Reassessment of the Nature of Beach Erosion North of Dar Es Salaam, Tanzania

Abstract

An assessment of the status of coastal erosion along the coastline of Tanzania, including the Zanzibar islands, through field surveys, revealed that the problem is widespread but more serious along the Kunduchi beach, to the north of Dar es Salaam city. Analysis of wave climate data, drogue tracking, current measurements and beach profiles obtained during both the NE-monsoon and SE-monsoon periods shows that the prevailing erosion is mainly a result of combined effects of high tidal levels and strong waves. Wave heights at Kunduchi beach are about 0.5m during the NE-monsoon period and increase to about 0.9m during the SE-monsoon. Such wave climate in combination with high tidal range of up to 4m results in increased wave erosion during high spring tides. Beach profiles made during selected periods indicate that erosion is generally more severe during the SE-monsoon period, especially during July/August/September. It was also found that erosion intensity depends on both shoreline geology and nearshore bathymetry. Owing to the nature of the bathymetry, waves are focused selectively at some sections of the shoreline while the rock type of each section determines the ease with which it erodes. Tidal circulation in the area appears, as a rule, to flood from the north and ebb in the reverse direction. Analysis of the various drogue tracks cast seaward of the breaker zone during both the NE and SE monsoon periods shows a net southward transport which is confirmed by current measurements. Grain size data and sedimentary structures from the beach and within the breaker zone on the other hand point to a net northward transport. The contrast shows that whereas the beach and breaker zone sediment dynamics is largely driven by the action of breaking waves which are stronger during the SE monsoon, the dynamics outside the breaker zone is driven mainly by tidal circulation. The supply of sand from rivers and creeks draining into the area contributes the bulk of the sand that is being moved along the beach. The growth of large spits across creek mouths are attributable to the alongshore deflection of the ebb jets out of the creeks.