and the existing vegetation consists of native grasses. The existing on-site
excess runoff drains to the south/southeast into the Cherry Street gutter, which flows to the
east and then to the north along Franklin Street to the West Vine Drive Drainage, thence to
the Poudre River. No “prominent features” including streams, canals, ponds, wetlands, or
major utilities are present on the property.
Figure 3. Property Aerial Photograph
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c. Aside from the aforementioned West Vine Basin drainage that exists along Cherry Street on
the south side of the project, no other major drainageways exist in this vicinity.
d. The proposed landscaping will include a retaining wall located coincident with the existing
mapped floodway along the south sides of the property, curving to the north within Tract A,
the eastern-most lot. There will be no improvements within the floodway boundaries, but
rather, Tract A is dedicated for the stormwater detention pond to be situated outside of the
floodway, but within the identified floodplain.
A Landscape Plan has been prepared in accordance with the PDP submittal requirements,
and is submitted as a separate document from this Drainage Plan. Briefly, the Landscape
Plan calls for an assortment of flowering plants and shrub beds and trees. As required by
the Fort Collins Land Use Code, low-water-use plants and grasses are used as much as
possible. The City Forester, Tim Buchanan, provided key input in preparing the landscape
plan.
e. All proposed building footprints (building envelopes) and sidewalks are provided on the
Cherry Street Cottages Site Plan, submitted as a separate part of this PDP application.
Additionally, the Landscape Plan clearly shows all building pad sites, as well as the retaining
wall, berms, planters, shrub beds, trees, etc.
The proposed Cherry Street Cottages will include nine (9) dwelling units, including primary
dwelling units and secondary dwelling units on the three (3) western-most lots (Lots 1, 2,
and 3), with one primary dwelling unit on the remaining three lots (Lots 4, 5, and 6). An
access driveway/alley, dedicated as Tract B, will be located along the north side of the
property, with rear garages for each lot. The secondary dwelling units on Lots 1,2, and 3 will
consist of upstairs townhomes over the garages, and standard, single-story garages will be
built in the rear portion of Lots 4, 5, and 6. Tract A will be dedicated for a stormwater
detention pond, as shown in the Drainage Plan.
f. The 100-year floodplain and floodway boundaries as well as the base flood elevations
across the property are clearly depicted on the attached Drainage Plan. The on-site
detention pond and retaining wall and berm are also shown on the Drainage Plan.
g. Soils encountered on the property generally consist of the Nunn–Fort Collins–Ulm
association: deep, nearly level, well drained clay loams and loams that formed in alluvium.
More site-specific mapping indicates a Table Mountain Loam at 0 to 1% slope. A
representative profile would consist of a surface layer of grayish brown loam about 36
inches thick, with underlying material of brown fine sandy loam about 10 inches thick and
yellowish brown, fine sandy loam about 5 inches thick. Below that is sand and gravel (Soil
Survey of Larimer County Area Colorado, US Soil Conservation Service and Forest Service,
1980).
h. Aside from the aforementioned West Vine Drainage floodplain, no major drainage ways are
present on or adjacent to the property.
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i. There are no irrigation ditches, laterals, or structures located on the property.
j. The proposed Cherry Street Cottages project will consist of six buildable lots, plus Tract A
(eastern-most lot dedicated for the detention pond), and Tract B (northern portion of the
site dedicated for the access driveway and drainage). Tract A, the eastern-most lot, will be
dedicated for an on-site stormwater detention pond. Lots 1 through 6 (from west to east)
will be accessed from a rear alley/driveway via Tract B. Each lot will consist of primary living
units on the front half with garages built in the back half. Secondary living units will be built
on the second floor of the garages on lots 1, 2, and 3, for a total of 9 living units on this 1-
acre parcel.
Other improvements that are included with the Cherry Street Cottages project will be
landscaping, as presented in the Landscape Plan, and a short retaining wall running the
length of the project from Lyons Street to the detention pond in Tract A. The retaining wall
will serve to elevate the lots as needed to place the bottom floor at approximately 18 inches
above the Base Flood Elevation, discussed below.
k. The proposed Cherry Street Cottages project is located in Low Density Mixed-Use
Neighborhood (LMN) zoning, which allows 4–9 dwelling units per acre.
