HomeMy WebLinkAboutAPPENDICES - City of Auburn Comprehensive Sewer Plan_FINAL.pdfCity of Auburn Comprehensive Sewer Plan
A-1
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Inter-local Agreements and Outside Appendix A:
Agency Correspondence
A1. King County
• Sewage Disposal Service with Metro (Ordinance 2774, and Resolutions 1727 and 2090)
• Franchise Agreement No. 14458
A2. Soos Creek Water and Sewer District
• Service Area Boundaries (Resolution 3321)
A3. City of Kent
• Sewer Service Boundaries (Resolution 3322)
A4. City of Pacific
• Sewer Service Boundaries (Resolutions 4335 and 730)
A5. Muckleshoot Indian Tribe
• Sewer Service Boundaries (Resolution 4902)
• Wastewater Conveyance Cost Sharing (Resolution 3660)
• Temporary Sewage Lift Station Operation (Resolution 3502)
A6. Lakehaven Utility District
• Sewer Service Boundaries (Resolutions 3651, 3824, and 2005-1038)
A7. City of Algona
• Sewer Service Boundaries (Resolution 3589)
A8. City of Bonney Lake
• Sewer Service Boundaries (Resolutions 3760 and 3796)
• Right of Way Use Permits (Resolutions 3873 and 1471)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A1: Inter-local Agreements and Outside
Agency Correspondence
King County
x Sewage Disposal Service with Metro (Ordinance 2774, and Resolutions
1727 and 2090)
x Franchise Agreement No. 14458
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A2: Inter-local Agreements and Outside
Agency Correspondence
Soos Creek Water and Sewer District
x Service Area Boundaries (Resolution 3321)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A3: Inter-local Agreements and Outside
Agency Correspondence
City of Kent
x Sewer Service Boundaries (Resolution 3322)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A4: Inter-local Agreements and Outside
Agency Correspondence
City of Pacific
x Sewer Service Boundaries (Resolutions 4335 and 730)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A5: Inter-local Agreements and Outside
Agency Correspondence
Muckleshoot Indian Tribe
x Sewer Service Boundaries (Resolution 4902)
x Wastewater Conveyance Cost Sharing (Resolution 3660)
x Temporary Sewage Lift Station Operation (Resolution 3502)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A6: Inter-local Agreements and Outside
Agency Correspondence
Lakehaven Utility District
x Sewer Service Boundaries (Resolutions 3651, 3824, and 2005-1038)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A7: Inter-local Agreements and Outside
Agency Correspondence
City of Algona
x Sewer Service Boundaries (Resolution 3589)
City of Auburn Comprehensive Sewer Plan
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145308 Auburn Comprehensive Sewer Plan_Appendix A Flysheets.docx
Appendix A8: Inter-local Agreements and Outside
Agency Correspondence
City of Bonney Lake
x Sewer Service Boundaries (Resolutions 3760 and 3796)
x Right of Way Use Permits (Resolutions 3873 and 1471)
City of Auburn Comprehensive Sewer Plan
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Hydraulic Capacity Analysis Appendix B:
Technical Memorandum
Limitations:
This document was prepared solely for the City of Auburn in accordance with professional standards at the time the services were performed and in
accordance with the contract between the City of Auburn and Brown and Caldwell dated December 6, 2013. This document is governed by the
specific scope of work authorized by the City of Auburn; it is not intended to be relied upon by any other party except for regulatory authorities
contemplated by the scope of work. We have relied on information or instructions provided by the City of Auburn and other parties and, unless
otherwise expressly indicated, have made no independent investigation as to the validity, completeness, or accuracy of such information.
701 Pike Street Suite 1200
Seattle, WA 98101
T: 206.621.0100
F: 206.749.2200
Prepared for: City of Auburn
Project Title: Sanitary Sewer Comprehensive Plan
Project No.: 145308
Technical Memorandum
Subject: Sanitary Sewer Model Development
Date: March 9, 2015
To: Robert Elwell, P.E.
From: Justin Twenter, P.E.
Sanitary Sewer Model Development
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Table of Contents
List of Figures ........................................................................................................................................................... iii
List of Tables ............................................................................................................................................................. iv
List of Abbreviations ................................................................................................................................................. iv
Section 1: Introduction ............................................................................................................................................. 1
Section 2: Modeling Scenarios ................................................................................................................................ 1
Section 3: Flow Monitoring Data ............................................................................................................................. 1
Section 4: Climatic Data ........................................................................................................................................... 4
4.1 Rainfall....................................................................................................................................................... 4
4.2 Evapotranspiration.................................................................................................................................... 5
Section 5: Hydraulic Model Development ............................................................................................................... 5
5.1 Software Platform ..................................................................................................................................... 5
5.2 Model Extent ............................................................................................................................................. 6
5.3 Infrastructure Data ................................................................................................................................... 8
5.3.1 Geographic Information System ................................................................................................. 8
5.3.2 Pump Station ............................................................................................................................... 9
5.3.3 Stuck River Trunk ...................................................................................................................... 12
5.4 Boundary Conditions .............................................................................................................................. 13
Section 6: Hydrologic Model Development ........................................................................................................... 13
6.1 Calibration Period Dry Weather Flow ..................................................................................................... 13
6.2 Development of Subcatchments ........................................................................................................... 16
6.3 Calibration Period Wet Weather Flow .................................................................................................... 18
Section 7: Long-Term Simulations......................................................................................................................... 21
Section 8: Future Conditions ................................................................................................................................. 23
8.1 Future Dry Weather Flow ........................................................................................................................ 23
8.1.1 Dry Weather Flow from Population Expansion ........................................................................ 23
8.1.2 Dry Weather Flow from Sewer Extension ................................................................................. 26
8.2 Future Wet Weather Flow ....................................................................................................................... 28
8.3 Future Hydraulic Improvements............................................................................................................. 29
8.4 Future-Conditions Summary ................................................................................................................... 32
Section 9: Model Results ....................................................................................................................................... 32
9.1 Baseline Conditions ................................................................................................................................ 32
9.2 2020 Conditions ..................................................................................................................................... 35
9.3 2034 Conditions ..................................................................................................................................... 37
Section 10: Conclusions ........................................................................................................................................ 40
References .............................................................................................................................................................. 41
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Attachment A: Modifications to Collection System GIS ....................................................................................... A-1
Attachment B: Pump Station Data ....................................................................................................................... B-1
List of Figures
Figure 3-1. Auburn area flow monitors.................................................................................................................... 3
Figure 5-1. Auburn MIKE URBAN model extent ...................................................................................................... 7
Figure 5-2. Example pipe profile with interpolated manhole invert elevation ...................................................... 9
Figure 5-3. Ellingson PS comparison..................................................................................................................... 11
Figure 5-4. Location and flow path of SRT ............................................................................................................ 12
Figure 6-1. AUBRN53 monitor flow data and observed rainfall .......................................................................... 14
Figure 6-2. AUBRN53 monitor DWF calculation ................................................................................................... 15
Figure 6-3. Dry weather flow schematic ................................................................................................................ 16
Figure 6-4. Upstream area of influence example ................................................................................................. 17
Figure 6-5. MIKE URBAN RDI engine schematic .................................................................................................. 19
Figure 6-6. AUBRN53 wet weather calibration ..................................................................................................... 21
Figure 7-1. Citywide peak RDII Cunnane plot ....................................................................................................... 22
Figure 8-1. TAZ polygons within the vicinity of Auburn ......................................................................................... 24
Figure 8-2. Proposed sewer extensions and development percentages ............................................................ 27
Figure 8-3. High and low volume time series comparison example.................................................................... 29
Figure 8-4. Pacific PS re-route ............................................................................................................................... 31
Figure 9-1. Baseline-conditions minimum freeboard ........................................................................................... 33
Figure 9-2. Low-freeboard short manhole ............................................................................................................ 34
Figure 9-3. Flooding along Boundary Boulevard SW ............................................................................................ 34
Figure 9-4. 2020 conditions minimum freeboard ................................................................................................ 36
Figure 9-5. 2020 surcharge of the KC-owned Auburn West Interceptor ............................................................ 37
Figure 9-6. 2034 conditions minimum freeboard ................................................................................................ 38
Figure 9-7. Surcharged line upstream of Verdana PS .......................................................................................... 39
Figure 9-8. Hydraulic profile of KC-owned Auburn West Interceptor at Perimeter Road ................................... 39
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List of Tables
Table 3-1. King County Flow Monitors..................................................................................................................... 4
Table 4-1. Long Term Rainfall Sources and Dates ................................................................................................. 5
Table 5-1. Auburn Area Pump Station Capacities ................................................................................................ 10
Table 5-2. Auburn Area Pump Station Upgrades .................................................................................................. 11
Table 6-1. MIKE URBAN Hydrologic Model Calibration Parameters .................................................................... 19
Table 7-1. Peak RDII Cunnane Estimated Flow Frequency ................................................................................ 22
Table 7-2. Peak RDII per Monitoring Basin ........................................................................................................... 23
Table 8-1. Future Population Estimates by TAZ Polygon ...................................................................................... 25
Table 8-2. Analysis Period Model Modifications ................................................................................................... 32
List of Abbreviations
BC Brown and Caldwell
CIP capital improvement project
City City of Auburn
DHI Danish Hydraulic Institute
DWF dry weather flow
ET evapotranspiration
ft3 cubic foot/feet
GIS Geographic Information System
gpcd gallons per capita per day
HGL hydraulic grade line
H&H hydrologic and hydraulic
ID identification
KC King County
LOS level of service
mgd million gallon(s) per day
MH manhole
PS pump station
RDII rainfall-derived inflow and infiltration
SR State Route
SRT Stuck River Trunk
SSA sewer service area
TAZ Traffic Analysis Zone
WWHM Western Washington Hydrology Model
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Section 1: Introduction
As part of the 6-year comprehensive planning effort, Brown and Caldwell (BC) developed a hydrologic and
hydraulic (H&H) model of the City of Auburn’s (City) sewer collection system. The objective of the modeling
effort was to characterize the magnitude and volume of rainfall-derived inflow and infiltration (RDII) entering
the sewer system during wet weather events and to evaluate whether that RDII causes surcharge beyond the
established level of service (LOS) and/or surface flooding during a 20-year storm in baseline (year 2014)
conditions as well as future conditions in year 2020 (6-year scenario) and year 2034 (20-year scenario)
conditions. This document describes the analysis periods covered in this effort, driving data used to build the
model, calibration of the model, and hydraulic results from the 20-year storm simulations. In general, the
City’s sewer system infrastructure performs well in a 20-year storm. The modeling analysis indicated that no
capital improvement projects (CIPs) are required to address hydraulic capacity restrictions in City-owned
pipes in the existing or future 6-year conditions. The future 20-year condition indicates an area of hydraulic
restriction upstream of the Verdana Pump Station (PS); however, CIPs are not planned from 20-year condi-
tion simulation results.
