HomeMy WebLinkAboutDrainage Reports - 05/18/2022 (3)FINAL DRAINAGE REPORT
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A Portion of Tract A
Seven Lakes Business Park P.U.D
Fort Collins, CO
Prepared for:
Mountain View Community Church
2330 East Prospect Road
Fort Collins, CO 80525
Date:
December 18, 2020
City of Fort Collins Approved Plans
Approved by:
Date:
Matt Simpson
05/18/2022
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December 18, 2020
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Mountain View Community Church Improvements
Fort Collins, Colorado
Project Number: U20004
Dear Staff:
United Civil Design Group, LLC. is pleased to submit this Final Drainage Report for the Mountain View Community Church site
in Fort Collins, Colorado. In general, this report serves to document the stormwater impacts associated with the proposed
improvements related to the existing site.
We understand that review by the City of Fort Collins is to assure general compliance with standardized criteria contained in
the Fort Collins Stormwater Criteria Manual. This report was prepared in compliance with technical criteria set forth in the
Fort Collins Stormwater Criteria Manual.
If you should have any questions or comments as you review this report, please feel free to contact us at your convenience.
Sincerely,
United Civil Design Group
Colton Beck, PE Sam Eliason, PE
Project Engineer Principal
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TABLE OF CONTENTS
I. General Location and Description......................................................................................................................1
A. Location and Project Description................................................................................................. 1
B. Description of Property................................................................................................................ 2
C. Floodplains................................................................................................................................... 2
II. Drainage Basins and Sub-Basins.........................................................................................................................3
A. Major Basin Description............................................................................................................... 3
B. Sub-Basin Description.................................................................................................................. 3
III. Drainage Design Criteria....................................................................................................................................3
A. Regulations................................................................................................................................... 3
B. Directly Connected Impervious Area (DCIA)................................................................................ 3
C. Hydrological Criteria..................................................................................................................... 4
D. Hydraulic Criteria.......................................................................................................................... 4
E. Modifications of Criteria.............................................................................................................. 4
IV. Drainage Facility Design.....................................................................................................................................4
A. General Concept........................................................................................................................... 4
B. Specific Details............................................................................................................................. 4
V. Erosion Control...................................................................................................................................................7
VI. Conclusions........................................................................................................................................................7
A. Compliance with Standards ......................................................................................................... 7
B. Drainage Concept......................................................................................................................... 7
C. Stormwater Quality...................................................................................................................... 7
VII. References......................................................................................................................................................8
APPENDICES
APPENDIX A Hydrology Calculations
APPENDIX B Hydraulic Calculations
B.1 Low Impact Development Calculations
B.2 Water Quality Calculations
B.3 Inlet Sizing Calculations (Reserved for Final Submittal)
B.4 Storm Pipe Calculations (Reserved for Final Submittal)
B.5 Curb Channel Calculations (Reserved for Final Submittal)
B.6 Weir Calculation (Reserved for Final Submittal)
APPENDIX C Referenced Materials
APPENDIX D Drainage Exhibits
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I.GENERAL LOCATION AND DESCRIPTION
A. LOCATION AND PROJECT DESCRIPTION
The Mountain View Community Church site the site exists as a portion of Tract A, Seven Lakes Business
Park P.U.D, located in the southwest quarter of Section 17, T7N, R698W of the 6th P.M., City of Fort Collins, Larimer County,
Colorado. The property, consisting of approximately 2.90 acres, is located north of East Prospect Road and east of South
Timberline Road. The site currently exists as a commercial property with adjacent parking. The proposed Mountain View
Community Church modified site improvements are limited to 0.56 acres of disturbed area.
The property is bounded by the Spring Creek Trail to the north and west, and commercial properties to the south and east.
Stormwater on the site currently drains in several directions away from the existing building, however, all runoff is ultimately
collected by storm sewer infrastructure and conveyed to the Cattail Chorus Natural Area ponds to the north or a drainage
channel to the east of the site. Ultimately both areas drain to the Cache La Poudre River, which is approximately 2000 feet
east of the site.
The proposed improvements to the site consist of reconstructing a portion of the building along with associated landscaping,
walks, and parking around the perimeter of the building. Additionally, water quality measures are proposed with the
reconstruction to improve drainage function and water quality.
FIGURE 1:SITE VICINITY MAP
This drainage report presents the overall drainage plan for the development. In general, this report serves to provide an
analysis of the drainage impacts associated with the development of site as it relates to existing and proposed drainage
facilities on-site.
