8  Model Description    8.2.1  Supply System    8.2.2  Reservoir Operations    8.2.4  Instream Flow Requirements    8.2.5  Demand Forecasts    8.2.6  Conservation    8.2.7  Future Sources    8.2.8  Hydrologic Data    8.3.7  TSI/SSP

The north embankment along Masonry Pool is connected to a glacial moraine.  This complicates the water balance of the Masonry Pool and Chester Morse Lake.  Water seeps from the Masonry Pool into the moraine and then leaves the basin or recharges the Cedar River, as shown in Figure 8.12.

As discussed in the Seattle Operations section, the Cedar Reservoir is modeled as two systems.  Chester Morse Lake is impounded by the Overflow Dike, and does not contribute significantly to moraine seepage.  The Masonry Dam impounds the Masonry Pool, and as the elevation in the Masonry Pool increases the rates of seepage into the Cedar Moraine increases due to the higher pressure gradient.  Once water has seeped into the moraine, gage data show that a portion flows via natural recharge to the Snoqualmie Basin, Rattlesnake Ditch, and Boxley Creek while the remaining water flows to the Cedar River via Canyon Creek and natural recharge.

Extensive modeling has been completed for the moraine aquifer.  URS Consultants and the US Geological Survey (USGS) completed two models.  Hydrocomp Consultants developed the third model.  These three were combined to form the "default" aquifer model, which predicts moraine seepage and return flows associated with the Cedar Reservoir (Seattle Water Department, 1995). This volume is represented in CRYSTAL as a function of the elevation of the Masonry Pool.  The portion of the volume that seeps into the moraine and returns to the Cedar as recharge is calculated in CRYSTAL as follows:

Cedar Recharge = Seepage Volume -

(Flow to Boxley Creek + Flow to Snoqualmie Aquifer +

Flow to Rattlesnake Ditch + Flow to Canyon Creek)

Based on studies conducted by Seattle, the lag between water leaving the Masonry Pool via seepage and that same water returning to the Cedar River via recharge is approximately 1.5 months.  This lag has been incorporated into the CRYSTAL model.

Table of Contents    8.1.1  Everett Supply System    8.3.1  Tacoma Supply System