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October 27, 2020
Wes Lamarque
Development Review Engineer
Water Utilities Engineering
City of Fort Collins
700 Wood Street
Fort Collins, Colorado 80522
RE: 608 Sycamore Street Drainage Memo
Parcel # 9711217038
Dear Wes:
KEEFE CIVIL, INC
825 Union Street
Golden, Colorado 80401
(970) 215-6808
meg@keefecivil.com
Keefe Civil Project Number: 2020-021
The purpose of this letter is to describe the proposed site improveinents and potential drainage
impacts at 608 Sycamore Street (Exhibit A). The 0.195-acre project site is a developed single-
family residential lot with established landscaping located on Sycamore Street between Whitcomb
Street and Lincoln Street in Fort Collins, Colorado. Current conditions at the site include a single-
story house, a small garden shed with wooden deck, gravel and flagstone pathways, established
trees, several garden areas, and a concrete driveway.
The lot is bound by existing single-family residential properties on the west and east lot lines,
Sycamore Street right-of-way to the south, and a dirt alley way to the north. Per the NRCS Soil
Survey of Larimer County (E�ibit B), the site entirely consists of Paoli fine sandy loams and is
classified as being within hydrologic soil group A. These soils are anticipated to have a high
infiltration rate when thoroughly wet and a high rate of water transinission.
Per the flood insurance rate map, the lot is located within Zone X. Zone X is defined as an area of
miniinal flood hazard and does not have a base flood elevation associated with it. Also, the site is
outside of all City of Fort Collins mapped floodplains. (Exhibit C)
Historically, site stormwater has drained with gentle slopes either from the north to south into
Syca�nore Street right-of-way (Basin A) or from the southwest to northeast via overland flows in
the backyard (Basin B). When stormwater does not infiltrate the established backyard, it eventually
flows into the alley way to the north. Based on field survey and visual inspections, the alley drains
at a gentle slope into North Whitcomb Street to the east. Historic and developed drainage basins
are shown in Exhibit D.
The proposed site improvements include a two-story house addition with basement and a detached
garage with access off the alley. The existing house, garden shed, and concrete driveway will be
removed prior to construction. Proposed roof drain directions are noted on the grading plan and are
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608 Sycamore Street Drainage Memo Keefe Civil Project No.: 2020-021
Page 2 of 3 October 27, 2020
reflected in the drainage design presented in this memo. Most the house's roof drains are proposed
to drain towards Sycamore Street right-of-way, minimizing the stormwater flows to the alley way.
Since the finished floor of the existing offsite garage to the north of the alley (4997.4) is lower than
the proposed detached garage with this development, all proposed garage roof drains shall be
directed either to the south or the east. Proposed site drainage flows shall be directed towards the
northeast corner of the property (4997.0).
Based on a recent site visit and field survey (Exhibit A), developed stormwater from Basin B will
drain east to the Whitcomb Street right-of-way (4996.3) with minimal impacts, if any, to existing
offsite structures adjacent to the alley east of the project site (4997.4 and 4998.2).
The lot's total adjusted iulpervious area will increase by 2,465 square feet (sfl, as indicated in
calculations shown below.
Adjusted Developed Adjusted
Historic Lot Historic Lot Area Developed
Hardsca e or Hard Surface Area s Area s s Area s
Concrete 3 50 3 50 3 65 3 65
Rooftop 1,565 1,565 4,120 4,120
Patio and Pavers (40%) 355 142 93 37
Total Impervious Area 2,270 2,057 4,578 4,522
Adjusted Basin 2,465 sf
Im erviousness
The composite runoff coefficient and drainage basin runoff calculations for the historic and
developed site are included in the enclosed tables (Exhibit E). Using the Fort Collins Stormwater
Criteria Manual (2018), the proposed stormwater runoff will increase towards both Sycamore
Street right-of-way and the alley way, as indicated in the surrllnary table below.
2-year 10-year 100-year
Basin A Historic Runoff 0.08 cfs 0.14 cfs 0.35 cfs
Basin A Develo ed Runoff 0.15 cfs 0.26 cfs 0.65 cfs
Basin B Historic Runoff 0.20 cfs 0.34 cfs 0.98 cfs
Basin B Developed Runoff 0.24 cfs 0.40 cfs 1.03 cfs
Based on the enclosed design, it is my professional opinion the additional runoff generated by
these site improvements will not cause adverse impacts to downstream private properties.
I herby attest that this letter for the final drainage design for 608 Sycamore Street was prepared
by me or under my direct supervision, in accordance with the provisions of the Fort Collins
Stormwater Criteria Manual. T understand that the City of Fort Collins does not and will not
assume liability for drainage facilities designed by others.