E Prospect Rd
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B. DESCRIPTION OF PROPERTY
The project site currently exists as a fully developed commercial lot, including concrete and asphalt pavement, sidewalks,
rooftop, and landscaping, comprising of an existingimperviousness of 65.3%. In its existingcondition, by means of sheet flow,
concrete pans, curb and gutter, inlets, and storm sewer,the site ultimatelydrains off-site toCattail Chorus Natural Area ponds
north the site or to a drainage channel to the east.
Below are summaries of key components of the site in its existing conditions.
Land Use - The site commercial.
Ground Cover -The site exists as a commercial development with concrete and asphalt pavement, sidewalk, rooftop,
and surrounding grass landscaping. The grass cover is good (ie., heavy or dense cover with nearly all ground surfaces
protected by vegetation).
Existing Topography The site slopes in a multitude of directions away from the existing on-site building, however,
runoff ultimately drains north to an existing downstream pond associated with the Cattail Chorus Natural Area.
Grades In general, the western portion of the site is sloped westerly and northerly at approximately 1.0% to 5.0%;
the southern and eastern portions of the site are sloped easterly and northerly at approximately 1.0% to 5.0%. The
northern portion of the side drains northerly off-site.
Soil Type -largely C , namely Loveland clay
loam (0 to 1% slopes). The Web Soil Survey also indicates the site is comprised of Table Mountain loam (0 to 1%
slopes). The on-site soils provide moderate infiltration and are suitable for development.
Utilities The following dry utility lines run along the south side of the site: gas, electric, cable TV, fiber optic. Water
mains are also present on the south side of the site within West Prospect Road. A recently constructed sanitary
sewer service exists at the northwest portion of the site.
Drainage Features and Storm Sewer An off-site pond exists north of the site. On-site and off-site storm sewer
infrastructure conveys runoff to the mentioned downstream pond.
C. FLOODPLAINS
The existing site is within the Spring Creek
Floodplain which is a FEMA designated 100-
year floodplain and floodway. In addition, the
existing site and building is located within the
Spring Creek moderate risk floodplain. The
FEMA FIRM Panel # is 08069C0983H effective
5/2/2012. A Letter of Map Amendment
(LOMA) Determination Document #19-08-
0473A dated 3/27/2019 removed the building
from the 100-year floodplain. The current
FEMA FIRM Map along with the LOMA is
included in the appendix.
The proposed building will be used as a place of
Worship. The proposed improvements will
comply with Chapter 10 of the Municipal Code.
FIGURE 2:FLOODPLAIN MAP
Existing Building
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II.DRAINAGE BASINS AND SUB-BASINS
A. MAJOR BASIN DESCRIPTION
The existing site is located within both the Spring Creek and Cache La Poudre River master drainage basins. The northern
portion of the site drains downstream within the Spring Creek Basin, while the southern portion of the site drains east,
ultimately conveyed to the Cache La Poudre River. No known master planning improvements are associated with or adjacent
to the site.
B. SUB-BASIN DESCRIPTION
The site, along with the rest of Tract A, is included within the Seven Lakes Business Park P.U.D completed by Parsons &
Associates in January 1982. A Site Drainage and Grading Plan associated with this P.U.D is included in the Appendix. A
drainage report could not be located. The approved P.U.D provides context for how the overall Tract A portion of the Seven
Lakes Business Park was designed to drain and is somewhat similar to existing drainage patterns. The site exists within Basins
A3 and B of the mentioned Site Drainage and Grading Plan. Basin A3 drains north within the Spring Creek drainage basin;
Basin B drains south and east within the Cache La Poudre drainage basin. More recent drainage reports and letters related to
the Seven Lakes Business Park are recorded with the City of Fort Collins, however, documents specifically related to this site
are not recorded.
III.DRAINAGE DESIGN CRITERIA
A. REGULATIONS
The design criteria for this study are directly from the City of Fort Collins Storm Drainage Design Criteria and Construction
Standards Manual and the Urban Storm Drainage Criteria Manuals Volumes 2, and 3 (referred to herein as USDCM).
B. DIRECTLY CONNECTED IMPERVIOUS AREA (DCIA)
The redeveloping
urban areas. The following portions of this summary describe each step and how it has been utilized for this project:
Step 1 Employ Runoff Reduction Practices
Routing the roof and pavement flows through bioretention facilities and vegetated buffers to increase the time of
concentration, promote infiltration and provide water quality.
