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DRAINAGE REPORT
FOR
OAK 140
AT
140 EAST OAK STREET
FORT COLLINS, CO 80524
PREPARED FOR
DECEMBER 22, 2020
December 22, 2020
Dan Mogen
City of Fort Collins Utilities – Stormwater Engineering
700 Wood Street
Fort Collins, CO 80521
RE: Oak 140 – Drainage Report
JVA Job No. 3258c
Dear Dan:
The following Drainage Report and attached drainage maps have been prepared for the Oak 140
redevelopment project. The report and drainage maps have been produced in accordance with the
City of Fort Collins and the latest local Mile High Flood District recommendations.
It is our understanding that the information provided herein meets all requirements of the City of
Fort Collins.
Please contact us if you have any questions regarding this submission.
Sincerely,
JVA, Inc.
__________________________
Erik T. Nakos, P.E.
Project Manager
Oak 140 Drainage Report
Page 1 of 7
TABLE OF CONTENTS
GENERAL LOCATION AND EXISTING SITE INFORMATION ......................................................... 3
DRAINAGE BASINS AND HISTORIC RUNOFF .......................................................................... 4
HISTORIC DRAINAGE ..................................................................................................... 4
PROPOSED DRAINAGE .................................................................................................. 4
DRAINAGE DESIGN CRITERIA ............................................................................................ 5
HYDROLOGIC METHOD AND DESIGN STORM FREQUENCIES ................................................. 5
HYDRAULIC CRITERIA .................................................................................................... 5
VARIANCES FROM THE CRITERIA......................................................................................... 5
FOUR STEP PROCESS ........................................................................................................ 5
DRAINAGE FACILITY DESIGN ............................................................................................. 6
GENERAL CONCEPT ..................................................................................................... 6
STORMWATER POLLUTION PREVENTION/EROSION CONTROL .................................................. 6
TEMPORARY EROSION CONTROL .................................................................................... 6
PERMANENT EROSION CONTROL .................................................................................... 6
CONCLUSIONS................................................................................................................ 7
REFERENCES ................................................................................................................... 7
Appendix A – Referenced Information
1. Vicinity Map
2. Geotechnical Report Summary Logs of Exploratory borings
3. FEMA FIRM Map
4. NRCS Websoil Survey
5. Figure 1: Historic percentage of Imperviousness
6. Figure 2: Proposed Percentage of Imperviousness
Appendix B – Hydrologic/Hydraulic Computations
1. Rational Method Calculations
2. BMP Calculations
Oak 140 Drainage Report
Page 2 of 7
ENGINEER’S STATEMENT:
“I hereby certify that this report (plan) for the Drainage Design for the Oak 140 redevelopment
was prepared by me (or under my direct supervision) in accordance with the provisions of the City
of Fort Collins Standards for the Responsible Parties thereof. I understand that the City of Fort
Collins does not and shall not assume liability for drainage facilities designed by others.”
______________________________
Erik T. Nakos
Registered Professional Engineer
State of Colorado No. 40776
Oak 140 Drainage Report
Page 3 of 7
GENERAL LOCATION AND EXISTING SITE INFORMATION
The Oak 140 (site) is located in Section 12, Township 7 North, Range 69 West of the 6th Principal
Meridian, in Larimer County, Fort Collins, Colorado. Refer to Appendix A for the site vicinity
map. The site is bordered to the south by E Oak Street, to the east by Remington Street, and to the
east by Montezuma Fuller Alley. Developed residential areas are to the north, south, east and west
of the site. The existing site, platted as Lots 23-26, 29 and W 33 Ft Lot 30 all in Block 121, FTC
(same owner: Fort Collins Downtown Development Authority), is predominantly gravel and
asphalt from a previously demolished building. This site is part of the master drainage plan for the
Old Town Drainage Basin and designed to conform to the drainage basin design.
