Water AnalysisCity of Yelm Potable Water System
Tahoma Terra Hydraulic Analysis
Project #05111
Skillings-Connolly, Inc.
Consulting Engineers
5016 Lacey Blvd SE
Lacey, WA 98503
Reviewed by:
Thomas E. Skillings, PE
Principal
Andrew C. Perez, PE
September 2005
Yelm Potable Water System Analysis
The following potable water system model of the City of Yelm has been developed using
WaterCAD® software as a tool for evaluating the water 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 and Thurston Highlands master planned
communities. The following narrative details the modeling methods for the Yelm potable
water system pipe network.
Model Development
System Demands: The City of Yelm potable water system currently serves approximately
1860 Equivalent Residential Users (ERU's). Using information taken from the City of
Yelm Comprehensive Water Plan, which was approved in September of 2002, the Peak
Hourly Demand (PHD) on the system is approximately 1,800 gpmt
Distribution Svstem: The piping layout was created using the potable water system map
as per the City of Yelm Comprehensive Water Plan, approved in September 2002 along
with information on additions and proposed extensions provided by the City. The line
sizes vary from 2" to 12" in diameter. Likewise, the piping system is approximately 74%
PVC, 26% asbestos concrete (AC) and 0.01 % ductile iron. The friction factors for each
pipe range from 90 for the older sections of AC to 150 for the PVC pipe.
Storm The potable water system has three storage reservoirs with a total capacity of
approximately 1,050,000 gallons. For the scenarios analyzed in this report, each tank's
level was set at 12 feet from full, which is at the bottom of the equalization storage and
just above the fire suppression storage levels.
Source: The potable water system is currently served by Wells 1 and 2, which pump
alternately. Each pump is capable of delivering approximately 1,200 gpm. For the
scenarios analyzed, the pumps were turned off.
Svstem Anal
A steady state analysis of the potable water system 20 psi fire flows with PHD, using
WaterCAD, yields results that indicate the maximum amount of fire flow with a residual
zone pressure of 20 psi that can be delivered to each node/hydrant in system with PHD.
Model Calibration: The model was previously calibrated for the 2002 comprehensive
water plan update.
Scenarios:
SCENARIO 1:20 psi fire flow with PHD at Node A (see Figure 1) with pipes B
and C.
i City of Yelm Comprehensive Water Plan, September 2005 pg. 3-23
Skillings-Connolly, Inc. 1 of 2 Tahoma Terra
Consulting Engineers Potable Water System Analysis
SCENARIO la: Same as above, with the exception ofpipe C as a10" diameter.
SCENARIO 2: 20 psi fire flow with PHD at Node A with pipes B, C and D.
SCENARIO 2a: Same as above, with the exception of pipe C as a10" diameter.
SCENARIO 3: 20 psi fire flow with PHD at Node A with pipes A, B, and C.
SCENARIO 3a: Same as above, with the exception of pipe C as a10" diameter.
SCENARIO 4: 20 psi fire flow with PHD at Node A with pipes A, B, C and D.
SCENARIO 4a: Same as above, with the exception of pipe C as a10" diameter.
Model Scenario Results
Table 1 contains the results for all Scenarios.
Table 1
Scenario 1 Results
Scenario 20 psi Fire Flow vv~ithl'TTn at Node`A
1 2200
la 2200
2 2200
2a 2300
3 3000
3a 3100
4 3000
4a 3100
Conclusions and Recommendations:
Based upon the results of the fire flow analysis, the addition of Tahoma Terra to the City
of Yelm's potable water system requires the installation of a 10" diameter line connecting
Tahoma Terra Boulevard and along Longmire to McKenzie Street. The line would
provide adequate fire flow to the new subdivision. Accordingly, the water main
connecting Nisqually Estates to Tahoma Terra should be a 10" diameter line which will
provide approximately 100 gpm more fire flow than an 8" diameter main.
Skillings-Connolly, Inc. 2 of 2 Tahoma Terra
Consulting Engineers Potable Water System Analysis