HomeMy WebLinkAbout609 S COLLEGE AVE MIXED USE - BDR200004 - SUBMITTAL DOCUMENTS - ROUND 3 - DRAINAGE REPORTDrainage Memo
August 26, 2020
Project: 609 South College Avenue Project No. 564-005
Attn: Stormwater Staff
City of Fort Collins Stormwater Utility
700 Wood Street
Fort Collins, CO 80521
Stormwater Staff:
This letter serves to document the proposed drainage impacts of the addition to the existing building
proposed at 609 South College Avenue in Fort Collins, Colorado.
Project Overview
The project proposes to add an addition to the rear (west side) of the existing building adding ~800 sf.
Construction activities will also include the removal of the existing garage. A concrete patio area, concrete
walks, and alley parking will also be added as a part of these improvements, as well as a rain garden
similar to what was used at the Music District to provide LID treatment to the site. A dedicated fire service
and a single-phase transformer will also be added as part of this project.
Existing Site
The existing site is relatively flat (0% - 1%) in the front and rear yards, and functions as an “A” Lot, with
drainage generally draining from back to front along the side lot lines. In addition to the existing home, the
lot consists of a garage, a concrete driveway, concrete entry walk, and concrete walks around the entire
home. The remainder of the site is Kentucky Bluegrass lawn, with small landscaping beds and mature
trees.
The total existing impervious area for the lot is 2,781 sf.
Proposed Site
The proposed site will generally maintain the existing site drainage. Drainage from the rear yard, patio
area, and roof will be directed overland via concrete pan along the southern lot line to a rain garden that
will provide LID treatment. In the event of a large storm, water will exit out to College Avenue through a
proposed sidewalk chase. The parking area will maintain the existing drainage pattern and flow west to
Dalzell Alley. Existing grades outside of our improvements will remain unchanged, including along the
north side of the existing house where no improvements are proposed.
The total proposed impervious area for the lot is 4,725 sf. Of the existing impervious area, 1,505 sf is
unmodified, resulting in 3,220 sf of new and modified impervious area. The proposed change in
imperviousness will add an estimated 0.46 cfs to College Avenue and 0.1 cfs to Dalzell Alley in a 100-yr
event. We do not foresee the additional runoff causing any issues with the existing inlets intercepting the
flow at Myrtle Street and College Avenue, but additional information regarding the inlet’s capacity will need
to be obtained from the City and verified. No adverse effects to adjacent properties are anticipated due to
the increased imperviousness of the site.
Detention
Since the impervious area increase is than the 5,000 sf allowed in the Old Town Master Drainage Basin, no
detention is required for the site.
Floodplain
The property is not located in a FEMA regulated flood hazard zone. The project site is, however, located
partially within the City of Fort Collins regulated West Myrtle Street floodplain, which is part of a larger
basin study, the Old Town Basin, as shown in Figure 1, below. A LOMA (Letter of Map Amendment) is
proposed with the site, based on existing elevation data obtained with recent survey work conducted for the
project. The LOMA will be submitted by separate document, and is anticipated to show, based on existing
ground elevations that the current project site is in fact elevated above current Base (100-year) Flood
Elevation.
Current floodplain mapping of the area has been recently revised by the City of Fort Collins. The original
Old Town Basin study was entitled, “Old Town Basin Master Drainage Plan”, which was conducted by
Anderson Consulting Engineers in May of 2002. We have been in discussion City Staff, and have been
directed by Staff to utilize current draft information as best available data.
The question of inadvertent detention has been discussed, and initially, it was thought that compensatory
storage may be necessary with any fill placed within the development site. However, per correspondence
with City Staff, this area is not within the inadvertent detention ponding area, which was referred to as the
“College and Mulberry Pond”. Per City Staff, based on recent updated 2D modeling, the Base Flood
Elevation (BFE) for this site is 4995.50 (NAVD 88).
