HomeMy WebLinkAboutCHRISTIAN BROTHERS AUTOMOTIVE (CBA) REPAIR STORE - PDP - PDP150017 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTFINAL DRAINAGE AND EROSION CONTROL REPORT
FOR THE PROPOSED
CHRISTIAN BROTHERS AUTOMOTIVE
LOCATED AT THE NORTHEAST CORNER OF
S. COLLEGE AVENUE & FOSSIL CREEK PARKWAY
FORT COLLINS, CO
PREPARED FOR
CHRISTIAN BROTHERS AUTOMOTIVE CORPORATION
17725 KATY FREEWAY, SUITE 200
HOUSTON, TX 77094
PREPARED BY
17725 KATY FREEWAY, SUITE 103
HOUSTON, TX 77094
TABLE OF CONTENTS
Cover Letter
1. GENERAL LOCATION AND DESCRIPTION
A. LOCATION
B. PROPERTY DESCRIPTION
C. FLOODPLAIN
2. DRAINAGE BASINS AND SUB-BASINS
A. MAJOR BASIN DESCRIPTION
B. MINOR BASIN DESCRIPTION
3. DRAINAGE DESIGN CRITERIA
A. REGULATIONS
B. FOUR STEP PROCESS
C. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINT
D. HYDROLOGICAL CRITERIA
E. HYDRAULIC CRITERIA
F. FLOODPLAIN REGULATIONS COMPLIANCE
G. MODIFICATIONS OF CRITERIA
4. FACILITY DRAINAGE DESIGN
A. GENERAL CONCEPT
B. SPECIFIC DETAILS
5. CONCLUSIONS
A. COMPLIANCE WITH STANDARDS
B. DRAINAGE CONCEPT
APPENDICES:
APPENDIX A - Hydrologic, Hydraulic, Storm Sewer, Detention and Outfall Computations
1. Existing Conditions Runoff Calculations
2. Proposed Drainage Area A Runoff Calculations
3. Proposed Drainage Area B1 Runoff Calculations
4. Proposed Drainage Area B2 Runoff Calculations
5. Proposed Pond Drainage Area Runoff Calculations
6. Storm Sewer Calculations
7. Detention Pond Calculations
8. WQCV (Water Quality Capture Volume) Calculations
9. Detention Pond Volume Calculations
10. Offsite Runoff Estimate for 36” Pipe in Snead Drive ROW
11. Offsite Runoff Estimate from 2 - 16" Pipe under S. College Avenue ROW
12. Inlet Calculations
APPENDIX B - Floodplain Map
LIST OF TABLES AND FIGURES
Figure 1 – Vicinity Map
Figure 2 – Aerial Photograph
Figure 3 – FEMA Regulatory Map
August 24, 2015
Mr. Wes Lamarque
City of Fort Collins
PO Box 580
281 North College Avenue
Fort Collins, CO 80522
RE: Christian Brothers Automotive – Lot 1, Fossil Creek Basin
Dear Mr. Lamarque:
This project consists of one commercial building with associated parking and stormwater detention pond.
Construction is scheduled to begin spring 2016. This project is in the Fossil Creek Drainage Basin.
The project is approximately 1.86 acres located at the northeast corner of S. College Avenue and Fossil
Creek Parkway. The site is located in the Southwest Quarter of Section 1, Township 6 North, Range 69
West of the 6th P.M., City of Fort Collins, County of Larimer, State of Colorado.
All designs and calculations within this report are in compliance with the Urban Storm Drainage Criteria
Manual, Volume 3 and the Fort Collins Stormwater Manual. All basins in this project use the rational
method for runoff calculations.
If you have any questions or need any additional information regarding this report, please call.
Sincerely,
Dylan Hedrick, PE
Colorado License #46228
TABLE OF CONTENTS
6. GENERAL LOCATION AND DESCRIPTION
D. LOCATION
E. PROPERTY DESCRIPTION
F. FLOODPLAIN
7. DRAINAGE BASINS AND SUB-BASINS
C. MAJOR BASIN DESCRIPTION
D. MINOR BASIN DESCRIPTION
8. DRAINAGE DESIGN CRITERIA
H. REGULATIONS
I. FOUR STEP PROCESS
J. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINT
K. HYDROLOGICAL CRITERIA
L. HYDRAULIC CRITERIA
M. FLOODPLAIN REGULATIONS COMPLIANCE
N. MODIFICATIONS OF CRITERIA
9. FACILITY DRAINAGE DESIGN
A. GENERAL CONCEPT
B. SPECIFIC DETAILS
10. CONCLUSIONS
A. COMPLIANCE WITH STANDARDS
B. DRAINAGE CONCEPT
APPENDICES:
APPENDIX A - Hydrologic, Hydraulic, Storm Sewer, Detention and Outfall Computations
13. Existing Conditions Runoff Calculations
14. Proposed Drainage Area A Runoff Calculations
15. Proposed Drainage Area B1 Runoff Calculations
16. Proposed Drainage Area B2 Runoff Calculations
17. Proposed Pond Drainage Area Runoff Calculations
18. Storm Sewer Calculations
19. Detention Pond Calculations
20. WQCV (Water Quality Capture Volume) Calculations
21. Detention Pond Volume Calculations
22. Offsite Runoff Estimate for 36” Pipe in Snead Drive ROW
23. Offsite Runoff Estimate from 2 - 16" Pipe under S. College Avenue ROW
24. Inlet Calculations
APPENDIX B - Floodplain Map
LIST OF TABLES AND FIGURES
Figure 1 – Vicinity Map
Figure 2 – Aerial Photograph
Figure 3 – FEMA Regulatory Map
1. GENERAL LOCATION AND DESCRIPTION
A. LOCATION
1. VICINITY MAP
Figure 1. Vicinity Map
2. The project consists of a single-story commercial building, associated parking areas and
a detention pond with water quality controls for the developed portion of the site.
