An offline unstructured biogeochemical model (UBM) for complex estuarine and coastal environments

Due to increased pollutant loads and water use from coastal development and population growth, occurrences of low-dissolved oxygen and “hypoxic zones” have increased. Reports of fish kills and water quality impairment are also becoming more frequent in many coastal waters. Water quality managers and regulatory agencies rely on numerical modeling tools to quantify the relative contributions of anthropogenic and “natural” pollutant loads (nutrients and biochemical oxygen demand) on dissolved oxygen levels and use the results for remedial activities and source control. The ability to conduct season-long simulations with sufficient nearshore resolution is therefore a key requirement. Mesh flexibility and the ability to increase site specific resolution without disturbing the larger domain setup and calibration are critical. The objective of this effort was to develop a robust biogeochemical model suitable for simulation of water quality dynamics including dissolved oxygen in complex coastal environments with multiple tidal channels, tidal flats, and density-driven circulation using unstructured-grid formulation. This paper presents an offline unstructured biogeochemical model that uses the Finite Volume Coastal Ocean Model (FVCOM) discretization of the study domain and the corresponding hydrodynamic solution to drive biogeochemical kinetics based on a water quality model CE-QUAL-ICM. In this paper, the linkage between selected hydrodynamic and water quality models is subjected to several scalar transport and biogeochemical module tests (plume transport and dilution, BOD/DO sag, and phytoplankton/nutrients reaction), and results are compared to their analytical solutions as part of model validation. A preliminary application of the biogeochemical model with a year-long simulation of Hood Canal basin in Puget Sound, USA, is presented as an example and a test of the tool in a real estuary setting. The model reproduced the dynamics and seasonal variations in the biogeochemical state variables and was used to test short-term wind-driven dynamics that could influence dissolved oxygen concentrations in Hood Canal.