HomeMy WebLinkAbout3516 S MASON MEDICAL AND RETAIL MARIJUANA STORE - BDR200011 - SUBMITTAL DOCUMENTS - ROUND 3 - DRAINAGE REPORT
July 1, 2021
Dan Mogen, P.E.
City of Fort Collins Utilities – Stormwater Engineering
700 Wood Street
Fort Collins, CO 80521
RE: 3516 Mason Street BDR – Drainage Letter
JVA Job No. 3233c
Dear Dan:
This letter has been prepared to summarize the drainage concept, solutions and changes in
impervious area associated with the site improvements for the 3516 Mason Street BDR
redevelopment project located at 3516 South Mason Street in the City of Fort Collins.
Existing Drainage Patterns
The existing site, re-platted as Lot 15 of the South Mesa Subdivision and adjoining tract, is
predominantly paved asphalt and consists of the existing Mason Street Plaza building, drive aisles,
and parking areas. The existing ground surface is relatively flat, generally sloping either west or
east at grades ranging from 0.5% to 4.5%. All stormwater runoff is conveyed via surface flow
either to the west into the public right of way or east into the Larimer County No. 2 Canal. See
Figure 1: Historic Drainage Map.
The site is located in FEMA Flood Zone X, area of minimal hazard, per FEMA Flood Insurance
Rate Map number 08069C0987G, effective date 05/02/2012.
Proposed Drainage Patterns
Proposed drainage patterns are to remain the same as the existing drainage patterns. In general, the
proposed redevelopment of the site consists of reusing the existing building, restriping the south
parking area, removing the northern access to the site and the northwest parking area, repairing
public curb, gutter, and sidewalk, and constructing a courtyard and greenspaces on the west side
of the building. The site has been divided into three hydrologic basins for analysis. Basin A
represents the area of the site draining to the right of way of Mason Street. Basin B represents the
portion of the site draining to the east into the Larimer County No. 2 Canal. Basin C represents the
small area of roof and landscaping along the north which drains offsite, and eventually into the
Larimer County No. 2 Canal.
All basins eventually convey stormwater to Fossil Creek. It follows that the proposed
improvements will substantially decrease the total impervious area on the site, from 87.6% to
66.4%. See Figure 2: Developed Drainage Map for the drainage basin boundaries and a summary
of the existing and proposed impervious areas. Hydrologic and Hydraulic calculations are
attached.
The proposed development of this site will serve to improve water quality and reduce the peak
stormwater runoff events by removing a large portion of paved areas and replacing with vegetated
buffer zones. The significant reduction in impervious area will facilitate sedimentation and
infiltration while limiting erosion.
3516 Mason Street BRD - Preliminary Drainage Letter
July 1, 2021
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The site is severely constrained by its small size, shared parking and access, the lack of public
stormwater infrastructure, the existing developments surrounding it, and the Larimer County No.
2 Canal. Due to these constraints, the redevelopment proposes providing alternative measures as
best feasible to comply with the City of Fort Collins water quality and stormwater standards. Four
landscape buffers will be constructed to replace what was previously asphalt and improve upon
existing conditions. These reductions in asphalt will reduce the areas contributing to pollution from
vehicular traffic such as oils, gas, and other pollutants, reduce the heat island effect, and will be
replaced with vegetated buffers that provide natural and manicured landscape areas. The removal
of paved areas also contributes to improvements in storm water quality by minimizing directly
connected impervious areas (MDCIA) as per the USDCM Volume 3 for employing runoff
reduction practices.
The first landscape buffer is created by removing the northwestern asphalt area where there is
currently a drive access and parking area. This landscape buffer is approximately 1,900 square
feet, slopes at just under 2% towards Mason Street and is 50’ across from north to south and 20’
deep from east to west. This buffer will accept flows from approximately 620 square feet including
the concrete patio on the west side of the building. Flows entering this buffer will be sheet flow.
This vegetated buffer zone will allow infiltration, sedimentation, and pollutant uptake in the
grasses, mulch, and plantings before draining to the right-of-way. The area on the northwestern
side of the site proposes a large area of shrubs which will greatly increase the soil stabilization in
this area increasing the quality of stormwater by decreasing the amount of potential erosion.
The second landscape buffer is created by removing the existing crushed asphalt and paved area
on the east side of the site. This landscape buffer is approximately 1,820 square feet, slopes at just
over 2% towards the canal, and is 75’ across from north to south and 22’ deep from east to west.
This buffer will accept flows from approximately 5,000 square feet of roof and 4,100 square feet
of parking area. Flows entering this buffer will be both concentrated and sheet flow. This area also
includes the Natural Habitat Buffer Zone (NHBZ) with native seed per recommendations of the
environmental report. The eastern area of the site proposes a large area of native shrubs and grasses
as well as a deciduous tree which will greatly increase the stabilization of the soil increasing the
quality and clarity of stormwater before entering the Canal. The plantings in this area also serve to
improve stormwater quality by absorbing pollutants from the site.
The third landscape buffer is created on the north side of the site. The existing site provides a few
trees, but nothing in addition to slow flow rates and allow for sedimentation and infiltration before
flowing offsite and ultimately into the Canal. The development proposes to line the northern
property line with shrubs. The shrubs in this landscape buffer provide a means of slowing
stormwater flows to provide increased infiltration and sedimentation not previously provided. The
slowed flows allow for the plantings in this area to take in additional pollutants from the site before
they drain offsite to the Canal. This buffer is approximately 920 square feet, slopes at 1.5% towards
the northeast, and is 115’ across from east to west and about 10’ deep from north to south. This
buffer will accept flows via downspouts from the roof area of approximately 6,000 square feet. A
portion of this buffer connects to the eastern buffer and NHBZ.
The fourth landscape buffer is created by removing asphalt at the southwestern side of the site
south of the new sidewalk connection. This buffer is approximately 340 square feet, slopes to the
east at 1.3% and is 22’ across from north to south and 16’ deep from east to west. This buffer
accepts flow from approximately 300 square feet of the sidewalk and plaza area. Replacing asphalt
with a landscape buffer discourages parking on the eastern side of the site which previously
accommodated parking, reducing the stormwater quality impacts of vehicular pollutants.
3516 Mason Street BRD - Preliminary Drainage Letter
July 1, 2021
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To the maximum extent practical, this project has implemented landscaping buffer zones and
minimized directly connected impervious areas as alternative compliance methods to improve
water quality runoff, promote infiltration, and reduce peak runoff volume. Approximately 82% of
the site area will enter a landscape buffer area prior to discharging from the site.
Erosion and Sediment Control
During construction, temporary erosion and sediment control practices will be used to limit soil
erosion and sediment discharge off the site.
Please feel free to contact me to discuss any questions you may have.
Sincerely,
JVA, INCORPORATED
By: ________________________________
Erik Nakos, P.E.
Senior Project Manager
Associate
Enclosures: Figure 1: Historic Drainage Map, Figure 2: Developed Drainage Map, Hydrologic
Soil Rating, FEMA Firm Map, Rational Calculations, Variance Request