3. The site is bound on the west by South College Avenue (U.S. 287) and on the south by
Fossil Creek Parkway.
4. The project site is located in Lot 1 of the Discount Tire plat for Larimer County.
B. PROPERTY DESCRIPTION
Figure 2. Aerial Photograph
1. The subject property is approximately 1.875 acres.
2. The subject property currently consists of open area. Ground cover generally consists of
grass. An existing asphalt access drive provides access from Fossil Creek Parkway to
the parcel to the north of the subject property.
3. General topography slopes from the northwest corner of the site to an existing drainage
ditch which runs along the eastern edge of the site. The ditch flows through a headwall
and culverts which flow south under the Fossil Creek right-of-way.
4. Fall from the northwest corner to the southwest corner of the site to the existing drainage
ditch headwall is approximately 22’ (elevation 4958’ to approximately 4936’). Ground
slopes are gentle across the site (±2%).
5. The site receives offsite stormwater flow through a headwall structure with two 16” steel
pipes along the S. College Avenue right-of-way and flows south through the existing tree
strand, then east parallel to Fossil Creek Parkway. The water then outfalls off the site
through a 46”x30” CMP under Fossil Creek Parkway.
6. According to the United States Department of Agriculture (USDA) Natural Resources
Conservation Service (NRCS) Soil Survey, the site consists largely of Fort Collins loam
(Hydrologic Soil Group B) with Renohill clay loam (Hydrologic Soil Group D) at the
northwest corner of the site.
7. The proposed project will develop the majority of the existing site, constructing an
automotive service center. Parking areas and associated utilities will be constructed. A
water detention pond will be constructed at the southeast corner of the site.
C. FLOODPLAIN
1. The site is located in Zone X (Area of Minimal Flood Hazard) according to FEMA Panel
08069C1000F.
2. The proposed structure is outside of the 100-year and 500-year floodplain.
3. The vertical datum utilized for site survey work is the City of Fort Collins benchmarks City
of Fort Collins 7-94 Elevation = 4989.27 NAVD 1988 and City of Fort Collins 9-94
Elevation=4978.57 NAVD 1988.
4. The FEMA floodplain map is contained in Appendix B.
2. DRAINAGE BASINS AND SUB-BASINS
A. MAJOR BASIN DESCRIPTION
1. The project is located within the Fossil Creek Drainage Basin. Fossil Creek runs south of
Fossil Creek Parkway approximately 500’ south of the site.
2. Developed areas west of S. College Avenue drain historically through the project site.
These off-site drainage flow patterns will be maintained and accounted for with the
proposed development.
3. The design will maintain the existing on-site flow pattern to the maximum extent
practicable.
B. MINOR BASIN DESCRIPTION
1. The site has been divided into several smaller drainage basins for analysis.
2. The basins include the north side of the building and parking lot (Drainage Basin A), The
south side of the building and driveway (Drainage Basins B1 & B2), the detention pond
(Pond Drainage Basin), and the offsite flow contained in a swale.
3. The proposed basins are described in greater detail in IV. Facility Drainage Design.
3. DRAINAGE DESIGN CRITERIA
A. REGULATIONS
Design of the site must conform to the drainage basin design of the Fossil Creek Basin Master
Drainage Plan and the Fort Collins Stormwater Manual.
B. FOUR STEP PROCESS
The overall stormwater management strategy employed with the proposed project utilizes the
“Four Step Process” to minimize adverse impacts of development and urbanization on receiving
waters. The following is a description of how the proposed development has incorporated each
step.
Step 1 – Employ Runoff Reduction Practices
Several techniques have been utilized within the proposed project to reduce runoff peak flow,
runoff volumes and pollutant loads as the site is developed from the current state by
implementing multiple Low Impact Development (LID) strategies including:
Conserving existing vegetation amenities within the site including existing vegetated
areas and large trees around the perimeter of the site.
Providing large vegetated open areas throughout the site to reduce the overall
impervious area and to minimize directly connected impervious area (MDCIA).
Routing flows, to the maximum extent feasible, through vegetated swales to increase
time of concentration, promote infiltration and provide initial water quality.
