HomeMy WebLinkAboutReports - Drainage - 08/06/2025FINAL DRAINAGE REPORT
GILMARTIN SUBDIVISION
341 SOUTH TAFT HILL ROAD
FORT COLLINS, COLORADO
Project Number FDP250005
Prepared for
Evan Gilmartin
Owner
Prepared by
Sorensen Engineering & Construction, Inc.
1901 Bear Court
Fort Collins, CO
August 2025
1901 Bear Court / Fort Collins, Colorado 80525
(970) 590-1579 / paul@secengineering.net
Sorensen Engineering & Construction, Inc.
Civil/Environmental Engineering
August 6, 2025
City of Fort Collins Stormwater
Development Review Engineer
222 Laporte Ave.
Fort Collins, CO 80524
Subject: PDP/FP Drainage Report
Gilmartin Subdivision
341 South Taft Hill Road
Larimer County Parcel No. 9709404005
Project Number FDP250005
Dear City of Fort Collins Stormwater,
On behalf of Evan Gilmartin, owner of the referenced property, Sorensen Engineering &
Construction, Inc. (SEC – the “Design Engineer”) is pleased to submit this Final
Drainage Report for the proposed Gilmartin Subdivision. This Drainage Report follows
the City of Fort Collins Stormwater Criteria Manual (FCSCM) PDP/FP Drainage Report
Checklist.
STATEMENT OF COMPLIANCE: The Applicant commits that all erosion control
measures that are provided with the final plans will be implemented. This Final Drainage
Report complies with the Fort Collins Stormwater Criteria Manual and floodplain
regulations.
Please do not hesitate to contact me directly if you have any questions or concerns
regarding this Final Drainage Report, or if you require any additional information.
Respectfully,
Paul C. Sorensen, PE
Attachments
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“I hereby attest that this report for the final drainage design for the Gilmartin Subdivision was
prepared by me or under my direct supervision, in accordance with the provisions of the Fort
Collins Stormwater Criteria Manual. I understand that the City of Fort Collins does not and will
not assume liability for drainage facilities designed by others.”
Paul C. Sorensen, PE
8/06/2025
4
Sorensen Engineering & Construction, Inc. (SEC) is pleased to submit this Final Drainage
Report for the proposed Gilmartin Subdivision. This Final Drainage Report is prepared in
accordance with the City of Fort Collins Stormwater Criteria Manual.
I. GENERAL LOCATION & EXISTING SITE INFORMATION
As shown on Figure 1, Vicinity Map, the site is located at 341 South Taft Hill Road, Fort Collins,
Colorado. The site is located in the SE ¼, Section 9, Township 7 North, Range 69 West of the
6th Principal Meridian, City of Fort Collins, County of Larimer, State of Colorado. The project
site is located in the Mulberry Hill neighborhood and is bordered to the north by an existing
single-family residential property, to the east by South Taft Hill Road and City Park Golf Course,
to the west by single-family homes (Mulberry Hill) and Poudre High School, and to the south by
single family homes (Mulberry Hill) and a church, and Mulberry Street. The site is currently
covered with native grasses, with approximately 80% vegetative cover.
The Site is located in the Canal Importation Basin. There are no existing stormwater drainage
facilities on the subject property. The site is relatively flat, with existing drainage patterns
flowing gently from west to east. The New Mercer Ditch is pres ent about ¼-mile to the east;
running through the City Park Nine golf course. As shown on Figure 2, Existing Conditions and
Demolition Plan, the Site is currently vacant ground with native grasses and weeds across the
property. As required, a Floodplain Use Permit will be acquired prior to any construction
activities on the subject site. Additionally, in accordance with City of Fort Collins comments, the
following notes shall apply:
1) This property is currently located in the city regulated, 100-year Canal Importation flood
fringe and must comply with the safety regulations of Chapter 10 of City Municipal Code (in
fact, the entire property lies within the 100-year Canal Importation Flood Fringe area).
2) Any construction activities in the flood fringe (e.g. structures, sidewalk or curb & gutter
installation/replacement, utility work, landscaping, etc.) must be preceded by an approved
floodplain use permit, the appropriate permit application fees, and approved plans.
3) The lowest finished floor elevation of a new residential structure, and all duct work, heating,
ventilation, electrical systems, etc. must be elevated 18 inches above the highest Base Flood
Elevation (BFE) within the footprint of the structure.
4) Any new crawl space must comply with venting and crawl space requirements from Sections
10 39 and 10 40 of City Municipal Code.”
The NRCS Custom Soil Resource Report for the site is provided herein in Appendix A. As
shown, the on-site soil is primarily a Nunn clay loam at 1 to 3% slope with a small section on the
west side of the lot (west of the future Pennsylvania ROW) consisting of Altvan-Satanta loams at
0 to 3% slope. The entire area for this proposed project lies within the Nunn clay loam soils with
a Hydrologic Soil Group C.
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II. MASTER DRAINAGE BASIN INFORMATION
This project is located in the Canal Importation Basin in Fort Collins. The Canal Importation
Basin encompasses almost five square miles (3,200 acres) of west-central Fort Collins. The
basin is bounded generally by Laporte Avenue on the north, the foothills on the west, Drake
Road to the south and extends almost to Shields Street on the east. Across the basin, a 100 -year
storm event could lead to flood-related damage to homes and other structures.
There are no master planning improvements on or adjacent to the site. Historic drainage patterns
within the basin are from west to east. There have been significant precipitation and flood events
over the last 100 years that have caused property damage w ithin the basin. There are no
irrigation ditches, reservoirs, or other facilities that influence or are influenced by the local
drainage.
III. FLOODPLAIN INFORMATION
The project property lies within the City of Fort Collins 100-year Canal Importation Basin Flood
Fringe. The flood fringe area is relatively flat, sloping gently from west to east.
