HomeMy WebLinkAboutREGISTRY RIDGE PUD, SECOND FILING - PRELIMINARY / FINAL - 32-95F - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORT3.2.4 Swale Design
The drainage swale from Design Point 6 conveys water from Subbasin 1D to the street in
Subbasin 1C. The drainage swale in Subbasin lE conveys water from the Subbasin IF curb
opening to the area inlet at Design Point 7. These swales have been designed to function as a
mowed grass swale with a concrete trickle pan.
In addition, several swales with cobble trickle pans direct flow around buildings and discharge
into the street via curb openings. The multi -family tract is relatively flat and the common areas
typically are over irrigated. To prevent saturation of soils in the swales and at other locations
cobble trickle pans are proposed. These pans will consist of 2" to 4" cobble overlaying a concrete
valley pan. We have used this method of installing nuisance channels successfully in the past and
believe they function adequately in this case as well. These swales and the associated pipes under
the sidewalks were not evaluated as part of this report because they are intended to only provide
small storm and nuisance protection. Runoff from larger storms will obviously utilize these
conveyances as well but will act more as sheet flow once these facilities are inundated.
The results of the swale analysis can be found with supporting calculations in Appendix E.
3.3 Erosion Control
During construction sediment will be contained on site with silt fence around the site. Additional
measures will be used in the basins by installing gravel filters over all of the inlets and curb
openings. The site will be reseeded and mulched in areas not being paved to provide soil
stabilization until build out. It is anticipated that once the buildings are completed an irrigation
system will be installed and the common areas sodded. If there is to be a delay of more than 30
days in 'the construction of the buildings, the disturbed areas will be reseeded and mulched.
Erosion of soils in the onsite channels and swales are not a concern. Velocities are very low due
to the flat slopes, so low in fact that cobble trickle pans were necessary to prevent constantly
saturated soils.
TST, Inc. 13 January 18, 2000
0890-001
TABLE 5. SUMMARY OF STORM SEWER DESIGN
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Street encroachment criteria for the public streets was taken from Table 4-1 (minor storm) and
Table 4-2 (major storm) of the SDDC. 100-yr. flow depths in. The same general criteria was
used for the private streets within the multi -family tract with a notable exception; the private
streets have a valley pan section and the allowable flow depth is determined by the elevation of
attached garages which is the same for both the minor and major storms. All of the public and
private streets meet these requirements and will function below the allowable capacities. The
results of the Street Capacity Analysis can be found in Table 3 with supporting calculations
presented in Appendix B.
3.2.2 Inlet Design
Area inlets, curb inlets, and curb openings were used to collect 100-yr. runoff from low points.
Design Points 6 & 8 utilize curb openings to convey the storm runoff to adjacent swales. Ponding
depths were limited to prevent inundation of structures. The openings will convey the 2-yr runoff
without overtopping the curb, with the 100-yr overtopping the curb but runoff still being directed
to the appropriate swales without inundating any buildings.
An area inlet was designed for Design Point 7. This inlet was sized according to Figure 5-3 of the
SDDC manual and is sized to convey the 100-yr runoff.
CDOT Type "R" curb inlets were selected at Design Points 2, 4, 5, 12, & 13. These inlets are
connected to storm sewer systems that convey the runoff to First Filing Storm Lines 10 & 32.
The results of the Inlet Analysis and Design can be found in Table 4 with supporting calculations
presented in Appendix C.
3.2.3 Storm Sewer Design
Storm Sewer Lines ST-1, ST-IA, and ST-113 were designed with UDSEWER to convey the 100-
yr. runoff in Basin 403. ST-1 connects to First Filing Storm Line 32 at the downstream end, and
Storm Line 26 at the upstream. The water surface at the downstream end of ST-1 was
determined from the First Filing Report. The entire system, existing and proposed, was modeled
to assure no negative impacts on the existing system. Information was obtained from the First
Filing report for this purpose. The pipes were sized such that the hydraulic grade remains below
the flow line of the proposed inlets.
