HomeMy WebLinkAboutSTRAUSS CABIN CHURCH - BDR200003 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTDRAINAGE LETTER
STRAUSS CABIN CHURCH SUBDIVISION
FORT COLLINS, CO
1501 ACADEMY CT. STE 203 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com
February 25, 2020
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE.: STRAUSS CABIN CHURCH SUBDIVISION
FORT COLLINS, COLORADO
PROJECT NUMBER: U19035
Dear Staff,
United Civil Design Group, LLC. is pleased to submit this Drainage Letter for the Strauss Cabin Church Subdivision site in Fort
Collins, Colorado. In general, this summary serves to document the stormwater impacts associated with the proposed site
improvements.
SITE LOCATION AND PROJECT DESCRIPTION
The Strauss Cabin Church Subdivision site (referred herein as ”the site”) is situated in the Southwest Quarter of Section 3,
Township 6 North, Range 68 West of the 6th Principal Meridian, City of Fort Collins, Larimer County, State of Colorado. More
specifically, the site is located immediately east of Strauss Cabin Road, just north of the intersection with Big Dipper Drive
and just south of the intersection with Rock Creek Drive. The planned development for the site will consist of the following:
The construction of a 225 sf addition to the existing building.
The construction of a fire hydrant and new water service on the site.
The construction of sidewalk in front of the building addition, replacing existing asphalt parking area.
HYDROLOGIC CRITERIA
City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, provided by Figure RA-16 of the Fort Collins Stormwater
Criteria Manual, are utilized for all hydrologic computations related to the site in its existing/historic and proposed conditions.
Since this site is relatively small and does not have complex drainage basins, the peak flow rates for design points have been
calculated based on the Rational Method as described in the Urban Storm Drainage Criteria Manual (USDCM) and the City of
Fort Collins Stormwater Criteria Manual (FCSCM) with storm duration set equal to the time of concentration for each sub-
basin. This method was used to analyze the developed runoff from the 2-year (minor) and the 100-year (major) storm events.
The Rational Method is widely accepted for drainage design involving small drainage areas (less than 160 acres) and short
time of concentrations. Runoff coefficients are assumed based on impervious area and are given in the Appendices.
EXISTING CONDITIONS
Hydrology
The site, located within the McClellands Drainage Basin, is approximately 1.82 acres and currently consists of two buildings
housing the Mountain Life Church, a paved parking lot, associated utilities, and drainage infrastructure. The existing site is
divided into two basins in existing drainage patterns, generally delineated as the parking lot on the west side of the building
and the remainder of the site including the building, eastern parking area, and landscaped areas. The eastern basin currently
drains via overland flow to the Fossil Creek Inlet Canal immediately east of the site. The western basin drains to a roadside
ditch along the east side of Strauss Cabin Road. This roadside ditch appears to drain south along Strauss Cabin Road and
ultimately to the Fossil Creek Inlet Canal.
Floodplain
The site is not located within a FEMA or City-regulatory floodway or floodplain.
DRAINAGE LETTER
STRAUSS CABIN CHURCH SUBDIVISION
FORT COLLINS, CO
1501 ACADEMY CT. STE 203 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com
IMPROVED CONDITIONS
The proposed site improvements include a new fire hydrant on site, reconfigured sidewalk, a building expansion of
approximately 225 feet and an addition of a second floor to an existing building. Nearly all the proposed improvements
involve the removal and replacement of existing impervious surfaces, resulting in an increase of impervious area of 245
square-feet on site. The table below summarizes the hydrologic impact associated with the proposed site improvements
relative to existing conditions. Refer to the Drainage Plan and Hydrology calculations attached for additional improvements.
HYDROLOGIC SUMMARY TABLE
Existing Proposed
Overall Area (acre) 1.82 1.82
*Roof (sf) 4,062 4,479
*Asphalt (sf) 26,676 26,340
*Concrete (sf) 7,487 7,494
*Gravel (sf) 275 275
*Landscape (sf) 40,901 40,813
% Imperviousness 47.9% 47.9%
Composite C2 0.59 0.59
Composite C100 0.73 0.74
Q2 (cfs) 3.05 3.06
Q100 (cfs) 12.72 12.75
Runoff flows for the improved condition remain largely unchanged from existing conditions. Drainage patterns are unchanged
with the roof, eastern parking lot, and landscape areas draining east to the Fossil Creek Inlet Canal and the western parking
lot draining to the roadside ditch on the east side of Strauss Cabin Road.
