HomeMy WebLinkAboutStorm Drainage Report
Storm Drainage
Design Report
Regional Park Maintenance Building
Bozeman, Montana
September 27, 2022
Prepared by:
Madeline Good, EI
Rory Romey, PE
Gallatin County Regional Park – Maintenance Building
September 2022
Project: 22‐002
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 Ponds ................................................................................................................................ 3
4 Maintenance Plan .................................................................................................................... 5
Table 1 - Pre-Development Hydrology ........................................................................................... 2
Table 2 - Post Development Hydrology ......................................................................................... 3
Table 3 - Inlet and Gutter Summary (25-year event) ...................................................................... 3
Table 4 – Retention Pond Sizing .................................................................................................... 4
Table 5 – Pond Stage-Storage ......................................................................................................... 4
Attachments:
Exhibit 1.1 – Vicinity Map - Topo
Exhibit 1.2 – Aerial Map
Exhibit 2.1 – Post Development Drainage
Appendix A – Supporting Calculations
Appendix B – Storm Drainage Plans
Gallatin County Regional Park – Maintenance Building
September 2022
Project: 22‐002
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 known as the Gallatin County Regional Park, located in the NE ¼ of
Section 03, Township 02S, Range 5E. Vicinity maps showing the project location and extents are
provided with Exhibits 1.1 and 1.2. The proposed site improvements will involve construction of
a maintenance building and associated parking lot along Vaquero Parkway, west of the existing
Dinosaur Park Playground. A new retention pond will be provided east of the proposed
maintenance building to meet the 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), gravel (C=0.8), 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 basin data is provided in Appendix A. Basins, flow paths, and the general site layout for the
post-development condition can be found on Exhibit 2.1.
2.1 Pre‐Development
A single basin (Basin A) was assumed for the proposed improvement area with consistent basin
boundaries between the pre- and post-development site conditions for the purpose of pond sizing.
The maintenance building site currently consists of a gravel parking lot and undeveloped open
space. Basin A was conservatively assumed to be entirely open space prior to the proposed
development. Stormwater runoff sheet flows northwest across the gravel lot to a minor drainage
path paralleling the northern property boundary. From here, runoff continues to flow
west/northwest. 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.
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 0.36 0 15,487 0.20 14 0.12 0.14
Gallatin County Regional Park – Maintenance Building
September 2022
Project: 22‐002
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 3
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 areas and infrastructure is provided on Exhibit 2.1. Basin
A encompasses the proposed site improvements, including the maintenance building, gravel
storage area, and proposed parking lot. Basin A will be routed to a proposed retention pond located
east of the proposed parking lot.
Table 2 - Post Development Hydrology
Basin Area
(acres)
Impervious
(sf)
Gravel
(sf)
Open
Space
(sf)
Weighted
C
Tc
(min)
10-yr
Peak
Runoff
(cfs)
25-yr
Peak
Runoff
(cfs)
A 0.36 6,280 5,775 3,432 0.73 6 0.74 0.88
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 proposed parking lot has a max cross slope of 1.2%; the maximum allowable spread width is
then approximately 20-ft. The gutter flow capacity is based on the HEC-22 gutter flow
methodology for a composite section. Street/gutter capacity is 11.25-cfs for a cross slope of 1.2%
and a longitudinal grade of 1.86%. The calculated peak flow rate is less than the street/gutter
capacity. A 3-ft wide curb cut will be used to convey runoff from Basin A to the proposed retention
pond. The curb cut was modeled as a 3-ft wide weir; the calculated flow depth was 0.20-ft. Sample
calculations are provided in Appendix A. A summary of inlet and gutter flow is provided in Table
3. The proposed curb cut is located at a sag in the curb line.
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)
Curb Cut A.1 1.2 0.88 4736.90 4736.75 4736.65 11.7
3.2 Ponds
The pre-development 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 site. The retention pond was sized for the 10-year, 2-hour storm intensity as
Gallatin County Regional Park – Maintenance Building
September 2022
Project: 22‐002
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 4
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)
Actual Pond
Bottom Area
(SF)
Actual Storage
Volume
(CF)
1 760 502 379 1,028
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 the post-development runoff rates for the 10-year, 2-hour
event. In the event that the pond overtops, runoff will sheet flow south/southwest across the
existing gravel lot and either infiltrate or evaporate.
