HomeMy WebLinkAboutSPRING CREEK FARMS NORTH, FILING THREE - FDP - FDP120016 - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTmodeling by Aspen Engineering. Please refer to Appendix C, of this memorandum, for additional details on
the Pond Sizing, Rating Curves, and Storm Sewer Pipe Sizing for Joseph Allen Drive and the outfall for
Ponds 100 and 200.
The provided information in Appendix C, in relation to release rates for Ponds 100 and 200 and for the design
and sizing of Pond 100 is for informational purposes only and .may be modified in the future when
development occurs within the LMN Zone. Pond 200 is being built to its ultimate condition with the Park/
Pond project and is not anticipated to change size in the future. However, modifications to the outlet structure
may occur, if needed, including the installation of an orifice plate to restrict flows.
DRAINAGE SUMMARY
In summary, the proposed 1-acre Private Park/ Detention Pond (Pond 200) meets the intent of the Spring
Creek Farms North Drainage Masterplan and subsequent reports, including the "Spring Creek Farms North
Drainage and Erosion Control Plan", by Stantec Consulting, dated October 4, 2005, and the Spring Creek
Farms North ODP Exhibits and Report, by Aspen Engineering, dated February 22, 2011.
The proposed water quality and detention pond (Pond 200) and associated outlet structure and storm sewer
outfall pipe for the proposed Park exceed that required for the interim condition, with the interim runoff from
the 10.07 acre basin (Basin 1) having little effect on downstream systems, due to the "actual" `C' value being
0.26, versus the historic `C' value of 0.25. Furthermore, the proposed Pond 200 and storm outfall structure
and pipe have been sized for the anticipated, future development of the 10.07 acres basin (C=0.60) within the
LMN Zone, thus allowing "site ready" status for future development within the north end of the LMN Zone.
Please refer to sheets C-004 and C-005 of the Civil Utility Plans, as well as this memorandum and attached
calculations and exhibits for detailed information on the Park/ Pond 200. Should you have any questions or
concerns, please contact me.
Sincerely,
DFtADO .. .
GOB; • FtYA
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JOHN GOOCH, P.E. 325
ASPEN ENGINEERING
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approximately 4940.05 (in ultimate LMN build -out condition), thus rendering the majority of the Park non -
usable during major storm events. This approach not only allows utilization of green space by individuals
who might use the Private Park, but also meets the site's and future LMN Zone's needs in relation to
stormwater quality and management for the area. Please refer to the Modified FAA Rational Pond Sizing and
Water Quality Calculations in Appendix B for additional details.
EROSION CONTROL & BMP's
Erosion Control for the Park/ Pond area will include installation of silt fence, straw wattles/ gravel inlet
filters, vehicle tracking pads, and seeding and mulching. BMP's for the site will include a sod/grass-lined
Park/ Pond area and a cobble -lined swale and underlying micro -pool. The BMP mechanisms will allow
pollutants the opportunity to settle out of storm runoff, prior to flows being directed downstream to receiving
basins. Please refer to sheets C-004 and C-005 of the Civil Drawings for additional details and information.
Because the site and proposed improvements are greater than 1 acre, a Stormwater Management Plan will be
required. The SWMP will be provided to the Contractor(s) with the expectation that the Contractor(s) will
adhere to the SWMP and proposed erosion control measures shown on the proposed Erosion Control Plan,
which will mitigate erosion control problems until the proposed landscaping/seeding becomes established.
ADDITIONAL MODELING INFORMATION
It should be noted that Aspen Engineering has provided hydrologic and hydraulic modeling for the entire area
west of Joseph Allen Drive (From Nancy Grave south to Drake Road) to allow for sufficient design and
construction of Joseph Allen Drive and the associated inlets and storm sewer in Joseph Allen Drive, as well as
for the anticipated, future development within the LMN Zone (west of Joseph Allen Drive). The modeling is
for informational and preliminary design purposes only and focuses on two ponds on the west side of Joseph
Allen Drive: Pond 200 is the north pond described above and will be constructed with the Park. The second
Pond is Pond 100, which is located due west of the anticipated low point in Joseph Allen Drive and treats
runoff from the west side of Joseph Allen Drive, per the requirements and recommendations called for in the
ODP Drainage Exhibit by Aspen Engineering, dated February 22, 2011.
