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The Henry (AKA Building 3-4)
Site, Water, Sewer, Storm Water Engineering Report
June 8th, 2021, Site Plan
Prepared for: HomeBase Montana
Prepared by: Stahly Engineering and Associates
Engineer of Record: Cordell D. Pool, PE
Quality Control Reviewer: Zach Lowe, PE
Introduction
The Henry, part of the North Central Master Plan, is an infill redevelopment project of Lots 1-6, Block
3, of Beall’s Third Addition. The site is currently occupied by a parking lot that serves the existing
Medical Arts buildings also located in Block 3. The total lot size is 24,440 square feet. Existing
conditions on site consist primarily of the paved parking lot, with some minimal landscaping and a
small portion of an existing one-story building. As part of development, the lots will be aggregated
so that The Henry is contained within its own single lot.
Figure 1 – North Central Master Plan Project Vicinity
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The location of The Henry can be seen outlined in blue in Figure 1 above (note: Figure 1 shows the
North Central Master Plan at proposed full build-out). The Henry and related improvements are
proposed to be completed while the existing Medical Arts Building to the south and Tower Building to
the west remain occupied. The Henry is a 92,645 sf, 5-story over lobby with podium residential
building featuring 6 one-bedroom studio apartments and 38 2/3 bedroom residential units. The
building also contains 18,960 sf of basement level parking and 1,150 sf of bike parking, for a total
gross square footage of 118,080 sf. The proposed building and site improvements are shown on
Figure 2, excerpted from the North Central Master Plan submittal.
Civil Specifications and Design Standards
The civil specifications for the project are the Montana Public Works Standard Specifications
(MPWSS) and the City of Bozeman Modifications to MPWSS (COB Mods). Construction plans are
developed in accordance with the City of Bozeman Design Standards.
Demolition/Replacement on Adjacent Property
Demolition and replacement of existing site improvements on adjacent Block 3 property will be
required as part of The Henry construction. The existing “Same Day Surgery” building currently
attached to the Tower will be demolished. This demolition activity will also include the sidewalk and
awning of the Medical Arts Building south of The Henry. The Same Day Surgery sewer service that
runs underneath the The Henry site will require demolition.
After demolition, new accesses to existing buildings are required. This includes both the ground level
access to the Tower and Medical Arts Building as well as basement access to the Tower Building. A
retaining wall will be constructed at the Tower Building to allow the existing street-level grading of the
Tower Building’s back of house access to be maintained.
Off-site Improvements
Off-site infrastructure improvements are required as part of The Henry project. Off-site stormwater
infrastructure improvements consist of a 15” storm drain main and curb inlets in Tracy Avenue. Off-
site street improvements consist of replacement of existing curb and gutter along Villard and Tracy
frontages, as well as new intersection crosswalk markings. No off-site street lighting improvements
are proposed with this project.
Site Improvements
Site improvements consist of site hardscape improvements, franchise utility services, building water
and sewer services, and stormwater mitigation.
The Henry hardscape improvements consist of new perimeter public sidewalks, internal pathways
and plazas, and building accesses. Perimeter sidewalks are 5’ wide with 5.5’ planted boulevards as
proposed with the North Central Master Plan. A new pedestrian landing will be installed at the
southwest corner of the Villard and Tracy intersection. Internal pathways will provide connections to
the rear of The Henry and existing Block 3 buildings. A street access is proposed on the pathway
south of The Henry to allow maintenance vehicles into the site interior. A ground level courtyard is
proposed in the southwest pocket of the building. Entry stoops and steps are required at ground
floor entries fronting streets. ADA compliant ramps are provided at the building entries.
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On-site parking is provided underground within the basement of the building. The basement parking
was originally proposed to be accessed from Tracy Ave but is now proposed to be from Villard.
Basement parking provides 45 parking stalls, with an additional 5 on-street spaces available.
Figure 2 – The Henry North Central Master Plan Phase 2 Site Layout
Trash is stored within the building and rolled out to the Villard Street sidewalk for loading. This
requires street level access to the building’s trash room and a heated slab between the building and
loading area. An oversized access on Villard is proposed to allow trucks to angle load trash without
blocking on-coming traffic.
Gas, communication, and power are extended from the alley to the north. An existing gas line
currently extends under Villard to serve the existing “Tower” Building adjacent to the site and will
only require a short service. Communication lines and power will be extended underground
underneath Villard to serve The Henry. Franchise utilities will be arranged to allow future extension
to Building 3-5. Utility meters will be housed in a semi-enclosed space on the northwest corner of the
building, screening the meters from public view.
