HomeMy WebLinkAboutRAINTREE TOWNHOMES PUD FINAL - 42-93A - SUBMITTAL DOCUMENTS - ROUND 1 - DRAINAGE REPORTdescribed above. Detailed descriptions of the Preserve and Raintree Townhomes SWMM
' analyses can be found in the drainage reports for those developments.
The results of this hydraulic evaluation indicate that, based on the capacity of storm
sewer outfall, the allowable release rates for Ponds A and B are 6.1 and 5.9 cfs, respectively.
' The 5.9 cfs release rate for Pond B is commensurate with that previously defined for that pond.
The allowable release of 6.1 cfs for Pond A is an increase of 1.7 cfs over the original rate.
These outflows would result in 0.30 and 0.11 feet of freeboard in the downstream pipes at Pond
A and B outlets. Avoiding a pressurized condition at the outlet orifice plates ensures that the
ponds would not be hydraulically connected in the sense that one pond would not be able to back
flows into the other. At these levels, and for the conditions defined in this report, the available
freeboard in Ponds A and B would be 0.66 and 1.0 feet, respectively.
5
specified freeboard requirement in the pipe was met. The final freeboard in the outlet pipes for
Ponds A and B was identified as 0.30 and 0.11 feet, respectively. Although freeboard is still
available downstream of Pond A, additional releases are not possible as they result in backwater
in the Pond B outfall pipe thereby violating the freeboard requirement at that location.
Based on these results, it was determined that the allowable release rate from Pond A,
based on downstream storm sewer capacity, is actually 6.1 cfs, rather than the 4.37 cfs specified
in the 1980 drainage report. The final UDSewer analysis for the outfall pipe system is included
in the technical appendix provided at the end of this report.
An assessment was made of the validity of The Preserve and Raintree Townhomes
SWMM analyses in light of the results of the hydraulic analysis of the storm sewer outfall. The
tailwater in the pipe at the downstream face of the Pond A outlet orifice would be identical to
that shown on the utility plans. Since the tailwater elevation used to develop the original Pond
A rating curve was taken from the utility plans, this would not constitute a change to the
Raintree Townhomes SWMM model. However, the change in allowable release rate would
require an increase in orifice size. With the increase in potential outflows from Pond A, the
SWMM model developed in conjunction with the Raintree Townhomes drainage report was
modified to reflect the altered storage -discharge curve. Results of this analysis indicated that
under the conditions defined above, Pond A would have 0.66 feet of freeboard during the 100-
year event.
The hydraulic grade line elevation at the downstream face of the Pond B outlet orifice
would be 0.1 feet higher than that given on the 1985 utility plans. Therefore, the orifice plate
originally designed as part of The Preserve drainage report was enlarged slightly to meet the
actual modeled release rate of 5.9 cfs. This modified version of The Preserve SWMM model
was used in the analyses for the current study.
All SWMM computer output, (i.e., that associated with the Spring Creek Master Plan,
and the Raintree Townhomes and The Preserve P.U.D.s, modified herein) is provided in the
technical appendix to this report.
SUMMARY AND CONCLUSIONS
The hydraulic evaluation of the storm sewer outfall associated with the Raintree P.U.D.
detention facilities included an assessment of. (a) hydrologic modeling of Spring Creek, per the
Master Plan; (b) hydrologic modeling of the areas tributary to the two regional detention ponds,
performed in conjunction with The Preserve and Raintree Townhomes drainage reports; (c)
tailwater conditions in Spring Creek; and (d) timing of outflows from the two detention ponds.
The limitations and assumptions inherent in the hydraulic analysis of the storm sewer outfall are
4
The Preserve SWMM analysis used for this study included the three proposed on -site
detention ponds and the proposed expansion of Pond B. The Raintree Townhomes SWMM
analysis used for this study included the proposed expansion of Pond A, and conceptual storage
for Tract B of the Fort Collins Senior Center site to allow a peak release of 4.09 cfs under
developed conditions. This release rate was documented in the "Final Drainage and Erosion
Control Report for Fort Collins Senior Center" [TST, 1993]. An evaluation of the results of
these two hydrologic analyses indicated that the combined peak release would be 12.0 cfs
occurring at 3 hours 0 minutes (after rainfall commences). The peak outflows from the
individual ponds would both occur at this time. The discharges would be 6.1 and 5.9 cfs for
Ponds A and B, respectively.
The results of the developed condition SWMM analysis conducted for the Spring Creek
basin (as reported in the "Spring Creek Master Drainageway Plan Technical Addendum" [EPI,
1988]) were reviewed as a part of the current study. It was found that the peak discharge in
Spring Creek, at the confluence with the Raintree outfall, would occur at 1 hour 25 minutes and
would be 2,040 cfs. Furthermore, the discharge in Spring Creek at 3 hours 0 minutes (the time
of the peak outflow from the detention ponds) would be 882 cfs. Based on the HEC-2 analyses
conducted for the Master Plan, water surface elevations in Spring Creek were interpolated for
both 2,040 and 882 cfs. The resulting estimated water surface elevations are 5033.66 and
5032.45, respectively.
The Preserve and Raintree Townhomes SWMM results were reviewed to determine the
pond discharges at 1 hour 25 minutes (the time of peak flow in Spring Creek). The total
discharge at that time would be 11.5 cfs, with Ponds A and B contributing 5.9 and 5.6_ cfs,
respectively.
In order to identify the worst -case condition, two initial UDSewer analyses were
conducted, one each for: (a) the maximum 100-year water surface elevation in Spring Creek,
with the reduced pond outflows at 1 hour 25 minutes; and (b) the peak release from the ponds,
with the reduced water surface elevation in Spring Creek at 3 hours 0 minutes corresponding to
a discharge of 882 cfs. The results of these analyses indicated that Case (a) would result in the
most limiting hydraulic condition in the storm sewer outfall. Consequently, all subsequent
analyses were conducted for that case.