Step 2 – Implement BMPs that Provide a Water Quality Capture Volume (WQCF) with Slow
Release
The efforts described above in Step 1 will facilitate the reduction of runoff; however, urban
development such as the proposed project generates stormwater runoff that requires
additional BMPs and water quality structures. Dry wells, permeable pavement, and a rain
garden will be utilized as BMPs for runoff from pervious pavement and buildings. All
stormwater runoff from the site will ultimately be routed to the southeast corner of the site
where it is intercepted and treated by a water quality pond prior to exiting the site.
Step 3 – Stabilize Drainageways
The site accepts offsite stormwater runoff from a headwall and culverts running under S.
College Avenue as described above. The project will provide a grassed swale sized
appropriately for the 100-year flow to convey the runoff to the ultimate outfall at the southeast
corner of the site. Additionally, the site’s BMPs provide water quality treatment where none
previously existed thus removing erosion and sediment potential from onsite drainageways.
Furthermore, this project will pay one-time stormwater development fees, as well as ongoing
monthly stormwater utility fees, which are used by the City to achieve city-wide drainageway
stability.
Step 4 – Implement Site Specific and Other Source Control BMPs
The proposed project includes an automotive service center which require the need for site
specific source controls including:
A localized trash enclosure for the disposal of waste
Regular street sweeping of parking areas to control dust and sediment
Frequent watering of landscaping to prevent erosion through wind
C. DEVELOPMENT CRITERIA REFERENCE AND CONSTRAINT
The subject property is an “in-fill” development as the project is currently surrounded by developed
properties. As such, constraints have been identified during the course of this analysis that will impact
the proposed drainage system including:
Existing elevations along the perimeter of the site which the proposed grades must match
Stormwater runoff from offsite sources which must be conveyed
As previously mentioned, overall drainage patterns of the existing site will be maintained.
D. HYDROLOGICAL CRITERIA
1. The City of Fort Collins Rainfall Intensity-Duration-Frequency Curves, as depicted in Figure
RA-16 of the FCSCM, serve as the source for all hydrologic computations associated with the
proposed development. Tabulated data contained in Table RA-7 have been utilized for
Rational Method Runoff Calculations.
2. The Rational Method has been used to compute stormwater runoff using coefficients
contained in Tables RO-11 and RO-12 of the FCSCM.
3. Three separate design storms have been used to address distinct drainage scenarios. A
fourth design storm has been computed for comparison purposes. The first design storm is
considered the 80th percentile rain event, which has been used to design the project’s water
quality features. The second event analyzed is the “Minor” or “Initial” storm which has a 2-
year recurrence interval. The third event is the “Major” storm, which has a 100-year
recurrence interval. The fourth storm is the 10-year storm event computed for comparison
purposes only.
4. No other assumptions or calculation methods were used with this development which are not
referenced by the current City of Fort Collins criteria.
E. HYDRAULIC CRITERIA
1. As previously noted, the subject property maintains historic drainage patterns.
2. All drainage facilities proposed with the project are designed in accordance with criteria
outlined in the FCSCM and/or the Urban Drainage and Flood Control District (UDFCD) Urban
Storm Drainage Criteria Manual.
3. As stated above, the subject property is not located in either a FEMA regulatory or a City of
Fort Collins designated floodplain.
4. The subject project does not propose to modify any natural drainageways.
F. FLOODPLAIN REGULATIONS COMPLIANCE
1. As previously mentioned, this project is not subject to any floodplain regulations. However,
the building has been designed to ensure that the proposed building will not suffer damage
during the 100-year storm event. The proposed detention and water quality pond will provide
a rip-rap emergency spillway which coveys flow to the proposed drainage ditch conveying
offsite flow south of the site.
G. MODIFICATIONS OF CRITERIA
1. The owner is not requesting any modifications to the drainage criteria at this time.
4. FACILITY DRAINAGE DESIGN
A. GENERAL CONCEPT
1. The main objective of the drainage design is to maintain existing drainage patterns, ensure
no adverse impacts to any adjacent properties and to maintain the drainage concepts
outlined in the UDFCD.
2. Developed areas to the northwest of the site historically drain through the project site. These
offsite drainage flows and patterns will be maintained and accounted for with the proposed
development.
3. A list of tables and figures used within this report can be found in the Table of Contents at the
front of the document. The tables and figures are located within sections to which the content
best applies.
4. The drainage patterns anticipated for proposed drainage basins are described below:
Drainage Area A
Basin 1 consists of the northern half of the site including the parking lot and northern half of
the building. The basin drains towards the southeast through curb cuts into a rain garden
south of the vehicle storage area. Excess flow drains into a proposed grate inlet in the
southeast corner of the vehicle storage area.
Drainage Areas B1 & B2
Drainage Areas B1 & B2 consist of the southwest portion of the site including the southern
half of the proposed building. The basin drains east to a proposed inlet south of the proposed
building which then flows to a proposed gutter inlet in the driveway entrance to the site. The
runoff then flows into the detention pond and ultimately out through the detention pond outfall.