3. Floodplain Submittal Requirements
The City of Fort Collins Floodplain Review Checklist—50% Development Review Checklist was
reviewed, and each listed item has been completed and included with this submittal or marked
as “Not Applicable (N/A).”
As shown on Figure 4, Cherry Street Cottages Floodplain Map, based on the City Flood Risk
Map, the southern portion of the subject property is located in or adjacent to the City
regulatory West Vine Basin 100-year floodplain, and the southeast corner of Tract A (detention
pond area) is located in the 100-year Floodway and High Risk Flood Fringe of the West Vine
Drainage Basin. Accordingly, no improvements are proposed for the area within the 100-year
Floodway, and the aforementioned retaining wall as proposed is coincident with the floodway
boundary. FEMA FIRM Panel 0978G, Panel 978 of 1420, shows that the subject property is not
within a FEMA-designated floodplain (See Appendix D).
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Figure 4. West Vine Drainage Floodplain Map
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Homes and garages built on Lots 3, 4, 5, and 6 that are partially or totally within the City-
designated floodplain will have spread footing foundations with slab-on-grade concrete floors.
The lowest floor level will be at least 18-inches above the base flood elevation. As discussed
below, this project will comply with safety regulations of Chapter 10 of City Code.
a. No new residential structures will be built in the floodway.
b. Non-structural development (fill, sidewalks, retaining wall, and vegetation) will be
completed within the floodway, and such improvements will not cause a rise to the Base
Flood Elevation (BFE) or a change to the boundaries of the floodway or flood fringe.
c. Proposed residential structures within the flood fringe will be completed with the lowest
finished floor (including duct work, heating, ventilation and air-conditioning systems, etc.)
elevated at least 18 inches above the BFE—also referred to as the Regulatory Flood
Protection Elevation (RFPE). All houses located within the flood fringe will have spread
footing foundations with slab on grade concrete floors. The floor elevations will be set at
the RFPE, and no improvements, i.e. HVAC equipment, will be situated below this elevation.
Table 1 provides base flood elevation, regulatory flood protection elevation, finished floor
elevations, and HVAC elevations for all structures located within the floodplain. All
elevations given represent the estimated elevation at the upstream edge of each structure.
Table 1. BFE, RFPE, FF, and HVAC Elevations for All Structures Located within the
Floodplain
Lot Number Structure BFE RFPE FF HVAC
3 Home 5024.5 5026.0 5028.3 5028.3
4 Home 5024.3 5025.8 5028.2 5028.2
5 Home 5024.2 5025.7 5028.0 5028.0
6 Home
Garage
5023.3
5023.8
5024.8
5025.3
5027.5
5027.5
5027.5
n/a
d. Basements are not proposed below the RFPE in any structure located in the flood fringe.
e. There are no critical facilities proposed within the 100-year floodplain.
f. A Floodplain Use Permit will be applied for prior to construction of any structures within the
flood fringe, along with the appropriate permit application fee. The Applicant understands
that a FEMA Elevation Certificate is required prior to issuance of the Certificate of
Occupancy (CO) for any structures built in the 100-year floodplain.
g. As required by City regulatory requirements, No-Rise Certifications will be obtained for all
non-structural development in the floodway before any development work in the floodway
(sidewalks, retaining wall, detention pond berm) is initiated. However, discrepancies do
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exist between the Base Flood Elevations (BFE), as mapped by the City (see Figure 2), and
actual surveyed elevations—both tied to the NAVD 88 vertical datum—exist. For example,
consider the following two Flood Risk Map cross-sections:
XS #704 5024.4 NAVD 88: Location: diagonally crosses the subject property from Cherry
Street to the northwest at the approximate midpoint along Cherry Street of the proposed
development. The recent survey of the property, also tied to the NAVD 88 datum, shows a
ground surface elevation at this cross-section to be approximately 5024.2 in the center of
Cherry Street, and roughly 5024.5 at the top of the curb on the north side of Cherry Street.