Section 2: Modeling Scenarios
Four scenarios were analyzed for this modeling effort. For each case, the model was modified to represent
H&H conditions at a particular point in time. Each of the modeling scenarios are described below:
• Calibration period: Flow data used to calibrate the model were collected between September 2009 and
May 2011. In order to calibrate the model’s hydrology appropriately, the modeled collection system flow
paths needed to be representative of that time period. Trunk line and pump station upgrades made be-
tween the calibration period and current conditions were left out of the model for model calibration.
• Baseline conditions: The major change between the calibration period and baseline 2014 conditions
came from the construction of the Stuck River Trunk (SRT) and four pump station modifications. The SRT
diverts flow east from the intersection of K Street SE and 17th Street SE to the Auburn West Interceptor
along C Street SW (see additional description in Section 5.3.3. Before 2014, the Valley Meadows and
White Mountain pump stations were decommissioned, the Verdnana pump station was constructed, and
the Ellingson and Dogwood pump stations were upgraded. All of these changes were reflected in the
model to represent baseline conditions.
• 6-year planning horizon: Wastewater planners plan 6 years out, so, based on 2014 data, population
growth and RDII increases in the system are projected for 2020. Plans to re-route the discharges from
King County’s (KC’s) Pacific PS are also included in this simulation because it is expected to be re-routed
in the near future.
• 20-year planning horizon: Wastewater planners also plan 20 years out, so, based on 2014 data,
population growth and RDII increases in the system are projected for 2034. The 20-year planning hori-
zon looks for long term changes in the sewer system and the results are used to indicate areas to ob-
serve, rather than inform immediate capital improvement projects.
Section 3: Flow Monitoring Data
The KC Flow Monitoring program deployed between 80 and 120 flow monitors throughout the KC convey-
ance system to measure sanitary sewer flows for system management and capital facilities planning.
Fourteen of those monitors measure flows from KC mainlines that service areas within the city of Auburn.
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The data from those monitors are used in this study to characterize dry weather flow (DWF) patterns and to
perform model calibration. The flow monitoring locations are described in Table 3-1. Figure 3-1 presents a
map of the city of Auburn with the flow monitors located and their tributary upstream basins delineated.
These basins are called “monitoring basins.”
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Figure 3-1. Auburn area flow monitors
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Table 3-1 provides a location description for each of the flow monitors used in this study and the date
ranges of the monitors’ available data. In general, each flow monitor provided two wet seasons of data.
Table 3-1. King County Flow Monitors
Monitor ID Location Date range
ABN008 20th St. NW at West Valley Hwy. 7/29/2009–5/23/2011
ABN022 37th St. NW east of 1st St. NW 7/30/2009–5/23/2011
ABN023 30th St. NW west of C St. NE 9/1/2009–5/17/2011
ABN027 29th St. NW at Interurban Trail 8/10/2009–5/22/2011
ABN032 Between Clay St. NW and H St. NW south of 6th St. NW 7/28/2009–5/16/2011
AUBRN53 44th St. NW east of I St. NW 9/1/2009–5/16/2011
AUBWV016 Boundary Blvd. SW at O Street SW 9/1/2009–5/18/2011
LAKELANDHILLS_WW Lakeland Hills PS northwest of Oravetz Rd. SE 3/4/2010–7/25/2011
LKH001A 37th St. NW west of 1st St. NW 9/17/2009–5/16/2011
MSTTR02A 23rd Street NE at E Street NE 8/6/2009–5/23/2011
MSTTR22A Henry Rd. NE north of 6th St. NE 8/3/2009–5/16/2011
MSTTR48 K St. SE north of 17th St. SE 9/1/2009–5/18/2011
WINT003 B St. NW north of 16th St. NW 9/1/2009–5/18/2011
WINT038 Interurban Trail north of 15th St. SW 9/28/2009–5/23/2011
Because the model’s hydrologic parameter sets are defined by monitoring basin, the boundaries in Figure
3-1 also define the extent of each of the calibration models. In other words, the portions of the model
defined by the monitoring basin boundaries were calibrated independently of each other as calibration
basins. This is described in further detail in Section 6.3.
Section 4: Climatic Data
Rainfall and evapotranspiration (ET) time series data are required to simulate RDII processes within the
hydrologic model. The following sections describe the development of these data for use in hydrologic model
calibration and long-term model simulations.
4.1 Rainfall
BC developed a rainfall time series with 15-minute time increments based on data from several rain gauges.
Rainfall measurements recorded at the City’s rain gauge located at Auburn City Hall were used for the model
calibration period (August 2009 through May 2011) because the City Hall gauge is located closest in proxim-
ity to the flow monitoring basins.
For the times when the City Hall gauge was uninstalled or malfunctioning, the data gaps were filled with
rainfall collected at the Lakeland Hills PS, which is operated by KC and made available for free at the King
County Hydrologic Information Center (King County, 2014). A comparison of rainfall totals for the Auburn City
Hall gauge and the Lakeland Hills gauge over the period January 1, 2010, through April 30, 2011 (where
data exist for both gauges) indicated that the Lakeland Hills gauge recorded 8.6 percent more rainfall than
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the City Hall gauge, which is considered acceptable based on best practices and engineering judgment for
comprehensive planning analysis.
Local rainfall data were appended with 61 years of rainfall from Sea-Tac International Airport to create a
long-term rainfall record that can be used to analyze wet weather frequency. The Sea-Tac data were extract-
ed from the Western Washington Hydrology Model (WWHM 2012). Data priority in the record was given to
the nearest gauges, with gauges farther outside of Auburn used only when necessary to complete the long-
term record. Table 4-1 lists the rainfall data sources, date ranges used to compile the long-term record, and
notes associated with the use of the data.
Table 4-1. Long Term Rainfall Sources and Dates
Gauge location Data source Start date End date Notes
Sea-Tac Airport WWHM 2012 10/1/1948 12/31/2009 15-minute rainfall, used only in long-term simulations
Auburn City Hall City of Auburn 1/1/2010 12/31/2010 15-minute local rainfall
Auburn City Hall City of Auburn 1/1/2011 5/1/2011 5-minute rainfall aggregated to 15-minute time step
Lakeland Hills PS King County Hydrologic
Information Center
5/2/2011 5/31/2011 15-minute rainfall to fill the gap in the Auburn record with
KC rainfall data
Auburn City Hall City of Auburn 6/1/2011 11/13/2012 15-minute local rainfall
Lakeland Hills PS King County Hydrologic
Information Center
11/14/2012 12/5/2012 15-minute rainfall to fill the gap in the Auburn record with
KC rainfall data
4.2 Evapotranspiration
ET data are required to estimate evaporation and transpiration losses from the land surface. The Washing-
ton State University Puyallup, Washington, extension operates the AgWeatherNet website, which is a reposi-
tory for numerous climatological data sets throughout Washington State, including grass reference ET
calculated for Puyallup. Grass reference ET from AgWeatherNet was acquired for the same time period as
the long-term rainfall record (WSU, 2014). Given that the cities of Puyallup and Auburn are situated at
roughly the same elevation in the eastern Puget Sound region, their daily ET values are likely similar; there-
fore, the Puyallup Reference ET data set was considered applicable to the city of Auburn.
Section 5: Hydraulic Model Development
The following sections describe the software platform chosen for this modeling effort, the hydraulic model
extent, as well as the infrastructure data used to create the model.
5.1 Software Platform
Auburn’s sanitary sewer collection system discharges to KC mainlines at various locations throughout the
city. KC has performed sanitary sewer modeling at a coarse spatial resolution in the Auburn area using MIKE
URBAN 1 software, which is the County’s preferred modeling platform. Therefore, the City has chosen to use
1 MIKE URBAN is a software package developed and sold by the Danish Hydraulic Institute (DHI). More information can be found at
http://www.mikepoweredbydhi.com/products/mike-urban.
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MIKE URBAN to be consistent with the County’s modeling approach. The latest version of MIKE URBAN
(version 2014) was used.
KC uses the MIKE URBAN Runoff Model A for surface flows and the RDI model for groundwater infiltration to
estimate RDII. KC also uses the MOUSE hydraulic engine within MIKE URBAN to solve the hydraulic flow
routing equations. The MOUSE engine uses the full Saint Venant equations to solve for water levels and
velocities in piped systems. The Saint Venant equations provide more accurate hydraulic solutions in
complicated hydraulic environments that include changing flow rates, pipe surcharging, and back water
effects than simpler calculations such as the Kinematic Wave, which cannot solve flow rates in backwater
conditions. For consistency with KC, Runoff Model A and the RDI model were used in conjunction with the
MOUSE hydraulic engine in this modeling effort.
5.2 Model Extent
The hydraulic extent of the MIKE URBAN model was chosen to be consistent with previous comprehensive
planning modeling efforts. The same pipes, manholes (MHs), and pump stations included in the 2008
Comprehensive Plan (Brown and Caldwell, 2009) model were included in the current model. System modifi-
cations since then (new pump stations, conduits, and force mains) were reflected in the current model as
well.
In general, all pipes 10 inches in diameter and larger within the sanitary sewer service area (SSA) were
included in the model; smaller pipes were included only where needed to connect larger pipes to the main
network and force mains. Pipes smaller than 10 inches in diameter are less likely to be under capacity
because they are located predominantly in neighborhoods at the headwaters of the collection system and
they convey small flows to the mainline system. Ignoring these pipes greatly improves model run times
because of the reduced number of pipes requiring hydraulic calculations. Figure 3-1 provides a map of
Auburn’s collection system, the pipes included in the MIKE URBAN model, and the boundary of the SSA.
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Figure 5-1. Auburn MIKE URBAN model extent
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5.3 Infrastructure Data
Three sources of data were used to develop inputs for the collection and conveyance system: (1) the City’s
sewer utility geodatabase, (2) requested data from the City, and (3) the previous Comprehensive Plan model.
The following sections describe the data hierarchy and assumptions made in hydraulic model development.
5.3.1 Geographic Information System
The City’s Geographic Information System (GIS) Department provided BC with a 2014 sewer utility geodata-
base titled “Sewer.gdb.” The geodatabase contains geospatial locations and attribute data for sanitary sewer
structures such as mainlines, manholes, pump stations, and other appurtenances. The geodatabase was
used as the primary data source for constructing the collection and conveyance system model.
