HomeMy WebLinkAboutJOHNSON DRIVE APARTMENTS - PDP - PDP170034 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTPRELIMINARY DRAINAGE REPORT
Johnson Drive Apartments
Fort Collins, Colorado
September 6, 2017
Prepared for:
Next Chapter Properties
217 S. Neil Street, Suite B
Champaign IL, 61820
Prepared by:
301 North Howes Street, Suite 100
Fort Collins, Colorado 80521
Phone: 970.221.4158 Fax: 970.221.4159
www.northernengineering.com
Project Number: 1285-001
This Drainage Report is consciously provided as a PDF.
Please consider the environment before printing this document in its entirety.
When a hard copy is absolutely necessary, we recommend double-sided printing.
September 6, 2017
City of Fort Collins
Stormwater Utility
700 Wood Street
Fort Collins, Colorado 80521
RE: Preliminary Drainage Report for
Johnson Drive Apartments
Dear Staff:
Northern Engineering is pleased to submit this Preliminary Drainage Report for your review.
This report has been prepared in accordance with the City of Fort Collins Stormwater Criteria
Manual (FCSCM) and the Urban Drainage and Flood Control District (UDFCD) Urban Storm
Drainage Criteria Manual and serves to document the stormwater impacts associated with the
proposed Johnson Drive Apartments project. We understand that review by the City of Fort Collins
is to assure general compliance with standardized criteria.
If you should have any questions as you review this report, please feel free to contact us.
Sincerely,
NORTHERN ENGINEERING SERVICES, INC.
Cody Snowdon, PE Blaine Mathisen
Project Manager Project Engineer
Johnson Drive Apartments
Preliminary Drainage Report
TABLE OF CONTENTS
I. GENERAL LOCATION AND DESCRIPTION ................................................................... 1
A. Location ............................................................................................................................................. 1
B. Description of Property ..................................................................................................................... 2
C. Floodplain.......................................................................................................................................... 3
II. DRAINAGE BASINS AND SUB-BASINS ....................................................................... 5
A. Major Basin Description .................................................................................................................... 5
B. Sub-Basin Description ....................................................................................................................... 5
III. DRAINAGE DESIGN CRITERIA ................................................................................... 7
A. Regulations........................................................................................................................................ 7
B. Four Step Process .............................................................................................................................. 7
C. Development Criteria Reference and Constraints ............................................................................ 8
D. Hydrological Criteria ......................................................................................................................... 8
E. Hydraulic Criteria .............................................................................................................................. 8
F. Floodplain Regulations Compliance .................................................................................................. 9
G. Modifications of Criteria ................................................................................................................... 9
IV. DRAINAGE FACILITY DESIGN .................................................................................... 9
A. General Concept ............................................................................................................................... 9
B. Specific Details ................................................................................................................................ 10
V. CONCLUSIONS ...................................................................................................... 11
A. Compliance with Standards ............................................................................................................ 11
B. Drainage Concept ............................................................................................................................ 12
References ....................................................................................................................... 13
APPENDICES:
APPENDIX A – Hydrologic Computations
APPENDIX B – Hydraulic Computations
B.1 – Storm Sewers (Future)
B.2 – Inlets (Future)
B.3 – Detention Facilities
APPENDIX C – Water Quality Design Computations
APPENDIX D – Erosion Control Report
APPENDIX E – Soils Resource Report
Johnson Drive Apartments
Preliminary Drainage Report
LIST OF TABLES AND FIGURES:
Figure 1 Vicinity Map .......................................................................................................... 1
Figure 2 Aerial Photograph ................................................................................................... 2
Figure 3 Proposed Site Plan ................................................................................................. 3
Figure 4 FEMA Map ............................................................................................................ 4
Figure 5 City Floodplain Mapping .......................................................................................... 5
MAP POCKET:
C4.00 – Proposed Drainage Exhibit
Historic Drainage Exhibit
Johnson Drive Apartments
Preliminary Drainage Report 1
I. GENERAL LOCATION AND DESCRIPTION
A. Location
1. Vicinity Map
Figure 1 Vicinity Map
2. Johnson Drive Apartments project site is located in the replat of lots 13, 14, 15, 16,
and 17 Spring Court Subdivision which is located in the northeat quarter of Section
23, Township 7 North, Range 69 West of the 6th Principal Meridian, City of Fort
Collins, County of Larimer, State of Colorado.
3. The project site is located at 255 Johnson Drive at southwest corner of the
intersection of Johnson Drive and Spring Court. The property is bounded to the north
by Johnson Drive and further north by Spring Creek, to the east by Spring Court, to
the south by and irrigation lateral and to the west by BNSF Railway and the Max
route.
4. The project site lies within the Spring Creek Basin. The project site has historically
drained towards Spring Court and Johnson Drive via overland flow from south to
northeast. The site must provide current City Low-Impact Design (LID) requirements
and water quality treatment as well. Water quality treatment methods are described
Johnson Drive Apartments
Preliminary Drainage Report 2
in further detail below.
5. As this is an in-fill site, the area surrounding the property is fully developed.
6. There are no off-site flows that enter the project area.
B. Description of Property
1. Johnson Drive Apartments project is approximately 2.80 net acres.
Figure 2 Aerial Photograph
2. There are currently five properties being utilized for this project; one commercial lot
(Self-Storage) and four residential lots. The existing commercial lot routes
stormwater via overland and gutter flow and releases undetained into Johnson Drive.
Once in Johnson Drive, stormwater travels via gutter flow to the east where it is
intercepted by an existing inlet and conveyed directly to Spring Creek. The four
residential lots route stormwater via overland flow and releases undetained into Spring
Court. Once in Spring Court, stormwater travel via gutter flow to the north to Johnson
Drive, with the ultimate destination being Spring Court. The overall existing
impervious area located within in the four properties is 76,857 sq. ft. and was
released undetained. There are currently no water quality measures located on-site.
3. According to the United States Department of Agriculture (USDA) Natural Resources
Conservation Service (NRCS) Soil Survey, the site consists of Altvan-Satanta and Kim
loams, which fall into Hydrologic Soil Groups B. The NRCS soils report is provided in
Appendix E.
4. The proposed development is primarily composed of a proposed student housing
building. Associated site work that will be constructed with this project include water
services, sanitary sewer services, storm sewer, landscaped swales and walls, and
sidewalks along the north, south and east side of the building. Current City Low-
Impact Design (LID) requirements will be implemented with the project, and will
consist of sand filter, which is further discussed in Section IV, below.
