HomeMy WebLinkAbout010_Stormwater Engineering ReportEngineering Report
Stormwater Design
701 N. Ferguson Avenue
February 2023
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 2/17/2023
Engineering Report
Table of Contents
General Information and Design Criteria
Existing Site Conditions
Storm Drainage Plan Description
Estimation of Runoff
Retention Facilities
Storm Event Analysis
Pipe and Curb Conveyance Capacity
Facilities Maintenance
Figures and Tables
Figure 1. Vicinity Map
Figure 2. Stormwater Plan
Table 1. Storm Runoff Calculations
Table 2. Retention Basin Capacities
Table 3. Enhanced Infiltration Capacities
Table 4. Stormwater Runoff Rate Calculations
Table 5. Pipe Conveyance Capacities
Table 6. Curb and Gutter Conveyance Calculations
Table 7. Bozeman Rainfall Analysis (1990-2020)
Stormwater Plan 2 2/17/2023
General Information and Design Criteria
The proposed Trinity Church Development is located northwest of the
intersection of Durston Road and Ferguson Avenue. The proposed project
consists of a 2-phase site plan to construct a church and related accessory
facilities. Phase One includes initial construction of the church building and
associated facilities; Phase 2 includes a large addition to the building and
additional accessory facilities. The project is proposed on the currently
undeveloped 8.818-acre Lot 1 of Diamond Estates Subdivision Number 2 Phase
1.
The proposed two-phase Church building will consist of typical uses for a church
of this size. Phase 1 will consist of 10,100 square feet of floor space containing a
ministry center and offices for church staff. Phase 2 will include 30,565 square
feet of floor space and will include classrooms, a nursery, and a large fellowship
hall for worship services. Each phase will include associated parking, sidewalk,
signage, landscaping, etc. to serve the function of the building.
Stormwater mitigation is achieved by 5 traditional retention ponds in addition to
using efficient stormwater management design utilizing Low Impact Design (LID)
methodology to keep stormwater runoff to pre-development levels. 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 Owner’s contact information is:
Gallatin Valley Presbyterian Church a/k/a “Trinity Church”
4720 Classical Way
Bozeman, MT 59715
Stormwater Plan 3 2/17/2023
Figure 1: Vicinity Map, Bozeman, Montana
Existing Site Conditions
The Trinity Church project is located on the northwest quadrant of Durston Rd.
and Ferguson Ave. intersection and is currently undeveloped. The site slopes
slightly to the north but is generally flat within the project limits. Stormwater is
currently unmitigated on this site and generally flows to a drainage ditch on the
west side of the property.
Storm Drainage Plan Description
Due to high groundwater, the storm drainage plan for the Trinity Church consists
of a combination of traditional stormwater retention ponds designed to infiltrate
stormwater and replicate the natural hydrology of the site.
The overall storm drainage plan for the Trinity Church is depicted on Figure 2,
attached to this report. The plan shows proposed drainage flow paths, drainage
basins, and retention ponds. With development, the site will be divided into 5
primary drainage basins. Basin 1 contains the far western portion of the site.
Basin 2 contains west-central portion of the site. Basins 3 and 4 contains the
north-central of the site. Basin 5 is located along the eastern boundary of the site.
Stormwater Plan 4 2/17/2023
Storm runoff from each basin will be mitigated by detention/infiltration systems
located on common open spaces. A 10-yr, 2-hr design storm is utilized for these
basins.
Estimation of Runoff
Runoff estimates were obtained for each drainage basin using City of Bozeman
standards. For the stormwater detention 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).
Table 1 – Storm Runoff Calculations
Basin Characteristics Basin 1 Basin 2 Basin 3 Basin 4 Basin 5
Area (ft²) 96,665 164,777 48,044 30,455 40,790
Area (acre) 2.22 3.78 1.10 0.70 0.94
Pervious Area (ft^2) 19333 49433 9609 6091 8158
Impervious Area (ft^2) 77332 115344 38435 24364 32632
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 ) 0.76 0.69 0.76 0.76 0.76
Runoff Volume (cf) 10-yr 2-
hr storm 5020 7769 2495 1582 2118
Flow Rate Q (cfs) 25-yr 5-
min Storm 6.453 9.986 3.207 2.033 2.723
Retention Facilities
The site contains 5 retention facilities for stormwater management. Table 2
describes the characteristics of each retention basin and total site storage.
