8.0  Model Description    8.3.1  Tacoma Supply System    8.3.3  Groundwater Sources    8.3.4  Instream Flow Requirements    8.3.5  Demand Forecasts    8.3.6  Conservation    8.3.7  TSI Intertie    8.3.8  Future Sources    8.3.9  Hydrologic Data

The current rule curves used in the CRYSTAL model were obtained from the Draft Environmental Impact Statement for the Howard A. Hanson (HAH) Additional Water Storage project, which was issued by the U.S. Army Corps of Engineers (Corps) in April 1998.

The HAH Rule Curve was developed by the Corps to prevent floods between December and mid-April (Figure 8.19).  During this period, the reservoir is drafted to its dead storage volume of 1,200 acre-feet.  The current rule curve begins to refill in early April and completes refill to the maximum current reservoir volume of 24,200 acre-feet by June 1.  An additional 5,000 acre-feet of storage is available for a one-month period during May for fish flow augmentation under pending Section 1135 authorization.  The current rule curve begins to drawdown during July to augment fish flows and prepare for the fall/winter flood season, with the goal of 1,200 acre-ft of storage by December 1.  In addition, the Corps can elect to fill HAH above the rule curve for 1-2 weeks in the spring to remove shoreline debris; however, this volume is not available specifically for fish flow augmentation so it is not incorporated into the CRYSTAL model.

The Corps has operated HAH using a flexible refill schedule for the last two years.  The refill goals are determined as a percentage of inflows, with the initiation and percentage level of refill dependent on snowpack conditions.  An attempt to incorporate this dynamic rule curve into the model will be completed in the future.

Flood Operations

HAH was originally constructed, in part, to provide flood storage during winter high flows.  During the winter, when the reservoir is maintained near the dead storage level of 1,200 acre-ft, the following assumptions are used in CRYSTAL to control floods downstream:

• The Corps operate HAH to prevent flows greater than 12,000 cfs at the Auburn gage. In CRYSTAL, releases from HAH plus the inflows between Palmer and Auburn are compared to the flood threshold.  If releases from the reservoir would cause flows at Auburn to be above this threshold, the model releases only the amount of water needed to meet municipal demands (i.e., water that will be diverted under the first or second water right) and the Corps 110 cfs baseflow at Palmer. Because CRYSTAL runs on a weekly timestep this trigger is set to 8,000 cfs, which corresponds to the historical weekly flows associated with daily peak flows of 12,000 cfs or greater.  Retained volumes for flood control are released when expected flood levels recede.
• Historical data from 1986-1993 indicate that the Corps also retains a percentage of high inflows during the winter to allow turbid water to settle.  CRYSTAL is configured so if winter inflows are above 500 cfs, the following percent of inflow will be retained:
• Percent retained = -0.6146 * ln (inflows in acre-ft/week) + 1.8044
• If water is retained because of high, presumably turbid, inflows then CRYSTAL is configured to turn on the North Fork well field until the retained flows are released from HAH.

Table of Contents    8.1.1  Everett Supply System    8.2.1  Seattle Supply System