HomeMy WebLinkAboutSPRING CREEK FARMS NORTH SECOND FILING (TRAILS @ TIMBERLINE) - PDP - PDP120004 - REPORTS - DRAINAGE REPORT (3)Preliminary Development Plan
Drainage Report
For
Spring Creek Farms North Filing No. 2
submitted to:
City of Fort Collins,
Colorado
April 25, 2012
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
TABLE OF CONTENTS
1.0 Introduction
1.1 Scope and Purpose ..................................................................................................... 1
1.2 Project Location and Description ................................................................................. 1
1.3 Previous Studies ......................................................................................................... 1
2.0 Existing/Historic Conditions ............................................................................................. 3
3.0 Developed Conditions Plan
3.1 Design Criteria ............................................................................................................. 4
3.2 Drainage Plan Development ....................................................................................... 4
3.2.1 Inlet & Storm Sewer Capacity ............................................................................ 5
3.2.2 Street Capacity ................................................................................................... 5
3.3 Erosion and Sediment Control .................................................................................... 6
4.0 Conclusion .......................................................................................................................... 6
5.0 References .......................................................................................................................... 6
Figures
Figure 1: Vicinity Map ................................................................................................................... 2
Appendices
Appendix A – Rainfall Data, City of Fort Collins
Appendix B – Rational Method Analysis
Appendix C – Excerpt from Previous Reports
Sheets
Drainage & Erosion Control Plan ........................................................................................ Sheet 1
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Page 1
1.0 Introduction
1.1 Scope and Purpose
This report presents the results of a preliminary drainage evaluation for Spring Creek Farms
North Filing No. 2 which consists of Outlot A of the Spring Creek Farms North Subdivision. A
developed hydrologic analysis of the area is presented to show adherence to the overall
preliminary analyses presented to the City by Stantec in 2008 and then by Aspen Engineering in
2011. The allowable release rates, as set forth in those previous studies, will be adhered to and
are used to size the detention facility for the developed area.
1.2 Project Location and Description
Spring Creek Farms North Filing No. 2 is located in the Southeast Quarter of Section 19,
Township 7 North, Range 68 West of the 6th Principal Meridian. More specifically, the project is
located in Parcel B1 of the Amended Overall Development Plan for Spring Creek Farms North
located at the northwest quadrant of Timberline and Drake Roads in Fort Collins, Colorado. The
site is bounded on the north by Charles Brockman Drive (adjacent to the Police Station), on the
east by Timberline Road, on the south by Drake Road, and on the west by the future extension
of Joseph Allen Drive.
The project is planned for a multi-family community, consisting of approximately 314 residential
units in 11 buildings situated on a 16-acre site. These are apartment-style residential buildings
fronting a main loop road and parking areas within the site. The site will also include a club
house with pool and other active outdoor amenities such as community gardens and park areas.
A vicinity map illustrating the project location is provided in Figure 1. The proposed site is
located within the Foothills Master Drainage Basin.
The area undergoing development with this project includes the 16-acre site and the future
Joseph Allen Drive, resulting in a total developed acreage of 17.9 acres. Also included in the
drainage analysis presented here are the remaining areas of the Spring Creek Farms North
development that are anticipated to contribute runoff toward the North Tributary Storm Sewer
System. These area are west of Joseph Allen Drive and south of Nancy Gray Avenue (approx.
20.3 acres), the west half of Timberline Road as it fronts the property and the north half of Drake
Road as it fronts the property.
1.3 Previous Studies
The “Spring Creek Farms North Overall Drainage Exhibit”, prepared by Aspen Engineering and
dated February 22, 2011 presented drainage basin information for the entire Spring Creek
Farms North development area. This Drainage Exhibit referred to the approved Spring Creek
Farms North Drainage and Erosion Control Plan, prepared by Stantec and dated October 4,
2005.
The “100% Design Drainage Report for Interim Timberline Road Widening Drake to Prospect”
prepared by North Star Design and dated July 20, 2005 provided an overview of the master
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Page 2
drainage planning for the area along Timberline Road and for the Spring Creek Farms area.
The “Conceptual Drainage Report for Spring Creek Farms”, prepared by Manhard Consulting
Ltd and dated December 11, 2000 was also referred to for overall release rate allowances for
the area.
