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Stormwater Design Report Greg Matthews Studio Post 14 625 Boardwalk Ave.
Lot 2, Block 3, Ferguson Farm Subdivision
Prepared for: Greg Matthews Studio, Inc. 620 E. Cottonwood, Ste. 103 Bozeman, MT 59718
Prepared by: Bridger Engineers, Inc. 2150 Analysis Drive, Ste. B Bozeman, MT 59718
Introduction The proposed project consists of a new office building to be located at 625 Boardwalk Avenue on Lot 2, Block 3 of the Ferguson Farm Subdivision. This lot was a part of the original Spring Creek Village Resort Lot 4 Subdivision. The lot currently contains a 13’ wide sidewalk along Boardwalk Avenue and an existing paved shared parking lot with integral concrete curb and gutter on the west half of the lot. The proposed improvements consist of a one-story office building with a partial lower-level storage and mechanical area, adjacent sidewalks, landscaping and new utility service connections. There are two separate drainage areas for the Lot 2 stormwater runoff: Drainage Area (A) consists of all onsite paving, hardscape and landscape. Runoff from this drainage area flows overland via existing curb & gutter to the existing Town & County (T&C) Retention Pond 1. Drainage Area (B) consists of the proposed building roof and interior courtyard. Runoff shall be captured via gutters and a trench drain and piped to proposed underground StormTech retention chambers below the slab in the southwest corner of the building. The existing stormwater infrastructure for Area (A) was designed and constructed per the Design
Report for Storm Water Management, Lot 4 Spring Creek Village Resort Subdivision prepared by C&H Engineering & Surveying and dated July 2013. The existing infrastructure consists of paved roads and parking lots with curb & gutter and the Retention Pond 1. Recently Retention Pond 1 was reconstructed according to the Stormwater Design Report & Calculations prepared by Madison Engineering for Town & County Foods revised 1/14/2021. The report verifies that the reconstructed Retention Pond 1 provides the required capacity for stormwater runoff from Lot 2 (Drainage Area A) as originally intended and designed. Applicable pages in both reports referenced above are included in Appendix A & B of this report for reference. In both reports, retention ponds were sized per the City of Bozeman’s design standards for a 10-year 2-hour storm event. Existing stormwater infrastructure also exists for Area (B). This infrastructure consists of a drywell (or percolation basin) located at the southeast corner of the onsite parking lot. This
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drywell was intended to receive stormwater runoff for the future building to be located on this lot. However, since the drywell cannot provide the required stormwater retention volume or the necessary percolation rate, it will not be used for retention. Instead, it shall remain and be used for access to the new proposed underground retention chambers. Stormwater Design Drainage Area A – Parking Lot Runoff to Retention Pond 1 Per the Spring Creek Village Resort drainage plan, the parking lot for this project is located within
Drainage Area 12 (DA12). Runoff from this drainage area was designed to flow overland to perimeter curb and gutter where it would be directed to Retention Pond #1 (RP1). RP1 is located within an Open Space parcel directly to the west of the Town & County development. The T&C development recently reconstructed RP1 to provide capacity for the T&C Phase 1 development as well as the existing off-site basins that drain to RP1.
The original C&H design report included 24,712 sf of hardscape and landscape area from DA12 for RP1 (of which 7,950 sf of runoff comes from this Lot 2). Table II of the T&C stormwater report shows that the original DA12 runoff area of 24,712 sf has been included in the new volume calculations for the reconstructed pond.
In conclusion, both the original Retention Pond 1 and the reconstructed Retention Pond 1 were
designed to retain stormwater runoff from the parking lot of the proposed project via existing curb & gutter. Drainage Area B – Building Runoff to Retention Chambers The building roofs slope inward to a central courtyard. The perimeter of the roof around the
courtyard contains a gutter to collect and direct the runoff to the east courtyard wall. The gutter along this wall will allow water to fall to the ground via a rainscreen. The base of the rainscreen will have a trench drain to capture the runoff from above as well as the runoff from the courtyard and direct it below ground to a 6” stormwater pipe. The 6” stormwater pipe will carry the runoff to an underground StormTech retention chamber system below the slab of the building in the southwest corner. The StormTech chambers shall be connected to the existing drywell manhole
with an 18” diameter pipe for inspection and maintenance access. The existing drywell consists of a 48” diameter by 8’ deep concrete slotted manhole with a 24” solid steel manhole cover. A new opening shall be cored into the wall for the new 18” pipe. Retention Calculations Retention calculations are provided in Appendix C of this report for the underground retention chambers. Calculations are provided as required by the City of Bozeman Design Standards and Specifications Policy using a 10-year 2-hour storm event. The total area of the building roof and interior courtyard is 7,457 sf. The calculations show that the building produces approximately 455 cubic feet of required storage volume. Four (4) Stormtech SC-740 chambers with 48” of clean crushed drain rock below will be installed providing 489 cf of storage. Details of the retention system are provided on Sheet C2.1. Details & Specifications Included with the Grading & Drainage Plan (C1.1) is a Stormwater Detail Sheet (C2.1) which contains details and specifications of the underground retention system.
