HomeMy WebLinkAbout008StormDrainageReportStillwater Hospice – Nikles Business Park
May 2022
Project: 21‐076
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582‐0221 . Fax: (406) 582‐5770 Page 1
Table of Contents
1 Project Overview and Background ......................................................................................... 2
2 Hydrology ............................................................................................................................... 2
2.1 Pre-Development .............................................................................................................. 2
2.2 Post Development ............................................................................................................ 3
3 Post Development Hydraulics ................................................................................................. 3
3.1 Inlet and Gutter Capacity ................................................................................................. 3
3.2 Storm Drain ..................................................................................................................... 4
3.3 Ponds ................................................................................................................................ 4
4 Maintenance Plan .................................................................................................................... 6
Table 1 - Pre-Development Hydrology ........................................................................................... 3
Table 2 - Post Development Drainage Basins ................................................................................ 3
Table 3 - Inlet and Gutter Summary (25-year event) ...................................................................... 4
Table 4 - Outfall Summary ............................................................................................................. 4
Table 5 - Minimum Sizing .............................................................................................................. 5
Table 6 – Pond A Pond Stage-Storage-Discharge .......................................................................... 5
Attachments:
Exhibit 1.1 – Vicinity Map - Topo
Exhibit 1.2 – Aerial Map
Exhibit 2.1 – Pre-Development Drainage
Exhibit 2.2 – Post Development Drainage
Appendix A – Supporting Calculations
Appendix B – Storm Drainage Plans
Stillwater Hospice – Nikles Business Park
May 2022
Project: 21‐076
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582‐0221 . Fax: (406) 582‐5770 Page 2
1 Project Overview and Background
The subject property is located at the southwest corner of the intersection of Nikles Drive and
Evergreen Drive. Vicinity maps showing the project location and extents are provided in Exhibits
1.1 and 1.2. The proposed site improvements will involve construction of two office buildings.
The site is currently undeveloped. The improvements are anticipated to be completed in two
phases. The first office building will be constructed near the intersection of Nikles Drive and
Evergreen Drive and is intended to provide office space for Stillwater Hospice. The second
structure is set back further in the site and is planned for permitting and construction upon securing
a lease tenant. A parking lot is included as part of the improvements. An existing ditch is located
along the southern and western borders of the project site. Findings related to the ditch were
provided as part of a separate site plan in 2015. An excerpt from the previous review comments is
provided below. The Gallatin Conservation District has determined the water feature on the
western edge of the property is non-jurisdictional. Therefore, the water feature does not trigger
the requirements found in Section 38.23.100, BMC, Watercourse Setbacks. The existing 30-foot
utility and drainage easement shown on the plat applies. A new detention pond at the west side of
the site will be provided to meet the water quality and peak runoff rate requirements. The
stormwater pond and collection/conveyance infrastructure are designed to provide capacity for
both phases of development.
The following sections describe the proposed stormwater design and its compliance with the
current City of Bozeman (COB) storm drainage standards. Current regulations dictate that peak
runoff rates for the post-development condition are mitigated to pre-development for the 10-year
event. Stormwater collection and conveyance infrastructure is sized for the 25-year event.
2 Hydrology
The rational method was utilized to estimate peak flow rates for conveyance sizing as outlined by
the City of Bozeman Design Standards and Specifications. A weighted runoff coefficient “C” was
determined based on a combination of open space (C=0.2) and pavement (C=0.95). The time to
concentration was estimated using the overland flow method from the City of Bozeman Design
Standards and TR-55 for shallow concentrated flow. A more detailed breakout of the data for each
basin is provided in Appendix A. Basins, flow paths, and the general site layout for pre- and post-
development conditions can be found on Exhibits 2.1 and 2.2, respectively.
2.1 Pre‐Development
An overview of the pre-development drainage is provided on Exhibit 2.1. A single basin was
assumed for the site with consistent basin boundaries between the pre- and post-development site
conditions for the purpose of pond sizing. Runoff from the site ultimately outfalls to the existing
ditch. A summary of the pre-development hydrologic conditions is provided in Table 1; more
detailed information providing a time to concentration breakout and intensity is provided in
Appendix A.
