HomeMy WebLinkAboutDrainage Reports - 07/11/2022HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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HCI ENGINEERING
A division of
HABERER CARPENTRY INC.
DRAINAGE REPORT
FOR:
Planetscale Living Affordable Housing Project
906 E. Stuart Street, Fort Collins, CO
Owner:
Planetscale Limited
ATTN: Serdar Badem
12933 Regan Lane, Saratoga, CA
Serdar.Badem@Gmail.com
Engineer Contact(s):
HCI Engineering
T. Garrett Goodlin, P.E.
621 Southpark Dr., Suite 1600
Littleton, CO 80120
PH: 303-979-3900
FX: 303-278-7814
6/2/2022
Estimated Project Dates:
Project Start Date: 7/1/2022 Project Completion Date: 12/31/2022
City of Fort Collins Approved Plans
Approved by:
Date:
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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Table of Contents:
I. General Location & Description ................................................................................................................3
A. Project Location .............................................................................................................................. 3
B. Description of Property .................................................................................................................. 3
C. Topographic Data............................................................................................................................ 4
D. FEMA and Flood Zone Data ......................................................................................................... 4
E. Soils Information ............................................................................................................................. 4
II. Drainage Facility Design ............................................................................................................................4
A. Hydrological Criteria ....................................................................................................................... 4
III. Stormwater Management Facility Design .................................................................................................5
A. Stormwater Conveyance Facilities ................................................................................................. 5
B. Easements ....................................................................................................................................... 5
C. Compliance with Standards ............................................................................................................ 6
D. Maintenance Plan ....................................................................................................................... 6
E. Conclusion ....................................................................................................................................... 6
IV. References .................................................................................................................................................6
Appendix A, Maps & Reference Materials .............................................................................................................7
Appendix B, Hydrology Calculations ......................................................................................................................8
Appendix C, Drainage Plan .....................................................................................................................................9
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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I. General Location & Description
A. Project Location
The Site, an area consisting of 0.32 acres, is located at 906 E. Stuart Street, Fort Collins, CO,
which is more precisely located in a part of Tract 7, East Acres Subdivision, lying in the NE
Quarter of Section 24, Township 7 North, Range 69 west of the 6th Principal Meridian, City of
Fort Collins, County of Larimer, State of Colorado. The Site is bordered by Stuart Street Right-of-
Way to the south, and existing residential land uses to the west, north and east. Refer to the
following map.
SITE MAP
The Site is approximately located at
Latitude, 40 33 49 Longitude, 105 03 40
B. Description of Property
The Site is currently developed with an existing two-story residence with associated driveway
and yard. Site improvements will consist of a proposed Four Unit townhome with associated
permeable paver drive and parking, with yard. As part of this development, construction
activities will consist of a new structure built on the existing foundation, utility installation, ,
paving, fine grading and landscaping. In addition to this, the site driveway and parking lot are
proposed to be constructed of permeable pavers. The Site disturbed area and construction
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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limits total approximately 0.28 acres. The existing and developed Site and basin percent
impervious values are listed in Table 1 below:
Table 1
ONSITE % Impervious
Existing Site
Proposed Site
50.8% (0.32 acres)
40.5% (0.32 acres)
C. Topographic Data
The General Existing Site Topographic Conditions for The Site are itemized in Table 2 below:
Table 2
Description Result Location of Occurrence
Highest Elevation 4982 South of Site
Lowest Elevation 4980 North of Site
Steepest Slope 25% East limits
Average Slope 1 - 3% Most of The Site
D. FEMA and Flood Zone Data
According to the FEMA Flood Insurance Rate Map: Community-Panel No. 08069C0983H,
Effective May 2, 2012, The Site is located in Zone X unshaded. These are areas of 0.2% annual
chance of flood; areas of 1% annual chance flood with average depths of less than 1 foot or
with drainage areas less than 1 square mile; and areas protected by levees from 1% annual
chance flood. The Regulatory Flood Elevation adjacent to The Site is coincident with the project
survey elevation of 4953 (worst-case scenario).
