HomeMy WebLinkAbout20 - Design Report - Bridger View PUD - StormwaterEngineering Report
Stormwater Design
Bridger View PUD
October 2020
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 10/8/2020
Engineering Report
Table of Contents
General Information and Design Criteria
Existing Site Conditions
Storm Drainage Plan Description
Estimation of Runoff
Retention Facilities
Woonerf 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-basin 1-2)
Table 3 Stormwater System Capacity (sub-basin 3-5)
Table 4 Woonerf Conveyance Capacity Calculations
Appendices
Stormwater Maintenance Plan
Stormwater Plan 2 10/8/2020
General Information and Design Criteria
The proposed Bridger View Planned Unit Development (PUD) is located
southwest of the intersection of Bridger Drive (MT 86) and Story Mill Road.
Bridger View PUD is a 62-home neighborhood consisting of modest homes with
a compact urban, design. This project is located on approximately 8 acres which
was formerly occupied by 40 mobile homes within the Bridger View Trailer Park.
The compact, efficient neighborhood utilizes, Low Impact Design (LID) methods
to provide decentralized stormwater mitigation, where numerous infiltration areas
can; replicate a natural hydrology, provide enhanced stormwater treatment, and
function as site amenities. The LID stormwater mitigation will be designed in
accordance with City of Bozeman Design Standards and Specifications Policy for
retention/infiltration facilities.
Plans are provided showing proposed grading, drainage flow paths, and
stormwater retention/infiltration details.
The developer’s contact information is:
HRDC
Heather Grenier
32 South Tracy Ave.
Bozeman, MT 59715
Figure 1: Vicinity Map, Bozeman, Montana
Stormwater Plan 3 10/8/2020
Existing Site Conditions
The Bridger View PUD site is located on the former site of the Bridger View
Trailer Park. The trailer park had some minor cut/fill grading to establish level
trailer sites; but previous site work did not significantly impact the soils since the
mobile homes did not have foundations. The existing asphalt drives and concrete
parking spaces will be removed during site construction. Beyond the minor site
grading, the soils are typical of the Bozeman area.
A geotechnical evaluation was conducted and is provided in a separate appendix
of the submittal. The soils can generally be described as a deep topsoil
overlaying a fine-grained subsoil, with gravelly sand material at a depth of
approximately 8 feet. Because basements are proposed for the new homes,
groundwater monitoring was conducted throughout the summer of 2019.
Monitoring results are provided with the geotechnical report. Groundwater depths
were generally greater than 10 feet and should not impact homes or utilities. The
underlying gravelly sand material will provide long-term infiltration of stormwater.
The site is not located within a 100-year floodplain.
The topography of the existing site is generally flattened due to past grading but
gradually slopes to the northwest. Runoff from the site ultimately makes its way
to the East Gallatin River through the Story Mill Community Park and the existing
drainage along the south side of Bridger Drive.
Storm Drainage Plan Description
The storm drainage plan for the Bridger View PUD consists of several LID
solutions designed to infiltrate stormwater and replicate the natural hydrology of
the site.
The overall storm drainage plan for the Bridger View PUD is depicted on Figure 2
at the end of this report. This plan shows proposed drainage flow paths,
mitigation areas, and drainage basins. With final grading, the site will be divided
into 5 sub-basins as shown on Figure 2. Sub-basin 1 consists of the Hillside Lane
right-of-way. Sub-basin 2 is a small basin located in the southern portion of the
site draining to Hillside Green. Sub-basin 3 is the eastern portion of the site
draining to the Flourhouse Court. Sub-basin 4 is the central portion of the site
draining to the Millworks Court. Sub-basin 5 is in the western portion of the site
draining to the Blue Silos Court.
Storm runoff from Hillside Lane (sub-basin 1) will be mitigated by bio-retention
(bioswales) located in the street boulevards between the curb and sidewalk. The
Hillside Lane right-of-way is proposed to be 64’ wide, or four feet wider than
typical to provide an additional two-foot width for each boulevard. The proposed
grading separates Hillside Lane runoff from the remainder of the site.
