HomeMy WebLinkAbout180137 Stormwater Design Report_5-13-2020
DESIGN REPORT
STORMWATER MANAGEMENT
PALISADES APARTMENT COMPLEX
LOT 5B
AM. PLAT OF LOT 5, ANNIE SUBDIVISION, PHASE 2
Prepared for:
Bechtle Architects
4515 Valley Commons Drive Ste. 201
Bozeman, MT 59718
Prepared by:
Project Number: 180137
March 2020
INTRODUCTION
The Palisades Apartment Complex proposes to develop Lot 5B of the Amended Plat of Lot 5,
Annie Subdivision, Phase 2. The proposed layout of Lot 5B consists of a 3 story, 18 unit apartment
building, 7 individual row house units, an access driveway and parking, and associated
landscaping areas and retention facilities. The property is located within the Bozeman city limits
and is currently zoned R-4. A combination of site grading, curb and gutter, and retention ponds
will be used to manage stormwater runoff from the site. Supporting stormwater calculations are
enclosed in Appendix B for reference. Installation of N. 25th from Tschache to Oak Street and
Breeze Lane from N. 27th to N. 25th will happen concurrently with the Palisades Construction. A
separate retention facility will be constructed in the Park for road runoff from Breeze Lane and
North 25th Avenue. See the approved N. 25th Avenue and Breeze Lane storm water design report
for details.
LOT 5B STORMWATER DESIGN Lot 5B was divided into four drainage areas shown on the drainage area map in Appendix A.
Drainage Area 1 has a total area of 17,545 sf (2,300 sf of landscape and 15,245 sf of hardscape).
Drainage Area 2 has a total area of 17,618 sf (8,114 sf of landscape and 9,504 sf of hardscape).
Using the City of Bozeman design storm, Drainage Areas 1 and 2 on Lot 5B generate a total runoff
volume of 1,725 ft3. Drainage Area 1 will drain to a curb inlet at the low point in the curb and
gutter to the south of the proposed 18 unit apartment. Said curb inlet will drain west via a 10-inch
PVC pipe to a manhole and then north via a 10-inch PVC to another curb inlet located near the end
of the drive aisle and then west via a 12-inch PVC pipe to Retention Pond 1. Retention Pond 1 will
have a volume of 1,726 ft3.
Drainage Area 3 will consist of the roof drainage from the 18 unit apartment building and a small
landscaped area. The roof will have an area of 9,811 sf and the adjacent landscaped area to the
south will have an area of 1,649 sf, which generates 651 ft3 of runoff. A dry well will retain and
infiltrate the runoff produced by the roof. The drywell will have a capacity of 678 ft3 not including
infiltration.
Drainage Area 4 will consist of the remainder of the site that will drain into N. 25th Avenue to the
proposed retention pond in Rose Park, totaling 12,977 sf (6,268 sf of landscape and 6,709 sf of
hardscape). This project will be constructed concurrently with the N. 25th Ave and Breeze Lane
construction, and the runoff volumes have been coordinated to ensure the pond has the capacity to
handle the additional runoff from this site. The proposed retention pond within Rose Park,
including runoff from this site, is required to be 4,820 ft3. This pond provides 5,527 ft3 of storage,
which is adequate to handle the additional runoff from this site. See Appendix B for pond/drywell
calculations.