Stillwater Hospice – Nikles Business Park
May 2022
Project: 21‐076
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582‐0221 . Fax: (406) 582‐5770 Page 3
Table 1 - Pre-Development Hydrology
Basin Total
Area (ac)
Impervious
(sf)
Open
Space (sf)
Weighted
C
Tc
(min)
10-yr Peak
Runoff
(cfs)
25-yr Peak
Runoff (cfs)
A.1 0.99 0 43,262 0.20 12 0.37 0.44
2.2 Post Development
Input parameters and results for the post development drainage basins are summarized in Table 2.
An overview plan showing the drainage areas and infrastructure is provided on Exhibit 2.2. Both
basins A.1 and A.2 are routed through the proposed detention pond to the existing ditch along the
western border of the subject property.
Table 2 - Post Development Drainage Basins
Basin
Total
Area
(acres)
Impervious
(sf)
Open
Space (sf)
Weighted
C
Tc
(min)
10-yr Peak
Runoff
(cfs)
25-yr Peak
Runoff
(cfs)
A.1 0.65 13,511 14,665 0.56 6 1.03 1.23
A.2 0.35 10,006 5,080 0.70 6 0.69 0.82
3 Post Development Hydraulics
The post development collection and conveyance infrastructure considers the 25-year design event
as outlined in the City of Bozeman Design Standards. Capacity calculations for the proposed storm
drainage infrastructure are provided in Appendix A.
3.1 Inlet and Gutter Capacity
The maximum allowable water surface for the 25-year event is 0.15-ft below the top back of curb.
The gutter flow capacity is based on the HEC-22 gutter flow methodology for a uniform section.
Street capacity is 1.58-cfs for an assumed cross slope of 1.5% and a longitudinal grade of 1%. A
3-ft wide sidewalk chase and 3-ft wide curb cut will be used to convey runoff from Basin A.1 and
Basin A.2, respectively, to the proposed detention pond. Both the sidewalk chase and curb cut
were modeled as a 3-ft wide weir, calculated flow depths are 0.25-ft and 0.19-ft, respectively. All
calculated peak flow rates are less than the typical street and inlet capacity. Sample calculations
are provided in Appendix A. A summary of inlet and gutter flow is provided in Table 3. All inlets
are located at a sag in the curb line.
Stillwater Hospice – Nikles Business Park
May 2022
Project: 21‐076
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582‐0221 . Fax: (406) 582‐5770 Page 4
Table 3 - Inlet and Gutter Summary (25-year event)
Inlet
Cross
Slope
(%)
Calc. Peak
Flow (cfs)
TBC
(ft)
TBC-0.15
(ft)
Calculated WSE
(ft)
Spread
(ft)
Sidewalk
Chase A.1
1.0 1.23 4730.37 4730.22 4730.17 9.02
Curb Cut A.2 1.5 0.82 4730.37 4730.22 4730.11 7.43
The concrete channel conveying drainage through the landscape island was modeled based on weir
flow. The calculations conservatively assumed the entire basin drains though the channel. The
channel width is 2.25-ft with a corresponding flow depth of approximately 0.30-ft. Supporting
calculations are included in Appendix A.
3.2 Storm Drain
Roof drainage from the phase 1 structure at the north end of the property will be piped to the
proposed detention pond. Roof downspouts will be collected and conveyed within the structure to
the storm drain pipe located on the south side of the structure. Overflow spouts are provided on
the side of the structure in case pipes become plugged. The proposed storm pipe is 8-inches and
will have a minimum grade of 1%. The estimated capacity of an 8-inch pipe flowing full is 1.05-
cfs. The peak runoff rate generated from the roof area is 0.39-cfs based on the minimum time to
concentration of 6-minutes. Therefore, the conveyance pipe is adequately sized for conveyance of
the roof drainage. Sample calculations are provided in Appendix A.
3.3 Ponds
The pre-development and post development basin summaries were previously provided in Section
2. The post development drainage is broken into two basins, A.1 and A.2, both of which drain to
the proposed detention pond. A summary of the combined pond basin is provided in Table 4.