Stormwater Plan 4 10/8/2020
Storm runoff from the interior streets and lots will be mitigated by
retention/infiltration systems located on common open spaces. In general,
building sites will be sloped from the building to the adjacent common areas or
interior street woonerfs and alley woonerfs. A small portion of site runoff from
sub-basin 2 will be mitigated by bio-retention located in the Hillside Green
common open space. For sub-basins 3-5, woonerfs, which are valleyed, will
carry runoff to inlets and storm drains running to the retention/infiltration systems
located in the courts. Prior to infiltration into the soil, stormwater runoff will be
treated through sediment pretreatment and filtration in below-ground chambers.
Because the on-site stormwater retention/infiltration systems will also be utilized
as site amenity spaces, enough underground storage capacity is provided to
allow this area to be used after common rain events, such as the design storm.
For larger storm events, which are less common, the courtyards can
accommodate shallow ponding for a short period after the storm. This provides
mitigation of storms exceeding the design requirements, with only infrequent
impacts on the site.
Survivability of LID systems through conventional subdivision and subsequent lot
development is difficult. The concurrent subdivision/lot development permitted by
the PUD allows the LID systems to be installed with landscaping after building
construction and associated site impacts are completed. This ensures that the
LID systems survive the lot development and will perform reliably. 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.
Estimation of Runoff
Runoff estimates were obtained for each sub-basin using City of Bozeman
standards. For the stormwater retention calculations, the design storm is a 10-
year, 2-hour storm. 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 0.81-inch storm event. For the purposes of sizing conveyance facilities,
the design storm is a 25-year event at a time of concentration of 5 minutes. This
storm has an intensity of 3.826 in/hour, but due to its short duration, only
represents a 0.32-inch event.
Table 1 shows the storm runoff calculations for the 5 drainage sub-basins.
Specifically, the design uses the volume of the 10-year, 2-hour storm (for
retention calculations), and the flow rate from the 25-year event (for conveyance
calculations).
Stormwater Plan 5 10/8/2020
Table 1 – Storm Runoff Calculations
Basin
Characteristics
Sub-basin
1
Sub-basin
2
Sub-basin
3
Sub-basin
4
Sub-basin
5
Area (ft²) 43,067 17,115 64,114 90,946 134,314
Area (acre) 0.99 0.39 1.47 2.09 3.08
Pervious Area (ft²) 15,073 5,990 22,440 31,831 47,010
Impervious Area (ft²) 27,994 11,125 41,674 59,115 87,304
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-yr 2-hr storm 1,928 766 2,870 4,071 6,012
Flow Rate Q (cfs)
25-yr 5-min Storm
2.48 0.99 3.69 5.23 7.73
Retention Facilities
The site is divided into 5 sub-basins; each with their own stormwater
management facilities.
Sub-Basin 1 and 2:
Sub-basin 1 is located along Hillside Lane. As depicted in attached Figure
2, the stormwater management facilities proposed for this sub-basin
consist of bio-swales located on either side and at the end of Hillside
Lane. The bio-swales are intended to treat runoff through filtration and
reduce runoff volumes through infiltration. Runoff from Hillside Lane is
conveyed via curb and gutter to bioswale inlet chases for sediment
capture before entering the bio-swales. Sub-basin 2 is located north of
Hillside Lane and consists of a single, shallow bio-retention system
designed to treat overland flow runoff from nearby roofs, sidewalks, and
vegetated areas. All stormwater management facilities located in sub-
basin 1 and 2 contain approximately two feet of bio-swale media storage.
Table 2 below shows the storage capacity of sub-basins 1 and 2 exceed
the 10-year, 2-hour retention requirement.