IV. PROJECT DESCRIPTION
As shown on Figure 3, Overall Site Plan, the proposed project contains approximately 0.71
acres; including the following characteristics:
• Lot One for a single-family home with a detached additional dwelling unit (ADU):
.60+/- acre (excludes the Pennsylvania Street ROW);
• Dedication of Pennsylvania Street: .09 acre;
• Outlot: .15 acre;
• Dedication of Additional Public Right-of-Way for S. Taft Hill Road (Two-Lane Arteria):
12’; and
• Dedication of Utility Easement along S. Taft Hill Road: 15’
V. PROPOSED DRAINAGE FACILITIES
The project site is currently vacant, covered with grass and weed vegetation, with approximately
2% impervious area. Drainage patterns are from west to east across the site, discharging in
general via sheet flow to S. Taft Hill Road. No existing improvements are found on the subject
property. Proposed site improvements and specific water quality (Low Impact Development –
LID) measures include the following:
• A total estimated area of disturbance, including a staging area on the west side of the
proposed home and ADU buildings is approximately 26,163 square feet (0.60 acres) (the
disturbed area is limited to Lot 1);
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• Construction of the access driveway along the north side of the subject property. The
driveway will be constructed of concrete. The driveway will encompass an approximate
area of 150-feet long by 12-feet wide (1,800-sf). Additional gravel or recycled asphalt
surface will include the parking area on the west side of the home and ADU of
approximately 50-ft x 50-ft (2,500 sf), for a total area of approximately 4,300 square feet
(sf), or 0.10 acres(ac);
• One single family home, with a building footprint of 35-feet x 60-feet (2,100 sf), plus one
ADU with a building footprint of 25-feet x 40-feet (1,000 sf), including area within that
footprint for a 10-ft x 10-ft “room” for the fire suppressant sprinkler system tank and
pump for a total rooftop area of 3,100-sf (0.07 AC);
• Direct building downspouts into landscaped areas;
• Site grading will be accomplished to provide disconnection of impervious areas to the
extent possible;
• Flood vents will be installed in the foundation walls of both structures. Vents are sized to
provide 1 square inch of opening per 1 square foot of floor space; and
• The remaining portions of the subject property will be landscaped with grass and
plantings or remain as native, undisturbed grass.
As indicated above, this site is in the Fort Collins Canal Importation Basin. Figure 4, City of
Fort Collins Flood Risk Map, provides the corresponding flood map, showing the City-provided
100-year flood fringe depth of flow across the site. Figure 5, Grading and Drainage Plan
presents details of the proposed drainage facilities for this project. As shown, the proposed
structures are structurally independent and are elevated so that the lowest floor, including HVAC
ductwork, is at least 18-inches above the 100-year Base Flood Elevation (BFE) as defined. The
basic drainage plan for this project is to minimize grading and maintain the existing drainage
pattern: sheet flow across the property from west to east.
As shown on Figure 6, Erosion Control and Floodproofing Plan, flood vents will be installed in
the upstream and downstream sides of both the proposed ADU and single family home
foundations to allow flood water to flow via its natural course unobstructed by the structures,
thence discharging via sheet flow across landscaped areas into the curb and gutter flow in Taft
Hill Road and flowing to the existing storm sewer inlet south of the site along the west side on
Taft Hill Road. The flood vents will be sized to provide at least 1-square inch of vent area per 1-
square foot of floor area, and the crawlspace in each unit will be graded to promote flood water
drainage following a major precipitation event. One sump pump shall be installed near the
downstream (east) side of each structure to ensure complete evacuation of runoff water following
major precipitation events. Off-site flow to adjacent properties will be mitigated by grading to
direct flow from the west side of the proposed housing structures to the inverted crown driveway,
and thence to S Taft Hill Road.
Maintenance access for drainage facilities associated with this project shall remain open as nay
be needed.
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VI. DRAINAGE DESIGN CRITERIA
a. Four-Step Process
Management of runoff from frequently occurring storm events shall include consideration of the
following four step process that may be applicable for this project:
i. Employ runoff reduction practices: With the exception of building
footprints, the concrete driveway and the gravel/recycled asphalt parking
area, the remainder of the site will retain in pervious nature, allowing for
most precipitation from small, frequent events to infiltrate on-site. The
relatively flat nature of this site does not encourage off-site runoff from
small, more frequent precipitation events. Site grading and landscaping
will serve to minimize small precipitation event runoff.
ii. Implementing best management practices (BMPs) that provide a water
quality capture volume with slow release and/or infiltration : This process
in not applicable for this site; no detention storage is required, therefore
consideration of the Water Quality Capture Volume (WQCV) is not
applicable.
iii. Stabilizing Streams: This process is also not applicable for this site – no
streams are present on the site or adjacent to the site.
iv. Implementing site specific and other source control BMPs: Site-specific
BMPs to be implemented at this site include the following (discussed in
detail below in the “Erosion and Sediment Control” section of this
Drainage Report):
1. Silt Fencing
2. Vehicle Tracking Pad
3. Concrete Washout Pit
4. Sediment Straining
5. Controlled Staging Area
6. Stockpile Management
b. Rainfall Data
CoFC rainfall data are used for the 2-year and 100-year storm events. Rainfall runoff data used
are as described in the Fort Collins Stormwater Criteria Manual (FCSCM), Chapter 5, Table 3.4-
1. The “minor storm” analysis is the 2-year event, and the “major storm” even is the 100-year
event. The 2-year drainage system is designed to transport stormwater runoff from the site with
minimal disruption to neighbors or the urban environment. This minor storm runoff will be
conveyed in the curb and gutter of Taft Hill Road to the nearby storm sewer inlet. The 100 -year
drainage system is designed to eliminate the loss of life and to minimize adjacent property
damage and to convey runoff to the Taft Hill curb and gutter and thence to the storm sewer inlet.
The hydrologic calculations for this drainage investigation are based on the historic and proposed
(composite) runoff coefficients as described in FCSCM Chapter 5, Table 3.2-2 as shown in Table
1. The runoff analysis discussed below uses runoff coefficients that employ the Frequency
Adjustment Factors as presented in Table 3.2-3 for respective storm frequencies. For this very
small project, the runoff coefficient presented below is for the entire site as a single basin.
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Calculations of the composite runoff coefficients are included in Appendix B, Hydrologic
Calculations.
