Review of the Lake Washington Ship Canal and Ballard Locks model, Seattle, Washington, 2014–20

Abstract

Executive Summary The Hiram M. Chittenden (Ballard) Locks and Lake Washington Ship Canal connect freshwater Lake Washington and saline Shilshole Bay of Puget Sound in Seattle, Washington. The locks and canal allow for ships to traverse this reach. Anadromous salmonids also migrate through, transitioning between saline and freshwater environments, and making use of a fish ladder at the locks when traveling upstream. WEST Consultants, Inc., constructed a two-dimensional hydrodynamic and water-quality model (CE-QUAL-W2) simulating flow, water temperature, and salinity for the Ballard Locks and the Lake Washington Ship Canal. An initial model was built for calendar years 2014–15, and the model was updated using a more recent and modern dataset for calendar years 2016–20. The U.S. Army Corps of Engineers requested that the U.S. Geological Survey review this model and its documentation to evaluate the technical aspects of its development and calibration. Findings from this review include the following: Overall, the Lake Washington Ship Canal CE-QUAL-W2 model was well-documented and constructed largely following typical model-development methods. The Lake Washington Ship Canal model was built with CE-QUAL-W2 model version 4.5, compiled and released by Portland State University in April 2021. CE-QUAL-W2 updates and improvements are regularly released with bug fixes and new features, so any model updates would benefit from the use of the most-recent software release. The model grid that represents the Lake Washington Ship Canal bathymetry was 9.2 kilometers (5.7 miles) long, matching the expected length of the waterway. The deepest model segments were near sampling site LLLW (Large Locks site) near the locks. Lake Union is reported to constitute most of the volume of the Lake Washington Ship Canal and is depicted as such in the model grid. The model includes several water outflows at Ballard Locks, including the large and small locks, a saltwater drain, a spillway, smolt flumes, and a fish ladder. Flows from the spillway, smolt flumes, and fish ladder were combined into one structure outflow in the model and assigned one withdrawal elevation from the Lake Washington Ship Canal. The smolt flume and spillway withdraw from the same elevation, but the fish ladder flow withdraws from a higher elevation in Lake Washington Ship Canal, and that flow could be separated into its own withdrawal. The model input files were created using the Coordinated Universal Time standard instead of the more typical choice of using local standard time. This is not incorrect, but sub-daily results would need to be converted to local time for science-communication purposes. The meteorological dataset had some unexpected anomalies, such as a baseline shift in the wind-speed dataset. Other nearby meteorological datasets could be used instead or used to correct the current meteorological inputs. The upstream boundary was configured with water-temperature data from a continuous monitor buoy in Lake Washington. The boundary salinity was set at 0 parts per thousand for the duration of the model simulation. A more realistic estimate of salinity at the upstream boundary could be constructed using data from the same buoy. Saline inflow at the downstream boundary of the Lake Washington Ship Canal model through lock exchanges at the large lock was included as a tributary in the model. Salinity and temperature inputs in this tributary at the large locks were set as constant values for the entire simulation. Saline inflow through the small lock was not included in the model because few data were available, and the input was likely to be small because of the smaller surface area and volume of the small lock relative to the large lock. The model did not include any flow, water temperature, or salinity inputs to the Lake Washington Ship Canal other than at the locks and at the upstream boundary. Any point sources, small tributaries, or stormwater inputs were omitted from the model. It is unclear whether this is a substantial omission relative to model results. Most model parameters were set as defaults or to reasonable values. However, the value of the WINDH parameter, the height of the wind speed measurement, was different than the height of the meteorological site. Compared to measured data, the model simulated water-surface elevations and water temperatures with reasonable accuracy. Differences in the modeled and measured salinities revealed some opportunities to improve the simulation of salinity, both baseline salinity and the salinity maxima in summer and autumn.