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Block 4
Water, Sewer, Storm Water Engineering Report
April 6, 2021
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 Block 4 Building is an infill redevelopment of Lots 1-10 of Block 4 of Beall’s Third Addition. The
project is a portion of the North Central Master Plan. The site is currently occupied by a parking lot
that serves the buildings in the surrounding area. The total lot size is 32,645 square feet. Existing
conditions on site consist primarily of the paved parking lot, with some minimal landscaping. The
existing lots will be aggregated into a single common lot (Lot 1A) so that the project is self contained
within a single lot.
Figure 1 – North Central Master Plan Project Vicinity
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The Block 4 Building is a 6-story residential building featuring 70 one-bedroom studio apartments
and 29 2/3 bedroom residential units for a total of 99 units. The building will also contain 6,052 sf of
retail space of the 1st floor, and 53,628 gross sf of parking, split between public parking in the
basement and residential parking on levels 1 and 2. The total building gross square footage is
168,105 sf. The proposed building, site, and off-site improvements are shown on the Civil plans
provided with the Site Plan submittal. This Engineering Report supports the design of the water and
fire service connection, sewer service line, and storm water mitigation system.
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.
Off-site Improvements
Per the North Central Master Plan, off-site infrastructure improvements are planned as part of the
Block 4 project, corresponding to Phase 1 of the Master Plan. Off-site improvements will be
submitted for review to the Engineering Department after Site Plan review.
Planned off-site stormwater improvements consist of a new 15” storm sewer main in Villard Street
extending west from the Tracy Avenue and Villard Street intersection. Four new curb inlets are
proposed to improve intersection and street drainage. To accommodate proposed widening of the
west side of Willson Avenue the existing curb inlet at the southeast corner of Block 4 will be removed
and replaced as part of the project.
Planned off-site street improvements include development of the unimproved section of Villard Street
directly north of the site, improvements to the alley west of the site, intersection striping in the project
vicinity, and widening of Willson Avenue from 37’ to 39’. Development of the currently unimproved
section of Villard Street consists of re-grading, using the existing sidewalk on the north side of the
road as a design basis, installation of curb and gutter, and road construction and paving. Willson
Avenue will be widened by sawcut of existing pavement and demolition of existing curb on the west
side of the street. A mid-block crosswalk in Willson, facilitating pedestrian traffic between Block 3 on
the east side of Willson and Block 4 is planned with future phase development of the North Central
Master Plan. New pedestrian landings will be installed at the west side of Willson Avenue
intersections as part of the curb reconstruction. Improvements to the alley west of the site include
installing a 16’ wide paved alley. Alley grading will be changed from the existing crown to a valleyed
design for drainage control. A 5’ access easement along the alley is proposed to allow the 16’
pavement width. Intersection marking consisting of crosswalks, stop bar, and yield triangles will be
installed at both the intersection of Villard and Willson as well as Beall and Willson.
Planned off-site water improvements consist of installation of a new fire hydrant at the northeast
corner of the site, pending Fire Department approval.
Street lighting improvements are proposed for the west side of Willson Avenue from the Willson-
Villard intersection to the Willson-Lamme intersection. Lighting improvements for the east side of
Willson Avenue are proposed in subsequent development phases.
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Site Improvements
Block 4 site improvements consist of streetscape improvements between the building and the curb.
Streetscape improvements are in accordance with the North Central Master Plan. The streetscape
includes a 6’ (min) concrete sidewalk, and 6’ of permeable pavers between the building face and the
curb. This design provides a flexible urban streetscape that accommodates increased summer
pedestrian use, and winter snow storage and drainage on the paver strip. Curbed planting beds are
proposed instead of tree-grates to incorporate additional boulevard plantings. Amenities such as
benches and bike racks will be located in the paver boulevard strip.
On-site parking is located in the basement, level 1, and level 2. Basement parking, for non-
residential uses, will be accessed via Villard Street. Residential parking on levels 1 and 2 is
accessed via the alley. A total of 177 on-site parking spaces are provided.
