2.4.1 Coastal Structures
Coastal structures are constructed features along a coastline that serve various purposes.
Groins and breakwaters form part of coastal defence schemes whereas ports and harbours form part of a cities infrastructural network. With coastal cities growing in both population and physical assets, the need for coastal defences has greatly increased (Genovese and Green, 2015). These structures are typically installed following a major storm event in a process referred to as “coastal armouring” (Dugan et al., 2011).
Structures are generally installed along beaches experiencing erosion in an attempt to halt the process. Dugan et al. (2011) state that the placement of a coastal structure fundamentally alters hydrodynamics and the flow of water together with sediment dynamics and depositional processes which proves advantageous to eroding coasts.
Although they have proven to be effective in protecting infrastructure from storms and erosion, concerns still remain relating to their construction.
Dugan et al. (2011) infer that little is understood regarding the ecological conse- quences of these structures on native environments. Furthermore, even less is known about the impacts of these structures on open-coast systems such as beaches. This notion is shared by Airoldi et al. (2005) who add that construction of defences may lead to disruption of soft-bottom environments together with the introduction of artificial hard-bottom environments. This also has a knock-on effect on species diversity in the area, allowing for the spread of non-native species which increases habitat homogeneity (Airoldi et al., 2005). In addition to habitat destruction and alteration, coastal defence structures impact coastline evolution by altering natural sediment transport processes.
Dugan et al. (2011) identified potential changes to coastlines following the installation of varying defence structures;
• Initial reduction of beach widths seaward of shore-parallel structures responding to placement losses as well as continuous processes of active and passive erosion.
• Beach area reduction and passive erosion.
• Scour up-drift of the structure together with flanking erosion due to stronger physical processes induced by wave reflection and surf zone narrowing during storms.
• Accelerated erosion of coastlines down-drift of groins and jetties as a result of abrupt discontinuities in littoral sediment transport.
2.4 Impacts on Sediment Supplies Dugan et al. (2011) add that these effects scale based on the amount of interaction between the structure and the wave climate. In addition to coastal defence structures, port and harbour developments are known to cause local environmental problems (Davis and Macknight, 1990). These structures alter wave conditions and act as a sink for sediment, limiting supply to longshore drift systems and causing down-drift coastal regions to become vulnerable (Haslett, 2016). Storm conditions with high surge levels result in hard structures such as groins and breakwaters providing little relief (Van Rijn, 2011). Furthermore, hard structures may cause an increase in coastal variability.
Figure 2.2 illustrates the interaction of littoral transport with a harbour breakwater and the formation of a discontinuity.
Fig. 2.2 Sediment transport around a harbour mouth (Bosboom and Stive, 2015) It is however important to make a distinction between infrastructural installations such as harbours and coastal defence structures such as groins. The former commonly has negative implications whereas the latter is introduced to remediate ongoing issues.
A study by Bernatchez and Fraser (2012) found that the effects of coastal defence structures are compounded on beaches with a high level of sediment transport due to the reflective nature of the structure. Vaidya et al. (2015) suggest that decisions on whether to build structures should include a thorough analysis of past shoreline behaviour and projected future developments.
2.4.2 Fluvial Sediment Yields
Rivers are a key pathway for land-ocean sediment transfer, forming an integral part of the coastline evolution scheme. On a global scale, Theron et al. (2008) found that coastlines rely on rivers for approximately 80% of their sediment load. Contemporary
changes in sediment fluxes as a result of human activity (Walling, 2006). A range of human activities occur within drainage basins, with Chu et al. (2009) identifying dams, reservoirs, water conservation schemes, water consumption and sand mining as the primary causes behind sediment load reductions. Given increased demand for sand required in construction and infrastructure sectors, sand mining has become common practice along the flood-plains of rivers (Amponsah-Dacosta and Mathada, 2017).
Sand mining refers to the extraction of sand predominantly from an open pit or by means of dredging along ocean floors and river beds (Amponsah-Dacosta and Mathada, 2017). Syvitski and Saito (2007) demonstrated that sand mining had caused mega-deltas along the Asian coastline to shrink due to reduced sediment yields whereas pre-mining studies showed consistent accretion. With sand mining experiencing a rise in popularity, regulation of the activity has proved troublesome in many countries which compounds the problem. South Africa serves as an example, where sand mining regulations are split into three categories; mineral, environmental and land use planning regulations. It was found that mineral regulations are favoured ahead of the others, with Green (2012) implying that all three classes should hold equal importance. This notion is shared by Amponsah-Dacosta and Mathada (2017) who found that most mining activities have little regard for environmental protection. Furthermore, South Africa is experiencing a high amount of illegal sand mining where miners have not received official permits for their operations (Theron et al., 2008). This worsens resource exploits and also makes prediction of sediment budgets inaccurate.
Dams have also played a significant role in sediment load reductions. Snoussi et al.
(2002) studied the impacts of dam construction on water and sediment fluxes. Findings showed that the cumulative volume of sand trapped by three dams along the Sebou River in Morocco resulted in a 95% reduction in sediment load. Yang et al. (2018) present similar findings showing significant sediment load reductions due to damming.
In contrast, Yang et al. (2018) initially found a 30% increase in load due to surface erosion resulting from an increase in human population, inferring that certain activities lead to increases in sediment load.
The combined effect of dams and sand mining have demonstrated how severely sediment yields of rivers may be affected. Rivers also intermittently transact sand to the ocean, typically during flooding events (Michaelides and Singer, 2014). This behaviour is exacerbated when considering the sedimentation pattern of the river. Estuaries that are in a state of non-progressive sedimentation maintain a state of long-term dynamic equilibrium. This means that a constant sediment volume is always retained within the estuary (Theron et al., 2008). These are also referred to as river-dominated estuaries,
2.5 Coastal Management & Planning