Methodology

The development of Fort Dauphin's coastal degradation was studied by comparing satellite imagery taken at different times, namely 1998, 2004, 2009, and 2011. These satellite images enabled us to measure and observe the variation of the beaches and the coastal area width (Cambers 1998; Brebbia et al. 2009; Y Wang 2010). In this survey, the most damaged areas in each of the years were analyzed, such as Bevava Beach, Galions Bay, and Fort Dauphin Bay, as well as Libanona

Fig. 13.2 Geology of Fort Dauphin and its surrounding areas

Beach. The dunes of Galions Bay were also studied due to the combination of coastal and aerial erosion. The analysis involved measuring a fixed 20 m portion of the bays for each of these periods; 1998 was taken as the reference point in these measurements, and the analyses incorporated some measurements of water levels, waves, and currents made by QIT Madagascar Minerals (QMM).

These cartographical analyses were coupled with field work and villagers' observations, where coastal erosion was observed for each measurement at different periods. Some photos and video were obtained from the fieldwork, facilitating the formulation of appropriate solutions to stop or mitigate the coastal erosion Phenomenon (Auckland Regional Council 2000; DGENV European Commission 2004; Brebbia et al. 2009; NSW Government 2010; Y Wang 2010).

Data Processing

Numerical models were also used to assess and analyze the waves in the surrounding area and around Ehoala Port. The results of these models were checked using satellite, recorded wave, and pressure data. Based on satellite imagery analyses, Orthophoto, Quickbird, and worldview respectively for 1998, 2004, 2009, and 2011 were treated with GIS assessment methods using Arc gis.

Results and Discussion

Table 13.1 summarizes the development of coastal erosion in Fort Dauphin, which manifests by reduction of beach width in the four selected portions that are most affected by coastal erosion (Fig. 13.3). In some periods, the phenomenon of accretion was also apparent, but in general, diminishing of the beach width ranged from 2.6 m (Galions bay) to 9 m (Ambinanibe). The accretion measured during the study periods was insignificant compared to the sediment loss.

Table 13.1 Development of coastal erosion in Fort Dauphin

Period

Ambinanibe Beach (630 m)

Galions Bay (400 m)

Libanona Beach (430 m)

Fort Dauphin Bay (1,000 m)

2004–

2009

Accretion = 9.4 cm/year

Loss = 0.87 m/year

Accretion =

29 cm/year

Accretion

=30.4cm/year

2009–

2011

Loss = 9.165 m/year

Loss = 2.64 m/year

Loss =

4.81 m/year

Loss = 5.265 m/year

Fig. 13.3 Selected portions showing coastal erosion and wind erosion respectively in blue and yellow

Impacts and Causes of Coastal Erosion

The impacts of coastal erosion most frequently encountered in Fort Dauphin can be grouped into three categories: coastal flooding as a result of dune erosion, undermining of sea defenses associated with foreshore and subaerial erosion, and retreating cliffs, beaches, and dunes causing loss of land (Fig. 13.4).

Coastal erosion is derived from numerous causes but wind and current are particularly significant. These two parameters play important roles in coastal abrasion. Due to the similarity in the locations of the most affected areas, and the wind

Fig. 13.4 Impacts of coastal erosion in Fort Dauphin

Fig. 13.5 Wave simulations in Fort Dauphin showing wave directions (Rio Tinto-QMM 2008)

and current directions, the results of numerical models of waves and current direction show that wave action and ocean currents are among the most important factors causing coastal erosion in Fort Dauphin. As shown in Figs. 13.5, 13.6, 13.7, and 13.8, the majority of the strongest waves and currents come from the southwest and east.

The manifestation and activity of these two parameters on beach and dunes could be explained by combinations of various natural forces such as the wave direction approach, as well as the dredging (digging) phenomenon in the coastal area. Before explaining the coastal erosion process in Fort Dauphin, it is interesting to remember that the surf zone is the area where waves break. It is a turbulent zone, as waves smash and dissipate their energy in this area while producing intense local currents that eventually reach the coastal shores (Hyndman 2006). During this turbulent time, water removes sediment in its path and then local currents carry it to the sea when leaving the coast. The same process occurs in the wave zone; in this zone, the depth of breaking varies depending on wave size (Hyndman 2006). This phenomenon is observed on all the beaches in Fort Dauphin, and beach and dune dredging appears in the same manner as shown in Fig. 13.9.

During the study, natural sand transport into the deep sea was observed in almost all the beaches of the city after dredging. Sand content in sea water varies depending on the area, but it seemed greater in Bevava Bay (Ambinanibe) and Galions Bay.

