HomeMy WebLinkAboutGrading Plans - 11/15/2024
KEEFE CIVIL, INC
3125 Crockett Street
Fort Collins, Colorado 80526
(970) 215-6808
meg@keefecivil.com
November 12, 2024 Keefe Civil Project Number: 2023-027
Water Utilities Engineering
City of Fort Collins
700 Wood Street
Fort Collins, Colorado 80522
RE: 116 First Street Drainage Memo
Parcel 9712107022
To Whom it May Concern:
The purpose of this letter is to describe the proposed site improvements and potential drainage
impacts at 116 First Street (Exhibit A). The 0.146-acre project site is an undeveloped single-family
residential lot located on the east side of First Street between East Lincoln Avenue and Logan
Avenue in Fort Collins, Colorado (Figure 1). The site is located within the Poudre River drainage
basin, directly east of the river.
Figure 1: Vicinity Map
Grading/Stormwater Plan Accepted
(Building Permit Released)
Accepted by:
Date:
Dan Mogen
11/15/2024
116 First Street Drainage Memo Keefe Civil Project No.: 2023-027
Page 2 of 4 November 12, 2024
The lot is bound by an existing single-family residence to the north, First Street right-of-way to the
west, a gravel alley way to the east, and a concrete alley way to the south. Per the NRCS Soil
Survey of Larimer County (Exhibit B), the site entirely consists of Loveland clay 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 with reduced flood risk due to a
levee. Zone X is defined as an area of minimal flood hazard and does not have a base flood
elevation associated with it. However, the site is within a City of Fort Collins mapped moderate
risk floodplain, which indicates there is a 1% annual chance storm likely to produce shallow
flooding of up to one foot in depth (Exhibit C).
Historically, the site’s stormwater has drained with flat to average slopes towards either First Street
or one of the alleys to the south and east. Based on visual field inspections, the dirt alley to the east
does not adequately drain stormwater to public right-of-way before impacting private property;
therefore, no new drainage is proposed in that direction. Based on a field survey completed by
Majestic Surveying, the concrete alley has a high point adjacent to the southeast corner of the
property. The concrete alley is in good condition and adequately drains stormwater either to the
east or west into public right-of-way prior to negatively impacted private property (Figure 2). See
Exhibit D for drainage basins and stormwater flow patterns.
Figure 2: Southern alley
During the first phase of this project, the proposed site improvements include a single-family home
with basement, curb, gutter, and sidewalk improvements, and the removal of an existing concrete
pad. The second phase of the project will involve a future garage and concrete driveway. The
grading plan was developed to match existing drainage patterns, and the developed basins are
similar to historic conditions. 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.
Less area will drain to the eastern dirt alley and First Street, while a larger area will drain into the
concrete alley and out to Second Street. It is my professional opinion proposed improvements
presented in this drainage memo will not negatively impact downstream private properties adjacent
to the alley before draining into either First or Second Street right-of-way to the west and east,
respectively.
116 First Street Drainage Memo Keefe Civil Project No.: 2023-027
Page 3 of 4 November 12, 2024
The total site area, including the adjacent half right-of-way, is ~9,408 square feet (sf). As indicated
in the table below, the adjusted site imperviousness during the first phase of site development will
increase by 2,260 sf . During this first phase of the project, prior to construction of the future
garage and concrete driveway, the site will have a 48% adjusted site imperviousness.
Site Hardscape or Hard Surface
Draining Towards Alley
Historic Site
Area (sf)
Adjusted
Historic Area
(sf)
Phase One
Developed Site
Area (sf)
Adjusted Phase One
Developed Area (sf)
Concrete/Asphalt (100%) 2,140 2,140 2,240 2,240
Rooftop (100%) 0 0 2,160 2,160
Gravel (40%) 265 106 265 106
Total Impervious Area 2,405 2,246 4,665 4,506
Adjusted Site Imperviousness 2,260
As indicated in the table below, the adjusted site imperviousness will increase by 3,490 sf with the
full build-out of the project. The garage plans were preliminary at the time of the report, but the
roof area was assumed to be 1,040 sf and a concrete driveway area outside of the roof line was
estimated at 190 sf. With construction of the proposed home and future garage, the final adjusted
site imperviousness for the site, including the adjacent half right-of-way, is 61%.