APPENDIX A
DRAINAGE AREA MAP
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APPENDIX B
DRAINAGE AREA CALCULATIONS
DRAINAGE AREA #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 15245 14483
Landscape 0.20 2300 460
Total 17545 14943
A = Area (acres) 0.4028
C = Weighted C Factor 0.85
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.75% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 135 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.6
Tc Total = 5.0 (5 minute minimum)
DRAINAGE AREA #2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 9504 9029
Landscape 0.20 8114 1623
Total 17618 10652
A = Area (acres) 0.4045
C = Weighted C Factor 0.60
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 1.70% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 26 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)5.7
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 1.00%
L = length of gutter (ft) 98
V = mean velocity (ft/s) 3.00
Tc Gutter Flow (minutes) =0.5
Tc Total = 6.3 (5 minute minimum)
DRAINAGE AREA #2
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 9811 9320
Landscape 0.20 1649 330
Total 11460 9650
A = Area (acres) 0.2631
C = Weighted C Factor 0.84
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 5.18% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.35 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 79 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)6.9
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 71.00%
L = length of gutter (ft) 367
V = mean velocity (ft/s) 25.26
Tc Gutter Flow (minutes) =0.2
Tc Total = 7.1 (5 minute minimum)
DRAINAGE AREA #4
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2)C * Area
Hardscape 0.95 6709 6373
Landscape 0.20 6268 1254
Total 12977 7627
A = Area (acres) 0.2979
C = Weighted C Factor 0.59
2. Calculate Tc (Time to Concentration)
Tc Overland Flow
Tc = 1.87 (1.1-CCf)D1/2/S1/3
Storm
S = Slope of Basin (%) 2.80% Return (yrs)Cf
C = Rational Method Runoff Coefficient 0.95 2 to 10 1
Cf = Frequency Adjustment Factor 1.1 11 to 25 1.1
D = Length of Basin (ft) 94 26 to 50 1.2
51 to 100 1.25
Tc Overland Flow (minutes)1.3
Tc Gutter Flow
Tc = L/V/60
V = (1.486/n)R2/3 S1/2
n = Mannings Coefficient 0.013
R = Hydraulic Radius A/P (ft) 0.13 (0.15' below top of curb)
S = slope (%) 0.84%
L = length of gutter (ft) 389
V = mean velocity (ft/s) 2.75
Tc Gutter Flow (minutes) =2.4
Tc Total = 5.0 (5 minute minimum)
APPENDIX C
POND SIZING CALCULATIONS
RETENTION POND #1
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 24749 23512
Landscape 0.20 10414 2083
Total 35163 25595
C=Weighted C Factor 0.73
2. Calculate Additional Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.73
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.81
Q = runoff (cfs) 0.24
V = REQUIRED VOL (ft3)1725
RETENTION POND - ROSE PARK
REQUIRED VOLUME
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 6709 6373
Landscape 0.20 6268 1254
Total 12977 7627
C=Weighted C Factor 0.59
2. Calculate Additional Required Volume
Q = CIA
V=7200Q
C = Weighted C Factor 0.59
I = intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.30
Q = runoff (cfs) 0.07
V = REQUIRED VOL (ft3)514 *
*Total runoff from Lot 5B contributing to the pond proposed within
Rose Park. Pond sizing has been coordinated with the 25th and
Breeze Lane construction.
DRYWELL #1
1. Calculate Area and Weighted C Factor
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 9811 9320
Landscape 0.20 1649 330
Total 11460 9650
A = Area (acres) 0.2631
C = Weighted C Factor 0.84
2. Calculate Required Volume
Q=CIA
V=7200Q
C = Weighted C Factor 0.84
I = Intensity (in/hr) 0.41 (10 yr, 2hr storm)
A = Area (acres) 0.26
Q = Runoff (cfs) 0.09
V = REQUIRED VOL (ft3)651
3. Calculate Drywell Volume
Existing Soil Condition Gravel
Percolation Rate (min/in) 6 (see Circular DEQ 4,
Percolation Rate (ft/hr) 0.83 Table 2.1-1)
Porous Media in Drywell Gravel
Void Ratio of Media 30.00%
Gravel Offset Dist. From Drywell (ft) 7
Infiltration
Drywell Gravel Area (ft2)278.5
Infilitration Volume (ft3)464.13
Gravel Void Volume
Gravel Bed Depth (below MH) 3.50
Gravel Volume (ft3)2015.29
Gravel Storage Volume (ft3)604.59
Manhole Volume
Manhole Depth (ft) 4.00
Manhole Volume (ft3)73.29
Provided Volume Inc. Perc. (ft3)678
*Infiltration calculations are shown but are not included in the total provided
volume
APPENDIX D
STORM SEWER FACILITIES
CALCULATIONS
PIPEs #1 & 2 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 15245 14483
Landscape 0.2 2300 460
Total 17545 14943
A = Area (acres) 0.4028
C = Weighted C Factor 0.85
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.08 DA1 #EX1
i = rainfall intensity (in./hr.) 3.83
