Impact of improvement of the entrance channel on the rate of sediment deposition into the Dar es Salaam Harbour
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The study reported here sought to investigate the effect that the dredging of the harbour basins and the entrance channel has had on sediment deposition rate into the Dar es Salaam harbour. Echo-sounding was carried out along berths No 2, 3, 4, 6, 7 and 8 as a means of detecting changes to their bathymetry in the four years since 1998, when the dredging commenced. Concentrations of transported sediments into and out of the harbour were determined using sediment traps. Short-term measurement of waves and currents were also carried out. Sediment traps, a wave-gauge and a current-meter were deployed in the entrance channel during the South East and North East monsoon seasons respectively. Our results reveal that the sedimentation rates after improvement of the channel are between 13 and 43 cm/year. Rates of sedimentation before dredging were relatively low, at between 7 and 25 cm/year. There are two reasons for the difference in rates. First, the settling of fine particles as a result of dredging activities, and second, the change of channel alignment and subsequent erosion on Ras Makabe. The eroded sediments were probably deposited into the harbour basin by the flood tide. This trend has the implication that the harbour basin will need more frequent dredging than it did before improvement of the entrance channel: In the past, with a maximum sedimentation rate of 25cm/year, it took 20 years before the harbour basin needed to be dredged. At the present sedimentation rate (maximum of 43cm/year), it will take only 11 years before re-dredging is necessary.