The initial analysis for Case (a) also indicated that water surface elevations at the orifice
plates are actually lower than those shown on the 1985 utility plans. However, due to the
relatively limited incremental freeboard available at the Pond B outlet, the previously prescribed
allowable release rate of 5.94 cfs (actual peak release rate of 5.9 cfs) appears to be appropriate
for that pond. On the other hand, the somewhat larger incremental freeboard available
downstream of Pond A suggests that a higher release rate may be possible. Multiple UDSewer
analyses were then conducted using incrementally larger release rates from Pond A until the
K,
developed condition runoff from the 100-year storm. These drainage studies have determined
' that both ponds would have to be enlarged in order to adequately detain flows from the 100-year
event. The results of these studies have pointed to the possibility that the existing detention
system may be inadequate even without development of The Preserve and/or Raintree
' Townhomes. Consequently, this study was initiated by the City of Fort Collins Stormwater
Utility to verify the capacity of the storm sewer outfall system for Ponds A and B; with the
' purpose of defining higher release rates from the ponds, if possible, to the extent the system's
capacity will allow.
The primary constraint in defining the capacity of the storm sewer system, as specified
' by the Stormwater Utility, is that outflows from the ponds not pressurize the pipes at the
downstream face of the orifice plates. This requirement is commensurate with the current
condition as indicated by the existing hydraulic grade lines. In order to ensure that this
constraint is met, the minimum freeboard requirement in the pipes was set to be 0.1 feet.
The scope of this study included an evaluation of the Stormwater Management Models
(SWMM) for the Spring Creek Master Drainageway Plan [EPI, 1988], The Preserve P.U.D.
[LA, 1993], and Raintree Townhomes P.U.D. [LA, 1993] to determine the worst -case hydraulic
condition for the outfall pipe; i.e., maximum tailwater in Spring Creek versus maximum
outflows from the ponds. This study then involved, iteratively, UDSewer and SWMM analyses
to determine maximum allowable release rates for the two ponds based on the capacity of the
downstream storm sewer system.
HYDRAULIC AND HYDROLOGIC EVALUATION
Due to its connection to Detention Ponds A and B, the hydraulic analysis of the Raintree
storm sewer outfall is actually a UDSewer analysis of the pipe, performed in conjunction with
a hydrologic (SWAM) analysis of the ponds and Spring Creek basin. It is noted that all
hydrologic analyses either investigated or conducted for this study were based on the 100-year
developed condition event.
The SWMM analyses associated with the final drainage reports for The Preserve and
Raintree Townhomes (as of October 6, 1993) were the basis used by the current study for
determining outflows from Ponds A and B. It is noted that these reports have not yet received
final approval by the City of Fort Collins Stormwater Utility. Consequently, any modifications
to the reports which impact detention must conform to the assumptions and. results of this study.
Any adverse impacts to the storm sewer outfall system arising from future detention
modifications would need to mitigated.
2
BACKGROUND
' Development of the Raintree P.U.D. began in the early 1980s with the commercial area
which now exists adjacent to the northwest corner of Shields Street and Drake Road, south and
' east of Raintree Drive. Two regional detention ponds (Ponds A and B) were constructed to
serve the Raintree area. These ponds are both located between the New Mercer Ditch and
Larimer County Canal No. 2. The outfall pipes for these two ponds connect at a manhole
located south of Larimer County Canal No. 2. From this manhole, outflows are conveyed under
the canal via a siphon, and then west in a pipe to Spring Creek. The outfall pipe confluences
with Spring Creek at a point approximately 175 feet downstream of the canal.
As a part of the original drainage study conducted in conjunction with the Raintree
development, allowable release rates were established for the two ponds. In the document
"Storm Water Drainage Report for the Raintree P.U.D." [TST, 1980], release rates of 4.37 and
5.94 cfs were specified for Ponds A and B, respectively. An hydraulic grade line was
established for the downstream pipe system. The resulting hydraulic grade line is documented
on the utility plans for the "Stormwater Detention Facilities for Raintree P.U.D." [dated 1985]
which are on file with the City of Fort Collins Engineering Department. (It is noted that in the
1980 report, the Pond A and B designations were reversed relative to current conventions. The
1985 utility plans show Pond A to be located east of Pond B; this is the currently recognized
labeling scheme.)
Orifice plates were designed and constructed to meter outflows from the ponds. The
utility plans for the outfall system indicate that the hydraulic grade line at the downstream face
of the orifice plates for Ponds A and B leave 0.30 and 0.21 feet of freeboard (below crown of
pipe), respectively, in the pipes.
PURPOSE AND SCOPE OF THIS STUDY
It is currently proposed that Pond A would serve the new Raintree Townhomes and Fort
Collins Senior Center developments, as well as a portion of the Raintree Commercial site. An
existing storm sewer directs runoff from a portion of the commercial site to Pond A. It is also
proposed that Pond B would receive runoff from The Preserve development and a portion of the
Raintree Commercial site. Similarly, an existing pipe conveys runoff from the western fringe
of the commercial area to Pond B.
Drainage studies currently being conducted for additional development within the Raintree
area, notably The Preserve P.U.D. [TST & LA, 1993] and the Raintree Townhomes P.U.D.
[LA, 1993], have found that the existing detention system is under -designed to accommodate
LIDSTONE & ANDERSON, INC.
Water Resources and Environmental Consultants
736 Whalers Way, Suite F-200
Fort Collins, Colorado 80525
(303) 226-0120
October 6, 1993
Ms. Kate Malers
City of Fort Collins
Stormwater Utility
235 Mathews Street
Fort Collins, CO 80524
Re: Hydraulic Evaluation of the Storm Sewer Outfall for the Raintree P.U.D. Detention
' Facilities (LA Project No. CO-FC-93.09)
' Dear Kate,
Lidstone & Anderson, Inc. (LA) has completed the analyses associated with referenced study
' and is pleased to submit the attached letter report. We believe that the analyses and report are
complete and provide an accurate description of the. hydrologic conditions which affect the
Raintree detention facilities storm sewer outfall, and the hydraulic conditions in the pipe system
for the 100-year event.
We hope that this report will meet your needs in explaining the storm drainage situation
associated with the outfall. If during the course of your review of this report, you have any
questions concerning the study please do not hesitate to call me.
' Sincerely,
Gre o J och, P.E.
Senior ngineer
' GJK/tlt
' Enclosure
II
I' Branch Office: Box 27, Savery, Wyoming 82332
HYDRAULIC EVALUATION OF THE
STORM SEWER OUTFALL FOR THE
RAINTREE P.U.D. DETENTION FACILITIES
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
(LA Project No. CO-FC-93.09)
October 6, 1993
APPENDIX A
LA 1993 LETTER REPORT:
°HYDRAULIC EVALUATION OF THE STORM SEWER OUTFALL
FOR THE RAINTREE P.U.D. DETENTION FACILITIES"
1
1
1
1
1
I 1
TST, INC.