DETENTION & WATER QUALITY
The total site increase of impervious area is 245 square feet. This area includes the proposed building addition and
reconfigured sidewalks. Due to the minimal impacts related to the site’s overall increase in imperviousness (i.e. less than
1,000 sf), the site improvements are exempt of detention and water quality requirements per City of Fort Collins criteria.
LOW IMPACT DEVELOPMENT (LID)
In February 2013, City Council adopted the LID policy and criteria. Due to the minimal impacts related to the site’s overall
increase in imperviousness (i.e. less than 1,000 sf), the site improvements are exempt of LID requirements per City of Fort
Collins criteria.
EROSION CONTROL
Erosion control, both temporary and permanent, is a vital part of any development project. For this project, the site
disturbance is less than 1 acre; therefore, a CDPHE Stormwater Management Plan (SWMP) is not required. However,
comprehensive erosion control measures are included with the site improvements. Refer to the Utility Plans for additional
information.
CONCLUSIONS
The drainage system proposed within this Drainage Letter provides adequate conveyance of the developed stormwater
runoff from the proposed improvements to existing drainageways adjacent to the site. A minor increase in the overall
imperviousness is proposed with the site improvements; however, existing drainage patterns are unchanged and the added
impervious area is less than 1,000 square feet, meaning the project is exempt from City of Fort Collins detention, water
quality, or LID requirements per City criteria.
DRAINAGE LETTER
STRAUSS CABIN CHURCH SUBDIVISION
FORT COLLINS, CO
1501 ACADEMY CT. STE 203 | FORT COLLINS, CO 80524 | 970-530-4044 | www.unitedcivil.com
We understand that review by the City of Fort Collins to assure general compliance with standardized criteria contained in
the Fort Collins Stormwater Criteria Manual (FCSCM). This summary was prepared in compliance with technical criteria set
forth in both the Urban Drainage and Flood control District (UDFCD) Urban Storm Drainage Criteria Manual and the FCSCM.
If you should have any questions or comments as you review this drainage summary, please feel free to contact us at your
convenience.
Sincerely,
United Civil Design Group, LLC
Kevin Brazelton, P.E.
Principal
APPENDICES
APPENDIX A – Hydrologic Calculations
APPENDIX B – Drainage Exhibits
APPENDIX A
HYDROLOGIC CALCULATIONS
RUNOFF COEFFICIENTS AND % IMPERVIOUS
Strauss Cabin Church Subdivision, Fort Collins, CO
Basin Design Pt. Composite NRCS Soil
Total Total Roof
(1) Asphalt Concrete
(1)
Gravel
(1)
Lawns
(1) Imperviousness Type C
2 C
100
%I = 90% %I = 100% %I = 90% %I =40% %I=2% (%I)
acres sf sf sf sf sf sf
EX-WEST EX-WEST 0.54 23,598 1,481 18,462 1,641 - 2,014 90.3% C/D 0.89 1.00
EX-EAST EX-EAST 1.28 55,803 2,581 8,215 5,846 275 38,887 29.9% B 0.46 0.57
- - - - -
1.82 79,401 4,062 26,676 7,487 275 40,901 47.9% 0.59 0.73
Basin Design Pt. Composite NRCS Soil
Total Total Roof
(1) Asphalt Concrete
(1)
Gravel Lawns
(1) Imperviousness Type
C2
C100
acres sf %I = 90% %I = 100% %I = 90% %I =40% %I=2% (%I)
sf sf sf sf sf
PR-WEST PR-WEST 0.54 23,598 1,550 18,335 1,632 - 2,081 90.0% C/D 0.89 1.00
PR-EAST PR-EAST 1.28 55,803 2,929 8,006 5,862 275 38,733 30.1% B 0.46 0.58
- 0.00 - - - -
1.82 79,401 4,479 26,340 7,494 275 40,813 47.9% 0.59 0.74
Notes:
(1) Recommended % Imperviousness Values per USDCM Volume 1, Table 6-3.
(2) Runoff C is based on %I and NRCS soil group per UDFCD Volume 1, Table 6-4.
