HomeMy WebLinkAboutSupporting Documentation - Drainage - 03/14/20251901 Bear Court / Fort Collins, Colorado 80525
(970) 590-1579 / paul@secengineering.net
Sorensen Engineering & Construction, Inc.
Civil/Environmental Engineering
March 14, 2025
City of Fort Collins Stormwater
Development Review Engineer
222 Laporte Ave.
Fort Collins, CO 80524
Subject: Drainage Narrative
Gilmartin Subdivision
341 South Taft Hill Road
Larimer County Parcel No. 9709404005
Dear City of Fort Collins Stormwater,
Sorensen Engineering & Construction, Inc. (SEC) is pleased to submit this Drainage Narrative for
the proposed Gilmartin Subdivision. 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 present about ¼-mile to the east;
running through the City Park Nine golf course. 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) A portion of 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.
2
4) Any new crawl space must comply with venting and crawl space requirements from Sections
10 39 and 10 40 of City Municipal Code.”
Hydrological Soil Type
The NRCS Custom Soil Resource Report for the site is provided herein in Attachment 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 B.
Drainage Basin Description
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 within the basin. There
are no irrigation ditches, reservoirs, or other facilities that influence or are influenced by the
local drainage.
Project Description
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): .58 acre
(includes the Pennsylvania Street ROW)
Dedication of Pennsylvania Street: .09 acre
Outlot: .13 acre
Dedication of Public Right-of-Way for S. Taft Hill Road (Two-Lane Arteria): 12’
Dedication of Utility Easement along S. Taft Hill Road: 15’
The proposed project is to create one lot for a single-family home plus one additional dwelling
unit (ADU). The proposed home and ADU are on Lot One, 0.58 acre, equaling approximately
3.45 dwelling units per acre which exceeds the LMN required minimum density of 3.00 dwelling
units per acre. Existing Public Improvements in the project vicinity include Taft Hill Road to the
east: a two-Lane arterial that includes a four-foot attached walk, vertical curb, gutter and striped
bike lane. There is currently 30 feet of half-width as measured from the centerline. There are
no overhead electric lines. The nearest fire hydrant is at the northeast corner of Taft Hill Road
and Mulberry Street.
The proposed home and ADU would be under one ownership. Lot One and the Outlot would be
owned by a single entity. Therefore, there are no common areas and, therefore, no
Homeowners’ Association.
Right-of-Way for Pennsylvania Street is dedicated for 57 feet wide per the requirement for a
Residential Local Street in accordance with LCUASS 7-9F, and aligns with three previous
dedications (Farmstead, Stead and Cherokee Flying Heights Subdivisions). The proposed Outlot
lies west of Pennsylvania Street and is not intended to be served by utilities until the
3
construction of this public street. As an Outlot, it is not intended to be a parcel eligible for a
building permit at this time.
Existing and Proposed Drainage Patterns
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. Existing site conditions are presented on the
preliminary plat, provided herein as Figure 2. As shown, 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 21,375.65 square feet (0.49 acres)
(the disturbed area is limited to Lot 1, minus the proposed Pennsylvania Street ROW);
• Construction of the access driveway along the north side of the subject property. The
driveway will be constructed of gravel or recycled asphalt with subbase preparation as
detailed on the construction civil plan set. The driveway will encompass an 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; 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 3, 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 4, Grading, Drainage, and Erosion Control
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, are at least 18-inches above the 100-year Base Flood Elevation (BFE) as defined.
The basic drainage plan for this project is to maintain the existing drainage pattern : sheet flow
across the property from west to east.
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. 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.
4
Hydrologic Criteria
Rainfall runoff date used are as described in the Fort Collins Stormwater Criteria Manual,
Chapter 5, Hydrology Standards. 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 percent impervious areas shown in Table 1.