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.85 (calculated above)
i = rainfall intensity (in./hr.) 3.83 (calculated above)
A = Area (acres) 0.40 (calculated above)
Q = 25-yr Flow Rate (cfs) 1.31
MANNING'S EQUATION FOR PIPE FLOW
Project: Pipes 1 & 2 Location: Inlet 1 & Manhole 1
INPUT
D= 10 inches
d= 9.38 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.004 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3S1/2
S=slope of channel Q=V x A
n=Manning's roughness coefficient
Solution to Mannings Equation
Area,ft2 Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
0.53 2.20 0.24 2.81 1.49 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
Manning's n-values
d
D
PIPE #3 25-YR OUTFLOW RATE
REQUIRED CAPACITY
1. Calculate Area and Weighted C Factor (Post-Development)
Contributing Area C Area (ft 2 )C * Area
Hardscape 0.95 24749 23512
Landscape 0.2 10414 2083
Total 35163 25595
A = Area (acres) 0.8072
C = Weighted C Factor 0.73
2. Calculate Rainfall Intensity (Duration = Max Tc from Contributing Drainage Areas)
i = 0.78x-0.64 (10-yr Storm, Fig. I-3, COB Design Standards)
x = storm duration (hrs) 0.10 DA2 #EX1
i = rainfall intensity (in./hr.) 3.31
3. Calculate 25-yr Pond Outflow Rate
Q = CiA
C = Rational Method Runoff Coefficient 0.73 (calculated above)
i = rainfall intensity (in./hr.) 3.31 (calculated above)
A = Area (acres) 0.81 (calculated above)
Q = 25-yr Flow Rate (cfs) 1.95
MANNING'S EQUATION FOR PIPE FLOW
Project: Pipe 3 Location: Inlet 2
INPUT
D= 12 inches
d= 11.26 inches
Mannings Formula n= 0.013 mannings
57.7 degrees
Q=(1.486/n)ARh2/3S1/2 S= 0.004 slope in/in
R=A/P
A=cross sectional area
P=wetted perimeter V=(1.49/n)Rh2/3S1/2
S=slope of channel Q=V x A
n=Manning's roughness coefficient
Solution to Mannings Equation
Area,ft2 Wetted
Perimeter, ft
Hydraulic
Radius, ft velocity ft/s flow, cfs PVC 0.013
0.77 2.64 0.29 3.17 2.42 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
Manning's n-values
d
D
APPENDIX E
25TH AND BREEZE REPORT
INTRODUCTION
The proposed infrastructure improvements will include the construction of N. 25th Avenue from
Oak Street to Tschache Lane and the construction Breeze Lane from N. 27th Avenue to N. 25th
Avenue.
Stormwater runoff from these improvements will be conveyed via curb and gutter channel flow to
storm sewer inlets. It will then be routed through storm drainage pipes to a retention pond located
in Rose Park and a Stormtech underground retention system located beneath the access alley on
Lot 5C. A Drainage Area Map is included in Appendix A, and calculations for each individual
drainage area (total area, weighted C factor, and time of concentration) are included in Appendix
B.
In addition to the right of way drainage, there will be portions of Lot 5C and Lot 5B included in the
stormwater capacity calculations. The remaining portions of the Lot 5B and 5C will retain
stormwater runoff on site (see the Palisades Apartments Stormwater Design Report for details).
Lot 5A will handle all of the runoff generated from the proposed development on site (see Silver
Creek Apartments Stormwater Design Report for details).
STORM SEWER FACILITIES DESIGN
Storm sewer facilities were sized for the 25-yr storm using Manning’s Equation. For each inlet, the
contributing area, weighted C factor, and time of concentration were calculated. These values were
input into Manning’s Equation to check capacity and flow characteristics for inlets, storm drain
pipes, and curb gutters. All curbs are designed to maintain 0.15’ freeboard per C.O.B. Design
Manual Section IV.C.5. For the purposes of this report, each pipe section was named to match the
associated upstream structure. Pipe sizing calculations are included in Appendix C.
RETENTION POND DESIGN
The proposed retention pond has been sized according to City of Bozeman Design Standards. The
proposed Retention Pond is sized to retain runoff from the 10-year storm event. Calculations used
for sizing each pond can be found in Appendix D. Design pond capacities were calculated using
volume surfaces in AutoCAD Civil 3D.
Proposed Retention Pond #1
Retention Pond #1 is the Southern retention pond located in Rose Park. Retention Pond 1 will
receive runoff from the southern 78 percent (approx.) of N. 25th Avenue, Breeze Lane, and
portions of Lot 5C and Lot 5B. Runoff from the lots and boulevard will drain to the curb and gutter
system and discharge to the storm inlets on the East and West side of N. 25th. From there, a 12”
PVC storm main will transport the storm water to Retention Pond 1. The required pond volume
was calculated to be 4,820 cubic feet. The provided pond volume is 5,527 cubic feet at a water
depth of 1.5’. Supporting calculations for the pond sizing can be found in Appendix C.
