HomeMy WebLinkAbout013 Appendix L - Stormwater Design Engineering ReportEngineering Report
Stormwater Design
South University District Phase 3 –
Block 2
Updated - September 2022
Site Plan RC #1 Submittal
Submitted to:
City of Bozeman Engineering Department
20 East Olive
Bozeman, MT 59715
Prepared by:
Stahly Engineering and Associates
851 Bridger Drive, Suite 1
Bozeman, MT 59715
(406) 522-9526
Stormwater Plan 1 9/15/2022
Engineering Report
Table of Contents
General Information and Design Criteria
Existing Site Conditions
Storm Drainage Plan Description
Estimation of Runoff
Retention Facilities
Groundwater Monitoring
Groundwater Impact Analysis
Storm Event Analysis
Conveyance Capacity
Facilities Maintenance
Figures and Tables
Figure 1. Vicinity Map
Figure 2. Stormwater Plan
Table 1. Storm Runoff Calculations
Table 2 Stormwater System Capacity (sub-drainage basin 1)
Table 3 Stormwater System Capacity (sub- drainage basin 2)
Table 4 Stormwater System Capacity (sub- drainage basin 3)
Table 5 Stormwater System Capacity (sub- drainage basin 4)
Table 6 Stormwater System Capacity (sub- drainage basin 5)
Table 7 Stormwater System Capacity (sub- drainage basins 6A, 7A & MDB)
Table 8 Groundwater Monitoring
Table 9 Groundwater Impact on System Capacity Analysis
Table 10 Bozeman Rainfall Analysis (1990-2020)
Table 11 Valley Drive Aisle Conveyance Calculations
Table 12 Curb and Gutter Conveyance Calculations
Table 13 Storm Drain Conveyance Calculations
Stormwater Plan 2 9/15/2022
General Information and Design Criteria
The proposed South University District Phase 3, Block 2 Development is located
northeast of the intersection of South 19th Avenue and Stucky Road. This development
will consist of 162 residential units on an 8.60-acre lot.
With 4 different building types; two rowhouse-style apartments, motor court apartments,
and flat style apartments. The intent of this project is to create 162 residential units over
a variety of building types to create a diverse neighborhood in a growing part of town
adjacent to the university. This application seeks feedback on the conceptual design
and associated site improvements.
Stormwater mitigation is achieved by using a compact, efficient stormwater
management design utilizing Low Impact Design (LID) methodology to mitigate the
impacts of stormwater runoff. The LID stormwater mitigation has been designed in
accordance with City of Bozeman Design Standards and Specifications Policy for
retention/infiltration facilities. Survivability of LID systems through conventional
development is difficult. Erosion control Best Management Practices (BMP’s) will be
utilized during construction to reduce the impacts of construction site runoff, until the
LID systems are constructed.
Plans are provided showing proposed grading, drainage flow paths, and stormwater
retention/infiltration details and locations.
The developer’s contact information is:
Wentworth Property Company
Ray Brown
802 North 3rd Avenue
Phoenix, Arizona 85003
Water supply and wastewater treatment will be provided by the City of Bozeman’s
municipal systems. The system owner’s contact information is:
City of Bozeman Public Service Department
PO Box 1230
Bozeman, MT 59771-1230
The location of the South University District Phase 3, Block 2 Development is
shown on the Vicinity Map in Figure 1.
Stormwater Plan 3 9/15/2022
Figure 1. Vicinity Map, Bozeman, Montana
Existing Site Conditions
SUD Phase 3 Block 2 is a block parcel created by the South University District Phase 3
Major Subdivision. The SUD Block 2 development site was previously used
agriculturally prior to its subdivision development. The existing ground slopes north at
grades between 1-3%. Existing offsite drainage is minimal, if present it would flow west
to an existing irrigation ditch on the east side of 19th Avenue Right-of-Way and
Mandeville Creek, east to Mandeville Creek or north to an irrigation ditch on the south
side of Kagy Boulevard and either infiltrates or makes its way to Mandeville Creek.
Block 2 cannot utilize Mandeville Creek as an outfall due to the topography, creek
location and adjacent properties. The Block 2 Development will have its own on-site
retention/infiltration facilities.
A geotechnical evaluation was conducted and is provided in a separate appendix of this
submittal. This site is typical of Bozeman with fine-grained soils overlying gravel
deposits. Geotechnical borings have determined the depth to gravel ranges from 3.5 to
5 feet below existing grade. The underlying gravelly sandy material will provide long-
term infiltration of stormwater.