Johnson Drive Apartments
Preliminary Drainage Report 3
Figure 3 Proposed Site Plan
5. There are no known irrigation laterals crossing the site. There is an irrigation lateral
bordering the south side of the property.
6. The project site is within a General Commercial District (C-G). The proposed use is
permitted within this zone district.
C. Floodplain
1. The project site is encroached by the FEMA designated 100- Year High Risk Flood
Fringe according to FIRM Panel 08069C87G for Larimer County, dated May 2, 2012.
Currently, this area is under review by FEMA which will be adopting the
recommendations set forth by the “Letter of Map Revision Application For The Mason
Street Transportation Corridor Crossing Of Spring Creek”, prepared by Anderson
Consulting Engineer, Inc. on August 10th 2016. This application process is in the
latter stages and the FIRM map will be updated per the approved recommendations
from Anderson Consulting Engineers, Inc.
Johnson Drive Apartments
Preliminary Drainage Report 4
2. Development is allowed within the flood fringe. It should be noted that the portion of
the structure that will be constructed within the floodplain with the development of
Johnson Drive Apartments will be elevated 18” above the Base Flood Elevation at all
opening and flood proofing will not be required.
Figure 4 FEMA Map
Johnson Drive Apartments
Preliminary Drainage Report 5
Figure 5 City Floodplain Mapping
II. DRAINAGE BASINS AND SUB-BASINS
A. Major Basin Description
1. The project site lies within the Spring Creek Basin. Detention requirements for this
site are to detain the difference between the 100-yr developed flow rate and the
historic 100-yr release of all existing impervious area and 2-yr release rate from the
existing landscaping areas. The project is composed of five different lots, one is a
commercial lot and the other four are single-family residential lots. Historically, the
lots sheet flow into the surrounding streets undetained. However, Johnson Drive
Apartments will release at a reduced historic rate and will also provide water quality.
The proposed release rate will be reduced to summation of the historic 2-year event
within the existing landscape areas and the historic 100-year event within all the
existing impervious areas. Water quality will be provided for all the paved areas via a
sand filter within the building. All other water quality will be provided by sheet
flowing across landscape areas.
2. There are no previous drainage studies for the area associated with Johnson Drive
Apartments project site.
B. Sub-Basin Description
Johnson Drive Apartments historically drains overland from south to north except for
the southwest corner of the site which is picked up by an irrigation lateral running
west to east. Runoff from the site has historically flowed overland north towards
Spring Creek and eventually enters the Cache La Poudre.
Johnson Drive Apartments
Preliminary Drainage Report 6
Basin HW1 Pervious
Historic Basin HW1 Pervious is 0.18 acres and is roughly the western 2/3 of the
property, and it is only comprised of landscaping area. Runoff generated in this basin
historically sheet flows north into Johnson Drive. Once in Johnson Drive runoff will be
conveyed west to east via curb and gutter until it enters a curb inlet and is further
conveyed to Spring Creek. Basin HW1 Pervious has a 2-yr runoff of 0.06 cfs.
Basin HW1 Impervious
Historic Basin HW1 Impervious is 1.56 acres and is roughly the western 2/3 of the
property, and it is only comprised of the paved and roof areas associated with
Johnson Drive Storage. Runoff generated in this basin historically sheet flows north
into Johnson Drive. Once in Johnson Drive runoff will be conveyed west to east via
curb and gutter until it enters a curb inlet and is further conveyed to Spring Creek.
Basin HW1 Impervious has a 100-yr runoff of 15.48 cfs.
Overall Basin HW is 1.73 acres and has a combined release rate of 15.54 cfs (0.06
cfs + 15.48 cfs) from Basins HW1 Pervious 2-year event and HE1 Impervious 100-
year event.
Basin HE1 Pervious
Historic Basin HE1 Pervious is 0.60 acres and is roughly the eastern 1/3 of the
property, and it is only comprised of landscaping area. Runoff generated in this basin
historically sheet flows directly into Spring Court and is then conveyed north via curb
and gutter until it enters a curb inlet and is conveyed to Spring Creek. Basin HE1
Pervious has a 2-yr runoff of 0.15 cfs.
Basin HE1 Impervious
Historic Basin HE1 Impervious is 0.21 acres and is roughly the eastern 1/3 of the
property, and it is comprised of the paved sidewalks and roof areas. Runoff generated
in this basin historically sheet flows directly into Spring Court and is then conveyed
north via curb and gutter until it enters a curb inlet and is conveyed to Spring Creek.
Basin HE1 Impervious has a 100-yr runoff of 2.08 cfs.
Overall Basin HE is 0.20 acres and has a combined release rate of 2.22 cfs (0.15 cfs
+ 2.08 cfs) from Basins HE1 Pervious 2-year event and HE1 Impervious 100-year
event.
Basin HS1 Pervious
Historic Basin HS1 Pervious is 0.27 acres and is situated in the southwest corner of
the project site. Runoff generated in this area will sheet flow into an existing irrigation
lateral that runs west to east. There is also a small detention pond in this area that is
serving the property to the south of the project site. This area will remain untouched
during the development of Johnson Drive Apartments. Basin HW1 Pervious has a 2-yr
runoff of 0.11 cfs.
Basin HS1 Impervious
Historic Basin HS1 Impervious is 0 acres because it is all vegetated. Basin HW1
Impervious has a 100-yr runoff of 0 cfs.
Overall Basin HS1 is 0.27 acres and has a combined release rate of 0.11 cfs (0.11
cfs + 0 cfs) from Basins HS1 Pervious and HS1 Impervious. Therefore, the overall
historic release rate from the site is 17.88 cfs (15.54 cfs + 2.22 cfs + 0.11 cfs).
However, due to uncapturable flows from Johnson Drive Apartments, which is
Johnson Drive Apartments
Preliminary Drainage Report 7
discussed in Section IV.A, the allowable release rate is reduced to 13.09 cfs.
1. A more detailed description of the project’s proposed drainage patterns follows in
Section IV.A.4.
A full-size copy of the Historic and Proposed Drainage Exhibits can be found in the
Map Pocket at the end of this report.