Water
An existing 6” cast iron City of Bozeman water main runs along Tracy Avenue. Domestic water and
fire service will be provided by connection to this existing main.
The City of Bozeman 2017 Water Facility Plan Update did not identify any fire protection limitation in
this area of town. Two fire hydrants are located within 75 feet of the building. Furthermore, as part
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of the 2017 Water Facility Plan Update, a fire flow test was performed on a hydrant approximately 75
feet away at the corner of northwest corner of the Villard Street and Tracy Avenue intersection. The
results for this test (test number 44) are included with this report. The static water pressure is
approximately 137.7 psi. Two adjacent hydrants were opened simultaneously at a 2.5” diameter
nozzle, the hydrant nearest to the site flowed at 1,744 gpm while the other flowed at 1,601 gpm for a
combined flowrate of 3,345 gpm. These flows resulted in a 24.8 drop in psi at the residual test
hydrant, for a residual pressure of 112.9 psi. This indicates that reasonable urban fire flows can be
met in the area. Specifically, each of the nearby 2 hydrants could be expected to provide similar
flows, resulting in approximately 3,500 gpm of fire flow available to The Henry. As mentioned
previously, a new fire hydrant may be required based on the final location of FDCs on the building.
Water and fire services have not yet been sized, but a 4” water service and a 6” fire service are
anticipated. The final design will be made by a Professional Engineer and included with future
submittals.
Irrigation water supply will be provided through the domestic water service. Preliminary irrigation
demands were provided by the landscape architect. Irrigation is anticipated to occur over an
approximately 100 -day season resulting in a total annual water use of 35,829 gallons, or 0.11 ac-ft.
The average daily irrigation flow is 358 gpd.
The estimated water use for The Henry is provided below in Table 1. Domestic water use estimates
are based on a usage rate of 65 gpd/capita with an estimated 2.17 persons per unit. Irrigation water
use estimates have been provided by the landscape architect. The estimated annual water use is
anticipated to be 2,301,092 gallons, or approximately 6,564 gpd, with a peak hour demand of 20.5
gpm.
Table 1. The Henry Annual Domestic Water Use
Annual Domestic Water Use
Residential #Units Gpd/capita Pop/unit Gallons/day
Apartment Unit 44 65.0 2.17 6206
Total Domestic Use 6,206
Domestic Annual Use (gallons) 2,265,263
Irrigation Annual Use (gallons) 35,829
Average Day Demand (gpd) 6,564
Peaking Factor 4.5
Peak Hour (gpm) 20.5
The North Central Master Plan anticipates water use that is lower than typical city design flow
estimates. However, there is not currently an established methodology for estimating water use in
this kind of urban, high-density residential use. High efficiency fixtures and efficient design reduces
the per capita water consumption, and this type of urban development typically does not attract
families which results in a lower water use per unit than is typical in the City. While data has not
been available for the requisite 3 years for consideration, nor are there enough comparable
developed sites, recent data from the similar Black Olive building indicate a water usage rate of
approximately 50 gallons/day/unit. It is anticipated that water use within North Central will be very
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similar to that of Black Olive. Comparing of the City accepted water use estimates to measured
Black Olive meter reading data show the Black Olive water use to be approximately 35% of the City
estimates.
City provided calculations for determining Cash-in-lieu-of water rights payment has estimated the
annual water use of The Henry as 5.94 ac ft. The total CILWR is estimated to be $35,655.
Sewer
An existing 8” PVC sewer main is located in Tracy Avenue. One new sewer service is proposed to
be connected to this main. The new sewer service has not yet been sized, but will be based on
preliminary fixture unit counts. An 8” sewer service in anticipated.
Daily wastewater generation was determined utilizing the same residential unit estimates as
described in the water use estimate. Applying this value results in an average daily flow for the
project of 4.31 gpm. Using a peaking factor of 4.5 the peak hour flow is estimated at 19.4 gpm.
These values and demands are summarized and tabulated below.
Table 2. The Henry Estimated Sewer Use and Demands
Estimated Sewer Use and Demands
Residential #Units Gpd/capita Pop/unit Gallons/day
Apartment Units 44 65.0 2.17 6206
Total Domestic Use 6,206
Average Day Demand (gpm) 4.31
Peaking Factor 4.5
Peak Hour (gpm) 19.4
As mentioned in the previous section, the anticipated domestic water use for The Henry will be much
lower than typical City design flow estimates, resulting in an average day demand and peak hour
demand substantially lower than presented above.
