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APPENDIX A
SWMM COMPUTER OUTPUT FOR THE DEVELOPED CONDITION
100-YEAR EVENT
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' storage value because the shape of the actual storage -discharge curve is such that, for a given
storage, more outflow will occur than indicated by the assumed straight line curve. The results
' of this analysis are also included in the SWMM output in Appendix A.
As discussed in Chapter II, slight modifications to Pond A could provide 1.0 ac-ft of
available storage. Conceptually, the bottom of the existing pond could be regraded to increase
' the potential storage to 1.3 ac-ft. Given that the estimate of required storage is 3.0 ac-ft,
additional volume is required for the pond. It is expected that Pond A would be expanded to
' the east to Shields Street, between the two irrigation canals. A simple comparison of maximum
surface area to pond capacity for the modified existing condition (1.3 ac-ft), and extrapolation
' to the proposed pond area indicates that roughly 3.1 ac-ft of storage is available. Since it is
probable that the estimated volume requirement of 3.0 ac-ft is conservatively high, it is apparent
that an expanded Pond A would be sufficient for meeting stormwater detention criteria, thereby
eliminating need for on -site detention in the Raintree Townhomes site.
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II
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While no on -site detention was modeled for any of the subcatchments, SWMM was
allowed to "create" storage over pipe inlets. In this manner, each entire attenuated hydrograph
is passed through its respective conveyance element, eventually reaching Pond A. This was
considered a reasonable approach in modeling this particular system, because storage has been
designed over the numerous inlets, with limited capacity storm sewer pipes. In addition, storage
requirements (greater than the actual storage provided) may be relieved by overland flow to
either the New Mercer Ditch or Shields Street thereby effectively removing runoff from the
system. As modeled, the hydrograph is attenuated while maintaining mass balance.
Table 3.3 presents a brief summary of the results of the hydrologic modeling. It is noted
that the majority of runoff in the area is generated in the Raintree P.U.D., south of Raintree
Drive. The SWMM output is contained in Appendix A.
' Table 3.3. Summary of Hydrologic Modeling Results for the Developed Condition 100-Year
Event.
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Location
Peak
Discharge
(cfs)
Volume
(ac-ft)
Townhomes Drainage Area (Off -Site)
6
0.27
Total From Townhomes Drainage Area (Including Off -Site)
12
0.65
Senior Center Drainage Area (Off -Site)
87
2.43
Total From Senior Center Drainage Area (Including Off -Site)
22
3.98
3.3 Preliminary Sizing of Major Drainage Facilities
Based on the output from the hydrologic model, preliminary drainage facility sizes were
' determined for the Raintree Townhomes site. The proposed pipe sizes are a 24-inch RCP to
drain both the inlets and the siphon, and a 15-inch RCP underdrain. These pipes are sized to
' operate without surcharging.
Preliminary re -sizing of Detention Pond A was accomplished using the hydrologic model
' and a simple iteration procedure. This technique involves assuming a two -point, straight-line,
storage -discharge curve. The first point on the curve represents zero discharge at zero storage;
the second point is the allowable release rate (4.4 cfs) and the minimum storage necessary to
' meet the release rate. The storage is varied and the model rerun until the appropriate storage
is obtained. This method produced a preliminary volume requirement of 3.0 ac-ft, plus
' freeboard. It is noted that, in this case, this method should result in a conservatively high
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TOWNHOMES
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Figure 3.3. Schematic Diagram of the Proposed Condition SWMM Model.
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Table 3.2. Summary of Conveyance Element Parameters.
Conveyance
Element
Description
Width or
Diameter
(ft)
Length
(ft)
Slope
(ft/ft)
Manning's
"n"
Left Side
Slope
ZH : IV
Right Side
Slope
ZH : 1 V
101
Channel
0.1
320
0.0130
0.016
0.0
12.0
102
Pipe
Overflow
2.0
0.0
100
100
0.0050
0.0050
0.016
0.250
0.0
50.0
0.0
50.0
103
Channel
0.0
150
0.0050
0.250
4.0
4.0
104
Pipe
Overflow
1.25
0.0
440
440
0.0050
0.0050
0.016
0.250
0.0
4.0
0.0
4.0
105
Pipe
2.0
100
0.0100
0.016
0.0
0.0
106
Pipe
1.25
485
0.0100
0.016
0.0
0.0
107
Pipe
2.0
478
0.0100
0.016
0.0
0.0
108
Pipe
1.25
400
0.0100
0.016
0.0
0.0
109
Pipe
2.0
70
0.0120
0.016
0.0
0.0
Table 3.1. Summary of Subcatchment Parameters.
