8.2.1 Seattle's Water Supply System

This section provides an overview of the Seattle water supply system. The topics of this discussion include a summary of the system, reservoir operations, current instream flow requirements, demand forecasts for this area, and future water supply options. A description of the hydrologic data incorporated into CRYSTAL concludes the Seattle information.

The Seattle portion of the CRYSTAL model is based on the Conjunctive Use Evaluation (CUE) model developed by SPU’s Water Management Section and actively used by SPU for yield forecasting. The following sections describe the assumptions used in the CRYSTAL model. These assumptions closely approximate actual operations, but actual operations may differ in many circumstances.

Seattle Public Utilities (SPU) obtains water from three primary sources: Chester Morse Reservoir, South Fork Tolt Reservoir, and the Highline Well Field (Figure 8.12). The user has the option of operating the reservoirs conjunctively or independently, and also the choice of how much water to pump from the wells.

Figure 8.8 SPU System Map

Cedar Reservoir

Cedar Reservoir is modeled as two reservoirs due to seepage that occurs between the Overflow Dike and the Masonry Dam (Figure 8.9). The water that seeps out of the reservoir system leaves the system through Boxley Creek and Rattlesnake Lake, or recharges the Cedar River through Canyon Creek and subsurface flow to the Cedar River above Landsburg Returns.

Figure 8.9 Cedar Reservoir Profile

Dead storage is water that is not available through the outlet structure, in this case, by gravity flow through the Overflow Dike. Dead storage elevation at Chester Morse Lake is below 1532 feet. Masonry Pool can be drained by gravity to 1500 feet or less. Normal minimum operating level is 1530 feet, but can be lowered to 1510 to reduce seepage during dry periods. The dead storage elevation drops to 1502 feet at Chester Morse Lake if the pumping alternative is activated.

The CRYSTAL model considers releases from the Chester Morse Reservoir as either reservoir release or penstock flows. The maximum capacity of the two penstocks, which is dependent on the elevation of Chester Morse Reservoir, is 750 cfs. The reservoir release includes any spill from the reservoir (through spillways or spill valve), instream flow releases and M&I releases to be diverted at Landsburg, excluding penstock flows.

Reservoir release, penstock flows, and intermediate flow combine to form the flow above Landsburg. Diversion to Lake Youngs is a function of the portion of the Cedar water that contributes to M&I demands. The flow above Landsburg minus the Diversion to Lake Youngs plus intermediate flow form the flow at Renton, which is used to meet instream flow requirements at Renton. Lake Young storage is not modeled.

South Fork Tolt Reservoir

There are two types of releases from the Tolt Reservoir: Tolt Draft and Tolt Flow. Tolt flow combines with intermediate inflow to meet the Tolt Instream flow requirements or exceed minimum by Tolt spill. The Tolt Draft is used to meet the Tolt portion of M&I demands.

Highline Wellfield

The user can define the groundwater-pumping rate of the Highline Well Field. The default value is 10 mgd. The wellfield can be pumped for a 14-week period, and becomes available in July. An available option is to increase pumping to 15 mgd over this period, when available. Highline Wellfield is drafted before the storage in the reservoirs.

Conjunctive Use of Reservoirs

The quantity of water taken from the reservoirs is calculated as either: 1) A specific percentage of demand each reservoir must release; or 2) A quantity defined in the "space rule." If a specific percent is used then that percent of demand is taken from the Cedar system, if available. The space rule is a more dynamic rule that attempts to minimize the spill from the system by proportioning releases based on reservoir storage, inflows, and demands.

Table of Contents 8.1.1 Everett Supply System 8.3.1 Tacoma Supply System