III. DRAINAGE DESIGN CRITERIA
A. Regulations
There are no optional provisions outside of the FCSCM proposed with the Johnson Drive
Apartments project.
B. Four Step Process
The overall stormwater management strategy employed with the Johnson Drive
Apartments project utilizes the “Four Step Process” to minimize adverse impacts of
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 with the proposed development to facilitate the
reduction of runoff peaks, volumes, and pollutant loads as the site is redeveloped from the
current use by implementing multiple Low-Impact Development (LID) strategies including:
Providing as much vegetated open areas as possible along the east, west and south
portion of the site to reduce the overall impervious area and to minimize directly
connected impervious areas (MDCIA).
Selecting a site that has been previously developed to reduce development impacts.
Routing flows, to the extent feasible, through a stormwater detention vault located
internal to the building for water quality purposes. Stormwater will be routed through a
sand filter to increase filtration.
Providing on-site detention to increase time of concentration, and reduce loads on
downstream storm infrastructure.
Routing runoff from the roof directly into the sand filter.
Step 2 – Implement BMPs That Provide a Water Quality Capture Volume (WQCV) with
Slow Release
The efforts taken in Step 1 will facilitate the reduction of runoff; however, urban
development of this intensity will still generate stormwater runoff that will require
additional BMPs and water quality. The stormwater runoff from the site will be
intercepted and treated using an internal stormwater vault with a water quality capture
volume.
Step 3 – Stabilize Drainageways
There are no major drainageways within the subject property. While this step may not
seem applicable to proposed development, the project indirectly helps achieve stabilized
drainageways nonetheless. By providing detention and water quality treatment where
none previously existed, sediment and flash flows with erosion potential are removed from
downstream drainageway systems. Furthermore, this project will pay one-time stormwater
Johnson Drive Apartments
Preliminary Drainage Report 8
development fees, as well as ongoing monthly stormwater utility fees, both of which help
achieve City-wide drainageway stability.
Step 4 – Implement Site Specific and Other Source Control BMPs.
The proposed project includes one multi-story building, covered parking areas within the
structure, retaining walls, and pedestrian sidewalks, all of which will require the need for
site specific source controls including:
A localized trash enclosure placed within the building.
The proposed development will provide LID features within the stormwater vault
internal to the building to provide enhanced water quality. The Sand Filter is designed
to capture the first flush storm event; thus, eliminating sources of potential pollution
previously left exposed to weathering and runoff processes.
C. Development Criteria Reference and Constraints
1. There are no known drainage studies for the existing property.
2. The subject property is a redevelopment project and is surrounded by currently
developed properties. As such, several constraints have been identified during the
course of this analysis that will impact the proposed drainage system including:
Existing elevations along the north, south, east, and west property lines will be
maintained.
Overall drainage patterns of the existing site will be maintained.
Release rates can not adversely impact existing infrastructure.
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 this development. Tabulated data contained in Table RA-7 has been
utilized for Rational Method runoff calculations.
2. The Rational Method has been employed to compute stormwater runoff utilizing
coefficients contained in Tables RO-11 and RO-12 of the FCSCM.
3. The Rational Formula-based Modified Federal Aviation Administration (FAA)
procedure has been utilized for detention storage calculations.
4. Two separate design storms have been utilized to address distinct drainage scenarios.
The first event analyzed is the “Minor,” or “Initial” Storm, which has a 2-yr recurrence
interval. The second event considered is the “Major Storm,” which has a 100-yr
recurrence interval.
5. No other assumptions or calculation methods have been used with this development
that are not referenced by 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 Johnson Drive Apartments project are
designed in accordance with criteria outlined in the FCSCM and/or the Urban
Drainage and Flood Control District’s (UDFCD) Urban Storm Drainage Criteria
Manual.
3. As stated in Section I.C.1, above, the subject property is encroaching a FEMA 100-
Year High Risk Flood Fringe.
Johnson Drive Apartments
Preliminary Drainage Report 9
F. Floodplain Regulations Compliance
As previously mentioned, the project site is encroaching a FEMA 100-Year High Risk Flood
Fringe along the northwest corner of the property. Development is allowed within the flood
fringe pursuant to Chapter 10 of the City of Fort Collins Municipal Code. It should be noted
that no structures will be constructed within the floodplain with the development of Johnson
Drive Apartments.
G. Modifications of Criteria
The proposed Johnson Drive Apartments development is not requesting any modifications.
IV. DRAINAGE FACILITY DESIGN
A. General Concept
1. The main objectives of the Johnson Drive Apartments drainage design are to maintain
existing drainage patterns and ensure no adverse impacts to any adjacent properties
or existing infrastructure.
2. As previously mentioned, there are no off-site flows draining onto the existing
property. Additionally, on-site LID feature will be provided which will enhance
downstream water quality. These measures are discussed further below.
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 the
sections to which the content best applies.
4. The proposed Johnson Drive Apartments project is has been divided into five (5)
major drainage basins, designated as Basins A1, OW1, OS1, OS2, and ON1.
Drainage patterns for basins shown on the Proposed Drainage Exhibit are described
below.
Basin A1
Basin A1 has a net area of 1.668 acres, which corresponds to the entire building
footprint. Runoff generated in Basin A1 will be generated by the roof which will then
convey the runoff to an internal storm vault via roof leaders. The internal storm vault
will also be a sand filter therefore, 100% of the treatable runoff is treated via LID.
Basin A1 will provide onsite detention that is required for the runoff volume difference
between the 100-year developed inflow rate and the 2-year historic release rate for
any net new impervious area. After Basin A1 is detained and treated, it will be
conveyed to Spring Creek via ADS pipe. The 100-year flow rate for Basin A1 is 16.59
cfs.
Basin OW1
Basin OW1 is located between the proposed apartment building and the Mason Street
corridor retaining wall and has a net area of 0.093 acres. This area is all landscaping
but due to existing grades this area will be getting free released into Spring Creek via a
Nyloplast inline drain that is connected to the outfall pipe of Basin A1. Since Basin
OW1 is leaving the site undetained and untreated the overall release rate will be
modified to reflect this. The 100-year release rate for Basin OW1 is 0.17 cfs.
