U.S. Geological Survey scientists have developed a model-based assessment of transport and deposition of residual oil that is causing shoreline reoiling within the northern Gulf of Mexico in the form of mixtures of sand and weathered oil, known as surface residual balls (SRBs). The results of the assessment can be used to provide guidance for ongoing response activities and data collection needs as well as to guide response for future oil spills or related events.

Results from these modeling efforts have identified spatial and temporal variations in alongshore currents, including locations where there are gradients resulting in convergences and decelerations in the flow. Likewise, the results identify spatial and temporal variations in the mobility of sand and SRBs. A striking example of convergence identified by the model was shown at Pensacola Pass, Fla., where trends in the orientation of the adjacent shoreline cause flow and potential SRB flux to be driven toward the inlet. This situation forces SRBs to interact with tidal currents, and the complexity of the situation is exacerbated by SRBs interacting with sandbars and channels which are themselves undergoing constant reconfiguration as wave and tide conditions change.

Modeling results suggest that, under the most commonly observed low-energy wave conditions, larger SRBs are not likely to move very far alongshore. This finding suggests that, under nonstorm condtions, large SRBs from one source location may not be redistributed to other alongshore locations. Under storm conditions, however, it is possible for larger SRBs to be mobilized. SRBs are likely to become buried and unburied under normal sand transport processes because they are less mobile than sand, thereby lengthening the time SRBs take to move onshore and adding a layer of complexity to the model.