Table 1. Existing and Proposed Composite Runoff Coefficients
Total Drainage Area = 0.71 Acres = 30,809 square feet (sf)
Surface Cover/Use Defined
Runoff
Coefficient1
Area (sf)
Historical/Existing Proposed
Historical Flow Analysis
(undeveloped clay-loam soil
type)
0.20 30,809 0
Parking area (gravel) 0.50 0 2,500
Driveway (concrete) 0.95 2,400
Rooftop 0.95 0 3,100
Undisturbed soil (assume 2%
slope)
0.20 0 15,000
Landscape/grasses (assume 2%
slope)
0.20 0 7,809
Historical
Runoff
Coefficienti
0.20
Proposed
Runoff
Coefficient
0.36
1. Reference: FCSCM Chapter 5, Table 3.2-2
c. Runoff Calculation Method
The Rational Method is used to calculate both the minor and major runoff events. The one-hour
point rainfall depth used for the 2-year event is 0.82 inches and for the 100-year event is 2.86
inches (FCSCM Chapter 5, Table 3.4-1). The hydrological analysis was computed using the
Peak Runoff Prediction by the Rational Method spreadsheet developed by the Urban Drainage
and Flood Control District (now the Mile High Flood District - MHFD) as described in the
Urban Storm Drainage Criteria Manual (USDCM), Volume 1. The MHFD model allows for user
override to enter the runoff coefficients presented above as well as Fort Collins -specific rainfall
depths as presented in the aforementioned Table 3.4-1.
The Rational Method is used to analyze the existing and fully developed conditions . The
Hydrologic Calculations are presented in Appendix B, the results of which are summarized in
Table 2.
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Table 2. Results of Peak Runoff Flow Prediction by the Rational Method
Basin
Runoff Coefficients Peak Runoff Flowrate (cfs)
Existing Developed Existing Developed
2-yr 100-yr 2-yr 100-yr 2-yr 100-yr 2-yr 100-yr
Project 0.20 0.25 0.36 0.45 0.19 0.83 0.44 1.93
As is demonstrated above, the proposed site improvements will increase the estimated peak
runoff flowrate from 0.19 to 0.44 cfs for the 2-year event, and from 0.83 to 1.93 cfs for the 100-
year event. This increase in runoff is typical of small projects like this and would not contribute
to off-site impacts during a large precipitation event.
d. Detention Calculation Method – not applicable; detention storage is not required
for this project.
VII. VARIANCE REQUESTS
There are no variance requests with this project.
VIII. EROSION AND SEDIMENT CONTROL PLAN
Proposed ground disturbance on this lot will basically be limited to the new access driveway and
for the construction of two new residential structures. Erosion control measures to be
implemented include the following:
• Silt fencing shall be placed along the property perimeter to control off-site stormwater
and sediment runoff.
• Vehicle Tracking Pad shall be placed at the proposed new access point off of Taft Hill
Road near the northeast corner of the lot.
• Concrete Washout Pits shall be placed adjacent to the new home construction sites. The
specific locations shall be determined by the building contractor.
• Sediment Straining will occur naturally with the maintenance of existing native grasses
along most of the western half of the property. With construction activities limited to the
central and eastern portion of the property, the natural grass buffer will filter runoff as it
flows across the existing native grasses, effectively straining and providing physical
removal or retention of particulates from the runoff.
• Controlled staging area shall be placed on the site and coordinated by the building
contractor;
• Stockpile management shall be exercised. This project entails minimal dirt moving
activities; site grading will be minimal, and because no basements are allowed, excavated
materials will be minimal as well. However, any dirt stockpile that is required will be
managed and controlled in accordance with good housekeeping and best professional
judgement.
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Disturbed areas of the project that will remain uncompacted and will be revegetated with native
grasses. Figure 6, Erosion Control and Floodproofing Plan presents details of the proposed
BMPs to be implemented, as summarized above.
IX. CONCLUSIONS
This Drainage Narrative was prepared in accordance with the Fort Collins Stormwater Criteria
Manual. As is demonstrated above, the proposed site improvements will increase the estimated
peak runoff flowrate from 0.19 to 0.83 cfs for the 2-year event, and from 0.44 to 1.93 cfs for the
100-year event. SEC believes that the increase in peak runoff flowrates will not impact adjacent
properties. The drainage plan as herein described was prepared in accordance with sound
engineering practices and with Best Professional Judgement and provides an effective drainage
system for this site that will effectively control stormwater runoff from this property and not
exacerbate flood-related damage to neighboring or downstream properties.
STATEMENT OF COMPLIANCE: The Applicant commits that all erosion control measures that
are provided with the final plans will be implemented. This Final Drainage Report complies
with the Fort Collins Stormwater Criteria Manual and floodplain regulations.
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REFERENCES
1. City of Fort Collins Stormwater Criteria Manual, Chapter 5, Hydrology Standards,
December 2018.
2. City of Fort Collins Flood Risk Map, February 2, 2022.
3. National Resources Conservation Service (NRCS) Custom Soil Resources Report for
Larimer County Area, Colorado, 341 Southy Taft Hill Road, March 16, 2023.
4. Urban Drainage and Flood Control District, a.k.a., Mile High Flood District (MHFD),
Peak Runoff Prediction by the Rational Method, Version 2.00 released May 2017.
CONTOUR INTERVAL: 10 FEET
PROJECT DATE: FEBRUARY 2022
MAP SOURCE:
USGS FORT COLLINS, COLO 2016
LARIMER COUNTY PARCEL NUMBER:
9709404005
341 S Taft Hill Rd
Fort Collins, CO FIGURE 1
VICINITY MAP
PROJECT
SORENSEN ENGINEERING & CONSTRUCTION, INC
CIVIL / ENVIRONMENTAL ENGINEERING
1901 BEAR COURT FORT COLLINS, CO 80525
PHONE:970 590-1579 paul@secengineering.net
N Scale in Miles
0 11/2
PROPERTY
LOCATION
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LOT 1
26162.75 Sq. Ft.
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AS NOTED
FIG 2
GILMARTIN MINOR SUBDIVISION
A TRACT OF LAND LOCATED IN THE SOUTHEAST QUARTER OF SECTION 9, TOWNSHIP 7 NORTH,
RANGE 69 WEST OF THE 6TH PRINCIPAL MERIDIAN, CITY OF FORT COLLINS, COUNTY OF LARIMER, STATE OF COLORADO
341 SOUTH TAFT HILL ROAD
FORT COLLINS, COLORADO
LARIMER COUNTY PARCEL # 9709404005
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PENNSYLVANIA
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15.00'
30.00'
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UTILITY
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SETBACK
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S Taft Hill Road
35'
2100 sf
GRAVEL OR
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10'
ADU -
DETACHED 25'
40'
1000 sf
5' SETBACK (min)139'
Penn St to ADU 139'
CONRETE
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AS NOTED
QTY
3
SYMBOL BOTANIC NAME COMMON NAME SIZE
2
ULMUS DAVIDIANA
JAPONICA "DISCOVERY"
PINU EDULIS
DISCOVERY ELM
PINYON PINE
2.5" CAL. MIN
6' HT
LANDSCAPING NOTES:
COVER IN LANDSCAPE AREAS -
ALL REQUIRED LANDSCAPED AREAS MUST CONTAIN A MINIMUM OF
SEVENTY FIVE PERCENT (75%) ORGANIC MATERIAL, WITH A MAXIMUM OF TWENTY FIVE (25%) NON-LIVING MATERIALS.
MIN. SIZE FOR TREES AND SHRUBS -
ORNAMENTAL, LESS THAN 20' MATURITY HEIGHT. 1"-1.5" CAL. SIZE AT PLANTING
GRADING STANDARD -
LAWN AND GRASSED AREAS, MAX SLOPE 4:1
MAINTENANCE STANDARDS -
ALL LANDSCAPING AND REQUIRED BUFFERING SHALL BE CONTINUALLY MAINTAINED INCLUDING IRRIGATION IF APPLICABLE,
WEEDING, PRUNING, AND REPLACING IN A SUBSTANTIALLY SIMILAR MANNER AS ORIGINALLY APPROVED. THE FOLLOWING
SURVIVAL STANDARDS SHALL APPLY TO ALL LANDSCAPING AND REQUIRED BUFFERS:
1. LIVING GROUND COVERS: LIVING GROUND COVERS MUST BE FIFTY PERCENT (50%) ESTABLISHED AFTER THE FIRST
GROWING SEASON,, AND NINETY PERCENT (90%) ESTABLISHED THEREAFTER.
2. NON-LIVING GROUND COVERS: NON-LIVING GROUND COVERS, SUCH AS ROCK OR MULCH MUST BE ONE HUNDRED
PERCENT (100%) INTACT AFTER ONE (1) YEAR AND EIGHTY PERCENT (80%) INTACT THEREAFTER.
3. TREES AND SHRUBS: TREES AND SHRUBS MUST HAVE A ONE HUNDRED PERCENT (100%) SURVIVAL RATE AFTER ONE (1)
YEAR AND A NINETY PERCENT (90%) SURVIVAL RATE THEREAFTER.
ALL DIMENSIONS AND LOCATIONS ARE ASSUMED AND SHOW DESIGN INTENT ONLY, VERIFY IN FIELD.
PROPERLY PREPARE SOILS FOR NEW PLANTINGS. PROVIDE GOOD BALANCE OF AIR, SAND, SILT, AND CLAY. USE WOVEN
FABRIC WEED BARRIERS AS DESIRED.
IRRIGATION: ALL LANDSCAPE AREAS WITHIN THE SITE INCLUDING TURF, SHRUB BEDS, AND TREE AREAS SHALL BE
IRRIGATED WITH AN AUTOMATIC IRRIGATION SYSTEM. THE IRRIGATION PLAN MUST BE REVIEWED AND APPROVED BY
THE CITY OF FORT COLLINS WATER UTILITIES DEPARTMENT PRIOR TO THE ISSUANCE OF A BUILDING PERMIT. ALL TURF
AREAS SHALL BE IRRIGATED WITH AN AUTOMATIC POP-UP IRRIGATION SYSTEM. ALL SHRUBS BEDS AND TREES,
INCLUDING IN NATIVE SEED AREAS, SHALL BE IRRIGATED WITH AN AUTOMATIC DRIP (TRICKLE) IRRIGATION SYSTEM, OR
WITH AN ACCEPTABLE ALTERNATIVE APPROVED BY THE CITY WITH THE IRRIGATION PLANS. THE IRRIGATION SYSTEM
SHALL BE ADJUSTED TO MEET THE WATER REQUIREMENTS OF THE INDIVIDUAL PLANT MATERIAL.
COST NOTES: XERISCAPE CONVERSION TYPICALLY RUNS AT $1.50 - $4.50 PER/SF FOR MATERIAL AND PLANTS. LABOR
NOT INCLUDED.
OTHER COSTS WOULD INCLUDE THE REMOVAL OF EXISTING MATERIAL FROM THE SITE, ANY NECESSARY SOIL
AMENDMENT AND ANY TREES/SHRUBS REQUIRED.
THE DEVELOPER OR ITS SUCCESSOR(S) SHALL BE RESPONSIBLE FOR THE ONGOING IRRIGATION AND MAINTENANCE OF
THE LANDSCAPING LOCATED WITHIN TEH PUBLIC RIGHT OF WAY ALONG THE PORTION OF S TAFT HILL ROAD THAT ABUTS
THE DEVELOPMENT.
NOT FOR CONSTRUCTION FIG 3
GILMARTIN MINOR SUBDIVISION
NOTE: THE BUILDING DESIGN SHALL MEET THE ZONE DISTRICT AND BUILDING TYPE STANDARDS
AS OUTLINED IN SECTION 2.2.1 AND DIVISION 3.1
NOTE: AT THE TIME PENNSYLVANIA STREET IS FULLY IMPROVED AS A RESIDENTIAL
LOCAL STREET (LCUASS 7-8F), ALONG THE ENTIRE FRONTAGE OF THE GILMARTIN SUBDIVISION,
WITH PENNSYLVANIA STREET ACHIEVING CONNECTIVITY TO THE ADJACENT PUBLIC STREET
SYSTEM, AND IN COMPLIANCE WITH ALL REQUIREMENTS OF THE POUDRE FIRE AUTHORITY. THE
EXISTING DRIVEWAY AT SOUTH TAFT HILL ROAD SHALL BE CLOSED AND ACCESS TO THE
GILMARTIN SUBDIVISION SHALL THEN BE GAINED VIA PENNSYLVANIA STREET.
XS
#
:
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XS
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D
W MULBERRY ST
Printed: 02/02/2022.