Consulting Engineers
Table 4.4. Erosion Control Cost Estimate. OPINION OF COST
Client: JOHN MCCOY
Prroject: RAINTREE TOWNHOMES P.U.D.
Job No. 10-742-001
By: KGS
Date:'10/05/93
Sheet 1 of I
No
Item
Quantity.
Units
Unit
Cost
Total
Comments
EROSION CONTROL
1
RESEEDIMULCH
2.75
A.C.
650.00
$1,787.50
$0.0149/S.F.
2
INLET FILTER
4
EA.
300
$1,200.00
3
SILT FENCE
770
L.F.
3.00
$2,310.00
CONSTRUCTION COST
$5,297.50
1.5 X COST
$7,946.25
TOTAL SECURITY
$7,946.25
CITY RESEEDING COST
1
RESEEDING
3.93
A.C.
500.94
$1,968.70
$0.0115/S.F.
1.5 X COST
$2,953.05
TOTAL SECURITY
$2,953.05
23
Table 4.3. Construction Sequence.
Project: Raintree Townhomes P.U.D. Standard Form C
Sequence for 19933 Only Completed By: KGS Date: 9/29/93
Indicate by use of a bar line or symbols when erosion control measures will be installed. Major
modifications to an approved schedule may require submitting a new schedule for approval by the City
FnainaPr
Year
93
94
Month
Nov
Dec
Jan
Feb
Mar
Apr
May
June
July
Demolition
Overlot Grading
Wind Erosion Control:
Soil Roughing
Perimeter Barrier
Additional Barriers
Vegetative Methods
Soil Sealant
Other
Rainfall Erosion Control
Structural:
Sediment Trap/Basin
1-4
Inlet Filters
Straw Barriers
Silt Fence Barriers
Sand Bags
Bare Soil Preparation
Contour Furrows
Terracing
Asphalt/Concrete Paving
Other
Vegetative:
Permanent Seed Planting
Mulching/Sealant
Temporary Seed Planting
Sod Installation
Nettings/Mats/Blankets
Other
Structures: Installed by CONTRACTOR Maintained by' OWNER
Vegetation/Mulching Contractor To Be Decided by Bid
Date Submitted: 9/29/1993 Approved by City of Fort Collins on
22
Table 4.2. Effectiveness Calculations (Continued).
Project: Raintree Townhomes P.U.D. STANDARD FORM B
Completed By: KGS Date: 9/29/93
Erosion Control C-Factor P-Factor
Method Value Value Comment
Sediment Trap 1.00 0.50 Detention Pond
Bare Ground 1.00 0.90 Roughened
Gravel Inlet Filter 1.00 0.80
Pavement 0.01 1.00
Established Grass 0.075 1.00 Undisturbed Ground
Reseed/Mulch 0.06 1.00
Major
Basin
PS
(%)
Subbasin
Area
(AC)
Calculations
(Calculations Are Shown in Appendix H)
A
79.2
1
0.98
100% Undisturbed Ground
(93.2)
Inlet Filter
Sediment Trap
Wt. C-Factor = 0.075 (.0075)
Wt. P-Factor = 0.40 (0.80)
Eff = 97% (94.0%)
2
1.14
55% Bare Ground (Reseed/Mulch)
45% Paved
Inlet Filter
Sediment Trap
Wt. C-Factor = 0.55 (.04)
Wt. P-Factor = 0.38 (1.00)
Eff = 79.1 % (96.0%)
3
0.84
100% Bare Ground (Reseed/Mulch)
Inlet Filter
Sediment Trap
Wt. C-Factor = 1.00 (.06)
Wt. P-Factor = 0.28 (1.00)
Eff = 72.1% (94.0%)
9
1.0
100% Bare Ground (Reseed/Mulch)
Inlet Filter
Sediment Trap
Wt. C-Factor = 1.00 (.06)
Wt. P-Factor = 0.36 (1.00)
Eff = 64% (94.0%)
EffNgr _ [(97)(0.98) + (79.1)(1.14) +
(72.1)(0.84) + (64)(1)j/3.96
= 78.0% (94.6%)
I, Note: Values in parenthesis are post -construction.
11
21
Table 4.2 Effectiveness Calculations.
Project: Raintree Townhomes P.U.D. STANDARD FORM B
Completed By: KGS Date: 9/29/93
Erosion Control C-Factor P-Factor
Method Value Value Comment
Sediment Trap 1.00 0.50 Detention Pond
Bare Ground 1.00 0.90 Roughened
Gravel Inlet Filter 1.00 0.80
Pavement 0.01 1.00
Established Grass 0.075 1.00 Undisturbed Ground
Reseed/Mulch 0.06 1.00
Major
Basin
PS
(%)
Subbasin
Area
(Ac)
Calculations
(Calculations Are Shown in Appendix H)
10
72.7
0.95
75% Undisturbed Ground
(85.5)
30% Bare Ground (Reseed/Mulch)
30% Silt Fence
Wt. C-Factor = 0.34 (0.071)
Wt. P-Factor = 0.83 (1.00)
Eff = 72 % (93 %)
Note: Values in parenthesis are post -construction.
20
Table 4.1. Rainfall Performance Standard Evaluation.
Project: Raintree Townhomes P.U.D. STANDARD FORM A
Completed By: KGS Date: 9/29/93
During
Post -
Developed
Subbasin
Erodibility
Zone
Asb
(ac)
Lsb
(ft)
Ssb
(`%)
Lb
(ft)
Sb
M
Construction
PS
Construction
PS
M
M
1
Moderate
0.98
230
2.66
2
Moderate
1.14
485
1.65
3
Moderate
0.84
375
2.22
9
Moderate
1.00
654
0.99
TOTAL
3.96
441
1.85
79.2
93.2
10
Moderate
0.95
480
0.66
TOTAL
0.95
480
0.66
72.7
85.5
19
IV. EROSION CONTROL PLAN
The Erosion Control Plan for this site was proposed using the criteria set forth in the
SDDC manual. Transportation of sediment will be controlled by the implementation of a silt
fence, sediment trap, and soil roughening at the start of construction. Inlet filters will be
installed shortly after construction on all proposed inlets, as well as at the Pond A outlet
structure, to remove any sediments which may be transported prior to seeding. The pond will
be used as a sediment trap during construction by temporarily over -excavating it. Finally,
reseeding and mulching will be used to prevent the transportation of sediments. The contractor
shall be responsible for maintaining all erosion control facilities for as long as they are required.