Table 4 - Outfall Summary
Desc. Basins Area
(ac)
Impervious
Area (sf)
Weighted
C
Tc
(min) Q (cfs) 10-yr
Pre-Dev. A Pre-A.1 0.99 0 0.2 12 0.37
Post A A.1, A.2 0.99 23,517 0.61 6 1.73
The proposed pond was sized and designed as a detention pond discharging to the existing ditch
on the west side of the subject property. The minimum required detention volume was determined
based on the method outlined in the COB Design Standards. The minimum area is based on 145-
sf per 1-cfs of outlet flow rate from the COB Design Standards. The detention pond sizing output
is provided in Appendix A. The water quality volume is required to be stored and infiltrated on
site. The water quality volume is based on 0.5-inches of rainfall applied over the total impervious
area. A summary of the minimum pond sizing is shown in Table 5.
Stillwater Hospice – Nikles Business Park
May 2022
Project: 21‐076
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582‐0221 . Fax: (406) 582‐5770 Page 5
Table 5 - Minimum Sizing
Outfall Detention
(cf)
Retention
(cf)
10-yr
allowable
discharge
(cfs)
Minimum
Area (sf)
Actual
Bot. Area
(sf)
Water
Quality
Vol. (CF)
Pond A 525 - 0.37 54 726 931
A summary of the pond stage-storage-discharge relationship for the proposed detention pond is
outlined in Table 6. A more detailed summary for the outflow component is provided in Appendix
A. A 6” wide weir is set 1-ft off the pond bottom to provide the required water quality volume
prior to discharging from the pond. The incremental storage from a depth of 1-ft to a depth of 1.5-
ft is greater than the minimum pond detention volume based on the COB sizing method outlined
in the design standards. The table shows the pond provides adequate storage to control post
development runoff rates.
Table 6 – Pond A Pond Stage-Storage-Discharge
Stage (ft) Elevation (ft) Storage (cf) Discharge (cfs)
0.00 4728.50 0 0.00
0.10 4728.60 82 0.00
0.20 4728.70 163 0.00
0.30 4728.80 249 0.00
0.40 4728.90 341 0.00
0.50 4729.00 438 0.00
0.60 4729.10 540 0.00
0.70 4729.20 649 0.00
0.80 4729.30 763 0.00
0.90 4729.40 883 0.00
1.00 4729.50 1009 0.00
1.10 4729.60 1140 0.05
1.20 4729.70 1278 0.15
1.30 4729.80 1423 0.27
1.40 4729.90 1573 0.42
1.50 4730.00 1730 0.59
Geotechnical test pits were completed for the site on May 14, 2021 by AESI. The test pits were
generally excavated to a depth of 6.0 to 8.0 ft. The pits on the western side of the property (general
location of the proposed pond) encountered groundwater between 5.50-5.75 ft below ground
surface. A monitor well was installed near the proposed structure footprint at the north end of the
site. No water was observed in the monitor well when checked in the summer and fall of 2021.
The proposed pond will be approximately 4-ft below the existing ground surface.
Stillwater Hospice – Nikles Business Park
May 2022
Project: 21‐076
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582‐0221 . Fax: (406) 582‐5770 Page 6
4 Maintenance Plan
All proposed storm drainage features will be privately owned and maintained by the Nikles
Business Park LLC. Storm infrastructure generally consists of curb cuts, a sidewalk chase, a
detention pond, and pond outlet. The Nikles Business Park LLC (property owner) will be
responsible for the inspection and maintenance of all stormwater facilities located within the
project limits. The following maintenance plan outlines the proposed inspection and cleaning
schedule for the storm drainage facilities at the Nikles Business Park.
Detention Basin
o Inspection: Every 6-months and after large rainfall events
o Cleaning/Maintenance Schedule: Annually or as needed based on inspection
Remove sediment & litter/debris from all components of the detention
basin.
Inspect the basin for signs of erosion and repair eroded areas accordingly.
Perform spot-reseeding if necessary.
Observe drain time following rainfall events to determine if the facility is
clogged. If the observed drain time is longer than 72 hours, corrective action
must be taken to return the detention basin to the design drain time.
Regularly mow/manage all vegetation associated with the detention basin
and remove all clippings or other organic matter.
Remove sediment/debris from inlet pipes, in-flow paths, outlet pipes and
overflow paths.
The pond outlet structure will collect debris. A sump is provided on the
outlet structure and should be cleaned out by hand or a vac truck.
