Hydrodynamic and Sediment Transport Model Application for OSAT3 Guidance: Ratio of the wave- and current-induced shear stress to the critical value for oil-tar balls and sediment mobilization over a tidal cycle

The U.S. Geological Survey has developed a method for estimating the mobility and potential alongshore transport of heavier-than-water sand and oil agglomerates (tarballs or surface residual balls, SRBs). During the Deepwater Horizon spill, some oil that reached the surf zone of the northern Gulf of Mexico mixed with suspended sediment and sank to form sub-tidal mats. If not removed, these mats can break apart to form SRBs and subsequently re-oil the beach. A method was developed for estimating SRB mobilization and alongshore movement. A representative suite of wave conditions was identified from buoy data for April, 2010, until August, 2012, and used to drive a numerical model of the spatially-variant alongshore currents. Potential mobilization of SRBs was estimated by comparing combined wave- and current-induced shear stress from the model to critical stress values for several sized SRBs. Potential alongshore flux of SRBs was also estimated to identify regions more or less likely to have SRBs deposited under each scenario. This methodology was developed to explain SRB movement and redistribution in the alongshore, interpret observed re-oiling events, and thus inform re-oiling mitigation efforts.美国地质调查局（U.S. Geological Survey）研发了一种用于估算重于水的沙子与石油团聚体（焦油球或表面残留球（surface residual balls, SRBs））的迁移特性及潜在沿岸输运能力的方法。在深水地平线（Deepwater Horizon）漏油事件期间，部分抵达墨西哥湾北部近岸冲浪带的油污与悬浮沉积物混合沉降，形成潮下垫。若未被清除，此类垫状物可碎裂为SRBs，进而重新污染海滩。研究团队开发了一套估算SRBs迁移及沿岸运移的方法：从2010年4月至2012年8月的浮标数据中筛选出一套具有代表性的波浪条件，以此驱动空间变化沿岸流的数值模型；通过将模型计算得到的波浪与流共同诱导的剪切应力，与不同粒径SRBs的临界剪切应力进行比对，估算了SRBs的潜在迁移可能性；同时还估算了SRBs的潜在沿岸通量，以识别各情景下SRBs沉积概率更高或更低的区域。本方法论旨在阐释SRBs的沿岸运移与再分布过程，解读已观测到的复油事件，从而为复油防控工作提供科学依据。