For the purpose of the erosion control calculations, Basin A includes Subbasins 1, 2, 3
and 9. Subbasin lengths within Basin A were calculated by adding the overland travel lengths
and gutter lengths. Due to varying grades and overland flow distances within Subbasin 3, the
length and slope were calculated by length weighted averages. Within Basin A, sediment will
be controlled by: (a) a silt fence adjacent to New Mercer Ditch; (b) inlet filters on all proposed
inlets as well as on the existing Pond A outlet structure; and (c) a sediment trap within Pond A.
Subbasin 10 was analyzed separately due to the fact that it is hydraulically removed from
the other subbasins within the development area. The subbasin length was determined by only
the overland travel. A silt fence will be placed to eliminate sediment runoff into Larimer
County Canal No. 2 from the northern slope of the proposed Pond A expansion.
Tables 4.1 and 4.2 detail the rainfall performance and effectiveness of this erosion control
plan, respectively; associated calculations can be found in Appendix H of this report. It is noted
that the erosion effectiveness calculations are 1.2 and 0.7 percent lower than the calculated
performance standard for Basin A and Subbasin 10, respectively. The post -construction
performance levels are met and exceeded for the entire Raintree Townhomes site. Although the
effectiveness for the entire site is slightly below performance standards only during the
construction period, site specific and seasonal aspects need to be considered. Since the site is
relatively small, the erosion potential is also small. During the winter months the ground is
generally wet or frozen, thus reducing the opportunity for erosion. In addition, the chance for
heavy rain and runoff are considerably less likely to occur during this period of time.
Table 4.3 outlines the construction sequence for the erosion control plan; this table is also
included on Sheet 2. The erosion control cost estimate for Raintree Townhomes is provided in
Table 4.4.
18
' 3.4.7 Statement of Maintenance Responsibility
' The City of Fort Collins would be responsible for maintenance of all storm sewers
located in the City rights -of -way and easements. The Raintree Townhomes Homeowners
' Association would be responsible for maintaining all other on -site and off -site drainage facilities
which would be built or modified in conjunction with this project.
[1
II
11
17
plate, the pond outlet structure would remain in its existing configuration. A new 8.1-inch
square orifice opening would be required to limit the peak discharge to 6.1 cfs for the 100-year
event.
The results of the SWMM model detention routing analysis indicate that the peak
discharge from Pond A would be 6.1 cfs. The maximum active volume would be 2.3 ac-ft; this
corresponds to a water surface elevation of 5042.34 feet. It is noted that the total freeboard for
the major storm would be 0.66 feet. A variance is being requested associated with the 1.0 foot
of freeboard required per the SDDC Manual.
In order to reduce the possibility of uncontrolled pond overtopping to Larimer County
Canal No. 2, a 20-foot wide overflow weir section is proposed at elevation 5042.5 feet to divert
flow to Pond B. At water surface elevations of 5042.75 and 5042.9 feet, the overflow section
would have capacities of 6.5 and 13.2 cfs, respectively. It is noted that an overflow spillway
has also been proposed for Pond B, in conjunction with The Preserve P.U.D., which would
direct overflow to the Spring Creek floodplain. Design calculations for Pond A and all
associated outlet structures are provided in Appendix C.
Adverse impacts related to seepage from the New Mercer Ditch and Larimer County
Canal No. 2 are not anticipated as a result of the proposed Pond A expansion. Field
observations indicate that seepage into the pond does not presently occur. Furthermore, the
minimum ground elevation in the pond is not proposed to be lowered below the existing level.
Therefore, since the potential head differential would not be increased, seepage is not expected
for the future pond configuration.
Ditch access would be retained in the proposed condition. The existing access road along
the south side of the New Mercer Ditch would be reconfigured in the form of the 18-foot wide,
1-foot deep drainage swale. The existing access road for Larimer County Canal No. 2 which
is located between the canals would be altered in a manner which would increase the access road
frontage along the canal. The existing road is adjacent to the New Mercer Ditch at Shields
Street, and for a distance of approximately 800 feet west of Shields Street. The road currently
shifts from its alignment along the north side of the New Mercer Ditch to the south side of
Larimer County Canal No. 2 at the east end of existing Pond A (see Sheet 1). The relocated
access road (shown on Sheet 2) would be moved away from the New Mercer Ditch at the east
end of the proposed Pond A, approximately 400 feet east of Shields Street. This would increase
the access road frontage along Larimer County Canal No. 2 by about 400 feet.
16
of 4.3 feet of pressure head (1.9 psi) at the siphon outlet. Therefore all pipe joints within this
reach must have a pressure seal application which complies with ASTM Standard 361.
The results of the UDSewer analysis also show that the energy grade line in the storm
sewer is below the ground surface at all three inlets. Therefore, the pipe flow does not have an
adverse impact on the inlet designs which were previously described. All storm sewer design
calculations are shown in Appendix F.
3.4.5 Swale Design
The perimeter swale along the northern development boundary is designed in conjunction
with an improved access road for the New Mercer Ditch. The ditch has a top width of 18 feet
which corresponds to the width of the access road. A normal depth analysis was performed to
analyze the swale with a depth of 1.0 foot; this would provide swale side slopes of 9H:1V. The
flow to the eastern and western portions of the swale were determined to be 4 and 2 cfs,
respectively, by area weighting within Subbasin 3. The results of the normal depth analysis
indicate that the 100-year discharge from the eastern portion of Subbasin 3 can be conveyed in
the swale with a flow depth of 0.7 feet. This total swale depth provides sufficient freeboard for
conveying more than the extra one-third capacity (above the 100-year discharge) as required by
the SDDC Manual. The tributary area, and corresponding 100-year discharge, for the western
portion of the swale is smaller than the tributary area to the eastern portion of the swale.
Consequently, the western portion of the Swale (which has a design identical to the eastern
portion) would have ample capacity. Calculations for the Swale design are shown in Appendix
G.