Johnson Drive Apartments
Preliminary Drainage Report 10
Basin OS1
Basin OS1 has a net area of 0.301 acre, and is associated with the Sherwood
Irrigation lateral and detention pond that occupy the southwest corner of the site. This
area has historically bypassed the site flowing east via the irrigation lateral. Johnson
Drive apartments will not be developing anything in this area therefore this runoff will
continue to flow east via the irrigation lateral. Because Johnson Drive Apartments is
not able to capture and treat this area runoff the max allowable release rate will be
reduced to reflect this. The 100-year release rate for Basin OS1 is 1.03 cfs.
Basin OS2
Basin OS2 has a net area of 0.30 acres, and is located along the southern edge of the
proposed apartment building and northern side of the irrigation lateral. All the runoff
generated in Basin OS2 will flow east towards Spring Court via a swale. Runoff
generated in Basin OS2 will freely discharge into Spring Court via a concrete chase.
Therefore, the max allowable release rate will be reduced to reflect that Basin OS2 is
not being detained or treated. The 100-year release rate for Basin OS2 is 0.41 cfs.
Basin ON1
Basin ON1 has a net area of 0.48 acres, and is located along the eastern and
northern edge of the proposed apartment building. All the flow generated in Basin
ON1 will leave the site undetained and untreated via overland flow. The runoff will
either enter Spring Court or Johnson Drive. Once in the public right of way the runoff
will make its way to an existing inlet near the northeast corner of the intersection of
Johnson Drive and Spring Court. From there the runoff will be conveyed to Spring
Creek via pipe. The 100-year release rate for Basin ON1 is 3.18 cfs.
Max Allowable Release Rate
The max allowable release rate for Johnson Drive Apartments is 13.09 cfs. This was
calculated by summing up the historic release rates from Basins HW1, HE1, and HS1
2-year pervious and 100-year impervious rates. Following the calculations laid out in
Section II.B yields a historic release rate of 17.88 cfs. Due to the Basins OS1, OS2,
ON1, and OW1 free releasing off the site the 17.88 cfs was reduced by summation of
the aforementioned basins 100-year release rates which is 4.79 cfs. Therefore, the
max allowable release rate is 17.88 cfs – 4.79 cfs = 13.09 cfs.
B. Specific Details
1. The main drainage problems associated with this project site are the deficiency of
existing stormwater infrastructure present, steep existing grades, an existing irrigation
lateral running across the site, and FEMA floodplain. Currently the site drains to the
northeast. The proposed site will mitigate these issues by instituting the following
water quality and detention facilities:
Johnson Drive Apartments will be utilizing a sand filter to treat all the runoff
generated by Basin A1. The sand filter will be incorporated into the internal storm
chamber within the apartment building.
Johnson Drive Apartments
Preliminary Drainage Report 11
All the free released runoff will be generated by landscaping areas which will
provide minimal water quality via grass buffers.
2. Detention Pond Calculations
Using the FAA Method to detain Basin A1 with a reduced release rate of 13.09 cfs
yields an overall detention volume of 1946 cu. ft. This required volume will be stored
in conjunction with the WQCV within the internal storm chamber.
3. Water Quality Results
Following UDFCD criteria and using the characteristics from Basin A1 wields an
overall water quality capture volume (WQCV) of 2,187 cu. ft. This water quality will
be getting provided via sand filter. With the current configuration of the internal stomr
chamber the actual WQCV is 2,250 cu. ft. The treated runoff will be conveyed to
Spring Creek via HDPE pipe.
4. Below is a breakdown of the LID treatment of the proposed project site.
V. CONCLUSIONS
A. Compliance with Standards
1. The drainage design proposed with the Johnson Drive Apartments project complies
with the City of Fort Collins’ Stormwater Criteria Manual.
2. The drainage design proposed with the Johnson Drive Apartments project complies
with the City of Fort Collins’ Master Drainage Plan for the Spring Creek Basin.
3. The project site is encroached by a 100-Year High Floodplain Fringe along the
northwest corner of the property. However, the development will not adversely impact
any of the downstream infrastructure, as this floodplain is caused by the obstruction
of the College Avenue bridge.
4. The drainage plan and stormwater management measures proposed with the Johnson
Drive Apartments development are compliant with all applicable State and Federal
regulations governing stormwater discharge.
Basin ID
Basin
Impervious Area
Treatment Type
LID
System
Area
Treated by
Percent of
Impervious Area
A1 1.668 ac. Sand Filter Yes 1.668 ac. 84%
OW1 0.000 ac. Grass Buffer No N/A 0%
OS1 0.047 ac. Grass Buffer No N/A 0%
OS2 0.002 ac. Grass Buffer No N/A 0%
ON1 0.271 ac. Grass Buffer No N/A 0%
Total 1.988 ac. 1.668 ac. 84%
Johnson Drive Apartments
Preliminary Drainage Report 12
B. Drainage Concept
1. The drainage design proposed with this project will effectively limit potential damage
associated with its stormwater runoff. The total difference in impervious area between
historic and proposed conditions is only 9,801 sq. ft., which is only 8,801 sq. ft.
above the maximum threshold for detention according to the City of Fort Collins
Stormwater Criteria Manual.
2. Johnson Drive Apartments will be releasing at reduced runoff rates from the historic
condition to both Johnson Drive and Spring Court; therefore, there will be no
downstream impacts from this development.
3. Johnson Drive Apartments will provide water quality for a majority of the site (1.668
acres out of 2.80 acreas) with accepted LID treatments. The remaining portion of the
project site will receive water quality by crossing over landscaped areas. The areas
receiving water quality via landscaping were uncatchable and only include the
perimeter areas along the north, south, east, and west.
4. The proposed Johnson Drive Apartments development will not impact the Master
Drainage Plan recommendations for the Spring Creek major drainage basin.
Johnson Drive Apartments
Preliminary Drainage Report 13
References
1. City of Fort Collins Landscape Design Guidelines for Stormwater and Detention Facilities,
November 5, 2009, BHA Design, Inc. with City of Fort Collins Utility Services.
2. Fort Collins Stormwater Criteria Manual, City of Fort Collins, Colorado, as adopted by Ordinance No.
174, 2011, and referenced in Section 26-500 (c) of the City of Fort Collins Municipal Code.
3. Larimer County Urban Area Street Standards, Adopted January 2, 2001, Repealed and
Reenacted, Effective October 1, 2002, Repealed and Reenacted, Effective April 1, 2007.
4. Soils Resource Report for Larimer County Area, Colorado, Natural Resources Conservation
Service, United States Department of Agriculture.