UNDERGROUND RETENTION CHAMBER DESIGN
The remaining runoff will be conveyed to two curb inlets on the East and West side of the designed
low point in N. 25th. These catch basins will convey runoff to the Stormtech Retention Chambers
Located beneath the private access road on Lot 5C.
Chamber Design
The required storage volume for Drainage Area 2 is 1,103 cubic feet. The Stormtech system will
be have a 2 row design consisting of 4 chambers and 2 end caps per each row. Each chamber has a
storage volume of 179 cubic feet and each end cap has a storage volume of 47 cubic feet, yielding
a bare storage volume of 1,616 cubic feet. There is an additional 249 cubic feet of storage in the
perimeter gravel in the system. This yields a total system storage volume of 1,865 cubic feet.
Stormtech details and system information can be found in Appendix D and on sheet C5.2.
APPENDIX F
STORMWATER MAINTENANCE PLAN
STORMWATER MAINTENANCE PLAN FOR PALISADES APARTMENT COMPLEX
The following areas, facilities will be inspected and the identified deficiencies shall be corrected.
Clean-out must include the removal and legal disposal of any accumulated sediments and debris
from the entire stormwater management system. Storm Water Facilities: 1. Retention Ponds/Detention Ponds:
Inspect ponds annually and after major storms (following three days of dry weather after a storm event exceeding ¼ inch of precipitation.) and correct any identified deficiencies to ensure that it is working in its intended fashion and that it is free of trash and debris. Failure for water to percolate or pass through outlet structure within this time period
indicates clogging or poor-draining soils. Replace/repair as necessary. Remove and
properly dispose of any accumulated sediment or debris as needed. Inspect the embankments and drainage structures for settlement, slope erosion, and downstream swamping.
Mow the embankment to control woody vegetation. Remove woody debris from the bottom of the pond. Inspect for channelization at bottom of pond and remove any channels observed. Regrade as required if any channels are found that are not an element of the original design. Remove grass clippings, litter, and debris from ponds.
2. Vegetated Areas: Inspect slopes and embankments early in the growing season to identify active or potential erosion problems. Replant bare areas or areas with sparse growth. Where rill erosion is evident, armor the area with an appropriate lining or divert the erosive flows to on-site areas able to withstand the concentrated flows. The facilities will be inspected
after major storms and any identified deficiencies will be corrected. 3. Ditches, Swales, Stormwater Chases and other Open Stormwater Channels: Inspect 2 times per year (preferably in Spring and Fall) to ensure they are working in their intended fashion and that they are free of sediment and debris. Remove any
obstructions to flow, including accumulated sediments and debris and vegetated growth. Repair any erosion of the ditch lining. Vegetated ditches will be mowed at least annually or otherwise maintained to control the growth of woody vegetation and maintain flow capacity. Any woody vegetation growing through riprap linings must also be removed. Repair any slumping side slopes as soon as practicable. If the ditch has a riprap lining,
replace riprap on areas where any underlying filter fabric or underdrain gravel is showing through the stone or where stones have dislodged. Correct any erosion of the channel's bottom or side slopes. The facilities will be inspected after major storms and any identified deficiencies will be corrected. For concrete stormwater chase structures with metal “no slip” cover. Inspect covers to ensure metal edges aren’t protruding above the
concrete surface. Correct deficiencies if found. Repair damaged or badly cracked stormwater chases if observed. Standard Maintenance Summary: 1. Remove sediment and oil/grease from retention ponds.
2. Inspect and remove debris from drainage swales, catch basins, and detention ponds. 3. Monitor health of vegetation and revegetate as necessary to maintain full vegetative cover. 4. Inspect for the following issues: differential accumulation of sediment, drain time, signs of petroleum hydrocarbon contamination (odors, oil sheen in pond water), standing water,
trash and debris blockages. Note on sediment disposal: In most cases, sediment from a detention pond does not contain toxins at levels posing a hazardous concern. However, sediments should be tested for toxicants in compliance with current disposal
requirements and if land uses in the drainage area include commercial or industrial zones, or if visual or olfactory indications of pollution are noticed. Sediments containing high levels of pollutants should be disposed of in accordance with applicable regulations and the potential sources of contamination should be investigated and contamination practices terminated.
ACCEPTED BY PALISADES APARTMENT COMPLEX OWNERS ASSOCIATION, BY ________________________________________________
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