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 waves, surges and tides as well as the reef-lagoon-beach ecosystem as highlighted in The Study on Coastal Erosion in 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 during summer (November – March) months, with a number of storms reaching 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.

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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 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. 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. These waves will typically have peak wave periods of less than twelve seconds. The effect of these waves at the site under study is somewhat limited. November 2015 Page 10 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  Southern Hemisphere Swells. Swells are waves generated by distant storms that can propagate thousands of kilometres across the major oceans with little loss in energy. As these waves travel, the wave period increases significantly. Mauritius is strongly influenced by swells generated in the southern ocean due to forcing from the passage of extra-tropical cyclones. The swells typically approach Mauritius from the southeast to southwest and given the relatively north-facing orientation of the present site, these waves can affect and contribute to erosion at the site but with a however limited impacts as compared to a cyclone.  Tropical Cyclones. As previously mentioned, tropical cyclones develop in the southwest Indian ocean and typically approach Mauritius from the East and North. Tropical cyclones are small-scale storms of severe intensity that can generate extreme wave heights and surges. The season for tropical cyclones in Mauritius is from November to April with peak activity occurring during January and February. Tropical cyclones can have very high wind speeds and waves generated by severe can be extremely large with peak significant wave heights in excess of 15 m not being uncommon. They form the overriding consideration in the assessment of the design conditions for coastal infrastructure along the Mauritian coastline. The high wind speed and the low central depression of a tropical cyclone can induce large surge in coastal regions. These waves are largely responsible for the massive erosion that occurs along the coastal region of the island and such happening is usually over a very short period of time such as a few days. Long term wave hind cast data were acquired from the European Centre for Medium Range Weather Forecast (ECMWF). The ECMWF is an independent intergovernmental organisation supported by 34 states in the European Union. Specifically, we use data from the ERA-40 model which provides spectral wave data from 1979 to present on a 1 deg. x 1 deg. global grid. This wave data is used as boundary input information for local scale wave models (described in detail in the modelling section below).

Rose plots of hind cast significant wave height (HS) and peak wave period (TP) from a deep-water location offshore of the southern coast of Mauritius are shown below. The dominance of longer period swells from the south-westerly quarter is evident; however the effect of the persistent trade winds generating significant shorter period easterly swells is also clear.

Rose plots of long term (~33 year) hindcast wave model output for a point offshore of the southern coast of Mauritius.

November 2015 Page 11 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 2.1.5. Storm Surge Another important component of the overall water level is governed by atmospheric effects such as air pressure and wind speed. Low pressure systems cause sea levels to rise locally while higher atmospheric pressure causes a net decrease in the local sea level with a 1 hPa (1 hPa = 1 mb) pressure change corresponding to a rise or fall of sea level of approximately 1 cm.

In addition to the pressure driven variation in sea level there is also the surge caused by wind effects. Pressure gradients in sea level, associated with vertical circulation established by strong winds, are compressed as water depth decreases toward a land mass. The result is an increase in local sea level. The combination of wind-driven surge and inverse barometric effects is known as the storm surge. When storm surge and astronomical tide are combined, a value for storm tide can be determined.

The increased wave action associated with storms will also elevate mean sea level, where a coastline is exposed for a prolonged period of time to successive wave action, a build-up of wave-induced flux of horizontal momentum will occur, temporarily elevating the local sea level.

2.1.6. Nearshore wave transformation When the waves generated by cyclones, extra-tropical cyclones in more southerly latitudes and by the Southeast trade winds approach the shore, they are transformed by the near shore bathymetry through the following process:  Refraction as waves bend towards the shoreline on the reef front and flat  Shoaling as waves steepen and crests become larger moving into shallow water  Breaking over the reef front and reef flat dramatically reducing the wave height  Friction losses across the wide shallow lagoon particularly across areas of patch reef  Wave setup is generated and sustained inshore of the breaking zone as wave momentum is transferred shoreward  Wave runup occurs as the waves finally break on the beach  Wave overtopping of the shore occurs where the beach crest is low and/or the waves and water levels are high.

One of the major outcomes of the model is the figures for the dramatic drop in the significant wave height across the reef front and the reef flat which is around 10% of the height of the deep water waves. The drop, obtained from the model, is from 14 m to about 1.5 m for the 50-year cyclone wave and from under 12 m to a little over 1 m for the 5-year peak cyclone condition approaching from 30 degrees to the normal.

2.1.7. Longshore current longshore current is the movement of water or sediments, along a coast and parallel to its shoreline. Waves approaching the shore break in a region called the surf zone. In the surf zone, the waves dissipate all its energy to turbulences and sound. These turbulences can usually be observed up the shore in a white, frothy surge called the swash and will have for effect to put in suspension the sediment particles. Because of refraction and other nearshore processes, waves usually approach the shore at an angle, and as the wave train break along the shore, it will create a longshore current and will carry and deposit sediment both up and along the beach, but the backwash, acting under gravity, will always carry and deposit its sediment perpendicular to the shoreline, following the line of the steepest gradient. This produces a zig-zag movement of sediment along the beach known as longshore drift. The largest beach sediment is found where the process begins, updrift, and the smallest, most easily moved, downdrift.