3.4.6 Detention Pond Reconfiguration
As previously noted, in its existing configuration the total capacity of Pond A is 0.3 ac-ft
at elevation 5040.5 feet. As documented in Chapter II, a slight modification of the pond would
increase the capacity of the pond to 1.0 ac-ft at an elevation of 5042.0 feet. It is noted that this
configuration would provide 1.0 feet of freeboard. In order to maximize pond capacity in the
limited amount of space available, the existing pond will be regraded and expanded to the east.
A side slope of 311:1V was incorporated throughout the entire pond. A variance is requested
to allow the use of 3H:1 V side slopes as compared to 411:1 V side slopes mandated by the SDDC
Manual. The bottom slopes vary from 0.5 to 1.5 percent. With the exception of the orifice
15
1.8 cfs for the north and south sides of the street, respectively) are less than the allowable
discharge. For the major storm, the maximum allowable flow rate for the entire street section
is 30.4 cfs. It is noted that the peak discharge for the major storm event (12 cfs) is also less
than the allowable discharge. The calculations associated with the street capacity analysis are
included in Appendix D.
In addition to the street capacity calculations, the actual flow depth for the major storm
event was calculated for Shields Street to confirm that flow does not enter Evenstar Court. The
results of this analysis, also provided in Appendix D, show that flows on Shields Street do not
commingle with those on Evenstar Court for the major storm.
3.4.3 Inlet Design
As indicated in the previous section, it was determined that street capacities were not
exceeded within Raintree Townhomes. Therefore, curb inlets are necessary only at the sump
at the eastern end of Evenstar Court, near the intersection with Shields Street. An area inlet is
specified at the low point in the northern perimeter swale. Per SDDC Manual guidelines,
theoretical capacities of the curb inlets were reduced by 10 to 15 percent depending on the size
of the inlet. Inlets 1A and 1B are sized to be 6- and 8-foot Fort Collins standard curb inlets,
respectively, while a single standard area inlet is designed for Inlet 1C. The calculations
associated with the inlet design are provided in Appendix E.
3.4.4 Storm Sewer Design
The capacity of the pipe downstream of Inlets IA and 1B was designed for the total 100-
year discharge to the respective inlets. The pipe from Inlet 1A to 1B is sized as a 15-inch RCP
' to convey a discharge of 5.4 cfs (the 100-year discharge at Inlet 1A). The pipe from Inlet 1B
to Inlet 1C is designed as a 21-inch ADS pipe for a design discharge of 12 cfs (the total 100-
year discharge at Inlets 1A and 1B). Inlet 1C would collect runoff from Subbasin 3 as well as
' pipe flow from Inlets IA and 1B; the peak discharge from Inlet 1C to Pond A would be 14 cfs.
The siphon under New Mercer Ditch would be a 24-inch RCP.
I I A detailed hydraulic analysis of the pipe system was performed using the UDSewer pipe
hydraulic analysis model which was developed by the Urban Drainage and Flood Control
' District. The maximum water surface elevation in Pond A of 5042.34 feet was used as the
downstream tailwater elevation for the pipe network. The pipes from Manhole #1 (siphon outlet)
tto 27 feet upstream of Manhole #8 are operating in a pressurized condition, with a maximum
I' 14
1 3.4
Design of Drainage Improvements
3.4.1 General
The proposed drainage plan for Raintree Townhomes consists of a combination of street
flow, curb inlets, storm sewers, swales and an expansion of an existing detention pond. Final
lot grading details will ensure that each lot is graded and landscaped to provide positive drainage
around and away from building foundations. Drainage easements have been provided where
necessary, both within the Raintree Townhomes development and within the area proposed for
detention pond expansion between the New Mercer Ditch and Larimer County Canal No. 2.
It is noted that Pond A and the area proposed for expansion of the pond is owned by the
City of Fort Collins Parks and Recreation Department. An agreement has been reached between
the Raintree Townhomes developer and the Parks Department, whereby Raintree Townhomes
has been granted permission to expand the pond and direct additional runoff into it. This is
documented on the plat which shows an off -site easement, on Parks Department property,
between the ditches. Approval of this easement, an off -site use by Raintree Townhomes, is
indicated by the Parks Department signature on the plat.
3.4.2 Allowable Street Capacities
Evenstar Court is classified as a local street and incorporates a roadway width (flowline
to flowline) of 28 feet. It is further characterized by a 2 percent cross slope and a Fort Collins
standard 4.75-inch rollover curb. Allowable gutter flows and maximum street encroachments
for both the initial and major storms were estimated and evaluated based on specifications set
forth in the SDDC Manual.
Per the SDDC Manual, during the initial storm, runoff was not allowed to overtop the
curb or the crown. Criteria also dictates that the maximum flow depth during the major storm
is 6 inches over the roadway crown. However, since the southern property line coincides with
the back -of -walk, the maximum flow depth associated with the major storm was further
restricted to be no higher than the back -of -walk.
A normal depth analysis of the allowable street capacities was performed using the water
surface profile computer model HEC-2 (U.S. Army Corps of Engineers, 1991). A single cross
section, normal depth option was used in conjunction with the SDDC Manual capacity reduction
factors to find the flow rate associated with the allowable depth. The results of the analysis
indicate that the maximum allowable flow rate for the minor storm event is 6.0 cfs for either
half of Evenstar Court. The discharges to Evenstar Court for the minor storm event (2.2 and
13
Table 3.3. Summary of 2- and 100-Year Peak Runoff Values for Developed Conditions.
1
1
1
SWMM
Subbasin
1
Area
(acres)
0.98
Discharge (cfs)
2-Year 100-Year
< 0.54 3
2
1.14
3
9
3
0.84
1
6
4
2.90
6
19
5
1.73
3
11
6
9.66
21
67
7
0.98
2
8
8
2.40
5
16
9
1.00
2
8
'A value of 1 cfs was used for design of drainage facilities.
Table 3.4. Summary of Design Discharges at All Inlets and Other Pertinent Locations.
