HomeMy WebLinkAboutReports - Drainage - 03/14/2025 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 %, 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%-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.
1901 Bear Court/Fort Collins,Colorado 80525
(970)590-1579/Paul@secengineering.net
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
2
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.
3
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 % it 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/grasses' 0.20 0 7,809
Historical %i 2.00
Proposed %i 1 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
4
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
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CONTOUR INTERVAL: 10 FEET
LARIMER COUNTY PARCEL NUMBER:
9709404005 PROJECT DATE: FEBRUARY 2022
PROJECT
SORENSEN ENGINEERING&CONSTRUCTION,INC 341 S Taft Hill Rd
CIVIL/ENVIRONMENTAL ENGINEERING Fort Collins, CO FIGURE 1
1901 BEAR COURT FORT COLLINS,CO 80525 VICINITY MAP
:z PHONE:970 590-1579 paul@secengineering.net
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
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FORT COLLINS, COUNTY OF LARIMER, STATE OF COLORADO
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CERTIFICATE OF DEDICATION: STATEMENT OF OWNERSHIP AND SUBDIVISION - o
Know all persons by these presents, that the undersigned, being owners of the following described land:
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 A tract of land located in Section 9, Township 7 North, Range 69 West of the 6th Principal Meridian n ®
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 more particularly describes as: LAP ORT AVE �,R�m
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,
Beginning at the Southeast corner of Section 9, Township 7 North Range 69 West North 249.37 feet o ® ®®®®®®® �'
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 o ' ' IY ® 0
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 Thence S 89 35 30 E 42.00 feet to the True 9035'of,Beginning, thence N 89 west
30 W 444.05 feet, J
thence N 00 0215 W 69.38 feet thence S 89 35 30 E 444.10 feet to the west line of the South Taft Hill r L L�
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 ' _ �®� co
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acceptance of this dedication, y p y p y y Right-of-Way, thence South 69.38 feet to the True Point of Beginning. 9 PR JE T ITE ❑
the City does not accept the duty of maintenance of the Easements, or of improvements in the Easements that are not owned b the City. Owner will maintain the surface of the 3
i N Easements in a sanitary condition in compliance with any applicable weed, nuisance or other legal requirements. Q '�, w 25 o W
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 Cn CITY RKH Q❑ C) U X U)Q �° w
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, a°
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 be known as the Gilmartin Subdivision, subject to all easements and rights-of-way now of record or
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 existing or indicated on this plat. The rights and obligations of the Plat shall run with the land. L CRY --�
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 w UJ co
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to which they are attached. ATTORNEYS CERTIFICATION: `-'
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The rights granted to the City by this Plat inure to the benefit of the City's agents, licensees, permittees and assigns. 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 personsM41"Ll
MAINTENANCE GUARANTEE: 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 Q
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 Recorder of Larimer County, Colorado as of the date of execution of the plat and other information
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 discovered by me through reasonable inquiry and is limited as authorized by Section 2.2.3(C)(3)(f) of the Q Q
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 Land Use Code. VICINITY MAP O
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 Attorney: 1" = 600' n�
be coordinated with the owning utility company or department. Z LL
Address: _ J
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 J J
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 Registration No.: = O
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. SURVEYOR'S STATEMENT: LL
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I, Steven D Wood, a Colorado Registered Professional Land Surveyor, do hereby state that this Subdivision Plat �� NO�TI EC J Q U)
REPAIR GUARANTEE: was 1 -I�
prepared from an actual survey under my personal supervision, that the monumentation as indicated hereon were ALL RESPONSIBILITIES AND COSTS OF OPERATION, MAINTENANCE AND RECONSTRUCTION 0 z
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 found or set as shown, and that the forgoing plat is an accurate representation thereof, all this to the best of my OF THE PRIVATE STREETS AND/OR DRIVES LOCATED ON THE PRIVATE PROPERTY THAT IS J
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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 knowledge, information and belief. THE SUBJECT OF THIS PLAT SHALL BE BORNE BY THE OWNERS OF SAID PROPERTY, = J
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, EITHER INDIVIDUALLY, OR COLLECTIVELY, THROUGH A PROPERTY OWNERS' 0
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 ASSOCIATION, IF APPLICABLE. THE CITY OF FORT COLLINS SHALL HAVE NO OBLIGATION >
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. STEVEN D WOOD OF OPERATION, MAINTENANCE OR RECONSTRUCTION OF SUCH PRIVATE STREETS W O V
AND/OR DRIVES NOR SHALL THE CITY HAVE ANY OBLIGATION TO ACCEPT SUCH STREETS U
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 Colorado Registered Professional Land Surveyor# AND/OR DRIVES AS PUBLIC STREETS OR DRIVES.
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 T11 O
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 it
that I have the right to convey said land according to this Plat. APPROVED AS TO FORM, CITY ENGINEER: 1. Basis of bearings assuming the East line of the Southeast Quarter of Section 9 as bearing North 00000'00" Co LL
West, from a 2.5" aluminum cap at the Southeast corner of Section 9 stamped P.L.S. 34174 2012, to a 2Y2"
NOTICE OF OTHER DOCUMENTS: By the City Engineer of the City of Fort Collins, Colorado on this day, of A.D., aluminum cap stamped P.L.S. 29407 2008 at the East Quarter of Section 9 with all bearings contained
herein relative thereto:
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 City Engineer 2. Lineal units used in the preparation of this plat are based upon the U.S. Survey Foot.
