8 Model Description    8.2.1  Seattle Supply System    8.2.3  Moraine Seepage    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    8.2.7  TSI/SSP

The rule curves used in the CRYSTAL model were obtained from SPU and reflect the values currently assumed in yield modeling by that agency.

Figure 8.10 depicts the Cedar Rule Curve. The maximum reservoir volume in the winter months is 62,770 acre-feet with the refill period beginning the third week of February. The goal is to reach the 1560 feet (84,565 acre-feet) by the first week in May. The rule curve descends again during the third week of August to prepare for the fall flood season, with the goal of 1548 feet (62,770 acre-feet) by October 1.

Dead storage, the elevation below which water is not accessible, exists below 1532 feet.  Reservoir operators are able to pump from this dead storage in emergency situations.  An alternative under consideration is to install a permanent pumping system on the reservoir, which would lower the dead storage to elevation 1502 feet, and thereby increasing reservoir storage by 33,600 acre-feet.

Extensive research was completed in the 1998 Masonry Dam Flood Operations Study concerning the ability of the rule curve to prevent fall and spring floods from occurring downstream.  A dynamic flood season rule curve that is conditioned on snowpack and meteorological forecasts was deemed a more effective rule curve. The dynamic snow-pack rule will be incorporated in the future.

The maximum reservoir volume in the winter months is 48,800 acre-feet for the Tolt Reservoir, with the refill period beginning as early as the last week of January (Figure 8.11).  The refill period has two major steps, the first is to reach 56,100 acre-feet by the second week of February (elevation 1762 feet).  An additional 1800 acre-feet of water storage is obtained between March and the end of May via raising the ring gate to an elevation of 1765 feet.  The rule curve descends to an elevation of 1745 feet beginning the last week of August, with the goal of 48,800 by October 1.

The volume of dead storage in the South Fork Tolt Reservoir is dependent on the completion of the Tolt Treatment Facility (TTF).  Currently the dead storage is 27,464 acre-feet (Elevation 1730 feet), but the TTF will allow the reservoir to be drafted to 15,745 acre-feet (Elevation 1710 feet).  The TTF is expected to be online near the end of 2000. A dead storage elevation of 1660-1690 feet (1519 AF - 8228 AF) is under study, and is a water supply option for the future.

Table of Contents    8.1.1  Everett Supply System    8.3.1  Tacoma Supply System