Drainage Area Pond
Drainage Area Pond consists of the eastern half of the site including the detention and water
quality pond. The basin drains to the east through the pond’s stormwater quality outfall.
Excess flow drains through an emergency overflow from the pond to an inlet behind Snead
Drive right-of-way.
B. SPECIFIC DETAILS
1. Water quality treatment is provided for the proposed development through permeable
pavement, a rain garden and a water quality pond.
Final design details, construction documentation, and a Standard Operating Procedures
(SOP) manual shall be provided to the City of Fort Collins for review prior to Final
Development Plan approval. A final copy of the approved SOP manual shall be provided to
the City and must be maintained onsite by the entity responsible for the facility maintenance.
Annual reports must also be prepared and submitted to the City discussing the results of the
maintenance program (i.e. inspection dates, inspection frequency, volume loss due to
sedimentation, corrective actions taken, etc.)
2. The LID calculations for the proposed BMPs is included in Appendix A.
3. Proper maintenance of the drainage facilities designed with the proposed development is a
critical component of their ongoing performance and effectiveness. The BMPs may be easily
accessed by the maintenance staff via an observation well in the case of the dry wells or by
physical access in the case of the permeable pavement and rain garden.
4. The drainage features associated with the proposed project are all private facilities located on
private property with the exception of the grassed swale conveying public water along the
Fossil Creek Parkway ROW.
5. Stormwater runoff will flow generally to the southeast corner of the site through an
underground collection system. These systems will outfall to the detention pond and
eventually outfall to the underground storm sewer system in Snead Drive. Calculations for
Existing Runoff, Proposed Runoff, and the Detention Pond Volume are included in Appendix
A.
6. The proposed grassed swale is intercepted by a headwall a culvert under the proposed
driveway from Fossil Creek Drive and a headwall behind the curb to pick up the flow from the
swale to the underground drainage system located in the Snead Drive right-of-way.
5. CONCLUSIONS
A. COMPLIANCE WITH STANDARDS
1. The drainage design proposed complies with the City of Fort Collins’ Stormwater Criteria
Manual.
2. There are no regulatory floodplains within the proposed development. The outfall pipe for the
proposed detention pond connects to the proposed type “E” inlet within the Snead Drive right-
of-way. The emergency spillway allows overflow to flow out of the pond safely at a non-
erosive rate.
3. The drainage plan and stormwater management measures proposed are compliant with all
applicable State and Federal regulations governing stormwater discharge.
4. The proposed development is in compliance with Chapter 10 of City Code.
B. DRAINAGE CONCEPT
2. The drainage design proposed will effectively limit any potential damage associated with the
stormwater runoff by providing an extending detention water quality pond.
3. The BMPs in place treat the required 50% of stormwater from the new impervious areas as
per the LID requirements.
Appendix A – Hydrologic Computations
1. Existing Conditions Runoff Calculations
2. Proposed Drainage Area A Runoff Calculations
3. Proposed Drainage Area B1 Runoff Calculations
4. Proposed Drainage Area B2 Runoff Calculations
5. Proposed Pond Drainage Area Runoff Calculations
6. Storm Sewer Calculations
7. Detention Pond Calculations