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 3. Total area in square feet of easements dedicated to the public or the city by this plat. = 3954.54 Square
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 Feet. Total area in square feet of easements dedicated to the public or city that is being vacated by this plat. Sheet
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. PLANNING APPROVAL: = 0.00 Square Feet. FIGURE 2
4. At the request of the client no Title Commitment was obtained for this plat. PLAT
By the Director of Planning the City of Fort Collins, Colorado this day, of A.D., 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) 1 Of 1 Sheets
Director of Planning
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FIGURE 3 City Flood Risk Map
Hiah Risk
This information is based on the Federal Emergency Management
City Floodway-Area of 100-year floodplain with greatest depths and Agency(FEMA)Flood Insurance Rate Map(FIRM)and the City of
fastest velocities. Fort Collins Master Drainageway Plans. This letter does not imply
that the referenced property will or will not be free from flooding or
City Flood Fringe-May Include: damage. A property not in the Special Flood Hazard Area or in a
-Areas of FEMA 100-year floodplain(FEMA Zones A,AE,AO,and AH) City Designated Floodplain may be damaged by a flood greater
-Areas of City 100-year floodplain including ponding areas and sheet than that predicted on the map or from a local drainage problem
flow areas with average depths of 1-3 feet. 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
There is a 1%annual chance that these areas will be flooded. that results from reliance on this information.
Moderate Risk
All floodplain boundaries
May include: are approximate.
Areas of FEMA 500-year floodplain(FEMA Zone X-shaded). City of I
-Areas of FEMA or City 100-year floodplain(sheet flow)with Fort Collins
average depths of less than 1 foot. N FIGURE 3
Areas protected by levees from the 100-year flood. GIs
Low Risk w E 0 20 40 80 120 160
Areas outside of FEMA and City mapped 100-year and 500-year Feet
floodplains.Local drainage problems may still exist. S Printed:02/02/2022
SEAL
LEGEND
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ENVELOPE _
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Q o 26162.75 Sq. Ft. I Recycled Asphalt/ I / o I LL. c
U o 0.60 Acres Road Base I 5070.9' PRINCIPAL 5070.4' so. o' m Q
N ADU - x X TBACK
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PATTERN
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adjacent house
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SPACE TO PROMOTE (3 EACH END,
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FLOW TO VENTS BOTH STRUCTURES) `� RS O o Z
NOTES: (BOTH STRUCTURES)
(TYP) M M M M Q O
1. AT THE TIME PENNSYLVANIA STREET IS FULLY o O
CONSTRUCTED, ACCESS TO THIS PROPERTY MUST BE
� o W
TAKEN FROM PENNSYLVANIA STREET AND THE ACCESS z Q
TAKEN FROM S TAFT HILL ROAD MUST BE CLOSED. a Z
2. THE TOP OF FOUNDATIONS SHOWN ARE THE x o
MINIMUM ELEVATIONS REQUIRE FOR PROTECTION o
FROM THE 100-YEAR STORM. ADU - O w
PRINCIPAL
DEDATCHED � Z
BFE 5075.94 RESIDENCE M
5072
EXISTING 100-YR BASE FF 5076 Q
5078' GRADE FLOOD ELEVATION FF 5075.4 Z
n Q
5076' — _ — DUCT WORK w P4
J
z — MIN DUCT 5075 DUCT WORK 6" VERTICAL
O 5075' _ — — BFE 5071 .59'� 5074 — — — _ �— BFE 5073.5 MIN DUCT 5074.4 BFE 5070.72 CURB w
5074 — — — _ _ _ �— BFE 5072.9 _
w 5072' 5073' _ —
4%, 00*/
w —
5070' 5071 .9' 75071 .6 -
5071 .0'� 5071'
SPREAD FOOTING W/ FOUNDATION WALL 5070.4'
5068' (TOP OFIFOOTER MIN 30" BELOW EXIST GRADEI
I
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
SECTION A - A' o 0
co ELEVATION TABLE
SCALE: HORIZONTAL 1" = 20' FLOOD VENTS (1 SQ INCH VENT/1 SQ FOOT BUILDING): RECOMMEND FLOOD VENTS , , UJ
Cn
VERTICAL 1 = 4 BY SMART VENT OR EQUAL - ALL 2X2 MULTI-VENT CONFIGURATION o
UNIT EXISTING GRADE BFE FF (5:1 EXAGGERATION)
(UPSTREAM SIDE)
FLOOR AIR Q
STRUCTURE MODEL # COVERAGE VENTILATION # VENTS
PRIMARY EACH END
(SQ FT) (SQ IN) rzb1=11
RES 5071 5072.9 5075.4
PRINCIPAL 1540-
RESIDENCE 150202 800 204 3
ADU 5072.4 5073.5 5076 1540- DRAWN BY: SCALE:
510 200 51 1 PAS AS NOTED
EXISTING GRADE @ UPSTREAM SIDE OF UNIT ADU CHECKED BY: DATE:
1540- PS 03/06/2025
ESTIMATED ELEVATIONS - INTERPOLATED @UNIT N-S CL 511 400 102 2 SHEET NUMBER:
FIGURE 4
XS#.1477 AND XS#.1195 - BASE FLOOD ELEVATIONS CROSS-SECTION
SHEET INDEX:
JOB NUMBER:
Attachment A
NRCS Custom Soil Resources Report
6
USDA United States A product of the National Custom Soil Resource
Department of Cooperative Soil Survey,
Agriculture a joint effort of the United Report for
N
States Department of
RCS Agriculture and other Larimer County
Federal agencies, State
Natural agencies including the
Resources Agricultural Experiment Area, Colorado
Conservation Stations, and local
Service participants
341 SOUTH TAFT HILL RD
h
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March 16, 2023
Preface
Soil surveys contain information that affects land use planning in survey areas.
They highlight soil limitations that affect various land uses and provide information
about the properties of the soils in the survey areas. Soil surveys are designed for
many different users, including farmers, ranchers,foresters, agronomists, urban
planners, community officials, engineers, developers, builders, and home buyers.
Also, conservationists, teachers, students, and specialists in recreation, waste
disposal, and pollution control can use the surveys to help them understand,
protect, or enhance the environment.
