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Approve
Date:
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October 4, 2022
KEEFE CIVIL, INC
3125 Crockett Street
Gradin Fort Collins, Colarado 80526
Accepte�
Date:
❑ Escro�
(970) 215-6808
meg@keefecivil.com
Keefe Civil Project Number: 2022-017
Grading/Stormwater Plan Accepted
Water Utilities Engineering
City of Fort Collins (Building Permit Released)
700 Wood Street Accepted by: Dan Mogen
Date: 11/07/2022
Fort Collins, Colorado 80522
RE: 323 South Washington Avenue Drainage Memo
Parcel # 9711321007
Lots 7& 8, Block 5, Prospect Place
To Whom it May Concern: B2207488
The purpose of this letter is to describe the proposed site improvements and potential drainage
impacts at 323 South Washington Avenue (Exhibit A). The 0.325-acre project site has been
recently demoed and was a developed single-family residential lot with established landscaping
located on the northwest corner of South Washington Avenue and Woodfard Avenue in Fort
Collins, Colorado. The site is located within the Old Town drainage basin. Existing conditions at
the site included a single-story house, outbuildings, flagstone pathways, a wood patio, established
trees, and lawn area.
The lot is bound by an existing single-family residential property on the north, South Washington
Avenue right-of-way to the east, Woodford Avenue right-of-way to the south, and a crushed
asphalt alley to the west. Per the NRCS Soil Survey of Larimer County (Exhibit B), the site
entirely consists of Fort Collins loam and is classified as being within hydrologic soil group C.
These soils are anticipated to have a slow infiltration rate when thoroughly wet and a slow rate of
water transmission.
Per the flood insurance rate map, the lot is located within Zone X. Zone X is defined as an area of
minimal flood hazard and does not have a base flood elevation associated with it. The site is
located within a City of Fort Collins moderate risk floodplain area. (Exhibit C)
Historically, site stormwater in the front yard has drained with gentle slopes from the northwest to
the southeast into either Woodward Avenue ar Washington Avenue right-of-way (historic
basin A). See E�ibit D for historic and developed drainage basins.
Proposed site improvements include a one-story single-family residential home with a basement
and attached garage. The proposed driveway entrance off Woodward Avenue shall tie into an
existing concrete apron and loop out to a new access from the alley. All house roof drains are
proposed to drain towards either Woodward or Washington right-of-way, minimizing stormwater
flows to the alley way (developed basin B). Aside from a very small landscape area north of the
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323 South Washington Avenue
Pa�e 2 of 3
Keefe Civil Project No.: 2022-017
October 4, 2022
driveway (developed basin C), the remainder of the site will drain either out to the alley or public
right-of-way (developed basin A). Where possible, impervious areas and downspouts shall be
routed into pervious areas with landscaping to allow for project stormwater infiltration and
increased water quality with site runoff.
Based on visual inspections of the alley and field survey by others, it is my professional opinion
the proposed improvements presented in this drainage memo will not negatively impact
downstream private properties adjacent to the alley before draining into Woodford Avenue right-
of-way to the south. The alley has crushed asphalt, drains with a positive slope to the south, and the
home to the west is higl�er than the alley (Figure 1).
The site's total adjusted impervious area will increase by 4,221 sf, as indicated by calculations
shown below. Since the site's total adjusted impervious area increase is below 5,000 sf, water
quantity detention requirements are waived for this development.
Adjusted Developed Adjusted
Historic Lot Historic Lot Area Developed
Hardsca e or Hard Surface Area s Area s s Area s
Concrete 240 240 2,225 2,225
Rooitop (90%) 1,620 1,458 4,027 3,624
Deck and Pavers (40%) 1,005 402 1,180 472
Total Impervious Area 2,865 2,100 7,432 6,321
Adjusted Basin 4,221 sf
Im erviousness
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Figure 1: View of property to the west and alley conditions
323 South Washington Avenue
Pa�e 3 of 3
Keefe Civil Project No.: 2022-017
October 4, 2022
The proposed site imperviousness is �30%, which is less than the 50% assumed within the Old
Town drainage basin master plan (ICON Engineering 2017).
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 the adjacent street
right-of-way (developed basin A) and alley way (developed basin B), as indicated in the summary
table below.
2- ear 10- ear 100- ear
Historic Basin A Runoff 0.24 cfs 0.40 cfs 1.08 cfs
Developed Basin A Runoff 0.50 cfs 0.85 cfs 2.16 cfs
Develo ed Basin B Runoff 0.04 cfs 0.06 cfs 0.14 cfs
Develo ed Basin C Runoff 0.01 cfs 0.01 cfs 0.03 cfs
Conclusions
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 atiest thai this letter for ihe final drainage design for 323 South Washington Avenue 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.
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: Chandler Souther, Hammersmith Structures
Eric Johnson, Hammersmith Structures
Taylor Myer, VFLA Architecture + Interiors
References
Fort Collins Stormwater Criteria ManuaL 2018.