Figure 1. Vicinity Map
PROJECT
SITE
DRAKE ROAD
TIMBERLINE ROAD
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Page 3
2.0 Existing/Historic Conditions
The site generally slopes from northwest to the southeast at 0.5% to 1.0% toward the
southeastern most corner of the site at Timberline and Drake Roads where runoff is intercepted
in an existing area inlet at the North Tributary Storm Sewer system. The soil naturally occurring
on this site is classified as Type C by the Natural Resources Conservation Service (NRCS). The
existing condition of the land consists of natural grasses that appear to be regularly mowed
during summer months. Groundwater has been observed in geotechnical borings at depths
between 13 and 21 feet below the surface.
The multi-family site is surrounded by public roads: Timberline Road, Drake Road, Charles
Brockman Drive, and will be enclosed by the future Joseph Allen Drive on the west. Currently,
the west half of Timberline Road from Charles Brockman Drive to Drake Road drains onto the
site through a 10’ wide concrete sidewalk chase. The north half of Drake Road sheet flows
drains onto the site as there currently is no curb and gutter on the north side of Drake Road
from the railroad tracks to the Timberline intersection.
The allowed 100-year release from the entire Spring Creek Farms North area is 20 cfs over
approximately 55 acres (or 0.36 cfs/acre). However, since the original drainage planning
occurred for this area, a few basins located near the north end are now directing storm runoff to
the north toward the Spring Creek Basin. These basins are shown on the Overall Drainage
Exhibit prepared by Aspen Engineering and are basins 6, 7, 8, 10, 11, and 12 and make up
almost 10 acres of land. Please refer to Appendix C for further illustration.
The remaining basins still draining toward the North Tributary Storm Sewer system are 1-5, and
9. These make up 48.27 acres. Of the basins draining toward the south, basin 5 is now the
existing Police Station. This site makes up approximately 7.5 acres (8.6 acres total tributary
area) and has a detained release rate of 3.10 cfs.
Therefore, the remaining allowable release into the North Tributary Storm Sewer system is 16.9
cfs (20 cfs – 3.1 cfs) released at 0.36 cfs/acre.
Each site that develops within the Spring Creek Farms North area is required to detain onsite
and release stormwater at their prorated share of the outfall capacity of the North Tributary
Storm Sewer system.
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Page 4
3.0 Developed Conditions Plan
This preliminary drainage analysis for Spring Creek Farms North Filing No. 2 is intended to
demonstrate the conformance of the drainage concept with the previous analyses for the site
outlined in Stantec, North Star, and Aspen Engineering’s previous reports. Full analysis of
drainage facilities will be presented with the Final Development Plans.
3.1 Design Criteria
The drainage system presented in this report has been developed in accordance with the
criteria established by the City of Fort Collins Storm Drainage Design Criteria and Construction
Standards Manual (SDDC) dated May 1984 and revised in January 1997. Design guidelines
and information were also obtained from the Urban Drainage and Flood Control District, Urban
Storm Drainage Criteria Manual (USDCM) where applicable.
Peak design flows for the 2, 10, and 100 year storms were determined using the Rational
Method. City of Fort Collins rainfall data can be found in Appendix A of this report. A full
description of the Rational Method including the applicable runoff coefficients can be found in
Appendix B.
3.2 Drainage Plan Development
Each site that develops within the Spring Creek Farms North area is required to detain onsite
and release stormwater at their prorated share of the North Tributary Storm Sewer outfall
capacity. The developed site will consist of the onsite basins 1-4, and the Joseph Allen Drive
basins OS3 and OS4. For the purposes of this study, the western Joseph Allen Drive basin OS3
is assumed to be intercepted and detained with the sites that develop on the west side of
Joseph Allen Drive; therefore, basin OS3 is not included in the prorated share of the outfall
capacity for this multi-family project. Offsite basins OS6 & OS7 (1/2 of Timberline and Drake
respectively) are also included in the detention volume calculations.
Offsite basin OS1 is located on the west side of Joseph Allen Drive. When development on that
side of Joseph Allen Drive occurs, OS1 is anticipated to provide their own detention facilities
based on their prorated share of the outfall capacity. Detention capacity will need to include the
west half of Joseph Allen Drive, depicted as basin OS3 in these plans. The development(s) will
have the option to tie into the storm sewer system located in Joseph Allen Drive in two separate
locations at the back of the type R inlets located on the roadway, or to tie into the North
Tributary Storm Sewer System located along the north side of Drake Road.