All floodplain boundaries are approximate.
This information is based on the Federal Emergency Management Agency (FEMA) Flood Insurance Rate Map (FIRM) and the City of Fort Collins Master Drainageway Plans. This letter does not imply that the referenced property will or will not be free from flooding or damage. A property not in the Special Flood Hazard Area or in a City Designated Floodplain may be damaged by a flood greater than that predicted on the map or from a local drainage problem not shown on the map. This map does not create liability on the part of the City, or any officer or employee thereof, for any damage that results from reliance on this information.
City Flood Risk Map
Moderate Risk
May include:- Areas of FEMA 500-year floodplain (FEMA Zone X-shaded).- Areas of FEMA or City 100-year floodplain (sheet flow) withaverage depths of less than 1 foot.- Areas protected by levees from the 100-year flood.Low Risk
Areas outside of FEMA and City mapped 100-year and 500-year floodplains. Local drainage problems may still exist.
City Flood Fringe - May Include:- Areas of FEMA 100-year floodplain (FEMA Zones A, AE, AO, and AH)- Areas of City 100-year floodplain including ponding areas and sheetflow areas with average depths of 1-3 feet.There is a 1% annual chance that these areas will be flooded.
High Risk
City Floodway - Area of 100-year floodplain with greatest depths and fastest velocities.
0 40 80 120 16020Feet
FIGURE 4
SUBJECT PROPERTY
50
7
5
50
7
6
50
7
4
50
7
3
507
2
5071
50
7
0
50
7
1
50
7
0
507650
7
6
5077
507
7
adjacent house
adjacent garageadj
shedadjacent building
SO
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#
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#
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#
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8
8
LOT 1
26162.75 Sq. Ft.
0.60 Acres
5068'
5070'
5072'
5074'
5076'
5078'
5072'
0+00 0+20 0+40 0+60 0+80 1+00 1+20 1+40 1+60 1+80 2+00 2+20 2+40 2+60 2+80 3+00 3+20 3+40 3+60 3+80 4+00 4+20 4+40
DUCT WORK
DUCT WORK
PRINCIPAL
RESIDENCE
ADU -
DETATCHED
BFE 5070.72
FF 5075.4FF 5076
BFE 5072.9
MIN DUCT 5074.4BFE 5073.5
MIN DUCT 5075
100-YR BASE
FLOOD ELEVATIONEXISTING
GRADE
5075'
5074'
5073'
5071'
PROPOSED
5071.2'
PROPOSED
5071.7'
SPREAD FOOTING W/ FOUNDATION WALL
(TOP OF FOOTER MIN 30" BELOW EXIST GRADE)
BFE 5075.94
6" VERTICAL
CURB
SECTION A - A'
SCALE: HORIZONTAL 1" = 20'
VERTICAL 1" = 4'
(5:1 EXAGGERATION)
EL
E
V
A
T
I
O
N
1
"
=
4
'
OUTLOT
(EXIST GRADES/VEGETATION
TO REMAIN UNDISTURBED)
A'
A
BFE 5071.59'
QU
I
T
C
L
A
I
M
-
R
E
C
.
NO
.
2
0
0
7
0
0
1
5
2
3
5
GRADE FOR
NE FLOW
PATTERN
PENNSYLVANIA
STREET
57' RIGHT OF WAY
30.00'
SETBACK
42.00'
15.00'
UTILITY
EASEMENT
PR
O
P
E
R
T
Y
B
O
U
N
D
A
R
Y
Parking Spaces (10'x20')
Recycled Asphalt /
Road Base
NEW ACCESS TO
S TAFT HILL ROAD
(see detail sheet
U105 - Note 3)
EXISITNG ACCESS
TO BE REMOVED
AND REPLACED
WITH NEW CURB,
GUTTER, AND
SIDEWALK TO
MATCH EXISTING
(see detail drawing
701, Sheet U105)
BUILDING
ENVELOPE
PRINCIPAL
RESIDENCE
60'
35
'2100 sf
GRAVEL OR
RECYCLED
ASPHALT
ADU -
DETACHED
25
'
40'10'
1000 sf
5077
12.00'
5071.7'
X
5071.2'
50
7
0
.
9
'
5070.4'
X
X X5070.9'
9' UTILITY
EASEMENT
9' UTILITY
EASEMENT
5072.5'
X
5072.5'
X
5071.5'
X
FF 5075.4'
See Cross Section
LCUASS Fig 7-11F
(sheet U105)
Amended for site
conditions: 8' Utility
Easement on one
side onlyCONRETE
ALLEY
S
S
PROPOSED GRADE SHALL
REMAIN AS EXISTING WITH
THE FOLLOWING EXCEPTION:
PROPOSED GRADE DIPS SLIGHTLY
AT APPROXIMATE STRUCTURE
CENTERLINE TO PROMOTE FLOW
THROUGH VENT
S
S
PROPOSED
5070.9'
PROPOSED
5070.4'
ONE (1) SUMP PUMP SHALL BE INSTALLED
NEAR THE DOWNSTREAM (EAST) SIDE OF
EACH STRUCTURE. SUMP PUMP SHALL
BE SUPERIOR PUMP 92372, 1
3 HP
THERMOPLASTIC SUBMERSIBLE PUMP
WITH VERTICAL FLOAT CONTROL
SWITCH OR APPROVED EQUAL.
