Sand and Oil Agglomerates in the Surf Zone Using Science to Aid Deepwater Horizon Clean-up Efforts

Transcription

1 Sand and Oil Agglomerates in the Surf Zone Using Science to Aid Deepwater Horizon Clean-up Efforts P. Soupy Dalyander St. Petersburg Coastal and Marine Science Center DOI USGS Sand and Oil Agglomerates in the Surf Zone Using Science to Aid Deepwater Horizon Clean-up Efforts P. Soupy Dalyander St. Petersburg Coastal and Marine Science Center U.S. Department of the Interior U.S. Geological Survey Page 1

2 The Problem: April, 2010: Deepwater Horizon rig explodes July, 2010: well head capped September, 2010: well sealed Several million barrels of gas and oil released Credit: The Guardian Credit: NOAA Page 2

3 What Happened to the Oil? Credit: NOAA Oil came ashore as thick, weathered oil mousse Page 3

4 Sand and Oil Agglomerates: Submerged Oil Mat (SOM) and Surface Residual Ball (SRB) Formation In the surf and swash zones, oil and sand mixed to form dense (sinking) agglomerates Some oil/sand mats were 10 s of meters long Small pieces break off and can re-oil the beach (called surface residual balls, SRBs) Page 4

5 Clean-up Response Credit: Florida DEP Credit: USCG Credit: Florida DEP Shoreline Clean-up Assessment Technique (SCAT) teams searched for oil. When found, USCG operations removed as much material as possible. But it kept (keeps) coming back. Why? Federal On-Scene Coordinator (FOSC) chartered the Operational Science Advisory Team (OSAT3) to help figure out what s going on and improve response Page 5

6 Key Questions for Response Where could mats still remain? If we find SRBs, should we be looking for a mat nearby? Where is it all going to eventually go? Why do we keep finding SRBs in areas we ve cleaned? Page 6 Credit: NPR

7 Where could mats be hidden? Identify where they could have formed Examine bathymetry changes in zones of possible formation, identify areas sand may still hide mats Page 7

8 Key Questions for Response Where could mats still be hidden? If we find a bunch of SRBs, does that mean we should be go digging for a mat nearby? Where is it all going to eventually go? Why do we keep finding SRBs in the same places? Page 8 Credit: NPR

9 Numerical Approach: Hydrodynamics Hydrodynamic Model: Delft3D Nested models: Wind, waves, water levels, and currents Resolve surf zone and inlets Include large number of scenarios (80+) Page 9

10 Calculating SRB and Sediment Mobility Waves and currents generate shear stress (force per unit area) on the seafloor Symbols courtesy of the Integration and Application Network (ian.umces.edu/symbols/ Sediment and SRBs move when stress exceeds a grain-size and density specific threshold Also looked at integrated alongshore potential flux Class Size (cm) Density (kg/m 3 ) Crit. Stress (Pa) Sediment SRB SRB SRB SRB SRB SRB Page 10

11 SRB and Sediment Movement Probability Increasing Size Small Waves (0 0.5 m) Sediment 2.5cm SRB 10cm SRB ß Immobile Mobile à Page 11

12 SRB and Sediment Movement Probability Increasing Size Large Waves (> 2.0 m) Sediment 2.5cm SRB 10cm SRB ß Immobile Mobile à Page 12

13 SRB and Sediment Movement Probability Increasing Size Large Waves (> 2.0 m) Sediment SRBs: less mobile than sand (burial/exhumation likely) 2.5cm SRB not mobilized during low-energy wave conditions of northern Gulf of Mexico 10cm SRB ß Immobile Mobile à Page 13

14 Calculating Potential Flux Calculate flux (mass/time), assuming particles are present, for given size and density Integrate across surf zone Convergences or decelerations in flux are alongshore areas of likely deposition West East Page 14

15 Alongshore Current Convergences & Gradients Areas of likely deposition under given wave conditions identified (Pensacola Pass) Page 15

16 Inlet Dynamics: Little Lagoon Flood Tide Ebb Tide Page 16 Inlets trap SRBS

17 Numerical Model Results Greater mobility of sand than SRBs makes burial and exhumation likely Larger (cm-size) SRBs unlikely to move alongshore outside of storm events Eventual long-term distribution patterns predicted, but will take a long time to reach that state and for all SRBs to be uncovered/recovered Inlets serve as traps for SRBs Page 17

18 Results: Page 18 Methodology can be applied elsewhere

19 Model Assessment Completed after model run and results reported (i.e., no model tuning ) Starting in June, 2011 the Coast Guard collected data (poundage, shape) on SRB/mat material retrieved Spatially delineated in alongshore segment (~250 m) Some limits (resolution in space and time, doesn t indicate where SRBs weren t), but data collected as part of response was extremely valuable Page 19

20 Coast Guard Field Collection Increased collection moving west in segments from Little Lagoon to Pensacola Pass (consistent with model) Higher collection of SRBs in inlet Particularly high collection from Pensacola Pass Exception: Perdido Pass Page 20

21 Coast Guard Field Collection Page 21

22 Model Assessment & Improvement Artificial Sand and Oil Agglomerates Lab-made (paraffin + sand) Instrumentation Tripod (Waves, Images, Video) Page 22

23 Model Assessment & Improvement Artificial Sand and Oil Agglomerates 1-m Deployment Page 23 Swash Zone Deployment

24 Seafloor interaction SRB Burial Page 24 SRB Burial SRB Exhumation

25 Conclusions Identified offshore extent of possible mat formation Greater mobility of sand than SRBs makes burial and exhumation likely Larger (cm-size) SRBs unlikely to move alongshore outside of storm events Eventual long-term distribution patterns predicted, but will take a long time to reach that state and for all SRBs to be uncovered/recovered Inlets serve as traps for SRBs Results validated against (invaluable) USCG SRB collection data and pseudo-srb studies Page 25

26 Thank you! For additional information: Dalyander, P.S., Plant, N.G., Long, J.W., and M. McLaughlin (2015). Nearshore dynamics of artificial sand and oil agglomerates. Mar. Poll. Bull. 96(1-2), , doi: /j.marpolbul Dalyander, P.S., Long, J.W., Plant, N.G., McLaughlin, M.R., and R.C. Mickey (2015). Field Observations of Artificial Sand and Oil Agglomerates. USGS Open File Report OFR , doi: /ofr Dalyander, P.S., Long, J.W., Plant, N.G., and D.M. Thompson (2014). Use of Wave Scenarios to Assess Potential Submerged Oil Mat (SOM) Formation Along the Coast of Florida and Alabama. Appendix D in Operational Science Advisory Team (OSAT3), Investigation of Recurring Residual Oil in Discrete Shoreline Areas in the Eastern Area of Responsibility. Plant, N.G., Long, J.W., Dalyander, P.S., Thompson, D.M., and E.A. Raabe (2013). Application of a hydrodynamic and sediment transport model for guidance of response efforts related to the Deepwater Horizon oil spill in the Northern Gulf of Mexico along the coast of Alabama and Florida. USGS Open File Report OFR Page 26