Pipe attributes such as diameter, inlet and outlet elevations, and length, as well as manhole attributes of
invert and rim elevation, are all necessary to build out the hydraulic network. Some gaps existed in the
geodatabase that required an assumption to fill. The following describes the hierarchy of assumptions used
to assign missing data:
• Manholes:
− Invert elevation: If missing or suspect, the inlet invert elevation from the outgoing pipe was used, if
available, as it should be the lowest connecting element to the manhole. Otherwise, invert elevation
of the same node in the 2008 Comprehensive Plan model was used. If the data are not available
from those two sources, straight-line interpolation between the upstream and downstream manhole
invert elevations was used.
− Rim elevation: If missing or suspect, rim elevation of the same node in the 2008 Comprehensive
Plan model was used, if available. Otherwise, the rim elevation was estimated from land surface el-
evation contour data.
− Manhole ID: The “Structure” field was used to uniquely identify each manhole. If missing in the GIS,
the identification number from the 2008 Comprehensive Plan model ID was used.
• Conduits:
− Inlet and outlet elevations: If missing or suspect, the invert elevation from the connecting manhole
was used. Otherwise, the 2008 Comprehensive Plan model value was used. If both of those were
unavailable, straight-line interpolation between the nearest known upstream and downstream ele-
vations was used.
− Diameter: If missing or suspect, the 2008 Comprehensive Plan model value was used. Otherwise, it
was estimated based on the diameters of the adjacent pipes.
− Conduit ID: Identification numbers were verified for all conduits. If an ID was missing, the 2008
Comprehensive Plan model ID was used
Once the gaps in the data fields were filled, the database was imported to the MIKE URBAN model such that
hydraulic profiles could be plotted to inspect for erroneous data through visual inspection. Hydraulic profiles
plots were drawn for the entire modeled collection system and used to find incorrect diameters or invert
elevations by checking for severe and/or adverse slopes. Adjustments were made to correct elevations using
data from the 2008 Comprehensive Plan model where available. Otherwise, straight-line interpolation was
used for elevation data replacement (see example in Figure 5-2). No diameters appeared to require adjust-
ment in this process. In total, 110 manhole invert elevations were adjusted to remove GIS elevation errors
from the hydraulic model. These adjustments are documented in Attachment A.
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Figure 5-2 below shows a profile of a pipe section requiring elevation adjustment. The manhole in the middle
of the profile likely has an incorrect invert listed in the GIS as a small gravity pipe is unlikely to be so dramat-
ically inclined. Consequently, the manhole was assigned a new invert by interpolating between the two
manholes on either side. The connecting conduits were adjusted to have no inlet or outlet offset from the
adjusted manhole. Figure 5-2 provides the adjusted profile. This adjustment reduces the risk of simulating
system backups that are likely based on unconfirmed GIS data.
Figure 5-2. Example pipe profile with interpolated manhole invert elevation
5.3.2 Pump Station
BC asked the City for updated wet well volume and pump capacity information for the pump stations within
the study area. On March 14, 2014, the City provided an Excel table summary of the requested pump station
information, which is included in Attachment B.
Each pump station is composed of a lead pump and a lag pump. The single pump capacity was provided by
the Cityfor each, but a total combined pump capacity was not given for when both pumps are running. To
account for the reduction in capacity due to higher downstream head conditions from both pumps running,
but without empirical data to inform a reduction factor on the second pump’s capacity, a general assumption
of 50 percent of single pump capacity was assumed for the lag pump. Table 5-1 below provides the pump
station capacity information used in the hydraulic model.
Likely incorrect MH
invert elevation
Adjusted invert
elevation
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Table 5-1. Auburn Area Pump Station Capacities
Pump station Location Number of
pumps
Single pump capacity
(mgd)
Multiple pump capacity
(mgd)
22nd Street 22nd St. SE & Riverview Dr. 2 0.792 1.188
8th Street J St. NE & 8th St. NE 2 0.216 0.324
Area 19 Lake Tapps Pkwy. E & west of 72nd St. SE 2 0.468 0.702
D Street D St. NE & Auburn Way N. 2 0.576 0.864
Dogwood Dogwood St. SE 1500 & 15th St. SE 2 0.432 0.648
Ellingson Road 41st St. SE, east of A St. SE 2 2.199 3.298
F Street F St. SE & 17th St. SE 2 0.576 0.864
Lakeland Hills Oravetz Rd. SE north of Mill Pond Dr. SE 1* 1.732 N/A
North Tapps Lake Tapps Pkwy. E & west of 176th Ave. E. 2 0.734 1.102
Peasley Ridge S. 320th St. & 53rd Ave. S. 2 0.396 0.594
R Street R St. NE & 6th St. NE 2 0.144 0.216
Rainer Ridge 125th Pl. SE & south of SE 318th Way 2 0.288 0.432
Riverside 8th St. NE & 104th Ave. SE 2 0.576 0.864
Terrace View E Valley Hwy. E & north of Terrace View Dr. SE 2 0.972 1.458
Valley Meadows 4th St. SE & V St. SE 2 0.180 0.270
Verdana 118th Ave. SE & SE 296th Pl. 3** 2.520 3.780
* Lakeland Hills PS pump data not provided by the City as it is KC-owned. Single pump and parameters used from 2008 Comprehensive Plan
model as no new data were available.
**Third Verdana pump is an emergency pump and is not included in the hydraulic model.
The City was unable to provide updated data about the Lakeland Hills PS because it is owned and operated
by KC. Therefore, the 2008 Comprehensive Plan model values for the pump station were reused in this
modeling effort. The data available then were limited to one pump at 1.732 million gallons per day (mgd)
capacity, although the pump station has two discharge force mains and multiple pumps. Only one force main
is included in the model as only one pump capacity is known.
Since the previous Comprehensive Plan update, three pump stations were upgraded, a new pump station
was built, and two pump stations were decommissioned. The Dogwood and Ellingson PSs were upgraded
and are included in the model. The Auburn 40 PS, although upgraded, is not explicitly modeled. Rather, its
flow contribution and signature is accounted for within the model’s hydrologic calibration. Because the
hydraulic network is not extended up to the pump station, flow peaks are created within the hydrologic
model rather than using a pump station to augment the flow signature. The Verdana PS was constructed to
lift water that used to flow to the Valley Meadows and White Mountain PSs, both of which were decommis-
sioned. Table 5-2 below provides a summary of the modeled pump station upgrades since the previous
Comprehensive Plan. Following the table are discussions of how these changes were accounted for within
the model.
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Table 5-2. Auburn Area Pump Station Upgrades
Pump station Monitoring basin Previous capacity (mgd) New capacity (mgd) Previous wetwell
volume (ft3)
New wetwell
volume (ft3)
Dogwood MSTTR48 0.58 0.65 930 925
Ellingson WINT038 2.9 3.30 500 1,086
Verdana MSTTR02A
Valley Meadows = 0.36
3.78
Valley Meadows = 588
5,395 White Mountain = 0.36 White Mountain 333
Total = 0.72 Total = 921
The Dogwood PS was upgraded to a slightly higher pumping capacity after the monitoring period. The change
in peak discharge attributable to the pump station was assessed by examining the modeled flow at the
nearest downstream flow monitoring location (MSTTR48) given the two pump station capacities. Visual
inspection of the flow hydrograph at the monitor location indicated that the change in pumping capacity
made no appreciable difference in flow rates. Therefore, the new pump capacity was used during model
calibration.
The Ellingson PS, across the river from the Lakeland Hills PS, was also upgraded after the flow monitoring
period. Therefore, the old pump station parameters were used for calibration and adjusted parameters were
used to model baseline conditions. Both the Lakeland Hills and Ellingson PSs discharge to manhole 1208-
38 because they share a force main. The nearest flow monitor downstream is the WINT038 flow monitor,
which accepts flow from both pump stations. Figure 5-3 presents a comparison of discharges from the
Ellingson PS using previous and adjusted parameters. The use of a new constant-speed pump (orange)
produces flow spikes unlike the discharges from the previous variable-speed pump (blue). However, down at
the flow monitoring location, the pump station flows appear to attenuate and, consequently, the change in
flows at the monitor location are negligible (red = new pump parameters, green = old pump parameters).
The change in peak flow at the flow monitoring location for the December 2010 storm (largest storm in the
monitoring period) is nearly 5 percent, suggesting that the changes at the Ellingson PS did not greatly affect
the flow signature at the flow monitor.
Figure 5-3. Ellingson PS comparison
Orange: new constant-speed pump discharge; blue: old variable speed pump discharge;
green: modeled flows at monitor with new pump; red: modeled flows at monitor with old pump
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The Verdana PS replaced the Valley Meadows and White Mountain PSs for conveying wastewater from the
northeast portion of the city to the KC interceptor lines. The new Verdana PS has more capacity and wetwell
volume than the two older pump stations combined. Gravity pipe infrastructure in the area was modified to
bring wastewater flows to the Verdana PS; these modifications are reflected in the hydraulic model. Because
the subcatchment areas are a function of the pipe lengths within the system, a reworking of the model’s
hydrology would have been required to run the model in the different pumping conditions. Verdana PS is far
enough upstream from the next downstream flow monitor (MSTTR02A) that much of its flow signature can
attenuate before being observed. Furthermore, the Verdana PS’s discharges are a small proportion of the
total flow at the monitor because the MSTTR02A monitor also observes the MSTTR22A and MSTTR48
monitoring basins upstream. Consequently, reworking the model to run the decommissioned Valley Mead-
ows and White Mountain PSs for the sake of calibration was not pursued as the hydraulic modifications do
not make enough of a change in the flow signature at the monitor to warrant model reconstruction.
5.3.3 Stuck River Trunk
The SRT is approximately 4,000 feet long, with a diameter of 27 inches. Its purpose is to route flows from a
capacity-limited sewer line in southeast Auburn across to the Auburn West Interceptor, which has capacity to
convey additional flows. The SRT was constructed in 2013, which was after the monitoring period. Therefore,
the calibration model did not include the SRT. The SRT was built into the hydraulic model for baseline and
future conditions using design plan sets provided by KC. As-built drawings were not available; however,
Robert Elwell (Elwell, 2014) indicated in an e-mail that constructed conditions did not deviate much from the
original drawing set. Figure 5-4 below indicates the location of the SRT and the new flow path it provides.
Figure 5-4. Location and flow path of SRT
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5.4 Boundary Conditions
Boundary conditions at model outfalls can affect the hydraulic performance of a collection system. For a
collection system that discharges to a water body, such as a river, or a treatment plant the elevation of the
river’s water surface can cause backwater in the collection system if the river elevation is high. Knowledge of
the boundary condition is necessary to accurately replicate this phenomenon.
