HomeMy WebLinkAbout17 - Design Report - Rosa Apartments Master & Ph 1 - Stormwater DESIGN REPORT
STORMWATER MANAGEMENT
ROSA APARTMENTS MASTER SITE PLAN AND
PHASE 1 SITE PLAN
Prepared for:
Rosa Construction, Inc.
125 Central Avenue, Ste. 1 A
Bozeman, MT 59718
Prepared by:
Engineering and Surveying Inc.
1091 Stoneridge Drive • Bozeman, MT 59718
Phone (406) 587-1115 • Fax (406) 5B7-9768
www.chengineers.com • info@chengineers.com
Project Number: 16723
June 2017
INTRODUCTION
The Rosa Apartments Master Site Plan project proposes to develop Lot 1, Block 1 of Traditions
Subdivision Phase 3 into multiplex 3-story apartment buildings. The property is located in the
City of Bozeman and is zoned as R-4 with an Open Space lot along the western edge of the site.
Rosa Way, a future public street,is proposed with this project which will be constructed in Phase 2
of Rosa Apartments Master Site Plan. The roadway is a 31' cross section and it will be built to the
City of Bozeman standard specifications. The Roadway will follow the existing public utility and
access easement along the west edge of Lotl,Block 1, Traditions Phase 3 and will be located in a
60' wide public access and utility easement. Construction of Oak Street,the public street along the
north side of the site, is underway and stormwater improvements have been completed including
installation of a 36" by 58" RCP arch culvert that flows under the Oak Street right-of-way
conveying Baxter Ditch; these plans have been approved by the City of Bozeman under the Oak
Street Infrastructure project, Traditions Phase 3. There is an existing detention pond in the
northwest corner of the site located in Open Space 1 which will collect drainage from the site and
also from the Oak Street right of way and future Rosa Way public access and utility easement.
This pond discharges through an existing outlet structure to Baxter Ditch to the west. A
combination of site grading, curb and gutter, storm inlets and piping, and the existing detention
pond will be used to manage stormwater runoff on the site. Supporting stormwater calculations
and a breakdown of each drainage area are included in Appendix B for reference. Previous
reports pertinent to the Rosa Apartments Master Site plan have been provided at the end of this
report for reference; the following are included with this submittal: Rosa Subdivision (June 2005),
Flanders Creek (June 2006 revision) and Traditions Phase 3 Oak Street Improvements (August
2015).
CONTRIBUTING DRAINAGE AREAS TO EXISTING
STORMWATER DETENTION POND
The site was divided into three drainage basins as shown on the attached Drainage Area Map
included in Appendix A. All drainage from Drainage Areas I and 2, Oak Street, Twin Lakes
Avenue, Sherwood Way and Rosa Way flow into the existing detention pond located in Open
Space 1 to the west of the site. The weir in the existing detention pond outlet structure is 5 inches
in width and 3' in height. All gutter capacities were checked along Oak Street and Twin Lakes
Avenue in the original stormwater report submitted for the Oak Street Improvements project and
there is a gutter flow spreadsheet included in Appendix B which assesses the gutter capacity of
Rosa Way. The existing detention pond has a volume of 5,195 cubic feet at 1.5' in depth. This
volume calculation assumes the high groundwater elevation of the pond is at 4737.1' which was
the measured elevation of groundwater in the pond during a field survey conducted on May 25,
2017. This pond will be partially filled in along its eastern edge during Rosa Way construction,to
take place in Phase 2 of the project. Due to the roadway project and the vicinity of the Baxter
Ditch watercourse setback, the original pond shape will be retained except for some regrading on
the east side for Rosa Way and for the snow storage access road which will allow snow removal
equipment access to the entire pond for snow storage. The bottom of the pond will be filled with
washed drain rock to the extents shown on the plans during Phase 2 of the project which will
prevent compaction of the bottom of the pond during snow removal activity. The addition of
drain rock will preserve the infiltration rate of the pond. When the existing pond was constructed,
the actual depth was approximately 3 feet. The capacity of the pond will be increased by utilizing
the additional depth and installing a protective 4' tall chain link fence around the pond. The
internal slot will be modified to be adjustable for the increase in volume to meet storage and
outflow requirements for later phases. At 2.7' in depth the pond has a capacity of 11,139 CF;
taking into account the volume used up by high groundwater in the pond. The recorded ground
water elevation is 4737.1' (NAVD88).