Various land use regulations of Federal, State, and local governments may impose
special restrictions on land use or land treatment. Soil surveys identify soil
properties that are used in making various land use or land treatment decisions.
The information is intended to help the land users identify and reduce the effects of
soil limitations on various land uses. The landowner or user is responsible for
identifying and complying with existing laws and regulations.
Although soil survey information can be used for general farm, local, and wider area
planning, onsite investigation is needed to supplement this information in some
cases. Examples include soil quality assessments (http://www.nres.usda.gov/wps/
portal/nres/main/soils/health/)and certain conservation and engineering
applications. For more detailed information, contact your local USDA Service Center
(https://offices.sc.egov.usda.gov/locator/app?agency=nres)or your NRCS State Soil
Scientist(http://www.nres.usda.gov/wps/portal/nres/detail/soils/contactus/?
cid=nres142p2_053951).
Great differences in soil properties can occur within short distances. Some soils are
seasonally wet or subject to flooding. Some are too unstable to be used as a
foundation for buildings or roads. Clayey or wet soils are poorly suited to use as
septic tank absorption fields.A high water table makes a soil poorly suited to
basements or underground installations.
The National Cooperative Soil Survey is a joint effort of the United States
Department of Agriculture and other Federal agencies, State agencies including the
Agricultural Experiment Stations, and local agencies. The Natural Resources
Conservation Service (NRCS) has leadership for the Federal part of the National
Cooperative Soil Survey.
Information about soils is updated periodically. Updated information is available
through the NRCS Web Soil Survey, the site for official soil survey information.
The U.S. Department of Agriculture (USDA) prohibits discrimination in all its
programs and activities on the basis of race, color, national origin, age, disability,
and where applicable, sex, marital status, familial status, parental status, religion,
sexual orientation, genetic information, political beliefs, reprisal, or because all or a
part of an individual's income is derived from any public assistance program. (Not
all prohibited bases apply to all programs.) Persons with disabilities who require
2
alternative means for communication of program information (Braille, large print,
audiotape, etc.)should contact USDA's TARGET Center at(202)720-2600 (voice
and TDD). To file a complaint of discrimination, write to USDA, Director, Office of
Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410 or
call (800)795-3272 (voice)or(202) 720-6382 (TDD). USDA is an equal opportunity
provider and employer.
3
Contents
Preface....................................................................................................................2
HowSoil Surveys Are Made..................................................................................5
SoilMap.................................................................................................................. 8
SoilMap................................................................................................................9
Legend................................................................................................................10
MapUnit Legend................................................................................................ 11
MapUnit Descriptions.........................................................................................11
Larimer County Area, Colorado...................................................................... 13
3—Altvan-Satanta loams, 0 to 3 percent slopes......................................... 13
74—Nunn clay loam, 1 to 3 percent slopes.................................................15
References............................................................................................................17
4
How Soil Surveys Are Made
Soil surveys are made to provide information about the soils and miscellaneous
areas in a specific area. They include a description of the soils and miscellaneous
areas and their location on the landscape and tables that show soil properties and
limitations affecting various uses. Soil scientists observed the steepness, length,
and shape of the slopes; the general pattern of drainage; the kinds of crops and
native plants; and the kinds of bedrock. They observed and described many soil
profiles.A soil profile is the sequence of natural layers, or horizons, in a soil.The
profile extends from the surface down into the unconsolidated material in which the
soil formed or from the surface down to bedrock. The unconsolidated material is
devoid of roots and other living organisms and has not been changed by other
biological activity.
Currently, soils are mapped according to the boundaries of major land resource
areas (MLRAs). MLRAs are geographically associated land resource units that
share common characteristics related to physiography, geology, climate, water
resources, soils, biological resources, and land uses (USDA, 2006). Soil survey
areas typically consist of parts of one or more MLRA.
The soils and miscellaneous areas in a survey area occur in an orderly pattern that
is related to the geology, landforms, relief, climate, and natural vegetation of the
area. Each kind of soil and miscellaneous area is associated with a particular kind
of landform or with a segment of the landform. By observing the soils and
miscellaneous areas in the survey area and relating their position to specific
segments of the landform, a soil scientist develops a concept, or model, of how they
were formed. Thus, during mapping, this model enables the soil scientist to predict
with a considerable degree of accuracy the kind of soil or miscellaneous area at a
specific location on the landscape.
Commonly, individual soils on the landscape merge into one another as their
characteristics gradually change. To construct an accurate soil map, however, soil
scientists must determine the boundaries between the soils. They can observe only
a limited number of soil profiles. Nevertheless, these observations, supplemented
by an understanding of the soil-vegetation-landscape relationship, are sufficient to
verify predictions of the kinds of soil in an area and to determine the boundaries.
Soil scientists recorded the characteristics of the soil profiles that they studied. They
noted soil color, texture, size and shape of soil aggregates, kind and amount of rock
fragments, distribution of plant roots, reaction, and other features that enable them
to identify soils.After describing the soils in the survey area and determining their
properties, the soil scientists assigned the soils to taxonomic classes (units).
Taxonomic classes are concepts. Each taxonomic class has a set of soil
characteristics with precisely defined limits. The classes are used as a basis for
comparison to classify soils systematically. Soil taxonomy, the system of taxonomic
classification used in the United States, is based mainly on the kind and character
of soil properties and the arrangement of horizons within the profile. After the soil
5
Custom Soil Resource Report
scientists classified and named the soils in the survey area, they compared the
individual soils with similar soils in the same taxonomic class in other areas so that
they could confirm data and assemble additional data based on experience and
research.
