HomeMy WebLinkAboutA7. 2015 Wastewater Plan Update
Commission Memorandum
REPORT TO: Honorable Mayor and City Commission
FROM: Bob Murray, Project Engineer
Rick Hixson, City Engineer
SUBJECT: 2015 Wastewater Collection System Facility Plan Update
MEETING DATE: June 1st, 2015
AGENDA ITEM TYPE: Action
RECOMMENDATION: Having reviewed and considered the executive summary, presentation,
public comment, and all the information presented, I hereby adopt the 2015 Wastewater
Collection System Facility Plan Update.
BACKGROUND: Attached is a copy executive summary for the 2015 Wastewater Collection
System Facility Plan Update prepared by Morrison Maierle. A copy of the entire draft plan can
be found on the Engineering Division section of the City of Bozeman website at
www.bozeman.net.
Last year, three different subdivisions on the west side of the City requested that they be allowed to connect to existing sewer mains in different drainage zones then designated in the wastewater
facility plan in place at that time. In order to connect to the proper drainage zone would have
required installation of tens of millions of dollars worth of infrastructure. The Commission
reviewed and approved the requests with the condition that each development contribute $50,000
towards an update of the collection system portion of the Wastewater Facility Plan. Although development on the west end of the system was the primary driver of the update, the entire
system was reviewed and the hydraulic model updated to address short term capacity issues as
well as the long term extensions that need to be complete to serve continued growth.
There are three components of the update we want to emphasize.. The first is the wastewater flow rate used to develop the plan. A detailed review of the wastewater flows since the last update has shown that per capita wastewater flows have been reduced from 89 to 64 gallons per
day. Reduced wastewater flows effectively increase the capacity of the collection system.
Reduced wastewater flows demonstrate the effectiveness of the City’s system rehabilitation and
water conservation efforts. Maintaining these reduced flows over the life of the plan will require continued investments in these programs.
Second, the drainage zones have been reconfigured to serve the City as it grows. The
recommended capital improvements are based on strict adherence to these drainage zones.
Approval of this plan by the Commission will establish a a clear policy that connection to the
collection system outside of the zones defined in the plan will not be approved going forward.
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This policy will ensure adequate capacity for all properties within a given zone based on the
main sizes shown in the plan update.
Finally, the consultant has provided a list of short term and long term projects that need to be
complete in order for development to continue throughout the City. The short term projects are extensions that need to be complete for developments that are ongoing or imminent that will be stopped due to capacity issues without the extensions. The long term projects are necessary to
ultimately build out the entire service area included in the plan. These recommendation will be
incorporated into capital plans and rate requests.
UNRESOLVED ISSUES: None
ALTERNATIVES: As suggested by the City Commission
FISCAL EFFECTS: Estimates of all of the recommended projects are included in the plan.
Attachments: Executive Summary
Report compiled on: 5/21/15
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Bozeman Wastewater Collection Facilities Plan
Update
Chapter 1
Executive Summary
Prepared by:
Mike Hickman, P.E.
Reviewed by:
James Nickelson, P.E.
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Table of Contents
1.1 INTRODUCTION ........................................................................................................... 1-1
1.2 BASIS OF PLANNING ...................................................................................................... 1-1
1.3 EXISTING SYSTEM DATABASE ............................................................................... 1-2
1.3.1 Data Base Recommendations................................................................................... 1-2
1.4 MODEL DEVELOPMENT AND CALIBRATION ................................................ 1-3
1.4.1 Model Development .................................................................................................. 1-3
1.4.2 Model Calibration ...................................................................................................... 1-4
1.5 EXISTING SYSTEM EVALUATION ......................................................................... 1-4
1.5.1 Recommendations...................................................................................................... 1-4
1.6 FUTURE SYSTEM EVALUATION ............................................................................ 1-5
1.6.1 System Deficiencies ................................................................................................... 1-6
1.6.2 Recommended Improvements ................................................................................. 1-7
1.7 POLICY AND PROGRAM RECOMMENTATIONS ............................................. 1-8
List of Tables
Table 1-1 – Estimated Wastewater Generation - Average Day ................................................ 1-2
Table 1-2 – Category 1 Improvements ......................................................................................... 1-7
Table 1-3 – Category 2 Improvements ......................................................................................... 1-8
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1.1 INTRODUCTION
Periodically, the City of Bozeman undertakes a comprehensive planning effort to update and
evaluate its existing wastewater collection system, and to estimate and plan for future expansion
based on current population and land use trends. This document represents the most recent effort,
which was triggered by land development occurring on the west side of the community. This
executive summary briefly describes the plan’s contents, conclusions, and recommendations. The
goals of this Wastewater Plan are:
Define and evaluate the existing infrastructure in order to determine capacity and existing
flows.