Storm Water
The existing site conditions consist almost entirely of a single connected impervious area (paved
parking lot) without any stormwater mitigation. Currently, runoff from the site is directed north or east,
to Villard or Tracy, respectively. City storm sewer currently terminates at the intersection of Villard
and Tracy. Block 4 development to the west will extend new storm drain mains in Villard Street. The
Henry will require a new 15” storm drain and curb inlets to be installed in Tracy Avenue. This main
will be extended into Block 3 to serve The Henry and other Block 3 buildings.
At full build out all buildings in Block 3 will share a common stormwater water retention/infiltration
system in the east west alley running through the middle of the Block (Beall’s Alley). This shared
system cannot be constructed until the Medical Arts Building is demolished. In the interim, the
stormwater mitigation for The Henry will be temporarily constructed to the east of the Medical Arts
Building. The building outlet and overflow discharge line are designed to work for the both the
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interim and final stormwater systems. The overflow pipe will connect to the new storm drain main
installed Tracy Ave.
Storm water mitigation is based on the redevelopment project low impact design (LID) requirement in
the City Design Standards to “infiltrate, evapotranspire, or capture for reuse the runoff generated
from the first 0.5” of rainfall. Additionally, to reduce impacts to the existing storm drainage
infrastructure, the stormwater mitigation will reduce peak flows from larger storm events to below
existing values. The most limiting stormwater mitigation criteria for this site is to provide mitigation of
the larger storm events; ensuring that the post developed runoff is equal to or less than existing
condition runoff values. The proposed stormwater mitigation system will capture, retain, and infiltrate
the building rooftop runoff for storms up to and exceeding 0.5” of rainfall. The infiltration system will
capture rooftop runoff and pipe it to a subsurface chamber/gravel infiltration system located south of
the building. All stormwater piping will be sized to handle the 25-year design event.
A summary of the stormwater calculations showing the mitigation of the increased stormwater runoff
has been provided below in Table 3. The required storage volume for the 0.5” event is 857 cf. To
reduce flows to existing storm drainage infrastructure 1553 cf of storage volume is proposed. This
results in a system that will completely retain runoff from storms up to the 0.97-inch event, which
represents the 98th percentile of storm events in Bozeman. Storm runoff from larger events will
overflow into the City storm sewer network at a rate significantly lower than existing conditions.
Groundwater monitoring wells were installed as part of the geotechnical investigation for the North
Central Master Plan. The stormwater mitigation system is located approximately halfway between
two of these wells, labeled in the Geotechnical Report as Borehole #2 and Borehole #3. Based on
the observed seasonal high groundwater in these wells, it is anticipated that the seasonal high
groundwater rises to approximately 8.5’ below the bottom of the stormwater retention system.
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Table 3. The Henry Storm Water Calculations
Site Statistics
Land Classification C Existing Area (sf) Post Dev Area (sf)
Rooftops 0.9 982 15,107
Pavement 0.9 19,685 0
Sidewalk 0.9 1,499 4,763
Permeable Pavers 0.3 0 369
Landscape 0.2 3,081 5,008
Total 25,247 25,247
Weighted Runoff Coeff. (C ) 0.81 0.75
Design Storm Information
Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year
Drainage Area (acres) 0.580 0.580 0.580 0.580 0.580
Drainage Area (sf) 25247 25247 25247 25247 25247
Slope (%) 2 2 2 2 2
Time of Concentration (min) 5.0 5.0 5.0 5.0 5.0
24 Hour Precipitation Volumes (in) 0.50 1.84 2.16 2.42 2.67
Existing Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.52 1.81 2.24 2.52
Runoff Volume (cf) 857 3153 3702 4147 4576
Post Dev Peak Flow Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Intensity at Tc (Figure I-2 pg. 29) (in/hr) NA 3.22 3.83 4.74 5.34
Peak Runoff Rate at Tc (Q = CIA) (cfs) NA 1.40 1.67 2.07 2.33
Runoff Volume (cf) 791 2913 3419 3831 4226
Mitigation Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Runoff Volume Increase (cf) 0 -241 -283 -317 -349
Retention Volume (cf) 1553 1553 1553 1553 1553
Net Runoff Volume Post Mitigation (cf) 0.00 1360 1866 2278 2673
% Decrease in Runoff from Existing 100.00 56.9 49.6 45.1 41.6
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Storm Water Maintenance:
General Information
The proposed storm water conveyance and infiltration facilities will be operated and maintained by
the property manager.
Storm Water Facilities Maintenance Schedule
1. Site Housekeeping. (Continuously as needed)
The main cause of storm water facility damage is poor site housekeeping. Sediment tracked
onto pavement can be washed into storm water appurtenances and damage these facilities.