Pond 100 was sized to provide interim water quality and detention for the west half of Joseph Allen Drive and
will be constructed with the Joseph Allen Drive roadway and utility infrastructure improvements, by others.
A 10' Type R Inlet will be located at the low point and will convey storm runoff west and into Pond 100, via
a 24" RCP. The runoff will flow south to the proposed water quality/ detention pond Outlet Structure, where
it will discharge southeast and into the storm sewer outfall in Joseph Allen Drive. The outflow from Pond
100 will combine with that from Pond 200 and will be conveyed south and into the North Tributary Storm
Sewer. Please refer to the Ultimate Drainage Plan for the LMN parcel Exhibit and other exhibits in Appendix
C for further details.
Aspen Engineering has designed Pond 100 to serve the interim water quality and detention needs of Joseph
Allen Drive, with Pond 100 anticipated to be expanded west in the future to serve additional water quality and
detention that may be needed once the LMN parcel(s) are developed. Pond 200 has been sized for the build -
out of the north LMN area and will not be enlarged in the future. However, an orifice plate will be installed,
in the future, on Pond 200's outlet structure to restrict flow once the north LMN parcel is built out.
Aspen Engineering has provided hydrologic and hydraulic modeling for the approximate 20.18 acre area (21-
acre area used in calculations) along the west side and including the west half of Joseph Allen Drive. The
calculations provide for the interim and ultimate sizing of Pond 100, as well as the ultimate sizing for Pond
200 and the proposed storm sewer pipe in Joseph Allen Drive. The release rates from Pond 100 and Pond 200
have been set based on the parameters required by the Spring Creek Farms Drainage Masterplan and
The proposed Park/ Pond will replace the existing grass/native field area, with the east side of the park tying
in to the Joseph Allen Drive roadway improvements from the Spring Creek Farms North, Second Filing. It's
important to note that the slight increase in "actual" impervious area during the interim condition provides for
a composite `C' value of 0.26. However, Aspen Engineering is utilizing a `C' value of 0.60 to size the
proposed detention pond (Pond 200) and estimate runoff for future, anticipated development within the 10.07
acre basin of the LMN Zone.
DRAINAGE DETAILS
The 10.07 acre basin (Drainage Basin 1) is bounded on the west by the Railroad Tracks, to the North by the
future Charles Brockman Drive extension, to the east by Joseph Allen Drive, and to the south by the
remaining LMN Zoning area. Existing conditions provides for runoff from Basin I flowing to the southeast
at grades of approximately 0.6% to 1.2%. Construction of the Park will utilize the same existing grades for
the majority of Basin 1, with the exception of the Park/ Pond area. The majority of the Park/ Pond area will
utilize grades varying from 1.2% to 4.5% for grass and trail areas within the pond with side slopes being no
greater than 4:1.
Runoff from Basin 1 will flow southeast and into the proposed Park/ Pond, where flows will be directed
southeast to the water quality and detention pond outlet structure. Based on the `C' value of 0.60, the
calculated 2-year and 100-year time -of -concentrations for the Park are 23.8 minutes and 21.4 minutes, with
the 2-year and 100-year rational method flows being 8.88 cfs and 40.78 cfs, respectively. The calculated
historic 2-year time -of -concentration and 2-year rational method flow is 48.6 minutes and 2.36 cfs,
respectively (see attached rational calculations in Appendix A). Aspen Engineering has also provided the
"actual" time of concentration and rational runoff calculations for the "actual" `C' value of 0.26 to
demonstrate what will be seen in the interim condition when no development (housing; streets, other
impervious areas, etc....) exists, with these calculations found at the end of the rational calculations and
labeled as "For Reference Only".