If we assume that the mapped base flood elevation at this cross section of 5024.4 is
accurate, the flood would be approximately 0.2-feet deep (2.4-inches), and would not top
the curb but rather would be contained within Cherry Street curb flow, and there would
accordingly be no flooding on the subject property, and certainly no flooding above the
aforementioned retaining wall that will run the length of the project from Lyons Street to
the detention pond.
XS #702 5023.6 NAVD 88: Location: diagonally crosses the east side in the lower
approximate 1/3 of the property: The aforementioned recent survey of the property
indicates an approximate ground elevation at this cross section of 5024.0 on the property
adjacent to the proposed stormwater detention pond. As shown on the Drainage Plan Map
(24 x 36, attached), the approximate elevation at the curb outfall in Cherry Street is also
5023.6. Accordingly, again if we assume that both the Flood Risk Map and the recent survey
are correct, the floodplain would not even encroach onto the curb, and certainly would not
encroach onto the subject property.
Based on our discussions with the City, we understand that the City is aware of issues like
this for the West Vine Drainage area, and accordingly initiated the remapping effort. The
applicant also understands that the result of this remapping work may actually show a
higher floodplain elevation here, or, conversely, it may eliminate this area from the Flood
Risk area altogether. Notwithstanding this fact, the Applicant acknowledges that a “No-Rise
Certificate” will be required prior to beginning any work within the floodway.
In addition to the basic Flood Risk elevation discrepancies discussed above, SEC reviewed
the Fort Collins Master Plan Basin Recommended Improvements map, reproduced herein as
Figure 5. With the implementation of recommended basin improvements, this exhibit
shows that the future floodplain should be eliminated from the Cherry Street location
adjacent to the subject property. Accordingly, the Applicant contends that the measures
agreed to above for the development of Cherry Street Cottages will provide acceptable
flood protection for the development and cause no impairment or downstream damage to
properties during the 100-year precipitation event.
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Figure 5. Fort Collins Master Plan Basin Recommended Improvements
B. DRAINAGE BASINS AND SUB-BASINS
1. Major Basin Description
a. The Cherry Street Cottages project is located within the West Vine Basin, located in
northwest Fort Collins with a drainage area of approximately 2,350 acres. Accordingly, this
drainage plan conforms to the West Vine Basin Master Drainage Plan (currently being
updated) as well as the Fort Collins Stormwater Criteria Manual (FCSCM).
b. The City estimates that approximately 85% of the basin is in unincorporated Larimer
County, and in general it drains from west to east along five flow paths. Frequent drainage
problems have arisen on occasion because of sporadic development that occurred within
the basin with corresponding overall inadequate drainage facilities. Major storm events
over the last approximately 20 years have created flooding problems in the vicinity of Irish
Drive and Overland Trail, but not, to the Applicant’s knowledge, specifically in the Cherry-
Lyons Streets area. Moreover, the City of Fort Collins Master Plan Basin Recommended
Improvements, as shown by the Flooding Solution Map in the West Vine Basin Master Plan,
indicates that the current designated floodplain and floodway may be eliminated in the
Cherry-Lyons Streets area with future drainage improvements.
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c. Future improvements to upstream irrigation facilities within the basin may result in a
positive influence on the Cherry Street Cottages project and the immediate area of Cherry
and Lyons Streets. There are no other irrigation facilities within the basin that will influence
or be influenced by the Cherry Street Cottages project, and excess developed runoff from
the subject property will be released in a controlled manner, minimizing the impact of this
project on the overall drainage patterns within the West Vine Basin.
2. Sub-Basin Description
a. The Cherry Street Cottages project will occupy approximately 1 acre within the 2,350-acre
West Vine Drainage Basin. Although this is a minuscule portion of the overall basin, the
proposed project incorporates on-site detention for both water quality and discharge
quantity benefits. Specific Best Management Practices (BMPs) and Low Impact
Development (LID) requirements are met with this project, as discussed in detail below.
b. The southern flow path within the West Vine Basin contributes flow to Cherry Street on the
south side of the proposed project, as depicted in Figure 2. As shown in Figure 5, “City of
Fort Collins Master Plan Basin Recommended Improvements,” future basin improvements
appear to effectively remove the subject property from designated “problem” areas with
respect to basin flooding.
c. The project site receives very little runoff from off-site properties, with the primary source
of off-site runoff coming from the property to the north. A slight drainage divide exists just
north of the property, directing runoff from that area to the east.