E. Soils Information
Soil Classifications for The Site are reported by N.R.C.S. as consisting of Type C soil for 100% of
The Site (Santana loam, 1 to 3 percent slopes) Refer to Attachments for the includ .R.C.S.
Websoil Survey Repo averaging 0.32, The Site has a high to susceptibility to
sheet and rill erosion by water. With a Wind erodibility group of 5 The Site has a low
susceptibility to erosion due to wind.
II. Drainage Facility Design
A. Hydrological Criteria
Due to the size of this site, the rational method was used to determine the runoff for the 10-yr
and 100-yr storm event. (See Appendix B) The rainfall intensities given in the Urban Storm
Drainage Criteria Manual (USDCM) are as follows:
TABLE 3
1-HOUR POINT RAINFALL VALUES FOR CITY OF FORT COLLINS (INCHES)
2-YR 5-YR 10-YR 100-YR
0.83 1.09 1.40 2.86
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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III. Stormwater Management Facility Design
A. Stormwater Conveyance Facilities
There are no existing drainage structures or features on the site. Flows are directed east away
from the building toward the existing concrete driveway, or west to the existing side yard.
Flows then sheet flow to the northeast off the property. Flows then enter an existing
landscaped swale contained in the property to the north, which directs flows east to the
Springmeadows Ct. Right-of-Way. Flows are then routed by curb and gutter to the north where
they either enter a single curb inlet, or overtop the curb to Spring Creek.
The historic drainage patterns are mimicked in the proposed condition. Flows produced on the
north and east are directed to the new permeable pavers. Flows produced on the west and
south side of the property are directed away from the building to one of two new landscape
swales with underdrain to ensure adequate slope away from the structure. The underdrains
direct the flows to the permeable pavers. Once flows enter the permeable pavers they will
either infiltrate through the system or be directed to the northeast off of the property,
matching the historic condition. Due to the permeability of the new pavers, the volume and
rate of flows are both reduced in the proposed condition.
Historic Direct Runoff Comparison
The site has been evaluated as a single basin of 0.32 acres. The composite percent impervious
of the existing site is 50.8% and the 10-year and 100-year flows are 0.23 & 0.63 CFS,
respectively. The composite percent impervious of the proposed site is 40.5% (10.3% less) and
the 10-year and 100-year flows are 0.69 & 0.95 CFS (0.02 CFS and 0.02 CFS less), respectively.
The change in the Direct Peak Runoff values for The Site for the 10 and 100-year storm events
are shown in the following Table.
TABLE 4
DIRECT RUNOFF SUMMARY
Area Area % Imp.
Time of
Concentration
Runoff
Coefficient
Direct Basin
Runoff
Design (Acres) C10 C100 Q10 (cfs)
Q100
(cfs)
HISTORIC 0.32 50.8% 6.64 0.51 0.69 0.71 0.97
TOTAL PROPOSED 0.32 40.5% 6.75 0.50 0.68 0.69 0.95
B. Easements
There is an existing HOA easement directly east of this property, between Spring Meadow
Court ROW and The Site. The HOW has stated that it will not allow any construction and/or
stormwater work in this area and is also not willing to grant any drainage easements to
accommodate any increase in flows.
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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C. Compliance with Standards
The proposed Site has been evaluated using the rational method as set forth by UDSCM. Both
the 100-yr and 10-yr storms have been evaluated. The stormwater conveyance system has
been designed to comply with conveyance methods and capacities. This project will be
complying with the Low Impact Development drainage requirements set forth in the City of Fort
Collins Stormwater Criteria Manual LID Implementation Manual. (See Appendix A) The project
will comply with both options for the implementation of LID systems, as all new private
impervious area will be treated by the proposed permeable pavers. The required detached
public sidewalk drains south to the Stuart Street ROW.