The objective of soil mapping is not to delineate pure map unit components; the
objective is to separate the landscape into landforms or landform segments that
have similar use and management requirements. Each map unit is defined by a
unique combination of soil components and/or miscellaneous areas in predictable
proportions. Some components may be highly contrasting to the other components
of the map unit. The presence of minor components in a map unit in no way
diminishes the usefulness or accuracy of the data. The delineation of such
landforms and landform segments on the map provides sufficient information for the
development of resource plans. If intensive use of small areas is planned, onsite
investigation is needed to define and locate the soils and miscellaneous areas.
Soil scientists make many field observations in the process of producing a soil map.
The frequency of observation is dependent upon several factors, including scale of
mapping, intensity of mapping, design of map units, complexity of the landscape,
and experience of the soil scientist. Observations are made to test and refine the
soil-landscape model and predictions and to verify the classification of the soils at
specific locations. Once the soil-landscape model is refined, a significantly smaller
number of measurements of individual soil properties are made and recorded.
These measurements may include field measurements, such as those for color,
depth to bedrock, and texture, and laboratory measurements, such as those for
content of sand, silt, clay, salt, and other components. Properties of each soil
typically vary from one point to another across the landscape.
Observations for map unit components are aggregated to develop ranges of
characteristics for the components. The aggregated values are presented. Direct
measurements do not exist for every property presented for every map unit
component. Values for some properties are estimated from combinations of other
properties.
While a soil survey is in progress, samples of some of the soils in the area generally
are collected for laboratory analyses and for engineering tests. Soil scientists
interpret the data from these analyses and tests as well as the field-observed
characteristics and the soil properties to determine the expected behavior of the
soils under different uses. Interpretations for all of the soils are field tested through
observation of the soils in different uses and under different levels of management.
Some interpretations are modified to fit local conditions, and some new
interpretations are developed to meet local needs. Data are assembled from other
sources, such as research information, production records, and field experience of
specialists. For example, data on crop yields under defined levels of management
are assembled from farm records and from field or plot experiments on the same
kinds of soil.
Predictions about soil behavior are based not only on soil properties but also on
such variables as climate and biological activity. Soil conditions are predictable over
long periods of time, but they are not predictable from year to year. For example,
soil scientists can predict with a fairly high degree of accuracy that a given soil will
have a high water table within certain depths in most years, but they cannot predict
that a high water table will always be at a specific level in the soil on a specific date.
After soil scientists located and identified the significant natural bodies of soil in the
survey area, they drew the boundaries of these bodies on aerial photographs and
6
Custom Soil Resource Report
identified each as a specific map unit.Aerial photographs show trees, buildings,
fields, roads, and rivers, all of which help in locating boundaries accurately.
7
Soil Map
The soil map section includes the soil map for the defined area of interest, a list of
soil map units on the map and extent of each map unit, and cartographic symbols
displayed on the map.Also presented are various metadata about data used to
produce the map, and a description of each soil map unit.
8
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Custom Soil Resource Report
Map Unit Legend
Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI
3 Altvan-Satanta loams,0 to 3 0.1 16.8%
percent slopes
74 Nunn clay loam, 1 to 3 percent 0.7 83.2%
slopes
Totals for Area of Interest 0.8 100.0%
Map Unit Descriptions
The map units delineated on the detailed soil maps in a soil survey represent the
soils or miscellaneous areas in the survey area. The map unit descriptions, along
with the maps, can be used to determine the composition and properties of a unit.
A map unit delineation on a soil map represents an area dominated by one or more
major kinds of soil or miscellaneous areas.A map unit is identified and named
according to the taxonomic classification of the dominant soils. Within a taxonomic
class there are precisely defined limits for the properties of the soils. On the
landscape, however, the soils are natural phenomena, and they have the
characteristic variability of all natural phenomena. Thus, the range of some
observed properties may extend beyond the limits defined for a taxonomic class.
Areas of soils of a single taxonomic class rarely, if ever, can be mapped without
including areas of other taxonomic classes. Consequently, every map unit is made
up of the soils or miscellaneous areas for which it is named and some minor
components that belong to taxonomic classes other than those of the major soils.
Most minor soils have properties similar to those of the dominant soil or soils in the
map unit, and thus they do not affect use and management. These are called
noncontrasting, or similar, components. They may or may not be mentioned in a
particular map unit description. Other minor components, however, have properties
and behavioral characteristics divergent enough to affect use or to require different
management. These are called contrasting, or dissimilar, components. They
generally are in small areas and could not be mapped separately because of the
scale used. Some small areas of strongly contrasting soils or miscellaneous areas
are identified by a special symbol on the maps. If included in the database for a
given area, the contrasting minor components are identified in the map unit
descriptions along with some characteristics of each.A few areas of minor
components may not have been observed, and consequently they are not
mentioned in the descriptions, especially where the pattern was so complex that it
was impractical to make enough observations to identify all the soils and
miscellaneous areas on the landscape.
The presence of minor components in a map unit in no way diminishes the
usefulness or accuracy of the data. The objective of mapping is not to delineate
pure taxonomic classes but rather to separate the landscape into landforms or
landform segments that have similar use and management requirements. The
delineation of such segments on the map provides sufficient information for the
development of resource plans. If intensive use of small areas is planned, however,
11
Custom Soil Resource Report
onsite investigation is needed to define and locate the soils and miscellaneous
areas.
An identifying symbol precedes the map unit name in the map unit descriptions.
Each description includes general facts about the unit and gives important soil
properties and qualities.
Soils that have profiles that are almost alike make up a soil series. Except for
differences in texture of the surface layer, all the soils of a series have major
horizons that are similar in composition, thickness, and arrangement.
Soils of one series can differ in texture of the surface layer, slope, stoniness,
salinity, degree of erosion, and other characteristics that affect their use. On the
basis of such differences, a soil series is divided into soil phases. Most of the areas
shown on the detailed soil maps are phases of soil series. The name of a soil phase
commonly indicates a feature that affects use or management. For example, Alpha
silt loam, 0 to 2 percent slopes, is a phase of the Alpha series.
