HomeMy WebLinkAboutDrainage Reports - 04/24/2020January 20, 2020
Mr. Matt Simpson
City of Fort Collins Stormwater Department
700 Wood Street
Fort Collins, CO 80521
RE: Drainage Memorandum Poudre Valley Hospital Harmony Campus Cancer
Center LINAC Expansion
Dear Matt:
The purpose of this drainage memorandum is to summarize the proposed drainage design and
address the additional impervious area runoff being added by the 4,782 square foot Cancer
Center Expansion area and the addition of 19 parking stalls and associated 24 wide drive
aisle, which will be located off the northeast corner of the existing Redstone POB parking
lot. The building expansion will replace existing landscaping and hardscaping areas, which
includes rock, grass, and miscellaneous shrubs and plants, trees, existing concrete sidewalks
and curb and gutter, and asphalt. The parking stall addition will replace existing grass areas
that are not currently landscaped. An LID basin will be installed east of the northeast
parking stall addition area to provide LID treatment for approximately 11,881 sf of
impervious area and accounts for the new parking lot area as well as a portion of the existing
parking lot area, with both areas being tributary to the LID basin. This contributing area is
greater than the 8,769 sf of new impervious area, due to the LINAC building expansion and
for the 19-parking stall addition. Please refer to the attached Grading, Drainage, and Erosion
Control Plan, which accompanies this memorandum.
In general, the project will consist of an approximate 4,782 square foot building expansion
area (4,692 square feet measured footprint) on the northwest wing of the existing Poudre
Valley Hospital Building (Existing Radiation/ Oncology section of the building) as well as
the 19-parking stall addition to offset the 9 spaces lost by the LINAC building expansion.
The project will entail construction of the new building addition and adjacent private drive
re-alignment, along with minor site grading, sidewalk, landscaping, and drainage
improvements around and adjacent to the new addition. The 19-parking stall addition area
will entail new concrete curb and gutter and asphalt paving for the stalls and associated drive
aisle, with landscaping added to the interior islands and around and within the LID treatment
basin, as applicable.
City of Fort Collins Approved Plans
Approved by:
Date:
Matt Simpson
04/24/2020
The proposed building expansion will replace approximately 2,548 and 2,144 square feet of
existing landscaped and hardscaped areas, respectively. It is important to note that previous
major amendment plans (sheet 2B of 6 from BHA Design, Inc., revised November 19, 1999)
included this part Furthermore, the approved
grading and drainage masterplan and SWMM Model for the PVHS Campus (PVHS Second
Filing) provided for the runoff from the entire PVHS Campus being ultimately routed to the
southeast corner of the PVHS Campus to the existing detention/water quality pond,
constructed with the PVHS Second Filing project.
The masterplan SWMM model provided for the entire PVHS Campus having an
imperviousness of 80%. Therefore, it appears the 4,782 square foot building expansion and
6,952 square foot parking lot addition are within these parameters and have little effect on the
existing storm drainage system or existing appurtenances, due to the long path of travel (time
of concentration) in reaching the existing pond.
To evaluate the LID size requirements, the new construction impervious area was compared
to the actual tributary area draining to the LID. The additional impervious area for the
building expansion (2,548 sf) and the proposed east parking lot (6,221 sf) results in a total
area of 8,769 sf. The total area contributing runoff to the LID from the existing parking lot
(5,660 sf) and the parking lot expansion (6,221 sf) results in a total area of 11,881 sf.
Therefore, the greater area of 11,881 sf was used to size the LID. A basin drainage map is
provided as an attachment with this letter and included LID sizing information.
It s important to note that runoff from the existing parking lot and 19-stall parking lot
addition will sheet flow east, through an 8- inch wide curb cut, and discharge to the proposed
LID Basin. The LID Basin has been intentionally sized for approximately 11,881 SF of
impervious area, which is greater than the net increase in impervious area from the building
expansion and parking lot addition areas. There is no physical way to provide direct
conveyance of the newly added impervious area from the LINAC building expansion area to
the proposed LID Basin. However, flow from the LINAC expansion area will continue to
drain east and into the existing 25 Type R Inlet, located approximately 185 east of the
LINAC expansion in the existing parking lot, as historically occurs. The additional 0.44 cfs
flow from the newly added impervious areas of the LINAC expansion area, is considered
negligible, and will not adversely affect the capacity of the existing 25 Type R Inlet.
In speaking with the City of Fort Collins Stormwater Department, rational calculations
comparing historic versus the proposed condition are not required due to addition fitting
within the parameters of the existing SWMM Modeling and Drainage Report from the PVHS
Second Filing project. However, the runoff from the existing and proposed conditions were
compared using a total disturbance area of 16,061 square feet and a minimum time of
concentration of 5 minutes. The 100-year runoff under the exiting condition is calculated at
approximately 3.23 cfs (C=0.70, I = 9.95 in/hr), versus the proposed addition runoff being
approximately 3.67 cfs (C = 0.80, I = 9.95in/hr). Therefore, the proposed 4,782 square foot
addition will provide approximately 0.44 cfs of additional runoff, which is considered
negligible.
Stormwater runoff from the entire roof of the new addition will be captured by internal roof
drains. The new roof area runoff will combine with the existing roof drains and discharge to
Capacity (cfs)
1/16/2020
Project: Disturbed Acres:0.53
EROSION CONTROL BMPs Units
Estimated
Quantity
Unit
Price
Total
Price
EA 3 $40.00 $120.00
EA 5 $300.00 $1,500.00
EA 1 $1,100.00 $1,100.00
EA 2 $500.00 $1,000.00
AC 0.15 $500.00 $75.00
Sub-Total: $3,795.00
1.5 x Sub-Total: $5,692.50
Amount of security: $5,692.50
Total Acres x Price/acre: $530.00
$1,000.00 Sub-Total: $530.00
1.5 x Sub-Total: $795.00
Amount to Re-seed: $795.00
Minimum escrow amount: $3,000.00
Erosion Control Escrow:$5,692.50
Miniumum Escrow Amount
UCH Linac
Unit Price of Seeding per acre:
the cost to re-vegetate the disturbed land to dry land grasses based upon unit cost determined by the City's Annual Revegetation and Stabilization Bid,
whichever is greater. In no instance, will the amount of security be less than one thousand five hundred dollars ($1,500) for residential development or three
Inlet Protection (IP)
Concrete Washout (CW)
Final Escrow Amount
Erosion Control Escrow Estimate
BMP Amount
Gravel Curb Check Bags
Vehicle Tracking Control (VTC)
Hay or Straw Dry Mulch (1-5%)
Reseeding Amount
1/16/2020 3:42 PM A:\AspenProjects\400-003-UCH Harmony LINAC\Design\Drainage\Erosion_Escrow-Linac.xls