The Study of Sediment Characteristics and Nearshore Sediment Dynamicsin Coastal Tanzania
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The nearshore morphological features, its sediment dynamics and characteristics of the Tanzania Mainland coastal stretch between the rivers Pangani and Wami were investigated. The study is a continuation of other similar studies (e.g Shaghude 2001) which provides a detail account on the nearshore sediment dynamics and its characteristics, in the Tanzania mainland coastal stretch between the rivers Ruvu and Wami, south of the present investigated area. The study is inline with the guidelines on the studies of shoreline changes in the Eastern African region, as well the study of the Eastern African database of coastal resources. The former has recommended detailed studies of shore morphological features, with updating of old information, while the later has recommended establishment and updating databases of coastal resources for the purpose of sustainable management of the existing resources. The investigated coastal stretch between the rivers Pangani and Wami broadly exhibit major north-south variation. The northern coastal stretch between river Pangani and Mkwaja is dominantly a patch reef coast, with or without cliff, with offshore fossil reefs and islands and sometimes with sand spit shores, with few rivers, high water depths, low quartz (20-50% by volume) and feldspar (10-15% by volume) content in the lithogenic dominated sand sediments and high wave activity. From shore to offshore the sediment changes from siliciclastic dominated facie to carbonate dominated facie. The southern coastal section between Mkwaja and Wami river is dominantly a low-lying sandy coast, with relatively large number of rivers, low depths, high quartz (60-75% by volume) and feldspar (20-25% by volume) content in the lithogenic dominated sand sediments and low wave activity. The carbonate facie is generally missing and the sediments are dominated by siliciclastic facie. Mineralogical analyses of the sediments show that most of the lithogenic components, particularly the quartz and feldspar are of angular to sub-angular shape, suggesting that the sediments are texturally immature. Occurrence of hornblende in the sediments is another evidence of immaturity of the sediments. The siliciclastic sediments are therefore inferred to have been transported for a short distance before deposition. Most of the quartz minerals are also highly fractured , occasionally showing undulatory extinction suggesting that most of the lithogenic sediments are derived from a highly metamorphosed rocks. Two of the rivers, namely, the Pangani and Wami which drain through the crystalline metamorphic rocks of the Mozambican belt located on the hinterland of the coastal plateau are therefore considered to be the major contributors of the siliciclastic sediments. All the remaining rivers in the investigated area drain through the coastal plateau or coastal plain, consisting of younger sedimentary formations. The problem of shoreline changes, particularly coastal erosion is very serious in the Pangani river mouth and the former island of Maziwi, reported to have recently disappeared. The estimated rate of erosion at the Pangani river mouth is about 7 to 20 metres per year and the observed erosion is attributed to the high wave activity which is exacerbated by anthropogenic activities related with the upstream damming, mainly, the Nyumba ya Mungu. The recent disappearance of the Maziwi island has been attributed to the clearance of vegitation on the island during the 1970’s which has been exacerbated by the wave erosion. Significant salinity intrusion has been observed in the Pangani estuary, and this again has been related with the increased water abstraction, mainly due to irrigation along the Pangani catchment.
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