Some map units are made up of two or more major soils or miscellaneous areas.
These map units are complexes, associations, or undifferentiated groups.
A complex consists of two or more soils or miscellaneous areas in such an intricate
pattern or in such small areas that they cannot be shown separately on the maps.
The pattern and proportion of the soils or miscellaneous areas are somewhat similar
in all areas. Alpha-Beta complex, 0 to 6 percent slopes, is an example.
An association is made up of two or more geographically associated soils or
miscellaneous areas that are shown as one unit on the maps. Because of present
or anticipated uses of the map units in the survey area, it was not considered
practical or necessary to map the soils or miscellaneous areas separately.The
pattern and relative proportion of the soils or miscellaneous areas are somewhat
similar.Alpha-Beta association, 0 to 2 percent slopes, is an example.
An undifferentiated group is made up of two or more soils or miscellaneous areas
that could be mapped individually but are mapped as one unit because similar
interpretations can be made for use and management. The pattern and proportion
of the soils or miscellaneous areas in a mapped area are not uniform. An area can
be made up of only one of the major soils or miscellaneous areas, or it can be made
up of all of them. Alpha and Beta soils, 0 to 2 percent slopes, is an example.
Some surveys include miscellaneous areas. Such areas have little or no soil
material and support little or no vegetation. Rock outcrop is an example.
12
Custom Soil Resource Report
Larimer County Area, Colorado
3—Altvan-Satanta loams, 0 to 3 percent slopes
Map Unit Setting
National map unit symbol: jpw2
Elevation: 5,200 to 6,200 feet
Mean annual precipitation: 13 to 15 inches
Mean annual air temperature: 48 to 50 degrees F
Frost-free period: 135 to 150 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Altvan and similar soils:45 percent
Satanta and similar soils: 30 percent
Minor components:25 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Altvan
Setting
Landform: Benches, terraces
Landform position (three-dimensional): Side slope, tread
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Mixed alluvium
Typical profile
H1 -0 to 10 inches: loam
H2- 10 to 18 inches: clay loam
H3- 18 to 30 inches: loam
H4-30 to 60 inches: gravelly sand
Properties and qualities
Slope: 0 to 3 percent
Depth to restrictive feature: More than 80 inches
Drainage class:Well drained
Runoff class: Low
Capacity of the most limiting layer to transmit water(Ksat): Moderately high to high
(0.60 to 2.00 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum content: 10 percent
Available water supply, 0 to 60 inches: Low(about 5.4 inches)
Interpretive groups
Land capability classification (irrigated): 3e
Land capability classification (nonirrigated): 3e
Hydrologic Soil Group: B
Ecological site: R067BY002CO - Loamy Plains
Hydric soil rating: No
13
Custom Soil Resource Report
Description of Satanta
Setting
Landform:Terraces, structural benches
Landform position (three-dimensional): Side slope, tread
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Mixed alluvium and/or eolian deposits
Typical profile
H1 -0 to 9 inches: loam
H2-9 to 18 inches: loam
H3- 18 to 60 inches: loam
Properties and qualities
Slope:0 to 1 percent
Depth to restrictive feature: More than 80 inches
Drainage class:Well drained
Runoff class: Low
Capacity of the most limiting layer to transmit water(Ksat): Moderately high to high
(0.60 to 2.00 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum content: 10 percent
Available water supply, 0 to 60 inches: High (about 10.1 inches)
Interpretive groups
Land capability classification (irrigated): 1
Land capability classification (nonirrigated): 3c
Hydrologic Soil Group: B
Ecological site: R067BY002CO- Loamy Plains
Hydric soil rating: No
Minor Components
Nunn
Percent of map unit: 10 percent
Ecological site: R067BY002CO-Loamy Plains
Hydric soil rating: No
Larim
Percent of map unit: 10 percent
Ecological site: R067BY063CO-Gravel Breaks
Hydric soil rating: No
Stoneham
Percent of map unit: 5 percent
Ecological site: R067BY002CO-Loamy Plains
Hydric soil rating: No
14
Custom Soil Resource Report
74—Nunn clay loam, 1 to 3 percent slopes
Map Unit Setting
National map unit symbol: 2tlpl
Elevation: 3,900 to 5,840 feet
Mean annual precipitation: 13 to 17 inches
Mean annual air temperature: 50 to 54 degrees F
Frost-free period: 135 to 160 days
Farmland classification: Prime farmland if irrigated
Map Unit Composition
Nunn and similar soils:85 percent
Minor components: 15 percent
Estimates are based on observations, descriptions, and transects of the mapunit.
Description of Nunn
Setting
Landform:Terraces
Landform position (three-dimensional):Tread
Down-slope shape: Linear
Across-slope shape: Linear
Parent material: Pleistocene aged alluvium and/or eolian deposits
Typical profile
Ap-0 to 9 inches: clay loam
Bt-9 to 13 inches: clay loam
Btk- 13 to 25 inches: clay loam
Bk1 -25 to 38 inches: clay loam
Bk2-38 to 80 inches: clay loam
Properties and qualities
Slope: 1 to 3 percent
Depth to restrictive feature: More than 80 inches
Drainage class:Well drained
Runoff class: Medium
Capacity of the most limiting layer to transmit water(Ksat): Moderately low to
moderately high (0.06 to 0.20 in/hr)
Depth to water table: More than 80 inches
Frequency of flooding: None
Frequency of ponding: None
Calcium carbonate, maximum content:7 percent
Maximum salinity: Nonsaline to very slightly saline (0.1 to 2.0 mmhos/cm)
Sodium adsorption ratio, maximum:0.5
Available water supply, 0 to 60 inches: High (about 9.9 inches)
Interpretive groups
Land capability classification (irrigated): 2e
Land capability classification (nonirrigated): 3e
15
Custom Soil Resource Report
Hydrologic Soil Group: C
Ecological site: R067BY042CO-Clayey Plains
Hydric soil rating: No
Minor Components
Heldt
Percent of map unit: 10 percent
Landform:Terraces
Landform position (three-dimensional):Tread
Down-slope shape: Linear
Across-slope shape: Linear
Ecological site: R067BY042CO-Clayey Plains
Hydric soil rating: No
Satanta
Percent of map unit: 5 percent
Landform:Terraces
Landform position (three-dimensional):Tread
Down-slope shape: Linear
Across-slope shape: Linear
Ecological site: R067BY002CO- Loamy Plains
Hydric soil rating: No
16
References
American Association of State Highway and Transportation Officials (AASHTO).
