Sierra Leone Metocean Measurement Project

As metocean conditions (tides, currents, waves, sediment transport) in the estuary are poorly understood, metocean studies of the area are required to support the development of the port.

WorleyParsons conducted a dry-season metocean data collection programme in the estuary between February and June 2010. This programme included twelve moorings throughout the estuary and offshore, measuring tides, currents, waves, water quality and sediment transport. Discharge measurements using a vessel mounted ACDP, bathymetric survey and water quality profiles were also conducted.

Information gained in the dry season program was used to construct and calibrate hydrodynamic, wave and sediment transport models of the estuary. However, significantly different conditions were expected to occur in the wet season (June to Oct) with decreased salinity increased currents and discharges, so a second wet season program was needed.

MSI was the contractor appointed to undertake the wet season data collection programme and with offices in both Australia and South Africa, liaison between the client's project manager in Australia and MSI's field team in Sierra Leone was easy. The programme took place between 7 September and 15 October with two oceanographic engineers assigned full time to this project, which included the following components:
1. 30 day current profile and wave measurements at Tagrin Point
2. 30 day current profile measurements at Pepel
3. Current profile transects at 4 locations, for 12 hours each at spring and neap tides, through the lower estuary
4. Water quality profile measurements at 14 locations throughout the estuary at both spring and neap tides
5. Flow measurements of the Rokel river

For the measurements at Tagrin Point and Pepel, MSI designed and built custom bottom frames to house the TRDI ADCPs and RBR tide / wave recorder. Given the nature of fishing activity in the area, these were made as trawl resistant as possible, and each was fitted with a Seimac satellite beacon and a groundline to assist in recovery. Each of the frames was also fitted with an acoustic release for ranging and location purposes, and each groundline was fitted with an acoustic release and flotation for recovery purposes.

For the water quality measurements, MSI used a Valeport CTD fitted with pH, oxygen, turbidity and chlorophyll sensors whilst for the transect measurements, a TRDI 600kHz ADCP was mounted on a vessel provided by Class Diving. The data was collected using VMDas and vessel heading / position data recorded by a Hemisphere DGPS installed by MSI as well.

Measuring the flow of the Rokel river was achieved by taking discrete ADCP measurements at fourteen locations from a bridge across the river (image).

Despite the significant challenges of the project, particularly fishing activity which resulted in one of the frames being dragged 400m and 4km on subsequent occasions, no equipment was lost and we collected the required data.