Estimate location and nature of future population growth, associated increases in
wastewater quantities, and their effect on existing infrastructure
Develop a comprehensive plan to address deficiencies and meet present and future
requirements, while continuing to plan for and accommodate the City’s growth.
1.2 BASIS OF PLANNING
The basis of planning defines geographical planning limits, existing population and land uses,
projected future population increases including density and distribution, and associated projections
of wastewater flow increases throughout the wastewater collection system. The planning period is
through the year 2034. The planning basis is used to quantify inputs to the sewer model that is used
in subsequent chapters to evaluate the existing and future collection systems.
The study area covers approximately 42,400 acres. This area sets boundaries for future development
over the planning period. For planning purposes, this study assumes the service area will encompass
the entire study area.
Population projections are based on an average yearly population growth of 3.0 percent. For the
purposes of this plan, the population/land use distribution is based on a saturation density or full
build-out conditions. Projected wastewater flows are based on future land uses.
The current overall “per capita” wastewater flow rate of 128 gallons per capita per day was used as
the basis for characterizing current and projected wastewater flows. This value is within the range
experienced by most municipalities with predominantly domestic wastes. It is conservatively
assumed that water conservation efforts and continuing rehabilitation work on the collection system
to reduce infiltration will not have a significant impact in lowering per capita flows over the planning
period; these conservation efforts will be offset to some extent as the remaining infrastructure
continues to age and potentially deteriorate. Using the City’s data on potable water use during
non-irrigation months as a good proportional indicator of wastewater generation, the overall “per
capita” flow rate was allocated to different land uses in Table 1-1.
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Table 1-1 Estimated Wastewater Generation –Average Day
User Category
Percentage of
Wastewater Generation
Wastewater
(gal/capita/day)
Residential 50%64.4
Commercial 26%33.8
Top 8 Water Users 1 8%9.6
Montana State University 11%14.6
Government 3%4.0
Industry 1%1.7
Total 100.0% 128
1. Eight highest water consumers; includes hotels and hospital.
The 64.4 gallons per capita per day “residential user” category combined with a value of 2.17
persons per household based on the 2010 census data results in 140 gallons per day per dwelling
unit, which was used in wastewater generation estimates. Future commercial flows were allocated
based on an estimated flow per acre. Using the above wastewater generation rates, population
densities and other information, wastewater generation rates were calculated on a “per acre” basis
for a defined set of land use categories. The “per acre” values were then applied to existing uses;
future development areas with current land use designations or zoning; and future development of
areas with currently undefined land use. Details are provided in Chapter 2.
The wastewater flow value of 64.4 gallons per capita per day developed with this plan is significantly
lower than values used in previous planning efforts. The 2007 plan used a value of 89 gallons per
day per capita. The difference is attributed to a combination of using more precise data, reduced
levels of infiltration due to rehabilitation projects and an increase in percentage of homes that use
lower flow fixtures than were used prior to the Energy Policy Act of 1992 which was the first
substantial national legislation focused on lowering water use for fixtures.
1.3 EXISTING SYSTEM DATABASE
This study combines and updates inventories from the 1998 City of Bozeman Wastewater Facility
Plan, 2007 update, and the current GIS inventory. Additionally, the database was updated to include
data from the City’s record drawings for recent projects. Limitations to the database include some
missing manhole/pipe elevation and slope data, and data on a variety of elevation datums which
cannot be readily correlated. To fill in this missing information for modeling purposes, estimated
values were arrived at based on surrounding elevations. At the time of this study, the wastewater
collection system is made up of approximately 210 miles of gravity sewer mains, 4,200 manholes,
twelve lift stations, and associated force mains that route wastewater to the treatment plant.