Figure 1 – Current City of Fort Collins Floodplain Mapping
(Source: FCMaps, http://gisweb.fcgov.com)
Control points/benchmarks utilized with the survey completed for the project are as per the plan set for the
project and are shown in Figure 2, below.
Figure 2 – Current City of Fort Collins Floodplain Mapping
(Source: Utility Plans for 609 South College Avenue, Northern Engineering)
It is noted that some of the surrounding developments have used the NGVD 29 (unadjusted) datum. The
conversion from NAVD 88 to NGVD 29 (unadjusted) datum in this area is -3.18-ft.
Any portion of the site to be developed outside of the LOMA area must comply with all floodplain
regulations outlined in Chapter 10 of City Code. A floodplain use permit would be required for any site
construction element (detention ponds, bike paths, parking lots, utilities, etc.) that may still remain in the
floodplain once the LOMA is complete. It is also noted that Critical Facilities are not allowed in the
floodplain.
The applicant acknowledges that the building design does not comply with the current regulatory
floodplain. The applicant will apply for a Letter of Map Amendment showing that the addition area is
elevated above the Base Flood Elevation, however the City cannot guarantee approval of the LOMA without
a submittal. The applicant understands that building permits cannot be issued prior to the approval of the
LOMA.
Water Quality and LID
Water Quality for the site is being provided by the Udall Natural Area.
LID treatment requirements will be achieved by directing the concentrated flows from the roof and the patio
through concrete pan on the south side of the site into a rain garden in the front of the house. The rain
garden will contain Bioretention Soil and landscape. Water will infiltrate the soil, removing stormwater
contaminates.
Due to the limited area available for the rain garden, it is only sized to handle the required run-on area of
new or modified impervious area (%75 of 3,220 sf = 2,415 sf), though a majority of the new, modified,
and existing impervious area flows into the rain garden (3,464 sf). It is assumed that this will not be a
problem as the water flowing to the rain garden is not expected to be heavily contaminated. As an extra
measure of protection, a grouted cobble sediment basin will be added before the rain garden to further
remove contaminates. A variance regarding rain garden sizing design is being requested.
In the event of a large storm, a sidewalk chase will allow rain garden overflow to travel to College Avenue.
An exhibit has been provided to illustrate the existing and proposed impervious areas. Please feel free to
contact us if you have any questions.
Northern Engineering Services, Inc.
Benjamin Ruch, PE
Project Engineer
CHARACTER OF SURFACE:
Runoff
Coefficient
1
Percentage
Impervious
1
Project:
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By:
Asphalt …………………......................... ............................................................................... 0.95 100% Date:
Concrete ………………. ......................... ............................................................................... 0.95 100%
Gravel (packed) ………. ......................... ............................................................................... 0.50 40%
Roofs………………….. ......................... ............................................................................... 0.95 90%
Pavers…………………. ......................... ............................................................................... 0.50 40%
Recycled Asphalt ......................... ............................................................................... 0.80 80%
Lawns and Landscaping
Playgrounds 25%
Sandy Soil
Flat <2% ………….. ......................... ............................................................................... 0.10 2%
Average 2% to 7% ......................... ............................................................................... 0.15 2%
Steep >7% ………… ......................... ............................................................................... 0.20 2%
Clayey Soil
Flat <2% ………….. ......................... ............................................................................... 0.20 2%
Average 2% to 7% ......................... ............................................................................... 0.25 2%
Steep >7% ………… ......................... ............................................................................... 0.35 2% 2-year Cf
= 1.00 10-year Cf
= 1.00
USDA SOIL TYPE: C
Sub-Basin ID
Sub-Basin
Area
(sq. ft.)
Sub-Basin
Area
(ac.)
Area of Asphalt
& Concrete
(ac.)
Area of Roofs
(ac.)
Area of Gravel
(ac.)