Where waves are strong, the coast will be eroded and sediments will get carried away and where they are weak sediments will be deposited. This causes materials to be moved from areas of strong to areas November 2015 Page 12 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 of weak wave activity. The level of wave activity is determined by many factors such as the direction and fetch of the prevailing wind. Longshore drift can have undesirable effects on the geomorphology of the region, such as beach erosion.

Figure 2.2.: Schematics showing generation of longshore current

2.1.8. Tides and Sea Levels The sea level at a particular site is one of, if not the most important aspect of any oceanographic assessment. However, the variations in sea level over time are a complex interaction of many different factors. To aid in this assessment, long term (August 1986 – present) water level data from Port Louis was sourced from the Port Louis tide gauge which is part of the Global Sea Level Observing System (GLOSS), an international programme conducted under the auspices of the Joint Technical Commission for Oceanography and Marine Meteorology (JCOMM) of the World Meteorological Organisation (WMO) and the Intergovernmental Oceanographic Commission (IOC). The main component of GLOSS is the 'Global Core Network' (GCN) of 290 sea level stations around the world for long term climate change and oceanographic sea level monitoring.

Tidal Water Levels The most well-known type of sea level variation, the daily tides, are known as ‘astronomical’ tides since their magnitude and frequency are governed primarily by the gravitational effects of astronomical bodies such as the sun and the moon. The astronomical tidal fluctuations are at Port Louis are mixed, mainly semi diurnal with a maximum measured range of 1.33 m over the 27 years of measured data from the Port Louis tide gauge (maximum of +0.63 m and minimum of -0.7 relative to the mean over the full data range). Average deviations from chart datum (CD) for mean neap and spring tides are provided based on Admiralty Chart 711. Assuming a mean sea level of 0.45 m, we also present the deviation from MSL based on these values.

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Recorded sea level data from the Port Louis Tide Gauge as an example for a recent period.

Tidal statistics for Port Louis from Admiralty Chart 711.

Elevation above Elevation above Tide chart datum (m) MSL (m) Mean High Water Springs 0.70 +0.25 Mean High Water Neaps 0.50 +0.05 Mean Sea Level 0.45 0.00 Mean Low Water Neaps 0.40 -0.05 Mean Low Water Springs 0.20 -0.25

2.1.9. Tides at the site In Mauritius there are two tide gauges that are operational and that transmit data in near real time to the Sea Level Station Monitoring Facility, a programme run under the aegis of the UNESCO/IOC. The two stations are located at Port Louis with GPS position 20.15716º S and 057.5043º E and at Blue Bay with GPS position 20.4441333º S and 057.71095º E. The data from these tides station, while being administered by the Mauritius Meteorological Services, are available online and in near real time from the website www.vliz.be , the Vlaams Instituut Voor De Zee of Belgium.

It is acknowledge that there exist a minimum time lag in the tides between St Felix and Blue Bay, however the difference in the tide level is of the order or ± 1 to 2 cm and as such would not have any significant impacts on any of the data measured except for a shift of the same order in the bathymetry data. This has been considered as acceptable when considering the level of details and accuracy required for this present survey. The sea level from the tide gauge of Blue Bay was used both for planning and for correcting the bathymetry data.

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Figure 2.1. Tide data from the tide gauge in Blue Bay

2.2. Tropical Climatology As noted above, Mauritius is affected by tropical cyclones. During severe storms, waves can reach much higher levels than the long-term average wave heights. Furthermore, wind and pressure induced set up of the water level (storm surge) in combination with the high waves can exacerbate coastal inundation and/or coastal erosion.

2.2.1 Historical Events In the southern Indian Ocean, tropical cyclones generally form in the central and western subequatorial region then track to the south and west. They usually follow a curved track that moves them to the south and east as they pass near Mauritius before moving into cooler water and becoming extra tropical storms. Occasionally, as with Cyclone Bansi in January 2015, a storm may form to the northwest of Mauritius and track to the southeast. It is when storms are positioned to the north west of Mauritius that they pose the greatest threat to the west coast of the island. Storms that stay to the east of the island generally will not have a strong impact on the west coast.

The figure below shows tracks of the strongest cyclones known to have affected Mauritius and in the table below present a list of tropical storms and cyclones that have affected Mauritius. In 1960, intense Cyclone Carol reached maximum gusts of 256 km/h in 1996, wave heights of up to 5.5 m were recorded near Port Louis during cyclone Daniella which is considered to be one of the most severe cyclones for the Port Louis region. The wind speeds associated with tropical cyclone Gafilo in 2004 reached 258 km/h, with a minimum barometric pressure of 895 hPa, however it stayed well to the north of Mauritius.

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Cyclone tracks of events that significantly affected Mauritius (Mauritius Meteorological Services)

November 2015 Page 16 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 A list of significant cyclones affecting Mauritius. MD: Moderate Depression, SD: Severe Depression, STS: Severe Tropical Storm, TC: Tropical Cyclone, ITC: Intense Tropical Cyclone, VITC: Very Intense Tropical Cyclone, Max Min Closest point to Wind Year Dates Active Name Category Pressure Mauritius Speed (mb) (km/hr) 1979 21-23 Dec Claudette IC Over Mauritius 221 965 1980 24 - 28 Jan Hyacinthe IC 80 km NW 129 993 1980 3 - 4 Feb Jacinthe IC 150 km SE 129 992 1980 12 - 13 Mar Laure IC 30 km NE 201 989 1981 5 - 7 Jan Florine IC 80 km W 135 1003 1982 5 - 6 Feb Gabrielle MD 100 km NW 145 1001 1983 23 - 26 Dec Bakoly IC 55 km SW 198 992 1989 27 - 29 Jan Firinga TC 80 km NW 190 994 1989 4 - 6 Apr Krissy SD 30 km S 150 976 1994 9 - 11 Feb Hollanda IC 20 km NW 216 984 1995 7 - 8 Jan Christelle MD Over Mauritius 109 994 1995 24 - 27 Feb Ingrid TC 100 km NE 153 989 1995 8 - 13 Mar Kylie SD 135 km WNW 114 1005 1996 24 - 25 Feb Edwige MD 100 km N 162 1009 1996 14-16 Apr Itelle IC 275 km N 109 1011 1996 6 - 8 Dec Daniella IC 40 km SW 170 998 1998 10 - 11 Feb Anacelle TC 50 km E 121 985 1999 8 - 10 Mar Davina IC 25 km SE 173 974 2000 27 - 29 Jan Connie IC 200 km NW 122 1003 2000 13 - 15 Feb Eline SD 130 km N 129 1006 2002 20-22 Jan Dina VITC 50 km N 228 988 2003 12-13 Feb Gerry TC 100 km NNE 143 990 31 Dec 03 - 03 2003/04 Darius STS 40 km SE 113 994 Jan 04 2005 22-24 Mar Hennie STS 60 km SE 112 990 2006 03-04 Mar Diwa STS 220 km NNW 126 1005.7 2007 22-25 Feb Gamede TC 230 km NW 158 995.5 2008 02-29 Feb Hondo ITC 100 km N 215 915 2012 07-24 Feb Giovanna ITC 300 km N 195 935 2015 09-18 Jan Bansi VITC 280 km N 220 923

2.2.2.Tropical Cyclone Frequency Baird (2003) presents a detailed analysis of tropical storm frequency in for Mauritius. Using storm data from 1848-2002, they determined that on average 1.7 storms pass within 300 km of Mauritius each year. However, this metric has no relation to the storm intensity of its potential effect on the study site. They went on to note that when looking at storms from 1980 to 2002, the average is higher at 2.6 storms per year moving within 300 km. Additionally, storms of tropical cyclone intensity have passed within 100 km of Mauritius 9 times in that time period, or once every 2.5 years.

The Baird (2003) study numerically simulated wave heights from 25 historical cyclones, computing the storm generated wave heights on different sides of the island. Using this information in the context of a statistical extreme value analysis they computed the recurrence interval of cyclone induced wave heights as well as the recurrence interval for the storms themselves. This output is reproduced in the tables below

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Recurrence interval (RI) for cyclone induced waves on 3 sides of Mauritius reported by Baird (2003).

West South East RI H Tp H Tp H Tp (year) (m) (sec) (m) (sec) (m) (sec) 5 12.3 14 11.2 14 11.9 14 10 14.7 16 14.4 16 13.6 15 25 17.7 18 18.6 18 15.3 16 50 20.0 20 21.9 20 17.4 18 100 22.2 20 25.1 20 18.9 18

Estimated return periods for recent tropical cyclones as calculated by Baird (2003).

RI Event West South East 1980 - Laure 4.5 1981 - Florine 10.0 5.2 9.7 1983 - Bakoly 2.9 6.1 1989 - Firinga 6.3 4.6 1993 - Cecilia 5.0 1993 - Colina 6.5 3.0 1994 - Holanda 5.1 16.1 1995 - Ingrid 6.8 1996 - Bonita 6.8 1996 - Danielle 4.3 2.6 1996 - Flossy 5.7 1998 - Annacelle 4.0 1999 - Davina 4.0 3.6 2000 - Connie 4.0 6.3 2002 - Dina 11.2 18.6 2002 - Guillaume 12.0

It was observed that the wave heights and periods calculated by Baird (2003) are very conservative when compared to other probabilistic estimates of cyclone wave height in the southern hemisphere. For example, Stephens and Ramsay (2014) in their study of cyclones in the southwest Pacific computed wave heights of the order of 7.5, 10 and 11 m for the 10, 50 and 100 year ARI’s respectively.

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