In the case of Auburn’s collection system, a normal depth downstream boundary condition was used, which
assumes that the hydraulic grade line (HGL) in the most downstream link is set by the normal flow depth
rather than a special hydraulic circumstance such as a regulated interceptor where the level is set by
manual or automated controls (thus creating an HGL level that does not correlate with flow rate). In the case
of the Auburn system, the outlet sewer line was modeled beyond the AUBRN53 flow monitor, which is the
most downstream flow monitor, representing the boundary of the calibrated study area. Because the
interceptor line within which the AUBRN53 flow monitor is located is not regulated with controls, normal flow
calculations are adequate in ensuring that the HGL within the model is representative of field conditions. A
normal depth boundary condition also provides a representative downstream hydraulic condition during
long-term simulations where observed depths in the downstream system are not available.
Section 6: Hydrologic Model Development
This section describes the development of the hydrologic model, which produces the two components of
sewer flow:
• DWF, which is composed of wastewater from residential, commercial, and industrial water usage and is
relatively unaffected by climatic conditions
• RDII, which consists of groundwater (infiltration) seeps into sewer pipes through holes, cracks, joint
failures, and faulty connections, as well as runoff (inflow) from roof drain downspouts, foundation drains,
storm drain cross-connections, and through holes in manhole covers.
Subcatchments are created in the model to generate RDII flows to the collection system. The land surface
and subsurface parameters are then calibrated to produce simulated flows that reflect the conditions
observed through flow monitoring. Model simulations provide long-term flow hydrographs that can be
analyzed to quantify the magnitude and frequency of peak flow events for use in conveyance design.
6.1 Calibration Period Dry Weather Flow
DWF can be measured during prolonged dry periods when wet weather flows are relatively small. In the
Pacific Northwest, DWF is best measured in August and September after the aquifers recede and groundwa-
ter baseflows are lowest. Furthermore, these months experience comparatively lower chances of rainfall,
which further improves the likelihood that the observed flow is not influenced by wet weather.
Flow monitoring data across all monitors generally included two dry periods, during the two observed
summers, from which the DWF portion could be calculated. Figure 6-1 below provides the observed flow
data for the AUBRN53 monitor, as an example, with observed rainfall plotted at the top. The brackets
indicate the dry periods that were selected to represent typical DWF patterns. In both of these periods,
rainfall is minimal and the groundwater baseflow is assumed to be minimal as the hydrograph levels off from
its wet season recession. The lack of a recession indicates that the groundwater infiltration has likely ceased
and the only remaining component of the hydrograph is the flow attributable to DWF.
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Figure 6-1. AUBRN53 monitor flow data and observed rainfall
Calculating DWF for implementation into the model involves assessing an average flow magnitude that can
be scaled on an hourly basis to represent the daily use patterns observed in the flow data. Although an
average amount of DWF is created within the system, it enters the system in peaks and troughs based on
usage patterns that vary throughout the day. Hourly scaling factors multiply against the average flow magni-
tude to represent those troughs and peaks within the model. For example, the average flow magnitude for
the AUBWV016 monitoring basin is 1.15 mgd; however, the peak water usage on a weekday from 8 to 9
a.m. is 1.31 mgd. To account for that hour’s DWF, the model scales the 1.15 mgd average value by a factor
of 1.14 to achieve 1.31 mgd within the model from 8 to 9 a.m. Average flow magnitudes and hourly factors
for weekdays, Saturdays, and Sundays were calculated for each of the flow monitors within the area of study
and built into MIKE URBAN’s “cyclic patterns” engine to simulate Auburn’s DWF.
Flow monitors located downstream of other flow monitoring locations were used to quantify DWF for inter-
mediate areas. For example, flow monitor MSTTR22A is located downstream from the MSTTR48 flow
monitor. The DWF associated with the intermediate MSTTR22A monitoring basin is equal to the total DWF at
the MSTTR22A monitor minus the DWF at the MSTTR48 monitor. Figure 6-2 below illustrates the DWF
pattern calculated for weekday DWF at the AUBRN53 monitor. The observed flow data are presented in blue
and the simulated DWF (given the calculated average DWF magnitude [7.168 mgd] and the daily and hourly
factors) is presented in red.
Dry periods for DWF calculation
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Figure 6-2. AUBRN53 monitor DWF calculation and hourly pattern
Blue = observed; red = modeled
Figure 6-3Error! Reference source not found. is a schematic of the flow monitors used to calculated DWF
rates and patterns including calculated estimates for the average total and average incremental DWF rates
for each monitor. Note that the PACIFICPS_FM monitor was not used because it is located within a section of
KC-owned pipe that is not included in the hydraulic model. DWF for the area upstream of the PACIFICPS_FM
monitor was captured by the next monitor downstream: AUBWV016.
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Figure 6-3. Dry weather flow schematic
Total = total observed DWF at meter, Individual = DWF intermediate contribution from the monitoring basin
DWF can be loaded into a MIKE URBAN model in a variety of ways. For this modeling effort, DWF was loaded
as a geocoded network load using the “Load Allocation” tool. The benefit of using this methodology is that
the loads for an entire monitoring basin can be lumped into one network load, which can easily be switched
between active and inactive without having to change the properties of every node in the model to. This
makes for a more organized model and facilitates modeling different scenarios with ease and reduced risk
of error. DWF was loaded proportionally across all modeled nodes within each monitoring basin based on the
upstream pipe length weighting calculations detailed in Section 6.2. For the ABN008 monitoring basin, the
0.15 mgd average DWF flow magnitude was proportionally divided across all 63 of the modeled nodes using
the area factors described in Section 6.2.
As a high-level check, the total DWF for the SSA was used to estimate per capita water usage. In 2010, the
residential population of Auburn was 70,420. The total DWF generated within the City’s SSA was calculated
as the average DWF at the AUBRN53 monitor minus the average DWF at AUBWV016 (monitoring inflows
from KC-owned pipes). This calculates to 6.02 mgd of DWF, which equates to 86 gallons per capita per day
(gpcd) of potable water usage. This value is consistent with Robert Elwell’s understanding of the City’s water
consumption and indicates that the DWF values calculated for this modeling effort are reasonable.
6.2 Development of Subcatchments
The City of Auburn’s collection system is a separated system, which means runoff from land surfaces should
be routed into the stormwater conveyance system rather than into sewer pipes. However, monitoring data
indicate there is a wet weather flow signature in the sewers, indicating either groundwater, surface water, or
some combination of both is present.
Loading the land surface into the model involves the creation of subcatchments, which are model represen-
tations of the land surface that create wet weather flow. Each subcatchment has an assigned area (as well
as other hydrologic parameters) and loads RDII to a collection system node. Every node in the hydraulic
model was assigned a subcatchment representative of the contributing area upstream of each node. The
exception to this rule are dummy nodes, which are fictitious nodes used to connect links in complex hydrau-
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lic configurations. To account for the upstream contributing area at a given node, upstream pipe length was
multiplied by an assumed width. Upstream contributing area was calculated as the length of pipe between a
given modeled node and the next upstream modeled node and/or the length of upstream pipes not included
in the hydraulic model, multiplied by 200 feet of width (100 feet on each side of the conduit). For areas of
the collection system that are not included in the hydraulic model (such as a neighborhood with all 8-inch-
diameter pipes as depicted in Figure 6-4 below in red), this weighting system accounts for the proportionally
higher amount of system inflows assumed to be from that area. Conversely, a node along a mainline may
have a small area of influence that is simply representative of the length of one upstream link (the link in
blue in Figure 6-4). In a given wet weather event, proportionally more flow is expected to be loaded to the
mainline at MH 1409-29 than would be expected to enter the collection system at MH 1409-16. This
weighted approach attempts to account for this field process. ArcGIS software was used to calculate up-
stream pipe lengths. Those data were then brought over to Microsoft Excel, where subcatchment area
calculations were made. During calibration, the total area of some monitoring basins was adjusted to match
KC modeling efforts (described in Section 6.3). However, the area proportions defined by this method were
retained.
Figure 6-4. Upstream area of influence example
In addition to assigning areas to model subcatchments, the subcatchment areas can be divided against the
total area of a monitoring basin, for example, to create non-dimensional area factors for use in distributing
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other types of loads across the collection system on a monitoring basin basis. Subsequent sections below
describe the use of the area weighting factors to distribute those flows.
The development of the AUBWV016 basin’s hydrologic model deviated slightly from the monitoring basins
wholly within the City’s SSA. The AUBWV016 monitor observes flow entering the SSA within a KC main line
from KC’s collection system upstream. Although the KC pipes in this monitoring basin can be displayed
within a map (see Figure 3-1), elevation data for the pipes are missing in KC’s GIS and these pipes were not
included in the MIKE URBAN hydraulic model. A single model subcatchment instead represents the entire
monitoring basin and its area calculation follows the methodology established above. The KC-owned Pacific
PS within the monitoring basin is consequently not hydraulically modeled. Its flow signature is inherent to the
hydrologic calibration of the subcatchment.
6.3 Calibration Period Wet Weather Flow
Calibrating the RDII module of MIKE URBAN is a process of iteratively adjusting hydrologic modeling parame-
ters to match observed flows from flow monitoring records for each of the calibration basins. This modeling
effort used the MOUSE Time-Area model for surface water discharges in combination with the MOUSE RDI
groundwater modeling routine to calculate RDII flow rates. This combination of hydrologic routines is con-
sistent with the preferred methods used by KC.
The monitoring basin boundaries defined the boundaries for the breaking apart the MIKE URBAN model into
calibration basin models so hydrologic parameters could be assigned specific to the flow characteristics
observed for each flow monitoring basin. Calibration of the MIKE URBAN model was performed by moving
sequentially from the upstream monitoring basins to the most downstream basins. BC contacted KC to
inquire as to whether KC would be willing to share its parameters for this area to expedite our calibration. On
March 28, 2014, King County sent BC MOUSE models for each of the flow monitoring basins within the
Auburn study area. These models provided a set of initial model parameters from which to begin calibration.
In some cases, further calibration was not required as the KC parameters performed adequately within the
calibration basin models. A goal of meeting wet weather peak magnitudes and volumes within 10 percent
was the established calibration criterion for this modeling effort.
The KC MOUSE models assumed different total subcatchment areas for each monitoring basin, so the BC
subcatchment areas within each calibration basin model were scaled until the total area matched the area
in the KC models. This preserved the proportional loading of RDII established by the upstream pipe length
calculations while maintaining the water balance generated by the KC MOUSE models. By maintaining the
same water balance as KC, the calibration effort of the MIKE URBAN model was expedited.