Step 2 Provide Water Quality Capture Volume (WQCV)
The objective of providing WQCV is to reduce the sediment load and other pollutants that exit the site. For this project
WQCV is provided within the bioretention facilities.
Step 3 Stabilize Drainageways
The site is adjacent to Spring Creek and the use of LID will help slow runoff from the site and benefit the stabilization of the
Spring Creek drainageway. In addition, this project will pay stormwater development and stormwater utility fees which the
City uses, in part, to maintain the stability of the City drainageway systems.
Step 4 Consider Need for Site Specific and Source Control BMPs
Site specific and source control BMPs are generally considered for large industrial and commercial sites. The
redevelopment of the existing site will include multiple site specific and source controls, including:
Dedicated maintenance personnel providing landscape maintenance and snow and ice management.
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C. HYDROLOGICAL 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 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 10-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.
D. HYDRAULIC CRITERIA
The developed site will convey runoff to existing design points via swales, concrete pans, and pipes. The City of Fort Collins
Stormwater Criteria Manual (FCSCM) and USDCM are referenced for all hydraulic calculations. In addition, the following
computer programs are utilized:
Storm Sewer Extension for AutoCAD Civil3D
Hydraflow Express Extension for AutoCAD Civil3D
UD-Inlet by UDFCD
Drainage conveyance facility capacities proposed with the development project, including swales and bioretention ponds,
are designed in accordance with criteria outlined in the FCSCM and/or the Urban Drainage and Flood Control
Storm Drainage Criteria Manual (UDFCD).
E. MODIFICATIONS OF CRITERIA
A variance was approved for storm water quality requirements. The site was unable to meet City of Fort Collins stormwater
requirements due to existing site constraints further documented in the approved variance included with Appendix C.
IV.DRAINAGE FACILITY DESIGN
A. GENERAL CONCEPT
Developed runoff is designed to largely maintain existing drainage patterns. Existing conveyance methods include sheet flow,
concrete pans, curb and gutter, inlets, and storm sewer that ultimately drain runoff off-site to Cattail Chorus Natural Area
ponds north the site. Runoff that drains off-site to the east is ultimately conveyed to the Cache La Poudre River by means of
existing storm sewer infrastructure and drainage swales related to Tract B and Tract C of the Seven Lakes Business Park. Per
City standards, stormwater detention is not being provided because the increase in impervious surfaces is less than 1,000
square-feet. Per City standards, water quality and low impact development (LID) is being proposed with project to mitigate
the impervious areas that are being modified with the development. This includes a proposed bioretention pond on the
north side of the building and a new storm drain system that conveys runoff to the vegetative buffer on the west side of the
property.
B. SPECIFIC DETAILS
Hydrology
Site improvements include an increase of approximately 601-sf of additional impervious area relative to existing conditions.
Due to the proposed improvements
are not expected to negatively impact the existing nearby hydraulic features. The table on the following page summarizes
the hydrologic impact associated with the proposed site improvements relative to existing conditions. Refer to the drainage
exhibits and hydrology calculations attached for additional information.
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MVCC Site
Existing Proposed
Overall Area (acre)2.90 2.90
*Roof (sf)36,667 37,606
*Asphalt (sf)36,862 34,709
*Concrete (sf)13,123 14,482
*Gravel (sf)- 536
*Landscape (sf)39,608 38,927
% Imperviousness 66.4% 66.6%
Composite C2 0.73 0.73
Composite C100 0.91 0.92
TABLE 1-HYDROLOGIC SUMMARY
d
to 0.56 acres of site modifications) are included in Appendix B.
On-site Basins
The following basins provide drainage delineations for the site in its improved condition.Note that basin designations are not
bound by property lines; the site receives additional off-site runoff in its existing condition. Refer to Appendix A for hydrology
computations and Appendix B for calculations related to Water Quality, Low Impact Development, and other hydraulic
features.
Basin B
Sub-basins B1-B2 represent on-site and off-site drainage basins where runoff is captured and conveyed to an on-site
vegetative buffer area. These basins consist of roofs, concrete and asphalt paving, and landscaping. Sub-basin B1 largely
consists of a parking lot, and Basin B2 consists solely of roof area. Runoff within sub-basin B1 is conveyed to the vegetative
buffer via pans, curb and storm sewer infrastructure, while runoff within sub-basin B2 is conveyed to the vegetative buffer
via roof drains/storm sewer. The storm sewer and inlets are designed to convey the 2-year storm to the vegetative buffer
area. In a major storm event larger than the 2-year storm event, stormwater will continue northeast along the asphalt path
toward the Spring Creek trail.