Inspect all components of the detention basin in accordance with an
approved inspection form. Refer to the Montana Post-Construction Storm
Water BMP Design Manual for additional information and example forms.
Curb Cut/Sidewalk Chase
o Inspection: Every 6-months and after large rainfall events
o Cleaning/Maintenance Schedule: Annually or as needed based on inspection
Clear curb cuts and sidewalk chase when partially covered with sediment,
vegetation, and/or debris. Maintenance is often needed during fall after
leaves have fallen.
Ensure snow is not built up over curb cuts and/or sidewalk chase, clear snow
and ice as necessary during winter months.
Appendix A
Supporting Calculations
AreaArea Open (sf)paved (sf) Overland flow (ft)grade (%)CCf (25‐yr)Tc (min) Length Grade Time (min) avg grade (ft/ft)length (ft)Tc (min)PRE‐DEVELOPMENT10‐yr i25‐yr I 10‐yr Q25‐yr QA.1 1.86 2.23 0.37 0.44 43,2620.9943,262 00.20 120.19 76 2.32 0.2 1.1 10.8 104 2.13% 0.74Area Area POST DEVELOPMENT10‐yr i25‐yr i10‐yr Q25‐yr Q Open (sf) paved (sf) Overland flow (ft) grade (%) C Cf (25‐yr) Tc (min) Length Grade time avg grade (ft/ft) length (ft) Tc (min)A.1 2.86 3.40 1.03 1.23 28,1760.6514,665 13,5110.56 60.10 122 2.8 0.9 1.1 2 15 1.2 0.11A.2 2.86 3.40 0.69 0.82 15,0860.355,080 10,0060.70 60.10 70 1.3 0.9 1.1 2A Combined 2.86 3.40 1.73 2.05 43,262 0.99 19,745 23,5170.616 0.10Drainage Basin OverviewIntensity (in/hr) Peak Q (cfs)Area (sf) Area (acres)Area (acres)Area (sf)Manual ‐ HEC‐22 Gutter FlowC value Tc (min) Tc (hr)Overland Tc (COB Design Standards) Manual ‐ Shallow Concentrated (TR‐55)BasinOverland Tc (COB Design Standards) Manual ‐ HEC‐22 Gutter FlowTc (hr)Tc (min)C valueManual ‐ Shallow Concentrated (TR‐55)Intensity (in/hr) Peak Q (cfs)
Pre-Paved (sf) Post Paved (sf) Additional Pavement (sf) Water Quality Volume (cf)
0 23517 23517 931
Water Quality Volume
Calculation of Required Volume for Storm Detention Pond
Pond ID Proposed Pond
Accepts flow from basins A.1, A.2
(Reference: Bozeman Stormwater Master Plan - 1982)
Design Rainfall Freq. 10 year (see page III - 5 of master plan)
IDF coefficient a 0.64
IDF coefficient b
IDF coefficient n 0.65
Pre-development Calculations Post-development Calculations
CC
Areas (ft2):open space 43,262 0.20 Areas (ft2):open space 19,745 0.20
med. res. 0.35 Impervious 23,517 0.95
dense res. 0.50
comm. neigh. 0.60
comm. down. 0.80
industrial 0.80
Total: 43,262 Total: 43,262
total area: 0.99 acres total area: 0.99 acres
composite C: 0.20 composite C: 0.61
Overland tc Overland tc
average slope: percent average slope: percent
travel distance: feet travel distance: feet
tc:minutes tc:minutes
Channel tc Channel tc
channel tc:minutes channel tc:minutes
Total tc:12 minutes Total tc:6 minutes
intensity at tc (fig 23):1.86 in/hr intensity at tc (fig 23):2.86 in/hr
pre-devel peak runoff: 0.37 cfs post-devel peak runoff: 1.73 cfs
