HomeMy WebLinkAboutDrainage Reports - 05/18/2022 (2)
KEEFE CIVIL, INC
3125 Crockett Street
Fort Collins, Colorado 80526
(970) 215-6808
meg@keefecivil.com
July 14, 2021 Keefe Civil Project Number: 2021-006
Matt Simpson
Development Review Engineer
Water Utilities Engineering
City of Fort Collins
700 Wood Street
Fort Collins, Colorado 80522
RE: 613 South Meldrum Street Drainage Memo
FDP200028
Dear Matt:
The purpose of this letter is to describe the proposed site improvements and potential drainage
impacts at 613 South Meldrum (Exhibit A). The 0.217-acre project site is a developed single-
family residential lot with established landscaping located on South Meldrum Street between
Myrtle Street and Laurel Street in Fort Collins, Colorado. The site is located within the Old Town
drainage basin. Current conditions at the site include a single-story house, a small garden shed,
flagstone pathways, concrete and wood patio areas, established trees, several garden areas, and a
brick driveway.
The lot is bound by existing single-family residential properties on the north and south lot lines,
South Meldrum Street right-of-way to the east, and a dirt alley way 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. Also, the site is
outside of all City of Fort Collins mapped floodplains. (Exhibit C)
Historically, site stormwater has drained with gentle slopes from the west to east into South
Meldrum Street right-of-way, when not infiltrated into established landscaping (historic basin A).
Based on field survey and visual inspections, the alley drains north from a high point located
adjacent to the southwest corner of the site. Historic and developed drainage basins are shown in
Exhibit D.
The proposed site improvements include a two-story carriage house with a basement and an
attached garage with access off the alley. The proposed driveway shall be constructed with gravel
and the existing garden shed will be removed prior to construction. All carriage house roof drains,
aside from one, are proposed to drain towards South Meldrum Street right-of-way, minimizing
stormwater flows to the alley way. A small drainage basin (<350 sf adjusted impervious area) west
of the carriage house is proposed to drain into the existing alley.
613 South Meldrum Street Drainage Memo Keefe Civil Project No.: 2021-006
Page 2 of 3 July 14, 2021
Hardscape or Hard Surface Towards Alley Developed Site Area (sf) Adjusted Developed Area (sf)
Concrete 100 100
Rooftop (90%) 80 72
Gravel (40%) 440 176
Total Impervious Area 620 348
Based on visual inspections of the alley and field survey by others, 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 Myrtle Street right-of-way to the north.
Figure 1: Adjacent alley way looking north towards Myrtle Street
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. Since the lot’s total adjusted impervious area increase is below 5,000 square feet (sf), water
quantity detention requirements are not required for this development.
Overall Site Hardscape or Hard
Surface
Historic Site
Area (sf)
Adjusted
Historic
Area (sf)
Developed
Site Area (sf)
Adjusted
Developed
Area (sf)
Concrete 500 500 850 850
Rooftop (90%) 1,540 1,386 3,240 2,916
Gravel, Patio, and Pavers (40%) 1,150 460 1,320 528
Total Impervious Area 3,190 2,346 5,410 4,294
Adjusted Site Imperviousness 1,948
613 South Meldrum Street Drainage Memo Keefe Civil Project No.: 2021-006
Page 3 of 3 July 14, 2021
The total site imperviousness is 21%, which is less than the 55% allowed 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 from the site will increase with
development as indicated in the summary table below.
2-year 10-year 100-year
Basin A Historic Runoff 0.17 cfs 0.29 cfs 0.80 cfs
Basin A Developed Runoff 0.03 cfs 0.05 cfs 0.13 cfs
Basin B Developed Runoff 0.01 cfs 0.01 cfs 0.03 cfs
Basin C Developed Runoff 0.02 cfs 0.03 cfs 0.08 cfs
Basin D Developed Runoff 0.11 cfs 0.19 cfs 0.60 cfs
Basin E Developed Runoff 0.14 cfs 0.25 cfs 0.72 cfs
Developed drainage basins B and C will allow overland drainage flows onto private properties to
the north and south; however, there will be no concentrated flows from the proposed site
improvements and the flows will be minimal, as indicated above.
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 hereby attest that this letter for the final drainage design for 613 South Meldrum 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: Jeff Palomo, Property Owner
References
Fort Collins Stormwater Criteria Manual. 2018.
ICON Engineering. 2017. Old Town Drainage Basin EPA SWMM Conversion and Hydrologic
Update.
EXHIBIT A
PROPOSED BUILDING
TOF=5006.50
MIN GFF=5005.07
GARAGE TOF=5005.67
ROOF DRAINS MUST NOT DRAIN
ONTO PRIVATE PROPERTY TO
NORTH OR SOUTH
GRADING PLANFORT COLLINS, COLORADOSHEET613 SOUTH MELDRUM STREET UTILITY PLANSKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM3 OF 6SOUTHMELDRUMSTREETALLEYA
A
_________________________________
SECTION A-A
A
A
EXHIBIT B
Hydrologic Soil Group—Larimer County Area, Colorado
(613 South Meldrum St)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/8/2021
Page 1 of 444920624492070449207844920864492094449210244921104492062449207044920784492086449209444921024492110492936492944492952492960492968492976492984492992493000493008
492936 492944 492952 492960 492968 492976 492984 492992 493000 493008
40° 34' 47'' N 105° 5' 0'' W40° 34' 47'' N105° 4' 57'' W40° 34' 45'' N
105° 5' 0'' W40° 34' 45'' N
105° 4' 57'' WN
Map projection: Web Mercator Corner coordinates: WGS84 Edge tics: UTM Zone 13N WGS84
0 15 30 60 90
Feet
0 5 10 20 30
Meters
Map Scale: 1:355 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 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.