4' SIDEWALK
TOTAL DISTURBED AREA:
LOT 1 = 26,163 SF = 0.60 ACRES
0+79
1+
3
6
2+75 3+15 3+25
EXISTING
PROPOSED
4+60 4+80 5+00
0+
7
9
0+
0
0
2+
7
5
3+
1
5
3+
8
5
1+
3
6
0+
7
9
0+
0
0
2+
7
5
3+
1
5
3+
8
5
1+
0
4
PROTECT EXISTING
ACCESS FROM
NEIGHBORING
PROPERTY ONTO
S TAFT HILL RD
0
40
20
N
o
r
t
h
-
S
c
a
l
e
1
"
=
2
0
'
SEAL
LEGEND
DRAWN BY:
PS
PAS
CHECKED BY:
SHEET NUMBER:
SCALE:
DATE:
08/04/2025
SHEET INDEX:
JOB NUMBER:
RE
V
I
S
I
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N
S
PR
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NO
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DA
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19
0
1
B
E
A
R
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O
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FO
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T
C
O
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L
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S
,
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8
0
5
2
5
PH
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E
:
9
7
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1
5
7
9
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34
1
S
O
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T
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A
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H
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L
R
O
A
D
GR
A
D
I
N
G
A
N
D
D
R
A
I
N
A
G
E
P
L
A
N
AS NOTED
PROPOSED CONTOUR
FIG 5
ELEVATION TABLE
UNIT PROPOSED GRADE BFE FF
(UPSTREAM SIDE)
PRIMARY
RES 5070.9 5072.9 5075.4
ADU 5071.7 5073.5 5076
EXISTING GRADE @ UPSTREAM SIDE OF UNIT
ESTIMATED ELEVATIONS - INTERPOLATED @UNIT N-S ℄
XS#.1477 AND XS#.1195 - BASE FLOOD ELEVATIONS CROSS-SECTION
GILMARTIN MINOR SUBDIVISION
NOTES:
1. AT THE TIME PENNSYLVANIA STREET IS FULLY
CONSTRUCTED, ACCESS TO THIS PROPERTY MUST BE
TAKEN FROM PENNSYLVANIA STREET AND THE ACCESS
TAKEN FROM S TAFT HILL ROAD MUST BE CLOSED.
2. THE TOP OF FOUNDATIONS SHOWN ARE THE
MINIMUM ELEVATIONS REQUIRE FOR PROTECTION
FROM THE 100-YEAR STORM.
3. PROTECT EXISTING ACCESS FROM NEIGHBORING
PROPERTY ONTO S TAFT HILL ROAD.
4. NO TRACKING OF DEBRIS OR SEDIMENT IS ALLOWED
ON NEIGHBORING PROPERTYOR PUBLIC RIGHT OF WA. IF TRACKING
DOES OCCUR, CONTRACTOR SHALL CLEAN IT IMMEDIATELY.
PROPOSED RUNOFF
FLOW DIRECTION
S SUMP PUMP: ONE
EACH STRUCTURE
EXISTING CONTOUR5073
FLODPLAIN AND FLOODWAY NOTES
1. THE ENTIRE PROPERTY IS CURRENTLY LOCATED IN THE CITY REGULATED 100-YEAR
CANAL IMPORTATION FLOOD FRINGE AND MUST COMPLY WITH THE SAFETY REGULATIONS
OF CHAPTER 10 OF MUNICIPAL CODE.
2. ANY CONSTRUCTION ACTIVITIES IN THE FLOOD FRINGE (E.G. STRUCTURES, SIDEWALK OR
CURB & GUTTER INSTALLATION/REPLACEMENT, UTILITY WORK, LANDSCAPING,, ETC.) MUST
BE PRECEDED BY AN APPROVED FLOODPLAIN USE PERMIT, THE APPROPRIATE PERMIT
APPLICATION FEES, AND APPROVED PLANS.
3. THE LOWEST FINISHED FLOOR ELEVATION OF A NEW RESIDENTIAL STRUCTURE, AND ALL DUCT
WORK, HEATING, VENTILATION, ELECTRICAL SYSTEMS, ETC. MUST BE ELEVATED 18 INCHES.
4. ANY NEW CRAWL SPACE MUST COMPLY WITH VENTING AND CRAWL SPACE REQUIEMENTS
FROM SECTIONS 10 39 AND 10 40 OF CITY MUNICIPAL CODE.
ROOF DRAIN
50
7
5
50
7
6
50
7
4
50
7
3
507
2
5071
50
7
0
50
7
1
50
7
0
507650
7
6
5077
adjacent house
adjacent garageadj
shedadjacent building
SO
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#
.
1
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9
5
0
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0
.
7
2
f
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D
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8
XS
#
.
1
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4
6
5
0
7
0
.
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4
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XS
#
.
1
0
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1
5
0
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8
.
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D
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XS
#
.
1
4
7
7
5
0
7
5
.
9
4
f
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D
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XS
#
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5
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1
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5
9
f
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N
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D
8
8
LOT 1
26162.75 Sq. Ft.
0.60 Acres
5068'
5070'
5072'
5074'
5076'
5078'
5072'
0+00 0+20 0+40 0+60 0+80 1+00 1+20 1+40 1+60 1+80 2+00 2+20 2+40 2+60 2+80 3+00 3+20 3+40 3+60 3+80 4+00 4+20 4+40
DUCT WORK
DUCT WORK
PRINCIPAL
RESIDENCE
ADU -
DEDATCHED
BFE 5070.72
FF 5075.4FF 5076
BFE 5072.9
MIN DUCT 5074.4BFE 5073.5
MIN DUCT 5075
100-YR BASE
FLOOD ELEVATIONEXISTING
GRADE
5075'
5074'
5073'
5071'
PROPOSED
5071.2'
PROPOSED
5071.7'
SPREAD FOOTING W/ FOUNDATION WALL
(TOP OF FOOTER MIN 30" BELOW EXIST GRADE)
BFE 5075.94
6" VERTICAL
CURB
SECTION A - A'
SCALE: HORIZONTAL 1" = 20'
VERTICAL 1" = 4'
(5:1 EXAGGERATION)
EL
E
V
A
T
I
O
N
1
"
=
4
'
OUTLOT
(EXIST GRADES/VEGETATION
TO REMAIN UNDISTURBED)
A'
A
SF
SF SF
SF
BFE 5071.59'
QU
I
T
C
L
A
I
M
-
R
E
C
.
NO
.
2
0
0
7
0
0
1
5
2
3
5
GRADE FOR
NE FLOW
PATTERN
PENNSYLVANIA
STREET
57' RIGHT OF WAY
30.00'
SETBACK
42.00'
15.00'
UTILITY
EASEMENT
PR
O
P
E
R
T
Y
B
O
U
N
D
A
R
Y
Parking Spaces (10'x20')
Recycled Asphalt /
Road Base
NEW ACCESS TO
S TAFT HILL ROAD
(see detail sheet
U105 - Note 3)
EXISITNG ACCESS
TO BE REMOVED
AND REPLACED
WITH NEW CURB,
GUTTER, AND
SIDEWALK TO
MATCH EXISTING
(see detail drawing
701, Sheet U105)
BUILDING
ENVELOPE
PRINCIPAL
RESIDENCE
60'
35
'2100 sf
GRAVEL OR
RECYCLED
ASPHALT
ADU -
DETACHED
25
'
40'10'
1000 sf
VTC
FLOOD VENTS (1 SQ INCH VENT/1 SQ FOOT BUILDING): RECOMMEND FLOOD VENTS
BY SMART VENT OR EQUAL - ALL 2X2 MULTI-VENT CONFIGURATION
RS
CWA
CWA
CONCRETE WASHOUT
AREA - EXACT LOCATION
TBD BY CONTRACTOR.
CANNOT ENCROACH ON
ADJACENT PROPERTY.
5077
12.00'
STRUCTURE MODEL #
FLOOR
COVERAGE
PER VENT (SF)
AIR
VENTILATION
(SQ IN)
# VENTS
EACH END
PRINCIPAL
RESIDENCE
ADU
1540-
150202 800 204 3
1540-
510 200 51 1
1540-
511 400 102 2
FLOOD VENT
(3 EACH END,
BOTH STRUCTURES)
(TYP)
5071.7'
X
GRADE CRAWL
SPACE TO PROMOTE
FLOW TO VENTS
(BOTH STRUCTURES)
5071.2'
50
7
0
.
9
'
5070.4'
X
X X5070.9'
9' UTILITY
EASEMENT
9' UTILITY
EASEMENT
5072.5'
X
5072.5'
X
5071.5'
X
FF 5075.4'
See Cross Section
LCUASS Fig 7-11F
(sheet U105)
Amended for site
conditions: 8' Utility
Easement on one
side onlyCONRETE
ALLEY
S
S
PROPOSED GRADE SHALL
REMAIN AS EXISTING WITH
THE FOLLOWING EXCEPTION:
PROPOSED GRADE DIPS SLIGHTLY
AT APPROXIMATE STRUCTURE
CENTERLINE TO PROMOTE FLOW
THROUGH VENT
S
S
PROPOSED
5070.9'
PROPOSED
5070.4'
ONE (1) SUMP PUMP SHALL BE INSTALLED
NEAR THE DOWNSTREAM (EAST) SIDE OF
EACH STRUCTURE. SUMP PUMP SHALL
BE SUPERIOR PUMP 92372, 1
3 HP
THERMOPLASTIC SUBMERSIBLE PUMP
WITH VERTICAL FLOAT CONTROL
SWITCH OR APPROVED EQUAL.
VENT BOTTOM
ELEVATION
EAST: 5070.5'
WEST: 5071.0'
EAST: 5071.3'
WEST: 5071.8'
EAST: 5071.3'
WEST: 5071.8'
1. THESE ARE DUAL-FUNCTION VENTS, PROVIDING BOTH FLOOD PROTECTION
AND NATURAL AIR VENTILATION
4' SIDEWALK
TOTAL DISTURBED AREA:
LOT 1 = 26,163 SF = 0.60 ACRES
0+79 1+36 2+75 3+15 3+25
EXISTING
PROPOSED
4+60 4+80 5+00
SSA
SP
SF
1+
3
6
0+
7
9
0+
0
0
2+
7
5
3+
1
5
3+
8
5
1+
3
6
0+
7
9
0+
0
0
2+
7
5
3+
1
5
3+
8
5
1+
0
4
0
40
20
N
o
r
t
h
-
S
c
a
l
e
1
"
=
2
0
'
SEALLEGEND
DRAWN BY:
PS
PAS
CHECKED BY:
SHEET NUMBER:
SCALE:
DATE:
08/04/2025
SHEET INDEX:
JOB NUMBER:
RE
V
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S
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N
S
PR
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AS NOTED
PROPOSED CONTOUR
SILT FENCESF
FIG 6
GILMARTIN MINOR SUBDIVISION
VTC VEHICLE TRACKING
CONTROL
NOTES:
1. AT THE TIME PENNSYLVANIA STREET IS FULLY
CONSTRUCTED, ACCESS TO THIS PROPERTY MUST BE
TAKEN FROM PENNSYLVANIA STREET AND THE ACCESS
TAKEN FROM S TAFT HILL ROAD MUST BE CLOSED.
2. THE TOP OF FOUNDATIONS SHOWN ARE THE
MINIMUM ELEVATIONS REQUIRE FOR PROTECTION
FROM THE 100-YEAR STORM.
3. PROTECT EXISTING ACCESS FROM NEIGHBORING
PROPERTY ONTO S TAFT HILL ROAD.
4. NO TRACKING OF DEBRIS OR SEDIMENT IS ALLOWED
ON NEIGHBORING PROPERTYOR PUBLIC RIGHT OF WA. IF TRACKING
DOES OCCUR, CONTRACTOR SHALL CLEAN IT IMMEDIATELY.
RS ROCK SOCK
PROPOSED RUNOFF
FLOW DIRECTION
CWA CONCRETE WASHOUT
AREA
S SUMP PUMP: ONE
EACH STRUCTURE
EXISTING CONTOUR5073 SSA
SP
SF SEDIMENT STRAINING
STOCKPILE MANAGEMENT
STABILIZED STAGING AREA
FLODPLAIN AND FLOODWAY NOTES
1. THE ENTIRE PROPERTY IS CURRENTLY LOCATED IN THE CITY REGULATED 100-YEAR
CANAL IMPORTATION FLOOD FRINGE AND MUST COMPLY WITH THE SAFETY REGULATIONS
OF CHAPTER 10 OF MUNICIPAL CODE.
2. ANY CONSTRUCTION ACTIVITIES IN THE FLOOD FRINGE (E.G. STRUCTURES, SIDEWALK OR
CURB & GUTTER INSTALLATION/REPLACEMENT, UTILITY WORK, LANDSCAPING,, ETC.) MUST
BE PRECEDED BY AN APPROVED FLOODPLAIN USE PERMIT, THE APPROPRIATE PERMIT
APPLICATION FEES, AND APPROVED PLANS.
3. THE LOWEST FINISHED FLOOR ELEVATION OF A NEW RESIDENTIAL STRUCTURE, AND ALL DUCT
WORK, HEATING, VENTILATION, ELECTRICAL SYSTEMS, ETC. MUST BE ELEVATED 18 INCHES.
4. ANY NEW CRAWL SPACE MUST COMPLY WITH VENTING AND CRAWL SPACE REQUIEMENTS
FROM SECTIONS 10 39 AND 10 40 OF CITY MUNICIPAL CODE.
12
Appendix A
NCRS Custom Soil Resources Report
13
Appendix B
Hydrologic Calculations
Runoff Coefficient Calculations
Mile High Flood District
Peak Runoff Prediction by the Rational Method, Version 2.00
GILMARTIN SUBDIVISION
HISTORICAL AND PROPOSED RUNOFF COEFFICIENT CALCULATIONS
Total Drainage Area - 0.71 Acres =
Surface Cover/Use Defined %i1 Historical Proposed
2
Historical %i
Proposed %i
Area (sf)
Purpose:This workbook applies the Rational Method to estimate stormwater runoff and peak
flows from small urban catchments (typically less than 90 acres)
Function:1. To calculate the runoff coefficient, C for a catchment
2. To calculate the time of concentration, and then compare with the regional time
of concentration limit used for the Denver region. The smaller one is
recommended as the rainfall duration for use with the Rational Method.
3. To calculate the design rainfall intensity and resulting peak flow rate.
Content:The workbook consists of the following five sheets:
Intro Describes the purpose of each sheet in the workbook.
Rational Calcs Performs Rational Method calculations, Q = CIA
Weighted C Supporting tool to calculate area-weighted runoff coefficients from sub-areas.
Weighted Slope Supporting tool to calculate length-weighted slope from multiple flow reaches.
Weighted Tc Supporting tool to calculate reach-weighted time of concentration from multiple flow reaches.
Design Info Provides background information from the USDCM
Acknowledgements:Spreadsheet Development Team:
Derek N. Rapp, P.E.
Peak Stormwater Engineering, LLC
Holly Piza, P.E. and Ken MacKenzie, P.E.
Urban Drainage and Flood Control District
Comments?Direct all comments regarding this spreadsheet workbook to:UDFCD email
Revisions?Check for revised versions of this or any other workbook at:Downloads
PEAK RUNOFF PREDICTION BY THE RATIONAL METHOD
Denver, Colorado
Urban Drainage and Flood Control District
Version 2.00 released May 2017
, Intro ,
Designer:
Company:2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr
Date:1-hour rainfall depth, P1 (in) =0.82 1.20 1.40 2.00 2.40 2.86 3.00
Project:a b c
Location:Rainfall Intensity Equation Coefficients =28.50 10.00 0.786
2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr
Overland
Flow Length
Li (ft)
U/S
Elevation
(ft)
(Optional)
D/S
Elevation
(ft)
(Optional)
Overland
Flow Slope
Si (ft/ft)
Overland
Flow Time
ti (min)
Channelized
Flow Length
Lt (ft)
U/S
Elevation
(ft)
(Optional)
D/S
Elevation
(ft)
(Optional)
Channelized
Flow Slope
St (ft/ft)
NRCS
Conveyance
Factor K
Channelized
Flow
Velocity
Vt (ft/sec)
Channelized
Flow Time
tt (min)
Computed
tc (min)
Regional
tc (min)
Selected
tc (min)2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 500-yr
0.01 0.05 0.15 0.33 0.40 0.49 0.59 28.69 32.29 29.55 1.30 1.90 2.22 3.17 3.80 4.53 4.75 0.01 0.07 0.23 0.74 1.09 1.58 2.00
0.20 0.20 0.20 0.22 0.24 0.25 0.30 24.62 28.23 28.23 1.33 1.95 2.28 3.25 3.90 4.65 4.88 0.19 0.28 0.32 0.51 0.66 0.83 1.04
0.21 0.28 0.35 0.49 0.54 0.61 0.68 15.90 18.73 18.73 1.67 2.44 2.85 4.07 4.88 5.82 6.11 0.25 0.48 0.71 1.40 1.87 2.50 2.96
0.36 0.36 0.36 0.39 0.43 0.45 0.50 14.35 17.18 17.18 1.74 2.55 2.98 4.25 5.10 6.08 6.38 0.44 0.65 0.76 1.19 1.56 1.93 2.26
0.015300.00
Rainfall Intensity, I (in/hr)
1.22 3.61 29.55100.015265.00
Peak Flow, Q (cfs)
Calculation of Peak Runoff using Rational Method
Overland (Initial) Flow Time Channelized (Travel) Flow Time Time of ConcentrationRunoff Coefficient, C
Subcatchment
Name
Area
(ac)
NRCS
Hydrologic
Soil Group
Percent
Imperviousnes
s
EXISTING 0.71 C 2.0
Select UDFCD location for NOAA Atlas 14 Rainfall Depths from the pulldown list OR enter your own depths obtained from the NOAA website (click this link)
Cells of this color are for required user-input
Cells of this color are for optional override values
Cells of this color are for calculated results based on overrides
Paul Sorensen
Sorensen Engineering & Construction, Inc.
7/28/25
Taft Hill Subdivision: EXISTING & PROPOSED
341 South Taft Hill Road, Fort Collins, CO
Version 2.00 released May 2017
PROPOSED 0.71 C 29.8 150.00 0.015 415.00 0.015 20 2.45 2.82 25.22
I 𝑖𝑛/ℎ𝑟=a ∗P1
b +tc c
ti =0.395 1.1 −C5 Li
Si0.33
tt =Lt
60K St
=Lt
60Vt
Computed tc =ti +tt
Regional tc =26 −17i +Lt
60 14i +9 St
Selected tc =max tminimum ,min Computed tc ,Regional tc
tminimum=5 (urban)
tminimum=10 (non-urban)
Q 𝑐𝑓𝑠=CIA
Supplementary Design Information for UD-Rational Workbook
Urban Storm Drainage Criteria Manual (USDCM) Volume 1, Chapter 6 - Runoff (March 2017)
Version 2.00 released May 2017
Page 6
Supplementary Design Information for UD-Rational Workbook
Urban Storm Drainage Criteria Manual (USDCM) Volume 1, Chapter 6 - Runoff (March 2017)
Version 2.00 released May 2017
Page 7