5. Urban Storm Drainage Criteria Manual, Volumes 1-3, Urban Drainage and Flood Control
District, Wright-McLaughlin Engineers, Denver, Colorado, Revised April 2008.
Johnson Drive Apartments
Preliminary Drainage Report 14
APPENDIX A
HYDROLOGIC COMPUTATIONS
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Johnson Drive Apartments
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By: C. Snowdon
Asphalt ……....……………...……….....…...……………….………………………………….. 0.95 100% Date:
Concrete …….......……………….….……….………………..….………………………………… 0.95 90%
Gravel ……….…………………….….…………………………..……………………………….0.15 . 0.50 40%
Roofs …….…….………………..……………….…………………………………………….. 0.95 90%
Pavers…………………………...………………..…………………………………………….. 0.40 22%
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0%
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0% 2-year C
f = 1.00 100-year C
f = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
HW1 75478 1.73 0.86 0.01 0.69 0.00 0.18 0.87 0.87 1.00 86%
HE1 35074 0.81 0.00 0.07 0.14 0.00 0.60 0.36 0.36 0.45 23%
HS1 11546 0.27 0.00 0.00 0.00 0.00 0.27 0.15 0.15 0.19 0%
Total 122098 2.80 0.86 0.09 0.82 0.00 1.04 0.65 0.65 0.82 60%
HW1 (Impervious) 67772 1.56 0.86 0.01 0.69 0.00 0.00 0.95 0.95 1.00 96%
HW1 (Pervious) 7706 0.18 0.00 0.00 0.00 0.00 0.18 0.15 0.15 0.19 0%
HE1 (Impervious) 9085 0.21 0.00 0.07 0.14 0.00 0.00 0.95 0.95 1.00 90%
HE1 (Pervious) 25989 0.60 0.00 0.00 0.00 0.00 0.60 0.15 0.15 0.19 0%
HS1 (Impervious) 0 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0%
Overland Flow, Time of Concentration:
Project: Johnson Drive Apartments
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration: 0.15 0
Tt = L / 60V
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
Is Length
>500' ?
C*Cf
(2-yr
Cf=1.00)
C*Cf
(10-yr
Cf=1.00)
C*Cf
(100-yr
Cf=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
Rational Method Equation: Project: Johnson Drive Apartments
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
hw1 HW1 1.73 5 5 5 0.87 0.87 1.00 2.85 4.87 9.95 4.29 7.33 17.24
he1 HE1 0.81 16 16 15 0.36 0.36 0.45 1.81 3.08 6.62 0.52 0.89 2.38
hs1 HS1 0.27 5 5 5 0.15 0.15 0.19 2.85 4.87 9.95 0.11 0.19 0.49
N/A HW1 (Impervious) 1.56 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 4.21 7.20 15.48
N/A HW1 (Pervious) 0.18 10 10 10 0.15 0.15 0.19 2.26 3.86 7.88 0.06 0.10 0.26
N/A HE1 (Impervious) 0.21 6 6 5 0.95 0.95 1.00 2.67 4.56 9.95 0.53 0.90 2.08
N/A HE1 (Pervious) 0.60 19 19 18 0.15 0.15 0.19 1.65 2.82 5.92 0.15 0.25 0.66
N/A HS1 (Impervious) 0.00 5 5 5 0.00 0.00 0.00 2.85 4.87 9.95 0.00 0.00 0.00
N/A HS1 (Pervious) 0.27 5 5 5 0.15 0.15 0.19 2.85 4.87 9.95 0.11 0.19 0.49
Total Release = 17.88
HISTORIC RUNOFF COMPUTATIONS
Intensity,
i2
(in/hr)
Intensity,
i10
(in/hr)
Intensity,
i100
(in/hr)
Notes
C. Snowdon
September 6, 2017
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
Design
Point
Basin(s)
Area, A
(acres)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
Flow,
Q2
(cfs)
Total allowable release from Basin Hs1
= 0.11
C2
Total allowable release from Basin Hs1
= 2.22
Total allowable release from Basin HN1
= 15.54
Flow,
Q10
(cfs)
Flow,
Q100
(cfs)
C10
CHARACTER OF SURFACE:
Runoff
Coefficient
Percentage
Impervious Project: Johnson Drive Apartments
Streets, Parking Lots, Roofs, Alleys, and Drives: Calculations By:
Asphalt ……....……………...……….....…...……………….………………………………….0.. 95 100% Date:
Concrete …….......……………….….……….………………..….………………………………… 0.95 90%
Gravel ……….…………………….….………………………….0..……………………………….15 0.. 50 40%
Roofs …….…….………………..……………….…………………………………………….. 0.95 90%
Pavers…………………………...………………..…………………………………………….. 0.40 22%
Lawns and Landscaping
Sandy Soil ……..……………..……………….…………………………………………….. 0.15 0%
Clayey Soil ….….………….…….…………..………………………………………………. 0.25 0% 2-year C
f = 1.00 100-year C
f = 1.25
Basin ID
Basin Area
(s.f.)
Basin Area
(ac)
Area of
Asphalt
(ac)
Area of
Concrete
(ac)
Area of
Roofs
(ac)
Area of
Gravel
(ac)
Area of
Pavers
(ac)
Area of
Lawns and
Landscaping
(ac)
2-year
Composite
Runoff
Coefficient
10-year
Composite
Runoff
Coefficient
100-year
Composite
Runoff
Coefficient
Composite
% Imperv.
A1 72641 1.668 0.000 0.000 1.668 0.000 0.000 0.000 0.95 0.95 1.00 90%
OW1 4061 0.093 0.000 0.000 0.000 0.000 0.000 0.093 0.15 0.15 0.19 0%
OS1 13093 0.301 0.000 0.047 0.000 0.000 0.000 0.254 0.27 0.27 0.34 14%
OS2 11338 0.260 0.000 0.000 0.000 0.000 0.000 0.260 0.15 0.15 0.19 0%
ON1 20964 0.481 0.105 0.166 0.000 0.000 0.000 0.210 0.60 0.60 0.75 53%
TOTAL 122097 2.80 0.105 0.213 1.668 0.000 0.000 0.818 0.72 0.72 0.90 64%
Overland Flow, Time of Concentration:
Project: Johnson Drive Apartments
Calculations By:
Date:
Gutter/Swale Flow, Time of Concentration:
Tt
= L / 60V
Tc
= Ti
+ Tt
(Equation RO-2)
Velocity (Gutter Flow), V = 20·S
½
Velocity (Swale Flow), V = 15·S
½
NOTE: C-value for overland flows over grassy surfaces; C = 0.25
Is Length
>500' ?