The site is approximately 0.58 acres of previously developed property and has a historical percent
impervious of 92.6%. With the historical building removed years ago, the existing site has less
percent imperviousness with a gravel lot, however the drainage design was calculated to the
historical site. The existing ground surface is relatively flat, generally sloping to the east at grades
ranging from 0.1% to 3.0%.
From the geotechnical report dated May 5, 2020 the soil is mainly clayey sand. Groundwater was
encountered at 14 ft to 20 ft below the ground surface. The Summary Logs of Exploratory Borings
from the geotechnical report is attached in Appendix A.
The site is located outside FEMA and City of Fort Collins mapped 100-year and 500-year
floodplains, per FEMA Flood Risk Map Number 08069C0979H, effective date 05/02/2012. The
site is comprised of hydrologic soil group C/D per NRCS soil survey. The FEMA Firm Map and
NRCS Websoil Survey are included in Appendix A.
In general, the proposed redevelopment of the site consists of constructing a new building and
surrounding hardscape for proposed pedestrian pathways. In the new building, the roof drains will
be conveyed to 3rd floor LID planters to be used as LID treatment. Any excess flow will bypass
the system through a series of overflows and be conveyed to the public storm system to the south
in Oak street.
The site development will decrease the overall percent impervious slightly by 1.3% to a total of
91.3% from the historical 92.6%. The Old Town Drainage Basin requirements are that detention
is required if there is an increase of impervious area by an additional 5,000 SF from the historical
site. Since there is no proposed increase of imperviousness of the site from the historical site, there
is no detention required for this project. However, as part of the City of Fort Collins Storm Criteria,
there are water quality and LID treatment requirements because the project includes over 1,000 SF
of new or modified impervious area. The water quality requirement for the site is fulfilled by the
existing Udall water quality facility located to the east of the site by the Cache la Poudre River.
The Udall facility is considered to treat 50% of the site’s area. The LID requirement is to treat
50% of the added or modified impervious area. The proposed LID planters on the 3rd floor of the
building will treat the proposed roof (not including terrace areas) which will exceed the required
50%, and thus the entire site will be considered treated.
Oak 140 Drainage Report
Page 4 of 7
DRAINAGE BASINS AND HISTORIC RUNOFF
HISTORIC DRAINAGE
The existing site consists of one basin. The runoff from the existing site sheet flows to the east
through a sidewalk chase and into Remington Street. Then appears to flow north in Remington
Street to Mountain Ave and is then captured in the curb inlet on the south side of Mountain Ave,
west of Remington St.
PROPOSED DRAINAGE
Proposed drainage patterns are to remain generally the same as the current but will utilize a LID
treatment system for the roof area imperviousness before discharging to the public storm system.
The bioretention system will be utilized for low impact development (LID) treatment requirements
of the site. With the added or modified imperviousness area for the project being 0.69 acres, the
50% required area to treat is 0.34 acres. With the proposed roof area being treated, 0.42 acres, this
project will exceed the City requirement by treating 61% of the added or modified imperviousness.
The bioretention systems will reduce runoff, treat and slowly release the water quality capture
volume as part of the four step process for the City of Fort Collins criteria on BMP selection.
Detention will not be required as we are meeting the planned design outlined in the Old Town
Basin Master Drainage Plan and not increasing the site imperviousness as compared to historical
imperviousness.
For the purposes of calculations, the site was divided into 5 basins. Basin 1 includes the proposed
building roof that is conveyed to the LID planter on the 3rd floor. Basin 2 consists of the area of
the LID planter itself. Basin 3 is the roof/terrace areas that are not conveyed to the LID planter
and are conveyed to site overflow systems that discharges at ground level. Basin 4 consists of the
area within the property line to the north of the building that has modified impervious area. Basin
5 is the area outside the property line that has modified impervious area. Most of the proposed
building roof (Basin 1) is draining to the bioretention system (Basin 2) to be treated with the LID
planter, with larger storm events bypassing the system through the drain basin control structure
and overflow drains in the planter. The area of the building roofs and terraces that is not conveyed
to the LID (Basin 3) is combined downstream of the planter and discharges in an underground
storm drain into the inlet at Oak Street. Precautionary overflow from all building areas are
combined and conveyed to discharge at ground level during emergency situations. The
surrounding proposed hardscapes (Basins 4 & 5) will drain to either the south in the Oak storm
system or to Remington Street as the site has done historically and is not anticipated to have any
additional impacts to the surrounding area.
