Quantifying transport process uncertainty for oil spill modeling across the bay-shelf continuum
Oil spills are a common environmental issue in estuaries and coastal oceans. Such spills can be caused by ship collisions, offshore oil rig blowouts, or onshore leakage from production facilities. To minimize spill impacts, operational managers require reliable and rapid real-time prediction of oil transport paths through bays and inlets. Such modeling tools can be used for advanced planning, real-time decision-making, and post-event analysis of spill spatial extents. Advanced oil spill operational systems are commonly applied to predict the fate and trajectory of a spill – data that is needed as rapidly as possible during a spill event to set up containment equipment. In operational oil spill modeling, predictions require velocity currents provided by hydrodynamic models, which often employ a moderate domain size and coarse-resolution grids for computational efficiency and rapid predictions. However, coarse resolution models limit the accuracy of the modeling prediction. In particular, a practical coarse-grid resolution can introduce model structural errors when the small-scale flow features are poorly resolved. Such errors have been called “geometric uncertainty”, as the coarse-grid geometry of the model introduces uncertain error into the predictions. Of particular interest are starting jet vortices (tidal eddies) that are common at the inlet of bar-built estuaries with narrow inlet channels, where channel dredging and jetties have been employed to aid ship traffic. These eddies influence Lagrangian transport paths and hence the fate of an oil spill potentially entering or leaving an estuary. A further problem is that the multi-scale flows that combine bay and coastal shelf physics are not typically represented in estuary models, which limits the accuracy of oil spill predictions across the shelf-estuary interface. The errors introduced by neglecting the multi-scale flows can be significant, particularly when the alongshore currents on the shelf encountering strong tidal flows at the estuary entrance. At model scales relevant to the operational prediction of oil spills, this research quantifies: (i) effects of tidal eddies on mixing process and effects at a channel entrance, (ii) geometric uncertainty associated with bay oil spills, and (iii) oil spill transport across the shelf-estuary interface. These issues are addressed using Galveston Bay as the study site. It is demonstrated that an adequate eddy solution is obtained at the horizontal grid size of ∼140 m, and the model at a practical operational grid resolution (∼400 m) captures neither the eddies nor their effects on particle movement, despite showing a satisfactory prediction of net transport through the inlet. With regards to geometric uncertainty, the research shows that such uncertainty is variable in both space and time, and can increase during strong flow dynamics. It is further shown that multi-scale flows affect oil spill transport across the shelf-estuary interface, and models that are focused on either the shelf or the coastal region alone will poorly represent the transport. To address these issues, this study proposes: (i) an empirical Lagrangian eddy model to simulate eddy effects at a channel entrance when an operational model has insufficient grid resolution, (ii) a data-driven uncertainty model and a multi-model integration to operationally quantify the geometric uncertainty, and (iii) a new 3-dimensional Galveston Bay model to reproduce the multi-scale flows that control the shelf-estuary transport. The technologies developed herein are able to integrate small-scale physics and explicit information of estimated modeling errors, and thus improve oil spill predictions at operational grid scales. The approach integrates results from coarse-resolution and fine-resolution models, providing emergency managers a more reliable tool for rapid spill assessment and response. This research enhances our understanding of the oil transport across the threshold between two contiguous water systems and highlights the importance of resolving the multi-scale flows that affect the fate and transport of oil spills.