D. Maintenance Plan
The owner will compete a monthly maintenance inspection to clean and remove and trash,
debris and vegetation that is restricting or not allowing permeable pavers, landscape areas and
underdrains to drain. This will include mowing or trimming of landscaped areas. Any repairs to
the grading because of washout and/or sedimentation will be removed.
E. Conclusion
The proposed project has been analyzed utilizing the City of Fort Collins Stormwater Criteria
Manual. This drainage letter and associated calculations are limited to the project shown by
the attached Drainage Plan. Based on the above findings and attached calculations HCI
Engineering finds the proposed development to be in compliance with the drainage criteria set
forth in Report 1. No negative impacts are anticipated as a result of this development to
adjacent properties or downstream.
Sincerely,
T. Garrett Goodlin, P.E.
HCI Engineering
IV. References
1. City of Fort Collins Stormwater Criteria Manual (December 2018), by The City of Fort Collins.
2. Drainage Criteria Manual (Volume 1, 2 & 3), by Mile High and Flood Control District, current
versions.
3. FEMA website Flood Insurance Rate Maps.
4. USDA, Web Soil Survey, Soil Map Data
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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Appendix A, Maps & Reference Materials
FEMA Firmette
NRCS Soil Map
NRCS Soil Data
CFCSCM Excerpt
o Low Impact Development
o Runoff Coefficients
o Percent Impervious Values
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
3.0 Rational Method
3.2 Runoff Coefficients
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3.2 Runoff Coefficients
Runoff coefficients used for the Rational Method are determined based on either overall land use or
surface type across the drainage area. For Overall Drainage Plan (ODP) submittals, when surface types
may not yet be known, land use shall be used to estimate flow rates and volumes. Table 3.2-1 lists the
runoff coefficients for common types of land uses in the City.
Table 3.2-1. Zoning Classification - Runoff Coefficients
Land Use Runoff Coefficient (C)
Residential
Urban Estate 0.30
Low Density 0.55
Medium Density 0.65
High Density 0.85
Commercial
Commercial 0.85
Industrial 0.95
Undeveloped
Open Lands, Transition 0.20
Greenbelts, Agriculture 0.20
Reference: For further guidance regarding zoning classifications, refer to the Land Use
Code, Article 4.
For a Project Development Plan (PDP) or Final Plan (FP) submittals, runoff coefficients must be based on
the proposed land surface types. Since the actual runoff coefficients may be different from those
specified in Table 3.2-1, Table 3.2-2 lists coefficients for the specific types of land surfaces.
FORT COLLINS STORMWATER CRITERIA MANUAL Hydrology Standards (Ch. 5)
4.0 SWMM
4.1 Input Parameters
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Table 4.1-2. Land Use - Percent Impervious
Land Use
Percent Impervious
(%)
Residential
Urban Estate 30
Low Density 50
Medium Density 70
High Density 90
Commercial
Commercial 80
Industrial 90
Undeveloped
Open Lands, Transition 20
Greenbelts, Agriculture 2
Offsite Flow Analysis (when
Land Use not defined) 45
Reference: For further guidance regarding zoning classifications, refer to the Land Use
Code, Article 4.
For Final Plan (FP) submittals, impervious values must be based on the proposed land surface types.
Refer to Table 4.1-3 for recommended percent impervious values.
Table 4.1-3. Surface Type – Percent Impervious
Surface Type
Percent Impervious
(%)
Hardscape or Hard Surface
Asphalt, Concrete 100
Rooftop 90
Recycled Asphalt 80
Gravel 40
Pavers 40
Landscape or Pervious Surface
Playgrounds 25
Lawns, Sandy soil 2
Lawns, Clayey soil 2
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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Appendix B, Hydrology Calculations
Rational Calculations
o Percent Impervious
o Time of Concentration
o Direct Runoff 10-Year Storm
o Direct Runoff 100-Year Storm
o Direct Runoff Summary & routed Runoff Summary
HCI Engineering, - 621 SouthPark Dr., Suite 1600, Littleton CO, 80120 - 720-252-3484
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Appendix C, Drainage Plan