HomeMy WebLinkAboutDrainage Reports - 09/10/2021 (2)Final Drainage and Erosion Control
Report for
UCHealth-PVH Main
Entrance Improvements
Fort Collins, Colorado
April 9, 2021
City of Fort Collins Approved Plans
Approved by:
Date:
TABLE OF CONTENTS
DESCRIPTION PAGE
I. GENERAL LOCATION AND DESCRIPTION 5
A. LOCATION 5
B. DESCRIPTION OF PROPERTY 5
II. DRAINAGE BASINS 6
A. MAJOR BASIN DESCRIPTION 6
B. EXISTING SUB-BASIN DESCRIPTION 6
III. DRAINAGE DESIGN CRITERIA 7
A. REGULATIONS 7
B. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINTS 7
C. HYDROLOGIC CRITERIA 7
D. HYDRAULIC CRITERIA 7
E. VARIANCE 7
IV. DRAINAGE FACILITY DESIGN 8
A. GENERAL CONCEPT 8
B. SPECIFIC DETAILS 9
C. DETENTION POND 11
D. STREET CAPACITIES 12
V. STORM WATER QUALITY 12
A. GENERAL CONCEPT 12
B. SPECIFIC DETAILS 12
VI. EROSION CONTROL 13
A. GENERAL CONCEPT 13
VII. CONCLUSIONS 13
A. COMPLIANCE WITH STANDARDS 13
B. DRAINAGE CONCEPT 14
C. STORM WATER QUALITY 14
D. EROSION CONTROL CONCEPT 14
E. EROSION CONTROL ESCROW ESTIMATE 15
REFERENCES 15
APPENDIX
PAGE
VICINITY MAP & SOILS MAP A
RATIONAL METHOD HYDROLOGY B
STORM SEWER, CURB CUT, & INLET SIZING C
EROSION CONTROL CALCULATIONS & EROSION ESCROW D
PROPOSED DRAINAGE BASIN MAP, PROPOSED SITE PERVIOUS AREA MAP
& EXISTING SITE PERVIOUS AREA MAP E
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FINAL DRAINAGE
AND EROSION CONTROL REPORT
FOR THE UCHEALTH-PVH MAIN ENTRANCE IMPROVEMENTS
FORT COLLINS, COLORADO
I. GENERAL LOCATION AND DESCRIPTION
A. Location
The UCHealth-PVH Main Entrance Improvements are bounded by Lemay Avenue to
the west, the existing Therapy Garden to the north, the existing detention pond to the
east, and to existing residential homes on the south. The first seven houses, east of
Lemay Avenue, along East Pitkin Street, are owned by PVHS.
The project site can also be described as situated in the northwest quarter of Section
4, Township 6 North, Range 68 West of the 6 th P.M., of the City of Fort Collins,
Larimer County, Colorado. The UCHealth-PVH Main Entrance Improvements
project is comprised of a proposed new parking lot area, located north of the existing
detention pond, and proposed replacement of the existing PVHS Lemay Campus
parking lot south and east of the existing hospital building. Building improvements
will include removal and replacement of the existing main entry and surgery center
canopies, as well as a new MRI trailer parking area, at the south side of the building.
Proposed improvements are planned to occur over an area of approximately 6.70
acres.
B. Description of Property
The UCHealth-PVH Main Entrance Improvements site is comprised of building
façade improvements with an enhanced patient drop-off entry (for general public)
along the east side of the building, a new canopy at the out-patient surgery on the
south side of the building, MRI trailer parking and updated parking and access
drives on the south side of the building, parking lot replacement between the building
and the existing detention pond to the east, a new parking lot constructed within a
currently landscaped area (north of the existing detention pond), and associated site
and utility improvements to facilitate the building facade and parking lot
improvements.
The existing topography of the east parking lot generally slopes to the east at
approximately 3.3%, at the high side of the lot, to 1.0%, at the east side of the lot.
The parking lot, south of the building, slopes to the south and southwest at varying
slopes from 1.4% to 5%.
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II. DRAINAGE BASINS
A. Major Basin Description
The UCHealth-PVH Main Entrance Improvements site lies within the City of Fort
Collins Spring Creek Basin. In speaking with the City of Fort Collins Stormwater
Department, the proposed development is to be evaluated to determine the change in
imperviousness between existing and proposed conditions.
The proposed improvements will increase the site pervious area and therefore, will
not be required to provide any additional LID measures, detention, or water quality
volume.
