Flow velocity and implication on particle size of bottom sediment in the commodore channel lagos, southwestern nigeria

Abstract

Flow velocity generally influences the erosion, transport and deposition of sediments. Flow velocity and in relation to particle size distribution in the Commodore channel was investigated with the Acoustic doppler current profiler (ADCP), echosounder and veen van grab. Those equipment were deployed through a low draft survey boat to acquire acoustic and sediment data covering the study area. The ADCP measured the current velocity along fourteen established transect lines while twenty surface sediments samples were collected along the banks and center of the channel respectively. Sediment samples were analyzed for grain size distributions and mineralogicaly composition while the acoustics data were analyzed with Matlab software to produce velocity profiles for the channel area. The study aimed to determine the magnitude and direction of flow of water along the channel with a view to ascertain the sediment transport process.  Results indicated no significant difference in flow velocities along the different channel points. The flow velocity was however slightly higher around the channel mouth than in mid and upper reaches of the channel. The flow velocities showed negative correlation of (– 0.54 and – 0.28) with the sediment characteristics indicating that the sediment particle size distributions is unrelated to the flow velocity. This anomaly in the equilibrium flow velocity suggest  the impact of dredging and the continuous ebbing and flooding of tidal water which resulted to reworking of the sediment particle sizes prior to deposition. The sediment distributions were fine sand, moderately sorted, fine skewed with leptokurtic peaked. Significant fractions of the sediments were deposited in fluvial and shallow marine environments while a few were deposited in the beach and turbidity environment.  Sediments and acoustic data interpreted from the study suggest that sediment supply to the area was by fluvial processes through the barrier lagoon drainage basin.