Site Hardscape or Hard Surface
Draining Towards Alley
Historic Site
Area (sf)
Adjusted
Historic Area
(sf)
Full Build-Out
Developed Site
Area (sf)
Adjusted Full Build-
Out Developed Area
(sf)
Concrete/Asphalt (100%) 2,140 2,140 2,430 2,430
Rooftop (100%) 0 0 3,200 3,200
Gravel (40%) 265 106 265 106
Total Impervious Area 2,405 2,246 5,895 5,736
Adjusted Site Imperviousness 3,490
As provided by the City of Fort Collins’ engineering department, the current Poudre River Basin
RiskMap Study identified the project area as being within Basin 57. Per pages 491-493 of this
study, the curve number for Basin 57 was identified as 90. According to Exhibit B, the site is
classified within HSG C. Utilizing these preceding values and the following chart provided in the
study, the study assumed a 65% impervious area within Basin 57; therefore, the site will not
exceed the Poudre River Basin master-planned imperviousness and does not require detention. If
the combined garage roof and concrete driveway area exceeds 1,610 sf (65% imperviousness), the
site will be required to provide detention.
116 First Street Drainage Memo Keefe Civil Project No.: 2023-027
Page 4 of 4 November 12, 2024
Based on the enclosed design, it is my professional opinion that the additional runoff generated by
these site improvements will not cause adverse impacts to downstream private properties. I hereby
attest that this letter for the final drainage design for 116 First Street 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.
Enclosures
Cc: Mitchell A. Greeno, Philgreen Construction Inc.
11/12/24
EXHIBIT A
DSDS
DS
DS
DS DS
49
5
1
49
5
1
49
5
2
4952
49
5
1
49
5
0
49
5
1
49
5
2
N0
°
2
7
'
2
5
"
E
50
.
0
0
'
N89°32'35"W
127.00'
N89°32'35"W
127.00'
N0
°
2
7
'
2
5
"
E
50
.
0
0
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1 OF 1
GR
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PROPOSED BUILDING
FF=4953.98
TOF = 4953.00
PROPERTY LINE
PROPERTY LINE
PR
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Y
L
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AL
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PROPERTY LINE / ROW LIMITS
FUTURE GARAGE
TOF = 4952.44
ALLEY
DETAIL B
FI
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DETAIL A DETAIL C
11/12/24
EXHIBIT B
Hydrologic Soil Group—Larimer County Area, Colorado
(116 First Street)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
9/30/2024
Page 1 of 4
44
9
3
1
7
9
44
9
3
1
8
5
44
9
3
1
9
1
44
9
3
1
9
7
44
9
3
2
0
3
44
9
3
2
0
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44
9
3
1
7
9
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9
3
1
8
5
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9
3
1
9
1
44
9
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1
9
7
44
9
3
2
0
3
44
9
3
2
0
9
494308 494314 494320 494326 494332 494338 494344 494350 494356 494362
494308 494314 494320 494326 494332 494338 494344 494350 494356 494362
40° 35' 22'' N
10
5
°
4
'
2
'
'
W
40° 35' 22'' N
10
5
°
3
'
5
9
'
'
W
40° 35' 21'' N
10
5
°
4
'
2
'
'
W
40° 35' 21'' N
10
5
°
3
'
5
9
'
'
W
N
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 10 20 40 60
Feet
0 3 7 14 21
Meters
Map Scale: 1:254 if printed on A landscape (11" x 8.5") sheet.
Soil Map may not be valid at this scale.
MAP LEGEND MAP INFORMATION
Area of Interest (AOI)
Area of Interest (AOI)
Soils
Soil Rating Polygons
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Lines
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Soil Rating Points
A
A/D
B
B/D
C
C/D
D
Not rated or not available
Water Features
Streams and Canals
Transportation
Rails
Interstate Highways
US Routes
Major Roads
Local Roads
Background
Aerial Photography
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 18, Aug 24, 2023
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.
Hydrologic Soil Group—Larimer County Area, Colorado
(116 First Street)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
9/30/2024
Page 2 of 4
Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
64 Loveland clay loam, 0 to
1 percent slopes
C 0.2 100.0%
Totals for Area of Interest 0.2 100.0%
Description
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
Hydrologic Soil Group—Larimer County Area, Colorado 116 First Street
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
9/30/2024
Page 3 of 4
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.