Basin C
Sub-basin C1 is an on-site basin that consists of modified roof area. Similar to existing conditions, runoff is conveyed to the
access drive immediately south of the existing building. This runoff drains east to an existing, off-site inlet, and ultimately to
the Cache La Poudre River.
Stormwater Quality
Stormwater quality is required to be provided for the total new or modified impervious area on the site. The scope of the
MVCC site improvements do not include extensive parking lot work. City staff determined that the permeable paver
requirement does not apply to this project. The project is required to provide a minimum of 50% LID treatment, for all new
or modified impervious areas, using any approved LID method. The remaining 50% can be trea
water quality methods. A variance was approved to provide less than the required stormwater quality due to the existing
site constraints. See Appendix C for approved variance.
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WATER QUALITY REQUIRED:
TOTAL NEW OR MODIFIED IMPERVIOUS AREA = 18,850 SF
REQUIRED LID WQ TREATMENT = 9,425 SF (50% MIN)
WATER QUALITY PROVIDED:
VEGETATIVE BUFFER (LID WQ) FOR BASINS B1 & B2 = 11,198 SF (limited to 1:1 run-on ratio)
TOTAL IMPERVIOUS AREA TREATED = 11,198 SF (Approved Variance)
I. Vegetated Buffer
A vegetated buffer with an approximate area of 11,198-sf is utilized for water quality purposes with the site
improvements. This buffer treats basins B1 and B2 (see drainage plan attached) which has a total impervious area
of 29,332 sf. However, the water quality approved for LID credit is limited to the size of the vegetative buffer per
City standards.
This buffer is designed to improve stormwater runoff quality by straining sediment and promoting infiltration. To
distribute the concentrated flows of this buffer, an 84 lf level spreader is being provided downstream of the
concentrated flows. Refer to Appendix B for Grass Buffer calculations and minimum length of the level spreader.
Low Impact Development (LID)
In December of 2015, Fort Collins City Council adopted the revised Low Impact Development (LID) policy and criteria which
requires developments within City limits to meet certain enhanced stormwater treatment requirements in addition to more
standard treatment techniques. The scope of the MVCC site improvements do not include extensive parking lot disturbance
or construction. City of Fort Collins staff determined that the permeable paver requirement does not apply to this project.
The project is required to provide a minimum 50% LID treatment, for all new or modified impervious area, using any approved
LID method. The remaining 50% can be treated with LID or standard water quality methods.
The following measures are implemented with this proposed development:
I.Vegetative Buffer
A vegetated buffer with an approximate area of 11,198-sf is utilized for water quality purposes with the site
improvements. The inclusion of the vegetative buffer measures support that more than 50% of the modified site
area requiring the use of LID treatment is provided with the site improvements. Refer to Appendix B for calculations.
Detention
Detention is not being provided with the redevelopment of the site because the net increase in impervious surfaces is less
than 1,000 square-feet.
Standard Operating Procedures (SOPs)
In order for physical stormwater Best Management Practices (BMPs) to be effective, proper maintenance is essential.
Maintenance includes both routinely scheduled activities, as well as non-routine repairs that may be required after large
storms, or as a result of other unforeseen problems. Standard Operating Procedures should clearly identify BMP maintenance
responsibility. BMP maintenance is typically the responsibility of the entity owning the BMP.
Identifying who is responsible for maintenance of BMPs and ensuring that an adequate budget is allocated for maintenance
is critical to the long-term success of BMPs. Maintenance responsibility may be assigned either publicly or privately. For this
project, the privately owned BMPs including grass swales and the bioretention pond, are to be maintained by the property
owner.
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Storm Sewer
There are multiple storm sewers, roof drains and underdrains for the bioretention ponds proposed with the site
improvements. All storm sewers will be private and are typically sized to accommodate the flows from the 100-year storm
event. Storm system B is only sized to accommodate the flows from the 2-year storm event although all pipe upstream of
the Type C inlet is also able to accommodate the 100-year storm event. Hydraulic computations of these systems are included
in Appendix B.