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608 Sycamore Street Drainage Memo
Pa�e 3 of 3
Keefe Civil Project No.: 2020-021
October 27, 2020
Thank you for the opportunity to submit this drainage memo. Please do not hesitate to reach out
to me if you have any questions or comments regarding the above items.
Best regards,
Megan L. Keefe, PE / Manager
Keefe Civil, Inc.
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Enclosures
Cc: Steve Suchy, Renovative Resource
Sean A Tomlinson, Tomlinson Designs
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Page 1 of 4
Hydrologic Soil Group—Larimer County Area, Colorado
(608 Sycamore Soils Map)
MAPLEGEND
Area of Interest (A01)
Area of Interest (AOI)
Soils
Soil Rating Polygons
� A
Q A/D
0 B
� B/D
Q C
0 C/D
� D
0 Not rated or not availabie
Soil Rating Lines
r � A
� C
0 C/D
� D
� Not rated or not availabie
Water Features
Streams and Canais
Transportation
� Rails
�..� Interstate Highways
US Routes
Major Roads
Local Roads
Background
� Aeriai Photography
• r A/D
. r B
• r B/D
• r C
� r C/D
� � D
.,� Not rated or not availabie
Soil Rating Points
o A
� A��
O B
o g�o
MAP INFORMATION
The soil surveys that comprise your AOI were mapped at
1:24,000.
Warning: Soil Map may not be valid at this scale.
Enlargement of maps beyond the scale of mapping can cause
misunderstanding of the detail of mapping and accuracy of soil
line placement. The maps do not show the small areas of
contrasting soils that could have been shown at a more detailed
scale.
Please rely on the bar scale on each map sheet for map
measurements.
Source of Map: Natural Resources Conservation Service
Web Soil Survey URL:
Coordinate System: Web Mercator (EPSG:3857)
Maps from the Web Soil Survey are based on the Web Mercator
projection, which preserves direction and shape but distorts
distance and area. A projection that preserves area, such as the
Albers equal-area conic projection, should be used if more
accurate calculations of distance or area are required.
This product is generated from the USDA-NRCS certified data as
of the version date(s) listed below.
Soil Survey Area: Larimer County Area, Colorado
Survey Area Data: Version 15, Jun 9, 2020
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Aug 11, 2018—Aug
12, 2018
The orthophoto or other base map on which the soil lines were
compiled and digitized probably differs from the background
imagery displayed on these maps. As a result, some minor
shifting of map unit boundaries may be evident.
Natural Resources Web Soil Survey 10/8/2020
Conservation Service National Cooperative Soil Suroey Page 2 of 4
Hydrologic Soil Group—Larimer County Area, Colorado
Hydrologic Soil Group
Map unit symbol
81
Map unit name
Paoli fine sandy loam, 0 A
to 1 percent slopes
Totals for Area of Interest
Description
Rating
608 Sycamore Soils Map
Acres in AOI Percent of AOI
1.3 100.0%
1.3 100.0%
Hydrologic soil groups are based on estimates of runoff potential. Soils are
assigned to one of four groups according to the rate of water infiltration when the
soils are not protected by vegetation, are thoroughly wet, and receive
precipitation from long-duration storms.
The soils in the United States are assigned to four groups (A, B, C, and D) and
three dual classes (A/D, B/D, and C/D). The groups are defined as follows:
Group A. Soils having a high infiltration rate (low runoff potential) when
thoroughly wet. These consist mainly of deep, well drained to excessively
drained sands or gravelly sands. These soils have a high rate of water
transmission.
Group B. Soils having a moderate infiltration rate when thoroughly wet. These
consist chiefly of moderately deep or deep, moderately well drained or well
drained soils that have moderately fine texture to moderately coarse texture.
These soils have a moderate rate of water transmission.
Group C. Soils having a slow infiltration rate when thoroughly wet. These consist
chiefly of soils having a layer that impedes the downward movement of water or
soils of moderately fine texture or fine texture. These soils have a slow rate of
water transmission.
Group D. Soils having a very slow infiltration rate (high runoff potential) when
thoroughly wet. These consist chiefly of clays that have a high shrink-swell
potential, soils that have a high water table, soils that have a claypan or clay
layer at or near the surface, and soils that are shallow over nearly impervious
material. These soils have a very slow rate of water transmission.
If a soil is assigned to a dual hydrologic group (A/D, B/D, or C/D), the first letter is
for drained areas and the second is for undrained areas. Only the soils that in
their natural condition are in group D are assigned to dual classes.
Rating Options
Aggregation Method: Dominant Condition
Natural Resources Web Soil Survey 10/8/2020
Conservation Service National Cooperative Soil Survey Page 3 of 4
Hydrologic Soil Group—Larimer County Area, Colorado
Aggregation is the process by which a set of component attribute values is
reduced to a single value that represents the map unit as a whole.