Location or
SWMM Node
Contributing
Subbasins
Discharge (cfs)
2-Year
100-Year
Inlet IA
1, 2
1.8
5.4
Inlet 1B
2
2.2
6.6
Node 303
1, 2
3
11
Inlet 1C
3
<0.5
5
Node 304
1, 2, 3
3
14
Node 307
7
2
8
Pipe 10
5
3
5
Pipe 106
6
6
6
Node 305
4, 5, 6
13
31
Pipe 107
4, 5, 6
14
17
Node 306
4, 5, 6, 8
14
21
Pond Node 200
8
1
4.1
Pond A (Node 201)
1-6, 8, 9
4.9
6.1
1
12
LEGEND
OSUBBASIN
F�CHANNEL
ROUTING
O NODE
13 DETENTION
POND
RAINTREE
SENIOR
RAINTREE
OFFSITE
TOWNHOMES
CENTER
COMMERCIAL
n
n
n
n
301 ) (302
303 ) ( 3
1011 1102
OFFSITE
O 304
307
DISCHARGE TO
SHIELDS STREET
4 ) 11051 1106
305
0
FUTURE
SENIOR
CENTER
107 I2F 0 0
306
0
FDETENTION
POND A
ISCHARGE TO
PRING CREEK
Figure 3.1. Schematic Diagram of the Proposed Condition SWMM Model.
11
' subcatchments described above (Subbasins 2 and 3). The off -site area tributary to Pond A drains
through the proposed Senior Center. This area was divided into two Senior Center
subcatchments (Subbasins 4 and 5), an off -site subcatchment consisting of the commercial
development to the south (Subbasin 6), and on a conceptual basis, the future Senior Center
expansion (Subbasin 8). The off -site area tributary to Shields Street is represented by Subbasin
7, which is also the historical area contributing flow to Shields Street. Reference is made to
Sheet 1 which depicts the delineation of these subbasins and to Sheet 2 which provides greater
detail for the subbasins in and adjacent to Raintree Townhomes. Figure 3.1 provides a
schematic diagram of the SWMM model connectivity.
Referring to Sheet 1, Figure 3.1, and Table 3.1, it is evident that the tributary off -site
area to the south (Subbasin 1) has been modeled assuming existing, undeveloped conditions.
Consequently, any future development in that subbasin would be required, at a minimum, to
provide on -site detention to limit releases to the 100-year historical level as identified in the
attached SWMM results. This is a minimum requirement which addresses the peak outflow
only, and not runoff volumes. By meeting this peak release rate, the downstream curb inlets
and storm sewers would be sufficiently sized. However, it is recommended that future
development of the southern off -site area include a hydrologic routing analysis to ensure that
potential adverse impacts to Pond A are mitigated. Ultimately, the allowable release rate from
Subbasin 1 may be lower than the historical 100-year peak runoff in order to maintain the
prescribed freeboard in Pond A.
While on -site detention was incorporated into the drainage plans for both the Raintree
Commercial P.U.D. and the Senior Center, the current study determined that the detention
' provided is not adequate for containing the 100-year runoff. Therefore the on -site detention in
these areas (Subbasins 4, 5 and 6) was not explicitly modeled. Instead, the SWMM model was
allowed to "simulate" storage over pipe inlets. In this manner, each entire attenuated
' hydrograph is passed through its respective conveyance element, eventually reaching Pond A.
This approach was taken because in these areas the storm sewer outlet pipes associated with the
' designed detention ponds generally have limited capacity (relative to the contributing runoff),
thereby resulting in extensive and widespread ponding within these subbasins. As modeled, the
inflow hydrographs are attenuated while maintaining volumetric balance. As previously
' mentioned, on -site detention was modeled, at a conceptual level, for Tract B of the Senior
Center site (Subbasin 8).
' Table 3.3 presents a summary of the runoff results of the hydrologic modeling for each
subbasin. Table 3.4 summarizes the design discharges at all pertinent locations; i.e., existing
' and proposed inlets, and for critical SWMM elements. The SWMM output, which is the basis
for the information given in the tables, is contained in Appendix C.
1
I 1
10
Table 3.1. Summary of Subcatchment Parameters.
Subbasin
Basin
Width
(ft)
Area
(ac)
Percent
Impervious
Slope
(ft/ft)
Resistance Factors
Surface Retention Storage
Infiltration Rate
Impervious
n-value
Pervious
n-value
Impervious
(in)
Pervious
(in)
Maximum
(in/hr)
Minimum
(in/hr)
Decay
Rate
(see-')
1
230
0.98
10.0
0.006
0.020
0.250
0.10
0.30
0.90
0.43
0.0018
2
850
1.14
84.0
0.050
0.020
0.250
0.10
0.30
0.90
0.43
0.0018
3
700
0.84
26.0
0.050
0.020
0.250
0.10
0.30
0.90
0.43
0.0018
4
800
2.90
67.0
0.020
0.020
0.250
0.10
0.30
0.90
0.43
0.0018
5
320
1.73
70.0
0.013
0.020
0.250
0.10
0.30
0.90
0.43
0.0018
6
2,000
9.66
90.0
0.015
0.020
0.250
0.10
0.30
0.90
0.43
0.0018
7
700
0.98
70.0
0.020
0.020
0.250
0.10
0,30
0.90
0.43
0.0019
8
700
2.40
67.0
0.020
0.020
0.250
0.10
0.30
0.90
0.43
0.0019
9
2,000
1.00
1.0
0.200
0.020
0.250
0.001
ro,001
0.90
0.43
0.0018
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 : IV
101
Pipe
1.75
509
0.008
0.011
NA
NA
102
Swale
0.0
460
0.008
0.060
9
9
105
Pipe
1.25
487
0.005
0.013
NA
NA
106
Pipe
1.25
766
0.008
0.013
NA
NA
107
Pipe
2.00
525
0.005
0.013
NA
NA
' tributary to Raintree Townhomes or to Pond A, the 100-year peak discharge was
' calculated in order to confirm that flows on Shields Street would not enter
Evenstar Court.
' The entire area within Subbasin 8 is proposed for a future expansion of the
Senior Center. The drainage report for the Senior Center calls for on -site
' detention for the future expansion with a maximum release rate of 4.09 cfs for
the 100-year event. For this subbasin, a conceptual detention pond was modeled
for the current study to limit outflows to this maximum release rate. It was
assumed that outflows would be routed to Pond A through the existing siphon.
' Subbasin 9 has been included to account for direct rainfall on the expanded Pond
A. Accordingly, minimal surface retention storage depths were specified in the
' SWMM model for this subbasin.
' Subbasin 10 (as shown on Sheet 2 only) is located between the New Mercer
Ditch and Larimer County Canal No. 2, but excludes the area within Pond A.
This subbasin is not slated either for development or inclusion in an easement.
I' It has been delineated and included on Sheet 2 to allow a complete assessment of
erosion control considerations. Hydrologic calculations were not conducted for
' this subbasin.
