STEP Sewer Analsys-~ ~ L
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5016 Lacey Boulevard SE, Lacey, Washington 98503
1960) 491-3399 18001454-7545 Fax (360} 491-3857
wwwskilhngs.com
To: City of Yelm
PO Box 479
Yelm, WA 98597
ATTN: Jim Gibson, PE
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SEP 14 'O('~
LETTER OF TRANSMITTAL
Date: 9-13-2005
Project Number: 05111
RE:
WE ARE SENDIN"G YOU THE FOLLOWING ITEMS: ~ ~ ~ ,
COPIES ~ DATE ~~ ~~~'~ ~ ~ ~ ~ DESCRIPTION
1 9-13-2005 Tahoma Terra Water S stem Anal sis
1 9-13-2005 Tahoma Terra STEPS stem Anal sis
THESE ARE TRANSMITTED ^For your ^For action ^For review ®For your use ^ As requested
information specified below and comment
REMARKS:
Jim,
Call me with any questions.
Thanks,
Copies to: Signed: Andrew C. Perez, PE
If enclosures are not as noted, kindly notify us at once
City of Yelm STEP System
Tahoma Terra Hydraulic Analysis
Project #05111
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Skillings-Connolly, Inc.
Consulting Engineers
5016 Lacey 131vd SE
Lacey, WA 98503
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Reviewed by:
Thomas E. Skillings, PE
Principal
Andrew C. Perez, PE
September 2005
Yelm STEP System Analysis
The following Septic Tank Effluent Pump (STEP) system model of the City of Yelm has
been developed using WaterCAD® software as a tool for evaluating the STEP system's
future needs. The model will include potential future development within the City and
focus specifically on the proposed build-out of the Tahoma Terra subdivision. The main
objective of the analysis was to answer the following three questions: 1) What size STEP
line is required along Longmire Street from Tahoma Terra to McKenzie Court to handle
the STEP flows from the first two phases of Tahoma Terra, 2) How many total
equivalent residential users (ERU's) from Tahoma Terra can the existing STEP system
handle before a parallel STEP main is needed to the wastewater treatment plant, 3) What
is the required size of a parallel STEP main to the treatment plant at full build-out of
Tahoma Terra and the adjoining basins. The following narrative details the modeling
methods for the Yelm STEP system pipe network.
Model Development
_Svstem Demands: The City of Yelm STEP system currently serves approximately 1860
Equivalent Residential Users (ERU's). The City of Yelm Wastewater Treatment Facility
records show a peak hourly flow of 460 gpm and peak day flow of 280,000 gallons per
day (gpd). This equates to a peak hourly flow of 0.25 gpm/ERU (460 gpm / 1860 ERU)
and a per ERU volume of 150 gpd/ERU (280,000 gallons per day / 1850 ERU).
Accordingly, the 4450 residents provide a per capita daily volume is 63 gdpc (280,000
gallons / 4450 residents).
Distribution Svstem: The piping layout was created using the STEP system maps as per
the City of Yelm Water Reuse Project Facilities Plan, approved in September 1995 along
with proposed extensions and additions. The Facilities Plan divided the City into basins
(see attached system map) and distributed the ERU's between each basin based upon land
use zoning, which has remained largely unchanged.
Due to the City of Yelm's relatively flat topography, the entire piping system was set at a
uniform elevation. The only exception being the wastewater treatment plant's influent
control structure which was modeled at 15 feet above grade to mimic its elevation in
relation to the rest of the City.
Finally, each pipe is modeled as a PVC pipe with a C factor of 140 to incorporate any
possible wear and tear from use.
STEP Pumps: The STEP pumps were modeled using the Orenco 10 OSI OS HHF - 6
stage with'/4" flow controller pump curve, which is the City of Yelm's standard
residential STEP pump. The three control points entered into WaterCAD to create the
pump curve are found in Table 1:
Skiltings-Connolly, Inc. 1 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
Table 1
STEP Pump Curve Data
Flow ( m) Total Dynamic Head (ft)
0 238
4.6 190
9 0
Due to losses between the STEP pump and the conveyance piping to the STEP lines,
which the 1995 Facilities Plan calculated to be approximately 6 ft, the pump curve data
was adjusted according as seen in Table 2.
Table 2
Adjusted STEP Pump Curve Data
Flow ( m) Total D namic Head (ft)
0 232
4.6 184
9 0
For simplicity, one STEP tank was installed per basin instead of one STEP tank per ERU.
The single pump was then discharged into multiple nodes within the basin to balance the
flows. The pump curve was adjusted proportionally to reflect the number of ERUs,
thereby modeling the single pump as multiple pumps in parallel. For example, a basin
with 10 ERU's would have a single pump with the following pump curve points:
Table 3
STEP Pump Curve Data
Adjusted fora 10 ERU Basin
Flow ( m) Total D namic Head (ft)
0 232
46 184
90 0
Peak hourly flow per ERU for the system is calculated to be approximately
0.25gpm/ERU. Therefore, for each Basin pump, the three flow points for the pump curve
were multiplied by 0.25 gpm to mimic peak hourly flows.