Inlets
There are multiple inlets proposed with the site improvements. Inlets are utilized to support the proposed site modifications
in addition to existing site drainage insufficiencies. A Type 13 inlet associated with Basin B1 is proposed with site
improvements. Due to existing site constraints and the desire to only capture minor flows for water quality purposes, this
inlet is limited to capturing 1.5-cfs additional runoff related to Basin B1 will continue to drain downstream in accordance
with existing drainage patterns. Other proposed inlets are designed to convey the 100-year storm event. Hydraulic
computations of these systems are included in Appendix B.
V.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. -site erosion and
prevent sediment from traveling off-site during construction:
Silt Fence a woven synthetic fabric that filters runoff. The silt fence is a temporary barrier that is placed at the base
of a disturbed area.
Vehicle Tracking Control a stabilized stone pad located at points of ingress and egress on a construction site. The
stone pad is designed to reduce the amount of mud transported onto public roads by construction traffic.
Inlet Protection acts as a sediment filter. It is a temporary BMP and requires proper installation and maintenance
to ensure their performance.
Straw Wattles wattles act as a sediment filter in swales around inlets. They are a temporary BMP and require
proper installation and maintenance to ensure their performance.
The contractor shall store all construction materials and equipment and shall provide maintenance and fueling of equipment
in confined areas on-site from which runoff will be contained and filtered. T
will be inspected by the contractor at a minimum of once every two weeks and after each significant storm event.
VI.CONCLUSIONS
A. COMPLIANCE WITH STANDARDS
Storm drainage calculations have followed the guidelines provided by the Urban Storm Drainage Criteria Manuals Volumes
1, 2 and 3 and the City of Fort Collins Stormwater Criteria Manual.
B. DRAINAGE CONCEPT
The drainage system has been designed to convey the runoff to the designated design points and the existing public
infrastructure in an effective, safe manner. No negative impacts are anticipated to the City of Fort Collins Master Drainage
Plan or to downstream properties or infrastructure due to the proposed improvements.
C. STORMWATER QUALITY
A vegetated buffer is being provided to meet some of the water quality needs of the site. An approved variance is included
in Appendix C due to an inability to meet all City of Fort Collins water quality requirements.
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VII.REFERENCES
1. City of Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, November 2017.
2. Urban Storm Drainage Criteria Manual Volume 1 and 2, Urban Drainage and Flood Control District, Denver, Colorado,
June 2001, Revised April 2008.
3. Site Drainage and Grading Plan, Parsons & Associates, Fort Collins, Colorado, last revised 8/5/1985.
4. Natural Resources Conservation Service Web Soil Survey at: websoilsurvey.nrcs.usda.gov/app
5. Flood Insurance Rate Map, FEMA, Panel 08069C0983H, https://hazards.fema.gov/femaportal/
6. Stormwater Discharges Associated with Construction Activity, Stormwater Management Plan Preparation Guides, State
of Colorado, www.colorado.com
A PPENDIX A
H YDROLOGY C ALCULATIONS
A PPENDIX B
H YDRAULIC C ALCULATIONS
Sheet 1 of 1
Designer:
Company:
Date:
Project:
Location:
1. Design Discharge
A) 2-Year Peak Flow Rate of the Area Draining to the Grass Buffer Q2 =2.0 cfs
2.Minimum Width of Grass Buffer WG =40 ft
3.Length of Grass Buffer (14' or greater recommended)LG =45 ft
4.Buffer Slope (in the direction of flow, not to exceed 0.1 ft / ft)SG =0.010 ft / ft
5. Flow Characteristics (sheet or concentrated)
A) Does runoff flow into the grass buffer across the
entire width of the buffer?
B) Watershed Flow Length FL=ft
C) Interface Slope (normal to flow)SI=ft / ft
D) Type of Flow CONCENTRATED FLOW
Sheet Flow: FL * SI <1
Concentrated Flow: FL * SI > 1
6. Flow Distribution for Concentrated Flows
7 Soil Preparation
(Describe soil amendment)
8 Vegetation (Check the type used or describe "Other")
9. Irrigation
(*Select None if existing buffer area has 80% vegetation
AND will not be disturbed during construction.)
