2013 Water System Plan, Volume II - Seattle City Clerk's Office - City ...

2013 Water System Plan
Official Yield Estimate and
Long-Range Water Demand Forecast
Inputs and Assumptions for the Firm Yield Estimate
Firm yield of the water supply system is estimated using a simulation model developed by
Seattle Public Utilities called the Conjunctive Use Evaluation (CUE) model. Additional details of
the model and inputs are documented in the final report titled Firm Yield of Seattle’s Existing
Water Supply Sources, November 2011, prepared by Seattle Public Utilities.
Model Inputs and Assumptions
Firm yield is based on the 98% reliability standard—one shortfall occurs in the 81 years
of historic record.
Historic weekly inflows reconstructed for water year 1929 through 2009 are used.
Total system demand is shaped on a monthly demand pattern based on the average of
actual deliveries from calendar year 2005 through 2009.
Sources of supply are operated conjunctively as a single system.
Operational assumptions include:
• Cedar River System:
Meet Cedar River Habitat Conservation Plan instream flow commitments below
Landsburg, assuming flashboards in place on Overflow Dike.
Fixed rule curve for Cedar Reservoir of 1550’ for October-February and 1563’ for
May-August.
Minimum levels for Chester Morse Lake: 1532’; Masonry Pool: 1510’
Meet diversion limits specified by the 2006 Agreement with the Muckleshoot Indian
Tribe.
• South Fork Tolt System:
Meet instream flows from 1988 Tolt Settlement Agreement (with treatment project).
Fixed rule curve 1754’ for October-January; 1765’ for March-August.
Minimum level for South Fork Tolt Reservoir: 1710’
Treatment/Transmission capacity: 120 MGD
• Seattle Well Fields:
10 MGD withdrawn for 14 weeks as needed from July-December.
5 MGD recharged for 14 weeks from January-March.
Results
Based on the above, the system-wide firm yield is
172 million gallons per day.
FINAL Page 1 of 22 December 2011