2004. Standard specifications for transportation materials and methods of sampling
and testing. 24th edition.
American Society for Testing and Materials (ASTM). 2005. Standard classification of
soils for engineering purposes. ASTM Standard D2487-00.
Cowardin, L.M., V. Carter, F.C. Golet, and E.T. LaRoe. 1979. Classification of
wetlands and deep-water habitats of the United States. U.S. Fish and Wildlife
Service FWS/OBS-79/31.
Federal Register. July 13, 1994. Changes in hydric soils of the United States.
Federal Register. September 18, 2002. Hydric soils of the United States.
Hurt, G.W., and L.M. Vasilas, editors. Version 6.0, 2006. Field indicators of hydric
soils in the United States.
National Research Council. 1995. Wetlands: Characteristics and boundaries.
Soil Survey Division Staff. 1993. Soil survey manual. Soil Conservation Service.
U.S. Department of Agriculture Handbook 18. http://www.nres.usda.gov/wps/portal/
nres/d eta il/national/soils/?cid=nres 142p2_054262
Soil Survey Staff. 1999. Soil taxonomy: A basic system of soil classification for
making and interpreting soil surveys. 2nd edition. Natural Resources Conservation
Service, U.S. Department of Agriculture Handbook 436. http://
www.n res.usda.gov/wps/portal/nres/detail/national/soils/?cid=n res142p2_053577
Soil Survey Staff. 2010. Keys to soil taxonomy. 11th edition. U.S. Department of
Agriculture, Natural Resources Conservation Service. http://
www.nres.usda.gov/wps/portal/nres/detail/national/soils/?cid=nres142p2_053580
Tiner, R.W., Jr. 1985. Wetlands of Delaware. U.S. Fish and Wildlife Service and
Delaware Department of Natural Resources and Environmental Control, Wetlands
Section.
United States Army Corps of Engineers, Environmental Laboratory. 1987. Corps of
Engineers wetlands delineation manual. Waterways Experiment Station Technical
Report Y 87-1.
United States Department of Agriculture, Natural Resources Conservation Service.
National forestry manual. http://www.nres.usda.gov/wps/portal/nres/detail/soils/
home/?cid=n res 142p2_053374
United States Department of Agriculture, Natural Resources Conservation Service.
National range and pasture handbook. http://www.nres.usda.gov/wps/portal/nres/
detail/national/landuse/rangepasture/?cid=stelprdb 1043084
17
Custom Soil Resource Report
United States Department of Agriculture, Natural Resources Conservation Service.
National soil survey handbook, title 43041. http://www.nres.usda.gov/wps/portal/
nres/d eta il/soils/scientists/?cid=n res 142p2_054242
United States Department of Agriculture, Natural Resources Conservation Service.
2006. Land resource regions and major land resource areas of the United States,
the Caribbean, and the Pacific Basin. U.S. Department of Agriculture Handbook
296. http://www.nres.usda.gov/wps/portal/nres/detail/national/soils/?
cid=n res 142 p2_053624
United States Department of Agriculture, Soil Conservation Service. 1961. Land
capability classification. U.S. Department of Agriculture Handbook 210. http://
www.nrcs.usda.gov/lnternet/FSE—DOCUMENTS/nrcsl42p2_052290.pdf
18
Attachment B
Mile High Flood District
Hydrologic Calculations
7
GILMARTIN SUBDIVISION
HISTORICAL AND PROPOSED % IMPERVIOUS CALCULATIONS
Total Drainage Area - 0.71 Acres = 30,809 sf
Area (sf)
Surface Cover/Use Defined %il 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, Tabl,
12. Assume landscaped area around buildings
PEAK RUNOFF PREDICTION BY THE RATIONAL METHOD
Version 2.00 released May 2017
Urban Drainage and Flood Control District
Denver, Colorado
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
, Intro
Calculation of Peak Runoff using Rational Method
C mrpany S reroenEmm &CmaWehen l c Ve easetl hey ] 4_0395(7511—[s7 [y P d —4+[r rm �'nb,1 cI UDFCD ooalwn rZ DAA as l' om_gwe ,o. your ee see to At R n wn s enter own onLs
0 omanee hem me NDMwebane rtllctmsunxl
are 3114/25 C hl for tl I 1�M1our ramlall tlePtM1,P1 Iinl O.BB 1.OB 133 1369. 5. iS6ir Sa50
Pmjem Tea KII Subtl - STINGS—.; OtlIb dUWmknarabr �nvenAe.velrin.. Lr Lr