Table 1. Existing and Proposed % Impervious Calculations
Total Drainage Area = 0.71 Acres = 30,809 square feet (sf)
Surface Cover/Use Defined % i1 Area (sf)
Historical/Existing Proposed
Historical Flow Analysis 0.02 30,809 0
Parking and drive area (gravel) 0.80 0 4,900
Rooftop 0.95 0 3,100
Undisturbed or Decompacted soil 0.05 0 15,000
Landscape/grasses2 0.20 0 7,809
Historical % i 2.00
Proposed % i 29.8
1. Reference: MHFD Stormwater Criteria Manual, Volume 3, Chapter 6, Table 6-3.
2. Assume landscape area around buildings; overall impervious areas to be more precisely
defined and presented in an exhibit prior to building permit application.
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 0.98 inches and for the 100-year event is 3.67
inches. The Rational Method is used to analyze the existing and fully developed conditions. The
MHFD hydrologic runoff calculations are presented in Attachment B.
The hydrological analysis was computed using the Peak Runoff Prediction by the Rational
Method spreadsheet developed by the Urban Drainage and Flood Control District (Mile High
Flood District) as described in the Urban Storm Drainage Criteria Manual (USDCM), Volume 1.
The Hydrologic Calculations are presented in Appendix B, the results of which are summarized in
Table 2.
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 0.01 0.49 0.21 0.61 0.01 2.03 0.30 3.21
5
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 homes 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.
Disturbed areas of the project that will not be compacted gravel road surface will be
revegetated with native grasses.
Conclusions
As is demonstrated above, the proposed site improvements will increase the estimated peak
runoff flowrate from 0.01 to 0.30 cfs for the 2-year event, and from 2.03 to 3.21 cfs for the 100-
year event. This Drainage Narrative was prepared in accordance with the Fort Collins
Stormwater Criteria Manual, and SEC believes that the increase in peak runoff flowrates will not
impact adjacent properties.
“I hereby attest that this report for the preliminary &/or 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.”
Please do not hesitate to contact me directly if you have any questions or concerns regarding
this Drainage Narrative, or if you require any additional information.
Respectfully,
Paul C. Sorensen, PE
Attachments
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
GILMARTIN 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
N 89°35'30" W ANDERSON CECELIA M 15.00' REC. NO. 20100078322
--� S 89°35'30" E 444.1 O' •
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OUTLOT
5,482.02 Sq.
Ft. 0.13 Acres
,-._ 57.00' S 89'35'30" E 365.1 O'
PENNSYLVANI A z STREET 00 57' RIGHT-OF-WAY 0 0 DEDICATED BY THIS PLAT N 3954.54 Sq. Fl. LOT 1
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---57.00' N 89'35'30" W 365.05' \--N 89'35'30" W 200.00' ----N 89 °35'30" W 444.05'
N 89 °35'30" W
15.00'
THE PENTECOSTALS OF FORT COLLINS
REC. NO. 20060085357
20 0 ,n
( IN US FEET)
1 inch=
CERTIFICATE OF DEDICATION:
The Owner does hereby dedicate and convey to the City of Fort Collins, Colorado (hereafter "City"), for public use, forever, a permanent right-of-way for street purposes and the "Easements" as laid out and
designated on this Plat; provided, however, that (1) acceptance by the City of this dedication of Easements does not impose upon the City a duty to maintain the Easements so dedicated, and (2) acceptance by
the City of this dedication of streets does not impose upon the City a duty to maintain streets so dedicated until such time as the provisions of the Maintenance Guarantee have been fully satisfied. The streets
dedicated on this Plat are the fee property of the City as provided in Section 31-23-107 C.R.S. The City's rights under the Easements include the right to install, operate, access, maintain, repair, reconstruct,
remove and replace within the Easements public improvements consistent with the intended purpose of the Easements; the right to install, maintain and use gates in any fences that cross the Easements; the right
to mark the location of the Easements with suitable markers; and the right to permit other public utilities to exercise these same rights. Owner reserves the right to use the Easements for purposes that do not
interfere with the full enjoyment of the rights hereby granted. The City is responsible for maintenance of its own improvements and for repairing any damage caused by its activities in the Easements, but by
acceptance of this dedication, the City does not accept the duty of maintenance of the Easements, or of improvements in the Easements that are not owned by the City. Owner will maintain the surface of the
Easements in a sanitary condition in compliance with any applicable weed, nuisance or other legal requirements.