Stormwater Plan 5 2/17/2023
Table 2 – Retention Basin Capacities
Drainage Area 2 1 3 4 5
Retention Basin ID 1 1B 2 3 4
Mitigation Type Retention Retention Retention Retention Retention
Ponding Depth (ft) 1.5 1.5 1.5 1.5 1.5
Pond Bottom Area (ft^2) 4335 2959 3003 1340 1023
Pond Top Area (ft^2) 6680 4923 6074 2914 2122
Average Ponding Area (ft²) 5508 3941 4539 2127 1573
Total Ponding Storage (ft^3) 8261 5912 6808 3191 2359
Rain Event Captured (in) 1.35 1.35 0.72 1.05 1.22
Total System Storage Capacity (ft3)
25,107
In addition to providing adequate ponding storage, the retention facilities feature
enhanced infiltration to provide further stormwater mitigation. Approximately 50%
of the surface area of the bottom of each retention basin will have a strip of
gravel that extends to native gravel to facilitate infiltration. This design provides
superior infiltration as compared to a typical retention basin. The effects of
enhanced infiltration have not been included in design values, so it is possible for
these basins to provide greater mitigation performance than indicated in this
design report.
Detention Basin 1B is designated as a Phase 2 improvement. This will allow
adequate facilities to service higher runoff rates following Phase-2 development.
The other four basins are each proposed as Phase-1 improvements.
Table 3 – Enhanced Infiltration Calculations
Detention Basin ID 1 1B 2 3 4
Total Storage Capacity (cf) 8261 5912 6808 3191 2359
Gravel Strip Surface Area (ft^2) 2168 1480 1502 670 512
Gravel Infiltration Rate (in/hr) 1.67 1.67 1.67 1.67 1.67
Time for Complete Infiltration (hr) 27.4 28.7 32.6 34.2 33.1
As shown in Table 3, all basins will drain completely in under 48 hours following
the design storm event.
Conveyance Capacity
Stormwater runoff in each sub-basin will be collected into a storm sewer system
for conveyance to retention areas. The pipe network was designed to provide
adequate conveyance capacity for the 25-year, 5-minute design storm event.
Stormwater Plan 6 2/17/2023
Table 4 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).
Table 4 – Storm Runoff Rate Calculations
Basin Characteristics Basin 1 Basin 2 Basin 3 Basin 3 Basin 5
Area (ft^2) 96,665 164,777 48,044 30,455 40,790
Area (acre) 2.22 3.78 1.10 0.70 0.94
Weighted C 0.76 0.69 0.76 0.76 0.76
10-yr,2-hr Q (cfs) 0.69 1.07 0.34 0.22 0.29
25-yr,5-min Q(cfs) 6.45 9.99 3.21 2.03 2.72
Table 5 – Pipe Conveyance Calculations
The hydraulic capacity of the site 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 1.92 cfs of runoff.
Analysis was performed at a minimum slope of 0.5%. The capacity of a 15” storm
drain at 0.5% slope at 90% flow is 4.87 cfs which satisfies the required
conveyance capacity in all cases.
Manual Input Parameters
pipe diameter (in)15
depth in culvert (in)13.50
coefficient of roughness (n)0.013
slope (ft/ft)0.0050
Calculated Input Parameters
radius (in)7.50
radius (ft)0.63
depth of flow (ft)1.13
wetted perimeter 3.12
area of flow (ft2)1.16
hydraulic radius 0.37
Avg. Velocity (ft/sec)4.2
Max Velocity (ft/sec)6.3
Calculated Operating Parameters
Flowrate culvert (cfs)4.87
Stormwater Plan 7 2/17/2023
Table 6 – Curb and Gutter Conveyance Calculations
Curb and Gutter Capacity Calcs
Max Curb Conveyance Req'd (ft^3/sec) 1.45
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) 5.04
Curb and gutter & chases are primarily used on the site to direct runoff into the
storm sewer network. To ensure the standard curb design will provide adequate
conveyance capacity for peak flows, the length of curb with the greatest flow was
identified and analyzed for conveyance capacity.
The critical curb required to convey the maximum flow was identified as the
approximately 100’ length of curb channeling runoff to Retention Basin 2, serving
a catchment area of approximately 21,774 sf. This catchment area produces a
peak flow of 1.45 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 5.04
cfs.
Storm Event Analysis
To get a sense of how the site stormwater facilities would have performed in past
storm events, a storm event analysis was 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 below, the site’s stormwater system will retain the 97th percentile of
storms on site.
Stormwater Plan 8 2/17/2023
Table 7 – 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
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 will be
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.
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701 N FERGUSON AVEBOZEMAN, MT 59718FIGURE 2STORMWATERPLAN