By Tim Culmer & Simon Agass
There’s no such thing as a good news story when it comes to oil spills, but those involved in the response to the recent oil spill in Mauritius should be commended for their efforts in greatly reducing the effects of this tragic event. Using the MarineAware oil spill response platform, with its built-in modelling capability, we can see just how much worse the outcome could have been.
For more information on MarineAware: MarineAware: Integration & Collaboration
On 25 July the cargo ship, MV Wakashio, ran aground on a coral reef off of Point d’Esny in Mauritius. The area around Point d’Esny is a highly environmentally sensitive area, with nearby nature reserves and wetland sites, and is also economically important for the local population for fishing and as a tourism destination. 
The ship was exposed to rough seas on the reef but unable to be moved to safety. Satellite imagery captured on 1 August showed a small amount of oil beginning to leak from the ship, and after 12 days the increasing damage to its hull caused a significant amount of oil to be released from the ship’s starboard fuel tank. With around 3,900 tonnes of low-sulphur fuel oil onboard, as well as marine gas oil and lube oil, fears grew for the safety of the surrounding environments. 
Over the next nine days in excess of 1,000 m3 of oil spilled from the ship covering an area of 27km2 and contaminating approximately 32km of coastline. Currents and prevailing winds pushed the oil north into the Baie de Grand Port, where it threatened the bay’s fragile coral reef ecosystems and mangroves, as well as the sea birds, fish, turtles, dolphins, whales and shellfish that live there. 
How bad could it have been?
To see how effective the response to the MV Wakashio incident has been, we have used MarineAware’s modelling capability to simulate how the incident would have developed with no intervention and have compared these results to the outcome after the mitigation efforts.
THE INITIAL SPILL
In the days between the MV Wakashio being grounded and it starting to leak oil, the Mauritius government were able to prepare for the risk of a spill. Whilst they waited for help from foreign countries to arrive, organisations like the ITOPF provided remote advice on booming arrangements to protect sensitive coastal habitats. 
But due to high seas and limited resources immediately after the spill, responders were unable to surround the ship with booms to contain it. This meant that most of the oil began to travel towards the shores of the bay.
Figure 1 – Simulation showing oil spreading from the MV Wakashio shortly after it began leaking
By this time, booms were able to be deployed to reduce the amount of oil reaching and impacting the coast. Many of these were homemade; with hundreds of local volunteers constructing and deploying booms using dried sugar cane leaves, items of clothing and human hair to absorb the oil, with plastic bottles to keep them afloat. 
Whilst these booms were being deployed along the coastline, attempts were made to intercept the oil in the bay nearer the ship. However, the coral reef on which the ship was grounded forms a lagoon and its shallow depths presented a significant challenge to the response. Dispersants couldn’t be used to break up and sink the oil as there would not be enough water to ensure adequate dilution of the dispersed oil. The shallow water also hindered access to the oil so specialist “heli-skimmers”, which have a smaller draft for shallow water, had to be used to gather it.
Figure 2 – Simulated full extent of the oil after 9 days after the ship began leaking
Because there was not the resources to immediately contain the oil, the start of the MarineAware simulation closely matches what happened in reality, with the oil being pushed into the Baie de Grand Port by strong southerly winds.
The simulation goes on to demonstrate what would have happened if the booms and skimmers had never been deployed. Namely, large amounts of oil would be washed up directly on the coastline and far greater damage could be caused to the surrounding nature reserves, mangroves, estuaries and reefs.
Oil reaching the shoreline would then have been spread further along the coast by breaking waves and cross-shore currents. Although reports from the ground have recorded oil as far north as Bambous Virieux (11km north of the ship), the simulation shows that the levels of contamination in this area could have been far higher.
Figure 3 – Estimates from the ITOPF for the volumes of oil (of all types) spilled and recovered from the MV Wakashio 
When the waves began to reduce in size, booms were deployed nearer the ship and several salvage ships were deployed to remove the remaining oil from the MV Wakashio. It’s estimated that of the original 4,300 m3 of oil onboard (all types), only 1,000m3 entered the ocean.
Of the oil that entered the ocean, some of it would have been recovered by booms, skimmers and shoreline cleaning efforts. It is difficult to estimate exactly how much oil was recovered by these response efforts, but we do know nearly 1,200 m3 of liquid waste and 1,900 m3 of solid waste has been collected as of 25 August. 
The decision to remove the fuel oil from the ship proved to be a vital one, because on 15 August the ship broke in two depositing most of its remaining oil into the ocean. Thanks to the response, this was only a relatively small amount, but without it the 3,300m3 of oil pumped from the ship could now be in the ocean.
Using the MarineAware modelling, we can look at where this extra oil would have travelled.
Around the time the ship broke apart, the wind switched to predominantly blowing from the east, so any remaining oil released would have spread out to the west and reached previously uncontaminated shorelines. The simulation shows it would have travelled along the south side of the island, reaching over 14km west of the MV Wakashio, meaning far greater amounts of oil than in the real incident would have reached sites like the nature reserve of Blue Bay Marine Park and the popular tourist attraction of Pont Naturel.
Figure 4 – A change in wind direction after the ship broke apart could have caused widespread damage to the south coast of the island
What was the potential impact?
The MarineAware platform includes coastal Environmental Sensitivity Index (ESI) data provided by 4 Earth Intelligence (4EI), which can be used in unison with the modelling to assess the risk to different areas of the shore. 4EI’s ESI datasets are derived from the latest satellite imagery and use international standards to rank areas of coastline in terms of their sensitivity to marine pollution, such as oil spills, for both real-time emergencies and advanced planning. Viewing the modelling predictions alongside the ESI in MarineAware gives a clear picture of where the most at risk areas of coastline would have been.
Figure 5 – 4EI’s ESI data around the Blue Bay Marine Park with oil from the breakup of the ship, had it not been removed
The ESI data highlights that the coastlines near the oil spill are extremely sensitive. We can see that much of the coastline is lined with vegetation and mud/sand flats – areas that usually contain the most fauna and flora. This not only makes them the most sensitive to the effects of oil, but the shallow gradients of these ecosystems, combined with the higher number of obstructions, allow larger volumes of oil to build up.
Because many of the treatments for removing oil from sensitive coastlines, such as cleaning agents and flushing with water, can actually cause further damage, the best response is to stop the oil reaching them altogether. In total, the government and local volunteers deployed nearly 4km of booms to contain the oil and protect these areas.  We may yet not know the full extent of the damage caused by the MV Wakashio, but it’s clear these actions will have significantly reduced the amount these sensitive areas have been affected.
Could it happen again?
The MV Wakashio is not the only shipwreck to happen in Mauritius in recent history. According to Dr Vikash Tatayah of the Mauritian Wildlife Foundation, there have been five other wrecks in the last ten years. He also stated that 2,000 ships passed close to the island in the last month alone, indicating the high levels of traffic near this seemingly remote island. 
Although this event was undoubtedly a disaster, we can see from the modelling and ESI data that it could have been much worse. In addition to the dedication of those who helped mitigate the disaster, there was also some good fortune. For example, the relatively long amount of time it took for the ship to begin leaking oil gave responders time to prepare for the spill.
Given the amount of shipping in the area, this might not be the last oil spill disaster that we see in Mauritius. The MarineAware modelling provides a tangible warning that without an effective response in place, the next one could be far worse.