20ft.
Except as expressly permitted in an approved plan of development or other written agreement with the City, Owner will not install on the Easements, or permit the installation on the Easements, of any building,
structure, improvement, fence, retaining wall, sidewalk, tree or other landscaping (other than usual and customary grasses and other ground cover). In the event such obstacles are installed in the Easements, the
City has the right to require the Owner to remove such obstacles from the Easements. If Owner does not remove such obstacles, the City may remove such obstacles without any liability or obligation for repair
and replacement thereof, and charge the Owner the City's costs for such removal. If the City chooses not to remove the obstacles, the City will not be liable for any damage to the obstacles or any other property
to which they are attached.
The rights granted to the City by this Plat inure to the benefit of the City's agents, licensees, permittees and assigns.
MAINTENANCE GUARANTEE:
The Owner hereby warrants and guarantees to the City, for a period of two (2) years from the date of completion and first acceptance by the City of the improvements warranted hereunder, the full and complete
maintenance and repair of the improvements to be constructed in connection with the Development which is the subject of this Plat. This warranty and guarantee is made in accordance with the City Land Use
Code and/or the Transitional Land Use Regulations, as applicable. This guarantee applies to the streets and all other appurtenant structures and amenities lying within the rights-of-way, Easements and other
public properties, including, without limitation, all curbing, sidewalks, bike paths, drainage pipes, culverts, catch basins, drainage ditches and landscaping. Any maintenance and/or repair required on utilities shall
be coordinated with the owning utility company or department.
The Owner shall maintain said improvements in a manner that will assure compliance on a consistent basis with all construction standards, safety requirements and environmental protection requirements of the
City. The Owner shall also correct and repair, or cause to be corrected and repaired, all damages to said improvements resulting from development-related or building-related activities. In the event the Owner
fails to correct any damages within thirty (30) days after written notice thereof, then said damages may be corrected by the City and all costs and charges billed to and paid by the Owner. The City shall also have
any other remedies available to it as authorized by law. Any damages which occurred prior to the end of said two (2) year period and which are unrepaired at the termination of said period shall remain the
responsibility of the Owner.
REPAIR GUARANTEE:
In consideration of the approval of this final Plat and other valuable consideration, the Owner does hereby agree to hold the City harmless for a five (5) year period, commencing upon the date of completion and
first acceptance by the City of the improvements to be constructed in connection with the development which is the subject of this Plat, from any and all claims, damages, or demands arising on account of the
design and construction of public improvements of the property shown herein; and the Owner furthermore commits to make necessary repairs to said public improvements, to include, without limitation, the roads,
streets, fills, embankments, ditches, cross pans, sub-drains, culverts, walls and bridges within the right-of way, Easements and other public properties, resulting from failures caused by design and/or construction
defects. This agreement to hold the City harmless includes defects in materials and workmanship, as well as defects caused by or consisting of settling trenches, fills or excavations.
Further, the Owner warrants that he/she owns fee simple title to the property shown hereon and agrees that the City shall not be liable to the Owner or his/her successors in interest during the warranty period, for
any claim of damages resulting from negligence in exercising engineering techniques and due caution in the construction of cross drains, drives, structures or buildings, the changing of courses of streams and
rivers, flooding from natural creeks and rivers, and any other matter whatsoever on private property. Any and all monetary liability occurring under this paragraph shall be the liability of the Owner. I further warrant
that I have the right to convey said land according to this Plat.
NOTICE OF OTHER DOCUMENTS:
All persons take notice that the Owner has executed certain documents pertaining to this Development which create certain rights and obligations of the Development, the Owner and/or subsequent Owners of all
or portions of the Development site, many of which obligations constitute promises and covenants that, along with the obligations under this Plat, run with the land. The said documents may also be amended from
time to time and may include, without limitation, the Development Agreement, Site And Landscape Covenants, Final Site Plan, Final Landscape Plan, and Architectural Elevations, which documents are on file in
the office of the clerk of the City and should be closely examined by all persons interested in purchasing any portion of the Development site.
