Grain size distribution along the Msasani Beach, north of Dar es Salaam Harbour

Beach sediments collected from the tidal flat and beach slope at the Msasan i Beach, about 15 km north of the Dar es Salaam Harbour, are used to (1) es tablish the grain size distribution pattern, (2) assess the effect of man-m ade and natural structures (rivers, creeks, sea wall and groynes) on the gr ain size distribution, and (3) assess whether sediments are preferentially transported northward by longshore currents. Generally, the Msasani Beach s lope sediments have an average mean diameter of 1.55 phi, while those from the tidal flat average at 1.87 phi. There is a general trend of decreasing grain size from south to north that is associated with improvement in sorti ng, particularly in the tidal flat. The northward fining of the sediment su ggests that the sediment in the study area is transported from south to nor th. The trend of northward decrease in the mean grain size is locally disto rted at the river mouths, creeks end man-made structures. The contribution of the man-made structures to the observed distortion can not be separated from that of natural structures owing to the fact that these man-made struc tures were placed next to the natural features. The data collected from the Msasani Beach were subjected to factor analysis. Four factors account for more than 95% of the observed variations in the grain size distribution in the study area. These factors are: medium energy environments (66.5% of tot al variance) common at the beach slope; low energy environments found in mi cro-bays (16%); high energy environment found at the tidal creeks and river entrance (7.8%); and intermediate (between medium and low) energy environm ents common in the tidal flat (5.6%). Most of the beach slope samples are unimodal (87%), while a significant num ber of the tidal flat samples (46%) are bimodal, with the most common mode being 2.25 phi. The bimodality observed in this study can be attributed to the presence of shell fragments that were not removed during sieving, to su dden fluctuation in the energy strength at the beach, and to mixing of diff erent grain populations. (C) 2000 Elsevier Science Limited. All rights rese rved.