Groundwater monitoring was conducted in 2019 during the high groundwater season for
the SUD Phase 3 subdivision development. Groundwater monitoring found groundwater
depths ranged from 6.5’ to 10.0’ below existing grade in the areas of the site proposed
for development. The site is not located within a 100-year floodplain.
Stormwater Plan 4 9/15/2022
Storm Drainage Plan Description
The storm drainage plan for the SUD Phase 3 Block 2 development consists of fifteen
subsurface infiltration systems distributed evenly throughout the project site and are
designed to capture and infiltrate stormwater in a manner that more closely replicates
the natural hydrology of the site.
The overall storm drainage plan for Block 2 is depicted on attached Figure 2. This plan
shows proposed drainage basin areas, flow paths, and retention/infiltration system
locations. In general, grades will be sloped away from buildings towards interior drive
aisles. The sub drainage areas typically drain north via valleyed drive aisles that convey
runoff to inlets and storm drains connected to subsurface retention/infiltration systems
located within the drive aisles or landscape areas. Prior to infiltration into the native
gravels below the systems, stormwater runoff will be treated through sediment removal
in the pretreatment chambers. Where feasible, each subsurface infiltration system was
sized to capture and infiltrate the design storm event to reduce the chances of
overloading down gradient stormwater infrastructure. Stormwater runoff will bypass
retention systems once they are full and be conveyed to the next down gradient storm
system that has capacity remaining. The site will be divided into 15 sub-drainage areas
as shown on attached Figure 2 and described below.
Sub-drainage areas 1A through 1C are located on the west side of the site and drain
north to the Main Drainage Basin (MDB) through valleyed drive Aisle “A”. Sub-areas 2A
through 2C are located towards the center of the site and drain north to the main
drainage basin (MDB) through valleyed drive Aisle “B”. Sub-areas 3A and 3B are
located in the southeast corner of the site and drain west to the Main Drainage Basin
(MDB) through valleyed drive Aisle “1”. Sub-areas 4A and 4B are in the central portion
of the site and drain to the Main Drainage Basin (MDB) primarily through valleyed drive
Aisle “C”. Sub-areas 5A and 5B are located in the eastern most portion of the site
draining north to the Main Drainage Basin (MDB) through valleyed drive Aisle “E”, and
through storm drain piping. Sub-areas 6A and 7A are smaller drainage basin areas
located in the northwesterly portion of the site and drain south to the Main Drainage
Basin (MDB). The MDB (on-site collector basin) is the drainage area that encompasses
South 18th Avenue, Aisles 4 and 5 in addition to portions of Aisles 2 and 3 and conveys
runoff via curb and gutter and or inlets to storm pipe. Sub-basin MBD drains north to a
large subsurface infiltration system in the northeast corner of the site.
Estimation of Runoff
Runoff estimates were obtained for each sub-drainage area using City of Bozeman
standards. The sites stormwater facilities have been sized to fully capture the 10-year,
2-hour storm event. The Bozeman IDF (Intensity, Duration, and Frequency) curves
show a rainfall intensity of 0.41 in/hour for the 10-year, 2-hour storm, totaling a 0.82-
inch storm event. Table 1 shows the storm runoff calculations for the 15 drainage sub-
basins. Specifically, the design uses the volume of the 10-year, 2-hour storm for
retention calculations.
Stormwater Plan 5 9/15/2022
Table 1 – Storm Runoff Calculations
Drainage Area Characteristics 1A 1B 1C 2A 2B
Area (ft²) 16689 24294 25603 10952 24481
Area (acre) 0.38 0.56 0.59 0.25 0.56
Pervious Area (ft²) 5841 8503 8961 3833 8568
Impervious Area (ft²) 10848 15791 16642 7119 15913
Pervious ( C ) 0.2 0.2 0.2 0.2 0.2
Impervious ( C ) 0.9 0.9 0.9 0.9 0.9
Weighted ( C ) 0.66 0.66 0.66 0.66 0.66
Time of Concentration (min) 5 5 5 5 5
Runoff Volume (cf) 10yr 2hr storm 747 1087 1146 490 1096
Flowrate Q (cfs) 25yr 5min storm 0.96 1.40 1.47 0.63 1.41
Drainage Area Characteristics 2C 3A 3B 4A 4B
Area (ft²) 18355 12552 21517 23059 18074
Area (acre) 0.42 0.29 0.49 0.53 0.41
Pervious Area (ft²) 6424 4393 7531 8071 6326
Impervious Area (ft²) 11931 8159 13986 14988 11748
Pervious ( C ) 0.2 0.2 0.2 0.2 0.2
Impervious ( C ) 0.9 0.9 0.9 0.9 0.9
Weighted ( C ) 0.66 0.66 0.66 0.66 0.66
Time of Concentration (min) 5 5 5 5 5
Runoff Volume (cf) 10-year 2-hr storm 822 562 963 1032 809
Flowrate Q (cfs) 25yr 5min storm 1.06 0.72 1.24 1.33 1.04
Drainage Area Characteristics 5A 5B 6A 7A MDB
Area (ft²) 19308 37140 18100 12896 91590
Area (acre) 0.44 0.85 0.42 0.30 2.10
Pervious Area (ft²) 6758 12999 6335 4514 32057
Impervious Area (ft²) 12550 24141 11765 8382 59534
Pervious ( C ) 0.2 0.2 0.2 0.2 0.2
Impervious ( C ) 0.9 0.9 0.9 0.9 0.9
Weighted ( C ) 0.66 0.66 0.66 0.66 0.66
Time of Concentration (min) 5 5 5 5 5
Runoff Volume (cf) 10-year 2-hr storm 864 1662 810 577 4099
Flowrate Q (cfs) 25yr 5min storm 1.11 2.14 1.04 0.74 5.27
Stormwater Plan 6 9/15/2022
Retention Facilities
The site is divided into 15 sub-drainage areas, each with its own sub-surface infiltration
system designed to capture and infiltrate the design storm event.