C*Cf
(2-yr
Cf
=1.00)
C*Cf
(10-yr
Cf
=1.00)
C*Cf
(100-yr
Cf
=1.25)
Length,
L
(ft)
Slope,
S
(%)
Ti
2-yr
(min)
Ti
10-yr
(min)
Ti
100-yr
(min)
Length,
L
(ft)
Slope,
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
Length,
L
(ft)
Slope,
Rational Method Equation: Project: Johnson Drive Apartments
Calculations By:
Date:
From Section 3.2.1 of the CFCSDDC
Rainfall Intensity:
a1 A1 1.67 5 5 5 0.95 0.95 1.00 2.85 4.87 9.95 4.52 7.72 16.59 13.09
ow1 OW1 0.09 5 5 5 0.15 0.15 0.19 2.85 4.87 9.95 0.04 0.07 0.17 0.17
os1 OS1 0.30 5 5 5 0.27 0.27 0.34 2.85 4.87 9.95 0.24 0.40 1.03 1.03
os2 OS2 0.26 9 9 8 0.15 0.15 0.19 2.35 4.02 8.38 0.09 0.16 0.41 0.41
on1 ON1 0.48 9 9 7 0.60 0.60 0.75 2.30 3.93 8.80 0.66 1.14 3.18 3.18
Total 17.88
DEVELOPED 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)
Basin(s)
C. Snowdon
September 6, 2017
Intensity,
i10
(in/hr)
Rainfall Intensity taken from the City of Fort Collins Storm Drainage Design Criteria (CFCSDDC), Figure 3.1
C10
Area, A
(acres)
Intensity,
i2
(in/hr)
100-yr
Tc
(min)
100-year Release
Rate
Q = C f ( C )( i )( A )
Page 6 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\Direct-Runoff
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
C2 C100
2-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q100
(cfs)
a1 A1 1.668 0.95 1.00 5.0 5.0 4.52 16.59
ow1 OW1 0.093 0.15 0.19 5.0 5.0 0.04 0.17
os1 OS1 0.301 0.27 0.34 5.0 5.0 0.24 1.03
os2 OS2 0.260 0.15 0.19 8.7 8.4 0.09 0.41
on1 ON1 0.481 0.72 0.75 9.4 7.4 0.66 3.18
DESIGN
POINT
BASIN
ID
TOTAL
AREA
(acres)
C2 C100
2-yr
Tc
(min)
100-yr
Tc
(min)
Q2
(cfs)
Q100
(cfs)
hw1 HW1 1.733 0.87 1.00 5.2 5.0 4.29 17.24
he1 HE1 0.805 0.36 0.45 16.3 14.7 0.52 2.38
Page 7 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\SUMMARY-TABLE
APPENDIX B
HYDRAULIC COMPUTATIONS
B.1 – Storm Sewers (Future Use)
B.2 – Inlets (Future Use)
B.3 – Detention Facilities
APPENDIX B.1
STORM SEWERS (FOR FUTURE USE)
APPENDIX B.2
INLETS (FOR FUTURE USE)
APPENDIX B.3
DETENTION FACILITIES
Pond No :
A1
100-yr
1.00
5.00 min 1946 ft3
1.67 acres 0.045 ac-ft
Max Release Rate = 13.09 cfs
Time (min)
Ft Collins
100-yr
Intensity
(in/hr)
Inflow
Volume
(ft3)
Outflow
Adjustment
Factor
Qav
(cfs)
Outflow Volume
(ft3)
Storage
Volume
(ft3)
5 9.950 4985 1.00 13.09 3927 1058
10 7.720 7735 0.75 9.82 5891 1845
15 6.520 9800 0.67 8.73 7854 1946
20 5.600 11222 0.63 8.18 9818 1405
25 4.980 12475 0.60 7.85 11781 694
30 4.520 13587 0.58 7.64 13745 -157
35 4.080 14309 0.57 7.48 15708 -1399
40 3.740 14990 0.56 7.36 17672 -2682
45 3.460 15601 0.56 7.27 19635 -4034
50 3.230 16182 0.55 7.20 21599 -5416
55 3.030 16698 0.55 7.14 23562 -6864
60 2.860 17194 0.54 7.09 25526 -8331
65 2.720 17715 0.54 7.05 27489 -9774
70 2.590 18166 0.54 7.01 29453 -11286
75 2.480 18637 0.53 6.98 31416 -12779
80 2.380 19078 0.53 6.95 33380 -14301
85 2.290 19504 0.53 6.93 35343 -15839
90 2.210 19930 0.53 6.91 37307 -17377
95 2.130 20275 0.53 6.89 39270 -18995
100 2.060 20641 0.53 6.87 41234 -20592
105 2.000 21042 0.52 6.86 43197 -22155
110 1.940 21383 0.52 6.84 45161 -23778
115 1.890 21778 0.52 6.83 47124 -25346
120 1.840 22124 0.52 6.82 49088 -26963
*Note: Using the method described in Urban Storm Drainage Criteria Manual Volume 2.
A =
Tc =
Project Location :
Design Point
C =
Design Storm
DETENTION POND CALCULATION; MODIFIED FAA METHOD w/ Ft Collins IDF
Input Variables Results
Required Detention Volume
Fort Collins, Colorado
APPENDIX C
WATER QUALITY DESIGN COMPUTATIONS
Sheet 1 of 2
Designer:
Company:
Date:
Project:
Location:
1. Basin Storage Volume
A) Effective Imperviousness of Tributary Area, Ia Ia = 90.0 %
(100% if all paved and roofed areas upstream of sand filter)
B) Tributary Area's Imperviousness Ratio (i = Ia/100) i = 0.900
C) Water Quality Capture Volume (WQCV) Based on 12-hour Drain Time WQCV = 0.36 watershed inches
WQCV= 0.9 * (0.91* i3 - 1.19 * i2 + 0.78 * i)
D) Contributing Watershed Area (including sand filter area) Area = 72,641 sq ft
E) Water Quality Capture Volume (WQCV) Design Volume VWQCV = 2,187 cu ft
VWQCV = WQCV / 12 * Area
F) For Watersheds Outside of the Denver Region, Depth of d6 = 0.43 in
Average Runoff Producing Storm
G) For Watersheds Outside of the Denver Region, VWQCV OTHER = 2,187 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 DWQCV = 1.5 ft
B) Sand Filter Side Slopes (Horizontal distance per unit vertical, Z = 0.00 ft / ft
4:1 or flatter preferred). Use "0" if sand filter has vertical walls.