8. WQCV (Water Quality Capture Volume) Calculations
9. Detention Pond Volume Calculations
10. Offsite Runoff Estimate for 36” Pipe in Snead Drive ROW
11. Offsite Runoff Estimate from 2 - 16" Pipe under S. College Avenue ROW
12. Inlet Calculations
Existing Conditions
Soils A B C D Total
Grass SF 72068 684 72752
Acres 1.654 0.016 1.670
Paved SF 8130 8130
Acres 0.187 0.000 0.187
Total SF 80198 684 80882 SF
Acres 1.841 0.016 1.857 Acres
Percentage of Site Soils A B C D Total
Grass 0 89.10% 0 0.85% 89.95%
Paved 0 10.05% 0 0.00% 10.05%
Total 100%
5-year Runoff Calculations
Time of Concentration
Tc = Ti + Tt
Total TC length 725.62 ft
Initial Flow Time
ti = [(0.395)(1.1-C5)(sqrt(L)]/S^0.33
Runoff Coefficient Per Table RO-11
Average Runoff Coefficient (C): 0.23
L = 300 ft, maximum for urban land uses
Upstream Downstream Length
4953.8 4947.15 300
S = 0.0222 ft/ft
ti = 23.09 minutes
Overland Flow Time
Tt = L/V
V = Cv x Sw^0.5
Surface
Asphalt
Lawns (2 - 7%)
Runoff Coefficient
0.95
0.15
For 248.77 ft of shallow concentrated flow:
Cv = 7 (short pasture and lawn from Table RO-2)
Upstream Downstream Length
4947.15 4939.96 248.765
Sw = 0.0289 ft/ft
V = 1.19 ft/sec
Tt = 3.48 min
For 176.85 ft of channelized flow:
Cv = 7 (short pasture and lawn from Table RO-2)
Upstream Downstream Length
4939.96 4936.8 176.85
Sw = 0.0179 ft/ft
V = 0.94 ft/sec
Tt = 3.15 min
Total Time of Concentration
Tc = Ti + Tt
Tc = 29.72
Rainfall Intensity (using Equation RA-3)
I = 0.82 in/hr, per Table RA-7 per COFC Ammendments, Vol. 1, Ch. 4 - Rainfall
I = 0.82 in/hr, 2-year storm
1.40 in/hr, 10-year storm
2.86 in/hr, 100-year storm
Frequency Adjustment Factors
Runoff
Q = CIA
Q = 0.35 cfs, 2-year storm <-- Detention Release Rate
0.60 cfs, 10-year storm
1.53 cfs, 100-year storm
51 to 100 1.25
1
Cf
1.1
26 to 50 1.2
11 to 25
Storm Return Period Frequency Factor
(years)
2 to 10
Proposed Drainage Area A
Soils A B C D Total
Grass SF 3291 0 3291
Acres 0.076 0.000 0.076
Paved SF 16286 0 16286
Acres 0.374 0.000 0.374
Building SF 2526 0 2526
Acres 0.058 0.000 0.06
Total SF 22103 0 22103 SF
Acres 0.507 0.000 0.507 Acres
Percentage of Site Soils A B C D Total
Grass 0 14.89% 0 0.00% 14.89%
Paved 0 73.68% 0 0.00% 73.68%
Building 0 11.43% 0 0.00% 11.43%
Total 100%
Runoff Calculations
Time of Concentration
Tc = Ti + Tt
Total TC length 301.91 ft
Initial Flow Time
ti = [(0.395)(1.1-C5)(sqrt(L)]/S^0.33
Runoff Coefficient Per Table RO-11
Average Runoff Coefficient (C): 0.83
L = 300 ft, maximum for urban land uses
Upstream Downstream Length
4953.8 4947 300
S = 0.0227 ft/ft
ti = 22.92 minutes
Overland Flow Time
Tt = L/V
V = Cv x Sw^0.5
Surface Runoff Coefficient
Asphalt 0.95
Lawns (2 - 7%) 0.15
For 2 ft of shallow concentrated flow:
Cv = 7 (short pasture and lawn from Table RO-2)
Upstream Downstream Length
4947.15 4939.96 2
Sw = 3.5950 ft/ft
V = 13.27 ft/sec
Tt = 0.00 min
Total Time of Concentration
Tc = Ti + Tt
Tc = 22.92
Rainfall Intensity (using Equation RA-3)
I = 0.82 in/hr, per Table RA-7 per COFC Ammendments, Vol. 1, Ch. 4 - Rainfall
I = 0.82 in/hr, 2-year storm
1.40 in/hr, 10-year storm
2.86 in/hr, 100-year storm
Frequency Adjustment Factors
Runoff
Q = CIA
Q = 0.35 cfs, 2-year storm
0.59 cfs, 10-year storm
1.51 cfs, 100-year storm
11 to 25 1.1
26 to 50 1.2
51 to 100 1.25
Storm Return Period Frequency Factor
(years) Cf
2 to 10 1
Proposed Drainage Area B
Soils A B C D Total
Grass SF 0 0 0
Acres 0.000 0.000 0.000
Paved SF 2332 0 2332
Acres 0.054 0.000 0.054
Building SF 0 0 0
Acres 0.000 0.000 0.00
Total SF 2332 0 2332 SF
Acres 0.054 0.000 0.054 Acres
Percentage of Site Soils A B C D Total
Grass 0 0.00% 0 0.00% 0.00%
Paved 0 100.00% 0 0.00% 100.00%