Skillings-Connolly, Inc. 2 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
Table 4
STEP Pump Curve Data
Adjusted fora 10 ERU Basin at Peak Hourly Flow
Flow ( m) Total D namic Head (ft)
0 238
4.6x10x0.25=11.5 190
9x10x0.25=22.5 0
System Anal.~is
A steady state analysis of the STEP system, using WaterCAD, yields results that indicate
the maximum amount of flow that can be discharged from each basin into the distribution
system and thereafter onto the treatment plant. The balanced flow results from the steady
state analysis are used to identify areas where inadequate peak hourly flows and/or flow
velocities in excess of 5 feet per second are present.
Model Calibration: The steady state analysis at peak hourly demand was run using basin
ERU distributions and demands identical to the original system model from the 1995
Facilities Plan, which used Pipe-flo software. Flows were checked at intersections within
each basin and compared to the Pipe-flo results. Likewise, the results were checked
against the wastewater treatment plant data provided by the City for total system flows.
The WaterCAD model flows were within 4 percent of the Pipe-flo results and also
congruent with plant data.
Scenarios:
SCENARIO 1: All Basins, with the exception of Basin 1900 (Tahoma Terra) are
set at maximum build-out ERU levels (see Figure 1). 89 ERU's,
which represents the 89 homes in the first phase of the Tahoma
Terra subdivision, were implemented into Basin 1900. The piping
for Tahoma Terra discharges into a proposed 2 Inch diameter
STEP hne along Longmire Street from Tahoma Terra to the 6
Inch diameter STEP line along Washington Street. This line also
receives the flows from Basin 900.
SCENARIO 2: All Basins, with the exception of Basin 1900 (Tahoma Terra) are
set at maximum build-out ERU levels. 215 ERU's, to represent
the 215 homes in first and second phase of the Tahoma Terra
subdivision, were implemented into Basin 1900. The piping for
Tahoma Terra discharges into the 4 Inch diameter STEP line
along Longmire Street from Tahoma Terra to the 6 Inch diameter
STEP line along Washington Street. This line also receives the
flows from Basin 900.
Skiltings-Connolly, Inc. 3 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
SCENARIO 3: Same as Scenario 2, but with the ERU's for Basin 1900 (Tahoma
Terra) increased until the full capacity of the existing system is
reached.
SCENARIO 4a: All Basins are set at maximum build-out ERU's. A 12 Inch
parallel pipe to the treatment plant, connecting to the 12 Inch line
along Berry Valley Road at the intersection of Coates and Cullens
and continuing along Coates until the intersection of First and NP
where is up-sizes to a 16 Inch line all the way into the treatment
plant (see Figure 2).
SCENARIO 4b: All Basins are set at maximum build-out ERU's. A 14 Inch
parallel pipe to the treatment plant, connecting to the 12 Inch line
along Berry Valley Road at the intersection of Coates and Cullens
and continuing along Coates until the intersection of First and NP
where it is up-sized to a 16 Inch line to the treatment plant (see
Figure 2).
Model Scenario Results
SCENARIO 1: Table 5 contains the results of Scenario 1 for selected Basins
using a 2 Inch diameter line along Longmire Street from the
subdivision to the 6 Inch diameter STEP line along Washington
Street. The results show that a 2 Inch diameter line is not
sufficient to handle the flows from the first phase of the Tahoma
Terra subdivision when the adjoining Basins are at maximum
build-out.
Table 5
Scenario 1 Results
Basin No. No. of ERUs Required Peak
Hourly Flow (gpm) Calculated Capacity
(gpm)
900 170 43 23
1900 89 22 11
A second scenario was analyzed using a 4 Inch diameter line
along Longmire Street from the subdivision to the 6 Inch
diameter line along Washington Street. Likewise, the proposed 2
Inch STEP line along Terra Glenn ST SE was up-sized to a 4
Inch diameter pipe. The results show that the upsized lines
provide adequate volume for the 89 proposed homes when the
adjoining Basins are at maximum build-out. The results are
shown in Table 6.
Skillings-Connolly, Inc. 4 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
Table 6
Adjusted Scenario 1 Results
Basin No. No. of ERUs Required Peak
Hourly Flow (gpm) Calculated Capacity
(gpm)
900 170 43 49
1900 89 22 25
SCENARIO 2: Table 7 contains the results of Scenario 2 for selected Basins
using a 4 Inch diameter line along Longmire Street as previously
detailed. The results show that a 4 Inch diameter line provides
adequate volume for the 215 proposed homes when the adjoining
Basins are at maximum build-out.