Table 2 - Stormwater System Capacity (sub-basin 1-2)
Basin ID
Sub-basin
1
Sub-basin
2
10-year 2-hour Storm Runoff (Table 1) 1,928 cf 766 cf
Total Underground Storage [CF] 2,891 cf 858 cf
Storm captured before ponding [in] 1.23 in 0.92 in
Above Ground Ponding Storage [cf] - 541 cf
Rain Event Captured with ponding [cf] - 1.50 in
Stormwater Plan 6 10/8/2020
Sub-Basins 3-5:
The stormwater management facilities located in sub-basins 3-5 consist of
an underground infiltration facility for design storm events, and shallow
ponding up to 1 foot of water for storage of larger events. The
underground infiltration systems are approximately 7 feet in depth, down
to the native gravel soil horizon. Storage media is comprised of a
minimum of 3.5 feet of crushed gravel at the bottom and approximately 3.5
feet of bioswale media at the top of the system. Table 3 below shows that
sub-basins 3-5 exceed the 10-year, 2-hour retention requirement.
Table 3 - Stormwater System Capacity (sub-basin 3-5)
Basin ID
Sub-basin
3
Sub-basin
4
Sub-basin
5
10-year, 2-hour Storm Runoff (Table 1) 2,870 cf 4,071 cf 6,012 cf
Total Underground Storage [CF] 4,998 cf 7,671 cf 8,613 cf
Storm captured before ponding [in] 1.43 in 1.55 in 1.17 in
Above Ground Ponding Storage [cf] 2,201 cf 2,414 cf 3,804 cf
Rain Event Captured with ponding [cf] 2.06 in 2.03 in 1.69 in
Conveyance Capacity
Stormwater runoff in sub basins 3-5 will be collected in the woonerf streets and
conveyed to the common area court retention/infiltration system by storm drains.
The hydraulic capacity of the woonerf street 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 7.50 cfs of
runoff. The capacity of the woonerf at the lowest designed slope of 1% is 9.93 cfs
at a depth of 0.26 feet (3.12 inches) above the flow line in the woonerf.
Therefore, woonerf capacities in the development are adequate to carry the 25-
year, 5-minute storm event.
Table 4 – Woonerf 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) 0.26
Flow Area (sf) 3.38
Wetted Perimeter (ft) 26
Width (ft) 26
Hydraulic Radius (ft) 0.13
Manning's Roughness 0.013
Slope (ft/ft) 0.01
Average Velocity (ft/sec) 2.94
Flow (cfs) 9.93
Stormwater Plan 7 10/8/2020
The hydraulic capacity of the storm drains was 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 produces approximately 7.50 cfs of runoff.
The capacity of a 15” storm drain at 1.5% slope is 8.48 cfs at full flow. Therefore,
15” storm drains have the capacity to convey the 25-year peak runoff of any
drainage basin from the woonerfs to the retention systems.
Table 5 – Storm Drain Conveyance Calculations
Input Parameters
Pipe Diameter (in) 15
Flow Depth in Pipe (in) 13.75
Coefficient of Roughness (n) 0.013
Slope (ft/ft) 0.015
Calculated Parameters
Radius (in) 7.50
Radius (ft) 0.63
Flow Depth in Pipe (ft) 1.15
Wetted Perimeter (ft) 3.19
Area of Flow (sq ft) 1.18
Hydraulic Radius (ft) 0.37
Average Velocity (ft/sec) 7.20
Flowrate (cfs) 8.48
Facilities Maintenance
The proposed stormwater mitigation systems are located within public rights-of-
ways and common open spaces and will be operated and maintained by the
owners’ association. A detailed operations and maintenance plan has been
developed and will be 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.
Storm Water Maintenance Plan
(a) General Information. Bridger View utilizes Low Impact Development (“LID”) to
mitigate storm water impacts. The storm water facilities consist of curb and
gutter, drainage swales, bio-retention areas (bio-swales), conveyance piping, and
a final retention basin with an attached subsurface infiltration system. The system
is designed to accommodate all storm water from the Property.
The principal storm water conveyance and treatment facilities are located on the
Common Open Space Lots and are owned by Association. Privately owned LID
storm water facilities serving individual Lot development are encouraged and
private facilities are allowed to discharge to the Association facilities through the
Common Open Space Lots.