C. DRAINAGE DESIGN CRITERIA
1. Optional Provisions
There are no optional provisions or deviations from the FCSCM proposed with the Cherry
Street Cottages project.
2. Impervious Area
The scope of the proposed project does increase the impervious area of the property
through the addition of nine (9) residential dwelling units and the associated rear-entry
driveway and the access street. To the extent possible, Directly Connected Impervious Area
(DCIA) will be minimized, and the overall stormwater management strategy for this project
will utilize the “Four-Step Process” to minimize adverse impacts on receiving water quality,
as follows:
a. Step 1—Employ Runoff Reduction Practices. Because of the proximity of the project
property to the West Vine Drainage designated 100-year floodplain, the overall grading plan
includes the importing of fill material to elevate the ground floor elevation of the homes
within the designated floodplain to at least 18 inches above the BFE. This grading plan
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results in lot grading to create a slight southwest-to-northeast drainage pattern, directing
on-site runoff across grassy area to the driveway/alley access street on the north side of the
property, thence to the east and the detention pond. This changes the historic drainage
pattern from north to south, discharging to Cherry Street. Stormwater runoff will pass
through the detention pond with a controlled release back to Cherry Street, thereby
reducing and slowing off-site runoff.
The Cherry Street Cottages project will implement Low Impact Development (LID) strategies
to reduce runoff peaks, volumes, and pollutant loading from frequently occurring
precipitation events, i.e. 2-year storm events. To the extent practical, runoff will be routed
across vegetated areas (by sheet flow, if possible) to surface-based BMPs such as grass swales
and grass buffers. Implementing this LID strategy should serve to reduce the impacts of the
increased impervious areas while concurrently Minimizing Directly Connected Impervious
Areas (MDCIA). The MDCIA concept will be developed to reduce runoff peaks and volumes.
b. Step 2—Implement BMPs That Provide a Water Quality Capture Volume (WCQV) with Slow
Release. For this project, efforts presented in Step 1 should effectively minimize excess
runoff from frequently occurring storm events. After minimizing runoff, the remaining
runoff will be treated through capture and slow release of the WQCV from the on-site
detention pond.
c. Step 3—Stabilize Drainageways. As stated above, aside from the West Vine Basin Drainage,
no other major drainageways are present on or adjacent to the property. This project will
indirectly help to stabilize the West Vine Drainage through timed release of large
precipitation events, serving to reduce peak flows in the immediate vicinity. Moreover, the
project will pay not only onetime stormwater development fees, but also ongoing monthly
stormwater utility fees, serving to help achieve City-wide drainage-way stability.
d. Step 4—Implement Site-Specific and Other Source Control BMPs. This step typically applies
to industrial and commercial developments.
3. Development Criteria Reference and Constraints
a. There have been no previous drainage studies for this site.
b. There are no known drainage studies for any adjacent properties that will have any effect
on the Cherry Street Cottages project.
c. Site constraints exist on the south and west sides by existing streets, and on the north by a
minor drainage divide and the proposed access street/alley. The site is constrained on the
east by a fully developed residential neighborhood. No other site constraints currently exist
on the subject property.
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4. Hydrologic Criteria
a. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves for the 2-year and 100-
year design precipitation events, as provided in Figure RA-16 of the FCSCM, serve as the
data source for all hydrologic computations associated with the Cherry Street Cottages
project. Data provided in Table RA-7 are used for Rational Method runoff calculations.
b. The Rational Method is used to compute stormwater runoff with coefficients provided in
Tables RO-11 and RO-12 of the FCSCM.
c. The Rational Formula-Based Detention Volume by the Modified FAA Method is used for
detention storage calculations.
d. Design storm recurrence intervals of 2-years (Minor Storm) and 100-years (Major Storm)
are used for these analyses.
