60
4 Testing and Validating Against Historic Spills
The Deepwater Horizon oil spill and Exxon Valdez oil spill were selected as case
studies for comparing results from ERA Acute against historic spills.
• The Deepwater Horizon Oil Spill (DHOS) began on 20th April 2010 in the Gulf
of Mexico on the BP operated Macondo Prospect. Following the explosion and
sinking of the Deepwater Horizon oil rig, a seafloor oil gusher flowed for 87 days,
until it was capped on 15th July 2010. The US Government estimated the total
discharge to be approximately between 701,000 to 857,000 m
3 crude oil (US
Coast Guard 2011).
• The Exxon Valdez Oil Spill (EVOS) occurred 24th March 1989, when the tanker
Exxon Valdez ran aground on Bligh Reef in Prince William Sound, Alaska. The
vessel was traveling outside normal shipping lanes to avoid ice. Within six hours
of the grounding, the Exxon Valdez spilled approximately 40,000 m
3 Prudhoe
Bay crude oil (Exxon Valdez Oil Spill Trustee Council, https://www.evostc.state.
ak.us/index.cfm?FA=facts.details).
The comparison studies between the results of the ERA Acute analyses of the
two cases and the post-spill estimations of damages were part of the process of validating ERA Acute as a method suitable for ERA purposes. Quantitative comparison
studies against historical oil spills are not commonly performed for environmental
risk assessment methods but have been performed for e.g. the biological effects model
in SIMAP oil spill model (French-McCay, 2004; French and Rines, 1997). Following
an evaluation of data availability and quality, the EVOS and DHOS cases, limited
to surface and shoreline compartments, were chosen for comparison. ERA Acute
impact calculations were compared to injury estimates from post spill assessments.
For the sea surface compartment, both modelled and satellite oil drift data were used
in the study.
4.1.1 Analysis Areas
The analysis area for the DHOS case was set to cover the US Economic Exclusion
Zone (EEZ) of the Gulf of Mexico (Fig. 4.1). The area is represented by 10,792
surface grid cells of 10 × 10 km (cells containing water) covering approximately
1,014,789 km
2 sea surface area, divided into 188,989 km
2 coastal area (<40 km from
the coastline) and 825,800 km
2 offshore area (>40 km from the coastline).
The analysis area for the EVOS case was divided into the two areas, one that
covers the total impact area including Cook Inlet, Kenai Peninsula, Kodiak Island
and Alaska Peninsula and one that covers the Prince William Sound (Fig. 4.2).
The Prince William Sound is represented by 240 surface grid cells of 10 × 10 km
overing approximately 14,592 km
2 sea area. The analysis area for seabirds and marine
mammals was restricted to the Prince William Sound since the VEC dataset and injury
assessment estimates were most reliable in this area.
4 Testing and Validating Against Historic Spills
The Deepwater Horizon oil spill and Exxon Valdez oil spill were selected as case
studies for comparing results from ERA Acute against historic spills.
• The Deepwater Horizon Oil Spill (DHOS) began on 20th April 2010 in the Gulf
of Mexico on the BP operated Macondo Prospect. Following the explosion and
sinking of the Deepwater Horizon oil rig, a seafloor oil gusher flowed for 87 days,
until it was capped on 15th July 2010. The US Government estimated the total
discharge to be approximately between 701,000 to 857,000 m
3 crude oil (US
Coast Guard 2011).
• The Exxon Valdez Oil Spill (EVOS) occurred 24th March 1989, when the tanker
Exxon Valdez ran aground on Bligh Reef in Prince William Sound, Alaska. The
vessel was traveling outside normal shipping lanes to avoid ice. Within six hours
of the grounding, the Exxon Valdez spilled approximately 40,000 m
3 Prudhoe
Bay crude oil (Exxon Valdez Oil Spill Trustee Council, https://www.evostc.state.
ak.us/index.cfm?FA=facts.details).
The comparison studies between the results of the ERA Acute analyses of the
two cases and the post-spill estimations of damages were part of the process of validating ERA Acute as a method suitable for ERA purposes. Quantitative comparison
studies against historical oil spills are not commonly performed for environmental
risk assessment methods but have been performed for e.g. the biological effects model
in SIMAP oil spill model (French-McCay, 2004; French and Rines, 1997). Following
an evaluation of data availability and quality, the EVOS and DHOS cases, limited
to surface and shoreline compartments, were chosen for comparison. ERA Acute
impact calculations were compared to injury estimates from post spill assessments.
For the sea surface compartment, both modelled and satellite oil drift data were used
in the study.
4.1.1 Analysis Areas
The analysis area for the DHOS case was set to cover the US Economic Exclusion
Zone (EEZ) of the Gulf of Mexico (Fig. 4.1). The area is represented by 10,792
surface grid cells of 10 × 10 km (cells containing water) covering approximately
1,014,789 km
2 sea surface area, divided into 188,989 km
2 coastal area (<40 km from
the coastline) and 825,800 km
2 offshore area (>40 km from the coastline).
The analysis area for the EVOS case was divided into the two areas, one that
covers the total impact area including Cook Inlet, Kenai Peninsula, Kodiak Island
and Alaska Peninsula and one that covers the Prince William Sound (Fig. 4.2).
The Prince William Sound is represented by 240 surface grid cells of 10 × 10 km
overing approximately 14,592 km
2 sea area. The analysis area for seabirds and marine
mammals was restricted to the Prince William Sound since the VEC dataset and injury
assessment estimates were most reliable in this area.