STORMWATER PHASING
Upon completion of Phase 1 only part of the existing ponds' capacity will be required. At 1.5' in
depth the pond has an existing capacity of 5,195 CF; this volume excludes the volume of high
groundwater in the pond. Referring to the attached spreadsheet entitled Detention Pond -Phase 1,
the required pond capacity for Phase 1 is 3,558 CF and at 1.5' of depth the weir will need to be
sized to 6.2" to accommodate the existing pre-development release rate of 2.7 ft3/s coming from
the Flanders Creek Subdivision existing Detention Pond#3 located in the north of the subdivision;
as mentioned in the June 22,2006 revision to the Flanders Creek stormwater design report the weir
for the Flanders Creek Detention Pond is 5.3"and the pre-development release flowrate is 2.7 ft3/s.
A detail is provided on Sheet C3.0 of the Phase 1 Site Plan submittal which provides instructions
for converting the existing weir slot to the required width. With the new adjustable weir installed
the outlet structure can be adjusted accordingly in subsequent phases.At full buildout,the weir slot
can be readjusted to 3.3"in width to account for the increase in pond storage at the 2.7' depth level.
At this point the fence can be installed around the existing pond to account for the depth increase.
During Phase 1 a temporary grass lined swale will be used to route stormwater to the existing
detention pond. A detail of this swale is provided on Sheet C2.1 of the Phase 1 submittal.
At full buildout Drainage Area 2 will be collected by a catch basin (CB 1) located at the northwest
side of the north parking area and Drainage Area 1 will flow into storm inlets CB3 and CB4
located at the northwestern end of Drainage Area 1 near the parking area and Rosa Way
intersection. All catch basins and inlets will convey stormwater via a 15" pipe to the existing
detention pond. The 15"diameter PVC pipe has a 4.91 cubic feet per second capacity which will
be adequate to convey drainage from each individual catchment. The pipe capacity spreadsheet
has been included in Appendix B located at the end of this report.
APPENDIX A
DRAINAGE AREA MAP
APPENDIX
SUPPORTING CALCULATIONS
DETENTION POND (Twin Lakes, Rosa Way, Drainage Area 1, Sherwood Way,West Oak-All Phases)
REQUIRED VOLUME
2. Calculate Area and Weighted C Factor(Post-Development)
Contributing Area C Area(ft2) C*Area
Oak Street 0.82 26588 21802 Composite ROW: C
Twin Lakes Avenue 0.73 22204 16209 Coale= ((0.95*38)+(0.2*7))/45 0.83
Rosa Way 0.82 39298 32224 CTW;n� ((0.95*21.5)+(0.2*7))/30 0.73
Sherwood Way 0.82 15709 12881 Csha�°°d= ((0.95*21.5)+(0.2*7))/30 0.73
Drainage Area 2 0.73 144932 105752'
Drainage Area 1 0.72 150331 107686 Crosa= ((0.95*41)+(0.2*7))/60 0.67
Total 399062 296555
A=Area(acres) 9.1612'
C=Weighted C Factor 0.74
3. Calculate T° (Pre-Development)
Tc Overland Flow
Tc=1.87(1.1-CCf)D'/2/Sv3
---------------------------------------------
;Storm
S=Slope of Basin{%) 1 :Return(yrs) Cf
C=Rational Method Runoff Coefficient 0.2`f ;2 to 10 1
Cf=Frequency Adjustment Factor 1.1' 11 to 25 1.1
D=Length of Basin(ft) 886 :26 to 50 1.2
51 to 100 1.25 '
------------------------------------------
Tc(Pre-Development)(minutes) 49'
4. Calculate Rainfall Intensity(Duration=Pre-Development Tc)
i=0.64z066(10-yr Storm, Fig.1-3,COB Design Standards)