Proposed Total
Composite Runoff Coefficients
Areas (2)
Existing Basins
Proposed Basins
Existing Total
Areas Composite Runoff Coefficients
(2)
Date: 2/17/2020 P:\U19035 - Strauss Cabin Church\Reports\Drainage\Calculations\U19035-Drain Calcs Updated
2-YEAR TIME OF CONCENTRATION
Strauss Cabin Church Subdivision, Fort Collins, CO
Basin Design Pt. Area
C(1) Length Slope ti
(2) Length Slope Cv Velocity (3) tt
(4) Check tc? Total Length tc
(5)
acres ft % min ft % fps min min Urban? ft min min
EX-WEST EX-WEST 0.54 0.89 105 3.5% 2.6 15 1.0% 20
2.0 0.1 2.7 Yes 120 11 5.0
EX-EAST EX-EAST 1.28 0.46 70 10.0% 4.5 0 20.0% 10 4.5 0.0 4.5 Yes 70 10 5.0
Basin Design Pt. Area
C(1) Length Slope ti
(2) Length Slope Cv Velocity (3) tt
(4) Check tc? Total Length tc
(5)
acres ft % min ft % fps min min Urban? ft min min
PR-WEST PR-WEST 0.54 0.89 105 3.5% 2.6 15 1.0% 20
2.0 0.1 2.7 Yes 120 11 5.0
PR-EAST PR-EAST 1.28 0.46 70 10.0% 4.5 0 20.0% 10 4.5 0.0 4.5 Yes 70 10 5.0
Notes:
(1) C=Cx*Cf
(2)ti = [1.87(1.1-CxCf)L1/2]/S1/3, S= slope in %, L=length of overland flow (300' max)
(3)V=CvS0.5, S=watercourse slope in ft/ft, UDFCD Equation RO-4
(4)tt=L/(V*60 sec/min)
(5)tc check (for urban or developed areas only) = total length/180 + 10
(6)min tc = 5 min
Existing Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
Proposed Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
Date: 2/17/2020 P:\U19035 - Strauss Cabin Church\Reports\Drainage\Calculations\U19035-Drain Calcs Updated
100-YEAR TIME OF CONCENTRATION
Strauss Cabin Church Subdivision, Fort Collins, CO
Basin Design Pt. Area
C(1) Length Slope ti
(2) Length Slope Cv Velocity (3) tt
(4) Check tc? Total Length tc
(5)
acres ft % min ft % fps min min Urban? ft min min
EX-WEST EX-WEST 0.54 1.00 105 3.5% 1.2 15 1.0% 20
2.0 0.1 1.3 Yes 120 11 5.0
EX-EAST EX-EAST 1.28 0.57 70 10.0% 3.7 0 20.0% 10 4.5 0.0 3.7 Yes 70 10 5.0
Basin Design Pt. Area
C(1) Length Slope ti
(2) Length Slope Cv Velocity (3) tt
(4) Check tc? Total Length tc
(5)
acres ft % min ft % fps min min Urban? ft min min
PR-WEST PR-WEST 0.54 1.00 107 3.5% 1.2 15 1.0% 20
2.0 0.1 1.4 Yes 122 11 5.0
PR-EAST PR-EAST 1.28 0.58 70 10.0% 3.7 0 20.0% 10 4.5 0.0 3.7 Yes 70 10 5.0
Notes:
(1) C=Cx*Cf
(2)ti = [1.87(1.1-CxCf)L1/2]/S1/3, S= slope in %, L=length of overland flow (300' max)
(3)V=CvS0.5, S=watercourse slope in ft/ft, UDFCD Equation RO-4
(4)tt=L/(V*60 sec/min)
(5)tc check (for urban or developed areas only) = total length/180 + 10
(6)min tc = 5 min
Existing Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
Proposed Basins
Travel/Channelized Time of Flow (tt)
ti+tt
tc Check for Urbanized Basins
Final tc
(6)
Date: 2/17/2020 P:\U19035 - Strauss Cabin Church\Reports\Drainage\Calculations\U19035-Drain Calcs Updated
RATIONAL METHOD PEAK RUNOFF
Strauss Cabin Church Subdivision, Fort Collins, CO
Basin Design Pt. Area 2-Year 100-Year
acre tc tc C2 C100 I2 I100 Q2 Q100
min min in/hr in/hr cfs cfs
EX-WEST EX-WEST 0.54 5 5 0.89 1.00 2.85 9.95 1.37 5.39
EX-EAST EX-EAST 1.28 5 5 0.46 0.57 2.85 9.95 1.68 7.33