The Association is responsible for maintaining storm water facilities on Common
Open Space and Hillside Lane All storm water facilities located on privately
owned Lots shall be maintained by the Lot Owner.
The storm water facilities are designed to operate without excessive
maintenance. However, like all infrastructure, periodic maintenance will prevent
costly repair and replacement. The maintenance plan set forth in this Section has
been prepared in accordance with City of Bozeman guidelines. Over time,
recommended maintenance guidelines may evolve. Please contact the City of
Bozeman Storm Water Division if any questions arise.
(b) Storm Water Facilities Maintenance Schedule
(i) Site Housekeeping. Site housekeeping is to be conducted continuously as
needed. The main cause of storm water facility damage is poor site
housekeeping. Sediment tracked onto pavement can be washed into storm
water bio-retention basins and conveyance piping and damage these
facilities. Trash can clog pipes and inlet structures causing property damage.
Site housekeeping consists of the following:
• Keep sidewalk and pavement areas clean
• Pick up trash
• Restore damaged landscaping in order to prevent sediment runoff
(ii.) System Monitoring. System monitoring is to be conducted quarterly, except in
winter. The storm water facilities shall be inspected quarterly to quickly
identify small issues before expensive damage can occur. In addition to
regular monitoring, the best time to inspect the performance of storm water
facilities is during runoff events. System monitoring consists of the following:
• Observe system during runoff. Look for ponding outside of retention areas.
This can indicate a clogged inlet or pipe.
• Inspect bio-retention and retention basins
• Inspect inlets, manholes and pipes
• Inspect subsurface infiltration system through inspection ports and
manholes. Measure water depth and sediment depth if present.
(iii.) Bio-retention and Retention Basin Maintenance. Bio-retention and retention
maintenance is to be conducted quarterly. The bio-retention and retention
basins are designed to provide long-term sustainable treatment of storm
water. With poor housekeeping, however, sediment can clog these facilities
and reduce infiltration capacity. Proper and timely cleaning of incidental
sediment in these basins can prevent these facilities from being damaged.
The bio-retention and retention basins are intended to be vegetated, since the
vegetation provides microbiological communities that can treat common
pollutants in storm water. Unhealthy vegetation can diminish the performance
of these basins. The landscaping of the bio-retention basins shall follow
typical landscape maintenance guidelines. The vegetation on the bottom of
the retention basin should not be regularly mowed, since mowing can cause
plant debris to build up in the basin. Bio-retention and retention basin
maintenance consists of the following:
• Remove sediment, trash, and debris
• Inspect for healthy vegetation
• Inspect for uniform ponding and water disappears in 3 days
(iv.) Bio-retention and Retention Basin Maintenance. Bio-retention and retention
basin maintenance is to be conducted annually. The vegetation should be
maintained annually to reduce plant debris build-up. The retention basin
vegetation should be cut to a height of 6” in the fall with the clippings removed
from the basin. Bio-retention and retention basin maintenance consists of the
following:
• Remove dead plant materials from bio-retention basins.
• Cut retention basin vegetation and remove clippings.
(v.) Bio-retention and Retention Basin Maintenance. This maintenance is to be
conducted long-term if and when necessary. If regular housekeeping and
maintenance is not performed adequately, sediment and debris can
accumulate in the basin and reduce the required storage capacity. If this
occurs the basins should be excavated back down to original plan grade.
Long-term bio-retention and retention basin maintenance consists of the
following:
• Hire a contractor to inspect and return basin condition to initial design
found on City engineering plans.
• Dredge basin if sediment build-up is greater than 6”
(vi.) Inlets, Manholes, and Piping Maintenance. Inlets, chases, manholes, and
piping maintenance is to be conducted quarterly. All storm water inlets,
chases and manholes have a sump to capture sediment. If this sediment is
not periodically removed it can wash downstream and clog infiltration
facilities. The sump is typically visible from the surface through the inlet grate.