Subarea
Width
(ft)
Area
(ac)
Percent
Impervious
Slope
(ft/ft)
Resistance Factors
Surface Storage
Infiltration Rate
Impervious
Pervious
Impervious
(in)
Pervious
(in)
Maximum
(in/hr)
Minimum
(in/hr)
Decay
Rate
(sec')
1
320
1.8
29.4
0.006
0.02
0.250
0.10
0.30
0.90
0.43
0.0018
2
733
0.8
82.0
0.050
0.02
0.250
0.10
0.30
0.90
0.43
0.0018
3
740
1.2
25.0
0.020
0.02
0.250
0.10
0.30
0.90
0.43
0.0018
4
660
5.3
67.2
0.020
0.02
0.250
0.10
0.30
0.90
0.43
0.0018
5
320
1.7
69.9
0.013
0.02
0.250
0.10
0.30
0.90
0.43
0.0018
6
2,000
11.2
90.0
0.015
0.02
0.250
0.10
0.30
0.90
0.43
0.0018
N
SCALE: 1 "=60'
Figure 3.2. Preliminary Site Plan
for Raintree Townhomes.
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Drainage Configuration.
' M. PRELIMINARY DRAINAGE PLAN
3.1 General
' The drainage boundaries of the proposed Raintree Townhomes, as well as all off -site
drainage areas tributary to Pond A, are shown in Figure 3.1. Development of the Townhomes
and Senior Center site will alter historical drainage patterns throughout the area. As part of the
approved Senior Center drainage plan, flows from the Senior Center which previously drained
across this site will be collected and conveyed to Pond A. Also, drainage from the northwest
corner of Shields Street and Raintree Drive will now pass through the system proposed for the
Senior Center development.
Figure 3.2 shows the preliminary site plan for Raintree Townhomes. Runoff which
travels to the cul-de-sac and street will be collected by curb inlets near Shields Street. These
inlets will also collect the off -site flow from the south. The remainder of the Townhomes site
drains overland to the north and will be collected by a perimeter Swale with an underdrain. A
pipe will connect the inlets to the underdrain. All flows are to be conveyed to the low point on
the north perimeter and siphoned under the New Mercer Canal to an expanded Pond A.
3.2 Hydrologic Modeling of Proposed Drainage Conditions
SWMM was used to model the basin response to the 100-year rainfall event. The
rainfall, resistance factors, surface storage and infiltration information were taken from the
Spring Creek Master Drainageway Plan [EPI, 1988]. The remaining subcatchment parameters
(area, width, etc.) and conveyance parameters (diameter, length, etc.) were taken from the City
of Fort Collins topographic aerial photographs [dated March 1986], or previously cited reports
and plans. Tables 3.1 and 3.2 summarize the resulting basin and conveyance parameters,
respectively.
The hydrologic model consists of two major subareas. Each of these subareas was
divided into several subbasins to reflect physical drainage conditions. The subarea which
impacts Raintree Townhomes was divided into an off -site subcatchment (Subbasin 1), and the
two on -site subcatchments described above (Subbasins 2 and 3). The other major subarea drains
' through the proposed Senior Center. This subarea was divided into two on -site subcatchments
(Subbasins 4 and 5), and an off -site subcatchment consisting of the commercial development
to the south (Subbasin 6). The two major subareas are combined at Pond A. Reference is made
' to Sheet 1 which depicts the delineation of these subbasins. Figure 3.3 provides a schematic
diagram of the SWMM model connectivity.
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4 Figure 2.1. Historical Drainage Basin.
H. HISTORICAL DRAINAGE CONDITIONS
Historically, the Raintree Townhomes area was tributary to Spring Creek, but the existing
drainage patterns are such that the New Mercer Canal intercepts all runoff from the Townhomes
drainage area except that which flows east to Shields Street. The drainage to Shields Street is
conveyed north to Spring Creek. The proposed development limits and the historical drainage
boundary are depicted in Figure 2.1.
The existing Townhomes drainage area consists of the Raintree Townhomes site and an
off -site area to the south. The tributary area is presently undeveloped, with the exception of the
southeast corner of the off -site area, which is a single private residence. The project site slopes
downward to the north varying from 1.3 to 12 percent; off -site slopes range from 1 to 2.5
percent.
It is proposed that runoff from the Townhomes site and tributary off -site are be routed
through existing Pond A which has a pre -defined, allowable release rate of 4.4 cfs. It is
assumed this rate would remain unchanged despite the addition of the Townhomes drainage.
Therefore, the historical hydrology was not analyzed as part of this study. It is noted that
contribution of runoff from the Townhomes site to Pond A constitutes an off -site input to the
pond as the current study site was not included in the original Raintree P.U.D. drainage study.
Pond A is part of the storm sewer system designed for the Raintree P.U.D. The system
includes storm sewer pipes draining the commercial area on the northwest corner of Drake Road
and Shields Street, south of Raintree Drive. A single pipe conveys runoff to the north from 11.2
acres of the Raintree area. The proposed Senior Center storm sewer system will connect to this
pipe, which passes under the New Mercer Canal as a siphon into Pond A. The outlet from Pond
A connects to the outlet from Pond B (also part of the Raintree P.U.D.) in a manhole located
south of Larimer County Canal No. 2. The system downstream of this junction consists of a
siphon under Larimer County Canal No. 2 and a pipe to Spring Creek.