The KC-parameterized MIKE URBAN calibration basin models were run through the calibration period (fall
2009 through spring 2011) to ensure that the KC parameters, once transposed to the new BC models,
performed adequately against the flow monitor data. In most cases, small adjustments were made to the
hydrologic model parameters (indicated in Table 6-1) to refine the calibration. Such adjustments are ex-
pected because the KC MOUSE models were simplified models consisting of one large subcatchment and
one conduit, while the BC models account for the full length of travel throughout the collection system.
Adjustments were sometimes necessary to compensate for the peak flow attenuation effects of the collec-
tion system.
Adjustments were made to both the surface runoff and RDI engine parameters depending on the calibration
needs of the model. The RDI engine accounts for the predominant portion of the collection system’s wet
weather flow because Auburn’s collection systems directly connected inflow is minimal. Consequently,
calibration was focused primarily on the RDI engine’s response. Figure 6-5 below provides a schematic of
the RDI engine indicating the different storage zones and components of the flow hydrograph.
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Figure 6-5. MIKE URBAN RDI engine schematic
Source: DHI MIKE URBAN User Manual
Table 6-1 below provides the model parameters adjusted in both the surface and RDI engines and what
effect they have on the hydrograph.
Table 6-1. MIKE URBAN Hydrologic Model Calibration Parameters
Model Engine Parameter name Description Effect on calibration
Time Area A – Surface
Runoff
Imperviousness (%) Relative amount of impervious area Rapid inflow response peak and volume
Time of concentration (min) Time for runoff to travel from the distal end
of the subcatchment Rapid inflow response timing and shape
Initial loss (in) Initial abstraction depth before rapid
response can be discharged Rapid inflow peak timing
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Table 6-1. MIKE URBAN Hydrologic Model Calibration Parameters
Model Engine Parameter name Description Effect on calibration
RDI – Groundwater
Infiltration
Groundwater area (%) Percent of subcatchment area available
for groundwater storage and discharge to
collection system
Duration of groundwater response
Surface storage (in) Storage layer that must be filled before a
rapid response can be discharged and
before infiltration to subsurface zones can
begin
Timing of rapid response peak and volume of
subsurface response
Root zone storage (in) The zone below the surface and above the
groundwater storage layer that transitions
moisture between the two layers
Its depth affects the hydrologic responses of both the
surface and groundwater zones
Overland coefficient A fraction that determines the extent to
which excess rainfall (after the surface
storage is filled) runs off as overland flow
vs. infiltrating to the lower zone. A value of
0 sends all rainfall excess to the lower
zone.
Affects volume of overland flow
Groundwater coefficient The proportion of the groundwater
catchment to the surface catchment. A
value less than 1 makes the groundwater
catchment smaller than the associated
surface catchment.
Affects volume of groundwater response
Tc overland flow (hr) Time constant used to determine how fast
the surface flow responds to rainfall and
the total volume discharged.
Affects overland flow peak timing
Tc interflow (hr) Time constant used to determine how fast
the interflow responds to rainfall and the
total volume discharged.
Affects interflow peak timing.
Tc baseflow (hr) Time constant used to control hydrograph
recession during dry periods. Affects shape of groundwater response
Specific yield Determines the specific yield of the
groundwater aquifer.
Affects aquifer storage volume, and as a function,
volume and shape of groundwater response
Calibration focused on matching the seasonal rise and fall of wet-weather-induced baseflow as well as
matching peak response due to individual storms. Over the course of two wet seasons, there were nine large
storms against which to calibrate the model’s peak runoff response, with the December 12, 2010, storm
providing the largest peak flow. Preference was given to calibrating to the largest storms in the record
because the model’s primary use is to simulate large storms. Two rising baseflow limbs from the falls of
2009 and 2010 and the falling baseflow limb of 2010 provided a sufficient amount of data to calibrate the
baseflow response.
Figure 6-6 below shows the calibration plot at AUBRN53 for the December 12, 2010 storm. The AUBRN53
monitoring basin model is the farthest downstream monitoring basin, so its performance reflects the per-
formance of the entire model upstream, as those monitoring basin models flow into this model. The
AUBRN53 model nearly matches the observed peak of 19 mgd by simulating only 2.4 percent lower. The
recession out of the storm nearly matches and the DWF patterns visually appear to be well represented in
the model. This model slightly overestimates flows in the days leading up to the peak on December 12;
however, the error on the total volume is 6.6 percent, which is well below the 10 percent maximum error
goal for this modeling effort.
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Figure 6-6. AUBRN53 wet weather calibration
Blue = observed; red = modeled
Section 7: Long-Term Simulations
BC ran hydrologic model simulations to produce long-term flow hydrographs that can be used to analyze the
magnitude and frequency of historical wet weather events. The long-term rainfall record (described in
Section 4.1) provides enough data for the model to be run from January 1, 1949, through December 31,
2012 (64 years), including 3 months of “spin up” time to remove the influence of initial-conditions parame-
ter estimates. Identification of the 20-year storm is necessary as the stated LOS goal for the sewer system is
referenced to a recurrence of 20 years (see Chapter 3 of the 2014 Comprehensive Plan). Running the model
through a 20-year storm will indicate areas of the system that back up or flood, which can help to identify
areas that do not meet the stated LOS.
Each of the calibration basin models were run using the 64-year record and the results were summed to
create a citywide RDII time series, which does not include DWF. The summed time series represents the total
RDII entering the collection system at any moment in the 64-year period. The citywide time series was then
separated into discrete events using a 24-hour inter-event duration to isolate periods when the RDII peaked
above a threshold minimum flow value of 8 mgd. This means that only the events that produced more than 8
mgd of peak RDII were included, and smaller events were removed from the analysis.
The selected events were ordered from largest to smallest and assigned a rank. A rank of 1 was assigned to
the largest storm, 2 to the second-largest, and so on. Cunnane plotting parameters were used to estimate
the recurrence interval for each event in years, as follows (Maidment, 1992): 𝑇𝑅= 𝑖+0.2𝑅𝑅𝑅𝑅−0.4
Where: 𝑖=𝑅𝑛𝑛𝑛𝑛𝑛 𝑜𝑜 𝑠𝑖𝑛𝑛𝑠𝑅𝑠𝑖𝑜𝑅 𝑦𝑛𝑅𝑛𝑠 𝑅𝑅𝑅𝑅=𝑛𝑅𝑅𝑅 𝑜𝑜 𝑠𝑠𝑜𝑛𝑛 𝑤ℎ𝑛𝑛𝑛 1 𝑖𝑠 𝑠ℎ𝑛 𝑠𝑅𝑛𝑙𝑛𝑠𝑠 𝑠𝑠𝑜𝑛𝑛
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The above equation will not identify a historical storm event that has exactly a 20-year peak flow recurrence.
For a 64-year record, the third-largest event is estimated to have about a 25-year recurrence and the fourth-
largest event is estimated to have about an 18-year recurrence using Cunnane parameters. The third- and
fourth-largest events from the 64-year simulated record produced peak discharges within ±0.2 mgd of each
other; therefore, either event could be used to approximate a 20-year event recurrence. The larger of the two
events, occurring on February 5, 1996, was selected as the 20-year event to be used to evaluate collection
system capacity and identify deficiencies in the conveyance system that may affect the systems LOS.
Figure 7-1 and Table 7-1 below provide the peak RDII frequency for specific recurrence intervals based on
log-interpolation between plotted events. These flows represent the peak RDII entering the collection system
throughout the entire SSA. The aggregated RDII inflows neither account for system storage, nor do they
include DWF. The aggregated RDII inflow time series does provide a clear distinction between storms in their
hydrologic response as collection system factors such as hydraulic capacity, flooding, and travel time are not
able to distort the peak flow signature of wet weather events.
Figure 7-1. Citywide peak RDII Cunnane plot
Peak RDII frequency values were calculated for each calibration basin to examine the relative contributions
from each basin. Dividing the peak RDII by the total length of the upstream collection system to calculate
unit RDII values provides insight into the relative contribution of infiltration and inflow in each basin. Table
7-2 provides the peak 20-year RDII statistics for each monitoring basin. Peak RDII for monitoring basins
downstream of upland basins does not account for the inflows from the upstream basins. In other words, the
RDII values are specific to the RDII created solely within the monitoring basin regardless of the influence of
upstream basins. Monitoring basins ABN022 and ABN023 illustrate the importance of calculating unit RDII
values. Although ABN022 has a higher 20-year peak RDII than ABN023, the unit RDII per mile of pipe for
ABN023 is higher. This indicates that the pipes within the ABN023 monitoring basin may be in worse
physical condition than those in the ABN022 basin, as ABN023 pipes create more RDII per length of pipe.
0
5
10
15
20
25
30
35
0.1 1.0 10.0 100.0 1000.0
Pe
a
k
R
D
I
I
(
m
g
d
)
Recurrence Interval (years)
Table 7-1. Peak RDII Cunnane
Estimated Flow Frequency
Flow Threshold RDII (mgd)
Q 100 32.77
Q 50 30.25
Q 25 27.19
Q 20 27.03
Q 10 24.69
Q 5 21.76
Q 2 18.75
Q 1 15.36
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Table 7-2. Peak RDII per Monitoring Basin
Monitoring basin 20-year RDII (mgd) RDII/in.-diam-mile* (mgd/in.-mi) RDII/mi** (mgd/mi)
ABN008 0.44 0.006 0.051
ABN022 1.10 0.010 0.106
ABN023 0.47 0.015 0.171
ABN024 0.11 0.004 0.040
ABN027 0.58 0.003 0.045
ABN032 1.95 0.038 0.367
ABRN53 5.43 0.031 0.969
AUBWV016 6.90 0.030 0.263
LakelandHills 0.23 0.001 0.006
LKH001A 0.66 0.051 0.582
MSTTR02A 3.23 0.006 0.054
MSTTR22A 5.31 0.019 0.195
MSTTR48A 3.76 0.011 0.125
WINT003 0.77 0.008 0.202
WINT038 1.05 0.008 0.088
* RDII per inch-diameter mile is a calculation of peak RDII divided by the sum of the upstream pipe diameters multiplied by
their respective total length of pipe in miles. This accounts for the fact that larger-diameter pipes can provide more pathways
for infiltration to enter the collection system.
** RDII per mile is a calculation of the peak RDI divided by the total length of upstream pipe in miles without regard for the size
of those upstream pipes.
Section 8: Future Conditions
The calibrated model was modified to estimate future flows given anticipated population growth, develop-
ment, and hydraulic modifications within the SSA. In Auburn, it is anticipated that population growth will
contribute additional DWF to the system alongside additional RDII from extension of the sewer system to
previously unsewered and undeveloped areas. Modified modeling simulations were used to identify potential
capacity restrictions that will need to be eventually addressed with capital improvements. The following
sections describe how the baseline model was modified to represent the future conditions of the 6-year
(2020) and 20-year (2034) planning horizons.