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Groundwater Groundwater monitoring was conducted in Spring 2024. The high groundwater was recorded on 5/31/2024 at an elevation of 4823.71. The bottom of the proposed retention system (including the 48” of clean drain rock below the chambers) is at elevation 4824.75.
Maintenance Plan A stormwater maintenance plan is included in Appendix D of this report.
APPENDIX A
RETENTION POND 1
SUPPORTING DOCUMENTATION
(from Lot 4, Spring Creek Village Resort Sub.
Stormwater Design Report, July 2013)
APPENDIX B
RECONSTRUCTED RETENTION POND 1
SUPPORTING DOCUMENTATION
(from Ferguson Farms Sub., Block 5 Modification
Stormwater Design Report for Town & Country Foods,
July 2020, Revised 1/14/21)
APPENDIX C
RETENTION CALCULATIONS
Stormwater Retention Calculations
Project:GMS Post 14
Date:6/20/2024
Engineer:M. Dyba, Bridger Engineers
C (weighted C factor):see table
I (Intensity):0.41 in/hr Intensity per COB DSSP Fig. I-3:
A (area):in acres I = 0.41 for 10 yr, 2 hr storm
I = 0.64 for 100 yr, 2 hr storm
UNDERGROUND STORAGE (Runoff from Building)
Land Use sf A (acres)C A*C
Building 7457 0.171 0.9 0.154
Pavement 0 0.000 0.8 0.000
Landscape 0 0.000 0.2 0.000
Totals:0.171 0.154
C ave (A*C /A):0.90
Q (CIA):0.063 cfs
V (Qx7200):455 cf REQUIRED STORAGE VOLUME
Stormtech Chamber SC-740:
Height:30 in 2.5 ft
Width:51 in 4.25 ft
Length: (per chamber)90.7 in 7.6 ft
Length: (installed)85.4 in 7.1 ft
Storage/chamber*:74.9 cf *
Storage/additional inch of stone**:1.13 cf **
Additional inch(es) of stone foundation:42 in
*Value provided by manufacturer assuming 6" stone above, below & between chambers & 40% stone porosity.
** Add 1.13 cf of storage for each additional inch of stone foundation.
Req'd # of chambers:3.7 (Req'd storage volume / storage/chamber)
Chamber Sizing:
Use 1 row with 10 chambers
Total number of chambers: 4
Actual Storage Volume:489 cf (# of chambers x storage/chamber)
APPENDIX D
STORMWATER MAINTENANCE PLAN
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Stormwater Maintenance Plan Greg Matthews Studio Post 14 625 Boardwalk Ave.
Lot 2, Block 3, Ferguson Farm Subdivision
Prepared for: Greg Matthews Studio, Inc. 620 E. Cottonwood, Ste. 103 Bozeman, MT 59718
Prepared by: Bridger Engineers, Inc. 2150 Analysis Drive, Ste. B Bozeman, MT 59718
Stormwater facilities for the Greg Matthews Studio project consist of those listed below. Maintenance of the subdivision stormwater infrastructure i.e., the shared parking lot with curb & gutter and the retention pond is the responsibility of the Property Owners Association. Retention Pond 1 is located on a private common open space lot owned by the Ferguson Farms Property Owners Association. The shared parking lot is located within a Shared Parking, Drainage & Access Easement. A maintenance plan is included in the POA Covenants. The onsite stormwater facilities for the building, consisting of those listed below, are owned, funded and maintained by the property owner. These facilities should be inspected bi-annually with maintenance measures performed on an as-needed basis. StormTech® Underground Stormwater Retention Chambers A system of StormTech underground storm water retention chambers are buried beneath the building slab. These chambers are accessed by a concrete manhole with a cast iron solid lid and an 18” diameter connecting pipe. StormTech.com provides an inspection and maintenance manual (attached to this document) for cleaning the chambers. This manual should be used to perform bi-annual inspections and maintenance as required. Cleaning of the chambers is accomplished by using a flusher nozzle as described in the attached manual. A JetVac process may be used, as the chamber row is installed as an isolator row, lined with an AASHTO Class 1 Woven Geotextile. Trench Drain A trench drain is located within the courtyard to capture stormwater runoff from the roof and courtyard. The courtyard shall slope to this drain at a 1% minimum slope. The drain contains a removable grate inlet cover and sediment catch. The drain and sediment catch should be inspected bi-annually for debris and sediment build-up. To inspect, remove the grate and clean out any debris and sediment build-up. The drain pipe may be cleaned out by flushing as needed. Roof Gutters The building contains gutters which capture and direct stormwater from the roof to the rain screen which allows runoff to flow downward to the trench drain below. Ensure that the gutters are free of debris and inspected annually.
Call StormTech at 860.529.8188 or 888.892.2694 or visit our website at www.stormtech.com for technical and product information.26
12.0 Inspection and Maintenance
12.1 ISOLATOR ROW INSPECTION
Regular inspection and maintenance are essential to assure
a properly functioning stormwater system. Inspec tion is
easily accomplished through the manhole or optional
inspection ports of an Isolator Row. Please follow local
and OSHA rules for a confined space entry.
