HomeMy WebLinkAbout605 E Bryant - STORM DRAINAGE REPORTRanieri Site Plan
June 2023
Project: 22-074
Storm Drainage Report
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 Drainage Swale ................................................................................................................ 4
3.3 Ponds ................................................................................................................................ 4
4 Maintenance Plan .................................................................................................................... 6
Table 1 - Pre-Development Hydrology........................................................................................... 3
Table 2 - Post Development Hydrology ......................................................................................... 3
Table 3 - Inlet and Gutter Summary (25-year event) ...................................................................... 4
Table 4 – Retention Pond Sizing .................................................................................................... 4
Table 5 – Pond Stage-Storage ......................................................................................................... 5
Attachments:
Exhibit 1.1 – Vicinity Map - Topo
Exhibit 1.2 – Aerial Map
Exhibit 2.1 – Pre-Development Hydrology
Exhibit 2.2 – Post Development Hydrology
Appendix A – Supporting Calculations
Appendix B – Storm Drainage Plans
Ranieri Site Plan
June 2023
Project: 22-074
Storm Drainage Report
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 605 E Bryant Street in Bozeman, Montana. Vicinity maps
showing the project location are provided with Exhibits 1.1 and 1.2. The proposed site
improvements will involve construction of two adjoining shop units with garages, each with 1
ADU living space above. A new parking lot is included as part of the improvements. An existing
single-family residence is located on the property and will remain in its current condition. A new
retention pond will be provided east of the existing house to meet water quality and peak runoff
rate requirements.
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 runoff 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 impervious surfaces (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 basin
data is provided in Appendix A. Basins, flow paths, and the general site layout for the pre- and
post-development conditions can be found on Exhibits 2.1 and 2.2, respectively.
2.1 Pre-Development
A single basin (Basin Pre-A) was assumed for the site with consistent basin boundaries between
the pre- and post-development site conditions for the purpose of pond sizing. The site currently
consists of a single-family dwelling, gravel parking area, and open space. A significant portion of
the runoff is generated from the neighboring property to the west. Runoff from the neighboring
property is currently routed to the project site via an existing concrete valley gutter. Run-on from
the neighboring property is routed through the project site. There were no existing stormwater
facilities observed on the adjacent property. All on-site areas within Basin Pre-A were
conservatively assumed to be entirely open space prior to the proposed development. Existing
paved/impervious areas within the offsite area of Basin Pre-A (i.e., neighboring property to the
west) are included in the pre-development hydrology calculations. Bozeman Creek runs along the
eastern property line. Stormwater runoff generally flows east and outfalls to Bozeman Creek. 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.
An overview of the pre-development drainage is provided on Exhibit 2.1. It should be noted that
the stormwater facility sizing conservatively accounts for the adjacent property along with the
project site.
Ranieri Site Plan
June 2023
Project: 22-074
Storm Drainage Report
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)
Pre-A 0.75 9,035 23,835 0.41 12 0.56 0.67
*Impervious area only considers offsite areas. The subject property was conservatively assumed to be entirely open
space prior to the proposed development.
2.2 Post Development
Input parameters and results for the post development drainage basin are summarized in Table 2.
An overview plan showing the drainage area and infrastructure is provided on Exhibit 2.2. Basin
Post-A encompasses the existing and proposed site improvements, including the proposed
shop/ADU units, proposed parking lot, and existing house. Offsite drainage from the west is
included in the drainage area, which consists of a portion of an existing parking lot & building.
Basin Post-A will be routed to a proposed retention pond located east of the existing house.
Table 2 - Post 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)
Post-A 0.75 18,315 14,555 0.62 6 1.33 1.59
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
resulting in an allowable depth of 0.3-feet at the flow line. Based on the proposed parking lot cross
slope of 1.5%; the maximum allowable spread width is then approximately 16.25-ft. The gutter
flow capacity is based on the HEC-22 gutter flow methodology for a composite section.
Street/gutter capacity is 12.17-cfs for a cross slope of 1.5% and a longitudinal grade of 3.1%. The
calculated peak flow rate is about 2-cfs and is less than the street/gutter capacity. A 4-ft wide
sidewalk chase will be used to convey runoff from Basin Post-A to a proposed drainage swale,
which will then outfall to the proposed retention pond. The sidewalk chase was modeled as a 4-ft
wide weir; the calculated flow depth was 0.24-ft. Sample calculations are provided in Appendix
A. A summary of inlet and gutter flow is provided in Table 3. The proposed sidewalk chase is
located at a sag in the curb line.