Building 0 0.00% 0 0.00% 0.00%
Total 100%
Runoff Calculations
Time of Concentration
Tc = Ti + Tt
Total TC length 59.03 ft
Initial Flow Time
ti = [(0.395)(1.1-C5)(sqrt(L)]/S^0.33
Runoff Coefficient Per Table RO-11
Average Runoff Coefficient (C): 0.95
L = 59.033 ft, maximum for urban land uses
Upstream Downstream Length
4950.5 4949.3 59.03
S = 0.0203 ft/ft
ti = 10.54 minutes
Overland Flow Time
Tt = L/V
V = Cv x Sw^0.5
Surface Runoff Coefficient
Asphalt 0.95
Lawns (2 - 7%) 0.15
Total Time of Concentration
Tc = Ti + Tt
Tc = 10.54
Rainfall Intensity (using Equation RA-3)
I = 0.82 in/hr, per Table RA-7 per COFC Ammendments, Vol. 1, Ch. 4 - Rainfall
I = 0.82 in/hr, 2-year storm
1.40 in/hr, 10-year storm
2.86 in/hr, 100-year storm
Frequency Adjustment Factors
Runoff
Q = CIA
Q = 0.04 cfs, 2-year storm
0.07 cfs, 10-year storm
0.18 cfs, 100-year storm
Storm Return Period Frequency Factor
(years) Cf
2 to 10 1
11 to 25 1.1
26 to 50 1.2
51 to 100 1.25
Proposed Drainage Area B2
Soils A B C D Total
Grass SF 3799 0 3799
Acres 0.087 0.000 0.087
Paved SF 0 0 0
Acres 0.000 0.000 0.000
Building SF 2746 0 2746
Acres 0.063 0.000 0.06
Total SF 6545 0 6545 SF
Acres 0.150 0.000 0.150 Acres
Percentage of Site Soils A B C D Total
Grass 0 58.04% 0 0.00% 58.04%
Paved 0 0.00% 0 0.00% 0.00%
Building 0 41.96% 0 0.00% 41.96%
Total 100%
Runoff Calculations
Time of Concentration
Tc = Ti + Tt
Total TC length 173.62 ft
Initial Flow Time
ti = [(0.395)(1.1-C5)(sqrt(L)]/S^0.33
Runoff Coefficient Per Table RO-11
Average Runoff Coefficient (C): 0.49
L = 173.62 ft
Upstream Downstream Length
4954 4950 173.62
S = 0.0230 ft/ft
ti = 17.34 minutes
Overland Flow Time
Tt = L/V
V = Cv x Sw^0.5
Surface Runoff Coefficient
Asphalt 0.95
Lawns (2 - 7%) 0.15
Total Time of Concentration
Tc = Ti + Tt
Tc = 17.34
Rainfall Intensity (using Equation RA-3)
I = 0.82 in/hr, per Table RA-7 per COFC Ammendments, Vol. 1, Ch. 4 - Rainfall
I = 0.82 in/hr, 2-year storm
1.40 in/hr, 10-year storm
2.86 in/hr, 100-year storm
Frequency Adjustment Factors
Runoff
Q = CIA
Q = 0.06 cfs, 2-year storm
0.10 cfs, 10-year storm
0.26 cfs, 100-year storm
11 to 25 1.1
26 to 50 1.2
51 to 100 1.25
Storm Return Period Frequency Factor
(years) Cf
2 to 10 1
Proposed Pond Drainage Area
Soils A B C D Total
Grass SF 28412 0 28412
Acres 0.652 0.000 0.652
Paved SF 0 0 0
Acres 0.000 0.000 0.000
Building SF 0 0 0
Acres 0.000 0.000 0.00
Total SF 28412 0 28412 SF
Acres 0.652 0.000 0.652 Acres
Percentage of Site Soils A B C D Total
Grass 0 100.00% 0 0.00% 100.00%
Paved 0 0.00% 0 0.00% 0.00%
Building 0 0.00% 0 0.00% 0.00%
Total 100%
Runoff Calculations
Time of Concentration
Tc = Ti + Tt
Total TC length 192.41 ft
Initial Flow Time
ti = [(0.395)(1.1-C5)(sqrt(L)]/S^0.33
Runoff Coefficient Per Table RO-11
Average Runoff Coefficient (C): 0.15
L = 192.413 ft, maximum for urban land uses
Upstream Downstream Length
4946.35 4942.6 192.41
S = 0.0195 ft/ft
ti = 19.29 minutes
Overland Flow Time
Tt = L/V
V = Cv x Sw^0.5
Surface Runoff Coefficient
Asphalt 0.95
Lawns (2 - 7%) 0.15
Total Time of Concentration
Tc = Ti + Tt
Tc = 19.29
Rainfall Intensity (using Equation RA-3)
I = 0.82 in/hr, per Table RA-7 per COFC Ammendments, Vol. 1, Ch. 4 - Rainfall
I = 0.82 in/hr, 2-year storm
1.40 in/hr, 10-year storm
2.86 in/hr, 100-year storm
Frequency Adjustment Factors
Runoff
Q = CIA
Q = 0.08 cfs, 2-year storm
0.14 cfs, 10-year storm
0.35 cfs, 100-year storm
Storm Return Period Frequency Factor
(years) Cf
2 to 10 1
11 to 25 1.1
26 to 50 1.2
51 to 100 1.25
Storm Sewer Calculations
FLOWLINE
INPUT TOTAL INTENSITY LINE DESIGN DESIGN SLOPE M.H. UP- DOWN- ACTUAL