Area of
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
Existing A 6,085 0.140 0.014 0.025 0.000 0.101 0.25 0.25 0.31 28%
Existing B 488 0.011 0.002 0.000 0.000 0.009 0.25 0.25 0.31 20%
CHARACTER OF SURFACE:
Runoff
Coefficient
1
Percentage
Impervious
1
Project:
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By:
Asphalt ………………… ......................... ............................................................................. 0.95 100% Date:
Concrete ………………. ......................... ............................................................................. 0.95 100%
Gravel (packed) ………. ......................... ............................................................................. 0.50 40%
Roofs………………….. ......................... ............................................................................. 0.95 90%
Pavers…………………. ......................... ............................................................................. 0.50 40%
Recycled Asphalt ......................... ............................................................................. 0.80 80%
Lawns and Landscaping
Playgrounds 25%
Sandy Soil
Flat <2% ………….. ......................... ............................................................................. 0.10 2%
Average 2% to 7% ......................... ............................................................................. 0.15 2%
Steep >7% ………… ......................... ............................................................................. 0.20 2%
Clayey Soil
Flat <2% ………….. ......................... ............................................................................. 0.20 2%
Average 2% to 7% ......................... ............................................................................. 0.25 2%
Steep >7% ………… ......................... ............................................................................. 0.35 2% 2-year Cf = 1.00 10-year Cf = 1.00
USDA SOIL TYPE: C
Sub-Basin ID
Sub-Basin Area
(sq. ft.)
Sub-Basin Area
(ac.)
Area of Asphalt
& Concrete
(ac.)
Area of Roofs
(ac.)
Area of Pavers
(ac.)
Area of
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
Proposed A 6,206 0.14 0.038 0.038 0.005 0.062 0.55 0.55 0.69 53%
Proposed B 824 0.02 0.016 0.000 0.000 0.003 0.55 0.55 0.69 84%
Proposed OS1 1,139 0.03 0.002 0.012 0.000 0.013 0.60 0.60 0.75 47%
PROPOSED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
609 S. College Avenue
B. Ruch
Project: 609 S. College Avenue
Calculations By:
Date:
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
Existing A Existing A No 0.25 0.25 0.31 22.1 22.1 20.5 N/A N/A 22.1 22.1 20.5
Existing B Existing B No 0.25 0.25 0.31 5.9 5.9 5.5 N/A N/A 5.9 5.9 5.5
Existing OS Existing OS No 0.25 0.25 0.31 1.9 1.9 1.8 N/A N/A 5.0 5.0 5.0
EXISTING TIMES OF CONCENTRATION COMPUTATIONS
B. Ruch
July 1, 2020
Design
Point
Sub-Basin
Overland Flow Gutter/Swale Flow Time of Concentration
Velocity (Gutter Flow), V = 20·S½
Tt = L / 60V
Tc = Ti + Tt (Equation RO-2)
Velocity (Swale Flow), V = 15·S½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Overland Flow, Time of Concentration:
Gutter/Swale Flow, Time of Concentration:
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Project: 609 S. College Avenue
Calculations By:
Date:
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
Proposed A Proposed A No 0.25 0.25 0.31 12.0 12.0 11.1 1.87 1.2 13.2 13.2 12.3
Proposed B Proposed B No 0.35 0.35 0.44 6.1 6.1 5.4 N/A N/A 6.1 6.1 5.4
Proposed OS1 Proposed OS1 No 0.00 0.00 0.00 2.6 2.6 2.6 N/A N/A 5.0 5.0 5.0