5. Hydraulic Criteria
a. The proposed Cherry Street Cottages drainage plan is designed in accordance with criteria
outlined in the FCSCM and/or the Urban Drainage and Flood Control District (UDFCD) Urban
Storm Drainage Criteria Manual.
b. No other drainage facility design criteria are use for this design.
c. As previously stated, there are no natural drainageways within the subject property.
Therefore, no modifications to natural drainageways are proposed, and no “Floodplain
Modeling Study” would be required based on this criterion alone.
6. Floodplain Regulatory Compliance
a. As stated above, the City of Fort Collins Floodplain Review Checklist for 50% Submittals has
been checked to assure compliance. Accordingly, it is the intent of the Applicant to comply
with all applicable City of Fort Collins floodplain regulations.
7. Modifications of Criteria
a. No modifications are requested at this time.
D. DRAINAGE FACILITY DESIGN
1. General Concept
a. Drainage patterns on the subject property have historically consisted of runoff from the
semi-developed property to the north. This runoff drainage has typically flowed to the
south-southeast, and that basic drainage pattern will not be affected by this project. The
primary objective of the Cherry Street Cottages drainage design is to maintain existing
drainage patterns to the extent possible, while not adversely impacting adjacent properties.
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b. As previously discussed, the primary off-site runoff occurs to the east into a developed
residential property and south to Cherry Street. With the completion of the proposed
project, the Applicant believes that off-site runoff may be somewhat reduced to the
residential neighborhood to the east through the installation of on-site detention in Tract A
at the east end of the proposed project. Runoff that previously made its way directly to the
residential properties to the east will now be detained and slowly released to the Cherry
Street curb and gutter, potentially serving to somewhat mitigate flooding of the properties
to the east.
c. A list of Tables and Figures used in this report is provided in the Table of Contents at the
front of this document. Each table or figure therein listed is located within the applicable
section of this report.
d. Existing drainage patterns consist of primarily sheet flow to Cherry Street on the south. The
proposed drainage patterns direct on-site runoff back to the north side of the property and
thence to the east and the detention pond. Both on-site and off-site runoff from the north
will flow to the detention pond located at the east end of the development.
2. Specific Details
a. No specific design problems were encountered during design, aside from employing an
iterative process to design and situate the detention pond in best accordance with both
hydrologic/hydraulic factors and the owner’s preferred pond location and configuration. No
other specific or special design problems were encountered.
b. The Modified FAA Method was used to size the Detention Pond. Outlet design is in
accordance with UDCM design parameters, and applicable equations are referenced in the
design, as shown in Appendices A & B, Hydrologic and Hydraulic Computations,
respectively. The pond volume was calculated to be 0.195 acre-feet (8,507 cubic feet) for
the 100-year required detention that’s based on the 100 year rainfall criterion of 3.67
inches over a 2 hour period. The available Water Quality Capture Volume (WQCV) is 0.017
AF. The corresponding 100-year water surface elevation (WSEL) is 5025.6. The WQCD
storage volume of 0.017 AF WSEL is 5024.4, yielding a WQCV total maximum ponding depth
(at the outlet) of 0.5 ft.
An orifice plate outlet is provided for controlled release from the detention pond. Three (3)
rows of orifices, each with a 0.5-inch-diameter hole will restrict discharge to a cumulative
rate of 0.01 cfs at a pond surface elevation of 5,024.4.
c. Table 2 presents a summary of detention pond design.
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Table 2. Detention Pond Design Summary
Item Quantity
Stage-Storage Curve See Appendix B
Stage-Discharge Curve See Appendix B
Detention Pond Required Volume
(a-f)
0.195
Detention Pond Provided Volume
(a-f)
0.200
Water Quality Capture Volume
(a-f)
0.017
Water Quality Elevation 5,024.4
Spillway Elevation 5,026.9
Pond Freeboard 1’
Outlet Size Orifice Plate: 3 rows with 1 0.5-
inch hole per row; 12-inch
weir for overflow release; 1’
emergency overflow weir
d. Maintenance access is open with no access limitations to the detention pond and
associated outlet works.
e. There is an existing 9-foot utility easement along the south and west sides and a 6-foot
utility and drainage easement along the north and east sides of the property. Lot 7, the
eastern-most lot, will be dedicated for the detention pond and related drainage features.