There are four downspouts draining into the LID planter. Each of the downspouts will discharge
into the side of the drain basin control structure that will drain into the LID planter and be treated.
The discharge from the structure will have concrete splash blocks and cobble to act as a forebay
to provide further energy dissipation. The LID system will fill up to provide the required WQ
treatment and is designed to slowly drain in 12 hours per typical Rain Garden design. In larger
events, the volume will overtop the weir in the structure to flow to the plumbing primary. There
is also an overflow in the LID system that will flow to the plumbing overflow system. The primary
and overflow systems are designed for the 100-yr storm by the plumbing engineer per the
International Plumbing Code, as is required for building elements. Primary drains will be piped
Oak 140 Drainage Report
Page 5 of 7
to the public inlet at the south side of the site. Overflow drains will discharge at ground level to
be collected at area drains and connected to that storm drain line to the existing inlet.
DRAINAGE DESIGN CRITERIA
HYDROLOGIC METHOD AND DESIGN STORM FREQUENCIES
The drainage for the site was designed to meet or exceed the City of Fort Collins’s Standards and
Specifications, including the Stormwater Criterial Manual which incorporates most of the Mile
High Flood District’s (MFHD) Urban Storm Drainage Criteria Manual (USDCM).
The design one hour point rainfall used for the 2 year and 100 year storm events are 0.82 inches
and 2.86 inches respectively.
The Rational Method (Q=CIA) was used to determine the storm runoff (Q) from the areas
contributing to the new storm system, with composite runoff coefficients (C) and contributing
areas (A) given for design points in sub-basins. The runoff coefficients for various land usages
were obtained from FCDCM, Chapter 5. Intensities (I) were determined using the Time-Intensity-
Frequency equations, and a calculated Time of Concentration (tc). Hydraulic Criteria. See
Appendix B for the runoff coefficient calculations.
HYDRAULIC CRITERIA
The proposed drainage system has been designed to comply with the City of Fort Collins
Stormwater Criteria Manual. The MHFD UD-BMP spreadsheet version 3.07 was used to analyze
the designed bioretention. Area drains were sized using MHFD’s USDCM Section 3.3.6 and
calculations are included with the rational calculations in Appendix B. Complete drainage plans,
details, dimensions, etc. are included in the grading and drainage plans in the site’s construction
documents.
VARIANCES FROM THE CRITERIA
No variances are requested for this design.
FOUR STEP PROCESS
The Four Step Process implemented by the City of Fort Collins for stormwater quality
management is:
1. Reduce runoff through use of Low Impact Development (LID) and Minimizing Directly
Connected Impervious Area (MDCIA)
2. Implement BMPs that provide a WQCV with slow release.
3. Stabilize Streams
4. Implement site specific and other source control BMPs.
This project is addressing this Four Step Process in the following ways;
Oak 140 Drainage Report
Page 6 of 7
1. The reduction of impervious area compared to historical, as well as the use of LID
bioretention planters to reduce runoff
2. The use of the LID biorentention system to capture and slow release, as well as the use of
the existing Udall facility.
3. There is no negative impact to stream erosion with the development of this site as compared
to historical.
4. The dumpsters are located in the trash enclosure with walls/fences which will limit the risk
of trash and debris from leaving the dumpster area. The enclosure is also located far away
from the storm drainage facilities so that highly polluted runoff from that area opportunity
to be cleaned prior to runoff into the public storm drain.