Table 5 – Pond Stage-Storage
Stage (ft) Elevation (ft) Storage (cf)
0.00 4735.00 0
0.10 4735.10 39
0.20 4735.20 82
0.30 4735.30 129
0.40 4735.40 180
0.50 4735.50 234
0.60 4735.60 293
0.70 4735.70 357
0.80 4735.80 424
0.90 4735.90 496
1.00 4736.00 573
1.10 4736.10 654
1.20 4736.20 740
1.30 4736.30 831
1.40 4736.40 927
1.50 4736.50 1028
Geotechnical test pits were completed by AESI within the Regional Park fenced dog park area,
approximately 1,500 feet west of the proposed maintenance building, on October 20, 2014. The
test pits were generally excavated to a depth of 8-ft. Groundwater was encountered between 4.00
to 4.25 ft below ground surface. Clean, sandy gravel was encountered between 1.5 to 2 ft below
ground surface. Additional data was obtained from the Geotechnical Report included in the Park
View Crossing Major Subdivision Preliminary Plat application (COB project No. 21158). The test
bore location map included in the report shows borehole ST-32 is located within the Regional
Gallatin County Regional Park – Maintenance Building
September 2022
Project: 22‐002
Bozeman Office . 32 Discovery Drive . Bozeman, Montana 59718 . Ph: (406) 582-0221 . Fax: (406) 582-5770 Page 5
Park, in the general location of the proposed maintenance building. The boring was performed on
September 21st, 2001 and indicated a groundwater depth of 9-ft below existing ground. The bore
hole consisted of a “poorly graded sand with gravel” lens from 1.5-ft below ground surface (BGS)
extending to 3-ft BGS followed by a “poorly graded gravel with sand and silt” lens starting 3-ft
BGS and extending to the bottom of the borehole (approximately 10.5-ft below existing ground).
The proposed retention pond bottom is approximately 2-ft below existing ground and therefore,
groundwater is not anticipated to be an issue.
4 Maintenance Plan
All proposed storm drainage features will be privately owned and maintained by Gallatin County.
Storm infrastructure generally consists of a curb cut and retention pond. Gallatin County (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 Regional Park Maintenance Building.
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 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.
Curb Cut
o Inspection: Every 6-months and after large rainfall events
o Cleaning/Maintenance Schedule: Annually or as needed based on inspection
Clear curb cuts 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, clear snow and ice as necessary
during winter months.
FIGURECivil Engineering
Geotechnical Engineering
Land Surveying
32 DISCOVERY DRIVE . BOZEMAN, MT 59718
PHONE (406) 582-0221 . FAX (406) 582-5770
www.alliedengineering.com
REGIONAL PARK MAINTENANCE BLDG
VICINITY MAP
BOZEMAN, MT
1.1
N
FIGURECivil Engineering
Geotechnical Engineering
Land Surveying
32 DISCOVERY DRIVE . BOZEMAN, MT 59718
PHONE (406) 582-0221 . FAX (406) 582-5770
www.alliedengineering.com
REGIONAL PARK MAINTENANCE BLDG
AERIAL MAP
BOZEMAN, MT
1.2
N
Appendix A
Supporting Calculations
