SHORELINE DEFENCE
When assessing the coastal vulnerability and/or maintenance, a coastal management plan must consider all the processes that affect the coastline, and not only isolated events. The impacts of cyclones on the East coast of Mauritius are not as detrimental as the impacts caused by the continuous actions of the SE Trade Winds and the Southern sea swells [Baird, 2003]. Therefore, a holistic approach is required for the implementation of a suitable coastal protection measure which would also mitigate the effect a short-term event such as a tropical cyclone.
However, this chapter assesses the need and the merits for a suitable coastal defence to control the amount of erosion predicted at Belle Mare due to storms only, based on a theoretical approach. As mentioned in Section 2.11, no coastal protection structure was modelled as part of this dissertation. The report by Baird [2003] indicated that limited engineering analyses were done by the Government of Mauritius prior to the implementation of the existing coastal protection structures around Mauritius.
7.1 Assessment of various coastal defences for Belle Mare
The reef at Belle Mare, which extends some 900m with the average water depth of approximately 2.26m, is a typical representation of the coastal characteristics surrounding Mauritius (i.e. reef-lagoon-beach systems) and this contributes considerably to the energy dissipation of the storm waves. As a result, the beach at Belle Mare is moderately affected by the actions of the tropical cyclones. Baird [2003] stated that the recent erosion scarping at Belle Mare is due to severe tropical cyclones several decades ago. There is no existing protection measure at Belle Mare. XBEACH predicted a wave run-up of 2.05m for a 15% increase in intensity for a cyclone with a 5-year return period. Belle Mare beach has a slope of 10% which implies that a beach flooding of ±20m is expected.
7.1.1 Alternative 1: “Do-Nothing”
The impacts of tropical cyclones on Belle Mare beach are said to be reversible [Baird, 2003].
The general wave climate driven by the SE Trade Winds helps with the natural replacement of the eroded sand. No development has taken place in the coastal zone and hence, there is no need
7.1.2 Alternative 2: Vegetated dunes
Baird [2003] stated that the scarping of the beach was due to a severe tropical cyclone some 20 years ago. This can be mitigated by the construction of a vegetated dune using native coastal vegetation. The dune would lessen the wave run-up during severe storm conditions whilst the vegetation will stabilize the dunes.
7.1.3 Alternative 3: GSCs with vegetated dunes
The steep scarping observed along the top of the shoreline indicates that Belle Mare beach is subject to significant flooding. The construction of vegetated dunes only may result in the repeated erosion of the sediments during severe cyclones, especially if the vegetation is not well maintained. Therefore, the implementation of GSCs should be considered as these are less susceptible of being displaced due to their robustness. These can then be covered in sand and vegetation to give an aesthetical appearance. GSCs are very practical and flexible measures as minimal maintenance is required and can be easily cut opened if found unsuitable.
7.1.4 Alternative 4: Beach nourishment
Beach nourishment can be used as a reactive or proactive measure in the case of erosion. Beach nourishment acts as a wave energy absorber and also increases the beach width. The volume of sand required to replenish the eroded beach due to cyclones may be small compared to the annual loss of sediment as a result of the SE Trade Winds along the east coast of Mauritius.
Beach nourishment is not a popular practise in Mauritius and is regarded as an expensive process. However, this has been adopted by various hotels for obvious commercial benefits instead of relying on the natural recovery process which may take a long time.
7.1.5 Alternative 5: Hard measures
The provision of “hard” measures such as revetments and/or seawalls must only be considered when all “soft” measures have proven unsuccessful in mitigating erosion. Belle Mare beach, as well as other popular beaches in Mauritius, rely on its pristine and attractive sandy coasts for recreational activities by local residents and the tourism industry. Where “hard” measures are necessitated, these must be assessed as being environmentally, socially, aesthetically, economically and financially acceptable. The status of Belle Mare beach is not considered critical for the implementation of structural protection measures.
7.2 Selection of coastal defence alternative
Belle Mare is well protected from the impact of tropical cyclone by its reef-lagoon-beach system. XBEACH showed that a major transformation of the storm waves as these travelled over the reefs. For Scenario 4 (i.e. 15% increase in the intensity of a tropical cyclone having a 5- year return period), XBEACH predicted an erosion of 21.45m3/m and a wave run-up of 2.05m.
Although these values are significant, Baird [2003] reported that the beach recovery can be achieved through natural process. Also, Belle Mare relies on its pristine beach for a prosperous socio-economic development. Considering the above, a “hard” measure is not advisable for mitigating the erosion resulting from tropical cyclones. The implementation of vegetated dunes is more suitable for Belle Mare beach as very little capital and/or knowledge is required. GSCs can be used with vegetated dunes to offer a more robust countermeasure if repeated erosion occurs. Due to the lack of local expertise and capital investment, beach nourishment can be looked at as a last resort. It is imperative that an integrated coastal management plan be commissioned to address any development and/or maintenance in the coastal zone and severe cases can be treated individually.