8.1 Future Dry Weather Flow
Future increases in DWF are expected to come from two sources: population growth (both new development
and redevelopment) and extending the sewer to areas that are currently using septic systems to treat their
wastewater. The following two sections describe the source data and parameterization of these sources of
DWF for both the 6-year and 20-year planning horizons.
8.1.1 Dry Weather Flow from Population Expansion
The state of Washington is divided into Traffic Analysis Zone (TAZ) polygons to track current populations and
to estimate future populations on a small-area basis. These TAZ polygons are used predominantly to plan
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transportation improvements to accommodate increasing populations. The population estimates for each
TAZ can be used to estimate the anticipated populations within the SSA and, consequently, the increase in
DWF from those residents. Table 8-1 maps the TAZ polygons used in this modeling effort against the bound-
ary of the SSA (see the Sewer Comprehensive Plan for more information about the SSA). Note that the TAZ
polygons do not line up directly with the boundary of the proposed SSA.
Figure 8-1. TAZ polygons within the vicinity of Auburn
To account for the differing boundaries between the TAZ polygons and the SSA, an assumption of uniform
population distribution within the TAZ was made in order to perform an area-weighted approach to popula-
tion growth estimation based on the fractional area of the TAZs within the proposed SSA (the exception is
TAZ 748, which is described below). For example, approximately 10.5 percent of TAZ 448 is located within
the proposed SSA. Therefore, only 10.5 percent of the future population projection would be applied to the
estimated increase in DWF to the collection system. The area factor for TAZ 448 then becomes 10.5 per-
cent.
Two exceptions to this method were applied in this effort. The first exception deals with TAZ polygons 432,
444, 445, and 763, which are fully outside of the proposed SSA but whose residents discharge to KC’s
collection system in the southwest corner of the proposed SSA. These polygons use a 100 percent area
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factor and are assumed to fully discharge to KC’s pipes. The second exception accounts for the non-
uniformity of the existing population distribution in TAZ 748 in the north Lake Tapps area. Visual inspection
of the TAZ indicated that the population density was non-uniform across the proposed SSA boundary, so the
TAZ’s area factor was increased from 31.4 percent to 62.8 percent, which allowed the 2013 population
estimate within the city boundary using the factored TAZ polygons to equal the 2013 population estimate
from the City provided by the City of Auburn (Chamberlain, 2014). This step provides assurance that the TAZ
polygon area factors method matches the City’s own understanding of its population numbers in the base-
line scenario.
The TAZ calculations were used to estimate population increases both in the proposed SSA and on KC land
(portion that use sewer lines that run through the city) for the 2020 and 2034 planning horizons using linear
interpolation. The TAZ-based City of Auburn population estimates calculated by BC were within 1 percent of
the citywide estimates provided by the City for the two planning horizons, indicating that the TAZ calculations
were corroborating the work the City had already performed within the city’s boundary. Table 8-1 below
provides the population estimates and area factors for each TAZ polygon. Red, underlined text indicates TAZ
polygons outside of the proposed SSA but whose populations use KC sewer lines that run through the City.
Table 8-1. Future Population Estimates by TAZ Polygon
TAZ ID Area factor 2010 population 2013 population 2020 population 2034 population
404 10.2% 811 823 852 937
405 100% 4,678 4,871 5,320 6,240
406 10.2% 516 532 569 640
409 22.1% 2,068 2,122 2,247 2,542
411 100% 4,428 4,718 5,395 6,505
430 8.6% 678 712 790 912
432 100% 3,905 4,138 4,681 5,583
433 100% 1,576 2,589 4,952 7,348
434 100% 136 142 156 181
435 100% 83 86 94 111
436 100% 4,177 4,265 4,469 5,106
437 100% 4,479 4,499 4,546 5,068
438 83.2% 4,330 4,525 4,981 5,869
439 100% 2,376 2,386 2,410 2,686
440 100% 0 0 0 0
441 100% 12 12 12 13
442 100% 9,186 9,248 9,392 10,481
443 100% 1,296 1,307 1,332 1,494
444 100% 4,317 4,348 4,419 4,764
445 100% 4,905 4,889 4,851 5,028
446 100% 3,344 3,511 3,902 4,626
447 79.9% 6,299 6,355 6,484 7,275
448 10.4% 217 223 237 262
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Table 8-1. Future Population Estimates by TAZ Polygon
TAZ ID Area factor 2010 population 2013 population 2020 population 2034 population
449 1.1% 46 46 47 49
450 100% 7,544 7,930 8,829 10,481
451 71.9% 2,872 2,954 3,144 3,627
748 62.8%* 9,267 9,379 9,641 10,351
763 100% 349 365 402 463
Service area population** 83,896 86,974 94,155 108,642
TAZ calculated city population 70,420 73,235 79,802 92,804
City estimate N/A 73,235 80,532 N/A
Percent difference N/A 0.0% -0.9% N/A
* Area fraction for TAZ 748 (north Lake Tapps) increased to make the TAZ calculated city population match the City’s estimate for 2013.
** Service area refers to the area inside which all collected sewage routes through the city (including inside KC mainlines).
Red, underlined text indicates TAZ polygons outside of the proposed SSA with populations whose sewage is conveyed through the city in KC’s
mainlines.
An assumption of 80 gpcd was used to assign future DWF to the model for the additional future populations,
and all future population expansion is assumed to be connected to the sanitary sewer. By comparison, the
calculated average DWF for the city in 2013 is 86 gpcd (which includes industrial and commercial inputs as
well). Anecdotal information from the City indicates that future DWF estimates are near 60 gpcd. Given that
future development is likely to include higher-efficiency water features that reduce per capita water de-
mands, a planning-level value of 80 gpcd is considered conservative. Industrial and commercial inputs were
assumed to scale proportionally with population growth.
Future DWF from population growth was applied to the model in addition to existing DWF for both the 2020
and 2034 planning horizons. Application of the future DWF was performed using MIKE URBAN’s water load
boundary condition editor, which allows a specified flow magnitude to be loaded at any node and scaled or
manipulated by a factor or pattern. Water loads representing future DWF magnitudes were applied to the
model’s nodes within each TAZ with load distribution based on the upstream pipe length factors described in
Section 6.2. For example, TAZ 405 is estimated to experience population growth of 642 people within the
proposed SSA by 2020, which equates to 0.051 mgd of future DWF. That 0.051 mgd of DWF was then
distributed proportionally across the nodes within TAZ 405 based on the area factors from the upstream
pipe length calculations (described in Section 6.2).
8.1.2 Dry Weather Flow from Sewer Extension
The City plans to extend the sewer system into residential areas that currently use onsite septic systems.
These areas will contribute DWF to the collection system in addition to the DWF increases from population
growth described above. Accounting for the amount and source of the DWF from the sewer extension
involved planning the locations of the future sewers and estimating the chance they will be developed by
each of the planning horizons. BC identified areas where the sewer system was likely to be expanded to
serve both developed and undeveloped areas. Those locations were geocoded as proposed sewer lines
within ArcGIS. The City provided a “percent chance of development” for these sewer lines based on the
2020 and 2034 planning horizons (Table 8-2).
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Figure 8-2. Proposed sewer extensions and development percentages
Legend: (2020 Percentage, 2034 Percentage)
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Sewer lines were classified as serving either new development or existing development. To estimate the
magnitude of the DWF from existing development, the existing DWF from eight headwater-monitoring basins
was divided against the total length of pipe in those basins to get a value of 0.52 mgd of DWF per 100,000
feet of pipe. This value provides an estimate of the amount of DWF produced per length of pipe, which can
estimate DWF from sewer line extension to developed areas. DWF from new development is accounted for
in the population expansion statistics.
Applying the percent chance of development to the estimated flow magnitude for the lines to the existing
development adjusts that future DWF down to a value representative of the chance that the DWF will ever
exist in the sanitary system. Flows were loaded into the model at the nearest node located downstream of
the future line. A citywide average diurnal pattern was used to scale the DWF throughout the day.
8.2 Future Wet Weather Flow
The construction of new sewer lines will create additional pathways for RDII to enter the collection system
because of inevitable holes, cracks, joint failures, and faulty connections. Accounting for that future RDII in
the model is important to make a more reasonable estimate of the future HGL when additional RDII enters
the collection system from sewer extension. Figure 8-2 (Section 8.1.2) indicates the locations of the planned
sewer extensions, as well as the percent chance that they will be constructed by the 2020 and 2034
planning horizons. All of these planned lines, whether for new development or to connect existing develop-
ment, are subject to RDII; thus, the development type distinction is irrelevant in the calculation of future wet
weather flow. It is assumed that existing lines will have approximately the same amount of structural defects
in the future, so their RDII loading is unchanged for future conditions.
BC used KC’s planning-level peak RDII value of 1,500 gallons per acre per day (Earth Tech Team, 2005) to
estimate RDII into the new sewer lines for a 20-year storm. Calculating a contributing area to the proposed
sewer lines was performed by multiplying the sewer length by 200 feet of influence width (described in
Section 6.2). To account for the chance that the pipe segment will be in the ground by the planning horizon,
the percent chance of development factor was multiplied by the contributing area to scale it down. Equation
8-1 below describes the flow calculation for RDII from sewer extension.
Equation 8-1. Flow from sewer extension RDII 𝑄𝑖𝑖𝑖𝑖𝑖𝑖−𝑚𝑚𝑚=(1500 𝑙𝑔𝑅𝑔)∗(𝑔𝑖𝑔𝑛 𝑠𝑛𝑅𝑙𝑠ℎ 𝑜𝑠)∗(200 𝑜𝑠 𝑤𝑖𝑔𝑠ℎ)∗(𝑃𝑠𝑅𝑅𝑅𝑖𝑅𝑙 𝐻𝑜𝑛𝑖𝐻𝑜𝑅 𝐷𝑛𝐷𝑛𝑠𝑜𝑔𝑛𝑛𝑅𝑠 𝑃𝑛𝑛𝑃𝑛𝑅𝑠𝑅𝑙𝑛)∗(1 𝑜𝑠243,560 𝑅𝑃𝑛𝑛 )∗(1 𝑀𝑀1,000,000 𝑀𝑅𝑠)
The future wet weather RDII was loaded into the model using a scaled unit RDII time series and an applied
factor. The RDII time series of the ABN032 basin was selected because the hydrograph provides a large
volume of water to the system because of its elongated rising and recession limbs. A high-volume time series
will produce a conservative result when evaluating storage and conveyance.