Tt = L / 60V
PROPOSED TIME OF CONCENTRATION COMPUTATIONS
Overland Flow, Time of Concentration:
B. Ruch
July 1, 2020
Gutter/Swale Flow, Time of Concentration:
Tc = Ti + Tt (Equation RO-2)
Velocity (Gutter Flow), V = 20·S½
Velocity (Swale Flow), V = 15·S½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Design
Point
Sub-Basin
Overland Flow Gutter Flow Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
Project:
Calculations By:
Date:
Existing A Existing A 0.14 22 22 21 0.25 0.25 0.31 1.53 2.61 5.53 0.05 0.09 0.24
Existing B Existing B 0.01 6 6 5 0.25 0.25 0.31 2.76 4.72 9.95 0.01 0.01 0.03
Existing OS Existing OS 0.04 5 5 5 0.25 0.25 0.31 2.85 4.87 9.95 0.03 0.04 0.11
Rainfall Intensity:
EXISTING RUNOFF COMPUTATIONS
Rational Method Equation: 609 S. College Avenue
B. Ruch
July 1, 2020
Flow,
Q100
(cfs)
Rainfall Intensity taken from the Fort Collins Stormwater Criteria Manual (FCSCM), Tables RA-7 and RA-8
Design
Point
Sub-Basin(s)
Area, A
(acres)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
C2
C10
C100
Intensity,
i2
(in/hr)
Intensity,
i10
(in/hr)
Intensity,
i100
(in/hr)
Flow,
Q2
(cfs)
Flow,
Q10
(cfs)
Q = C f ( C )( i )( A )
Project:
Calculations By:
Date:
Proposed A Proposed A 0.14 13 13 12 0.55 0.55 0.69 1.98 3.39 7.16 0.16 0.27 0.70
Proposed B Proposed B 0.02 6 6 5 0.55 0.55 0.69 2.67 4.56 9.95 0.03 0.05 0.13
Proposed OS1 Proposed OS1 0.03 5 5 5 0.60 0.60 0.75 2.85 4.87 9.95 0.04 0.08 0.20
July 1, 2020
Intensity,
i10
(in/hr)
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
B. Ruch
609 S. College Avenue
PROPOSED RUNOFF COMPUTATIONS
C100
Design
Point
Flow,
Q100
(cfs)
Flow,
Q2
(cfs)
10-yr
Tc
(min)
2-yr
Tc
(min)
C2
Flow,
Q10
(cfs)
Intensity,
i100
(in/hr)
Sub-Basin(s)
Rational Method Equation:
Rainfall Intensity:
Rainfall Intensity taken from the Fort Collins Stormwater Criteria Manual (FCSCM), Tables RA-7 and RA-8
Q = C f ( C )( i )( A )
G
W
C
S
E
VAULT
ELEC
TELE
T
VAULT
ELEC
CONTROL
IRR
VAULT
ELEC
/ / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / / / / / / / / / /
X
E E E
W
SS
SS SS SS
605 COLLEGE LTD
605 S COLLEGE AVE
FORT COLLINS, CO 805243003
GATON III INC
DBA CANINOS ITALIAN RESTAURANT
613 S COLLEGE AVE
FORT COLLINS, CO 805243003
SOUTH COLLEGE AVENUE
DALZELL ALLEY
EXISTING 100-YR FLOODPLAIN
(CITY OF FORT COLLINS)
EX A
0.140 ac
EX A
EX B EX OS
EX OS
0.037 ac
EX B
0.011 ac
HISTORIC DRAINAGE EXHIBIT
FORT COLLINS, CO
609 S. COLLEGE
E NGINEER ING
N O R T H E RN
AUGUST 26, 2020
P:\564-005\DWG\DRNG\564-005_EXST DRNG.DWG
( IN FEET )
0
1 INCH = 20 FEET
20 20
DRAINAGE SUMMARY TABLE
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
G
W
C
S
E
VAULT
ELEC
TELE
T
VAULT
ELEC
VAULT
ELEC
/ / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / / / / / / / / / /
E
E
E
X
E E E
W
SS
SS SS SS
X
X X X
X X X X X
X
A
0.142 ac
A
OS
B OS
0.026 ac
B
0.019 ac
605 COLLEGE LTD
605 S COLLEGE AVE
FORT COLLINS, CO 805243003
GATON III INC
DBA CANINOS ITALIAN RESTAURANT
613 S COLLEGE AVE
FORT COLLINS, CO 805243003
SOUTH COLLEGE AVENUE
DALZELL ALLEY
EXISTING 100-YR FLOODPLAIN
(CITY OF FORT COLLINS)
PROPOSED DRAINAGE EXHIBIT
FORT COLLINS, CO
609 S. COLLEGE
E NGINEER ING
N O R T H E RN
AUGUST 26, 2020
P:\564-005\DWG\DRNG\564-005_DRNG.DWG
( IN FEET )
0
1 INCH = 20 FEET
20 20
DRAINAGE SUMMARY TABLE
DESIGN
POINT
XS#:1205 4990.5ft NGVD 29
XS#:1364 4990ft NGVD 29
XS#:1699 4992.8ft NGVD 29
XS#:1529 4992.5ft NGVD 29
XS#:1882 4993.7ft NGVD 29
XS#:1929 4992.3ft NGVD 29
S COLLEGE AVE
W MYRTLE ST
DALZELL ALY
E MYRTLE ST
CORBIN ALY
. Printed: 11/07/2019
All floodplain boundaries
are approximate.