DRAINAGE FACILITY DESIGN
GENERAL CONCEPT
The site’s private storm drainage system has been designed to convey the 100-year storm event
through a combination of bioretention, inlets and pipes that connect into the existing curb inlet in
Oak Street. The hardscape around the building will be graded to drain into the ROW as it has
historically done. The existing public system in Oak Street and Remington are assumed to have
sufficient capacity for the developed flow as the developed impervious and runoff rates are less
than the historical rates.
The proposed bioretention planters located on the 3rd floor of the building will treat storm runoff
for a minimum of 50% of the modified or added impervious area on the site for water quality
treatment prior to discharging into the public storm system, satisfying the City of Fort Collins LID
requirements. These features will facilitate sedimentation and filtration while limiting erosion,
providing both treatment and slowed release of the water quality capture volume. The proposed
redevelopment of this site will serve to limit water quality and quantity impacts to natural
drainageways by decreasing the frequency, rate, duration, and volume of runoff.
STORMWATER POLLUTION PREVENTION/EROSION CONTROL
As site is less than 1 acre of disturbed area, an erosion control report is not required per City of
Fort Collins Storm Criteria. However, an erosion control plan and escrow is submitted to meet
with City requirements. During construction, temporary erosion and sediment control practices
will be used to limit soil erosion and sediment discharge off the site and into public existing
stormwater infrastructure.
TEMPORARY EROSION CONTROL
A temporary erosion control plan is to be implemented for the site during construction. Temporary
erosion control measures include, but are not limited to, vehicle tracking control, concrete washout
areas, and inlet and slope protection provided using erosion control wattles/sediment control logs,
rock socks, etc. All temporary erosion control measures are to be removed after they are deemed
unnecessary. A general erosion control plan has been provided in the civil construction documents.
PERMANENT EROSION CONTROL
Chapter 2 of the Urban Drainage and Flood Control District’s Urban Storm Drainage Criteria
Manual Volume 3 (USDCM) provides guidelines for the selection of appropriate permanent
Oak 140 Drainage Report
Page 7 of 7
structural BMPs for a site that is to be developed or redeveloped. The Oak 140 is best characterized
as a “conventional redevelopment” with under 1 acre of impervious area on the project site. The
BMP decision tree for such sites is provided in Figure 2 -2 of the USDCM. As previously stated,
the site and surrounding vicinity is comprised of hydrologic soil group Type C/D soils, per the
NRCS soil survey.
The water quality capture volume (WQCV) for this site is accounted for in the existing Udall water
quality and detention facility. No detention is required on the site. Runoff will be collected in the
private storm system before connecting to the public storm system, or sheet flow into the ROW as
it has done historically. The existing storm system should have sufficient capacity for the sites
runoff flows as the imperviousness and runoff from the site is less than the historical flows.
CONCLUSIONS
This Drainage Report for the Oak 140 has been prepared to comply with the stormwater criteria
set by the City of Fort Collins and the Mile High Flood Control District.
The proposed drainage system presented in this report is designed to convey the 100-year peak
stormwater runoff through the site via the proposed and existing storm systems. Through
calculations, modeling, and review of the proposed storm drain system, there appears to be
adequate capacity to convey the proposed flows to the existing storm system.
It can therefore be concluded that development of the Oak 140 complies with all of the stormwater
jurisdictional criteria and will not adversely affect the existing streets, storm drain system and/or
detention/water quality facilities.
REFERENCES
1. City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, December 2018.
2. “Geotechnical Investigation Proposed Apartment Complex, 140 East Oak Street, Fort
Collins, Colorado”, CTL Thompson Incorporated, Dated May 5, 2020.
3. “Urban Storm Drainage Criteria Manual”, Mile High Flood District, August 2018 version.