Ranieri Site Plan
June 2023
Project: 22-074
Storm Drainage Report
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 1.5 1.59 4731.93 4731.78 4731.72 7.70
3.2 Drainage Swale
As mentioned previously, a drainage swale will be used to convey runoff from the sidewalk chase
to the retention pond. The proposed swale is approximately 40-ft long, 4-ft wide (top width), and
0.5-ft deep. Autodesk Hydraflow Express was used to estimate the drainage swale capacity;
outputs are provided in Appendix A. The 25-year peak runoff rate from basin Post-A was estimated
to be 1.59 cfs. The drainage swale has an estimated capacity of 1.73 cfs and is therefore adequately
sized.
3.3 Ponds
The pre- and post-development basin summaries were previously provided in Section 2.
Stormwater runoff from the proposed development will drain to a proposed retention pond on the
east side of the existing house. The retention pond was sized for the 10-year, 2-hour storm as
specified in the COB Design Standards. The water quality volume, which includes runoff
generated from the first 0.5-in of rainfall on impervious areas, is required to be stored and
infiltrated on site. The retention pond sizing calculations are provided in Appendix A. A summary
of the minimum pond sizing and design parameters is shown in Table 4.
Table 4 – Retention Pond Sizing
Pond
Minimum Required
Retention Volume
(cf)
Water Quality
Volume
(cf)
Proposed Pond
Bottom Area
(sf)
Proposed Storage
Volume
(cf)
1 1,369 369 488 1,419
A summary of the pond stage-storage relationship is outlined in Table 5. The storage volume was
determined based on a volume analysis completed in Autodesk Civil 3D. Table 5 shows the pond
provides adequate storage to control post-development runoff rates for the 10-year, 2-hour design
event. In the event that the pond overtops, runoff will flow east toward Bozeman Creek.
Ranieri Site Plan
June 2023
Project: 22-074
Storm Drainage Report
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 5
Table 5 – Pond Stage-Storage
Stage (ft) Elevation (ft) Contour Area (sf) Storage (cf)
0.00 4730.00 488 0
0.10 4730.10 545 52
0.20 4730.20 602 57
0.30 4730.30 661 63
0.40 4730.40 720 69
0.50 4730.50 781 75
0.60 4730.60 843 81
0.70 4730.70 905 87
0.80 4730.80 969 94
0.90 4730.90 1,033 678
1.00 4731.00 1,099 785
1.10 4731.10 1,165 898
1.20 4731.20 1,233 1,018
1.30 4731.30 1,302 1,145
1.40 4731.40 1,371 1,278
1.50 4731.50 1,442 1,419
Groundwater data from the Gallatin City County Environmental Health Services online GIS map
was utilized to estimate depth to groundwater at the site. Monitoring wells nearest to the site were
located approximately 1,000-ft from the proposed retention pond and were generally located near
Bozeman Creek. The highest recorded groundwater depth in the monitoring wells was 67” below
existing ground. An existing COB public detention pond is located just south of the subject
property, within the Bryant Street Right of Way. The detention pond is shown on record drawings
from SID 599 (as well as the City Infrastructure Viewer) and was observed during the topographic
survey of the site. The existing pond bottom is approximately 2-ft below adjacent ground
elevations and does not show ponding based on aerial imagery or site observations. The proposed
on-site retention pond bottom is approximately 2-ft below existing ground indicating that ground
water should not be an issue.
Ranieri Site Plan
June 2023
Project: 22-074
Storm Drainage Report
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 property
owner (Daniel & Simia Ranieri). Storm infrastructure generally consists of a sidewalk chase,
drainage swale, & retention pond. The 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 subject property.
Retention 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 retention basin.
Inspect the basin for signs of erosion and repair eroded areas accordingly.
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 retention basin to the design drain time.
Regularly mow/manage all vegetation associated with the retention basin
and remove all clippings or other organic matter.
Remove sediment/debris from in-flow and overflow paths.
Inspect all components of the retention 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.
Vegetated Drainage Swale
o Inspection: Every 6-months and after large rainfall events
o Cleaning/Maintenance Schedule: Annually or as needed based on inspection.
Remove litter/debris from all components of the swale.
Regularly mow/manage all vegetation within the swale and remove all
clippings or other organic matter.
Repair undercut and eroded areas as needed at swale inflow and outflow
locations.
Examine channel bottom for evidence of erosion, braiding, excessive
ponding or dead grass.
Check inflow points for clogging and remove any sediment.
Inspect side slopes for evidence of erosion.
Sidewalk Chase
o Inspection: Every 6-months and after large rainfall events
o Cleaning/Maintenance Schedule: Annually or as needed based on inspection.