A map unit is typically composed of one or more "components". A component is
either some type of soil or some nonsoil entity, e.g., rock outcrop. For the
attribute being aggregated, the first step of the aggregation process is to derive
one attribute value for each of a map unit's components. From this set of
component attributes, the next step of the aggregation process derives a single
value that represents the map unit as a whole. Once a single value for each map
unit is derived, a thematic map for soil map units can be rendered. Aggregation
must be done because, on any soil map, map units are delineated but
components are not.
For each of a map unit's components, a corresponding percent composition is
recorded. A percent composition of 60 indicates that the corresponding
component typically makes up approximately 60% of the map unit. Percent
composition is a critical factor in some, but not all, aggregation methods.
The aggregation method "Dominant Condition" first groups like attribute values
for the components in a map unit. For each group, percent composition is set to
the sum of the percent composition of all components participating in that group.
These groups now represent "conditions" rather than components. The attribute
value associated with the group with the highest cumulative percent composition
is returned. If more than one group shares the highest cumulative percent
composition, the corresponding "tie-break" rule determines which value should
be returned. The "tie-break" rule indicates whether the lower or higher group
value should be returned in the case of a percent composition tie. The result
returned by this aggregation method represents the dominant condition
throughout the map unit only when no tie has occurred.
Componenf Percent Cutoff.� None Specified
Components whose percent composition is below the cutoff value will not be
considered. If no cutoff value is specified, all components in the database will be
considered. The data for some contrasting soils of minor extent may not be in the
database, and therefore are not considered.
Tie-break Rule: Higher
The tie-break rule indicates which value should be selected from a set of multiple
candidate values, or which value should be selected in the event of a percent
composition tie.
608 Sycamore Soils Map
Natural Resources Web Soil Survey 10/8/2020
Conservation Service National Cooperative Soil Survey Page 4 of 4
National Flood Hazard Layer FI RMette
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SEE FIS REPORT FOR DEfAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOOD
NAZARD AREAS
OTHER AREAS OF
FLOOD HAZARD
Without Base Flood Elevation (BFE)
Zone A. V. A99
With BPE or Depth zo�,e ne. no, aH. ve. aR
Regulatory Floodway
0.2% Annual Chance Flood Hazard, Areas
of 1% annual chance flood with average
depth less than one foot or with drainage
areas of less than one square mile zo„e x
� - � Future Conditions 1% Annual
_-- Chance Flood Hazard zo�,� x
Area with Reduced Flood Risk due to
Levee. See Notes. zo�,e x
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NO SCREEN Area of Minimal Flood Hazard zo„e x
� Effective LOMRs
OTHER AREAS Area of Undetermined Flood Hazard zo„e o
GENERAL '--' Channel, Culvert, or Storm Sewer
STRUCTURES � i i t i i t Levee, Dike, or Floodwall
� Cross Sections with 1% Annual Chance
��.s Water Surface Elevation
�- - - Coastal Transect
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Limit of Study
Jurisdiction Boundary
--- --- Coastal Transect Baseline
OTHER _ — profile Baseline
FEATURES Hydrographic Feature
MAP PANELS
Q
Digital Data Available �
No Digital Data Available �
Unmapped
The pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
This map complies with FEMA's standards for the use of
digital flood maps if it is not void as described below.
The basemap shown complies with FEMA's basemap
accuracy standards
The flood hazard information is derived directly from the
authoritative NFHL web services provided by FEMA. This map
was exported on 10/8/2020 at 5:33 PM and does not
reflect changes or amendments subsequent to this date and
time. The NFHL and effective information may change or
become superseded by new data over time.
This map image is void if the one or more of the following map
elements do not appear: basemap imagery, flood zone labels,
legend, scale bar, map creation date, community identifiers,
FIRM panel number, and FIRM effective date. Map images for
unmapped and unmodernized areas cannot be used for
regulatory purposes.
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572.0 0 286.00 572.0 Feet
This map is a user generated static output from the City of Fort Collins FCMaps
WGS 1984 Web_Mercator_Auxiliary_Sphere Internet mapping site and is for reference only. Data layers that appear on this
map may or may not be accurate, current, or otherwise reliable.