10. Outflow Collection (Check the type used or describe "Other")
Notes:
Continues to sheet flow to Spring Creek
Fort Collins, CO
Design Procedure Form: Grass Buffer (GB)
Sam Eliason
United Civil Design Group
Mountain View Community Church
August 19, 2020
UD-BMP (Version 3.07, March 2018)
Existing Xeric Turf Grass
Irrigated Turf Grass
Other (Explain):
Choose One
Choose One
Grass Swale
Street Gutter
Storm Sewer Inlet
Other (Explain):
None (sheet flow)
Slotted Curbing
Level Spreader
Choose One
Other (Explain):
Choose One
Yes No
Choose One
Permanent
None*
Temporary
UD-BMP_v3.07 (2).xlsm, GB 8/19/2020, 5:47 PM
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IINLETCAPACITY
Mountain View CommunityChurch, FortCollins, CO
INLETID:Type C
Governing Equations:
Inlet capacity equation at low flows (weir calculation):
Where:
P = 2(L + W)
H = depth of water above the flowline
Inlet capacity equation at higher flows (orifice calculation):
Where:
A = open area of the inlet grate
H = depth of water above the centroid of the cross-sectional area (A)
InputParameters:
Grate: Type C
Wier Perimeter: 12.0
Open Area of Grate (ft
2):4.5
Grate Centroid Elevation (ft): 4906.45
Allowable Capacity: 50%
Depth vs. Flow:
Depth Elevation Shallow Orifice Actual
Above Inlet Weir Flow Flow Flow
(ft)(ft)(cfs)(cfs)(cfs)
0.00 4906.45 0.00 0.00 0.00
0.05 4906.50 0.20 2.70 0.20
0.10 4906.55 0.57 3.82 0.57
0.15 4906.60 1.05 4.68 1.05
0.20 4906.65 1.61 5.41 1.61
0.25 4906.70 2.25 6.05 2.25 <----Maximum Depth
0.30 4906.75 2.96 6.62 2.96
0.35 4906.80 3.73 7.15 3.73
0.40 4906.85 4.55 7.65 4.55
0.45 4906.90 5.43 8.11 5.43
0.50 4906.95 6.36 8.55 6.36
5.10.3 HPQ
5.0)2(67.0 gHAQ
Date: 11/18/2020 E:\United Civil Dropbox\Projects\U20004 - Mountain View Community Church\Reports\Drainage\Calculations\U20004-Drain Calcs.xlsm
A PPENDIX C
R EFERENCED M ATERIALS
Mountain View Community Church
Alternative Compliance/Variance Application Supplemental Sheet
What hardship prevents this site from meeting the requirement?
This is an existing developed site that has many constraints that make development of more typical LID
types such as permeable pavement, bioretention, sand filters, and underground filtration very difficult
to include with this development. These constraints include the following:
Existing large trees with large root zones that restrict construction area of LID.
No storm sewer or shallow storm sewer that restrict the locations of LID with an underdrain.
Adjacent Spring Creek trail, Natural Area pond and buffer, wetlands, and floodway that all limit
the location of construction in proximity to those natural features.
Constructing a new storm outlet pipe into a City of Fort Collins Natural Area is an arduous
process that requires that all other alternatives be exhausted. This alternative was looked at
early in the process.
There is limited space north of the building in the only location with an existing viable storm
sewer outlet. This location has steep side slopes that would require a large wall to construct a
bioretention facility and the outlet pipe is still near existing large trees.
What alternative is proposed for the site?
A vegetative buffer to meet standard water quality and LID requirements is being proposed. This is an
existing well-established native grass area on the northwest portion of the site that we are proposing to
utilize as a vegetative buffer. We are redirecting the runoff from almost 30,000 square feet of
pavement and roof areas to drain across this vegetative buffer. We believe this truly meets the intent of
Low Impact Development and the Key LID techniques listed in Ch. 7, Section 2 because it Conserves
Existing Amenities, Minimizes Impacts, and Minimizes Directly Connected Impervious Areas.
A vegetative buffer is allowed within Section 6.7 to meet the water quality and LID requirements.
However, we cannot meet all the Standard Design Criteria for Vegetative Buffers shown in Figure 6.7-1
including minimum cross slope of 2%, soil amendments, 1:1 run-on ratio, and soil types. By City of Fort
Collins requirements, the site is required to provide stormwater quality treatment for 18,850 square
feet of impervious area. The 1:1 ratio requirement limits the amount of treated area to the area of the
vegetative buffer which is approximately 11,200 square feet as measured in the direction of the
stormwater flow from the level spreader.
11/10/20
105°2'22.10"W
105°1'44.65"W
SPRING CREEK
CROSS SECTION 1335
A PPENDIX D
D RAINAGE E XHIBITS