Clear sidewalk chase openings 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 sidewalk chase openings, clear snow and
ice as necessary during winter months.
Appendix A
Supporting Calculations
Drainage Basin Summary Sheet
Water Quality Volume Calculation
Retention Pond Sizing Calculation
Gutter Flow Capacity Calculation
Hydraflow Express Swale Capacity Results
Pond State Storage
Hydraulic Calculations
Project: Ranieri Site PlanProject Number: 22-074Location: Bozeman, MTAreaArea Open (sf)paved (sf) Overland flow (ft)grade (%)CCf (25‐yr)Tc (min) Length Grade k Time (min)PRE‐DEVELOPMENT10‐yr i25‐yr I 10‐yr Q25‐yr QPre‐A 1.82 2.18 0.56 0.67 32,8700.7523,835 9,0350.41 12.00.20 44 0.93 0.2 1.1 11.18 127 2.31% 0.491 0.86Area Area POST DEVELOPMENT10‐yr i25‐yr i10‐yr Q25‐yr QOpen (sf) paved (sf) Overland flow (ft) grade (%) C Cf (25‐yr) Tc (min) Length Grade k timePost‐A 2.86 3.40 1.33 1.59 32,8700.7514,555 18,3150.62 60.10 18 3.3 0.9 1.1 0.58 195 0.97% 0.619 1.61Drainage Basin OverviewIntensity (in/hr) Peak Q (cfs)Area (sf) Area (acres)Area (acres)Area (sf)**Column Headers that are underlined and red are manual inputsC value Tc (min) Tc (hr)Overland Tc (COB Design Standards) Manual ‐ Shallow Concentrated (TR‐55)BasinOverland Tc (COB Design Standards)Tc (hr)Tc (min)C valueManual ‐ Shallow Concentrated (TR‐55)Intensity (in/hr) Peak Q (cfs)
Project: Ranieri Site Plan
Project Number: 22-074
Location: Bozeman, MT
Pre-Paved (sf) Post Paved (sf) Additional Pavement (sf) Water Quality Volume (cf)
0 8844 8844 369
Water Quality Volume
Note: Post-Development paved/impervious area only includes those areas within the subject
property (offsite areas not included)
Project: Ranieri Site Plan
Project Number: 22-074
Location: Bozeman, MT
Calculation of Required Volume for Storm Detention/Retention Pond
Pond ID Proposed Detention Pond
Accepts flow from basins Post-A
(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 23,835 0.20 Areas (ft2):open space 14,555 0.20
Impervious 9,035 0.95 Impervious 18,315 0.95
Total: 32,870 Total: 32,870
total area: 0.75 acres total area: 0.75 acres
composite C: 0.41 composite C: 0.62
Overland tc 11.18 minutes Overland tc 0.58 minutes
Channel tc 0.86 minutes Channel tc 1.61 minutes
Total tc:12.0 minutes Total tc:6 minutes
intensity at tc (fig 23):1.82 in/hr intensity at tc (fig 23):2.86 in/hr
pre-devel peak runoff: 0.56 cfs post-devel peak runoff: 1.33 cfs
Storm Duration Intensity Future Runoff Runoff Release Required
(minutes) (in/hr) Rate (cfs)Volume (cf)Volume (cf) Storage (cf)
6 2.86 1.33 480 201 279
8 2.37 1.11 531 267 263
10 2.05 0.96 574 334 240
12 1.82 0.85 612 401 210
14 1.65 0.77 646 468 178
16 1.51 0.70 676 535 142
18 1.40 0.65 705 602 103
20 1.31 0.61 731 669 63
22 1.23 0.57 756 735 21
24 1.16 0.54 780 802 -23
required detention storage (ft3) =279
Retention Pond Calculations:
Q = CIA
C = 0.62 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.75 acres
Q = 0.19 cfs
required retention storage (ft3) =1,369 ft3
detention pond COB 22-074
Street/Gutter Capacity ‐ Composite Section
See 4‐11 of HEC‐22 Third Edition
Qw Flow Rate in depressed section (gutter pan), cfs
Q Total Flow Rate, cfs
Qs Flow rate in raised section (street section), cfs
Eo Ratio of flow in the gutter to the total flow (Qw/Q)