City of Fort Collins - GIS
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Parcels
City Floodplains
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Composite Runoff Coefficient Calculations
608 Sycamore Street, Fort Collins, CO
�����-���, )
C- — ;-�
A,
Where: C= Composite Runoff Coefficient
C; = Runoff Coefficient for Specific Area (A;), dimensionless
A; = Area of Surface with Runoff Coefficient of C;, acres or square feet
n= Number of different surfaces to be considered
A, = Total Area over which C is applicable, acres or square feet
Equation 5-2
Historic Drainage Basin A Developed Drainage Basin A Nistoric Drainage Basin B Developed Drainage Basin B
Minor (2- and Minor (2- and Minor (2- and Minor (2- and
Runoff 10-yr� Major (100-yr) 10-yr) Major (100-yr) 10-yr) Major (100-yr) 10-yr� Major (100-yr)
Surface Type Coefficients' Area (sf) Composite CZ Composite C3 Area (sf) Composite CZ Composite C3 Area (sf) Composite CZ Composite C3 Area (sf) Composite CZ Composite C3
Hardscape or Hard Surface
Concrete 0.95 300 90 50 275
Rooftop 0.95 705 2,065 860 2,060
Flagstone Patio 0.50 130 43 225 50
Total Impervious Area 1,135 2,198 1,135 2,385
Landscape or Pervious Surface
Lawns, Clayey Soil, Flat Slope < 2% 0.20 1,005 1,102 5,230 2,820
Total 2,140 0.57 0.71 3,300 0.69 0.87 6,365 0.32 0.40 5,205 0.54 0.67
Notes:
1. Runoff coefficients per Fort Collins Stormwater Criteria Manual Table 3.2-2.
2. Composite runoff coefficient per Fort Collins Stormwater Criteria Manual Equation 5-1.
3. Frequency adjustment factor per Fort Collins Stormwater Criteria Manual Table 3.2-3.
Table 3.2-3. Frequency Adjustment Factors
Storm Return Period Frequency Adjustment
(years) Factor (Cr)
2, 5, 10 1.00
25 1.10
50 1.20
100 1.25
Table 3.2-2. Surface Type - Runoff Coefficients
Surface Type Runoff Coefficients
Hardscape or Hard Surface
Asphalt, Concrete 0.95
Rooftop 0.95
Recycled Asphalt 0.80
Gravel 0.50
Pavers 0.50
Landscape or Pervious Surface
Lawns, Sandy Soil, Flat Slope < 2% 0.10
Lawns, Sandy Soil, Avg Slope 2-7% 0.15
Lawns, Sandy Soil, Steep Slope >7% 0.20
Lawns, Clayey Soil, Ffat Slope < 2% 0.20
Lawns, Clayey Soil, Avg Slope 2-7% 0.25
Lawns, Clayey Soil, Steep Slope >7% 035
608 Sycamore - drainage.xlsx
Drainage Basin Runoff Calculations
608 Sycamore Street, Fort Collins, CO
Area Minor Major
Initial/Overland Timel Travel TimeZ Final T� Intensit in/hr)4 Basin Flow (cfs 5
Basin Length Slope Minor to Major to Length Slope Channel Velocity tt Minor Tc Major Tc
(acres) Composite C Composite C ft o � ( ) ( ) ( � �o' yP ( p � ( ) ( � ( � 2-yr 10-yr 100-yr 2-yr 10-yr 100-yr
() (/ min min ft / T e f s min min min
Historic
A 0.049 0.57 0.71 25 2 3.9 2.9 NA 5.0 5.0 2.85 4.87 9.95 0.08 0.14 0.35
B 0.146 0.57 0.71 105 2 8.1 5.9 NA 8.1 5.9 2.40 4.10 9.44 0.20 0.34 0.98
Developed
A 0.076 0.69 0.87 20 2 3.0 1.5 NA 5.0 5.0 2.85 4.87 9.95 0.15 0.26 0.65
B 0.119 0.69 0.87 25 2 3.0 1.7 65 2% swale 2.1 0.5 5.0 5.0 2.85 4.87 9.95 0.24 0.40 1.03
Notes:
1. Per Fort Collins Stormwater Criteria Manual Section 3.3.2.
�.s��i.i-cxcf��
T' 3�
Where: C= Runoff Coefficient, dimensionless
Cf = Frequency Adjustment Factor, dimensionless
L= Length of Overland Flow, feet
5 = Slope, percent
2. Per Fort Collins Stormwater Criteria Manual Section 3.3.3.
V _ 1.49 R2/3S1/2
n
Where: V = Velocity, feet/second
n = Roughness Coefficient, dimensionless
R= Hydraulic Radius, feet (Hydraulic Radius = area / wetted perimeter, feet)
S = Longitudinal Slope, feet/feet
And:
L
T� - vXeo
3. Per Fort Collins Stormwater Criteria Manual Section 3.3.4. A minimum T� of 5 minutes is required.
T�-iso+10
4. Per IDF Table for Rational Method, Table 3.4-1 Fort Collins Stormwater Criteria Manual.
5. Per Fort Collins Stormwater Criteria Manual Equation 5-1.
Q = CIA
Where: Q= Peak Rate of Runoff, cfs
C = Runoff Coefficient, dimensionless
I = Rainfall Intensity, in/hr
A= Area of the Basin or Sub-basin, acres
608 Sycamore - drainage.xlsx