4. FEMA Flood Insurance Rate Map, Map Number 08069C0979H, Map Revised May 2,
2012
5. USDA NRCS Web Soil Survey
APPENDIX A – REFERENCED INFORMATION
Oak 140
140 East Oak St
Fort Collins, CO
MAP NOT TO SCALE
(Image from Google Earth)
0
5
10
15
20
25
30
35
40
0
5
10
15
20
25
30
35
40
6/12
14/12
50/2
50/1
50/2
50/2
WC=12.4
DD=119
SW=0.0
SS=<0.01
WC=17.7
DD=113
SW=0.1
WC=12.4
DD=119
SW=0.0
SS=<0.01
WC=17.7
DD=113
SW=0.1
TH-1
8/12
50/11
50/8
50/1
50/2
50/2
50/2
WC=15.4
DD=112
LL=34 PI=19
-200=53
WC=2.6
-200=5
WC=15.7
DD=106
-200=20
WC=15.4
DD=112
LL=34 PI=19
-200=53
WC=2.6
-200=5
WC=15.7
DD=106
-200=20
TH-2
6/12
19/12
50/8
50/1
WC=15.0
DD=113
SW=0.1
WC=13.2
DD=117
SW=0.6
WC=8.7
-200=16
WC=15.0
DD=113
SW=0.1
WC=13.2
DD=117
SW=0.6
WC=8.7
-200=16
TH-3
10/12
14/12
50/7
50/2
WC=14.9
DD=115
SW=0.0
WC=11.4
DD=124
-200=30
WC=14.9
DD=115
SW=0.0
WC=11.4
DD=124
-200=30
TH-4
9/12
14/12
50/8
50/1
50/1
50/2
WC=14.5
DD=115
SW=0.0
SS=0.180
WC=12.3
DD=119
SW=0.1
WC=14.5
DD=115
SW=0.0
SS=0.180
WC=12.3
DD=119
SW=0.1
TH-5
DEPTH - FEETDRIVE SAMPLE. THE SYMBOL 6/12 INDICATES 6 BLOWS OF A 140-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE A 2.5-INCH O.D. SAMPLER 12 INCHES.
FILL, SAND, CLAYEY WITH OCCASIONAL GRAVEL, MOIST, LOOSE, BROWN, DARK BROWN
1.
NOTES:
THESE LOGS ARE SUBJECT TO THE EXPLANATIONS, LIMITATIONS AND CONCLUSIONS IN
THIS REPORT.
WATER LEVEL MEASURED SEVERAL DAYS AFTER DRILLING.
SAND, CLAYEY, MOIST, MEDIUM DENSE, BROWN (SC)
3.
LEGEND:
SAND AND GRAVEL, SLIGHTLY CLAYEY, MOIST TO WET, VERY DENSE, REDDISH BROWN,
BROWN (SC, SP, SW-SC, GP)
SANDSTONE, CLAYEY, MOIST TO WET, VERY HARD, BROWN, OLIVE
DEPTH - FEETWATER LEVEL MEASURED AT TIME OF DRILLING.
Summary Logs of
Exploratory Borings
THE BORINGS WERE DRILLED ON APRIL 24, 2020 USING 4-INCH DIAMETER
CONTINUOUS-FLIGHT AUGERS AND A TRUCK-MOUNTED DRILL RIG.
FIGURE 2
WC
DD
SW
-200
LL
PI
UC
SS
-
-
-
-
-
-
-
-
INDICATES MOISTURE CONTENT (%).
INDICATES DRY DENSITY (PCF).
INDICATES SWELL WHEN WETTED UNDER OVERBURDEN PRESSURE (%).
INDICATES PASSING NO. 200 SIEVE (%).
INDICATES LIQUID LIMIT.
INDICATES PLASTICITY INDEX.
INDICATES UNCONFINED COMPRESSIVE STRENGTH (PSF).
INDICATES SOLUBLE SULFATE CONTENT (%).
2.
HOUSING CATALYST
140 EAST OAK STREET
CTL | T PROJECT NO. FC09242-125