ICON Engineering. 2017. Old Town Drainage Basin EPA SWMM Conversion and Hydrologic
Update.
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9/19/2022
Page 1 of 4
Hydrologic Soil Group—Larimer County Area, Colorado
(323 South Washington Avenue)
MAP LEGEND
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
0 A
Q A/D
0 B
� B/D
Q C
0 C/D
� D
� Not rated or not available
Soil Rating Lines
�, ,k A
�-.� A/D
� B
� B/D
r r C
r::;r C/D
�-,✓ D
. r Not rated or not available
Soil Rating Points
o A
0 A/D
e B
� B�o
0 C
0 GD
0 D
0 Not rated or not available
Water Features
Streams and Canals
Transportation
� Rails
� Interstate Highways
US Routes
Major Roads
Local Roads
Background
� Aerial Photography
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 16, Sep 2, 2021
Soil map units are labeled (as space allows) for map scales
1:50,000 or larger.
Date(s) aerial images were photographed: Jul 2, 2021—Aug 25,
2021
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.
uSDa Natural Resources Web Soil Survey 9/19/2022
� Conservation Service National Cooperative Soil Survey Page 2 of 4
Hydrologic Soil Group—Larimer County Area, Colorado
Hydrologic Soil Group
Map unit symbol
35
Map unit name
Fort Collins loam, 0 to 3 C
percent slopes
Totals for Area of Interest
Description
Rating
323 South Washington Avenue
Acres in AOI Percent of AOI
0.5 100.0%
0.5 100.0%
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
USDn Natural Resources Web Soil Survey 9/19/2022
� 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.
323 South Washington Avenue
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.
Component 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.
USDn Natural Resources Web Soil Survey 9/19/2022
� Conservation Service National Cooperative Soil Survey Page 4 of 4
National Flood Hazard Layer FIRMette � �:��M� Legend
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HAZARD AREAS Regulatory Fbodwey
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Composite Runoff Coefficient Calculations
323 South Washington Avenue, Fort Collins, CO
�(C',-�;1, )
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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 Dreinage Basin A Developed Drainage Basin A Developed Drainage 8asin B Developed Drainage Basin C
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 C' Area (sf) Composite CZ Composite C3 Area (sf) Composite CZ Composite C' Area (sf) Composite C� Composite C3
Hardswpe or Hard Surface
Concrete 0.95 240 1,625 600 0
Rooftop 0.95 1,620 4,027 0 0
Deck 0.40 415 0 0 0
Pavers 0.50 590 1,180 0 0
Totallmpervious Area 2,565 6,832 600 0
Landscape or Pervious Surface
Lawns, Clayey Soil, flat Slope < 2% OZO 11,285 0 0 0
Lawns, Ciayey Soil, Avg Slope 2-7% OZS 0 6,458 0 0
Lawns, C�ayey Soil, Steep Slope>7% 0.35 0 0 0 260
Total 14,150 032 0.40 13,290 0.57 0.71 600 0.95 1.00 260 0.35 0.44
Notes:
l. Runoff coefficients per Fort Collins Stormwater Criteria Manual Table 3.2-2.
2. Composite runoff coefficient per Port Collins Stormwater Criteria Manual Equation 5-2.
3. Frequency adjustment factor per Fort Collins Stormwater Criteria Manual Table 3Z-3.
Table 3.2-3. Frequency Adjustment Factors
Storm Return Period Frequency Adjustment
(years) Fador (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
Recyded 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, Flat Slope < 2% 0.20
Lawns, Clayey Soil, Avg Slope 2-7% 0.25
Lawns, Clayey Soil, Steep Slope >7% 035
323 5 Washington St - drainage.xlsx
Drainage Basin Runoff Calculations
323 South Washington Avenue, 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 � ( ) ( , ( � (a � yp ( p � ( ) � ( � 2-yr 10-yr 100-yr 2-yr 10-yr 100-yr
() (/o min min ft /o T e f s min (min min
Historic
A 0.325 0.32 0.40 60 2 9.0 8.1 NA 9.0 8.1 2.30 3.93 8.38 0.24 0.40 1.08
Developed
A 0.305 0.57 0.71 85 3 1.5 4.6 NA 0.0 5.0 5.0 2.85 4.87 9.95 0.50 0.85 2.16
B 0.014 0.95 1.00 34 2 0.4 0.9 NA 0.0 5.0 5.0 2.85 4.87 9.95 0.04 0.06 0.14
C 0.006 0.35 0.44 10 10 2.1 1.8 NA 0.0 5.0 5.0 2.85 4.87 9.95 0.01 0.01 0.03
Notes:
1. Per Fort Collins Stormwater Criteria Manual Section 3.3.2.
i.a��i.l-cxcf��
T� - �r
Where: C= Runoff Coefficient, dimensionless
Cr = Frequency Adjustment Factor, dimensionless
L= Length of Overland Flow, feet
S = 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� Vx60
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
323 S Washington St - drainage.xlsx
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