The proposed basement parking access from Villard is closer that the 40’ code requirement from the
alley access. A relaxation from this standard is requested in accordance with the BMC 38.400.090
H. criteria with a separate document. City staff has indicated support of this relaxation during Master
Plan reviews.
Figure 2 – Block 4 Site Plan
Franchise Utilities
Franchise utilities will be supplied by extension of and connection to existing lines running along the
alley. As envisioned by the North Central Master Plan, existing overhead power and
communications lines in the alley will be buried. A 5’ utility easement is proposed along the alley to
provide additional space for underground utilities, with additional easements for new transformers
and junction boxes located adjacent to the alley. Gas will be supplied by a new service connection
to the existing underground gas main underneath the alley. Communication service will be provided
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by service connections from the underground communication lines in the alley. Utility meters will be
located along the alley and screened from view. Trash will be stored in the building and rolled out to
a 5’ wide concrete staging area along the alley where it will be picked up. It is envisioned that 2-3
yard dumpsters will be used that are approximately 3.5’ deep, so trash containers will not intrude into
the 16’ paved alley.
Water
Existing City of Bozeman water infrastructure includes a 6” cast iron main in Willson Avenue. An
existing service provides irrigation for the landscaping currently located on site, which will be
abandoned. Domestic water and fire service will be supplied by installation of new services on this
main to a riser room within the basement. The water service line size is based on the plumbing
fixture counts and the Uniform Plumbing Code flow rates for each of the fixture counts. The
domestic water fixture counts are 1459 wsfu (water supply fixture units), requiring a 4” water service.
The fire service is planned to be 6”. The water and fire service lines will be designed by a PE and
submitted for review with the building permit.
The City of Bozeman 2017 Water Facility Plan Update did not identify any fire protection limitation in
this area of town. Three fire hydrants are located within 150 feet of the building. Furthermore, as
part of the 2017 Water Facility Plan Update, a fire flow test was performed on a hydrant
approximately 400 feet away at the corner of northwest corner of the Villard Street and Tracy
Avenue intersection. One of the fire hydrants used in the test is just a block away from the site. 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 Building 4.
Water service will be provided by a new service installed on the existing main in Willson Avenue.
The water service is anticipated to be a 4” service. The required meter size is based on the number
of water fixture units in the building. The preliminary water fixture unit count is 1459, which requires a
4” water meter.
The estimated water use for Building 4 is provided below in Table 1. Domestic water use estimates
are based on a usage rate of 65 gpd/c with an estimated 2.17 persons per unit. The retail space was
estimated based on an assumed rate of 30 gpd/1000 sf of retail floor space. Irrigation water use
estimates have been provided by the landscape architect. The estimated water use is anticipated to
be 5,213,766 gallons, or approximately 14,300 gpd, with a peak hour demand of 44.7 gpm.
Table 1. Block 4 Estimated Water Use
Annual Domestic Water Use
Residential #Units Gpd/capita Pop/unit Gallons/day
Apartment Unit 99 65.0 2.17 13964
Non-Residential Area (sf) Gpd/1000sf Gallons/day
Retail Space 6,052 30.0 182
Total Domestic Use 14,146
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Domestic Annual Use (gallons) 5,163,111
Irrigation Annual Use (gallons) 54,400
Average Day Demand (gpd) 14,295
Peaking Factor 4.5
Peak Hour (gpm) 44.7
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
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.
Sewer
An existing 10” Vitrified Clay sewer main is located in North Willson Avenue. This main was recently
lined with Cured in Place Pipe (CIPP). Wastewater services will be provided by a new service
connection on this main. The old vitrified clay pipe around the new CIPP will be removed during the
installation.
The new sewer service size will be based on the plumbing fixture counts in the building and the
Uniform Plumbing Code flow rate for this number of fixture counts. The preliminary drainage fixture
unit count is a total of 1167 dfu (drainage fixture units). For this number of fixture units, the plumbing
code requires an 8” sewer service line.
Daily wastewater generation was determined utilizing the same methodology as the domestic water
use estimate, minus irrigation flows that will not enter the sewer system. Applying this value results
in an average daily flow for the project of 9.82 gpm. Using a peaking factor of 4.5, the peak hour
flow is estimated at 44.2 gpm. These values and demands are summarized and tabulated below.