Trash can clog conveyance structures, potentially causing property damage.
x Keep sidewalk, permeable pavers, and parking areas clean.
x Pick up trash.
x Restore damaged landscaping in order to prevent sediment runoff.
2. Curb, Sidewalk Chase, and Infiltration System Maintenance. (Quarterly)
All storm water conveyance structures can acquire sediment and debris buildup. If this
sediment and debris is not periodically removed, it can cause undesired ponding and
clogging. These conveyance structures need to be inspected and cleaned if required.
x Inspect for sediment or debris in the structures and remove if present.
x Inspect infiltration system through inspection ports for sediment accumulation.
Sediment depth less than 3” is acceptable.
x Check for damage, repair as needed.
3. Curb and Infiltration System Maintenance. (Long-term)
If regular housekeeping and maintenance is not performed adequately, sediment and debris
can accumulate in the storm water conveyance structures and infiltration system and clog
them beyond repair.
x If greater than 3” of sediment is present in infiltration system, hire a contractor with a
Jet-Vac chamber cleaning system to remove the sediment from the infiltration system.
x If original system performance can be achieved through maintenance, hire a
contractor to repair and return conveyance structures and infiltration system to the
initial design condition found on City engineering plans.
4. System Monitoring. (Quarterly, except in winter)
The storm water facilities shall be inspected quarterly to quickly identify small issues before
expensive damage can occur. In addition to regular monitoring, the best time to inspect the
performance of storm water facilities is during runoff events.
x Observe system during runoff. Look for ponding on permeable pavers or inlet
structures. This can indicate a clogged paver infiltration and/or clogged conveyance
structure.
x Open infiltration system inspection ports within 24-hours of a storm event and look for
ponded water in the infiltration system. This can indicate clogged infiltration system.
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If clogged hire a contractor with a Jet-Vac chamber cleaning system to remove the
sediment from the infiltration system.
5. PERMEABLE INTERLOCKING CONCRETE PAVEMENT (PICP) Inspection & Maintenance
Guidelines. Service inspection and maintenance shall include the following activities:
x Winter Maintenance:
o Ensure only joint aggregate stone (typically # 8, #89 or #9 washed chip stone)
is used for traction as needed. Sand should not be used for winter traction.
x Normal Maintenance:
o Inspect surface for ponding after large rain events. If ponding is observed,
identify areas with severe sediment loading and vacuum to remove and
replace with new washed joint aggregate (typically # 8, #89, or # 9 washed
chip stone).
o Note any sediment laden run-off from adjacent areas onto permeable
pavement. If needed, correct with erosion control measures.
x Annual inspection and maintenance shall include the following activities:
o Replenish paver joints with additional aggregate if level is more than ½ in.
below chamfer bottoms.
o Inspect vegetation around PICP perimeter for cover & soil stability,
repair/replant as needed.
o Inspect and repair all paver surface deformations (depressions/settlement)
exceeding 1/2 in.
o Repair paver heights offset by more than 1/4 in. above or below adjacent units
or offset by more than 1/8” lippage from paver-to-paver.
o Replace cracked paver units impairing surface structural integrity.
o Check drains and outfalls (if existing) for free flow of water. Remove any
obstructions.
o Check observation wells (if existing) to confirm reservoir is draining (based on
size of last rain event).
o Vacuum surface (typically spring), adjust vacuuming schedule per sediment
loading. Once a year sweeping is normal unless excessive silts and fines are
present in joints.
o Test surface infiltration rate using ASTM C1781. If pavement infiltration rate is
< 100 in/hr. employ remedial maintenance procedure utilizing a vacuum
sweeper/method to extract affected clogged joints/voids and replace joint/void
areas with #8, #89 or #9 washed chip aggregates and retest infiltration rate to
confirm reinstated areas exceed 100 in/hr. flow rate. Repeat remedial process
as needed to exceed the 100 in/hr. criteria.
x Additional Normal Maintenance Notes:
o A dry mechanical or regenerative air-type sweeper may be used during dry
periods to remove encrusted sediment, leaves, grass clippings, etc. Vacuum
or sweeper settings may require adjustments to prevent uptake of aggregate
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from the paver voids or joints. Leaf blowers or other standard onsite manual
methods that are used for standard pavement maintenance may be employed
to remove this surface debris.
o It is not recommended to utilize a pressure washer to clean joints.
o Remove snow with standard plow/snow blowing equipment.
o Deicing salt may be used on permeable pavers (proper application and
appropriate salt type) but consult property owner or project engineer before
usage. In some regions deicing salt use is restricted. Salt use can affect water
quality and have environmental impact.