1.3.1 Data Base Recommendations
As the wastewater collection system expands, it is important to routinely update the inventory data
base. The data base provided with this plan does not include the most recently constructed portions
of the collection system and it will need to be updated routinely as development continues to occur.
A process should be established to ensure that GPS data is collected and that the data from the
record drawings is processed and placed in the data base on a routine basis. Standard procedures
should be developed regarding data acquisition and reporting in order to maintain an accurate data
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base. Ideally, and particularly for interceptors, trunk sewers and connection points for collectors
and laterals, “as-built” surveyed manhole rim elevations, and measured drops to pipe inverts would
greatly aid in resolving inconsistent elevation datums found in the database, and therefore providing
more accurate identification of deficiencies in the flow models.
The data base improvements recommended in Chapter 3 can be phased in over time as staff time or
outside contracts allow. Care needs to be taken to maintain the model as the data is revised or
expanded. A program should be developed so that both physical and condition information is
collected whenever a pipe or manhole is inspected.
1.4 MODEL DEVELOPMENT AND CALIBRATION
Since the 2007 Facility Plan was completed, the collection system has increased by approximately
55 miles of gravity sewer mains (37 percent increase), 850 manholes (26 percent increase), and
6 additional lift stations and associated force mains (100 percent increase). The Davis-Fowler
Interceptor was the most significant addition; it provides relief to the Baxter Interceptor and diverts
flows into the 27th Ave/Cattail Creek Interceptor.
The basis for evaluations of the existing and future collection systems is a revised wastewater
collection system model, which was established for this study using Innovyze’s InfoSWMM version
12, service pack 1, update 3 sewer modeling software.
1.4.1 Model Development
As discussed in Chapter 3, City GIS information was compiled with as-recorded drawings and
system databases for import into the collection system model. To remain consistent with the City’s
design standards, a Manning’s roughness coefficient of 0.013 was assigned to all pipes regardless of
material and age. All force mains were assigned a Hazen-Williams roughness coefficient (C value)
of 120.
For modeling purposes, the gravity wastewater collection system has been subdivided into three
categories: collectors/laterals, which primarily feed into trunk sewers, which then feed primarily into
interceptor sewers. Although collectors and laterals comprise about 75 percent of the total pipe length
of the system, their importance is limited to the point where they connect into trunk sewers. In
these upstream portions of the system, high variability in the timing and related volume of the actual
system flows correlates to low confidence in model flows and related system capacity. System
evaluation is therefore focused on the trunk lines and interceptors. Trunk sewers in Bozeman’s
system generally range in size from 10 to 18 inches in diameter, and comprise about 15 percent of
the total pipe length of the system; Interceptors, the largest pipes in the system, make up the
remaining 10 percent of the system’s gravity pipes. This study identifies eight interceptors.
The model includes eleven existing lift stations within the City’s wastewater system; seven of these
are owned and operated by the City. Three of the City-operated lift stations have been installed
since the 2007 Facility Plan, and all of these are located on the west side of the City.
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1.4.2 Model Calibration
In the spring of 2014, flow was monitored over a period of 1½ to 2 months near the discharge
points of four of the interceptors: 27th Avenue/Cattail Creek; Baxter; 19th Avenue/11th Avenue; and
North Frontage Road. Flow was also monitored at the influent of the wastewater treatment plant
during the first six months of 2014. During the period of flow monitoring, rainfall data was
monitored at three different locations within the City; this provided some spatial variability to the
rainfall data and allowed for different rainfall profiles to be applied to different parts of the system.
The rainfall data provided the basis for wet weather calibration of the model.
The model was calibrated using a rainfall event on May 18, 2014 and calibrating against the
associated flow monitoring data. The system manholes were split into seven different drainage
areas, each with its own hydrograph, based on which flow monitor they drain to and which rain
gauge is closest. The wet weather scenarios were calibrated by modifying the unit hydrographs until
a reasonable match was seen between the modeled flow and the monitored flow. The dry weather
and wet weather calibration results were generally within 10 percent of the monitor results and for
the purpose of this study, the model is a good indicator for the system performance. For the
purposes of evaluating the capacity of the existing and future system a 25-year design storm was
utilized.