Sub-Drainage Areas
Each sub-drainage area has a subsurface infiltration system designed to capture
and infiltrate stormwater runoff from hardscapes, buildings, and landscaped
areas. The underground infiltration systems are approximately 3.5 feet in depth
starting approximately 2 feet below finished grade and extend down to the native
gravel soil horizon. Storage media is comprised of a minimum of 3.5 feet of
crushed gravel with a minimum of 6 inches at both the top and bottom of the
system. Each retention system is hydraulicly connected to the down gradient
retention system via storm piping. Outlet elevation control will be provided on
each systems’ outlet pipe that allows for maximum ponding in each system
before flowing into the next system. Furthermore, each system is design to allow
for surfacing bypassing via valley gutters or curb and gutter drainage to be
captured by a down gradient retention system. Tables 2 – 7 below show that
sub-drainage areas in most cases exceed the 10-year, 2-hour storm event.
Table 2 - Stormwater System Capacity (sub-drainage area 1)
Basin ID 1A 1B 1C
10-year 2-hour Storm Runoff (Table 1) 747 1087 1146
Total Underground Storage (cf) 681 1451 1451
Storm Event Captured (in) 0.75 1.09 1.04
Table 3 - Stormwater System Capacity (sub-drainage areas 2)
Basin ID 2A 2B 2C
10-year 2-hour Storm Runoff (Table 1) 490 1096 822
Total Underground Storage (cf) 680 1451 1449
Storm Event Captured (in) 1.14 1.09 1.45
Table 4 - Stormwater System Capacity (sub-drainage areas 3)
Basin ID 3A 3B
10-year 2-hour Storm Runoff (Table 1) 562 963
Total Underground Storage (cf) 941 1418
Storm Event Captured (in) 1.37 1.21
Table 5 - Stormwater System Capacity (sub-basin 4)
Basin ID 4A 4B
10-year 2-hour Storm Runoff (Table 1) 1032 809
Total Underground Storage (cf) 1285 1847
Storm Event Captured (in) 1.02 1.87
Stormwater Plan 7 9/15/2022
Table 6 - Stormwater System Capacity (sub-drainage areas 5)
Basin ID 5A 5B
10-year 2-hour Storm Runoff (Table 1) 864 1662
Total Underground Storage (cf) 1269 3157
Storm Event Captured (in) 1.20 1.56
Table 7 - Stormwater System Capacity (sub-basin 6A, 7A & MDB)
Basin ID 6A 7A MDB
10-year 2-hour Storm Runoff (Table 1) 810 577 4099
Total Underground Storage (cf) 763 763 9337
Storm Event Captured (in) 0.77 1.08 1.87
Groundwater Monitoring
As part of the SUD Phase 3 subdivision, groundwater monitoring data was collected
and is summarized in Table 8 below. Due to ongoing construction and dewatering
activities associated with the subdivision, reliable groundwater monitoring has not been
able to take place since the start of subdivision construction.