AreaArea Open (sf)paved (sf) Overland flow (ft)grade (%)CCf (25‐yr)Tc (min) Length Grade Time (min)PRE‐DEVELOPMENT10‐yr i25‐yr I 10‐yr Q25‐yr QA 1.62 1.95 0.12 0.14 15,4870.3615,487 0 00.20 140.24 109 1.71 0.2 1.1 14.4Area 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 timeA 2.86 3.40 0.74 0.88 15,4870.363,432 6,280 5,7750.73 60.10 72 1.6 0.95 1.1 0.8 13 11.85 0.03C 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)Area Gravel (sf)Area Gravel (sf)Drainage Basin OverviewIntensity (in/hr) Peak Q (cfs)Area (sf) Area (acres)Area (acres)Area (sf)
Calculation of Required Volume for Stormwater Pond
Pond ID Proposed Pond
Accepts flow from basins 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 15,487 0.20 Areas (ft2):Gravel 5,775 0.80
Impervious 6,280 0.95
Open Space 3,432 0.20
Total: 15,487 Total: 15,487
total area: 0.36 acres total area: 0.36 acres
composite C: 0.20 composite C: 0.73
Overland tc Overland tc
average slope: 1.71 percent average slope: 1.56 percent
travel distance: 109 feet travel distance: 72 feet
C: 0.2 C: 0.95
Cf (25-year) 1.1 Cf (25-year) 1.1
Overland tc:14.4 minutes Overland tc:0.8 minutes
Shallow Concentrated tc Shallow Concentrated tc
average slope: average slope: 11.85 percent
travel distance: travel distance: 13 feet
shallow concentrated tc:minutes shallow concentrated tc:0.03 minutes
Total tc:14 minutes Total tc:6 minutes
intensity at tc (fig 23):1.62 in/hr intensity at tc (fig 23):2.86 in/hr
pre-devel peak runoff: 0.12 cfs post-devel peak runoff: 0.74 cfs
Storm Duration Intensity Future Runoff Runoff Release Required
(minutes) (in/hr) Rate (cfs) Volume (cf) Volume (cf) Storage (cf)
6 2.86 0.74 266 41 225
8 2.37 0.61 295 55 239
10 2.05 0.53 318 69 249
12 1.82 0.47 339 83 257
14 1.65 0.43 358 97 262
16 1.51 0.39 375 111 265
18 1.40 0.36 391 124 267
20 1.31 0.34 406 138 268
22 1.23 0.32 420 152 268
24 1.16 0.30 433 166 267
26 1.10 0.29 445 180 265
28 1.05 0.27 457 194 263
30 1.00 0.26 468 207 260
32 0.96 0.25 478 221 257
34 0.93 0.24 489 235 254
36 0.89 0.23 499 249 250
38 0.86 0.22 508 263 246
40 0.83 0.22 517 276 241
42 0.81 0.21 526 290 236
44 0.78 0.20 535 304 231
46 0.76 0.20 543 318 225
48 0.74 0.19 551 332 220
50 0.72 0.19 559 346 214
52 0.70 0.18 567 359 208
54 0.69 0.18 575 373 201
56 0.67 0.17 582 387 195
required detention storage (ft3) =268
Retention Pond Calculations:
Q = CIA
C = 0.73 (post-development)
I = 0.41 in/hr (10-yr, 2-hr storm)
A = 0.36 acres
Q = 0.11 cfs
required retention storage (ft3) =760 ft3
detention pond COB
Pre Paved/Gravel (sf) Post Paved/Gravel (sf) Additional Pavement/Gravel (sf) Water Quality Volume (cf)
0 12055 12055 502
Water Quality Volume
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 18.75 Spread Width on Street, ft
Sx 0.012 Street Cross Slope, ft/ft
SL 0.0186 Longitudinal Slope (ft/ft)
Street Flow Qs 9.12 Based on Triangular Section
Maximum Allowable Total flow rate
Sw 0.0625 Gutter pan cross slope, ft/ft
Sx 0.012 Street Cross Slope, ft/ft
W 1.25 Gutter Width from edge of pavement to flow line, ft
T 20 Overall Spread Width ‐ measured from flow line, ft
Sw/Sx 5.208333
T/W 16
Eo 0.1893
Ts 18.75
Q 11.254 Total Flow Rate (cfs)
Basin A
𝐸 ൌ 1
1
𝑆௪𝑆௫
1
𝑆௪𝑆௫𝑇𝑊െ 1
ଶ.
െ 1
𝑄௦ ൌሺ𝐾௨
𝑛ሻ𝑆௫ଵ.𝑆.ହ𝑇௦ଶ.
𝑄ൌ 𝑄௦
1 െ𝐸
Appendix B
Storm Drainage Plans