Assessing habitat damage caused by the grounding of the Shen Neng 1 on Douglas Shoal, southern Great Barrier Reef

Brief description

The key methods employed to assess damage to Douglas Shoal were high-resolution multibeam sonar bathymetry coupled with towed video (Towvid) surveys. In combination, these methods provide high quality data to assess damage over the very broad area of reef impacted by the ship grounding. Multibeam bathymetry is a form of echo sounding which uses pulses directed from the surface to the seafloor to measure the distance to the bottom by means of sound waves. Multibeam bathymetry and backscatter techniques are well suited to estimate areas of physical damage on coral reefs as they can detect flattening and gouging of the substratum as well as compacting of substratum expected following the grounding of large ships. Multibeam and backscatter techniques are best suited to detect physical damage on reefs with a relatively high degree of topographic complexity as this would provide the greatest contrast to reef flattened and/or gouged by a grounded vessel.Towed video (Towvid) surveys of benthic habitats are well suited to conditions where diving is impractical or where large areas require survey. A strengthened underwater housing containing a digital video camera mounted on a sled is tethered to and towed behind the ship with electromechanical cable controlled by a winch. The imagery is automatically geo-referenced with GPS data and both are simultaneously recorded on miniDV tapes aboard the vessel. Towvid is effective in detecting gross physical damage to habitat and /or widespread mortality at a broad taxonomic level. It is also well suited to estimating physical habitat types but again within broad classifications. Currents in the vicinity of Douglas Shoal were assessed using a satellite tracked drifter. The deployment of a satellite tracked drifter from the site of the grounding was to demonstrate the potential path of water-borne sediment plumes and oil spill contamination from shipping accidents at Douglas Shoal.Passive samplers were deployed and water samples collected The deployment of and the collection of water to assess potential water contamination at Douglas Shoal. The analysis of these samples is currently underway at the National Research Centre for Environmental Toxicology (ENTOX). The aims of the study were to assess the scale of the physical damage to Douglas Shoal and the severity of damage to the native biota, resulting from the grounding of the 225m bulk coal carrier Shen Neng 1 on 3 April 2010. The priority objectives of the impact assessment for determining physical impact on Douglas Shoal were to:1. map Douglas Shoal using high resolution multibeam bathymetry techniques to assess the extent of damage caused by the grounding of the Shen Neng 1.2. assess the benthic habitat and biota in undamaged and damaged areas of Douglas Shoal using towed video (Towvid).Additional objectives included:1. the deployment of a satellite tracked drifter at Douglas Shoal to assess local circulation and current drift away from Douglas Shoal. 2. the deployment of passive samplers and the collection of water to assess potential water contamination at Douglas Shoal.3. collect sediment samples to assess possible contamination by antifouling paint by SCUBA. This was not attempted due to poor weather conditions. On 3 April 2010, the 225 m bulk carrier Shen Neng 1 ran aground on Douglas Shoal in the southern section of the Great Barrier Reef, fully laden with approximately 65,000 tonnes of bulk coal and 977 tonnes of fuel oil (ATSB 2010, MSQ 2010).Douglas Shoal is a large, flat reef reaching between 9 and 15 m of the surface. The vessel was underway at a speed of approximately 8 knots when the grounding occurred and she sustained severe damage to her hull, and to the propeller and rudder. The grounding coincided with the afternoon low tide and the last of the spring tides. By half tide the ship had moved further onto the shoal and with the continuing tidal rise undertook a further westward drift of ~ 1.3 km before coming to rest on the falling tide. By the afternoon of the 2nd day, the ship moved ~ 1 km south towards the edge of the shoal where it remained for a week. During spring tides on 12 April she was re-floated with the aid of tugs following removal of approximately half of the fuel oil. A minor oil spill (~ 2 tonnes) that occurred earlier in the grounding was treated with chemical dispersants. During this 9 day event, the ship ground its way across the top of the shoal, rotating as alternate fore and aft sections of the hull resisted the drift. As major oil and coal spills did not occur, the most important impacts to Douglas Shoal were considered to be physical damage to the reef structure and its biota as well as potential contamination by antifouling paint which can affect invertebrate recruitment and reef recovery.

Lineage

Maintenance and Update Frequency: notPlanned

Statement: Statement: Adverse weather conditions reduced the sensitivity and resolution of both sonar and towed video surveys.Multibeam bathymetry:The echo sounder used here has 101 beams with a beam angle of 1.5° for each beam. These signals are reflected from the ocean floor at different angles and at different times, depending on the topography and depth of the sea floor. The precision of this technique can be within 10 cm under ideal conditions but the pitch and roll of a vessel due to poor weather conditions can reduce the sensitivity of this technique, despite compensation measures to account for vessel roll, pitch and yaw. Towed video (Towvid):Towvid became the primary method for benthic biota assessment on the RV Cape Ferguson voyage as diving surveys were ruled out due to poor weather conditions. While poor sea conditions are more favourable for Towvid than diving, the increased pitch and roll of the vessel along with strong currents can reduce the quality of captured imagery.

Notes

Credit
Negri, Andrew P, Dr (Principal Investigator)