3.05 12.72
Basin Design Pt. Area 2-Year 100-Year
acre tc tc C2 C100 I2 I100 Q2 Q100
min min in/hr in/hr cfs cfs
PR-WEST PR-WEST 0.54 5 5 0.89 1.00 2.85 9.95 1.37 5.39
PR-EAST PR-EAST 1.28 5 5 0.46 0.58 2.85 9.95 1.69 7.36
3.06 12.75
Existing Basins
Totals
Totals
Peak Discharge
Proposed Basins
Runoff Coefficients Rainfall Intensity Peak Discharge
RunoffIntensity Coefficients Rainfall
Date: 2/17/2020 P:\U19035 - Strauss Cabin Church\Reports\Drainage\Calculations\U19035-Drain Calcs Updated
APPENDIX B
DRAINAGE EXHIBITS
STRAUSS CABIN ROAD
LIMITS OF DISTURBANCE
FFE=4894.00
LIMITS OF DISTURBANCE
EAST
1.28 0.46
0.58
WEST
0.54 0.89
1.00
3.7%
5.7%
4.5%
3.0%
2.0%
1.2%
3.5%
10.1%
5.7%
7.7%
1.4%
6.5%
25.1%
12.0%
2.3%
3.2%
32.2%
33.3%
34.3%
31.3%
22.4%
15.8%
12.5%
22.9%
5.4%
2.5%
45.0%
37.5%
4.3%
1.5%
EXISTING DRAINAGE SWALE
FOSSIL CREEK INLET CANAL
FOSSIL CREEK CONCEPTS, LLC 02/25/2020
U19035
1" = 30'
1" = N/A
STRAUSS CABIN CHURCH SUBDIVISION
NORTH
2/17/2020 10:29:52 AM
P:\U19035 - STRAUSS CABIN CHURCH\CADD\ET\DRAINAGE PLAN.DWG
DRAINAGE PLAN
KRB JRS
1
1
DRAINAGE PLAN
0 30' 60'
SCALE: 1" = 30'
15'
1
LOT LINE
PROPERTY BOUNDARY
RIGHT OF WAY
EASEMENT
CENTERLINE
EXISTING MAJOR CONTOUR
EXISTING MINOR CONTOUR
LIMITS OF DISTURBANCE
LEGEND
EXISTING
CURB & GUTTER
ASPHALT PAVEMENT
CONCRETE PAVEMENT
OR SIDEWALK
SITE
NOTES
1. EXISTING UNDERGROUND AND OVERHEAD PUBLIC AND PRIVATE UTILITIES AS
SHOWN ARE INDICATED ACCORDING TO THE BEST INFORMATION AVAILABLE TO THE
ENGINEER. THE ENGINEER DOES NOT GUARANTEE THE ACCURACY OF SUCH
INFORMATION. EXISTING UTILITY MAINS AND SERVICES MAY NOT BE STRAIGHT
LINES OR AS INDICATED ON THESE DRAWINGS. THE CONTRACTOR SHALL BE
RESPONSIBLE TO CALL ALL UTILITY COMPANIES (PUBLIC AND PRIVATE) PRIOR TO
ANY CONSTRUCTION TO VERIFY EXACT UTILITY LOCATIONS.
2. THE CONTRACTOR SHALL PROTECT ALL EXISTING FEATURES THAT ARE NOT TO BE
REMOVED ADJACENT TO THE CONSTRUCTION AREA INCLUDING, BUT NOT LIMITED
TO, SIDEWALKS, PRIVATE FENCES, ABOVE GROUND OR UNDERGROUND UTILITIES,
STRUCTURES, AND UNDERGROUND FOUNDATIONS. THE CONTRACTOR SHALL BE
RESPONSIBLE FOR ANY DAMAGE THAT SHOULD OCCUR TO ANY ON-SITE, OFF-SITE,
PUBLIC OR PRIVATE FACILITY OR FEATURE AS A RESULT OF THE CONSTRUCTION
PROCESS FOR THIS PROJECT.
PROPOSED MAJOR
CONTOUR
PROPOSED MINOR
CONTOUR
GRADING
X.XX
XX.X X.XX
X BASIN DESIGNATION
BASIN AREA (ACRE)
5 - YR RUNOFF COEFF.
100 - YR RUNOFF COEFF.
FLOW DIRECTION
DP DESIGN POINT
D1
PREPARED FOR:
JOB NUMBER
SHEET NUMBER
OF SHEETS
DATE SUBMITTED:
The engineer preparing these plans will not be responsible
for, or liable for, unauthorized changes to or uses of these
plans. All changes to the plans must be in writing and
must be approved by the preparer of these plans.
NO. BY DATE
CAUTION
REVISIONS:
VERTICAL:
HORIZONTAL:
SCALE
PRELIMINARY
PLANS
NOT FOR CONSTRUCTION
1501 Academy Ct.
Ste. 203
Fort Collins, CO 80524
(970) 530-4044
www.unitedcivil.com
Civil Engineering &
Consulting
PROJ. MGR:
DRAWING NAME:
PATH:
DATE: TIME: DESIGNER:
UNITED CIVIL
Design Group LLC