B. Existing Sub-Basin Description
The existing basin, within the site and the limits of development, is a single basin
comprised of the existing parking lot, sidewalks, roof canopy areas, and landscaped
areas. The existing pervious surface area within the limits of work is 71,916 SF.
Please refer to the Existing Site Pervious Area Exhibit in Appendix D, which shows
the delineated existing pervious surface area within the limits of disturbance.
The existing basin contains approximately 6.7 acres of existing asphalt parking lots,
concrete and asphalt drive aisles, concrete sidewalks, and landscaped areas along the
east and south sides of the existing hospital building. The majority of the runoff is
conveyed over the surface to curb and gutter, chases, and storm pipes, before being
directed east and into the existing detention pond.
The residual runoff from the parking areas, drive aisles, and landscaped areas, in the
southwest corner of the site, drains west and southwest and out to Lemay Avenue.
Runoff flowing over and into the west access drive, flows west to the curb and gutter
on Lemay Avenue. Runoff, from the southwest drive aisle and parking lot,
concentrates at a curb cut in the existing parking lot, and is conveyed west through a
sidewalk chase that discharges to the curb and gutter in Lemay Avenue. Runoff from
the drives and parking lot area, south of the building, drains south through curb cuts
and chases to the curb and gutter and curb inlet, before being conveyed east in a
grass and cobble swale to the existing detention pond.
Runoff from the existing landscaped area, located north of the existing detention
pond, is split up, and drains east, west, and south to the existing, adjacent drive aisles
and into the existing detention pond.
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III. DRAINAGE DESIGN CRITERIA
A. Regulations
Since the subject site has been is acceptable to the City of Fort
Collins Stormwater Department to release runoff un-detained, if the impervious area
is not increased. The impervious area of the subject site has been reduced from the
existing condition, and therefore does not require additional LID improvements,
onsite water quality, or detention. Please refer to the Existing Pervious Area Map
and the Proposed Pervious Area Map in Appendix D for a comparison of the
pervious areas before and after construction improvements.
B. Development Criteria Reference and Constraints
The criteria and constraints from the City of Fort Collins will be met.
C. Hydrologic Criteria
The Rational Method for determining surface runoff was used for the project site.
The 2-year and 100-year storm event intensities were used in calculating runoff
values.
The City of Fort Collins intensity duration frequency curves were used to obtain
rainfall data for each storm specified. Detention pond sizing is not required as the
impervious area has been decreased with the proposed development. Please refer to
Appendix B and Appendix E for calculations.
D. Hydraulic Criteria
All hydraulic calculations, including final calculations and sizing for storm pipes,
curb cuts, swales, and erosion control and geotextile fabric BMP measures have been
provided in accordance with the City of Fort Collins Drainage Criteria and can be
found in the Appendices.
E. Variance
No variances are being requested for this project.
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IV. DRAINAGE FACILITY DESIGN
A. General Concept
Eleven drainage basins will provide conveyance of drainage for the 6.70-acre site to
existing water quality and detention pond facilities. Approximately 0.60 acres, in the
southwest corner of the site, will be routed west to the existing curb and gutter in
Lemay Avenue, as historically occurs. Approximately 0.10 acres will drain
southwest towards Lemay Avenue. A small amount of drainage may pass through
the northwest corner of the western residential lot along Pitkin Street, before flowing
to Lemay Avenue. However, this residence is owned by UCH, and the proposed
drainage pattern in this area is the same as the existing, historic drainage pattern seen
today. Approximately 0.09 acres will drain north to the Therapy Garden, which
ultimately drains to the existing detention pond. The drainage from remaining
parking lot, drive aisles, and landscaped areas will flow east across the parking lot
and drains to the existing east detention pond, as occurs under the existing condition.
In comparing existing conditions to proposed conditions for the subject site, the
pervious areas are 71,916 SF in the existing condition and 81,316 SF in the proposed
condition, resulting in a net increase of 9,400 SF of pervious area under the proposed
condition. Exhibits are provided in Appendix E showing the boundary for the limits
of disturbance and the existing and proposed pervious areas.
Because there is a net decrease in impervious area of the overall subject site, no LID
measures, water quality or detention volume is required for this site. Adequate
detention is provided for in the existing detention pond.
B. Specific Details
Basin 1
Basin 1 conveys runoff overland to the southwest across one lane of the access drive
to a 2 concrete chase in the median. Runoff passes through the 2 curb cut and
chase, across the southern drive aisle, and enters a cobble lined swale, that discharges
to a 2 concrete chase at design point 1. Runoff from basin 1 passes through the
chase and discharges to the curb and gutter in Lemay Avenue.