11 3.3 Hydrologic Modeling of Proposed Drainage Conditions
I I SWMM was used to model the basin response to both the 2- and 100-year rainfall events.
The rainfall, resistance factors, surface storage and infiltration information were taken from the
I
, Spring Creek Master Drainageway Plan [EPI, 1988]. The remaining subcatchment parameters
(area, width, etc.) and conveyance parameters (diameter, length, etc.) were taken from: (a) the
' proposed grading and drainage plans for Raintree Townhomes and the Fort Collins Senior
Center; (b) the grading and drainage plan for the Raintree Commercial P.U.D.; and (c) the City
of Fort Collins topographic aerial photographs [dated March 1986]. Tables 3.1 and 3.2
summarize the resulting basin and conveyance parameters, respectively.
The hydrologic model consists of three major areas: (a) on -site and tributary off -site; (b)
off -site tributary to Pond A; and (c) off -site tributary to Shields Street. Each of these areas was
' divided into several subbasins to reflect physical drainage conditions. The area which impacts
Raintree Townhomes was divided into an off -site subcatchment (Subbasin 1), and the two on -site
Runoff from Subbasin 2 would be collected along Evenstar Court and conveyed
to Inlets 1A and 1B at the east end of Evenstar Court The 100-year flows which
would be collected on the northern portion of the street would drain to Inlet 1B
(an 8-foot sump inlet), and would not commingle with the flows from the
southern portion of the street which would drain to Inlet IA. The 100-year flow
to Inlet IA would join the flow from Subbasin 1 and be conveyed by a 15-inch
RCP to Inlet 111, and then continue via a 21-inch ADS pipe to Manhole #2.
' Runoff from Subbasin 3, consisting of overland flow from the northern and
western portions of the development would be conveyed to a perimeter swale
along the northern boundary of the development. Flow from the 100-year event
' would then be conveyed via the swale to the low point at Inlet 1C (a standard
area inlet associated with Manhole #2). At this point, swale flows would join
I' runoff from Subbasins 1 and 2 and be conveyed to Pond A by way of the
proposed 24-inch RCP siphon under New Mercer Ditch.
Runoff from Subbasin 5 will be collected at a single inlet in the eastern portion
of the Senior Center, and then conveyed via a 15-inch RCP to the manhole
associated with SWMM Node 305. In addition, a majority of the flow from
Subbasin 4 will be conveyed to this manhole; runoff from the remaining portion
of Subbasin 4 collects at the manhole associated with SWMM Node 306. It is
noted that for the SWMM analysis, all runoff from Subbasin 4 is collected at
SWMM Node 305. The 100-year flow to the manhole from Subbasins 4, 5 and
6 will then be conveyed to the existing siphon via a 24-inch RCP where all flow
from these three subbasins will be conveyed to Pond A.
Runoff from Subbasin 6 consists of overland flow from the existing Raintree
Commercial P.U.D which collects at several inlets placed in the parking area and
along Raintree Drive. The drainage plan for this development showed four areas
of on -site detention within the parking areas of the development. Flow from the
100-year event is conveyed via a 15-inch RCP to an inlet associated with SWMM
Node 305 (see Sheet 1) where it joins flow from the Senior Center.
The area within Subbasin 7 is tributary to Shields Street; once in Shields Street,
runoff from this subbasin is conveyed north to Spring Creek. It is noted that this
is both an existing and proposed drainage pattern. Even though the flow is not
7
III. FINAL DRAINAGE PLAN FOR RAINTREE TOWNHOMES
3.1 General
The drainage boundaries for the proposed Raintree Townhomes P.U.D., as well as all
off -site drainage areas tributary to Pond A, are shown on Sheet 1. Development of the
Townhomes and Senior Center site would 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, a portion
of the drainage from the Raintree Commercial P.U.D. will now pass through the storm sewer
pipe system proposed for the Senior Center development.
Sheet 2 shows the final grading and site plan for Raintree Townhomes. Sheet 2 also
shows the proposed grading and drainage for the Senior Center as well as the general drainage
features of the undeveloped and residential property on the northwest corner of Shields Street
and Raintree Drive.
All on- and off -site runoff which travels to Evenstar Court will be collected by curb inlets
near Shields Street. A high point in the street at the entrance to Evenstar Court will preclude
off -site flows traveling south along Shields Street, from entering the development. This is
consistent with existing conditions along Shields Street, as the curb and gutter is continuous
(there is no existing curb cut) at the proposed location of Evenstar Court. The north and west
perimeter area of the Townhomes site drains overland to the north and will be collected by a
perimeter swale. A pipe will connect the inlets on Evenstar Court and convey flows underneath
the swale. 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 Proposed Drainage Plan
A qualitative summary of the flow conditions within each basin and at each design point
is provided in the following paragraphs. Discussion of the design of drainage facilities, which
are introduced in this section, is included in Section 3.4.
Runoff from Subbasin 1 consists of overland flow across the open and mostly
undeveloped portion of the adjacent property to the south. Flow from the 100-
year event would be collected on the south side of Evenstar Court and conveyed
to Inlet IA (a 6-foot sump inlet).
0
jy 't ` �. .yMr .✓r , b/; ' '
t► _ 7 1 . ifii ; ti
i>tNt',
1w.{ • , 1 \ S :i + Jam
a e r t *��"�,F .�•!F ��' _ 0 r j t ` ' � l � .� r 4 '`� �t l, , ;M1, .
J. p•
A.
x u It
.•7r'iY�ii a ' 'mil'` \. .. ' , �� Gx \`y J
ry
a C::.� .. aI.T.T+1 �.' f�� 4 j� �'r �.kefy I- a .`• F � �' .\ di�: =•- � �.
m lie
mot— J lwz
D'
F ' }
� Its =^ "s t _ • ,
' T\. �. lT•pyp�� I (�4 7 1 ' .'
1PW•- ��t. tom' s`. r` 1 �' s $`
Lit
-
ri
FIELDS STREW
O U Z
� i � .: , �, � �. .. � � �:.. +t..:�. � _� � _�►
' S Figure 2.1. Historical Drainage Basin.
