7.3.2 Seattle Water Supply System

The scenario evaluation, with respect to Seattle, has been separated into two sections, Seattle M&I and Instream conditions.

The following metrics describe the condition of Seattle M&I water supply with respect to the status quo and imminent scenarios during the years of investigation 2015, 2020, 2025, 2030, 2035, and 2040. Figures 7.2 and 7.3 portray simulation results found using the CRYSTAL model for Seattle annual M&I reliability, weekly reliability, M&I average shortfall, and M&I cumulative shortfall.

Seattle's annual M&I reliability decreases during both scenarios. The reliability begins at greater than 98% for both scenarios in the year 2015, but drops to 92% in the year 2020 during the status quo scenario. The imminent scenario provides a 98% reliability until the year 2030, when it drops to a reliability of 97%. Under the demands forecasted for the year 2040, the status quo and imminent scenarios provide reliabilities of 78% and 86%, respectively. The weekly reliability of Seattle decreases very little under both scenarios. The reliabilities are both near 98% in the year 2040.

These simulation results indicate that reliability of the Seattle system decreases as demand increases, as expected. The imminent scenario will yield unacceptable reliabilities, even with expanded water supply sources by the year 2030. The imminent scenario, though, will be more reliable for longer than the status quo scenario, even with the implementation of a habitat conservation plan and the corresponding increased fish flow requirements. The weekly reliabilities decrease very little, while annual reliability decreases significantly. This indicates that the number of M&I shortfalls increases, but they have short durations.

Figures 7.4 and 7.5 portray the shortfall events which occurred during the six model simulations. As expected, the total shortfall for each year of investigation increases as demand increases. The shortfalls occurring in the status quo scenario eventually become nearly 2.5 times greater in volume than those which occur during the imminent scenario. The graph depicting the average shortfall volumes shows that the average shortfall volume does not grow progressively larger with larger demand, as would be intuitive. The average volume is significantly larger in the year 2015 than in 2020. This decrease can be attributed to an increasing number of shortfalls in the system. In the year 2015, shortfalls occur less often and are larger than those in the year 2020. This situation also describes the decrease in average shortfall volume from the year 2035 to the year 2040, when the imminent scenario was modeled.

Instream Conditions

Figure 7.6 portrays the average amount of water flowing instream below Tolt Reservoir and Seattle's Diversion for M&I purposes. The imminent scenario, consistently has a lower average flow than that existing during the status quo scenario due to the Tolt Pipeline II, a second diversion from the South Fork of the Tolt River. As expected, the amount of water left instream decreases significantly as demand increases. Status Quo average flow decreases by 17% and the imminent scenario average flow decreases by 20% between the years 2015 and 2040.

Figure 7.7 shows that for each scenario, the volume of water in the Tolt River, which is greater than that required by the instream flow requirements decreases as demand increases. The imminent scenario provides less excess flow in the Tolt River. This difference in volume is due to the higher flow requirements, which are implemented along with the Tolt Filtration Facility and the higher fish flow requirements which are implemented on the Cedar River. Seattle's reliance on both these rivers creates a situation in which, once diversions from the Cedar Rivers are modified due HCP requirements, use of the Tolt River is modified to make up for this alteration.

Figure 7.8 portrays the percent of time, throughout each scenario simulation, that flows were greater than minimum requirements. As expected, this metric decreases as demands increase and the imminent scenario provides excess flows for a smaller amount of time.

Cedar River

Figure 7.9 portrays the amount of water instream below the Chester Morse Reservoir and Seattle's diversion from the Cedar River. For each of the year of investigation demands, flows supplied during the status quo scenario are significantly greater than those supplied during the imminent scenario. This difference is due to reservoir operation. The increased surface water available will be used to meet M&I demand with increased reliability, never reaching the Cedar River.

Figures 10 and 11 describe instream flows in the Cedar River for both scenarios. The situation is similar in that excess flows decrease with increased demand and the imminent scenario provides less excess than the status quo scenario. The discrepancy between the scenarios can be attributed to the higher flow requirements, which are implemented as part of the Cedar Habitat Conservation Plan.