Basin 2
Basin 2 conveys runoff from the landscaped area, in the southwest corner of the site,
overland to the southwest, and out to Lemay Avenue. It is important to note that the
lots immediately south of basins 1 and 2 are owned by UCH.
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Basin 3
Basin 3 conveys runoff overland to the south and across the proposed access drive,
and parking lot, and into the curb and gutter along the south bay of parking. Flow
captured in the curb and gutter, drains east and west to 2 curb cuts at each end of the
parking bay. Flow enters a swale and drains to a 24 Nyloplast drain basin with
standard H-20 grate at design point 3.
Basin 4
Basin 4 conveys runoff overland to the south and across the proposed parking area
and access drive before entering the curb and gutter along the south flowline. Flow
concentrates in sump conditions in the curb and gutter and flows through 2 and 4
curb cuts to enter basin 5. Flow enters a swale in basin 5 and drains to a 24
Nyloplast drain basin with standard H-20 grate at design point 5.
Basin 5
Basin 5 is comprised of landscaped area which concentrates in a swale, flowing from
the west and east ends of the basin, to a central sump condition. Concentrated flow
combines with the runoff from basin 4 and enters a 24 Nyloplast drain basin with
standard H-20 grate, at design point 5.
Basin 6
Basin 6 is comprised of landscaped area which drains to a swale. The west portion
of the basin drains to a sump inlet on a manhole constructed with a Neenah R2586-O
grate. The manhole is constructed on the existing 15 . The balance
of the landscaped basin area drains to a cobble-lined swale, which flows east, passes
through an existing concrete drainage pan, and discharges to the existing detention
pond, as historically occurs.
Basin 7
Basin 7 conveys runoff overland to the southeast and east across the access drives,
and parking lot.
Runoff from the south area of basin 7 concentrates in the curb and gutter along the
south side of the parking lot, while the runoff from the northern area of basin 7 sheet
flows east through the parking lot and drive aisles, and into the existing detention
pond, as historically occurs.
A small area of approximately 3,300 SF, located within a recessed area between the
building at the west side of basin 7, near the MRI trailer area, will drain to a 24
Nyloplast drain basin with standard H-20 grate. The intercepted runoff will be piped
west in a 12 HDPE storm pipe, then south in an existing 15 storm pipe, which
connects to the existing 15 RCP in basin 6. Runoff is conveyed east and into the
existing detention pond.
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Two proposed roof drain connections at the east side of the building, will be
conveyed south and southeast in 12 HDPE roof drain pipes connecting to a 15 RCP
storm pipe, which flows south and connects to the existing 15 RCP in basin 6. Flow
will ultimately discharge east and into the existing detention pond.
Basin 8
Basin 8 conveys runoff overland through the parking lot to the northeast and east,
then across the north-south access drive. Flow will concentrate in the drainage pans
at the east side of the north-south drive aisle and is conveyed south. A portion of the
new parking lot within basin 8, east of the north-south drive aisle, will also drain
west to the drainage pans.
Runoff in basin 8 is intercepted and discharged to the existing detention pond using
several conveyance methods. Flow in the north-south drainage pans will be
intercepted by two-24 Nyloplast drain basins with standard H-20 grates and piped to
a proposed manhole, constructed on the existing 18 RCP storm pipe, which
discharges to the existing detention pond.
Runoff will also be intercepted by a two 4 wide concrete chases with metal sidewalk
culverts at design point 8. Flow will discharge to swales, then enter the existing
detention pond. In a major storm event, carryover flow from basin 8 will overtop the
high point in the flowline, drain south to basin 7, and flow east and enter the existing
detention pond where the vertical curb ends.
Basin 9
Basin 9 conveys parking lot runoff overland to the northeast and southeast corners of
the basin. The north side of the basin drains to curb and gutter, then flows east in the
existing parking lot to a curb cut, located at the northeast corner of the existing
parking lot, which conveys runoff northeast and into the existing detention pond,
located northeast of the project improvement area. The south side of the basin drains
south to the existing parking lot, passes through an existing curb cut, and enters the
other existing detention pond, located in the southeast corner of the site.
Basin 10
Basin 10 conveys runoff from landscaped area and sidewalk areas overland to the
northeast then flows east, through the Therapy Garden, and ultimately discharges
east and into to the existing detention pond.