No structures will be placed within the drainage easement, including playground
equipment, but the area will be available for neighborhood activities during dry weather.
The detention pond, access drive/alley, and the area south of the proposed detention wall
will be dedicated for drainage and utility easements, and will be owned and maintained by
the future homeowners association.
f. No off-site facilities are proposed for conveyance of minor and major flows to the major
drainageway.
E. CONCLUSIONS
1. Compliance with Standards
a. This drainage plan is in compliance with City of Fort Collins Stormwater Criteria Manual.
b. This drainage plan is in compliance with the City of Fort Collins Master Drainage Plan and
the West Vine Drainage Plan.
e. This drainage plan is in compliance with the City of Fort Collins floodplain regulations.
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d. A Floodplain Use Permit application will be prepared in accordance with requirements
provided in Fort Collins City Code, Section 10-27, Floodplain Use Permit.
e. This drainage plan is in compliance with Chapter 10 of City Code, as well as applicable State
of Colorado and Federal regulations governing stormwater discharge.
2. Drainage Concept
a. The drainage design proposed for this project will effectively control damage associated
with stormwater runoff. Runoff from existing pervious areas that will be converted to
impervious areas will be controlled to release at the 2-year historic rate during the 100-year
event.
b. The proposed development will have no impact on either the City or West Vine Basin
Master Drainage Plans.
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REFERENCES
City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by
Ordinance No. 174, 2011.
Soil Survey of Larimer County Area, Colorado, United States Department of Agriculture Soil
Conservation Service, 1980.
Urban Storm Drainage Criteria Manual, Volumes 1–3, Urban Drainage and Flood Control
District, Wright-McLaughlin Engineers, Denver, Colorado, revised April 2008.
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APPENDIX A: HYDROLOGIC COMPUTATIONS
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1. Land use assumptions regarding adjacent properties
The Applicant assumes that existing land uses on adjacent properties to the south (Putnam
Elementary School) and east and west (established residential properties) will remain
unchanged from present uses. The land to the north currently contains one single-family
home, and may be subject to future residential development. In that eventuality, the
Applicant understands that no associated impact to the subject property should be realized,
and in fact, runoff from that area to the north may be mitigated to some extent.
2. Initial (2-year) and major (100-year) storm runoff calculations are attached, including the
historic and developed runoff computations using the UDFCD Rational Method.
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APPENDIX B: HYDRAULIC COMPUTATIONS
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1. Culvert Capacities: N/A. No culverts are proposed.
2. Storm Sewer Capacity: N/A. No new storm sewers are proposed.
3. Street flow calculations: N/A. Because the detention pond outlet is designed to control
release, no new stormwater runoff flow in excess of historic/existing flows will be
generated by this project.
4. Storm inlet capacity: N/A. No new storm inlets are designed for this project.
5. Open Channel Design: N/A. No new open channels are required with this project.
6. Check dam and/or channel drop design: N/A.
7. Detention facility design. The detention pond volume was calculated by the UDFCD
Modified FAA Method. Model printouts are attached, including the 2-year and 100-year
detention volumes, and stage-discharge graphs for both the detention basin and the WQCD
outlet.
8. Downstream/outfall system capacity to the major drainageway system. The detention pond
outfall will be to a concrete drain pan with rock to protect the pan edges during the 100-
year event discharge. The drain pan will transport outfall discharge via a sidewalk chase to
the existing Cherry Street curb and gutter.
9. Erosion protection measures for culverts, and storm sewer outlets: N/A.
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APPENDIX C: (RESERVED FOR FUTURE USE)
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APPENDIX D: FEMA FIRM MAP
Preliminary Drainage and Erosion Control Report
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APPENDIX E: EROSION CONTROL REPORT