Table 8 – Groundwater Monitoring
Measurement
Date
Depth from
Existing Ground
Groundwater
ELEVATION
5/3/2019 7.0 4922.10
5/6/2019 6.6 4922.50
5/13/2019 6.2 4922.90
5/22/2019 6.4 4922.70
5/29/2019 6.4 4922.70
6/5/2019 6.4 4922.70
6/12/2019 6.6 4922.46
6/24/2019 6.5 4922.63
7/12/2019 6.85 4922.25
7/17/2019 7.1 4922.00
8/26/2019 6.95 4922.15
Groundwater Impact Analysis
Table 10 below shows the chamber bottom elevations for each system in relation to the
extrapolated seasonally high groundwater elevation at that retention system’s location.
As shown in Table 10, the separation between chamber bottom and seasonally high
groundwater ranges from 1.3 feet to 3.5 feet with a site system average of 2 feet of
separation. Ideally, a separation of 3 feet from seasonally high groundwater is preferred
to ensure long-term survivability and performance of the retention system. The site
stormwater facilities were analyzed to determine the available capacity in the system if
storage capacity within 3 feet of the seasonally high groundwater elevation were
eliminated. As analyzed under that scenario, the on-site stormwater management
Stormwater Plan 8 9/15/2022
facilities would maintain capacity exceeding the design storm event of approximately
0.90-inch storm event, before escaping the system. Given the short-term nature of the
seasonal high groundwater, it is in the interest of the site to maximize storage and
maintainability of the system by installing the 2.5-foot-tall chambers as opposed to the
shallower 1.5 foot tall chambers.
Table 9 – Groundwater Impact on System Capacity Analysis
Storm
Facility ID
Chamber
Bottom
Elevation
Seasonal
High GW at
Chamber
GW
Separation
FT
Reduced Storage
Capacity
(3ft above High GW)
1A 4933.12 4930.49 2.63 597 cf (0.66” storm)
1B 4929.50 4927.50 2.00 967 cf (0.73” storm)
1C 4926.80 4924.75 2.05 961 cf (0.69” storm)
2A 4932.42 4930.49 1.93 438 cf (0.76” storm)
2B 4928.95 4927.50 1.45 701 cf (0.52” storm)
2C 4926.12 4924.75 1.37 662 cf (0.66” storm)
3A 4932.81 4930.49 2.32 728 cf (1.06” storm)
3B 4932.00 4930.49 1.51 714 cf (0.61” storm)
4A 4928.59 4927.00 1.59 681 cf (0.54” storm)
4B 4925.55 4924.25 1.30 800 cf (0.81” storm)
5A 4929.54 4927.94 2.54 677 cf (0.64” storm)
5B 4926.72 4924.25 2.47 2,599 cf (1.28” storm)
6A 4926.49 4923.00 3.49 842 cf (0.85” storm)
7A 4925.75 4923.00 2.75 700 cf (0.99” storm)
MDB 4924.30 4922.25 2.05 6,380 cf (1.28” storm)
TOTAL AVG 2.0 18,467 cf (0.90” storm)
Storm Event Analysis
A storm event analysis was previously conducted to determine the percentile storm
events in the City of Bozeman. The exercise analyzed 30 years of recorded 24-hour
rainfall events using precipitation data collected at MSU rain gauge Station
USC00241044. The analysis excluded snow fall events and 24-hour rainfall events that
accumulated less than 0.1-inches. The table below summarizes the percentile storm
events produced by the analysis. As indicated by the table, the site stormwater facilities
as design would completely retain the 99th percentile storm event.
Table 10 – Bozeman Rainfall Analysis (1990-2020)
Percentile Storm
Event Volume (in)
80th 0.41
85th 0.49
90th 0.57
95th 0.74
99th 1.2
100th 2.29
Stormwater Plan 9 9/15/2022
Conveyance Capacity
As designed, each retention system has the capacity to completely capture the 100-
year 5-minute (0.44 inch) storm event. The distributed retention systems throughout the
site reduce the likelihood of overloading any single conveyance system. Each retention
system has a primary overflow and a secondary overflow. Each overflow has adequate
capacity to convey the designed high intensity storm event without overloading the
system. The stormwater retention systems work in tandem to reduce the compounding
effects that come with a single large drainage basin. The site conveyance systems were
each individually analyzed to ensure adequate capacity exists in the event there is a
failure within the system (i.e., clogging).
Valley Drive Aisle
Stormwater runoff in most sub-drainage basins will be collected in valleyed drive
aisles and conveyed to an underground retention system. The hydraulic capacity
of the most limited valley drive aisle was analyzed to determine if the pavement
would be overtopped in a 25-year, 5-minute storm event. The maximum 25-year
event for any of the basins produces approximately 5.26 cfs of runoff, assuming
infiltration systems are full and being bypassed. The capacity of the aisle at the
lowest designed slope of 1.0% is 8.95 cfs at a depth of 0.25 feet (3.0 inches)
above the flow line in the drive aisle. Therefore, drive aisle capacities within the
development are adequate to carry in excess of the 25-year, 5-minute storm
event.
