Source, transport, and deposition of residual Deepwater Horizon oil that is causing shoreline reoiling along the coastlines of Alabama, Florida, Louisiana, and Mississippi is not well understood, and existing scientific knowledge has proven insufficient to fully support cleanup response and management decisions. Therefore, in May 2012, the U.S. Coast Guard, acting as the Federal On-Scene Coordinator chartered the third Operational Science Advisory Team (OSAT3) to provide science-based review of data collected (for example, morphologic elevations and oiling locations) and to conduct additional directed studies and sampling as necessary. A steering committee cochaired by British Petroleum Corporation (BP) and the National Oceanic and Atmospheric Administration (NOAA) is overseeing the project and includes State on-scene coordinators from four States (Alabama, Florida, Louisiana, and Mississippi), trustees of the U.S. Department of the Interior (DOI), and representatives from the U.S. Coast Guard.

Residual oil from the Deepwater Horizon spill is found in the shallow surf-zone in the northern Gulf of Mexico in two primary forms: submerged oil mats and surface residual balls (SRBs). Mats formed when weathered oil at the surface reached a shallow, energetic-enough location for wave-stirred sediment to mix with the oil and create a sand and oil mixture. This mixture can become heavier than water and settle to the the sea floor where more sediment can be incorporated, forming a submerged mat. Mats encountered as part of the Deepwater Horizon response efforts are generally meters in cross-shore width, meters to tens of meters in alongshore length, and a few to tens of centimeters thick. Under sufficiently energetic wave conditions, buried mats can be exhumed and pieces of the mats can break off and form SRBs (Shoreline Cleanup and Assessment Team, oral commun., 2012). The SRBs are observed in the surf zone and may wash up on beaches. SRBs and mats in the surf zone and on the beach are targets of ongoing cleanup response efforts. This report presents the results of an OSAT3 subgroup that developed hydrodynamic and sediment transport models and developed techniques for analyzing potential SRB redistribution and burial and exhumation to provide a better understanding of alongshore processes and movement of SRBs along the coastline of Alabama and Florida.

Overall OSAT3 Objectives

To provide improved understanding and guidance in support of the operational response to shoreline re-oiling, OSAT3 undertook five main tasks:

1. Evaluate the trends observed in frequency, rate, and potential for remobilization on beach segments in the areas affected by re-oiling.
2. Determine and record the locations and typical shoreline profiles and morphology for likely sources of residual oil or origin of the SRBs.
3. Define or determine the mechanisms whereby re-oiling phenomena may be occurring.
4. Investigate the potential for mitigation actions that may be taken to reduce these potential occurrences and, to the extent reoiling phenomena are identified, evaluate the feasibility of the mitigation actions and net environmental benefit of employing such actions.
5. Recommend a path forward to reach shoreline cleanup completion plan guidelines or appropriately manage identified areas through alternative methods.

To achieve these overall objectives, the OSAT3 subgroup collected and analyzed shoreline cleanup data (for example, SRB and mat recovery records and photographs showing location and size of SRBs) and geomorphic data (beach profiles, aerial imagery, shorelines, topography, and bathymetry) and conducted numerical modeling analysis of waves, water levels, currents, and local surficial sediment, and SRB mobility and transport. This report specifically addresses the numerical modeling aspect of the OSAT3 objectives along the coastline of Alabama and Florida; overall OSAT3 findings are not within the scope of the analysis of this report.

Objectives of USGS Hydrodynamic and Sediment Transport Modeling

This report documents the approach, results, and conclusions from the USGS OSAT3 subgroup that developed hydrodynamic and sediment (including SRBs) transport models to provide better understanding and prediction of alongshore processes and movement of SRBs. The subgroup was split into two teams tasked with analyzing specific regions of the affected coastline. One team covered the Louisiana and Mississippi coastlines (results not presented in this report), and the other team covered the Alabama and Florida coastlines. The latter region was modeled by researchers from the USGS Coastal and Marine Geology Program. The specific objectives of this subgroup's effort were to:

1. identify spatial patterns in alongshore current directions and velocity;
2. identify zones of convergence and alongshore current reversal;
3. identify potential sediment and SRB sinks;
4. estimate SRB movement along the coast; and,
5. determine the influence of tidal currents on SRB mobility and transport in the vicinity of tidal inlets.

These objectives were achieved through modeling scenarios covering the range of weather and wave variability since the Deepwater Horizon spill; extracting wave-driven alongshore currents, wave-orbital stirring, and tidal currents; and computing associated sediment and SRB mobility and alongshore transport patterns. The approach to selecting scenarios, hydrodynamic modeling, and SRB mobility and transport modeling is described in the Methods section. In the Results section, the modeled hydrodynamics are evaluated against observations and the alongshore current and SRB transport results from the scenarios are described. The impact of the results on achieving the overall OSAT3 objectives is described in the Discussion section. An analysis of our assumptions along with implications of our methods and findings for future operations associated with reoiling response are also discussed. The major findings from this study are summarized in the Conclusions section.