Table 2. One 11 2.0 Estimated Sewer Use and Demands
Estimated Sewer Use and Demands
Residential #Units Gpd/capita Pop/unit Gallons/day
Apartment Units 99 65.0 2.17 13964
Non-Residential Area (sf) Gpd/1000sf Gallons/day
Retail Space 6,052 30.0 182
Total Domestic Use 14,146
Average Day Demand (gpm) 9.82
Peaking Factor 4.5
Peak Hour (gpm) 44.2
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As mentioned in the previous section, the anticipated domestic water use for the Block 4 Building will
be much lower than typical City design flow estimates, resulting in a 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 (parking
lot) without any stormwater mitigation. There is a minimal amount of landscaping on existing on site.
Currently, runoff from the site is primarily directed north or west towards Villard and Willson Avenue,
respectively.
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 within the
building footprint. Per the Uniform Plumbing Code, building stormwater piping is sized to
accommodate the 100-year design storm. Permeable pavers will be installed between the sidewalk
and curb to further reduce stormwater runoff generated from the City sidewalks, provide space for
sidewalk and street snow storage, and infiltrate water from snowmelt.
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 1,177 cf and
the required storage volume to mitigate peak flows is 258 cf. In order to further reduce runoff
entering City infrastructure, the proposed net retention storage volume is 2,233 cf, exceeding
minimum requirements. The proposed infiltration system will completely retain runoff from storms up
to the 0.95-inch event, which represents the 97th percentile of storm events in Bozeman. In addition
to containing the volume of the first 0.5”, the proposed storage also contains the runoff volume
increase from pre-existing to post-development in the 10, 25, 50, and 100-year 24-hour events.
Considering the similar level of impervious area in pre and post development, the introduction of
Block 4’s stormwater mitigation system will greatly reduce volumes introduced to the existing City
infrastructure from the site. Any flows generated in storms that exceed the storage volume will be
conveyed to the City of Bozeman storm sewer via an overflow pipe that directs water to the
proposed off-site storm sewer in Villard.
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Table 3. Block 4 Storm Water Calculations
Site Statistics
Land Classification C Existing Area (sf) Post Dev Area (sf)
Rooftops 0.9 0 29321
Pavement 0.9 29,364 875
Sidewalk 0.9 0 825
Permeable Pavers 0.3 0 0
Landscape 0.2 3,281 1624
Total 32,645 32,645
Weighted Runoff Coeff. (C ) 0.83 0.87
Design Storm Information
Design Storm 0.5-Inch 10-Year 25-Year 50-Year 100-Year
Drainage Area (acres) 0.749 0.749 0.749 0.749 0.749
Drainage Area (sf) 32645 32645 32645 32645 32645
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 2.00 2.38 2.95 3.32
Runoff Volume (cf) 1128 4153 4875 5462 6026
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 2.09 2.48 3.08 3.46
Runoff Volume (cf) 1177 4331 5084 5696 6284
Mitigation Calculations
Design Storm 0.5 Inch 10-Year 25-Year 50-Year 100-Year
Runoff Volume Increase (cf) 48 178 209 234 258
Retention Volume (cf) 2233 2233 2233 2233 2233
Net Runoff Volume Post Mitigation (cf) 0.00 2098 2851 3463 4051
% Decrease in Runoff from Existing 100 50 42 37 33
Stormwater pipes within the building are designed per the Uniform Plumbing Code to accommodate
the 100-year storm event. This code requires an 8” pipe at a 2% slope for building drainage. The
stormwater service line leaving the building has a slightly flatter 1% slope so a 10” line is proposed,
which exceeds the capacity of the building roof drain piping. All runoff leaving the site is below
ground, preventing runoff impacts to adjacent property.
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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.
Keep sidewalk, permeable pavers, and parking areas clean.
Pick up trash.
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.
Inspect for sediment or debris in the structures and remove if present.
Inspect infiltration system through inspection ports for sediment accumulation.
Sediment depth less than 3” is acceptable.
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.
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.
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.
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.
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:
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.
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.
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.
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.