Inspection ports can allow inspection to be accomplished
completely from the surface without the need for a con-
fined space entry. Inspection ports provide visual access
to the system with the use of a flashlight. A stadia rod
may be inserted to determine the depth of sediment.
If upon visual inspection it is found that sediment has
accumulated to an average depth exceeding 3" (76 mm),
cleanout is required.
A StormTech Isolator Row should initially be inspected
immediately after completion of the site’s construction.
While every effort should be made to prevent sediment
from entering the system during construction, it is during
this time that excess amounts of sediments are most
likely to enter any stormwater system. Inspection and
maintenance, if necessary, should be performed prior
to passing responsibility over to the site’s owner. Once
in normal service, a StormTech Isolator Row should be
inspected bi-annually until an understanding of the sites
characteristics is developed. The site’s maintenance
manager can then revise the inspection schedule based
on experience or local requirements.
12.2 ISOLATOR ROW MAINTENANCE
JetVac maintenance is recommended if sediment has
been collected to an average depth of 3" (76 mm) inside
the Isolator Row. More frequent maintenance may be
required to maintain minimum flow rates through the
Isolator Row. The JetVac process utilizes a high pressure
water nozzle to propel itself down the Isolator Row while
scouring and suspending sediments. As the nozzle is
retrieved, a wave of suspended sediments is flushed back
into the manhole for vacuuming. Most sewer and pipe
maintenance companies have vacuum/ JetVac combi-
nation vehicles. Fixed nozzles designed for culverts or
large dia meter pipe cleaning are preferable. Rear facing
jets with an effective spread of at least 45" (1143 mm)
are best. The JetVac process shall only be performed
on StormTech Rows that have AASHTO class 1 woven
geotextile over the foundation stone (ADS 315ST or
equal).
Looking down the Isolator Row.
A typical JetVac truck. (This is not a StormTech product.)
Examples of culvert cleaning nozzles appropriate for Isolator Row
maintenance. (These are not StormTech products.)
27 Call StormTech at 860.529.8188 or 888.892.2694 or visit our website at www.stormtech.com for technical and product information.
12.0 Inspection & Maintenance
STORMTECH ISOLATOR™ROW - STEP-BY-STEP
MAINTENANCE PROCEDURES
Step 1)Inspect Isolator Row for sediment
A) Inspection ports (if present)
i. Remove lid from floor box frame
ii. Remove cap from inspection riser
iii. Using a flashlight and stadia rod, measure
depth of sediment
iv. If sediment is at, or above, 3" (76 mm)
depth proceed to Step 2. If not proceed
to Step 3.
B) All Isolator Rows
i. Remove cover from manhole at upstream
end of Isolator Row
ii. Using a flashlight, inspect down Isolator
Row through outlet pipe
1. Follow OSHA regulations for confined
space entry if entering manhole
2. Mirrors on poles or cameras may be
used to avoid a confined space entry
iii. If sediment is at or above the lower row of
sidewall holes [approximately 3" (76 mm)]
proceed to Step 2. If not proceed to Step 3.
Step 2)Clean out Isolator Row using the JetVac process
A) A fixed floor cleaning nozzle with rear
facing nozzle spread of 45" (1143 mm) or
more is preferable
B) Apply multiple passes of JetVac until back-
flush water is clean
C) Vacuum manhole sump as required during
jetting
Step 3) Replace all caps, lids and covers
Step 4)Inspect and clean catch basins and manholes
upstream of the StormTech system following local
guidelines.
4
21) B)1) A)
Figure 20 –StormTech Isolator Row (not to scale)
Please contact StormTech’s Technical Services
Department at 888-892-2894 for a spreadsheet to
estimate cleaning intervals.
12.3 ECCENTRIC PIPE HEADER INSPECTION
Theses guidelines do not supercede a pipe manufac-
turer’s recommended I&M procedures. Consult with the
manufacturer of the pipe header system for specific I&M
procedures. Inspection of the header system should
be carried out quarterly. On sites which generate higher
levels of sediment more frequent inspections may be
necessary. Headers may be accessed through risers,
access ports or manholes. Measurement of sediment
may be taken with a stadia rod or similar device. Clean -
out of sediment should occur when the sediment volume
has reduced the storage area by 25% or the depth
of sediment has reached approximately 25% of the
diameter of the structure.
12.4 ECCENTRIC PIPE MANIFOLD MAINTENANCE
Cleanout of accumulated material should be accom-
plished by vacuum pumping the material from the head-
er. Cleanout should be accomplished during dry weath-
er. Care should be taken to avoid flushing sediments out
through the outlet pipes and into the chamber rows.
Eccentric Header Step-by-Step MaintenanceProcedures
1.Locate manholes connected to the manifold system
2.Remove grates or covers
3.Using a stadia rod, measure the depth of sediment
4.If sediment is at a depth of about 25% pipe volume
or 25% pipe diameter proceed to step 5. If not
proceed to step 6.
5.Vacuum pump the sediment. Do not flush sediment
out inlet pipes.
6.Replace grates and covers
7.Record depth and date and schedule next inspection
1, 2, 6
3, 4, 5
Figure 21 –Eccentric Manifold Maintenance