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8 Model Description 8.3.1 Tacoma Supply System 8.3.2 Reservoir Operations 8.3.3 Groundwater Sources 8.3.4 Instream Flow Requirements 8.3.5 Demand Forecasts 8.3.6 Conservation 8.3.8 Future Sources 8.3.9 Hydrologic Data 8.3.7 Tacoma Second Supply Project/Tacoma-Seattle IntertieThe Second Supply Project has been proposed as a new source of regional municipal water for over 20 years. The project consists of expanding use of HAH Reservoir, diverting water under Tacoma's second water right (100 cfs) and providing a portion of this water to Tacoma, Seattle, and some of the members of the South King County Regional Water Association (SKCRWA). Participating members of the SKCRWA currently include the Covington Water District, City of Kent, and the Lakehaven Utility District. Conceptually, Pipeline 5 would be used to transmit the second supply water from Tacoma's existing headworks facility and this pipeline would be used to deliver water to Seattle via the Tacoma-Seattle Intertie (TSI) and to SKCRWA via various spurs (Figure 8.27).
Figure 8.27 Proposed Intertie System
HAH Expansion Alternatives The second supply project under consideration could expand use of HAH in two phases: Phase I would store an additional 20,000 acre-ft to meet Tacoma's second
water right and an additional 5,000 acre-ft for low flow augmentation, as
depicted in Figure 8.28.
Figure 8.28 S = Seattle, T = Tacoma, and P = SKCRWA purveyors Phase II stores an additional 2,400 acre-ft (for a total of 22,400) for municipal diversion, and an additional 9,600 acre-ft for fish flows (for a total of 14,600 acre-ft) in the Green River, as shown in Figure 8.29.
S = Seattle, T = Tacoma, and P = SKCRWA purveyors Figure 8.30 presents the proposed refill curves (dry years are used in the model to determine system performance under low flow conditions). Under both scenarios, refill begins by March to meet the additional storage requirements. After July 1, drawdown under these scenarios is controlled by the Corps flood curves. All of the curves effectively empty the reservoir by mid-December to prepare for winter floods.
Figure 8.30 Proposed HAH Rule Curves
Dedicated Block Storage Under future operating scenarios, inflows into the reservoir will be managed using dedicated block storage referred to as Fish Dam 1 (used to meet Corps baseflows), Diversion Dam (used to store Tacoma's second water right water), and Fish Dam 2 (used to meet Green River (Tacoma's) HCP instream flow targets). For Phase I of the project in CRYSTAL, inflows into HAH are either released or stored in a dedicated block as follows:
Management of the dedicated blocks of water ensures that water is not inadvertently diverted from municipal supply storage for fish flows and vice versa. Phase II of the project, which has slightly different refill priorities, has not yet been incorporated into the model logic.
Tacoma-Seattle Intertie This section of CRYSTAL is based on the Conceptual Agreement signed by Tacoma Public Utilities and Seattle Public Utilities regarding Tacoma's Second Supply Project. The TSI agreement between Tacoma and Seattle has two categories of water: "stored" water within HAH and the "run-of-river" water. Although detailed operating rules are expected in the final agreement, the following operating rules from the conceptual storage agreement are incorporated into CRYSTAL: Water is only available, and thus transferred through the TSI, under the second water right (100 cfs) if Tacoma’s primary diversion requirements and instream baseflows have been met as stipulated in Tacoma's Habitat Conservation Plan. All three parties (Seattle, SKCRWA, and Tacoma) each receive up to 3,333 acre-ft of fixed storage every year. The model assumes that Seattle and SKCRWA use all of the available allocation, while Tacoma only uses the portion of its fixed water allocation that is necessary to meet demands. Every year, Seattle, Tacoma, and SKCWRA have rights for up to 3,333 acre-ft of variable storage. Seattle can use both its and Tacoma’s variable storage allotment (i.e., for a total of up to 6,667 acre-ft) in 1 out of 3 years. If Seattle makes a call on its variable water, Seattle will use all available variable water. Otherwise, Tacoma receives all of the available variable water and uses the water only as necessary to meet remaining demands. The model assumes that if historic hydrologic flows are low, these would be the conditions under which Seattle would call on the entire variable water volume. Historical years with the lowest cumulative flows (i.e., 1929, 1933, 1937, 1941, 1944, 1957, 1963, 1978, 1987, 1989, and 1992), are hard-wired into the model. Operating rules will be developed based on hydrologic characteristics of these years (e.g., snowmelt) so that a decision is made in the spring of each year. Timing of Seattle’s request for variable storage occurs on July 1, in order to allow Tacoma adequate time to use the variable storage if Seattle doesn't request it. The model assumes that the SKCRWA will consistently demand 15 mgd (as specified in Tacoma's most recent water supply plan) and that the SKCRWA will always use all its allotted fixed and variable water storage (up to 6,667 acre-ft). In addition to the storage rules above, CRYSTAL also allocates run-of-river water, which is defined as any spill from HAH reservoir that is not allocated to first water rights, instream flows, or storage allocation. The maximum volume of run-of-river water that can be diverted is constrained by Tacoma's second diversion right (100 cfs) and is divided equally between the three parties: Seattle, SKCRWA, and Tacoma. The model assumes that available water under the second water right is stored in Diversion Dam between 15 Feb-30 June and that no run-of-river water is available during this period unless HAH is full.
Pipeline Capacities CRYSTAL allocates pipeline capacities according to which water is being transferred. Pipeline 5 is expected to have a maximum capacity of 95 mgd and convey water to Seattle (via the TSI), to SKCWRA (via numerous unnamed spurs), and to Tacoma's service area (Figure 8.31). The TSI pipeline has a maximum capacity of 40 mgd.
Figure 8.31 Proposed Pipeline Capacities
The capacity of Pipeline 5 is normally divided equally into thirds. However, when Seattle has made a call on variable water, the model diverts 40 mgd through the TSI because Seattle would want to convey the maximum stored allotment (10,000 acre-ft) at a higher delivery rate. Because this action leaves a maximum of 55 mgd of "non-Seattle" water, the model assumes the remaining 55 mgd of Pipeline 5 capacity will be divided equally between Tacoma and SKCWRA.
Table of Contents 8.1.1 Everett Supply System 8.2.1 Seattle Supply System
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