HomeMy WebLinkAboutMAPLE HILL, PHASE ONE - PDP - 29-00A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTexceeds 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.
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 and common areas, then concentrate at
proposed streets. Gutter flow in streets will be collected at low points via curb inlets or area
inlets and then conveyed via a storm sewer system or drainage pans to the detention ponds.
Subbasins were delineated based on the proposed grading. Final grading and basin
delineation is shown on the Rational Drainage Plan Sheet, which can be found in the back of
this report.
The site has been graded to accommodate two (2) detention ponds, however, they have been
hydraulically linked to function as one detention pond. Most of the site runoff will be directed
through this pond prior to discharge to the Larimer/Weld No.8 Ditch (No.8). The development
has been restricted to a 10 cfs release rate, except at the northeast corner of the site where the
Lind Farm property will discharge into the ditch through a series of swales and storm sewer
pipe.
Basin A encompasses the western portion of the site.
Basin B encompasses the middle/right portion of the site. This includes the proposed multi-
family area and the neighborhood park.
Basin C encompasses the southwest part of the site.
Basin D encompasses the northeast portion of the site.
Basin E encompasses the southeast portion of the site.
Runoff from each of the basins will be collected by a storm sewer system and directed to the
proposed detention pond.
TST, Inc. 7 November 18, 2002
0953-003
10
Developed Conditions Plan
3.1 Deslan 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).
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.
Because of the limited size of the subbasins on the site, the Rational Method was selected to
calculate runoff. However, the detention pond was analyzed and designed using UDSWM,
which will be discussed in Section 3.2.5. The Rational Method utilizes the SDDC manual
equation:
Q = CCAA
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 interpolated from
the rainfall intensity duration table in Figure 3-1 of the SDDC manual dated 1999. To obtain the
rainfall intensity, the time of concentration had to be determined. The following equation was
utilized to determine the time of concentration:
tc = ti + tt
where tc is the time of concentration in minutes, Q 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:
t. = [1.87(1.1 - CCf)Lo.$)/(S)0.33
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, and Cf is the storm
frequency coefficient. The formula limits the product of CCf to 1.0 and when the product
TST, Inc. 8 November 18, 2002
11 0953-003
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Introduction
1.1 Scope and Purpose
This report presents the results of a Final Drainage Evaluation for Maple Hill. In accordance
with the requirements of the Fort Collins Storm Drainage Design Criteria and Construction
Standards Manual (SDDC), the purpose of this report is to present a storm drainage plan that
identifies peak runoff conditions and provides a means by which to safely collect and convey
runoff across the site. This report will evaluate hydrologic conditions for the proposed
development to determine the location and magnitude of the storm runoff, and will use that
information for hydraulic analysis of the proposed streets and conveyance facilities.
1.2 Project Location and Description
Maple Hill is a proposed 451 single family, 76 duplex dwelling unit and 25 building (8-plex), 200
multi -family dwelling unit development located in the Northwest Quarter of Section 32,
Township 8 North, Range 68 West of the 6"' P.M., City of Fort Collins, State of Colorado. The
site is bounded on the west by County Road 11, the south by existing agricultural land, the east
by the Larimer/Weld No. 8 Irrigation Ditch (No. 8) and the north by County Road 52. A vicinity
map illustrating the project location is provided in Figure 1.
This project is located entirely within the Cooper Slough/Boxelder Drainage Basin. A master
drainage study currently does not exist for this portion of the basin. However, the existing
topography indicates that the proposed site drains to the No. 8 ditch and conveyed south.
TST, Inc. 1 November 18, 2002
0953-003
TABLE OF CONTENTS
1.0 Introduction
Page
1.1 Scope and Purpose...................................................................................................1
1.2 Project Location and Description...............................................................................1
2.0 Historic Conditions..........................................................................................................3
3.0 Developed Conditions Plan rtl
3.1
Design Criteria
...........................................................................................................6
3.2
Drainage
Plan Development......................................................................................7
3.2.1
Street Capacity
14
3.2.2
........................................................................................:...
Inlet Design..................................................................................................14
3.2.3
Storm Sewer Design....................................................................................19
3.2.4
Swale Design ................ :.................................................................
............. 19
3.2.5
Riprap Design
3.2.6
..............................................................................................19
Detention Pond Design................................................................................
24
3.3
Erosion/Sediment Control Plan................................................................................25
Figures
Figure1 - Vicinity Map................................................................................................................2
Figure 2 - Historic Drainage Map................................................................................................4
Figure 3 — Lind Property Historic Drainage Map.........................................................................5
Tables
Table 1 — Hydrologic Calculations Worksheet........................................................................8-11
Table 2 — Summary of Attenuated Runoff ....... :—'.... .............. ............... .. ... ............. ....-- .... 12-13
Table 3 — Summary of Street Capacity Analysis..................................................................15-16
Table 4 — Summary of, Inlet Analysis and Design.................................................................17-18
Table 5 — Summary of Storm Sewer Design........................................................................20-22
Table 6 — Summary of Riprap Design.......................................................................................23
Technical Appendices
Appendix A — Rational Method Analysis
Appendix B — UDSW M Model and Detention Pond Analysis
Appendix C — Street Capacity and Inlet Analysis Design
Appendix D — Storm Sewer and Riprap Design
Appendix E — Swale Design
Appendix F — Erosion Control
Sheets
RationalDrainage Plan..............................................................................................Sheet 1 of 1
UDSWMDrainage Plan.............................................................................................Sheet 1 of 1
November 18, 2002
Mr. Basil Hamden
City of Fort Collins Storm Water Division
P.O. Box 580
Fort Collins, CO 80522
Re: Maple Hill
Project No. 0953-003
Dear Mr. Hamden:
t We are pleased to present you with this Final Drainage Report for Maple Hill. This
report was prepared based on Urban Storm Drainage Criteria and City of Fort Collins
criteria. We believe it satisfies all criteria for a final report. This report also includes
I discussion of erosion and sediment control measures that will be utilized during and
after construction.
We look forward to your review and comment and will gladly answer any questions you
may have.
Sincerely,
IINC. NSULTING ENGINEERS
t
arsono
SD/sjk
Enclosures
TST, INC.
Consulting Engineers
748 Whalers Way - Building D
Fart Collins, CO 80525
(970)226-0557
Metro (303) 595-9103
Fax (970) 226-0204
Email info@tstinc.com
www.tstinc.com
FINAL DRAINAGE REPORT
FOR
MAPLE HILL
Submitted to:
CITY OF FORT COLLINS
November 18, 2002