Storm Duration Intensity Future Runoff Runoff Release Required
(minutes) (in/hr) Rate (cfs) Volume (cf) Volume (cf) Storage (cf)
6 2.86 1.73 621 133 488
8 2.37 1.43 687 178 509
10 2.05 1.24 743 222 521
12 1.82 1.10 792 266 525
14 1.65 0.99 836 311 525
16 1.51 0.91 876 355 520
18 1.40 0.84 912 400 513
20 1.31 0.79 947 444 503
22 1.23 0.74 979 488 490
24 1.16 0.70 1009 533 476
26 1.10 0.67 1038 577 461
28 1.05 0.63 1065 621 443
30 1.00 0.61 1091 666 425
32 0.96 0.58 1116 710 406
34 0.93 0.56 1140 755 385
36 0.89 0.54 1163 799 364
38 0.86 0.52 1185 843 342
40 0.83 0.50 1207 888 319
42 0.81 0.49 1227 932 295
44 0.78 0.47 1248 977 271
46 0.76 0.46 1267 1021 246
48 0.74 0.45 1286 1065 221
50 0.72 0.43 1305 1110 195
52 0.70 0.42 1323 1154 168
54 0.69 0.41 1340 1199 142
56 0.67 0.40 1357 1243 114
required detention storage (ft3) =525
detention pond COB
Pond Stage‐Storage‐Outflow Summary
POND A SUMMARY
Pond Bottom 4728.50 DO NOT TOUCH
Orifice Elevation
Orifice Size (ft) 0.00
Orifice Area (sf) 0.00 Pre‐Development Target Rate (10‐yr for COB) 0.37 cfs
Weir 1 Elevation 4729.50 Bottom Area 726 sf
Weir 1 Width (ft) 0.50 Water Quality Vol 931 cf
Weir 2 Elevation 4730.00 Minimum Required Detention Storage 525 cf
Weir 2 Width (ft) 3.10
Stage Elev Storage (cf) Orifice Head Orifice Outflow Weir 1 Head Weir 1 Outflow Weir 2 Head Weir 2 Ouflow Total Outflow Stage Elev
0.00 4728.50 0 4728.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 4728.50
0.10 4728.60 82 4728.60 0.00 0.00 0.00 0.00 0.00 0.00 0.10 4728.60
0.20 4728.70 163 4728.70 0.00 0.00 0.00 0.00 0.00 0.00 0.20 4728.70
0.30 4728.80 249 4728.80 0.00 0.00 0.00 0.00 0.00 0.00 0.30 4728.80
0.40 4728.90 341 4728.90 0.00 0.00 0.00 0.00 0.00 0.00 0.40 4728.90
0.50 4729.00 438 4729.00 0.00 0.00 0.00 0.00 0.00 0.00 0.50 4729.00
0.60 4729.10 540 4729.10 0.00 0.00 0.00 0.00 0.00 0.00 0.60 4729.10
0.70 4729.20 649 4729.20 0.00 0.00 0.00 0.00 0.00 0.00 0.70 4729.20
0.80 4729.30 763 4729.30 0.00 0.00 0.00 0.00 0.00 0.00 0.80 4729.30
0.90 4729.40 883 4729.40 0.00 0.00 0.00 0.00 0.00 0.00 0.90 4729.40
1.00 4729.50 1009 4729.50 0.00 0.00 0.00 0.00 0.00 0.00 1.00 4729.50
1.10 4729.60 1140 4729.60 0.00 0.10 0.05 0.00 0.00 0.05 1.10 4729.60
1.20 4729.70 1278 4729.70 0.00 0.20 0.15 0.00 0.00 0.15 1.20 4729.70
1.30 4729.80 1423 4729.80 0.00 0.30 0.27 0.00 0.00 0.27 1.30 4729.80
1.40 4729.90 1573 4729.90 0.00 0.40 0.42 0.00 0.00 0.42 1.40 4729.90
1.50 4730.00 1730 4730.00 0.00 0.50 0.59 0.00 0.00 0.59 1.50 4730.00
1.60 4730.10 1893 4730.10 0.00 0.60 0.77 0.10 0.27 1.05 1.60 4730.10
1.70 4730.20 2063 4730.20 0.00 0.70 0.98 0.20 0.77 1.75 1.70 4730.20
1.80 4730.30 2239 4730.30 0.00 0.80 1.19 0.30 1.42 2.61 1.80 4730.30
1.90 4730.40 2299 4730.40 0.00 0.90 1.42 0.40 2.19 3.61 1.90 4730.40
2.00 4730.50 2484 4730.50 0.00 1.00 1.67 0.50 3.06 4.73 2.00 4730.50
Appendix B
Storm Drainage Plans