x=storm duration(hrs) 0.82 (Tc Pre-Development)
i=rainfall intensity(in./hr.) 0.73
5. Calculate Runoff Rate(Pre-Development)
Q=CiA
C=Rational Method Runoff Coefficient 0.2';(open land)
i=rainfall intensity(in./hr.) 0.73'(calculated above)
A=Area(acres) 9.16 (calculated above)
Q=Runoff Rate (Pre-Development)(cfs) 1.34
6. Calculate Required Pond Volume
Total Area(acres)= 9.16 acres
Weighted C= 0.74'i
Discharge Rate(cfs)= 1,34'cfs(Equal to Pre-Development Runoff Rate)
Duration(min) Duration(hrs) Intensity QI�(cfs) Runoff Release Required
(inlhr) Volume Volume Storage(ft3)
59 0.98 065 4.40 15594 4736 10857
70 1.17 0.58 3.94 16555' 5619 10936'
71 1.18 0.57 3.91 16637' 5700 10938'
72 1.20 0.57 3.87 16719 5780 10939
73 1.22 0.56 3.84 16800' 5860 10940
74 1.23 0.56 3.80 16880 5940 10940
75i 1.25 0.55 3.77 16960i 6021 10939
76 1.27 0,55 3.74 17039! 6101 10938'
77 1.28 0.54 3.70 17117 6181 10935
78 1.30 0.54 3.67 17194, 6262 10933'
79 1.32 0.54 3.64 17271 6342 10929`
80 1.33 0.53 3.61 17347' 6422 10925'
81 1.35 0.53 3.58 17423' 6502 10920
82 1.37 0.52 3.56 17498 6583 10915
83 1.38 0.52 3.53 17572' 6663 10909;
OUTLET STRUCTURE SLOT
Q=CLH 312
QEXISTING POND=Discharge(cfs) 2.71 pre-development release rate from Detention Pond#3 of Flanders Creek
(June 22,2006 revision)
QPOND I =Discharge(cfs) '.1.34 pre-development release rate for Rosa Apartments all phases,
Twin Lakes,Rosa Way,Sherwood Way and Oak Street
QTOTAL AT POND 1 WEIR=Discharge(cfs) 4.05'.total Of QEXISTING POND and QPOND 1
C=Weir Coefficient 3.33;(per COB Design Standards)
H=Head(ft) 2.7'
L=Horizontal Length(ft) 0.27
L=Slot Width(inches) 3.3
DETENTION POND (Twin Lakes, Rosa Way, Rosa Apartments -Phase 1)
REQUIRED VOLUME
2. Calculate Area and Weighted C Factor(Post-Development)
Contributing Area C Area(ft2) C `Area
Oak Street 0.82 26588 21802 Composite ROW: C
Twin Lakes Avenue 0.73 22204 16209 Cock= ((0.95'38)+(0.2`7))/45 0.83
Drainage Area Phase 1 0.69 86955 59959 CTW;n� ((0.95-21.5)+(0.2"7))/30 0.73
Total 135747 97970
A=Area(acres) 3.1163
C=Weighted C Factor 0.72
3. Calculate T, (Pre-Development)
Tc Overland Flow
Tc= 1.87(1.1-CCf)D1/2/Sv3
I Storm I
S=Slope of Basin(%) 1`' :Return(yrs) Cf
C=Rational Method Runoff Coefficient 0.2 12 to 10 1
Cf=Frequency Adjustment Factor 1.1 ill to 25 1.1
D=Length of Basin(ft) 886' 1,26 to 50 1.2
51 to 100 1.25 I
Tc(Pre-Development)(minutes) 49'
4.Calculate Rainfall Intensity(Duration=Pre-Development Tc)
i=0.64xo.61(10-yr Storm, Fig.1-3,COB Design Standards)
x=storm duration(hrs) 0.82 (Tc Pre-Development)
f=rainfall intensity(in./hr.) 0.73
5. Calculate Runoff Rate(Pre-Development)
Q=CiA
C=Rational Method Runoff Coefficient 0.2 (open land)
i=rainfall intensity(in./hr.) 0.73 (calculated above)
A=Area(acres) 3.12!(calculated above)
Q=Runoff Rate (Pre-Development)(cfs) 0.46
6. Calculate Required Pond Volume
Total Area(acres)= 3.12 acres
Weighted C= 0.72
Discharge Rate(cfs)= 0.46 cfs(Equal to Pre-Development Runoff Rate)
Duration(min) Duration(hrs) Intensity Qln(cfs) Runoff Release Required
(in/hr) Volume Volume Storage(ft)
59 0.98 0.65 1.46 5151+ 1611 3540'
70 + 1.17 0.58 1.30 5469 1911 3558
71 1.18 0.57 1.29 5496` 1939 3558i
72 1.20 0.57 1,28 5523 1966 3557'
73 1.22 0,56 1.27 5550 1993 3557'
74 1.23 0.56 1.26 5577i 2021 3556!