1.5 EXISTING SYSTEM EVALUATION
The collection system was evaluated to identify deficiencies, defined by the City’s design criteria as a
depth of flow to pipe diameter (d/D) ratio exceeding 0.75. These deficiencies indicate potential
capacity issues in the collection system. Based on this criterion, the existing system did not exhibit
capacity deficiencies at average dry-weather flows or at peak wet-weather flows. Of the eight
interceptors, the model shows the North Frontage Road Interceptor to be nearest to capacity, with a
capacity of 7.0 MGD and a peak wet weather design flow of 4.6 MGD in the section of parallel
20-inch diameter pipes.
Under existing conditions, all of the lift stations have enough capacity to pump the peak wet weather
design flow, and no deficiencies were identified. In conjunction with planning for future expansion
and build-out, the existing lift stations will be evaluated further to compare the costs of constructing
larger regional lift stations, as proposed in the 2007 Facility Plan, to using smaller, localized lift
stations similar to the lift stations that have been built since the 2007 Facility Plan.
1.5.1 Recommendations
The highest confidence in the model results was in the interceptors in the vicinity of monitoring
locations; confidence is reduced upstream in the trunk and collector sewers. The capacity of the
interceptors and the capacity modeled both depend on the slope of the interceptor. There is high
uncertainty in the elevations used for inverts in the model due to various vertical datum values used
over time in new development areas within the City. As more accurate pipe invert elevations are
collected, the model should be re-evaluated for capacity.
The model assumes the diameter of each pipe is accurate based on GIS data provided by the City.
The internal diameter may be significantly smaller than the reported diameter and this may cause
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capacity constraints. This is especially true in large diameter PVC and HDPE pipes as well as pipes
that have been lined. If there are pipes with actual internal diameters not in agreement with the
current GIS data, the GIS data should be updated in the model to re-evaluate pipe capacities.
Although not used in the evaluation of the existing system, the pipe flow velocities calculated in the
modeling effort can be used in the City’s maintenance program; pipes with velocities less than 2 fps
should be regularly cleaned due to potential for settling/line blockage that could cause reduced
capacity, and pipes with velocities greater than 5 fps should be regularly checked for structural
problems such as excessive scouring.
The City should continue with the recommendations from the previous master plan to reduce
infiltration and inflow into the system. Specifically, the City should take the following steps:
Enforce existing ordinances to prevent crawl space sump pumps from discharging to the
sewer system.
Rehab or replace old clay pipe in the older parts of the City.
Continue the program to CCTV and record approximately 20 percent of the existing
collection system each year to identify specific problem areas associated with aging pipe.
The City should implement a routine flow monitoring program to track wastewater flows in the
collection system. It is recommended that, at a minimum, the four flow monitoring sites utilized in
this study be monitored on a biannual basis. As the city expands, a more robust flow monitoring
would be appropriate. Project specific flow monitoring should be considered for larger scale
development projects to confirm model data, particularly in areas located far from the study’s flow
monitoring sites.
1.6 FUTURE SYSTEM EVALUATION
Expansion and improvements to the existing wastewater infrastructure will be needed to serve
population growth within the service area. Most of the undeveloped and vacant land within the
existing city limits is in the west, north and northeast portions of the City. As these areas develop,
they will add flows to the interceptors on the east and west side of the system and new collectors
will need to be constructed to connect the new development to the existing interceptors.
Future interceptors, trunk lines and lift stations were added to the existing system model to evaluate
future system needs. The gravity sewer main extensions were planned to follow existing roadways
where roads existed and followed drainage basins where roads did not exist, and to follow existing
topography to maximize gravity flow. Lift stations and force mains were planned where gravity flow
would not be feasible. The model was used to preliminarily size proposed extensions, including lift
stations and force mains. In particular, the areas of expansion to the north and west are at a lower
elevation than the existing infrastructure and will require lift stations.
