Use of Acoustic Techniques for the Determination of Net Sediment Transport and Design of Safe Navigation


David Williams
Australian Institute of Marine Science 


The Marine Supply Base channel in Darwin Harbour, Northern Territory Australia provides access to berthing and loading at East Arm wharf for vessels that support the offshore energy industry.

Sedimentation in the channel due to complex currents in the area is reducing navigational efficiency and safety.

A combination of multi-beam echo sounder, utilising an Odom MB1 and Acoustic Doppler Current Profiler, using a network of TRDI Workshorse and Sentinel V, surveys were conducted over a 12 month period encompassing both dry and wet seasons. The MBES surveys mapped the extent of the MSB channel and the East Arm sandbar adjacent to the channel. Tidal current patterns determined from the ADCP surveys show that tidal currents in the area are highly variable and are stronger in the ebb tide direction except at the entrance to the navigation channel. Tidal current are more variable in the flood tide direction at all remaining sites along the channel. The strength and direction of the tidal currents indicate that sediment movement is along the channel toward the deeper seaward navigation channel.

Fine sediments have deposited in the MSB berthing area slightly reducing the volume of the berthing pocket and also depositing toward the end of the channel. An annual sediment deposition rate of 100 mm over the berth area was determined via observations and modelling.

Non-cohesive sand transport modelling, configured using data from the MBES and ADCP surveys, has indicated slow movement of the sandbar in a dominantly south west direction at between 100 – 200 mm per month. Fine sediment modelling indicates deposition in the MSB berth area of 20 – 50 mm/year under average conditions.

Modelling indicates that if the MSB channel was realigned to be straighter the current directions would be more regular assisting navigation. Sediment accumulation would not present a major issue if the majority of the East Arm sandbar was dredged.

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