POND SIZING AND DETAILS
Utilizing a `C' of 0.60, Pond 200 was sized to provide the future, anticipated water quality and detention
needs for approximately 10.07 acres. The required 100-year detention volume (including 0.24 ac-ft of water
quality) for ultimate build -out of the LMN is 2.37 ac-ft. Pond 200 will provide approximately 2.37 ac-ft of
volume at an elevation of approximately 4940.05. Should the pond overtop, flows will spill east over the
existing sidewalk and onto Joseph Allen Drive, where flows will be conveyed south in the curb and gutter in
Joseph Allen Drive and eventually in to the North Tributary Storm Sewer. The sidewalk on the west side of
Joseph Allen Drive will serve as the overflow weir for the pond. Overtopping is dictated by the grades along
the back -of -sidewalk, which follow the prevailing street flowline grade. Therefore, Pond 200 may begin
overtopping at an elevation of 4939.70, with a controlled weir length and depth up to 131' and 0.35,
respectively. During the interim condition, the 100-yr water surface elevation could be approximately
4938.05, which corresponds to a storage volume of 0.785 ac-ft. Please refer to Appendix B for the water
quality and detention pond sizing, and rating curves for Pond 200.
It's important to note that a 5' wide by 134' long cobble -lined swale with micro -pool will be installed along
the south side of the Park/ Pond 200. The micro -pool will allow pollutants to settle out and promote
infiltration into the soil, prior to runoff passing through the water quality/ detention pond outlet structure and
into the storm sewer in Joseph Allen Drive. The proposed swale will have grades varying from 1.3% to
2.0%, thus providing positive conveyance of stormwater east to the outlet structure. *The grading design and
modeling for Pond 200 allows minor storm events to concentrate within the south and southeast corner of the
Park, with the water quality elevation set at 4936.90, thus allowing individuals to continue to utilize the Park
during small storm events. Major storm events may inundate the Park/ Pond up to an elevation of
r
ENGINEERING
September 5, 2012
Mr. Wes Lamarque
City of Fort Collins Stormwater Department
700 Wood Street
Fort Collins, CO 80521
RE: Drainage Memorandum — Spring Creek Farms North, Filing No. 3: Approximate 1-
acre Private Park Pond along the west side of Joseph Allen Drive
Dear Mr. Lamarque:
The purpose of this drainage memorandum is to summarize the proposed drainage design and provide
calculations for the slight increase in "actual" runoff due to the additional impervious area added by the
approximate I -acre Private Park within the Spring Creek Farms North property. The Park will serve as a
combination Park/ Detention Pond (Pond 200) and meet the water quality and detention needs for future,
anticipated development within the north end of the LMN Zone.
It's important to note that for the purpose of this memorandum, Aspen Engineering's calculations reflect the
future, anticipated development within the 10.07 acre basin of the LMN zone, with a 'C' of 0.60 being utilized
throughout the hydrological and hydraulic calculations for the Proposed 1-acre Park and remaining upstream,
offsite area, rather than the 'C' of 0.26, which will be seen during the interim condition (construction of the
Park/ Pond 200 only). Please refer to the Drainage Exhibit and Erosion Control Plan and Notes and Details
Sheets (sheets C-004 and C-005) within the Utility Plan set, and Appendices A-C of this memorandum, for
rational method and other hydrologic and hydraulic calculations.
PROJECT DESCRIPTION
In general, the project will consist of an approximate I -acre Private Park/ Pond, located on the west side of
Joseph Allen Drive and approximately 360 feet south of Charles Brockman Drive, within the Spring Creek
Farms North site. The park will be approximately 213 feet wide (W-E) by 218 feet long (N-S) and will entail
both pervious and impervious walking paths, as well as landscaping, sitting areas, and other site amenities.
The Park/ Pond is located on the south end of the 10.07 acre drainage basin and will provide the water quality
and detention needs for the future, developed condition of the 10.07 acre area. Pond 200 will exceed the
water quality and detention needs during the interim condition, which will only entail the construction of the
Park with the remaining upstream area remaining in existing ground cover.
ASPEN
ENGINEERING
DRAINAGE MEMORANDUM
FOR
SPRING CREEK FARMS NORTH,
FILING NO
FORT COLLINS, COLORADO
PREPARED FOR:
SC GROUP INVESTMENTS, LLC
SEPTEMBER 5, 2012