FROM TO AREA AREA AREA C Tc 10 - YR Q Q TOTAL REACH SIZE MANN. SLOPE Q V FALL DROP STREAM STREAM V
MH MH (AC) (AC) (AC) (MIN) (IN/HR) (CFS) (CFS) (FT) (IN) CONST. (%) (CFS) (FPS) (FT) (FT) (FT) (FT) (FPS)
A1 POND 0.51 0.00 0.51 0.83 22.92 1.40 0.59 0.59 26 12 0.013 0.330 2.1 2.61 0.08 0.00 4889.08 4889.00 0.75
B2 B1 0.15 0.00 0.15 0.49 17.34 1.40 0.10 0.10 57 12 0.013 0.330 2.1 2.61 0.19 0.10 4942.96 4942.77 0.13
B1 POND 0.05 0.15 0.20 0.95 10.54 1.40 0.07 0.17 22 12 0.013 0.330 2.1 2.61 0.07 0.00 4942.67 4942.60 0.22
POND OUTLET 0.65 0.00 0.65 0.15 19.29 1.40 0.14 0.90 64 24 0.013 0.180 9.6 3.06 0.12 0.00 4942.40 4942.28 0.29
OFFSITE OFFSITE 0.21 0.00 0.21 0.35 10.00 3.50 0.25 0.25
Detention Pond Calculation; FAA Method w/ Ft. Collins IDF
Project Number 0302-1405
Project Location Fort Collins, CO
Calculations By: Dylan Hedrick, PE
Pond Number:
Design Point
Design Storm 100-year Required Detention Volume
Undevleoped "C" = 0.23
Developed "C" = 0.50 12854 ft3
Area (A) = 1.857 acres 0.295 ac-ft
Max Release Rate = 0.35 cfs
Cf = 1.25
Ft. Collins Inflow Outflow Storage
Time Time 100-yr Q100 (Runoff) (Release) Detention
Intensity Volume Volume Volume
(mins) (secs) (in/hr) (cfs) (ft3
) (ft3
) (ft3
)
5 300 9.95 11.55 3464 105 3359
10 600 7.72 8.96 5375 210 5165
15 900 6.52 7.57 6810 315 6495
20 1200 5.6 6.50 7798 420 7378
25 1500 4.98 5.78 8669 525 8144
30 1800 4.52 5.25 9442 630 8812
35 2100 4.08 4.73 9943 735 9208
40 2400 3.74 4.34 10416 840 9576
45 2700 3.46 4.02 10841 945 9896
50 3000 3.23 3.75 11245 1050 10195
55 3300 3.03 3.52 11604 1155 10449
60 3600 2.86 3.32 11948 1260 10688
65 3900 2.72 3.16 12310 1365 10945
70 4200 2.59 3.01 12624 1470 11154
75 4500 2.48 2.88 12951 1575 11376
80 4800 2.38 2.76 13257 1680 11577
85 5100 2.29 2.66 13553 1785 11768
90 5400 2.21 2.56 13849 1890 11959
95 5700 2.13 2.47 14089 1995 12094
100 6000 2.06 2.39 14344 2100 12244
105 6300 2.00 2.32 14622 2205 12417
110 6600 1.94 2.25 14859 2310 12549
115 6900 1.89 2.19 15134 2415 12719
120 7200 1.84 2.14 15374 2520 12854
135 8100 1.60 1.86 15040 2835 12205
150 9000 1.45 1.68 15144 3150 11994
165 9900 1.15 1.33 13212 3465 9747
180 10800 1.05 1.22 13160 3780 9380
Input Variables Results
Detention Pond Volume
Required Volume = 12854 CF
Add 1390 CF for BMP storage
Total Volume = 14244 CF
Contour Area Average Interval Cummulative High Water
Area (SF) Volume (CF) Volume (CF) Line (FT)
4942.4 9152.5
9152.5 1830.5 1830.5
4942.6 9152.5
9270.5 1854.1 3684.6
4942.8 9388.4
9388.4 1877.7 5562.3
4943.0 9388.4
9627.7 1925.5 7487.8
4943.2 9867.0
9746.8 1949.4 9437.2
4943.4 9626.5
9868.1 1973.6 11410.8
4943.6 10109.6
10232.1 2046.4 13457.2
4943.8 10354.6 4943.88
10478.2 2095.6 15552.8
4944.0 10601.8
New HWL = 60554987.5
HWL = 4943.88 ft
High Water Line = Contour Elevation + (Vol. Req. - Vol. Low Cuml.)*3 / (Area Low
+ Area High) + (Area Low x Area High)^0.5)
WQCV (Water Quality Capture Volume)
WQCV = a (0.91 x I^3 − 1.19 x I^2 + 0.78 x I) (in inches)
a = Coefficient corresponding to WQCV drain time
(from Table 3-2 of Volume III, Chapter 3)
I = = Imperviousness (%/100)
Table 3-2. Drain Time Coefficients for WQCV Calculations
I = 0.50
Using a 40 hour Drain Time:
WQCV = 0.21 inches
BMP Storage Volume = (WQCV/12) x Site Area
Site Area = 1.857 acres
BMP Storage Volume = 0.032 acre-ft
1390 cf
12 hours
24 hours
40 hours
Draina Time (hrs) Coefficient,
0.9
0.8
1
Orifice Sizing
Allowable Release Rate 0.35
Pond HWL Elevation 4943.88
Flowline Orifice Elevation 4943.22
Orifice Coefficient 0.65
Vertical Cicrular Plate
Outlet Diameter (in) 4
Outlet Area (sq. in.) 12.57
Hydraulic Head (ft.) 0.66
Oriface Flow 0.37 cfs
Offsite Runoff Estimate from 36" Pipe in Snead Drive ROW