The ABN032 time series was scaled to a peak value of 1 mgd such that a factor within the model could be
used to multiply the time series to the appropriate value for each loading node based on Equation 8-1
above. For example, if the required flow at a node from a sewer line extension is 0.05 mgd peak, a factor of
0.05 is applied to the RDII time series to produce 0.05 mgd of peak flow to the model. This method provides
a representative hydrograph shape to use within the model as compared to using a constant RDII value at
each node, which would provide an overly conservative flow volume.
Figure 8-3 shows the difference between high and low volume time series. The two time series experience
identical peak flow rates; however, the purple time series puts significantly more water into the collection
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system and is more likely to indicate capacity deficiencies in the system than the red time series. Using a
high volume hydrograph within the model ensures that the peak is propagated downstream rather than
allowing for attenuation of the instantaneous peak which could leave downstream bottlenecks unexposed.
Figure 8-3. High and low volume time series comparison example
Red = low-volume storm; purple = high-volume storm
For areas of redevelopment where housing will become denser, an assumption that denser developments
will most likely use the existing sewer lateral rather than install new laterals prevents the need to load future
RDII from those areas. Consequently, RDII from new development was the only type of future RDII included
in the model.
8.3 Future Hydraulic Improvements
After the flow monitoring period between 2009 and 2011, KC embarked on a two-phase project to reduce
flooding risks in capacity-limited sections of its sanitary sewer lines. For Phase I, KC constructed the SRT in
2013, which routes wastewater flow from the MSTTR48 monitoring basin (diversion at the intersection of K
Street SE and 17th Street SE) to the Auburn West Interceptor, thereby alleviating capacity deficiencies in the
diversion area. This was included in the baseline-conditions model. Phase II of the project will route flow
from the Pacific PS to the Auburn West Interceptor, thereby reducing surcharging near the intersection of
Boundary Boulevard SW and O Street SW. This project needed to be included in the future-conditions
scenarios as it is planned but not yet designed or constructed.
At the time of modeling, design drawings were not available for the Pacific PS project. A conceptual layout of
the project indicated that a new force main would be constructed to discharge to an interceptor line that
runs parallel to the Auburn West Interceptor before the two lines join at MH 807-46. Information such as the
pump station’s capacity, operational changes, force main diameter, etc., was not available; therefore,
assumptions were made to fill in these gaps.
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In the calibration-conditions model, the Pacific PS was not modeled, nor was KC’s upstream collection
system. The area tributary to the Pacific PS is a part of the AUBWV016 monitoring basin. Because the pump
station belongs to KC, it was not explicitly included in the baseline-conditions hydraulic model but, rather, its
inflow hydrograph was captured in the calibration of the AUBWV016 hydrologic model. As described in
Section 6.2, the AUBWV016 hydrologic model was modeled with one subcatchment to load KC flows into the
City’s collection system without a full collection system model. Although GIS data exist that describe the
layout of the County’s pipes tributary to the AUBWV016 flow monitor, elevation data are lacking such that
the collection system could not be built without additional data. Therefore, the AUBWV016 model subcatch-
ment (representing the hydrology of the monitoring basin) was subdivided to isolate the area contributing to
the Pacific PS.
KC GIS data were used to calculate the total length of pipe upstream of the AUBWV016 flow monitor,
including the areas upstream of the Pacific PS. The ratio of the length of pipe upstream of the Pacific PS to
the total length of pipe within the AUBWV016 monitoring basin was used to divide flows from the AUBWV016
subcatchment into two subcatchments. Figure 8-4 below shows the pipes located within the AUBWV016
monitoring subcatchment and the proposed force main layout and discharge location. The green pipes
upstream of the Pacific PS account for 59 percent of the total pipe length within the monitoring basin.
Consequently, the AUBWV016 subcatchment in the hydrologic model was split and 59 percent of the area
was re-routed to MH 807-46.
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Figure 8-4. Pacific PS re-route
Existing Pacific PS
discharge location
Monitor
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8.4 Future-Conditions Summary
Preparing the MIKE URBAN model to simulate future conditions required adjustments to both the hydrologic
inputs and the hydraulic network. DWF and RDII must be increased to account for population growth and
future development. Hydraulic modifications to the collection system must reflect planned infrastructure
projects. Table 8-2 summarizes the three results-producing periods of analysis and the major changes
associated with them.
Table 8-2. Analysis Period Model Modifications
Scenario Dry weather flow Wet weather flow Hydraulic modifications
Baseline None None Stuck River Trunk (constructed 2013)
2020 (6-year planning horizon) Population expansion = +0.82 mgd
Sewer extension = 0.36 mgd Sewer extension = + 0.56 mgd Pacific PS new discharge location*
2034 (20-year planning horizon) Population expansion = + 1.98 mgd
Sewer extension = +0.78 mgd Sewer extension = + 1.23 mgd None*
*Analysis period includes hydraulic modifications from previous periods.
Section 9: Model Results
The following sections describe the results of the hydraulic capacity evaluations. A 20-year event was
simulated to identify locations where the sewer collection system does not have sufficient capacity to meet
the LOS standard. The City’s LOS standard for new sewers is defined as no surcharging of pipes during the
20-year storm (where surcharging is defined as the HGL rising above the pipe crown). For existing sewers,
the standard is relaxed to allow surcharging below an excessive level, although the magnitude of excess is
not defined. The maps in the subsequent sections identify the minimum freeboard calculated at each
modeled manhole. Minimum freeboard at a manhole is calculated as the depth from the maximum simulat-
ed HGL elevation to the surface elevation. Manholes with maximum HGL elevations that exceed the rim
elevation are considered to be in flooding condition. Assessment of minimum freeboard gives indication of
hydraulic restrictions as the system is forced to back up and surcharge when water cannot pass through
restricted sections. For the purposes of this analysis, manholes with 3 feet of minimum freeboard or less are
indicated as they are surcharged high enough to cause or nearly cause flooding because of hydraulic
restrictions.
Capacity evaluations were run with all manholes set as “sealed.” Sealing the manholes prevents water
losses due to flooded manholes and forces sewer flows to continue downstream. This retains flows for
evaluation of downstream pipe capacities so that the entire collection system can be evaluated for peak flow
capacity.
9.1 Baseline Conditions
Results from the baseline-conditions simulation indicate one area that floods (along Boundary Boulevard SW
west of O Street SW) and 20 additional locations with less than 3 feet of freeboard. Investigation of the 20
locations with less than 3 feet of freeboard indicates that all of the manholes are shallow with depths
between 2.5 and 3.5 feet, which means that even DWF alone will result in 3 feet of freeboard or less. These
minimum freeboard locations are therefore not to be interpreted as indicative of a hydraulic restriction that
causes surcharging induced by high amounts of RDII. Figure 9-1 presents the minimum freeboard at all
manholes within the model in the baseline condition.
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Figure 9-1. Baseline-conditions minimum freeboard
Boundary Blvd SW
flooding area
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Figure 9-2 shows MH 409-33 (one of the 20 identified manholes outside of the Boundary Boulevard area
with less than 3 feet of minimum freeboard), which has a calculated freeboard of 2.14 feet. The MH depth is
2.41 feet, which means the depth of flow is only 0.27 foot. Because only 27 percent of the pipe depth (1-
foot diameter pipe) is being used at the peak of the 20-year storm, the calculated minimum freeboard in this
MH is not indicative of a hydraulic restriction. This situation is similar at the 19 other shallow manhole low
freeboard locations throughout the city. (Note: MIKE URBAN’s results viewer displays in metric units.)
Figure 9-2. Low-freeboard short manhole
Figure 9-3 below shows the simulated HGL along Boundary Boulevard SW between State Route (SR) 167
and O Street SW and indicates that MH 906-26 and MH 906-12 would flood during the 20-year event. There
is only 2,400 feet of Auburn sewer upstream of this location (to the left of MH 906-14 in the figure), so the
flooding is induced primarily by RDII from the 139,000 feet of KC line upstream of the AUBWV016 flow
monitor (which discharges into MH 906-06 in the figure) rather than RDII from the Auburn line itself. Flows
from upstream of the AUBWV016 monitoring basin include the existing discharges from the Pacific PS.
Figure 9-3. Flooding along Boundary Boulevard SW
MH Depth = 2.41 ft.
Flooding predicted
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The baseline-conditions model run indicates that there is area with flooding and it is due to high flows from
KC. Twenty other locations have calculated minimum freeboard levels of less than 3 feet; however, this
calculation is due to short manhole depths. These 20 locations were inspected and hydraulic results of the
model did not indicate that there were instances of hydraulic restriction during the 20-year storm. These 20
locations can therefore be ignored as they do not represent a risk to LOS.
9.2 2020 Conditions
Results from the 2020 simulation indicate that re-routing flow from the Pacific PS to the Auburn West
Interceptor reduced the HGL along Boundary Boulevard SW such that no manholes show a minimum
freeboard of less than 3 feet. However, the additional flows in the KC-owned Auburn West Interceptor raise
the HGL enough to cause the minimum freeboard to fall below 3 feet in eight manholes between 15th Street
SW and 15th Street NW. Figure 9-4 provides the minimum freeboard map for the 2020 simulation.
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Figure 9-4. 2020 conditions minimum freeboard
Auburn West
Interceptor
surcharged line
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Figure 9-5 shows the surcharged segment of the Auburn West Interceptor from MH 907-01 to MH 607-20,
which includes the eight manholes with freeboard less than 3 feet (1 meter in the figure). This line is owned
by KC and is therefore not considered for CIP by the City.
Figure 9-5. 2020 surcharge of the KC-owned Auburn West Interceptor
9.3 2034 Conditions
Results from the 2034 simulation are considered to be more uncertain than the results of the baseline and
2020 scenarios. This is because accurately predicting the pace and location of development and population
expansion 20 years into the future is inherently difficult. Consequently, the following results should be
interpreted as indications of what could happen given best estimates, rather than predictions of what will
necessarily happen. The model results indicated that the area around the intersection of Perimeter Road SW
and 1st Street SW is likely to experience flooding due to increased flows within the Auburn West Interceptor.
Additionally, future sewerage of the existing development upstream of the Verdana PS is likely to produce
surcharge in the line along 118th Avenue SE. Figure 9-6 presents the results of the minimum freeboard
evaluation for the 2034 planning horizon.
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Figure 9-6. 2034 conditions minimum freeboard
Perimeter Rd.
flooding area
118th Ave SE.
high HGL
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Figure 9-7 shows the maximum HGL along 118th Avenue SE to the Verdana PS wetwell. MH 313-115 has a
minimum of 0.5 foot of freeboard at the peak of the event. This area, although already developed, is planned
to be sewered to bring the existing development onto the City’s collection system. The model indicates that
when this area is sewered, the shallow sloped section of 8-inch-diameter pipe upstream of the Verdana PS is
likely to surcharge to within 0.5 foot of the lowest manhole (313-116) rim elevation, indicating a high risk of
flooding at that location. This is attributable to both the shallow slope of the 8 inch line as well as a diameter
decrease to 6 inches just upstream of the Pump Station at 413-50. It is recommended that the City verify
this diameter decrease as the GIS database (the source of the diameter information) may be incorrect).