C) Mimimum Filter Area (Flat Surface Area) AMin = 486 sq ft
D) Actual Filter Area AActual = 1500 sq ft
E) Volume Provided VT = 2250 cu ft
3. Filter Material
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 = 1.0 ft
Volume to the Center of the Orifice
ii) Volume to Drain in 12 Hours Vol12 = 2,187 cu ft
iii) Orifice Diameter, 3/8" Minimum DO = 1 - 3 / 16 in
Johnson Drive Apartments - Basin A1
Fort Collins, CO
Design Procedure Form: Sand Filter (SF)
Cody Snowdon
Northern Engineering
September 6, 2017
Choose One
Choose One
18" CDOT Class C Filter Material
Other (Explain):
YES
NO
WQ - Basin A1 - UD-BMP_v3.03.xlsm, SF 9/6/2017, 10:01 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?
PROVIDE A 30 MIL (MIN) PVC GEOMEMBRANE PER TABLE
SF-4 WITH SEPARATOR FABRIC (PER TABLE SF-3) ABOVE IT.
PROVIDE SEPARATOR FABRIC BELOW THE GEOMEMBRANE
AS WELL IF SUBGRADE IS ANGULAR OR COULD OTHERWISE
PUNCTURE THE GEOMEMBRANE.
6-7. Inlet / Outlet Works
A) Describe the type of energy dissipation at inlet points and means of
conveying flows in excess of the WQCV through the outlet
Notes:
Design Procedure Form: Sand Filter (SF)
Cody Snowdon
Northern Engineering
September 6, 2017
Johnson Drive Apartments - Basin A1
Fort Collins, CO
Choose One
YES NO
WQ - Basin A1 - UD-BMP_v3.03.xlsm, SF 9/6/2017, 10:01 AM
APPENDIX D
EROSION CONTROL REPORT
A comprehensive Erosion and Sediment Control Plan (along with associated details) will be included
with the final construction drawings. It should be noted, however, that any such Erosion and
Sediment Control Plan serves only as a general guide to the Contractor. Staging and/or phasing of
the BMPs depicted, and additional or different BMPs from those included may be necessary during
construction, or as required by the authorities having jurisdiction.
It shall be the responsibility of the Contractor to ensure erosion control measures are properly
maintained and followed. The Erosion and Sediment Control Plan is intended to be a living
document, constantly adapting to site conditions and needs. The Contractor shall update the
location of BMPs as they are installed, removed or modified in conjunction with construction
activities. It is imperative to appropriately reflect the current site conditions at all times.
The Erosion and Sediment Control Plan shall address both temporary measures to be implemented
during construction, as well as permanent erosion control protection. Best Management Practices
from the Volume 3, Chapter 7 – Construction BMPs will be utilized. Measures may include, but are
not limited to, silt fencing along the disturbed perimeter, gutter protection in the adjacent roadways
and inlet protection at proposed storm inlets. Vehicle tracking control pads, spill containment and
clean-up procedures, designated concrete washout areas, dumpsters, and job site restrooms shall
also be provided by the Contractor.
Grading and Erosion Control Notes can be found on Sheet C0.01 and C3.00 of the Utility Plans.
The Utility Plans will also contain a full-size Erosion Control Plan, as well as a separate sheet
dedicated to Erosion Control Details. In addition to this report and the referenced plan sheets, the
Contractor shall be aware of, and adhere to, the applicable requirements outlined in any existing
Development Agreement(s) of record, as well as the Development Agreement, to be recorded prior
to issuance of the Development Construction Permit. Also, the Site Contractor for this project will
be required to secure a Stormwater Construction General Permit from the Colorado Department of
Public Health and Environment (CDPHE), Water Quality Control Division – Stormwater Program,
before commencing any earth disturbing activities. Prior to securing said permit, the Site Contractor
shall develop a comprehensive StormWater Management Plan (SWMP) pursuant to CDPHE
requirements and guidelines. The SWMP will further describe and document the ongoing activities,
inspections, and maintenance of construction BMPs.
MAP PACKET
st
st
st
st
st
D
B M
D
ST
X
X X X
X
X
X
X
X
X
JOHNSON DRIVE
SPRING COURT
BURLINGTON NORTHERN
RAILWAY AND MAX GUIDEWAY
PORTION OF LOT 3,
BANK CENTER SQUARE
2' CONCRETE PAN
a1
FFE=89.50
EXISTING DRAINAGE DITCH
EXISTING STORM INLET
EXISTING STORM MH
EXISTING 12" ADS
4:1
9:1
5:1
5:1
4:1
43:1
6:1
7:1
7:1
17:1
5:1
4:1
11:1
6:1
(1.7:1)
(1.7:1)
(1.6:1)
(1.5:1)
(1.5:1)
(1.4:1)
(2.2:1)
BFE=4985.17
BFE=4986.17
BFE=4987.17
100-YEAR FLOODPLAIN
A1
OS1
OW1
ON1
OS2
ow1
os2
No. Revisions: By: Date:
REVIEWED BY:
C. Snowdon
DESIGNED BY:
DRAWN BY:
SCALE:
DATE:
09/06/2017
PROJECT:
1285-001
Sheet
of 11
JOHNSON DRIVE APARTMENTS These drawings are
instruments of service
provided by Northern
Engineering Services, Inc.
and are not to be used for
any type of construction
unless signed and sealed by
a Professional Engineer in
the employ of Northern
Engineering Services, Inc.
NOT FOR CONSTRUCTION
REVIEW SET
301 North 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
CALL 2 BUSINESS DAYS IN ADVANCE BEFORE YOU
DIG, GRADE, OR EXCAVATE FOR THE MARKING OF
UNDERGROUND MEMBER UTILITIES.
CALL UTILITY NOTIFICATION CENTER OF
COLORADO
Know what'sbelow.
Call before you dig.