The existing storage capacity of Pond A was determined from design topography shown
in the Raintree P.U.D. utility plans. Although the 5043.0-foot contour is nearly continuous
around the pond, an existing low point of 5041.5 ft between Ponds A and B limits the maximum
allowable ponding elevation to 5040.5 ft; this corresponds to 0.3 ac-ft of storage. Minor grading
between the ponds could raise the low point to 5043.0 ft thereby allowing a ponded water
surface elevation of 5042.0 ft; this would increase the available storage volume to 1.0 ac-ft.
Backup information concerning storage characteristics of the existing pond are provided in
Appendix B.
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Figure 1.1. Vicinity Map for Raintree Townhomes.
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I. INTRODUCTION
1.1 Background
The Raintree Townhomes P.U.D. is a proposed residential development located in the
SE quarter of Section 22, Township 7 North, Range 69 West, in the City of Fort Collins,
Colorado. The development is bounded on the north and west by the New Mercer Canal, on
the east by Shields Street, and on the south by the proposed Fort Collins Senior Center and a
small undeveloped tract of land. This area is part of the Spring Creek drainage basin. Figure
1.1 is a vicinity map of the project.
The proposed development would consist of multi -family dwellings. The proposed plan
intends to utilize the existing Detention Pond A of the Raintree P.U.D. and Senior Center,
increasing the volume of the pond as necessary to offset increased flows due to development.
The Raintree Townhomes property was not included as part of the Raintree P.U.D., as
dictated by historical drainage boundaries. However, development of the area has changed
drainage patterns such that the drainage areas are interrelated. Drainage for the Raintree P.U.D.
and related development has been previously addressed in: "Storm Water Drainage Report for
the Raintree P.U.D." [TST, 1980], "Addendum to Stormwater Drainage Report for the Raintree
P.U.D." [Parsons & Associates, 1985], the utility plans for Raintree P.U.D. [TST, 1985], and
"Final Drainage and Erosion Control Report for Fort Collins Senior Center" [TST, 19931.
The proposed drainage facilities for the Fort Collins Senior Center (approved and to be
constructed in 1993) would convey Senior Center runoff, as well as runoff from the commercial
development to the south (part of the Raintree P.U.D.), to Pond A. The allowable release rate
from Pond A is 4.4 cfs, as defined in the 1980 report.
1.2 Purpose and Scope of Study
This study defines the proposed drainage plan for Raintree Townhomes in the context of
current and approved developed conditions adjacent to the site, and includes an evaluation of
stormwater detention requirements. The analysis accounts for drainage from and tributary to
the Townhomes site, as well as all areas tributary to Pond A.
The EPA Storm Water Management Model (SWMM) was used to determine hydrographs
at various points within the system. This information was then used to assess proposed
stormwater facilities including a preliminary re -sizing of the detention pond.
TABLE OF CONTENTS (CONTINUED)
TABLES
Table 3.1. Summary of Subcatchment Parameters ....................... 8
Table 3.2. Summary of Conveyance Element Parameters . ................. 9
Table 3.3. Summary of Hydrologic Modeling Results for the Developed Condition 100-Year
Event. ......................................... 11
APPENDIX
Appendix A: SWMM Computer Output for the Developed Condition 100-Year Event
Appendix B: Stage -Storage -Discharge Characteristics for Pond A
Sheet 1: Overall Drainage Plan
SHEETS
ii
TABLE OF CONTENTS
I. INTRODUCTION ....................................... 1
1.1 Background ....................................... 1
1.2 Purpose and Scope of Study ............................. 1
II. HISTORICAL DRAINAGE CONDITIONS ........................ 3
III. PRELIMINARY DRAINAGE PLAN ........................... 5
3.1 General ......................................... 5
3.2 Hydrologic Modeling of Proposed Drainage Conditions ............ 5
3.3 Preliminary Sizing of Major Drainage Facilities ................ 11
FIGURES/TABLES/APPENDIX/SHEETS
FIGURES
Figure 1.1. Vicinity Map for Raintree Townhomes....................... 2
Figure 2.1. Historical Drainage Basin ............................... 4
Figure 3.1. Developed Conditions Drainage Configuration . ................. 6
Figure 3.2. Preliminary Site Plan for Raintree Townhomes.................. 7
Figure 3.3. Schematic Diagram of the Proposed Condition
SWMM Model . ................................... 10
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PRELIMINARY DRAINAGE PLAN
FOR
RAINTREE TOWNHOMES
PREPARED FOR:
City of Fort Collins
Stormwater Utility
235 Mathews
Fort Collins, CO 80524
PREPARED BY:
Lidstone & Anderson, Inc.
736 Whalers Way, F-200
Fort Collins, CO 80525
July 6, 1993
PRELIMINARY DRAINAGE PLAN
FOR
RAINTREE TOWNHOMES