Basin 11
Basin 11 conveys runoff from landscaped areas, sidewalk, and pavement areas to the
southwest and west and discharges to the curb and gutter in Lemay Avenue, as
historically occurs. Under the existing condition, approximately 7,397 SF of
impervious area drains over the access drive and enters Lemay Avenue. Under the
proposed condition, approximately 4,886 SF will drain over the access drive and out
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to Lemay Avenue. The residual runoff from basin 11 will be conveyed to a cobble-
lined swale in basin 1, which will also discharge to Lemay Avenue.
Overflow Conditions
Basins 1 and 11
Should the curb cut or chase on the median within basin 1 plug, runoff will drain
northwest in the driveway, and enter the gutter in Lemay Avenue. Should the metal
sidewalk culvert in basin 1 plug, flow will overtop the walk, and drain to the gutter in
Lemay Avenue. Basin 11 will continue to flow through the driveway section,
regardless of whether the gutter is obstructed or not.
Basins 3, 4, 5 and 6
Should the curb cuts in basins 3 or 4 plug, or the area inlet plug in basin 3, runoff
will spill over the curb, drain south into basin 5, and enter an area inlet. If the area
inlet in basin 5 should plug, runoff will overtop the high point, east of the area inlet,
then flow east to a sump area inlet in basin 6. If the inlet in basin 6 were to plug,
runoff will overflow to a swale, then flow east to the existing detention pond.
Basin 7
Should the roof drains plug in basin 7, runoff will flow east on the surface through
the drive aisles and parking lot and will drain to the existing detention pond. Should
the yard drain in basin 7 plug, overflow will drain south, through a sidewalk chase
with metal culvert, south over the curb, then east in the curb and gutter along the
south boundary of basin 7, and will ultimately discharge to the existing detention
pond.
Basin 8
Should the area inlets in the north-south drive aisle plug in basin 8, runoff will flow
south in the drainage pans, along the east side of the drive aisle. Runoff will then
pass through an 8 concrete chase and 4 concrete chase with metal sidewalk culvert
to enter the existing detention pond. Should the chases plug, flow will drain south in
the drive aisle curb and gutter, then flow into the pond where the vertical curb ends.
Basins 9 & 10
Basins 9 and 10 flow on the surface and should drain as intended. Should the
concrete chase in the north parking lot island plug, runoff will pond up until it drains
around the parking lot island and will continue flowing east in the existing curb and
gutter.
C. Detention Pond
The existing southeast detention pond will be utilized for the subject site, as the
impervious area is less than the existing impervious
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area. Therefore, additional LID measures, water quality, and detention are not
required, and the existing pond will remain as-is.
D. Street Capacities
Street capacity calculations are not required for the subject site, as no new public
street construction will be built with this project, and the impervious area has been
reduced by this development. However, the street capacity was calculated for the
north-south drive aisle which conveys runoff from basin 8. The flow depth in the
major storm event was used to size inlets and curb cuts. Capacity calculations and
curb cut sizing is provided in the Appendices of this report.
V. STORM WATER QUALITY
A. General Concept
The water quality of stormwater runoff must be addressed on all final design utility
plans. The site improvements will provide a net increase in the pervious area on the
campus and therefore, is not required to provide any additional LID s, water quality,
or detention volume. Best Management Practices for the treatment of stormwater
runoff for the subject site will include cobble and grass-lined swales and the existing
grass lined detention pond. These water quality features will provide a mechanism
for pollutants to settle out of the stormwater runoff before flows are directed off-site
ultimately discharging to the Poudre River, as historically occurs.
B. Specific Details
Best Management Practices (BMP) for the treatment of storm water runoff have been
incorporated into the final design for this project.
Approximately 6.7 acres of the UCHealth-PVH Main Entrance Improvements will be
disturbed for the site improvements. By comparing existing conditions to proposed
conditions for the subject site, the pervious area is 71,916 SF in the existing
condition, and 81,316 SF in the proposed condition. This results in a net increase of
9,400 SF of pervious area under the proposed condition. Exhibits are provided in
Appendix E showing the boundary for the limits of disturbance, and the existing and
proposed pervious areas.
The existing drainage patterns will be maintained with the proposed site
improvements. The majority of the disturbed area will be conveyed on the surface
and in storm pipes to the existing grass lined detention pond, as historically occurs.