Hydrologic Soil Group—Larimer County Area, Colorado
(613 South Meldrum St)
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/8/2021
Page 2 of 4
Hydrologic Soil Group
Map unit symbol Map unit name Rating Acres in AOI Percent of AOI
35 Fort Collins loam, 0 to 3
percent slopes
C 0.3 100.0%
Totals for Area of Interest 0.3 100.0%
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 613 South Meldrum St
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/8/2021
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 613 South Meldrum St
Natural Resources
Conservation Service
Web Soil Survey
National Cooperative Soil Survey
3/8/2021
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 3/10/2021 at 11:54 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°5'17"W 40°35'N
105°4'40"W 40°34'33"N
Basemap: USGS National Map: Orthoimagery: Data refreshed October, 2020
2,257
376.2
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
286.0
1:
WGS_1984_Web_Mercator_Auxiliary_Sphere
Feet286.00143.00
Notes
Legend
1,715
Parcels
FEMA Floodplain
FEMA High Risk - Floodway
FEMA High Risk - 100 Year
FEMA Moderate Risk - 100 / 500 Year
FEMA Map Panel
City Floodplains
City High Risk - Floodway
City High Risk - 100 Year
City Moderate Risk - 100 Year
Growth Management Area
Parks
Schools
Natural Areas
City Limits
EXHIBIT D
DRAINAGE EXHIBITFORT COLLINS, COLORADOSHEET613 SOUTH MELDRUM STREET UTILITY PLANSKEEFE CIVIL, INC3125 CROCKETT STREETFORT COLLINS, CO 80526(970) 215-6808MEG@KEEFECIVIL.COM6 OF 6
PROPOSED
RESIDENCE
FOR DRAINAGE REVIEW ONLY
NOT FOR CONSTRUCTION
HISTORIC DRAINAGE BASINS
DEVELOPED DRAINAGE BASINS
EXISTING
RESIDENCE
EXISTING
RESIDENCE
SOUTH MELDRUM STREETALLEYSOUTH MELDRUM STREETALLEY
EXHIBIT E
Composite Runoff Coefficient Calculations613 South Meldrum Street, Fort Collins, COArea (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Area (sf)Minor (2- and 10-yr) Composite C2Major (100-yr) Composite C3Hardscape or Hard SurfaceConcrete0.955001000115160500Rooftop0.951,54080001,4651,540Gravel0.5004400000Patio and Pavers0.501,150020130565720Total Impervious Area3,190 620202452,1902,760Landscape or Pervious SurfaceLawns, Clayey Soil, Flat Slope < 2% 0.20 6,2520445001,500Lawns, Clayey Soil, Average Slope 2-7% 0.25 0280001,1150Lawns, Clayey Soil, Steep Slope >7% 0.35 00027000Total9,442 0.40 0.50 900 0.51 0.64 465 0.21 0.27 515 0.52 0.65 3,3050.64 0.80 4,260 0.61 0.76Notes:Surface TypeRunoff Coefficients1Historic Drainage Basin A3. Frequency adjustment factor per Fort Collins Stormwater Criteria Manual Table 3.2-3.1. 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.Developed Drainage Basin EDeveloped Drainage Basin A Developed Drainage Basin B Developed Drainage Basin C Developed Drainage Basin D613 Meldrum - drainage.xlsx
Drainage Basin Runoff Calculations613 South Meldrum Street, Fort Collins, COLength (ft)Slope (%)Minor to (min)Major to (min)Length (ft)Slope (%)Channel TypeVelocity (fps)tt (min)Minor Tc (min)Major Tc (min)2-yr 10-yr 100-yr 2-yr 10-yr 100-yrA 0.217 0.40 0.50 180 2 14.0 12.0 NA 14.0 12.0 1.93 3.30 7.42 0.17 0.29 0.80A 0.021 0.51 0.64 20 4 3.1 2.4 NA 5.0 5.0 2.85 4.87 9.95 0.03 0.050.13B 0.011 0.21 0.27 5 10 1.7 1.6 NA 5.0 5.0 2.85 4.87 9.95 0.01 0.010.03C 0.012 0.52 0.65 10 33 1.1 0.8 NA 5.0 5.0 2.85 4.87 9.95 0.02 0.03 0.08D 0.076 0.64 0.80 12 5 7.5 1.2 155 2% swale 2.1 1.2 8.7 5.0 2.33 3.98 9.95 0.11 0.19 0.60E 0.098 0.61 0.76 120 2 8.0 5.5 NA 8.0 5.5 2.41 4.12 9.69 0.14 0.25 0.72Notes:3. Per Fort Collins Stormwater Criteria Manual Section 3.3.4. A minimum Tc of 5 minutes is required.4. Per IDF Table for Rational Method, Table 3.4-1 Fort Collins Stormwater Criteria Manual.1. Per Fort Collins Stormwater Criteria Manual Section 3.3.2.2. Per Fort Collins Stormwater Criteria Manual Section 3.3.3.Initial/Overland Time1Travel Time2HistoricDevelopedFinal Tc3Intensity (in/hr)4BasinArea (acres)Minor Composite CMajor Composite CBasin Flow (cfs)55. Per Fort Collins Stormwater Criteria Manual Equation 5-1.613 Meldrum - drainage.xlsx