This information is based on the Federal Emergency Management
Agency (FEMA) Flood Insurance Rate Map (FIRM) and the City of
Fort Collins Master Drainageway Plans. This letter does not imply
that the referenced property will or will not be free from flooding or
damage. A property not in the Special Flood Hazard Area or in a
City Designated Floodplain may be damaged by a flood greater
than that predicted on the map or from a local drainage problem
not shown on the map. This map does not create liability on the
part of the City, or any officer or employee thereof, for any damage
that results from reliance on this information.
City Flood Risk Map
Moderate Risk
May include:
- Areas of FEMA 500-year floodplain (FEMA Zone X-shaded).
- Areas of FEMA or City 100-year floodplain (sheet flow) with
average depths of less than 1 foot.
- Areas protected by levees from the 100-year flood.
Low Risk
Areas outside of FEMA and City mapped 100-year and 500-year
floodplains. Local drainage problems may still exist.
City Flood Fringe - May Include:
- Areas of FEMA 100-year floodplain (FEMA Zones A, AE, AO, and AH)
- Areas of City 100-year floodplain including ponding areas and sheet
flow areas with average depths of 1-3 feet.
There is a 1% annual chance that these areas will be flooded.
High Risk
City Floodway - Area of 100-year floodplain with greatest depths and
fastest velocities.
0 20 40 80 120 160
Feet
G
W
C
S
C
E
VAULT
ELEC
TELE
T
VAULT
ELEC
CONTROL
IRR
VAULT
ELEC
/ / / / / / / /
/ / / / / / / / / / / / / / / /
G
G
SOUTH COLLEGE AVENUE
DALZELL ALLEY
X X X
X X X
G
W
C
S
E
VAULT
ELEC
TELE
T
VAULT
ELEC
VAULT
ELEC
/ / / / / / / /
/ / / / / / / / / / / / / / / /
SOUTH COLLEGE AVENUE
DALZELL ALLEY
ROOFTOP
CONCRETE
ASPHALT
PAVERS
SURFACE
AREA (SF) % IMPERV.
IMPERV.
AREA (SF)
1,928
853
0
0
100%
100%
100%
40% 86
TOTALS 2,781 TOTAL= 2,781
ROOFTOP
CONCRETE
G
E
VAULT
ELEC
VAULT
ELEC
/ / / / / / / / / / / / / / / /
/ / / / / / / / / / / / / / / / / / / / / / / /
X
OHU
X X X X
X X X
X
X
X
RAIN GARDEN
REQUIRED VOLUME: 81 CU. FT.
(TO TREAT NEW AND MODIFIED IMPERVIOUS AREA)
PROVIDED VOLUME: 83 CU. FT.