Offsite basin OS2 is also anticipated to provide their own detention facilities when development
occurs. A proposed storm pipe is shown stubbing into this basin to allow for stormwater release,
which will come through the multi-family site storm sewer network. The 100-yr undeveloped
flows calculated from this site are 7.67 cfs. Based on the current basin size shown on these
plans, the release anticipated from this site in the developed condition is 1.84 cfs (0.36 cfs/acre
x 5.1 acres).
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Page 5
For this multi-family project, the basins included in the prorated share for allowable release are
basins 1-4, OS4, OS6, and OS7, for a total of 20.70 acres. The resulting release rate at 0.36
cfs/acre is 7.45 cfs for the multi-family site. Onsite basins 3 & 4 direct runoff toward Timberline
Road and into the storm line along the western edge of Timberline Road. Because of the way
this storm line is currently configured at the intersection of Timberline and Drake, the runoff
entering this pipe will not be intercepted in the onsite detention pond. The amount of runoff
generated in the 100-year developed condition in these basins is 1.24 cfs. This amount of runoff
will need to be subtracted from the allowable release of 7.45 cfs for our site, resulting in 5.82 cfs
remaining to be allowed to be released from the onsite detention pond.
Detention capacity will be calculated using the 5.82 cfs release for the multi-family site. With
this release rate, the anticipated required volume is 4.3 ac-ft. A mass balance spreadsheet is
included in Appendix B.
Onsite basin 2 directs runoff toward Charles Brockman Drive and ultimately into the detention
pond at the Police Station. The amount of runoff generated in the 100-year developed condition
in this basin is 3.83 cfs. It has been determined that the existing detention pond capacity at the
Police Station is adequate to accept this flow. The original required design volume per the
Police Station drainage report was 1.82 ac-ft and 2.11 ac-ft was provided. Checking the volume
requirements with adding Onsite basin 2 increases the required volume to 1.89 ac-ft using the
current release rate from that pond of 3.1 cfs. A mass balance with the increased area is
included in Appendix B. The street and inlet capacities on Charles Brockman Drive will be
evaluated with the final analysis for this site to determine if there is adequate capacity to accept
some additional runoff from this multi-family site.
Storm lines within the project site will be designed to safely convey the 10-year developed flow
in conjunction with the streets and parking lots providing a flow path to convey the remainder of
the storm flows (100-year flows) into the detention area and subsequently releasing into the
existing North Tributary Storm Sewer System at the allowable release rate.
The flow arrows shown on the proposed drainage plan in the back pocket of this report reflect
preliminary lot grading that will direct flows to storm sewers, overflow conveyance channels, and
the detention basin.
3.2.1 Inlet & Storm Sewer Capacity
Proposed storm sewers and inlets will be sized with final design.
3.2.2 Street Capacity
Street capacity has been preliminarily analyzed to plan for proper inlet spacing along Joseph
Allen Drive. Street inlets have been shown accordingly.
Street capacity calculations for Joseph Allen Drive, Charles Brockman Drive and the internal
roadway network for the multi-family site will be provided with final design.
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Page 6
3.3 Erosion and Sediment Control
A discussion of erosion and sedimentation control will be provided with final design.
4.0 Conclusion
The standards in the City of Fort Collins Storm Drainage Design Criteria and Construction
Standards Manual (SDDC) and the Urban Storm Drainage Criteria Manual, along with the overall
allowable release requirements set forth in previous studies of the Spring Creek Farms North site
have been used to create an effective preliminary storm drainage design. The proposed design is
expected to collect and convey storm water runoff through and around the site safely and effectively
to the existing North Tributary Storm Sewer system.
According to the study presented, the site complies with the allowable releases into the North
Tributary Storm Sewer System and provides sufficient stormwater detention on-site to account for
the allowable releases from the site and adjacent roadway runoff. The North Tributary Storm Sewer
system will be tied into with a controlled release and water quality structure near the corner of
Timberline and Drake Roads.