The future system was evaluated for two planning periods. The near-term planning period, called
“Existing and Obligated,” assumes the entire area within the current city limits is completely
developed. The “Existing and Obligated” model will show if the existing infrastructure has capacity
to meet current obligations for service, and what improvements would be required. The long-term
planning period, called “Study Area Build-Out,” assumes the entire study area is completely
developed based on land uses in the Community Plan. The “Study Area Build-Out” model shows
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what improvements and expansions to the wastewater system will be required to serve the entire
future service area at full build-out. The “wet weather” models determine system capacity; gravity
main capacity deficiencies are noted based on the City’s criteria that peak-hourly wet-weather flow
depth should not exceed 75 percent of the pipe diameter.
1.6.1 System Deficiencies
The Existing and Obligated planning model identified several areas that, before full build-out of this
scenario, will become deficient and will require sewer main extensions, replacement, new parallel
main, or pumping. The major items are:
The existing trunk sewer from the intersection of I-90 and Main Street to the Rouse
Interceptor (contingent on growth on the east side of the City).
The North Frontage Road Interceptor, at two separate sections of parallel 20-inch pipes.
The South University area.
The Study Area Build-Out planning model, which would increase the service area from 11,700 to
42,500 acres, identified several areas that will become deficient before full build-out of this scenario
and will require sewer main extensions, replacement, new parallel main, or pumping. The major
items, additional to the above-described “Existing and Obligated” items, are:
Replace the older section of the Davis-Fowler Interceptor with a 24-inch diameter pipe.
Increase capacity of the entire length of the North Frontage Road Interceptor.
Replace the 30-inch WWTP interceptor, which receives flow from the entire City, with a
48-inch pipe.
The 15-inch section of the Norton East Ranch Outfall Sewer (at Baxter Lane and Flanders
Mill Road) will reach capacity as the Aajker Creek drainage basin develops. Improvements
could involve either a partial or full diversion of these flows, with pumping implications
described in Chapter 5 - Future Collection System Evaluation.
Much of the Aajker Creek basin drains to areas at lower elevation than any existing sewer
infrastructure. It is possible for the southern portion of the Aajker Creek basin to drain by
gravity to the Norton East Ranch Outfall Sewer Interceptor. However, in order to reduce
the ultimate flow to the existing infrastructure a new diversion can be built to divert some of
the flow north to the new lift station required to service the rest of the basin (the proposed
future Davis Lane Lift Station). The diversion will not be needed until approximately 40 to
60 percent of the basin to the south of this new diversion is developed.
Expand capacity of the Norton Ranch Lift Station before the Norton Ranch Subdivision is
fully developed.
Numerous new extensions are required to provide full service to the planning area as
identified in Chapter 5.
In order to serve areas north of the existing system, four lift stations are proposed to pump
flows to existing infrastructure: Gooch Hill, Hidden Valley, Davis Lane and Spring Hill.
The need and timing of these improvements will largely be driven by development, either to extend
service to an area or to provide adequate capacity in the existing system. The city should look for
opportunities to install the infrastructure in an economically efficient fashion where possible. One
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example of this is to install the Davis-Fowler Interceptor at the time the underlying property
annexes while it is still undeveloped instead of waiting for the timing to be driven by capacity needs.
1.6.2 Recommended Improvements
As identified in Chapter 5 and summarized in Chapter 6, a series of projects are recommended. Due
to uncertainty in development timing and locations within the service area, it is difficult to determine
when future improvements will be needed. System flows depend on what development has
occurred within the contributing area, and improvements should be timed based on development
timing.
The projects summarized in Table 1-2 are recommended to serve future growth within the existing
city limits. This includes providing new sewer to areas that currently aren’t developed and only
includes pipes sized 12-inches and greater. The analysis performed to identify the projects and more
details on the projects is provided in Chapter 5 of this report.
Table 1-2 – Category 1 Improvements
Project Name Improvements Description Probable Cost
Front Street Interceptor Replace or parallel 8,500 feet of sewer along Front St and Haggerty Ln
from E Tamarack St to Ellis St $ 2,180,000
North Frontage Road
Interceptor
Replace or parallel 11,500 feet of the North Frontage Rd Interceptor
between Springhill Rd and Bridger Dr $ 5,290,000
South University
District
New 5,000 feet of sewer to divert South University District
development flows to the Davis-Fowler Interceptor $ 1,120,000
Norton Ranch Lift
Station
Increase capacity at the Norton Ranch Lift Station to support further
development of the Norton Ranch Subdivision $ 500,000
Davis Lane Lift Station Construct small initial Davis Lane Lift Station to serve area north of the
Cattail Lake Lift Station $ 500,000
Bridger Drive
Extension
Install a new 2.5 mgd capacity sewer along Bridger Drive from Birdie
Drive to Story Mill Rd $ 300,000
Additionally, 29,800 feet of sewer extensions were identified to serve currently undeveloped areas
within the city limits that were less than 12-inches in diameter. The total probable cost of these
extensions was estimated at $6,680,000.
