Why do we need to prevent beach erosion?

15Apr 2020
The Guardian
Why do we need to prevent beach erosion?

A team of environment experts has been tasked to study causes of serious damage and waves on the coastal stretch where two prominent Dar es Salaam hotels are built. The Ministry of State in Vice-President’s Office was aware of the acrimonious relations between managements of the-

-two hotels as a result of the effects of raging waves.

Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landward retreat of the shoreline can measured and described over a temporal scale of tides, seasons, and other short-term cyclic processes. Coastal erosion may be caused by hydraulic action, abrasion, impact and corrosion by wind and water, and other forces, natural or unnatural.

On non-rocky coasts, coastal erosion results in rock formations in areas where the coastline contains rock layers or fracture zones with varying resistance to erosion. Softer areas become eroded much faster than harder ones, which typically result in landforms such as tunnels, bridges, columns, and pillars. Over time the coast generally evens out. The softer areas fill up with sediment eroded from hard areas, and rock formations are eroded away. Also abrasion commonly happens in areas where there are strong winds, loose sand, and soft rocks. The blowing of millions of sharp sand grains creates a sandblasting effect. This effect helps to erode, smooth and polish rocks. The definition of abrasion is grinding and wearing away of rock surfaces through the mechanical action of other rock or sand particles.

Surges in coastal erosion north of Dar es Salaam city have been documented from 1977 to the early 1980s and around 1997/98. Analysis of the wind data shows that the documented increase in coastal erosion coincided with increased wind speeds. Extreme winds in excess of 10-11 m s-1 were experienced during 1976-78, 1984 and after 1996. Their coincidence with extreme high tides produced the high erosion rates. Using beach profile data for the years 1998-2000 the evolution of the beach profile with the changing monsoon winds and sediment input by rivers could be demonstrated. The beach erosion took place during the Southeast Monsoon when the alongshore sediment transport is northwards. During the Northeast Monsoon, the reversed transport is smaller to rebuild the beach. According to a study incipient seaward shoreline shift was observed towards the end of the 1999 SE monsoon period and subsequent monitoring revealed a seaward shift by the end of the next NE monsoon. The shift of the shoreline was attributed to a redistribution of the large volumes of fluvial sand that was supplied during the El-Nino rains of April/May 1998. With the drop in annual wind strengths, coastal erosion has since stabilized.

Coastal erosion is a serious environmental problem along the East African coast including Tanzania. The exact cause of the problem has for a long time remained uncertain. Studies on coastal erosion in Tanzania have concentrated mainly on the description of the situation at areas affected by the problem for example Dar es Salaam area.

Erosion Monitoring Committee (1987) established the status of various factors thought to contribute to the erosion problems including sand mining from riverbeds, meteorological conditions, hydrodynamics as well as geomorphological response of the beaches. The relative importance of the various causes or any major cause of the observed erosion however, has not been determined. Starting April 1996 studies by researchers of the Institute of Marine Sciences and the Department of Mathematics of the University of Dar es Salaam have been carried out with the aim of understanding the cause of excessive beach erosion at Kunduchi and generating baseline data for the design of mitigation options. Some preliminary studies for the mitigation of coastal erosion at Kunduchi beach evaluated the various causes of coastal erosion including anthropogenic causes but again the leading cause was not clearly identified.