Looatloa..1SbuIM1 Taff HII Road FOR-Ima.CO /o c-al PeStoeal te + Selernd.4�mav{tm�w,,,min(Compured 4.Re&IonaLx�l Ra mall inte on Ca—i— 26.50 1000 0.]05 1Unjlvl y Q(rfs) LIA
r 60 fiOVr fi0(141+9)$r Sb}TJr
Subm- Arn NRCS Parun[ a Ovmlentl UIS CIB Oradantl Overlord CkannelMtl UIS CIS nn ne imxetl NRCS ChannNized CM1annaliretl o anrxn n ten r w,
NydroloSlc Imperviouann Elnation ElavaUan 81 Elevarlan Elevation Flew Cempubd Re11-1 Sal d
Name (ac) Soil Group a 2Nr Syr 10,yr 2fi-Yr 50-W 100-yr 500,yr Fow LenprM1 11[i Ifll ow o ow Time F w en (fll AI ow Bleee Con nn eloclH Flow.Tima r�(minl 41min1 6(min) 2, Syr 101r 25yr N, IN, 600yr 2yr Syr 10yr 25-yr 50-Yr I., 500�yr
L,Pr1 (Opdonall IOPtionp FlSi lruep FlV lminl 1e 41ft)P� loenanaq (oPuenap Flsr(HM) rK v,11Uen) 41mm1
EXISTING 0.71 C 300.90. 6.615 6500 0.015. a 10 1 22 3-61 3229 155 2.11 2.68 5.81 S01
PROPOSED 0.71 C 298 f50.00. 0.615 1500 0.015. 20 282 2522 2'71
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
Table 6-1.Applicability of hydrologic methods 'Table 6 2.NRCS Conveyance factors,K
Watershed Size(acres) Is the Rational Method Applicable? Is CUHP Applicable? Type of Land Surface Conveyance Facim K
01.90 Yes Yes Heavy meadow 2.5
90 to 160 No Yes Tillage/field 5
160 to 3.000 No Yosi Short pasture and lawns. 7
Cnenter than 3,000 No Yes(subdividing into smaller Nearly bare ground 10
catchments required)'
I_ Subrlivuhmg fats smraher 5uheemLwents aad rnu4ug Ebe rcsnhann hydrogrepbc using SWMIM may be needed to Grassed waterway 15
arceaately model.earennuet wan areas of d.fferem sort types or peme am of,o,pervnousness. Paved areas and shallow paved swales 20
The general procedure for Rational Method calcalatiouns for a single catchment is as follows:
1- Delineate the catclmunt boundary and determine its area. Table 6-3. Recommended percentage imper-iiousoess.values
2. Define the flow path fiotu the upper-most portion of the catchment to the design point.Divide the Land Use or Percentage Impenioumess
flow path into reaches of sumlar flow type(e.g..overland flow,shallow Swale tlow.matter flow,etc.). Surface Characteristics
Delennnte,the length and slope of each reach.
Business.
3. Dcteruniuc dre tune of concentration-t,,for the selected waterway.
Downtown Areas 95
4. Find the rainfall intensify.i.fur the design stern using the calculated re and the rainfall intensity- Submbmr Areas 75
duration-frequency trove(see P,nYtyall chapter).
S. Detenune the runoff coefficient.C.
Residential lots(lot area only):
6. Calculate the peak flow rate.U,from the catchnnent using Equation 6-1. suagi-finusly
2 5 act s.or larger 12
0.75—2.5 acres. 20
The basic assumptions for the application of the Rational Method include: 0 25—0 75— 30
1. the topirted maxinnuu rate of runoff to the design point is a l ctiou of the average raiofnll.rate 0 25 acres or lcss 45
dining the tine of concentration to that point. Apmtm is 75
2. The hydrologic losses in the catclunent are homogeneous mid uniform.The runoff coefRciens very Industrial:
with respect to type of soils,imperviousness percentage,and rainfall fiequeucies.These coefficients
represent the average antecedent soil moisture condition. Light areas 80
3. The depth of rainfall used is one that occurs from the star of the storm it)the bore of concentration. Heavy areas 90
The design rainfall depth dining that period is converted to the average rainfall intensity for that Parks,cemeteries 10
period.
Play-grounds 25
4. The tnaainnun mnoff trite occurs when the ettrim area is contributing flow.This assumption is not
valid where a more intensely developed portion of the catchmem with a shorter time of concentration Schools 55
produces a higher rate of nmoff than the entire catclnnnem with a longer time of concentration. Railroad card areas 50
Undeveloped Areas:
Table 64.Runoff coefficient equatiuns based on NRCS soil group and storm return period Historic.flovv-aualvicia 2
NRCS Storm Return Period Greenbelts,agniculaual 2
Soil 2_Year 5-Year 10-Year 25-Year 50-Year 100-Year 500-Yerr
Gre Off te flow analysis(wirer land use not 45
'� def ed)
A Ca= Ca= Ca= CA= CA= Ca= CA=
Streets:
084ir'°'- 0.86ii 211 0.87ir"' 0.84,1— 0.85i+0.025 0.78i+0.110 0.651+0.254 Paved 100
B Ca= Ca= C_ Ca= Co' Cs= Cs= Gre"ci(pa ked) 40
084i.— 086i1d°s 0.81i+0.057 063i=0.249 056i+0328 0-47i+042.6 037i+O536 Drive and aalks 90
C/f) Cc C Cc.n= Ccu= Ccu= C__ Ccv= Roofs 90
0.83iii22 0.82i+0.035 0.74i+0.132 0.56r+0.319 0.49rt0..393 0.41i+0.484 0..32,+0.588 dawns-sandy sod 2
Launs_clayey soil
VVh—
i="ia impenioicsne.ss(expressed as a decimal)
C.=Rnmoff coeffnznt for Natural Resomecs Caosery—Service(NRCS)HSG A soils
Ca=Runoff coefficient for NRCS MG 13 sails
Cen=Runoff' f&ciem for NRCS HSG C and D colds_
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
Table 6-5. Ruuaff coefficients..c
Tatal ar Effective MRCS Hvdroloolc Soil G-p A
%Imperious 2-Year 5-Year 100-Year 25-Year 50-Year 100-Year 500-Year
iD4
2% 0_01 0.01 0.01 0 01 0 04 013 027 '
5% 0.02 0.02 0.02 003 0,07 0,15 0.29 -
10% 004 q U5 0.05 0.07 011 0-19 032 0t80
15% 0-07 008 008 01 0.15 0-23 035
20% 0-1 Oil 0.12 0.14 0.2 027 038 0 -
25% 0-14 015 016 0.19 0.24 03 042
064
30% 0.18 0.19 0.2 0.23 0.28 034 0.45 = - - r -- -
-x-
35% 021 0.23 0.24 027 0.32 038 0.48 -
40% 025 0.27 0.29 0.32 037 012 0.51 'S --0-10 yr
45% 03 031 0.33 0.:36 0.41 0..46 0.54 `0 0.40 - - -ME-25-W
50% 0.34 0-36 037 041 0.45 0-5 058 �' _ - -<>-502r
55% 0.39 04 042 0 45 04-0 0.54 061 I+lv yr
60% 0 43 045 047 05 054 0.58 064 -
65% 0.48 0.5 0.51 0,54 0.,58 0.62 0.67 _
701% 0.53 0.55 0.56 0.59 C.62 0..65 0.71
75% 0.58 0.6 0.61 Ofi4 0.66 0..69 0.74 0-�
80% 0.63 0.65 0.66 0,69 0.71 073 0.77 D 10 20 30 40 50 60 70 60 90 too
85% 0-68 a 7 0.71 0.74 0.75 0-77 0.8 Watershed Percentage Impewrcusness,%
90% 0.73 0-75 077 0.79 0-79 0-91 084
95°�6 0 .8 3 084 0-85 087 3 084 0-85 087Figure 6-1.Runoff coefficient vs.watershed imperiousness NRCS MG
087 0.38 0.89 0.9
Taral or Effeeme 1RCS Hydrologic.Soil Group
N.Irnpenious 2-Ye- 5-Year 10Y"ear 25-Year 50-Year 100-1ear-w year 100
2% 0.01 001 007 026 0.34 044 0 54
5% 0.03 0.03 0.1 028 0.36 0.45 0.55 0-90
10% 006 007 014 0.31 0.38 0-47 057 0SO
15% 009 0.11 018 0.34 0.41 05 059
20% 013 a is 022 0.38 044 052 0.61 070
25% 0.17 0.19 0.26 0.41 0.47 0.54 0.63 0 O,uo -.*-2-yr
30% 02 0.23 03 0.44 0,49 057 0.65
35% 0.24 0.27 0.34 0..47 0.52 0.59 0.66 - 0.50
10-yr
4046 029 032 038 OS 055 0-61 0.68 `m 040 -IIE-25_Yr
45% 033 036 042 053 0.53 0.64 0 7
50% 0.37 04 046 0.56 0-61 0-66 072 - 0.30 �50=yr
359"b 0.42 0.45 0.5 0.6 0.63 06S 0.74 U-20 100yr
60% 046 049 054 063 066 0-71 076 010
65% 0.5 0.54 0.58 0.66 0..69 0.73 0.77
70% a55 058 062 0-69 0-72 0-75 0.79 U.00 X
75% 0.6 0.63 066 0.72 0.,75 0..78 0.81 0 10 20 30 40 50 CO 70 80 90 100
80% 0-64 0-67 0-7 0.75 0-77 0.8 083 Watershed Percentage Irnig- usness,%
85% 0:69 0.72 074 0.78 0.8 0.82 0.85
90°. 11 4 0-76 0711 0.81 0.83 0.84 087 Figure 6-2.Runoff coefficient ss.watershed imperiousness NRCS HSG B
95% 0.79 0.81 0.82 0.91 0.86 0.87 0.88
100% 0.94 O;96 0.86 0,88 0,89 0,89 0.9
Total or Eft five, NRCS Hvdro]a air Soil Gres C
%fmpeniaus 2-Year 5-y- 10-Y- 25-Year 50-Year 100-Year 500Year
2% 0.01 0.05 0.15 0-33 0.40 0.49 059
5% 003 0.08 017 0,35 042 05 0.6 1.00
Iv. 0.06 0.12 021 0.37 0.44 052 062 0.90
15% 01 0-16 0-24 04 047 055 064
20% 0.14 0.2 0.28 0.43 049 057 065 0.80 - - -
25% 0.18 C.24 0.32 0.46 0.52 059 067 0.70
35% 0.26 028 039 0.51 0.57 0..61 0.7 /* o?gr
35°A 026 032 039 0.51 0.57 063 0.7 U 0.60 - -
-x-5-yr
40% 03 0.36 0.43 054 059 0-65 071 0.50 10-yr
45% 0.34 0.4 0.46 0.57 0.62 0,67 0.73 0_40 _ _ is-25-yr
50% 0.38 0.44 05 0.6 0.64 0 69 075 �50yr
55% 0.43 0.48 0.54 0.63 0.66 0.71 0.76 U 0'30 -&100-yr
601% 047 052 057 065 069 0-73 079 0.20 -
65% 0.51 056 0.61 0.68 0.71 0.75 0.79
70°� 056 0.61 0.65 0.71 0-74 0.77 0.91 0.10 - - - -
75% 0-6 0.65 0.68 074 0.76 0.79 082 0.00
80% 065 0.69 0.72 0-77 0.79 0-81 084 q 10 20 30 40 50 50 70 s0 90 100
85% 0-7 0.73 0.76 079 0.81 0,83 086 Watershed Percentage Imperviousness,°6
901% 0.74 077 0.79 0.82 084 0..85 0.87
95% 0.79 0.81 083 085
0.86 0-87 089 Figure 6-3.Runnft rnefficient ss.watershed imper-innsrress MRCS HSG C and I)
100% 0.83 0,85 0.97 0 88 0.89 0-89 09
Page 7