R
NORTH
( IN FEET )
0
1 INCH = 20 FEET
20 20 40 60
C4.00
DRAINAGE EXHIBIT
LEGEND:
RUNOFF SUMMARY:
PROJECT BOUNDARY
DESIGN POINT
PROPOSED STORM SEWER
EXISTING STORM INLET
PROPOSED CURB & GUTTER
1. REFER TO "PRELIMINARY DRAINAGE REPORT FOR JOHNSON DRIVE APARTMENTS
DATED 09/05/2017" FOR ADDITIONAL INFORMATION
BASE FLOOD ELEVATION (BFE)
EXISTING STORM MANHOLE D
A2
a3
(4:1)
PROPOSED MAJOR CONTOUR
EXISTING MAJOR CONTOUR
EXISTING MINOR CONTOUR
PROPOSED MINOR CONTOUR
PROPOSED SLOPE (RISE:RUN) 4:1
DRAINAGE BASIN MINOR/MAJOR COEFF.
DRAINAGE BASIN ID
EXISTING SLOPE (RISE:RUN)
EXISTING CONCENTRATED FLOW
PROPOSED CONCENTRATED FLOW
PROPOSED OVERLAND FLOW
NOTES:
100-YEAR FLOODPLAIN
EXISTING STORM SEWER ST
BASIN
ID
TOTAL
AREA
(acres)
C2 C100
Q2
(cfs)
Q100
(cfs)
A1 1.668 0.95 1.00 4.52 16.59
OW1 0.093 0.15 0.19 0.04 0.17
OS1 0.301 0.27 0.34 0.24 1.03
OS2 0.260 0.15 0.19 0.09 0.41
ON1 0.481 0.72 0.75 0.66 3.18
FOR DRAINAGE REVIEW ONLY
NOT FOR CONSTRUCTION
11
1285-001
Johnson Drive Apartments
Project Number :
Project Name :
Pond A1
Page 1 of 1
1285-001_Pond A1_DetentionVolume_FAAModified Method.xls
S
(%)
Velocity,
V
(ft/s)
Tt
(min)
2-yr
Tc
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
a1 A1 No 0.95 0.95 1.00 N/A N/A N/A N/A 209 0.50% 1.41 2.5 N/A N/A N/A 5 5 5
ow1 OW1 No 0.15 0.15 0.19 20 25.00% 2.7 2.7 2.6 N/A N/A N/A 171 2.92% 2.56 1.1 5 5 5
os1 OS1 No 0.27 0.27 0.34 24 45.92% 2.1 2.1 1.9 N/A N/A N/A N/A N/A N/A 5 5 5
os2 OS2 No 0.15 0.15 0.19 34 5.11% 6.0 6.0 5.8 N/A N/A N/A 335 2.00% 2.12 2.6 9 9 8
on1 ON1 No 0.60 0.60 0.75 81 2.11% 6.6 6.6 4.6 264 0.59% 1.54 2.9 N/A N/A N/A 9 9 7
DEVELOPED TIME OF CONCENTRATION COMPUTATIONS
Gutter/Internal Building Flow Swale Flow
Design
Point
Basin
Overland Flow
C. Snowdon
September 6, 2017
Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
C Cf L
Ti
= −
Page 5 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\Tc-10-yr_&_100-yr
DEVELOPED COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
10-year Cf
= 1.00
September 6, 2017
**Soil Classification of site is Sandy Loam**
C. Snowdon
Page 4 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\C-Values
C100
Q = C f ( C )( i )( A )
Page 3 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\Hist-Direct-Runoff
(min)
10-yr
Tc
(min)
100-yr
Tc
(min)
hw1 HW1 No 0.87 0.87 1.00 35 5.00% 1.5 1.5 0.6 425 0.90% 1.90 3.7 N/A N/A N/A 5 5 5
he1 HE1 No 0.36 0.36 0.45 85 1.01% 12.8 12.8 11.2 298 0.50% 1.41 3.5 N/A N/A N/A 16 16 15
hs1 HS1 No 0.15 0.15 0.19 24 45.92% 2.4 2.4 2.3 299 100.50% 20.05 0.2 N/A N/A N/A 5 5 5
N/A HW1 (Impervious) No 0.95 0.95 1.00 35 5.00% 1.0 1.0 0.6 425 0.90% 1.90 3.7 N/A N/A N/A 5 5 5
N/A HW1 (Pervious) No 0.15 0.15 0.19 35 5.00% 6.1 6.1 5.9 425 0.90% 1.90 3.7 N/A N/A N/A 10 10 10
N/A HE1 (Impervious) No 0.95 0.95 1.00 85 1.01% 2.6 2.6 1.7 298 0.50% 1.41 3.5 N/A N/A N/A 6 6 5
N/A HE1 (Pervious) No 0.15 0.15 0.19 85 1.18% 15.5 15.5 14.9 298 0.50% 1.41 3.5 N/A N/A N/A 19 19 18
N/A HS1 (Impervious) No 0.00 0.00 0.00 24 45.92% 2.8 2.8 2.8 N/A N/A N/A N/A N/A N/A 5 5 5
N/A HS1 (Pervious) No 0.15 0.15 0.19 24 45.92% 2.4 2.4 2.3 N/A N/A N/A N/A N/A N/A 5 5 5
HISTORIC TIME OF CONCENTRATION COMPUTATIONS
C. Snowdon
September 6, 2017
Design
Point
Basin
Overland Flow Gutter Flow Swale Flow Time of Concentration
(Equation RO-4)
( )
3
1
1 . 87 1 . 1 *
S
C Cf L
Ti
= −
Page 2 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\Hist-Tc-10-yr_&_100-yr
HS1 (Pervious) 11546 0.27 0.00 0.00 0.00 0.00 0.27 0.15 0.15 0.19 0%
HISTORIC COMPOSITE % IMPERVIOUSNESS AND RUNOFF COEFFICIENT CALCULATIONS
Notes
September 6, 2017
10-year Cf
= 1.00
**Soil Classification of site is Sandy Loam**
Runoff Coefficients are taken from the City of Fort Collins Storm Drainage Design Criteria and Construction Standards, Table 3-3. % Impervious taken from UDFCD USDCM, Volume I.
Existing Impervious Area Breakdown
Page 1 of 23 D:\Projects\1285-001\Drainage\Hydrology\1285-001_Rational-Calcs.xlsx\Hist-C-Values