75 1,25 0.55 1.25 5603 2048 3555f
76 1.27 0.55 1.23 5629' 2075 3554
77 1.28 0,54 1,22 5655' 2103 3552'
78 f 1.30 0.54 1.21 5680 2130 3550
79 1.32 0.54 1.20 5706' 2157 3548'
80 1.33 0.53 1.19 "' 5731' 2185 3546
81 1.35 0.53 1.18 5756' 2212 3544'
82 1.37 0.52 1.17 5781 2239 3541
83 1.38 0.52 1 A 7 5805' 2266 3539;
OUTLET STRUCTURE SLOT
Q=CLH 12
QEXISTING POND=Discharge(cfs) 2.71 pre-development release rate from Detention Pond#3 of Flanders Creek
(June 22,2006 revision)
QPOND I=Discharge(cfs) 0.46I pre-development release rate for phase 1,Twin Lakes and Oak Street
QTOTAL AT POND 1 WEIR=Discharge(cfs) 'I 3,17 total Of QEXISTING POND and QPOND 1
C=Weir Coefficient 3.33I(per COB Design Standards)
H=Head(ft) 1.5
L=Horizontal Length(ft) 0.52
L=Slot Width(inches) 6.2
DRAINAGE AREA ROSA WAY
Contributing Area C Area(ft 2) C * Area
ROW 0.82 35358 2894
Total 35358 289936
C =Weighted C Factor
A =Area (acres)
Required Gutter/Pipe Capacity (25-yr Storm)
Tc Overland Flow
Tc = 1.87 (1.1-CC)D1/2/S1/3
Storm
S = Slope of Basin (%) 0.5 Return Cf
C =Rational Method Runoff Coefficient Q.35. 2 to 10 1
Cf=Frequency Adjustment Factor 1 �1. 11 to 25 1.1
D =Length of Basin (ft) 15 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes) = 6.52
Tc Gutter Flow
Tc =L/V/60
V= (1.486/n)R2/3 S 1/2
n= Mannings Coefficient : 0
R = Hydraulic Radius A/P (ft) '� 01 �;(0.15' below top of curb)
S = slope (ft/ft) 0.010
L = length of gutter(ft) 520
V = mean velocity (ft/s) �,3: 17'
Tc Gutter Flow (minutes) 89,
Tc Total (Overland +Gutter)
Q= CIA
C =Weighted C Factor 0$ '(calculated above)
I= 0.78 Tc 0'64(in/hr) 2 55
A = area(acres) 0 81,
Qrequired WS) _ "� w
Provided Gutter Capacity (flowing at 0.15' below top of curb)
Q = (1.486/n)AR2/3S U2
n =Mannings Coefficient o 013
A = area(ft) 14
P = wetted perimeter(ft) 92
R= Hydraulic Radius A/P (ft) 01 .
S =slope (ft/ft) 0.005
Qprovided WS)
GUTTER HAS ADEQUATE CAPACITY
MANNING'S EQUATION FOR PIPE FLOW
Project: 15"Storm pipe Location: Storm Inlets
By: ADM Date: 7/23/2015
Chk. By: Date:
Clear Data
0 Entry Cells
INPUT
D= 15 inches
_ ______
d= 14.07 inches
Mannings Formula d n= 0.013 mannings
D 0= 57.7 degrees
Q=(1.486/n)ARh213S112 S= 0.005 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3SI12
S=slope of channel Q=V X A
n=Manning's roughness coefficient
Solution to Mannings Equation Manning's n-values
Wetted Hydraulic
Area,ft2 Perimeter,ft Radius,ft velocity ft/s flow,cfs PVC 0.013
1.20 3.30 0.36 4.11 4.91 PE(<9"dia) 0.015
PE(>12"dia) 0.02
PE(9-12"dia) 0.017
CMP 0.025
ADS N12 0.012
HCMP 0.023
Conc 0.013
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