Inlets, manholes, and piping maintenance consists of the following:
• Remove visible sediment from sump.
• Inspect pipe inlets and outlets for sediment in pipe and remove if present.
(vii.) Inlets, Chases, Manholes, and Piping Maintenance. This maintenance is to be
conducted long-term if and when necessary. If regular housekeeping and
maintenance is not performed adequately, sediment and debris can
accumulate in the storm water conveyance piping, and pipes may become
clogged. Long-term inlets, chases, manholes, and piping maintenance
consists of the following:
• Hire a contractor to clean pipes and remove all sediment from manhole
sumps and chases.
(viii.) Subsurface Infiltration System Maintenance. Subsurface Infiltration System
maintenance is to be conducted quarterly. Inspect infiltration system through
inspection ports and manholes for sediment. All manholes on the subsurface
infiltration system have a sump to capture sediment. If this sediment is not
periodically removed it can wash into the infiltration system and clog
infiltration facilities. The subsurface infiltration system is made up of multiple
chamber rows. Each chamber row has an inspection port allowing the
chamber bottom to be inspected. Subsurface infiltration system maintenance
consists of the following:
• Remove visible sediment from manhole sumps.
• Inspect pipe inlets and outlets for sediment in pipe and remove if present.
• Inspect that stormwater infiltrates from chambers within 3 days of rainfall
event.
• Inspect sediment depth in chambers. Sediment depth in chambers less
than 3” is acceptable.
(ix.) Infiltration System Maintenance. This maintenance is to be conducted long-
term if and when necessary. If greater than 3” of sediment is present in
infiltration system, clean out maintenance must be conducted.
• Hire a contractor with a Jet-Vac chamber cleaning system to remove the
sediment from the infiltration system.
(c) Record Keeping
(i) Records of quarterly and long-term monitoring and maintenance shall be
kept and made available to the City upon request.
STORY MILLSPUR TRAILSTORY MILL COMMUNITY PARKCITY OFBOZEMANPROPERTYSTORY MILL ROAD (60' ROW & 30' ESMT)BRIDGER DRIVE (Aka Hwy 86) - (110' ROW)HILLSIDE LANE(60' ESMT)HILLSIDE LANE (64' ROW)FLOURHOUSECOURTMILLWORKSCOURTLOT 56LOT 53LOT 49LOT 46LOT 21LOT 40LOT 35LOT 23LOT 13LOT 26LOT 27LOT 4LOT 16LOT 19LOT 47LOT 12LOT 44LOT 43LOT 39LOT 22LOT 38LOT 31LOT 33LOT 42LOT 2LOT 41LOT 37LOT 10LOT 9LOT 32LOT 29LOT 30LOT 5LOT 7LOT 8LOT 52LOT 55LOT 57LOT 54LOT 51LOT 50LOT 48LOT 45LOT 17LOT 25LOT 14LOT 15LOT 11LOT 20LOT 24LOT 36LOT 34LOT 1LOT 6LOT 3LOT 28HILLSIDE GREENHILLSIDE GROVEBLOCK 1BLOCK 2BLOCK 3BLOCK 4PATH TO THE "M"PARKSIDE WALKMILLWORKS ROW
FLOURHOUSE WAY (30' ROW)BLUE SILOS WAY (30' ROW)BLUE SILOSCOURTMILLWORKS WAY (30' ROW)ADDITIONAL ROW 1976 SFLOT 18LOTC-1LOTC-2LOTC-4LOTC-5LOTC-3LOTC-6OS 1OS 5OS 6OS 3OS 4OS 7OS 7OS 8OS 7SUB-BASIN:31.47 acSUB-BASIN: 42.09 acSUB-BASIN:53.08 acSUB-BASIN:10.99 acSUB-BASIN: 20.39 acFigure 2STORMWATERPLANPROFESSIONALENGINEERS &SURVEYORSBRIDGER VIEW STAHLYENGINEERING& ASSOCIATESHRDC BOZEMAN MONTANA