Fig. 13.6 Wave heights (offshore)

Fig. 13.7 Wave direction and current from eastern part of the study area (Source: QMM)

Usually, dredging activity depends on wave direction approaches. Wave direction approaches to the shore are important for coastal stability because the changes in angle lead to coastal erosion by removing beach sediment and transporting it into the sea. Normally, wave direction approaches should be perpendicular to the shore (Hyndman 2006). In that case, the energy produced by wave forces dissipates into

Fig. 13.8 Wave direction and current from southern part of the study area (Source: QMM)

Fig. 13.9 Image showing the actual state of Libanona in Galions Bay, 2012

the terrestrial zone and longshore current follows a parallel direction along the shore line. But if wave direction approaches are not perpendicular, a huge amount of energy is dissipated to the shore, leading to coastal destabilization. Hence, coastal zone is unable to resist this energy, and eventually longshore current direction at an angle along the shore line dredges sediment (Hyndman 2006). In Fort Dauphin, coastal erosion may have been caused by the irregularity of wave direction approaches because they were not perpendicular to the shoreline during the fieldwork in 2011 and 2012.

Dredging is always happening in coastal areas but its intensity is weak in the normal environment (Hyndman 2006). Coastal dredging, characterized by the lack of sediment supply or sand in the coastal zone, is probably due to the formation of canyons in the deep sea or atmospheric air disorders. When canyons have formed, nature tries to fill the gaps at the expense of beaches or sensitive areas whose sand can be transported. Both formation of canyons and atmospheric air disorders might have occurred in Fort Dauphin because human-induced dredging during the Ehoala port construction could be one of the causes leading to coastal erosion in this area.

The dredging of the ocean bottom on a superficies of 181,000 m2 (QMM S.A. 2009) might also have led to coastal erosion in Galions Bay between 2004 and 2009. This might also explain the beach diminishing in this area during that period as no major change was noticed at the other beaches.

Furthermore, waves and tides act on cliffs formed of solid rock in the same way on beaches and dunes, but their action is focused at the base and on arches. Some pieces of rock at these points are washed away each time leading finally to instability, or even to rocks being torn off and thrown into the sea at the foot of the cliffs. These rocks are later pulled into the sea, accelerating the erosion. It means that the results of wind-induced erosion and current-induced erosion are the same but the processes are different.

Moreover, during this period (2004–2009) the study area was threatened many times by tropical cyclones. As a result, heavy damage to the coastline was recorded between 2009 and 2011. The damage observed on the beach portions probably resulted from heavy waves with very active currents. In addition, traces of subaerial erosion characterize some places. Coastal erosion and particularly subaerial erosion could therefore be occurring at the same time with heavy rain. Overall, a loss of ten meters from the coastal area was measured between 2009 and 2011. These facts suggest that only cyclones are capable of causing storm surges, winds, and currents at the highest levels that can destroy 10 m of coastal area in 2 years.

Looking at the weather events that have occurred recently in Madagascar, the island has experienced frequent tropical cyclone passage during the past ten years (Rakotondravony 2012) (Fig. 13.10). Generally, tropical cyclone passage is accompanied by heavy rain and violent storms. Tropical cyclones that attacked the country were strongly associated with marine movements in the immediate vicinity of Fort Dauphin. Although the path of a storm may not directly affect the coastal city of Fort Dauphin, this area sustains heavy rains, high waves, and storm surges along the coast (Donque 1974; Météorologie Nationale 1975; Direction de la météorologie 1984). Moreover, these factors have an influence on the local atmospheric circulation and the influence remains even a few years after the cyclone event. It takes approximately 4–6 years before normal conditions return (Nicholson 1997), but due to climate change that disrupts the air circulation, normal conditions may never be recovered (Rasmusson and Wallace 1983). It is also surprising that many tropical cyclones affecting Madagascar, eventually reach Fort Dauphin and dissipate there, or nearby.

During the period from 2004 to 2009 coastal accretion occurred although longshore currents did not significantly affect the beach; the waves were certainly less aggressive due to less precipitation. Nevertheless, residents along the shoreline found that the sea level increased significantly after the tsunami event in 2004. According to villagers, they noticed that the width of the beach had declined by about 80 m since 2004. One factor that can amplify the action of cyclone is a rise in sea level. This takes place extremely slowly and seems minimal, but also causes the removal of shoreline.

Fig. 13.10 Habitual trajectories of cyclones in Madagascar

Fig. 13.10 (continued)

 
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