The projects summarized in Table 1-2 are recommended to serve future growth outside the existing
city limits. The analysis performed to identify the projects and more details on the projects is
provided in Chapter 5 of this report. Many of these projects will be required outside of the City’s
normal 20 year planning period and should be re-evaluated as the land use and growth patterns are
updated. It is important to note that if development is concentrated in a basin contributing to the
project, the project may be required sooner to serve new development.
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Table 1-3 – Category 2 Improvements
Project Name Improvements Description Probable Cost
Davis-Fowler Interceptor Replace or parallel 2700 feet of the Davis-Fowler Interceptor
between Durston Rd and W Oak St $ 760,000
WWTP Interceptor Replace or parallel 1200 feet of sewer from I-90 to the WRF $ 420,000
27th Avenue/Cattail Creek Replace or parallel 3300 feet of the WWTP Interceptor Sewer $ 950,000
Bridger Creek Golf Course to
North Frontage Rd Sewer
Replace or parallel 7700 feet of sewer around the Bridger Creek
Golf Course and from the Course to North Frontage Rd $ 1,590,000
Bridger Dr to North Rouse Ave Replace or parallel 1300 ft of sewer along Bridger Drive
between Birdie Drive and North Rouse Avenue $ 320,000
Norton East Ranch Diversion Divert flow from the Norton East Ranch Sewer to the Davis
Lane Lift Station $ 3,320,000
Davis Lane Lift Station
Expansion
Expand the Davis Lane Lift Station for the Norton East Ranch
Diversion flows $ 5,300,000
Gooch Hill Lift Station and
Forcemain Construct a 6.2 mgd lift station and forcemain $ 7,820,000
Hidden Valley Lift Station and
Forcemain Construct a 1.5 mgd lift station and forcemain $ 5,190,000
Spring Hill Lift Station and
Forcemain Construct a 4.2 mgd lift station and forcemain $ 4,650,000
In addition to the improvements described above, an additional 380,000 feet of trunk lines at an
estimated cost of $78,000,000 will be required to serve areas outside of the current city limits and
within the Community Plan Boundary. All of the expansions to the City is heavily dependent on
where development occurs and service is required.
1.7 POLICY AND PROGRAM RECOMMENDATIONS
A number of policy and program related recommendations were identified as part of the planning
process. These include both new policies and important existing policies. The following is a list of
those recommendations:
1. Drainage Basin Integrity
It is important that the drainage basins be maintained as outlined in the report to avoid
impacting the available capacity assigned to each drainage basin. The proposed
improvements are sized based on the contributing area defined within each drainage basin
and any significant changes in the basin boundaries may result in a lack of capacity in the
planned extensions. It is recommended that the city implement a policy to prohibit
transferring flow from one basin to another.
2. Collect Accurate Elevation Data – Existing System
For the existing system, particularly for trunk and interceptor sewers, obtaining accurate
manhole rim and invert elevation would improve the accuracy of future wastewater
modelling efforts.
3. Standardize Elevation Datum – Future System
The standard process for establishing project specific datums has historically relied on the
city’s fire hydrant benchmark program. This method served the city well for many years;
however, as the city has expanded the benchmark system is prone to compounding errors.
With current survey technology it is recommended that a new standard system be developed.
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4. Sump Pumps and Roof Drains
Continuing to enforce current policies to prevent sump pump and roof drain connections to
the sewer system is critical to maintaining capacity for wastewater flows.
5. Collection System Rehabilitation
Continuing an annual rehabilitation program at current or higher levels is important in order
to continue to reduce infiltration and maintain system integrity as the overall system ages.
6. Flow Monitoring
A routine program of flow monitoring in the collection system is recommended.
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