Line ST-2 was also analyzed with UDSEWER. It discharges runoff from Basin 402 and connects
to First Filing Storm Line 10 at the existing inlet. The 100-yr water surface was determined from
the First Filing report.
The results of the Storm Sewer Design can be found in Table 5 with supporting UDSEWER and
HY-8 results presented in Appendix D.
TST, Inc. 9 January 18, 2000
0890-001
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where L is the length of overland flow in feet (limited to a maximum of 500 feet), S is the average
basin slope in percent, C is the composite runoff coefficient for the area of overland flow
(different from the composite runoff coefficient for the entire basin and used to match the
methodology of the First Filing report), and Cr is the storm frequency coefficient. The formula
limits the product of CCr to 1.0 and when the product exceeds this value 1.0 is used in its place.
Gutter (or channel) travel times were determined by utilizing Figure 3-3 for the flow velocity
within the conveyance element. The travel time was then determined by dividing the gutter flow
length by the velocity. This procedure for computing time of concentration allows for overland
flow as well as travel time for runoff collected in streets, gutters, channels, or ditches. After the
peak runoff was calculated, attenuated runoff was calculated. This was done by combining all
contributing areas upstream of a given design point. The time of concentration for the design
point was taken as the greatest time of all the contributing subbasins and adding in pipe travel
time if necessary.
3.2 Drainage Plan Development
The proposed drainage plan consists of a combination of overland flow and gutter flow. The
runoff will sheet flow across landscaped yards, common areas and parking lots, then concentrate
at proposed streets or swales. Gutter flow in streets will be collected at low points via curb inlets
or curb openings and then conveyed via a storm sewer system to the water quality ponds of First
Filing. Subbasins were delineated based on the proposed grading. Final grading and basin
delineation is shown on the Drainage & Erosion Control Plan sheets, which can be found in the
back of this report.
All subbasins are proposed to discharge from the site undetained. The runoff from the site was
compared to the runoff assumed in the Final Drainage Report for Registry Ridge P.U.D. First
Filing, and was found to be in compliance with that report. Basin delineation used in this report
matches that of the First Filing report to aid in comparisons.
Basin 403 contains the majority of the site and drains to the south. This basin also includes the
offsite area to the west of the site that discharges onto the site via First Filing Storm Line 26.
Storm drainage from this basin will be collected via storm sewer and routed to First Filing Storm
Line 32.
Basin 402 contains the northeast portion of the site and discharges to the north. Storm drainage
from this basin will be collected via storm sewer and routed to First Filing Storm Line 10.
The results of the Rational Method Hydrologic Analysis can be found in Table 1 with the
methodology of calculations shown in Appendix A. Table 2 shows the results of the runoff
attenuation described previously.
TST, Inc. 5 January 18, 2000
0890-001
3.0
Developed Conditions Plan
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. Where applicable,
design guidelines and information were also obtained from the Denver Regional Council of
Government Urban Storm Drainage Criteria Manual (USDCM). The drainage system also meets
conditions as presented in the Final Drainage Report for Registry Ridge P.U.D. First Filing (First
Filing).
Developed condition hydrology was evaluated based on the 2-year and 100-year storm
frequencies as dictated by Table 3-1 of the SDDC manual. Detention for the site has been
accounted for in detention ponds in First Filing, therefore, no onsite detention is proposed.