Assuming the diameter information is correct, the modeling results indicate that although the Verdana PS
has been sized and built to handle increased flows associated with future sewerage, the existing sewer lines
may not have enough capacity to convey that sewage to the pump station in 2034.
Figure 9-7. Surcharged line upstream of Verdana PS
Figure 9-8 presents the hydraulic profile of the Perimeter Road flooding area along the Auburn West Inter-
ceptor. The added flows from the SRT, the diversion of Pacific PS, as well as the increased flows from
population growth and new sewer lines all increase the HGL in this line, resulting in flooding at two locations.
Although this flooding violates the City’s LOS, the line is owned by KC and is therefore not considered for CIP
development by the City.
Figure 9-8. Hydraulic profile of KC-owned Auburn West Interceptor at Perimeter Road
0.5 ft freeboard
Verdana P.S. wetwell
flooding predicted
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Section 10: Conclusions
BC used MIKE URBAN to construct and calibrate a model of the City’s sewer collection system including
outlying areas that drain into the city. The model was used to evaluate conveyance capacity deficiencies for
existing baseline conditions, as well as the future 6-year and 20-year conditions corresponding with the
planning horizons of the Comprehensive Plan. The model was calibrated using 2 years of flow data from 14
KC flow monitoring sites that were within the Auburn vicinity. Calibrating over this long period of time helps
to reduce model calibration uncertainty as a variety of storm sizes and durations are used to adjust the
model parameters.
BC modeled future H&H conditions for the 6-year and 20-year planning horizons using population growth
and sewer extension estimates, which add dry and wet weather flow to the collection system by adding new
users and new pipe. Hydraulic features constructed after the flow monitoring period, such as new pump
stations and a trunk line, were included in the model to accurately represent baseline conditions. A future
modification to King County’s Pacific PS, although still in conceptual design, was modeled in both future
conditions to estimate the effect of that modification on hydraulic conveyance.
BC analyzed long-term hydrographs to identify an event in the 64-year rainfall record that is closest to a 20-
year event. The 20-year event, which took place on February 5, 1996, was simulated in the existing-
conditions, future 6-year, and future 20-year conditions models to evaluate the LOS of the collection system
in all three conditions. Although LOS is defined stringently for new construction as no surcharging of the pipe
crown during a 20-year storm, surcharge below an excessive amount is allowed for the existing system
before LOS is considered to be violated. Modeling results analysis identified manholes with less than 3 feet
of minimum freeboard during the 20-year storm as an indicator of pipe sections with hydraulic restriction
that cause surcharge of the system.
In general, the City of Auburn’s sanitary collection system has no capacity-related issues. Although the
baseline-conditions modeling indicates flooding along Boundary Boulevard, this issue will be alleviated by
the re-routing of discharge from KC’s Pacific PS in the coming years. The 6-year planning horizon simulation,
which accounts for the Pacific PS’s proposed new discharge location, indicates that the Auburn West
Interceptor will experience surcharge as the HGL will rise to within 3 feet of minimum freeboard because of
increased discharge from the pump station. The interceptor is owned by KC and is not considered for CIP.
The 20-year planning horizon simulation indicates that flooding is likely to occur along the Auburn West
Interceptor and surcharge is likely upstream of the Verdana PS. The sewer lines upstream of the Verdana PS
are owned by the City; however, CIP is not planned around results from this scenario because of the uncer-
tainty associated with the assumptions for 20 years into the future.
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References
Brown and Caldwell, December 2009. City of Auburn Comprehensive Sewer Plan. Prepared for the City of Auburn by Brown
and Caldwell.
Chamberlain, Elizabeth “RE: TAZ data.” April 18, 2014. E-mail. City of Auburn, Wash.
Earth Tech Team, Regional Needs Assessment Report, Regional Infiltration and Inflow Control Program, Appendix A5 Assump-
tions for Regional I/I Control Program, King County, 2005 (pg. 7)
Elwell, Robert. “County Plans.” June 12, 2014. E-mail. Sewer Utility Engineer, City of Auburn, Wash.
King County,”Hydrologic Data Download Page,” Hydrologic Information Center Home, 2014,
http://green.kingcounty.gov/wlr/waterres/hydrology/ Maidment, David R., ed. Handbook of Hydrology. New York, N.Y.:
McGraw-Hill, 1992. Print.
Washington State University Puyallup Extension, “Historic Data,” AgWeatherNet, 2014, http://weather.wsu.edu/awn.php
Western Washington Hydrologic Model 2012. http://www.ecy.wa.gov/programs/wq/stormwater/wwhmtraining/index.html
Sanitary Sewer Model Development
A-1
Use of contents on this sheet is subject to the limitations specified at the beginning of this document.
AuburnSewer_ModelTM_Final.docx
Attachment A: Modifications to Collection System GIS
MH IDIssue
807-28Elevation likely too low, appears to look like a syphon. Rim elevation much lower than nearby contour
908-19Invert likely too high. Profile jumps up for this MH. Interpolate the elevation for a smooth profile
808-80,
708-12
Elevation likely too low, appears to look like a syphon. Interpolate the elevation for a smooth profile. MH elevations in area
do not match 2008 Comp Plan model
508-28 Elevation likely too low, appears to look like a syphon. MH not in 2008 comp plan model. Interpolated to a new elevation for
consistent profile.
1009-91 Elevation too low, appears to look like a syphon. Use invert from 909-56 in 2008 comp plan (same location, name appears to
have changed).
1012-69Elevation was low for outlet node of a forcemain. Use 2008 comp plan model for elevations
1009-95Elevation likely too high. Interpolate to constant slope to match rest of trunk. Node not in 2008 comp plan
1009-44Elevation likely too high. Use 2008 comp plan model value to prevent adverse slopes
1009-101Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value.
909-52Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value.
909-102Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value.
809-91Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value.
709-40Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value.
709-80 Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value for invert. Rim elevation was
below pipe crown, so interpolate to rim elevation.
709-28Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
810-20Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
710-25Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value
710-34Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
710-32Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
610-48Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value
610-123,
610-125,
310-124
Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
610-12,
610-117 Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value
610-09Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value
611-56Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
511-54Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
510-76Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
511-26,
511-27 Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value
511-41,
511-39,
411-68,
410-77,
410-76
Elevations adjusted to 2008 Comp Plan to remove adverse slopes
509-19,
509-18
Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value. Set 509-19 Rim to same as
the next MH downstream in the intersection as the comp plan value is illogically high (almost 30 feet higher than same
intersection MH)
509-07Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value.
908-19Elevation likely too high. Creates adverse slope on inlet pipe. Interpolate the invert as MH does not exist in 2008 comp plan.
708-29Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
808-80Elevation likely too low. Creates adverse slope on outlet pipe. Interpolate a value as MH not in 2008 Comp Plan
1009-100Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value.
1010-91Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
811-13Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
508-28Elevation likely too low. Creates adverse slope on outlet pipe. Interpolate a value as MH not in 2008 Comp Plan
614-90Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
714-05Elevation likely too low. Creates adverse slope on outlet pipe. Interpolate value as MH not in 2008 comp plan
713-18Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan model value.
413-66Elevation likely too high. Creates adverse slope on inlet pipe. Invert interpolated as MH is not in 2008 comp plan.
512-91Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
611-07Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan value.
709-28Rim elevation much higher than nearby contours. Interpolated value used.
409-40Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
410-78Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
410-01Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan value.
410-25Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
409-55Invert likely too high. Creates adverse slope on outlet pipe. Interpolate an invert as MH not in 2008 comp plan model.
606-86MH Depth 1.2', not likely. Rim elevation interpolated
506-07Invert too high. Creates adverse slope on inlet pipe. Use 2008 comp plan value.
906-14,
906-26,
1006-02,
1006-04
Inverts too low, use comp plan values. Otherwise, water would not leave the pipes to travel downstream (steep adverse slope
after 1006-04)
906-05Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
807-28Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
307-07Elevation likely too high. Creates adverse slope on inlet pipe. Use 2008 comp plan value.
409-72,
309-11,
309-10
Elevations incorrect. Use 2008 Comp Plan values
409-01Elevation too low, creates adverse slope outlet pipe. Use 2008 Comp Plan value
309-75,
309-74 Elevations too low, creates adverse sloped mainline. Use 2008 comp plan values
309-68Elevation too low. Use 2008 comp plan value
309-76,
309-49,
309-48,
309-47,
309-46,
309-68
Elevations need adjustment to 2008 comp plan values as pipeline is adverse
207-11,
207-05 Elevations need adjustment down as pipeline is adverse. Use 2008 comp plan values
713-22Invert too high, interpolate a lower value as 2008 comp plan value is integer.
713-14,
713-13 Elevations too high, create adverse slopes. Use 2008 comp plan values
709-84,
709-87,
709-68,
709-67,
709-63
Adjust all values to 2008 Comp Plan values as this section is adverse
710-72,
710-73,
710-74
Adjust all values to 2008 Comp Plan values as this section is adverse
608-32,
508-13 Adjust all values to 2008 Comp Plan values as this section is adverse
509-12Elevation too high, creates adverse slope. Interpolate value as comp plan value is a copy of upstream value.
409-51Elevation too low, creates adverse slope. Interpolate value.
512-10Elevation likely too low. Creates adverse slope on outlet pipe. Use 2008 comp plan model value
407-01Value likely too high, interpolated down so mainline is constant slope.
307-18Value likely too high, interpolated down so mainline is constant slope.
1208-38,
1108-09,
1108-07,
1108-08,
1008-09,
908-24,
908-25,
908-26
Interpolated MH inverts based on 0.002 ft/ft slope upstream of known elevation at 908-15. Rim elevations estimated from
contours
Sanitary Sewer Model Development
B-1
Use of contents on this sheet is subject to the limitations specified at the beginning of this document.
AuburnSewer_ModelTM_Final.docx
Attachment B: Pump Station Data
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City of Auburn Comprehensive Sewer Plan
C-1
Use of contents on this sheet is subject to the limitations specified at the end of this document.
City of Auburn Comprehensive Sewer Plan_FINAL.docx
Pump Station InformationAppendix C:
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C-2
City of Auburn Comprehensive Sewer Plan
D-1
Use of contents on this sheet is subject to the limitations specified at the end of this document.
City of Auburn Comprehensive Sewer Plan_FINAL.docx
SEPA Compliance Appendix D: