Proposal for the rearrangement of loose rocks and creation of a semi submerged beach in front of Shanti Maurice – A Nira Resort, St Felix November 2015 Chapter 2 The Physical Environment at the site The coastal zone is a delicate, dynamic balance between the powerful driving forces of the ocean such as cyclones waves, surges and tides as well as the reef-lagoon-beach ecosystem as highlighted in The Study on Coastal Erosion in Mauritius by Baird and Associates in 2003 (hereinafter Baird 2003). The coastal zone further, offers protection against these processes, as well as, producing sediments for the beaches. This chapter deals with the coastal conditions and the processes that are general for Mauritius and to the project site in particular. The climatological factors like winds, rainfall, tides, sea level rise and waves have been briefly addressed. An overview of the climatological factors and physical oceanographic processes that affect the coastline of Mauritius is given in this section and is based on recent studies and reports that were made especially under the Adaptation Fund Project for Riviere Des Galets and Mon Choisy, 2015 and also the Study of Coastal Erosion in 2003. Pertinent extracts from these reports are given below. 2.1. Climatological Factors General Overview Mauritius has a moderate tropical climate characterized by a summer season from around November to April and a winter season from May to October. The island is dominated by trade-winds from the east-southeast, generally reaching 20-30 km/h during the winter season. Much stronger winds – exceeding 250 km/h – have been recorded during tropical cyclones. Tropical storms are common in the Indian Ocean during summer (November – March) months, with a number of storms reaching cyclone intensity (Jury et al., 1999). Tropical cyclones generally form to the northeast of Mauritius, in the Intertropical Convergence Zone, between 5°S and 10°S. The frequency of tropical cyclones in the vicinity of Mauritius has been both related to El Niño – Southern Oscillation (ENSO) and quasi- biennial oscillation (QBO) (Jury, 1993; Jury et al., 1999). Rainfall patterns generally follow the tropical season with higher rainfall from December through April and less rainfall from May through November. Winds Long-term wind data comes from the National Centres for Environmental Prediction (NCEP) of the US Government’s National Oceanographic and Atmospheric Administration (NOAA). Thirty three years of wind speed and direction data were sourced from the NOAA’s global 0.5 by 0.5 degree reanalysis model and were interpolated to a latitude and longitude corresponding to Riviere des Galets. The wind climate is summarised by the wind rose shown below. November 2015 Page 8 of 48 Proposal for the rearrangement of loose rocks and creation of a semi submerged beach in front of Shanti Maurice – A Nira Resort, St Felix November 2015 Rose plots of long term (~33 year) hindcast wind data offshore from Riviere Des Galets, Southern Coast of Mauritius. 2.1.1. Rainfall St Felix lies mainly in a semi-dry region and rainfall patterns generally follow the tropical season with higher rainfall from December through April and less rainfall from May through November. Usually torrential rain would occur whenever a cyclone or very active cloud bands are in the vicinity. Table 2.1. shows the mean monthly rainfall near the site. The Rivière des Galets has its mouth some 2 km to the west of the project site while Rivière Savanne gain access to the sea at its river mouth at Souillac some 6 km to the east of the project site. The sea water in front of the site is minimally influenced with fresh runoff water from cyclone episodes and after heavy rain events. However, a small rivulet does have its exit to the sea at the St Felix public beach. 2.1.2. Sea-surface temperature Sea surface temperature varies from 28.1ºC (March) to 23.7 ºC (September). However, sea surface temperature, at times, rises up to 29 ºC in the summer months and can fall below 22 ºC during the winter months. MONTH TEMPERATURE HUMIDITY WIND SUNSHINE RAINFALL Mean LTM NO OF DAYS Highest NO OF DAYS Mean Highest Mean Lowest Wind Daily WITH Gust WITH Max Max Min Min Speed Hrs Mean % 1971- 1971- per Monthly 2000 Recorded 2000 Recorded day 1971- RAINFALL > RAINFALL > Km/h Km/h 2000 1 MM 5 MM January 29.8 35.9 23.0 17.7 81 11.4 219 7.7 239.7 236.6 16 8 February 29.5 34.4 23.1 17.4 83 9.5 209 7.1 198.7 266.6 16 10 March 29.1 33.4 22.7 15.9 83 9.5 169 6.9 212.9 203.1 17 8 April 28.2 32.8 21.8 15.0 83 9.5 153 6.5 194.1 211.5 17 8 May 26.8 30.6 20.2 13.3 81 11.4 69 6.6 203.2 153.4 14 6 June 25.2 30.4 18.7 11.5 78 11.4 105 6.1 182.1 95.2 14 5 July 24.2 27.7 18.1 11.0 77 13.3 87 5.5 170.9 100.2 16 5 August 24.2 28.8 17.8 11.0 78 15.2 89 5.9 181.4 87.9 15 5 September 25.1 29.1 18.0 11.7 77 13.3 82 6.7 200.8 59.6 10 3 October 26.4 29.9 19.2 11.0 77 11.4 69 7.6 236.3 60.1 9 3 November 28.0 32.6 20.4 12.2 78 11.4 92 8.8 265.4 76.9 10 4 December 29.2 33.8 21.9 16.1 80 11.4 221 8.4 259.7 171.8 12 6 Table 2.1.: The annual mean values for various climatological parameters, source: Mauritius Meteorological Services November 2015 Page 9 of 48 Proposal for the rearrangement of loose rocks and creation of a semi submerged beach in front of Shanti Maurice – A Nira Resort, St Felix November 2015 2.1.3. Atmospheric pressure The lowest atmospheric pressure occurs during the month of February which is one of the most active months of the cyclone season. Highest atmospheric pressure occurs in August when strong anticyclones influence weather over the region. 2.1.4. Waves Located in the south western Indian Ocean, Mauritius is exposed to swells created by westward moving low pressure systems between 30° S and 60° S, cyclone swells forming between 15 and 25° S, and localised wind swells caused by the persistent trade winds. A good and detailed description of the wave climate around Mauritius is given in Chapter 3 of Baird 2003. For the purpose of this present report, only the gist of information on waves from Baird 2003 which are pertinent to the site and which are important to the implementation of the various improvement works have been detailed out along with some basic information regarding waves and storm surges. Wind generated waves are one of the fundamental driving forces for the movement of sediments in the coastal environment and definition of the wave climatology is one of the starting points in any coastal analysis. Waves generated by wind are primarily a function of the wind speed, the duration of the wind and the distance over water which the wind blows, i.e. the fetch. The wind speed and direction are not constant and since the wind continues to generate new waves over the whole length of the fetch, waves of many heights, lengths and periods are generated, resulting in what is commonly known as ‘irregular waves’. To get a meaningful way of measuring these irregular waves, significant wave height is used. The significant wave height is defined as the average height of the highest one-third of all waves in a given series of waves. Despite the predominance of southeast trade winds in Mauritius, the major wave generating systems resulting in large wave conditions are due to the passage of cold fronts and their associated low pressure systems that pass to the south of the African Continent. Extratropical cyclones affecting Mauritius usually originate in the mid-latitudes of the Southern Indian Ocean between 30ºS and 60ºS. During the summer months, November to April, the situation is more complex: the oceanic anticyclone weakens and subdivides and a ridge of the Arabian anticyclone intermittently affects the north of the Mozambique Channel while the inter-tropical convergence zone extends its influence. The trade-wind circulation becomes less regular and convective instability develops almost daily in all regions. It is during this season that the depressions and tropical cyclones spawned in the southwest Indian Ocean can, if conditions are right, affect Mauritius. Tropical cyclones are generated in the southern equatorial belt of the Indian Ocean, generally travelling westward and southward. The cyclones often curve to the south and East prior to reaching the Island of Mauritius and the cyclone intensity typically diminishes with latitude. As a result of the above climatic influences, the waves affecting the coastal areas of Mauritius may be generated through several different meteorological phenomena. Given that Mauritius is an island and occupies a relatively isolated position in the Indian Ocean, the wave-generation fetches are exceptionally long from any direction. No part of the coastline is immune from direct impacts of any of the waves generated as detailed below. Local generated seas. Waves may be generated in the immediate vicinity of Mauritius by the southeast trade winds.