GROUTED
COBBLE
SEDIMENT
BASIN
P:\564-005\DWG\DRNG\564-005_LID.DWG
609 S. COLLEGE AVE.
FORT COLLINS
COLORADO
301 N. Howes Street, Suite 100
Fort Collins, Colorado 80521
ENGINEER ING
N O R T H E RN
PHONE: 970.221.4158
www.northernengineering.com
DESCRIPTION
LID TREATMENT EXHIBIT
DRAWN BY
B. RUCH
DATE
August 26, 2020
PROJECT
564-005 LID-1
SCALE DRAWING
1"=20'
( IN FEET )
1 inch = ft.
30 0 30 Feet
30
LEGEND
RAIN GARDEN
TREATMENT AREA
RAIN GARDEN
UNTREATED AREA
609 S. College Ave.
Project Summary
Total Proposed Impervious Area 4,725 sf
Existing Impervious Area 2,781 sf
Unmodified Existing Impervious Area (Roof and Walks) 1,505 sf
New Impervious Area 1,944 sf
New and Modified Impervious Area 3,220 sf
Target Treatment Percentage New and Modified Impervious Area 75%
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia = 100.0 %
(100% if all paved and roofed areas upstream of rain garden)
B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 1.000
C) Water Quality Capture Volume (WQCV) for a 12-hour Drain Time WQCV = 0.40 watershed inches
(WQCV= 0.8 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including rain garden area) Area = 2,415 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 81 cu ft
Vol = (WQCV / 12) * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER = cu ft
Water Quality Capture Volume (WQCV) Design Volume
H) User Input of Water Quality Capture Volume (WQCV) Design Volume VWQCV USER = cu ft
(Only if a different WQCV Design Volume is desired)
2. Basin Geometry
A) WQCV Depth (12-inch maximum) DWQCV = 8 in
B) Rain Garden Side Slopes (Z = 4 min., horiz. dist per unit vertical) Z = 4.00 ft / ft
(Use "0" if rain garden has vertical walls)
C) Mimimum Flat Surface Area AMin = 48 sq ft
D) Actual Flat Surface Area AActual = 51 sq ft
E) Area at Design Depth (Top Surface Area) ATop = 199 sq ft
F) Rain Garden Total Volume VT= 83 cu ft
(VT= ((ATop + AActual) / 2) * Depth)
3. Growing Media
4. Underdrain System
A) Are underdrains provided?
B) Underdrain system orifice diameter for 12 hour drain time
i) Distance From Lowest Elevation of the Storage y = N/A ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 = N/A cu ft
iii) Orifice Diameter, 3/8" Minimum DO = N/A in
Design Procedure Form: Rain Garden (RG)
Benjamin Ruch
Northern Engineering
August 24, 2020
609 S. College
Fort Collins, CO
UD-BMP (Version 3.06, November 2016)
Choose One
Choose One
18" Rain Garden Growing Media
Other (Explain):
YES
NO
RG_UD-BMP_v3.06.xlsm, RG 8/24/2020, 6:12 AM
Sheet 2 of 2
Designer:
Company:
Date:
Project:
Location:
5. Impermeable Geomembrane Liner and Geotextile Separator Fabric
A) Is an impermeable liner provided due to proximity
of structures or groundwater contamination?
6. Inlet / Outlet Control
A) Inlet Control
7. Vegetation
8. Irrigation
A) Will the rain garden be irrigated?
Notes:
Design Procedure Form: Rain Garden (RG)
Benjamin Ruch
Northern Engineering
August 24, 2020
609 S. College
Fort Collins, CO
Choose One
Choose One
Choose One
Sheet Flow- No Energy Dissipation Required
Concentrated Flow- Energy Dissipation Provided
Plantings
Seed (Plan for frequent weed control)
Sand Grown or Other High Infiltration Sod
Choose One
YES
NO
YES
NO
RG_UD-BMP_v3.06.xlsm, RG 8/24/2020, 6:12 AM
Minimum Area to be Treated by LID measures 2,415 sf
Rain Garden
Total Rain Garden Run on Area (Existing and New/Modified Impervious Area)** 3,464 sf
**Rain Garden only sized to treat required impervious area (2,415 sf).
Total Treatment Area 2,415 sf
New and Modified Impervious Area Treatment Percentage** 75%
Percent Total Project Immpervious Area Treated** 73.3%
**Rain Garden only sized to treat 75% new and modified impervious area (2,415 sf).
** WATER QUALITY FOR ENTIRE SITE PROVIDED BY UDALL NATURAL AREA
ASPHALT
PAVERS
SURFACE
AREA (SF) % IMPERV.