Orifice Sizing Manning Equation
Top of Pipe Elevation 4938.53 Pipe Diameter 36 inches
Flowline Pipe Elevation 4937.2 Manning Coefficient (n) 0.013
Orifice Coefficient 0.65 Hydraulic Radius 9 inches
Slope (ft/ft) 0.001
Vertical Cicrular Plate
Outlet Diameter (in) 36 Velocity 3.0 ft/sec
Outlet Area (sq. in.) 1017.88 Flow 21.1 cfs
Hydraulic Head (ft.) 1.33
Oriface Flow 42.58 cfs
Orifice Equation 42.58 cfs > 21.09 cfs
With one 36" pipe, the total flow is 42.58 cfs
Offsite Runoff Estimate from 2 - 16" Pipe under S. College Avenue ROW
Orifice Sizing Manning Equation
Top of Pipe Elevation 4955.13 Pipe Diameter 16 inches
Flowline Pipe Elevation 4953.8 Manning Coefficient (n) 0.013
Orifice Coefficient 0.65 Hydraulic Radius 4 inches
Slope (ft/ft) 0.02
Vertical Cicrular Plate
Outlet Diameter (in) 16 Velocity 7.8 ft/sec
Outlet Area (sq. in.) 201.06 Flow 10.9 cfs
Hydraulic Head (ft.) 1.33
Oriface Flow 8.41 cfs
Use Manning Equation since 8.41 cfs > 10.85 cfs
With two 16" pipes, the total flow is 21.70 cfs
Culverts and Outfall Pipe Sizing for Offsite Outfall
Orifice Sizing Manning Equation
Top of Pipe Elevation 4938.23 Pipe Diameter 30 inches
Flowline Pipe Elevation 4936.9 Manning Coefficient (n) 0.013
Orifice Coefficient 0.65 Hydraulic Radius 0.625 feet
Slope (ft/ft) 0.003
Vertical Cicrular Plate
Outlet Diameter (in) 30 Velocity 4.6 ft/sec
Outlet Area (sq. in.) 706.86 Flow 22.5 cfs
Hydraulic Head (ft.) 1.33
Oriface Flow 29.57 cfs
Use Manning Equation since 29.57 cfs > 22.47 cfs
With one 30" pipe, the total flow is 29.57 cfs
Inlet Calculations
Governing Equations
At low flow depths, the inlet will act like a weir governed by the following equation:
Q = 3.0 x P x H ^ (1.5)
- where L = Perimeter of Grate
- where H corresponds to the depth of water above the flowline
At higher flow depths, the inlet will act like an orifice governed by the following equation:
Q = 0.67 x A x (2 x g x H) ^ 0.5
- where A equals the open area of the inlet grate
- where H correseponds to the depth of water above the centroid of the cross-sectional area (A)
Input Parameters
Type of Grate: Grate Inlet
Width of Grate: 16.25 in 1.35 ft
Length of Grate: 23.75 in 1.98 ft
Open Area of Grate: 2.68 sf
Flowline Elevation (ft): 4944 ft
Reduction Factor: 50%
The exact depth at which the inlet ceases to act like a weir, and begins to act like an orifice is unknown.
However, what is known, is that the stage discharge curves of the weir equation and orifice equation will
cross at a certain flow depth. The two curves can be found below:
0.00
5.00
10.00
15.00
20.00
25.00
0.00 5.00 10.00 15.00 20.00 25.00
Depth vs. Flow
Elevation
Shallow Weir
Flow (cfs)
Orifice
Flow (cfs)
Actual
Flow (cfs)
4944.00 0.00 0.00 0.00
4944.05 0.22 3.22 0.22
4944.10 0.63 4.56 0.63
4944.15 1.16 5.58 1.16
4944.20 1.79 6.44 1.79
4944.25 2.50 7.21 2.50
4944.30 3.29 7.89 3.29
4944.35 4.14 8.53 4.14
4944.40 5.06 9.11 5.06
4944.45 6.04 9.67 6.04
4944.50 7.07 10.19 7.07
4944.55 8.16 10.69 8.16
4944.60 9.30 11.16 9.30
4944.65 10.48 11.62 10.48
4944.70 11.71 12.06 11.71
4944.75 12.99 12.48 12.48
4944.80 14.31 12.89 12.89
4944.85 15.67 13.29 13.29
4944.90 17.08 13.67 13.67
4944.95 18.52 14.05 14.05
4945.00 20.00 14.41 14.41
0.15
Depth Above Inlet (ft)
0.00
0.05
0.10
0.70
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60 <-- Depth of 100-year
0.channel65 flow in
0.75
0.80
0.85
0.90
0.95
1.00
Appendix B - FEMA Regulatory Map
Figure 5. FEMA Regulatory Map
The FEMA Panel 08069C1000F is not printed. The above map is a screen shot from FEMA’s
GeoPlatform online program. http://fema.maps.arcgis.com/home/webmap/viewer.html?useExisting=1