Coastal processes and beach erosion of North Dar es Salaam

The hydrodynamical factors at the stretch of beaches north of Dares Salaam was investigated to establish their magnitude during the Northeast monsoon and possible effect on the beaches. Series of coastal surveys was conducted to measure currents, waves winds, tides, etc.The dominant wind direction was from sea to land during th day with a maximum speed of 20m/s observed in February. At nigh the breeze blew seaward.Waves, with significant height and period of about 1 metre and 8 seconds respectively at breaking point were observed at Kunduchi beach. Other beaches experienced wave effect deminishing in amplitude in South-North direction. Compared with breaker zone values, offshore waves had smaller amplitude and period. The currents in the surf zone and offshore have values not exceeding 0.5m/s. The estimated sediment transport by longshore current pe annum is about 228m3. This precludes sediment input from seasonal streams which have maximum and minimum inputs during the long and short rains respectively. Analysis of the profile data from 1986-1989 confirms sediment during the long and short rains respectively. Analysis of the profile data from 1986-1989 confirms sediment accretion during rainy seasons and erosion commences as soon as the rains cease. Alternate accretion and erosion effects are significant within the first 50m of the surf zone. Aerial photographs confirm shoreline recession of about 110m at the Kunduchi-Manyema creek between 1966 and 1982. Within the same period a maximum recession of 30m occurred at Kunduchi with a gradual decrease in the recession northwards to about 5m at Bahari beach. In March 1991 there was a shoreline retreat of over 1.5m at Kunduchi beach. Reconstruction of the groynes according to the recommended transition, sizes of blocks, lengths and spacings between the compartments will obviate the ineffectiveness associated with the present groynes. Riprap blocks will protect the berm slopes from erosion. Beach nourishment will help restore the beach at Bahari. In addition, annual beach nourishment requirement of 0.21m3/m is needed because of sea level rise in all the beaches.