U. 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 a small portion 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 tributary 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 from south to 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 area be routed
through existing Pond A which has a pre -defined, allowable peak release rate of 6.1 cfs [LA,
1993]. Since the downstream release rate is dictated by the capacity of the receiving storm
sewer outfall, as opposed to the 2-year historical peak runoff, the historical hydrology was not
analyzed as part of this 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 at the northwest corner of Drake Road
and Shields Street, south of Raintree Drive. A single pipe conveys runoff to the north from 9.7
acres of the Raintree area. The proposed Senior Center storm sewer system will connect to this
pipe, with the combined runoff conveyed to an existing siphon passing under the New Mercer
Canal 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. Sheet 1 presents the existing storm sewers system for Raintree P.U.D. and
the proposed system for the'Senior Center.
' The existing storage capacity of Pond A was determined from design topography shown
in the Raintree P.U.D. utility plans. Although an elevation of 5043.0 feet is nearly continuous
' around the pond, an existing low point of 5041.5 feet between Ponds A and B limits the
maximum allowable ponding elevation to 5040.5 feet; this corresponds to 0.3 ac-ft of storage.
Minor grading between the ponds could raise the low point to 5043.0 feet thereby allowing a
' ponded water surface elevation of 5042.0 feet with one foot of freeboard; 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.
stormwater detention requirements in accordance with the previously mentioned studies. The
analysis accounts for drainage from and tributary to the Townhomes site, as well as all areas
tributary to Pond A.
It is noted that all drainage facilities proposed herein are designed in accordance with,
and meet the specifications and requirements set forth in the City of Fort Collins Storm Drainage
Design Criteria and Construction Standards (SDDC) Manual.
The EPA Storm Water Management Model (SWMM) was used to determine hydrographs
at various points within the system. This information was then used to prepare a stormwater
management plan that addresses overall storm drainage issues and provides detailed solutions to
specific drainage conditions within the site. The SWMM analysis was conducted to provide the
information necessary to design specific drainage measures including curb inlets and storm sewer
pipes, and allow re -sizing of Detention Pond A.
3
m
UNIVEIS
F. r
ST
rd Ing
4rl
w ad
a 9 &1073
El
4W
41
Ell]
High
lar
5056
... ........ .
.. .........
to
9 9 4
R
J
z
TV -
rakes
509
ORA AD
YSOb
E - --
TO NH
ME
p
CT 11
Rocky Moue
High h
9
•IL
27
I fa
j7F
All,
cc
•
CA 0
IP•
•
40
Tim,
1,?7
T HORS TH ROAD•
I.
Figure 1.1. Vicinity Map for Raintree Townhomes.
N
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 for the project site.
The proposed development would consist of six duplexes. The proposed plan intends to
utilize the existing Detention Pond A which was designed in conjunction with the Raintree
P.U.D. and Senior Center, increasing the volume of the pond as necessary to offset increased
flows due to the Raintree Townhomes development.
The Raintree Townhomes property was not included as part of the original 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, 19801, "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, 1993].
The proposed drainage facilities for the Fort Collins Senior Center (approved with
construction pending) 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 was defined as 4.4 cfs in the 1980 report. However, a recent analysis performed
for the Fort Collins Stormwater Utility ("Hydraulic Evaluation of the Storm Sewer Outfall for
the Raintree P.U.D. Detention Facilities" [LA, 1993]) indicates that the allowable release rate
is actually 6.1 cfs based on the capacity of the downstream storm sewer system. The letter
report describing the hydraulic evaluation of the Raintree outfall pipe is included in this report
as Appendix A.
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
1
TALBE OF CONTENTS (CONTINUED)
Table 4.3. Construction Sequence ................................ 22
Table 4.4. Erosion Control Cost Estimate . .......................... 23
APPENDICES
Appendix A: LA 1993 Letter Report: "Hydraulic Evaluation of the Storm Sewer Outfall
for the Raintree P.U.D. Detention Facilities"
Appendix B: Existing Condition Stage -Storage Characteristics for Pond A
Appendix C: Developed Condition Hydrologic Modeling
Appendix D: Street Capacity Calculations
Appendix E: Inlet Hydraulic Design Calculations
Appendix F: Pipe Hydraulic Design Calculations
Appendix G: Swale Design Calculations
Appendix H: Erosion Control Plan Calculations
SHEETS
Sheet 1: Overall Drainage Plan
Sheet 2: Grading, Drainage and Erosion Control Plan
ii
TABLE OF CONTENTS
I. INTRODUCTION ....................................... 1
1.1 Background ....................................... 1
1.2 Purpose and Scope of Study ............................. 1
II. HISTORICAL DRAINAGE CONDITIONS ........................ 4
III. FINAL DRAINAGE PLAN FOR RAINTREE TOWNHOMES ............ 6
3.1 General .......................................... 6
3.2 Proposed Drainage Plan ................................ 6
3.3 Hydrologic Modeling of Proposed Drainage Conditions ............. 8
3.4 Design of Drainage Improvements ........................ 13
3.4.1 General .................................... 13
3.4.2 Allowable Street Capacities ........................ 13
3.4.3 Inlet Design ................................. 14
3.4.4 Storm Sewer Design ............................ 14
3.4.5 Swale Design ................................. 15
3.4.6 Detention Pond Reconfiguration ..................... 15
3.4.7 Statement of Maintenance Responsibility ................ 17
IV. EROSION CONTROL PLAN ............................... 18
FIGURES/TABLES/APPENDICES/SHEETS
FIGURES
Figure 1.1. Vicinity Map for Raintree Townhomes. ...................... 2
Figure 2.1. Historical Drainage Basin . .............................. 5
Figure 3.1. Schematic Diagram of the Proposed Condition SWMM Model. ....... 11
TABLES
Table 3.1.
Summary of Subcatchment Parameters . ......................
9
Table 3.2.
Summary of Conveyance Element Parameters ...................
9
Table 3.3.
Summary of 2- and 100-Year Peak Runoff
Values for Developed Conditions ..........................
12
Table 3.4.
Summary of Design Discharges at all Inlets and
Other Pertinent Locations . .............................
12
Table 4.1.
Rainfall Performance Standard Evaluation . ...................
19
Table 4.2.
Effectiveness Calculations ..............................
20
i
FINAL DRAINAGE REPORT
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
IN CONJUNCTION WITH:
TST, Inc.
748 Whalers Way, Building D
Fort Collins, CO 80525
October 6, 1993
FINAL DRAINAGE REPORT
FOR
RAINTREE TOWNHOMES