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.
Hydrologic Soil Group—Larimer County Area, Colorado 116 First Street
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
9/30/2024
Page 4 of 4
EXHIBIT C
National Flood Hazard Layer FIRMette
0 500 1,000 1,500 2,000250
Feet
Ü
SEE FIS REPORT FOR DETAILED LEGEND AND INDEX MAP FOR FIRM PANEL LAYOUT
SPECIAL FLOOD
HAZARD AREAS
Without Base Flood Elevation (BFE)
Zone A, V, A99
With BFE or DepthZone AE, AO, 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 mileZone X
Future Conditions 1% Annual
Chance Flood HazardZone X
Area with Reduced Flood Risk due to
Levee. See Notes.Zone X
Area with Flood Risk due to LeveeZone D
NO SCREEN Area of Minimal Flood Hazard Zone X
Area of Undetermined Flood HazardZone D
Channel, Culvert, or Storm Sewer
Levee, Dike, or Floodwall
Cross Sections with 1% Annual Chance
17.5 Water Surface Elevation
Coastal Transect
Coastal Transect Baseline
Profile Baseline
Hydrographic Feature
Base Flood Elevation Line (BFE)
Effective LOMRs
Limit of Study
Jurisdiction Boundary
Digital Data Available
No Digital Data Available
Unmapped
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 10/12/2023 at 10:07 AM 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.
Legend
OTHER AREAS OF
FLOOD HAZARD
OTHER AREAS
GENERAL
STRUCTURES
OTHER
FEATURES
MAP PANELS
8
B 20.2
The pin displayed on the map is an approximate
point selected by the user and does not represent
an authoritative property location.
1:6,000
105°4'20"W 40°35'35"N
105°3'43"W 40°35'7"N
Basemap Imagery Source: USGS National Map 2023
4,514
752.3
FCMaps
This map is a user generated static output from the City of Fort Collins FCMaps
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
572.0
1:
WGS_1984_Web_Mercator_Auxiliary_Sphere
Feet572.00286.00
Notes
Legend
3,430
Street Names
FEMA Floodplain
FEMA High Risk - Floodway
FEMA High Risk - 100 Year
FEMA Moderate Risk - 100 / 500 Year
City Floodplains
City High Risk - Floodway
City High Risk - 100 Year
City Moderate Risk - 100 Year
City Limits
EXHIBIT D
DSDS
DS
DS
DS DS
49
5
1
4952
4952
49
5
2
FIR
S
T
S
T
R
E
E
T
CONCRETE ALLEY
(16' ROW)
DIR
T
A
L
L
E
Y
(1
6
'
R
O
W
)
SE
C
O
N
D
S
T
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E
T
FIR
S
T
S
T
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E
E
T
CONCRETE ALLEY
(16' ROW)
DIR
T
A
L
L
E
Y
(1
6
'
R
O
W
)
SE
C
O
N
D
S
T
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DR
A
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N
A
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X
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LEGEND:
1 OF 1
FOR DRAINAGE REVIEW ONLY
NOT FOR CONSTRUCTION
HISTORIC DRAINAGE BASINS AND EXISTING ALLEY WAY CONDITIONS
DEVELOPED DRAINAGE BASINS
PROPOSED
BUILDING
PROPERTY LINES
PROPERTY LINES
FUTURE GARAGE
DSDS
DS
DS
DS DS
49
5
1
49
5
1
49
5
2
4952
49
5
1
49
5
0
49
5
1
49
5
2
N0
°
2
7
'
2
5
"
E
50
.
0
0
'
N89°32'35"W
127.00'
N89°32'35"W
127.00'
N0
°
2
7
'
2
5
"
E
50
.
0
0
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1 OF 1
GR
A
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P
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A
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PROPOSED BUILDING
FF=4953.98
TOF = 4953.00
PROPERTY LINE
PROPERTY LINE
PR
O
P
E
R
T
Y
L
I
N
E
AL
L
E
Y
PROPERTY LINE / ROW LIMITS
FUTURE GARAGE
TOF = 4952.44
ALLEY
DETAIL B
FI
R
S
T
S
T
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E
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T
DETAIL A DETAIL C
11/12/24
Grading/Stormwater Plan Accepted
(Building Permit Released)
Accepted by:
Date:
Dan Mogen
11/15/2024