Table 7
Scenario 2 Results
Basin No. No. of ERUs Required Peak
Hourly Flow (gpm) Calculated Capacity
(gpm)
900 170 43 46
1900 215 54 56
SCENARIO 3: From the results of Scenario 2 seen in Table 7, the hydraulic
capacity of the existing system feeding Basins 900 and 1900 to
the treatment plant is at or near capacity. Further analysis
revealed line velocities in excess of 7 feet/second entering the 12
Inch diameter line into the plant as a result of the additional
ERU's from Basin 1900. Therefore, the maximum number of
ERU's from Basin 1900 the system can handle before a parallel
STEP line is needed to the treatment plant from Tahoma Terra is
215.
SCENARIO 4a: Table 8 contains the results of Scenario 4a, for selected Basins.
The results show that the proposed piping along Berry Valley
Road with a 12 Inch diameter parallel STEP line to the
wastewater treatment plant, is adequate to handle flows from all
Basins during maximum build-out. It should be noted, however,
that flow velocities into the treatment plant as a result of the
additional ERU's from Basin 1900 are in excess of 5 feet/second
and therefore at the upper limit of their hydraulic capacity.
Skillings-Connolly, Inc. 5 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
Table 8
Scenario 4a Results
Basin No. No. of ERUs Required Peak
Hourly Flow (gpm) Calculated Capacity
(gpm)
900 170 43 57
1900 1100 275 431
SCENARIO 4b: Table 9 contains the results of Scenario 4b, for selected Basins.
The results show that the proposed piping along Berry Valley
Road with a 14 Inch diameter parallel STEP line to the
wastewater treatment plant, is adequate to handle flows from all
Basins during maximum build-out. The line velocities are lower
than 5 feet/second in the line and therefore there is additional
capacity in the line.
Table 9
Scenario 4b Results
Basin No. No. of ERUs Required Peak
Hourly Flow (gpm) Calculated Capacity
(gpm)
900 170 43 58
1900 1100 275 437
Conclusions
Based upon the results of the steady state analysis of the City of Yelm STEP system at
peak hourly demand, the following was determined:
1) A 4" diameter STEP line is required along Longmire Street from Tahoma Terra to
McKenzie Court to handle the peak flows from the Tahoma Terra subdivision.
Likewise, within Tahoma Terra, a 4" diameter STEP line is required along
Tahoma Glenn ST SE.
2) The aforementioned pipe sizes are adequate to handle the flows from the expected
215 ERU's from the first two phases of Tahoma Terra, without the need for a
parallel line to the treatment plant.
3) The maximum number of ERU's available in Basin 1900 (Tahoma) which the
existing STEP system at full build-out can handle before a parallel line to the
treatment plant is needed is 215 ERU's. This coincides to the full build-out of
phases 1 and 2 of Tahoma Terra.
4) A 12" diameter parallel STEP line (see Figure 2) to the wastewater treatment
plant is adequate to handle the flows from the full build out of phases ]and 2 of
Skillings-Connolly, Inc. 6 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
Tahoma Terra along with the full build-out of the rest of the Basins. However, the
12" parallel line experiences flow velocities greater than 4 feet/per second
through town and in excess of 5 feet/per second near the plant, which suggests it
is near it maximum hydraulic capacity.
5) A 14" diameter STEP line to the wastewater treatment plant is adequate to handle
the flows from the full build-out of phases 1 and 2 of Tahoma Terra and full
build-out of the rest of the City with capacity to spare. It should be noted,
however, that the flows into the existing plant at this stage of full build-out are
near or greater than the plant's capacity. Further addition of ERU's from Thurston
Highlands would require a new treatment plant. As a result, demands on the
system from Thurston Highlands were not incorporated into the above scenarios.
Based upon the steady state analysis results of the City of Yelm STEP system, the
following construction sequence is proposed:
A) Install a 4 Inch diameter STEP line along Longmire Street from the Tahoma Terra
subdivision to the 6 Inch diameter STEP line along Washington Street
B) Construct the first and second phases, respectively of the Tahoma Terra
subdivision.
C) Install the 12 Inch diameter parallel STEP line to the treatment plant and the
proposed 12 Inch diameter STEP line along Berry Valley Road and thereafter
connecting to the 16nch diameter STEP line to the treatment plant. Likewise,
connect the Tahoma Terra Subdivision to the 12 Inch diameter STEP line along
Berry Valley Road to loop the system.
D) Install the remaining STEP piping for the Thurston Highlands master planned
community as the construction develops. However, it should be noted that
additional treatment capacity either at the existing facility, or at a new facility that
will be required at that time.
Skillings-Connolly, Inc. 7 of 7 Tahoma Terra
Consulting Engineers STEP System Analysis
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