N 89"35'30" W 256.0'
LLOYD THOMAS C/CAROL YN F
REC. NO. 20060032629
40 6.0
STATEMENT OF OWNERSHIP AND SUBDIVISION
Know all persons by these presents, that the undersigned, being owners of the following described land:
A tract of land located in Section 9, Township 7 North, Range 69 West of the 6th Principal Meridian
more particularly describes as:
Beginning at the Southeast comer of Section 9, Township 7 North, Range 69 West North 249.37 feet,
Thence S 89°35'30" E 42.00 feet to the True Point of Beginning, thence N 89'35'30" W 444.05 feet,
thence N 00 °02'15" W 69.38 feet, thence S 89°35'30" E 444.10 feet to the west line of the South Taft Hill
Right--0f-Way, thence South 69.38 feet to the True Point of Beginning.
which above described tract contains 30,809.08 Sq. Ft. being 0.71 acres, more or less have caused the
above described land to be surveyed and subdivided into lots, tracts and streets as shown on this plat to
be known as the Gilmartin Subdivision, subject to all easements and rights-of-way now of record or
existing or indicated on this plat. The rights and obligations of the Plat shall run with the land.
ATTORNEY'S CERTIFICATION:
I hereby certify that this Subdivision Plat has been duly executed as required pursuant to Section
2.2.3(C)(3)(a) through (e) inclusive of the Land Use Code of the City of Fort Collins and that all persons
signing this Subdivision Plat on behalf of a corporation or other entity are duly authorized signatories
under the laws of the State of Colorado. This Certification is based upon the records of the Clerk and
Recorder of Larimer County, Colorado as of the date of execution of the plat and other information
discovered by me through reasonable inquiry and is limited as authorized by Section 2.2.3(C)(3)(f) of the
Land Use Code.
Attorney:
Address:
Registration No.:
SURVEYOR'S STATEMENT:
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FOUND 3/4" REBAR AND 2.5"
ALUMINUM CAP P.L.S. 29407
STAMPED PER MONUMENT
RECORD FILED AUGUST 31,
2008
LEGEND S 89 °35'30" E
30.00'
N 89 °35'30"
W 30.00'
30.0'
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14823 STAMPED 1.0 FT W.C.
la\= SET 5/8" REBAR AND GREEN � PLASTIC CAP P.L.S. 38547
� FOUND SECTION MONUMENT V-AS DESCRIBED
LOT LINE
EASEMENT LINE
FOUND 3/4" REBAR AND 2.5"
ALUMINUM CAP P.L.S. 34174
STAMPED PER MONUMENT
RECORD FILED FEBRUARY
28,2013
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VICINITY MAP
1" = 600'
I, Steven D Wood, a Colorado Registered Professional Land Surveyor, do hereby state that this Subdivision Plat
was ,----; NOTICE1------------------------------,
ALL RESPONSIBILITIES AND COSTS OF OPERATION, MAINTENANCE AND RECONSTRUCTION
OF THE PRIVATE STREETS AND/OR DRIVES LOCATED ON THE PRIVATE PROPERTY THAT IS
THE SUBJECT OF THIS PLAT SHALL BE BORNE BY THE OWNERS OF SAID PROPERTY,
prepared from an actual survey under my personal supervision, that the monumentation as indicated hereon were
found or set as shown, and that the forgoing plat is an accurate representation thereof, all this to the best of my
knowledge, information and belief.
STEVEN D WOOD
Colorado Registered Professional Land Surveyor#
APPROVED AS TO FORM, CITY ENGINEER:
By the City Engineer of the City of Fort Collins, Colorado on this day,
City Engineer
PLANNING APPROVAL:
By the Director of Planning the City of Fort Collins, Colorado this day,
Director of Plannin
of
of
A.O.,
A.O.,
EITHER INDIVIDUALLY, OR COLLECTIVELY, THROUGH A PROPERTY OWNERS'
ASSOCIATION, IF APPLICABLE. THE CITY OF FORT COLLINS SHALL HAVE NO OBLIGATION
OF OPERATION, MAINTENANCE OR RECONSTRUCTION OF SUCH PRIVATE STREETS
AND/OR DRIVES NOR SHALL THE CITY HAVE ANY OBLIGATION TO ACCEPT SUCH STREETS
AND/OR DRIVES AS PUBLIC STREETS OR DRIVES.
1.Basis of bearings assuming the East line of the Southeast Quarter of Section 9 as bearing North 00°00'00"
West, from a 2.5" aluminum cap at the Southeast corner of Section 9 stamped P.L.S. 34174 2012, to a 2½"
aluminum cap stamped P.L.S. 29407 2008 at the East Quarter of Section 9 with all bearings contained
herein relative thereto:
2.Lineal units used in the preparation of this plat are based upon the U.S. Survey Foot.
3.Total area in square feet of easements dedicated to the public or the city by this plat. = 3954.54 Square
Feet. Total area in square feet of easements dedicated to the public or city that is being vacated by this plat.
= 0.00 Square Feet.
4.At the request of the client no Tille Commitment was obtained for this plat.
5.Project Bemchmark Fort Collins 25-97 located at the Southwest corner Mulberry St. and Taft Hill Rd., on
a concrete Traffic Signal Base NAVD88 = 5071.25 (NGVD = 5068.08)
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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 3
SUBJECT PROPERTY
FIGURE 3
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
U
T
H
T
A
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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'
5071.6'5070.4'5071.0'
5071.9'
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
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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 C105)
NEW ACCESS TO
S TAFT HILL ROAD
(SEE DETAIL
SHEET CXXX)
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
12.00'
STRUCTURE MODEL #
FLOOR
COVERAGE
(SQ FT)
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'
0
40
20
N
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t
h
-
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c
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e
1
"
=
2
0
'
SEAL
LEGEND
DRAWN BY:
PS
PAS
CHECKED BY:
SHEET NUMBER:
SCALE:
DATE:
03/06/2025
SHEET INDEX:
JOB NUMBER:
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AS NOTED
NEW CONTOUR
SILT FENCESF
FIGURE 4
ELEVATION TABLE
UNIT EXISTING GRADE BFE FF
(UPSTREAM SIDE)
PRIMARY
RES 5071 5072.9 5075.4
ADU 5072.4 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
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.
RS ROCK SOCK
PROPOSED RUNOFF
FLOW DIRECTION
CWA CONCRETE WASHOUT
AREA
6
Attachment A
NRCS Custom Soil Resources Report
7
Attachment B
Mile High Flood District
Hydrologic Calculations
GILMARTIN SUBDIVISION
HISTORICAL AND PROPOSED % IMPERVIOUS CALCULATIONS
Total Drainage Area - 0.71 Acres =30,809 sf
Surface Cover/Use Defined %i1 Historical Proposed
Historical Flow Analsis 0.02 30809 0
Driveway (gravel_0.55 0 0
Parking lot and drive area
(gravel)0.80 0 4,900
Rooftop 0.95 0 3,100
Undisturbed or Decompacted
soil (native grasses and open
space)0.05 0 15,000
Landscape/grasses2 0.20 0 7,809
Historical %i 0.020
Proposed %i 0.298
1. Reference: Mile High Flood District Stormwater Criteria Manual; Volume 3, Chapter 6, Table 6-3.
2. Assume landscaped area around buildings
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) =0.98 1.09 1.33 1.69 1.99 3.67 4.50
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 1.55 1.73 2.11 2.68 3.15 5.81 7.12 0.01 0.06 0.22 0.63 0.90 2.03 3.01
0.21 0.28 0.35 0.49 0.54 0.61 0.68 15.90 18.73 18.73 1.99 2.22 2.71 3.44 4.05 7.47 9.16 0.30 0.44 0.67 1.19 1.55 3.21 4.4325.22
Cells of this color are for calculated results based on overrides
Paul Sorensen
Sorensen Engineering & Construction, Inc.
3/14/25
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
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 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