Coastline evolution around African seaports

Abstract

Seaports are important maritime commercial facilities and key hubs for national and global trades. There is a growing need for seaborne transport and, hence, seaport facilities, especially in emerging economies due to economic and population growth, such as in Africa. In sedimentary environments, port construction may induce coastal impacts regarding up-drift accretion and down-drift erosion. These coastal impacts potentially increase risks such as harbour siltation and coastal area erosion. To mitigate or even avoid these risks in the pre-construction stage, coastline evolution around ports needs to be well-understood. Analysis of long-term shoreline position data around existing ports can provide this understanding. However, long-term in-situ shoreline position data around ports are often unavailable or inaccessible, especially in emerging economies which are often data poor. Nevertheless, nowadays a growing database of satellite images provides these data on a global scale for the last decades. Furthermore, the launch of Google Earth Engine cloud computing platform in 2016 enabled accessibility and efficient processing of these satellite images. This development allows building an evidence database for shoreline positions around African seaports. With this database, coastline evolution trends around all ports can be analysed, inter-compared and related to environmental and port characteristics to derive lessons for future port development.
According to the World Port Index, 165 African seaports (after excluding river ports, offshore platforms and anchorages from 266 African ports) are identified. Only 125 ports at sandy coastlines, where SDS detection has been validated, are focused in this research. The results from this study show that coastline evolution, especially coastal area erosion, around the majority of African seaports is limited by the narrow sandy beach and rocky substratum of Afro-trailing coasts. However, there are still some ports at sediment-rich coasts having dramatic erosion and/or siltation hazards. After analysing common characteristics of these high-hazard ports, lessons are learnt for port development. Regarding site selection and breakwater design, ports with massive river sediment supply and/or ports at open coasts have larger negative coastal impacts. For coasts with river sediment supply, it is better to construct ports at the up-drift side of the river mouth to avoid interruption of river supplied sediment transport, which is found helpful for ports around West Mediterranean Sea. Shoreline management plan should be coupled with methods to maintain or increase river sediment supply, which can be learnt from shoreline retreating around ports in West Africa. Furthermore, to reduce coastal impacts around ports, port breakwaters at open coasts should be carefully designed regarding length and orientation to achieve a smaller shore-normal projected length, especially when the gross longshore wave power is massive. Regarding mitigation methods for coastline evolution impacts, shoreline protection structures are effective in reducing erosion hazards in the time scale of 30 years. Extension of the port breakwater (s) can be a temporary solution to mitigate the potential siltation problem but to reduce the down-drift erosion problem at the same time, sediment by-pass system, which is proved to be successful in South African ports, can be designed.