5.0 References
1. Aspen Engineering, February 22, 2011, “Spring Creek Farms North Overall Drainage
Exhibit”
2. North Star Design, Inc., July 20, 2005 “100% Design Drainage Report for Interim
Timberline Road Widening Drake to Prospect”
3. Manhard Consulting, Ltd., December 11, 2000. “Conceptual Drainage Report for
Spring Creek Farms Fort Collins, Colorado”
4. City of Fort Collins, May, 1984 (updated January, 1997). “Storm Drainage Design
Criteria and Construction Standards Manual (SDDC)”.
5. Urban Drainage and Flood Control District, 2001, “Urban Storm Drainage Criteria
Manual”.
APPENDIX A
Rainfall Data, City of Fort Collins
APPENDIX B
Rational Method Analysis
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Appendix B
Developed Runoff
Developed condition runoff was evaluated in accordance with the criteria established by the City
of Fort Collins Storm Drainage Design Criteria and Construction Standards Manual (SDDC)
dated May 1984 and revised in January 1997. Design guidelines and information were also
obtained from the Denver Regional Council of Government Urban Storm Drainage Criteria
Manual (USDCM) where applicable. A full description of this method is available through those
manuals; this appendix is to serve as a reference for Willow Brook Filing No. 3. The Rational
Method computes only the peak flow at a design point and does not provide hydrograph
information.
Rational Method
City of Fort Collins
The Rational Method calculates peak runoff using the equation:
Q = CCfIA
Where: Q = The maximum rate of runoff (cfs)
C = Rational Method Runoff Coefficient for the design storm frequency. City of Fort
Collins runoff coefficients are found in Table 3-3 of the SDDC manual.
Cf = Storm Frequency Coefficient found in Table 3-4 of the SDDC manual.
I = Average intensity of rainfall in inches per hour for a duration equal to the time of
concentration, Tc. City of Fort Collins rainfall intensity data are shown in this
Appendix.
A = Basin Area (acres)
Rational Method Runoff Coefficients, C, are a function of the basin land use and the design
storm frequency. They are listed in Table 3-3 of the Storm Drainage Design Criteria manual. For
basins containing more than one land use, a weighted average runoff coefficient has been
computed.
Time of Concentration, Tc, is the sum of the overland travel time, to, and the channel or conduit
flow time, tt. Time of concentration is used to select the correct rainfall intensity for the rational
method equation.
T c = t i + t t
where: Tc = Time of Concentration (min),
to = Overland Travel Time (min),
tt = Channel or Conduit flow time (min).
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Appendix B
Overland Travel Time, to, is computed using the frequency adjusted runoff coefficient
and is applicable to all design storm intervals.
( ) 3
1
1 . 8 ( 1 . 1 ) 0 . 5
S
t CC f L
o
−
=
Where:
to = Overland Travel Time (min)
C5 = The Rational Method runoff coefficient for the 5 – year storm
L = Length of overland flow (ft), Maximum = 400 feet.
S = Average basin slope (%)
Channel or Conduit Travel Time, tt, is determined from the velocity of flow computed
for the hydraulic properties of the channel, ditch, gutter, pipe or sewer. For the purposes
of this report, the following equation was used:
V
tt 60 L *
=
Where:
tt = Channel or Conduit Travel Time (min)
L = Length of channel or conduit flow (ft)
V = Velocity of flow (fps), determined from Figure 3-2
Spring Creek Farms North Filing No. 2 Preliminary Drainage Report
Appendix B
APPENDIX C
Excerpts from Previous Reports
760 Whalers Way Bldg C, Suite 200 Fort Collins, CO 80525 ideas@tstinc.com
970.226.0557 main 303.595.9103 metro 970.226.0204 fax www.tstinc.com
PROJ. NO.
DRAWN:
CHECKED:
DATE:
REVISIONS
SCALE:
SHEET NUMBER
SHEET TITLE:
ISSUED FOR:
DESIGNED:
Timberline Road & Drake Road
Fort Collins, Colorado 80525
1045.0039.00
H.E.M.
A.S.G.
04/25/2012
SCHEMATIC DESIGN
DRAINAGE & EROSION
PLAN
C-14
SPRING CREEK FARMS
NORTH - FILING NO. 2
H.E.M.
AS NOTED
A MCWHINNEY COMMUNITY