Because of the limited size of the subbasins on the site, the Rational Method was selected to
calculate runoff. The Rational Method utilizes the SDDC manual equation:
Q = CC fIA
where Q is the flow in cfs, C is the runoff coefficient, Cf is the storm frequency coefficient, I is the
rainfall intensity in inches per hour, and A is the total area of the basin in acres. The runoff
coefficient, C, was calculated from Table 3-3 of the SDDC manual based on the proposed
developed condition land use. A composite runoff coefficient was calculated for each sub -basin
based on the percentage of impervious surface (C = 0.95) and pervious surface (C = 0.25). Cf
was taken from Table 3-4 of the SDDC manual and was determined to be 1.0 for the 2-year storm
and 1.25 for the 100-year storm. The appropriate rainfall intensity was obtained from the
Intensity Formula for Registry Ridge P.U.D. determined by Northern Engineering Services and
presented in the First Filing report. To obtain the rainfall intensity, the time of concentration had
to be determined. The following equation was utilized to determine the time of concentration:
t,= ti+ tt
where U is the time of concentration in minutes, t; is the initial or overland flow time in minutes,
and tt is the travel time in the gutter in minutes. The initial or overland flow time was calculated
with the SDDC manual equation:
i
TST, Inc. 4 January 18, 2000
` 0890-001
I
0
2.0
Historic Conditions
Most of the runoff from the site currently sheet flows from the northwest to southeast with an
average slope of 1% and discharges into Storm Line 32 from Registry Ridge P.U.D. First Filing
(First Filing) at the southeast corner of the site. Some runoff discharges directly onto the
surrounding streets and is collected at existing inlets on Storm Lines 10 & 32 of First Filing. In
addition, runoff from First Filing Storm Line 26 is discharged into a swale that crosses the center
of the site and discharges in Storm Line 32.
TST, Inc. 3 January 18, 2000
0890-001
FIGURE 1
HARMONY ROAD
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TST, Inc. 2 January 18, 2000
0890-001
January 18, 2000
Mr. Basil Hamden
City of Fort Collins
Stormwater Utility Department
P.O. Box 580
Fort Collins, CO 80522-0580
Re: Registry Ridge P. U.D., Second Filing
Project No. 0890-001
Dear Mr. Hamden:
We are pleased to submit this Final Drainage Report for the Registry Ridge P.U.D.,
Second Filing. The report includes our evaluation of the proposed storm runoff
interception and conveyance facilities, and erosion control plan. This report was
prepared based on current City of Fort Collins criteria and we believe it meets the
requirements for a final submittal. This submittal includes revisions based on the City's
previous comments.
We look forward to your review and comments and will gladly answer any questions you
may have.
Sincerely,
TST, INC. CONSULTING ENGINEERS
Eric M. Fuhrman
EMF/tdy
TSTINC.
!
748 Whalers Way - Building D
Fort Collins, CO 80525
Consulting Engineers
(970) 226-0557
Metro (303) 595-9103
Fax (970) 226-0204
Email info@tstinc.com
www.tstinc.com
David B. Lindsay, P.E.
102 Inverness Terrace East
Suite 105
Englewood, CO 80112
(303) 792-0557
Fax (303) 792-9489
FINAL DRAINAGE REPORT
FOR
REGISTRY RIDGE P.U.D.,
SECOND FILING
Submitted to:
CITY OF FORT COLLINS
January 18, 2000
TABLE OF CONTENTS
Page
1.0 Introduction
1.1 Scope and Purpose...................................................................................................1
1.2 Project Location and Description............................................................................1
1.3 Previous Studies.......................................................................................................1
2.0 Historic Conditions.............................................................................................................3
3.0 Developed Conditions Plan.................................................................................................4
3.1 Design Criteria.........................................................................................................4
3.2 Drainage Plan Development....................................................................................5
3.2.1 Street Capacity .............................................................................................9
3.2.2 Inlet Design..................................................................................................9
3.2.3 Storm Sewer Design....................................................................................9
3.2.4 Swale Design.............................................................................................13
3.3 Erosion Control......................................................................................................13
Figures
a
Figure1 - Vicinity Map...................................................................................................................2
Tables
Table 1 - Hydrologic Calculations Worksheet..........................................................................
6&7
" Table 2 - Summary of Attenuated Runoff.......................................................................................8
- Table 3 - Summary of Street Capacity Analysis............................................................................10
Table 4 - Summary of Inlet Analysis and Design..........................................................................11
Table 5 - Summary of Storm Sewer Design..................................................................................12
Technical Appendices
- Appendix A - Rational Method Analysis
Appendix B - Street Capacity Analysis
_
Appendix C - Inlet Analysis and Design
- Appendix D - Storm Sewer Design
Appendix E - Swale Design
Appendix F - Erosion Control
- Appendix G - Registry Ridge P.U.D. First Filing In
Sheets
_ Drainage & Erosion Control Plan
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