Modeling water exchange and transport timescales in a multi-inlet bay system of Puget Sound, Washington

In this study, an integrated modeling framework consisting of the watershed model HSPF and the hydrodynamic model FVCOM is used to simulate the watershed hydrology and hydrodynamic circulations in a multi-inlet bay system of Puget Sound, which includes Liberty Bay, Port Orchard Bay, Dyes Inlet, and Sinclair Inlet. These interconnected inlets and bays form a unique subsystem that is connected to the Central Basin of Puget Sound through two narrow passages, namely Agate Passage and Rich Passage. Due to residential and industrial development in the watershed, degraded water quality has been observed in the system and manifests as bacteria contamination, low dissolved oxygen and harmful algal blooms. All these water quality issues require a good understanding of the pollutant sources from the watershed and the transport pathways in the receiving waters, and thus call for an integrated modeling approach based on watershed, hydrodynamic and water quality models. In this presentation, the high-resolution FVCOM model is used to simulate the general hydrodynamic circulation in the subsystem and quantify its physical transport timescales (e.g., residence time) with both neutral-buoyance tracers and Lagrangian particles under various hydrological conditions simulated by the HSPF model. The results demonstrate that hydrodynamic circulation plays an important role in explaining some of the water quality issues in the system. The transport timescales under various hydrological conditions provide useful indicators on how physical processes modulate pollutant transport and water quality in the subsystem