IMPERV.
AREA (SF)
2,164
2,482
0
198
100%
100%
100%
40%
TOTALS 4,844 TOTAL= 4,725
EXISTING PROPOSED
2,164
79
1,928
853
0
2,482
0
P:\564-005\DWG\DRNG\564-005_IMPERVIOUS.DWG
609 S. COLLEGE AVE.
FORT COLLINS
COLORADO
301 N. Howes Street, Suite 100
Fort Collins, Colorado 80521
E NGINEER ING
N O R T H E RN
PHONE: 970.221.4158
www.northernengineering.com
DESCRIPTION
EXISTING VS. PROPOSED
IMPERVIOUS AREA
DRAWN BY
B. Ruch
DATE
August 26, 2020
PROJECT
564-005 SK-C1
SCALE DRAWING
1"=30'
CHANGE IN IMPERVIOUS AREA = 1,944 SF
1,360 SF ROOF UNMODIFIED
+ 145 SF WALK UNMODIFIED
--------------------------------
1,505 SF EXISTING IMPERVIOUS UNMODIFIED
75% TREATMENT NEW AND MODIFIED IMPERV.
= 75% (4,725 -1,505) = 2,415 SF
WATER QUALITY PROVIDED BY
UDALL NATURAL AREA.
BASIN
ID
TOTAL
AREA
(acres)
C2 C100
2-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q100
(cfs)
Existing A Existing A 0.14 0.25 0.31 22.1 20.5 0.05 0.24
Existing B Existing B 0.01 0.25 0.31 5.9 5.5 0.01 0.03
Existing OS Existing OS 0.04 0.25 0.31 5.0 5.0 0.03 0.11
Proposed A Proposed A 0.14 0.55 0.69 13.2 12.3 0.16 0.70
Proposed B Proposed B 0.02 0.55 0.69 6.1 5.4 0.03 0.13
Proposed OS1 Proposed OS1 0.03 0.60 0.75 5.0 5.0 0.04 0.20
C2 C100
2-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q100
(cfs)
Existing A Existing A 0.14 0.25 0.31 22.1 20.5 0.05 0.24
Existing B Existing B 0.01 0.25 0.31 5.9 5.5 0.01 0.03
Existing OS Existing OS 0.04 0.25 0.31 5.0 5.0 0.03 0.11
Proposed A Proposed A 0.14 0.55 0.69 13.2 12.3 0.16 0.70
Proposed B Proposed B 0.02 0.55 0.69 6.1 5.4 0.03 0.13
Proposed OS1 Proposed OS1 0.03 0.60 0.75 5.0 5.0 0.04 0.20
C Cf L
Ti
= −
C Cf L
Ti
= −
July 1, 2020
1) Runoff coefficients are taken from the Fort Collins Stormwater Criteria Manaual, Table 4.1-3and Table 3.2-2.
100-year Cf = 1.25
Composite Runoff Coefficient with Adjustment
Existing OS 1,594 0.037 0.003 0.019 0.000 0.014 0.25 0.25 0.31 57%
1) Runoff coefficients are taken from the Fort Collins Stormwater Criteria Manaual, Table 4.1-3and Table 3.2-2.
EXISTING COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
609 S. College Avenue
B. Ruch
July 1, 2020
100-year Cf
= 1.25
Composite Runoff Coefficient with Adjustment