Sx Street cross slope, ft/ft
SL Longitudinal Slope, ft/ft
Sw Gutter pan cross slope, ft/ft
W Gutter Width from edge of pavement to flow line, ft
T Overall Spread Width ‐ measured from flow line, ft
Ts Spread Width on Street, ft
Ku 0.376 metric (0.56 English)
n Manning's Coefficient
Maximum Allowable Steet Flow Rate
Ku 0.56 0.376 metric (0.56 English)
n 0.013 Manning's Coefficient
Ts 15 Spread Width on Street, ft
Sx 0.015 Street Cross Slope, ft/ft
SL 0.031 Longitudinal Slope (ft/ft)
Street Flow Qs 9.42 Based on Triangular Section
Maximum Allowable Total flow rate
Sw 0.0625 Gutter pan cross slope, ft/ft
Sx 0.015 Street Cross Slope, ft/ft
W 1.25 Gutter Width from edge of pavement to flow line, ft
T 16.25 Overall Spread Width ‐ measured from flow line, ft
Sw/Sx 4.166667
T/W 13
Eo 0.2259
Ts 15
Q 12.173 Total Flow Rate (cfs)
Basin A
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Channel Report
Hydraflow Express Extension for Autodesk® Civil 3D® by Autodesk, Inc. Wednesday, Jun 28 2023
<Name>
Triangular
Side Slopes (z:1) = 4.00, 4.00
Total Depth (ft) = 0.50
Invert Elev (ft) = 4731.00
Slope (%) = 0.98
N-Value = 0.033
Calculations
Compute by: Q vs Depth
No. Increments = 10
Highlighted
Depth (ft) = 0.50
Q (cfs) = 1.733
Area (sqft) = 1.00
Velocity (ft/s) = 1.73
Wetted Perim (ft) = 4.12
Crit Depth, Yc (ft) = 0.42
Top Width (ft) = 4.00
EGL (ft) = 0.55
0 .5 1 1.5 2 2.5 3 3.5 4 4.5 5
Elev (ft) Depth (ft)Section
4730.75 -0.25
4731.00 0.00
4731.25 0.25
4731.50 0.50
4731.75 0.75
4732.00 1.00
Reach (ft)
Depth Q Area Veloc Wp Yc TopWidth Energy
(ft) (cfs) (sqft) (ft/s) (ft) (ft) (ft) (ft)
0.05 0.004 0.010 0.37 0.41 0.04 0.40 0.05
0.10 0.024 0.040 0.59 0.82 0.08 0.80 0.11
0.15 0.070 0.090 0.78 1.24 0.12 1.20 0.16
0.20 0.150 0.160 0.94 1.65 0.16 1.60 0.21
0.25 0.273 0.250 1.09 2.06 0.20 2.00 0.27
0.30 0.444 0.360 1.23 2.47 0.24 2.40 0.32
0.35 0.669 0.490 1.37 2.89 0.29 2.80 0.38
0.40 0.956 0.640 1.49 3.30 0.33 3.20 0.43
0.45 1.308 0.810 1.62 3.71 0.37 3.60 0.49
0.50 1.733 1.000 1.73 4.12 0.42 4.00 0.55
Project: Ranieri Site Plan
Project Number: 22-074
Location: Bozeman, MT
Pond Stage‐Storage Summary
POND A SUMMARY
Pond Bottom 4730.00
Stage Elev Contour Area (sf) Storage (cf)
0.00 4730.00 488 0
0.10 4730.10 545 52
0.20 4730.20 602 57
0.30 4730.30 661 63
0.40 4730.40 720 69
0.50 4730.50 781 75
0.60 4730.60 843 81
0.70 4730.70 905 87
0.80 4730.80 969 94
0.90 4730.90 1033 678
1.00 4731.00 1099 785
1.10 4731.10 1165 898
1.20 4731.20 1233 1018
1.30 4731.30 1302 1145
1.40 4731.40 1371 1278
1.50 4731.50 1442 1419
Gallatin County GIS
311 W. Main Room 305Bozeman, MT 59715
Phone: (406) 582-3049
Email: GIS@gallatin.mt.gov
June 20, 2023
Gallatin County GIS
±
0 0.035 0.07 0.105 0.14
Miles
Water
Addresses
Minor Roads
Numbered Highways
StreamsRiversLakes
Structure Addresses
Recent AddressesParcels
Cities
Quarter Sections
Subdivisions
Minor Subdivisions
"
"
Sections
Townships
Alley/Driveway
Unpaved
Paved
Rail Line
Montana
U.S.
Interstates
Public Lands
Local Government
State of Montana
U.S. Bureau of Land Management
U.S. Forest Service
U.S. Park Service
GW Monitor Well: 2016-25-01
Highest Recorded GW Depth = 68" BGS
GW Monitor Well: 2016-25-02
Highest Recorded GW Depth = 67" BGS
GWIC Well 220564
Static Water Level = 5-ft
Subject Property
Appendix B
Storm Drainage Plans