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VI. EROSION CONTROL
A. General Concept
The potential exists for erosion problems during construction, but should be minimal
after completion of proposed development. Silt fence will be installed along the
perimeter of the north, east, south, and west boundary of the site improvements to
prevent sediment from leaving the site. Where silt fence is shown to cross hard
surface areas, rock socks shall be utilized for these areas, as may be applicable during
construction. A tracking pad will also be placed at entrance/exit to the parking lot on
the north access drive, which connects to Doctors Lane. Gravel inlet filters will be
placed at proposed curb cuts, proposed storm inlets, and proposed sidewalk chases to
mitigate the build-up of sediment and transport of debris into existing streets and
downstream drainage systems. Geotextile fabric will also be utilized at concentration
points (curb cuts) and in swales as shown in the plans, to minimize erosion and
increase soil stabilization. Please refer to Appendix D for erosion control
calculations.
VII. CONCLUSIONS
A. Compliance with Standards
All computations within this report have been completed in compliance with the City
of Fort Collins Storm Drainage Design Criteria.
The City of Fort Collins Stormwater Utility will not maintain the on-site storm
drainage facilities within the subject site. The owners of the subject site will
maintain their on-site storm drainage facilities on a regular basis. The following
shall be implemented for the private stormwater im
maintenance procedures for the project on an annual or bi-annual basis:
1) Storm sewer inlets, pipes, curb cuts and concrete sidewalk culverts/chases,
structures, and manholes, shall be cleaned through the removal of debris and
sediment from the associated items to allow for adequate drainage through
the site to the existing detention facility or downstream systems
2) Sedimentation/ silting shall be removed to allow for adequate drainage along
the bottom of the swales and existing detention pond. Grass scrubbing along
the bottom of the existing detention pond (as necessary) to remove sediment
and promote grass growth.
3) Re-vegetation through the use of Natural Seeding/ Sod shall be provided for
disturbed areas and other permanent erosion controls shall be provided for
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areas where erosion has taken place and requires remediation back to the
proposed condition shown in the plans
The Stormwater Operating/ Maintenance Procedures listed above are guidelines to
the minimum procedures that shall be implemented for the site, with these and
additional measures being utilized on an as-needed basis.
B. Drainage Concept
The proposed drainage concepts presented in this study and shown on the utility
plans adequately provide for the transmission of developed on-site runoff to the
existing detention pond, the adjacent street (Lemay Avenue), and existing and
proposed storm sewer systems. The proposed roof drain connections and storm
sewers will connect to the existing storm pipes within the parking lot and
improvement area to convey flow to the existing detention pond, located southeast of
the existing hospital building and parking lot within the UCHealth-PVH Main
Entrance Improvements project.
The existing and proposed storm sewer system, swales, and paved parking lot and
drive aisles will provide for the 100-year developed flows to be conveyed to the
existing detention pond. The proposed improvements will increase the overall
perviousness of the site. Therefore, no additional LID measures, water quality, or
detention are required for the site.
If groundwater is encountered at the time of construction, a Colorado Department of
Health Construction Dewatering Permit will be required.
C. Storm Water Quality
The final design has addressed the water quality aspect of stormwater runoff. The net
increase of 9,400 SF of pervious area for the proposed site improvement results in no
need for additional LID treatment, water quality, or detention, prior to flows being
directed downstream.
D. Erosion Control Concept
Proposed erosion control concepts have been provided for and adequately provide for
the control of wind and rainfall erosion from the proposed development. Through
the construction of the proposed erosion control concepts, the City of Fort Collins
performance standards will be met. The proposed erosion control concepts presented
in this report and shown on the erosion control plan and are in compliance with the
City of Fort Collins Erosion Control Criteria.
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Erosion Control Escrow Estimate
The Erosion Control Escrow Estimate for the subject property is approximately
$18,210. Please refer to Appendix D for this calculation.
REFERENCES
1. Storm Drainage Design Criteria and Construction Standards by the City of Fort Collins,
Colorado, May 1984, Revised January 1997, & Associated UDFCD Updates and New City
of Fort Collins Stormwater Criteria manual, adopted January 2012.
2. Erosion Control Reference Manual for Construction Sites by the City of Fort Collins,
Colorado, January 1991, Revised January 1997, & Associated UDFCD Updates and New
City of Fort Collins Stormwater Criteria manual, adopted January 2012.
3. Urban Storm Drainage Criteria Manual Volume 1; Urban Drainage and Flood Control
District, Denver, Colorado, March 1969, Revised August 2018.
4. Urban Storm Drainage Criteria Manual Volume 2; Urban Drainage and Flood Control
District, Denver, Colorado, March 1969, Revised September 2017.
5. Urban Storm Drainage Criteria Manual Volume 3, Best Management Practices; Urban
Drainage and Flood Control District, Denver, Colorado, September 1992, Revised
November 2010.
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APPENDIX