Table 11 – Valley Drive Aisle Conveyance Capacity Calculations
Left Channel side slopes (ft:1) 50
Right Channel side slope (ft:1) 50
Channel bottom width (ft) 0
Flow Depth (ft) 0.25
Flow Area (sf) 3.125
Wetted Perimeter (ft) 25
Width (ft) 25
Hydraulic Radius (ft) 0.125
Manning's Roughness 0.013
Slope (ft/ft) 0.01
Average Velocity (ft/sec) 2.86
Flow (cfs) 8.95
Stormwater Plan 10 9/15/2022
Curb and Gutter
Curb and gutter are used on the site to direct runoff into the underground storm
pipe network. Curb and gutter is primarily used along S. 18th Ave and on east-
west running drive aisles. To ensure the standard curb design will provide
adequate conveyance capacity for peak flows, curb and gutter with the greatest
flows and least amount of redundant or secondary storm conveyance options
was identified and analyzed for conveyance capacity. The critical curb required to
convey the maximum flow was identified as the approximately 443’ length of curb
running from south to north along S. 18th Ave, serving a catchment area of
approximately 45,795 sf (i.e. half of MDB Drainage Area). This catchment area
produces a peak flow of 2.64 cfs for the 25-year design storm. As shown in Table
6 above, At the minimum slope of 0.5%, the curb on site has a conveyance
capacity of 5.03 cfs.
Table 12 – Curb and Gutter Conveyance Calculations
Curb and Gutter Capacity Calcs
Max Curb Conveyance Req'd (ft^3/sec) 2.63
Right-side Slope X:1 0.06
Left-side Slope X:1 33.00
Channel Bottom Width (ft) 0
Flow Depth (ft) 0.350
Flow Area (ft^2) 2.025
Wetted Perimeter (ft) 11.906
Width 0.044
Hydraulic Radius (ft) 0.170
Manning's Roughness 0.013
Slope (ft/ft) 0.005
Average Velocity (ft/sec) 2.49
Flow (ft^3/sec) 5.03
Storm Drains
The hydraulic capacity of the storm drains were analyzed to determine the
required pipe size to convey the 25-year, 5-minute storm event. The maximum
25-year event for any of the basins draining to a 15” storm pipe produces
approximately 5.27 cfs of runoff. The capacity of a 15” storm drain at 0.6% slope
is 5.33 cfs at 90% full pipe flow. Therefore, 15” storm drains have the capacity to
convey the 25-year peak runoff of any onsite defined drainage areas. The
maximum 25-year event for any of the basins draining to a 12” storm pipe
produces approximately 2.13 cfs of runoff. The capacity of a 12” storm drain at
0.5% slope is 2.71 cfs at 90% full pipe flow. Therefore, 12” storm drains have the
Stormwater Plan 11 9/15/2022
capacity to convey the 25-year peak runoff of any drainage basin from the
woonerfs to the retention systems.
Table 13 – Storm Drain Conveyance Calculations
Manual Input Parameters
Pipe Diameter (in) 12 15
Flow Depth in Pipe (in) 11.10 13.5
Coefficient of Roughness (n) 0.013 0.013
Slope (ft/ft) 0.005 0.006
Calculated Parameters
Radius (in) 6 7.50
Radius (ft) 0.50 0.63
Flow Depth in Pipe (ft) 0.93 1.13
Wetted Perimeter (ft) 2.59 3.12
Area of Flow (sq ft) 0.76 1.16
Hydraulic Radius (ft) 0.29 0.37
Average Velocity (ft/sec) 3.6 4.6
Flowrate (cfs) 2.71 5.33
Facilities Maintenance
The proposed stormwater mitigation systems are located within private rights-of-ways
and will be operated and maintained by the owners’ association. A detailed operations
and maintenance plan has been developed and included in the final covenants. At a
minimum, the owners’ association is responsible for quarterly inspection and annual
maintenance of all stormwater facilities. The inspection and maintenance
responsibilities include, but are not limited to, general housekeeping responsibilities,
visual inspection for performance, removal of sediment from mitigation facilities,
vegetative maintenance, as well as any necessary repairs to the facilities themselves.
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3 9/14/2022 SITE PLAN RESUBMITTAL ZWL CDP PROFESSIONALENGINEERS &SURVEYORSSTAHLYENGINEERING& ASSOCIATESSUD PH3
BLOCK 2
WENTWORTH PROPERTY
COMPANY