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The worlds coastlines, dividing the land from the sea, are geological environments which are unique in their composition and the physical process influencing them. The majority of these coastlines have beaches made up of loose sediments like gravel, sand, or mud that are continuously being acted upon by waves , currents and winds, redefining their shape without interruption. Nevertheless, in spite of the various wave climates that prevail around the world and the difference in the coastline composition, the nature and behavior of the beaches are frequently very similar. The winds blowing over huge extents of oceans provide the necessary energy and momentum to waves. This is accumulated energy is dispersed in the surf zone and the breaking of waves in this zone is largely responsible for the formation of turbulence, which traps and suspends the sediments from the beach.
Hence the beach profile shape is due to the action of waves and currents at the shoreline. The waves also form nearshore currents that transport the suspended sediments alongshore or crossshore. Nearshore currents can move enormous quantities of sediments along the shoreline and in the cross shore directions in volumes as large as hundreds of thousands of cubic meters of sand per year in some regions of the world. Longshore currents are formed by waves that break neither perpendicularly nor parallel to the shoreline and flow in the direction comparable to the wave direction. Many times, longshore current turns seaward to become rip current which carry sediment offshore. This movement of sediment is termed as the litteral drift and the quantity of sediment set in motion along the coast is the littoral transport. During a year, wave environment changes and so does the transport directions. But there is a dominant direction of the sediment transport that prevails at most coastlines. The direction that is coincident with this dominant transport direction is known as downdrift and updrift is the opposite direction. The cross shore transport is also caused by wave or wind induced mean cross shore flows and is mainly responsible for the presence of sand bars and other beach profile changes. These profile changes are usually slow, on the order of years in duration or can occur rapidly during storms, on the order of hours.
The profile of beaches is one feature of coastal areas that are often studied and analysed. These transverse sections through coastlines give a better idea as to the changes occurring over time at one point on the coast, either in the shape or size of a beach. The study of a number of profiles from the different points can gather data about the movement of sediment along a coast and to compare one area from another. Data collected can be plotted as a graph to produce the physical representative of the shape of the beach and can also be analysed statistically.
Figure 1.1 Beach profile terminology (adapted from the Shore Protection Manual 1984).
Figure 1.2 Typical Beach Profile
Figure 1.2 shows a typical beach profile, climbing down rapidly near the coast and then following the continental shelf's slope to depths over 40 M. Near the shore, the sand depth is about 2M and decreases to about 20 cm at depth of 40M.
Major beach erosion occurs during large storms. These storms also stir the sand from the deeper depths and bring it towards the surf zone. However, some beaches have 'sound pumps' with over capacity and are capable of self repair during bad times. But if this self repair mechanism becomes damaged, beaches are more reactive to weather conditions and climate cycles. Beach erosion of Westhampton Beach(USA) over a period of 40 years is shown in the diagram below. This trend is almost similar for many beaches all over the world.
Figure 1.3 Beach erosion of Westhampton Beach(USA)
While headlands and cliffs erode, beaches are able to hinder the action of waves on them thus making themselves a redoubtable defense against the anger of the sea. Its components of self repair are: (1) Drying of beach sand because of receding tide and by wind and sunshine; (2) Blowing of sand inland by sea wind; (3) Storage of sand in the dunes. Therefore a beach is able to store sand and grow during favorable meteorological conditions. During bad weather conditions, beaches erode and lie flatter.
The ocean's level was around 120 m lower than today as the termination of the last ice age blocked in the mountain and the polar ice caps. At that time the beaches were near to the border of the continental shelf. When the climate started warming, the sea level rose and moved the beaches landwards along with its sand. The ocean's level took some 4000 year to rise and this process was over about 6000 years ago.
Description of Mauritius Coastal Zone.
Mauritius is a small island state covering 1865 km2 of land. It is situated at latitude 200 South and longitude 580 East, around 1,242 miles off the South East coast of Africa. The coastline of Mauritius is 322 km in length, is bordered by fringing coral reefs which confine a lagoon area of 243km2 and has the above 93 miles (150km) of white sandy beaches. The country is constantly winds throughout the year, except for short periods in summer months (November to April) when tropical storms approach the island. These winds are stronger in winter (May to October) due to the presence of strong anticyclones passing close to Mauritius. The island also has protected bays and calm lagoons which have enabled the development of fishing and tourism.
Marine and coastal resources are one of the main pillar to the national economy. In 2000 the population of Mauritius was about 1.14 million and had a Gross Domestic Product (GDP) of Rs101.7 billion (Central Statistics Office, as reported in Halcrow 2003). The tourism strategy (Deloitte&Touche 2002) sees a growth from the actual 9,000 hotels rooms to around 20,000 in 2020. The coastal zones have witnessed a rapid boost in development over the past years and have been extensively exploited for various activities. This is due to the expansion in tourist arrivals, rising from 103000 in 1977 to 656450 in 2000. Coral sand removal(800,000 tonnes/yr) and sewage discharge in the lagoons mean that some beaches and lagoons have been severely impacted. Moreover there is the absence of a proper planning with respect to coastal development and inadequate enforcement have resulted in construction of buildings everywhere and structures such as jetties and groynes along the coast. Thus the coastal zone has become under severe stress.
Description of the 3 coastal sites chosen
Blue Bay - the bay with its nice deep blue color in the southeast of Mauritius. The colors of the sea are incredibly powerful and consist of a variety of many different blues. The Blue Bay has a very nice public beach which is perfectly suited for swimming and relaxing. Blue Bay, as well as the general area in the south and south-east is not so much touristically tapped. That is why the beach of Blue Bay is very quiet during the week. On weekends, however, one has the feeling that the half population of Mauritus pilgrimages here to spend some nice free days at seaside.
They are also very quiet at the weekends.
Blue Bay is particularly famous for its snorkeling trips. Since 1997, a 353-hectare area which is located in the sea just in front of the public beach is appointed as the first and until now the only Marine Park in Mauritius. The reasons for the foundation are the unusual and unique coral gardens, which are situated here. The marine ecosystem provides a rare beauty with its many different species of flora and fauna. These are especially the corals, which are in an incredibly good condition. Overall a figure of about 72 different corals and 32 different species of fishes are founded in this area. The corals are sometimes even up to 800 years old.
The public beach of St Félix is another beautiful beach of Mauritius. St. Felix is located also in the south between the towns of Belle Ombre and Soulliac. Actually St Félix has even two public beaches; however, the second beach from the direction of Le Morne is much more beautiful. To get there you have to continue the road after the direction sign to the public beach of St Félix a little bit until you reach a roundabout where you have to take the third exit.
The beach is wonderful, the exceptionally white sand, which flows into the turquoise green of the Indian Ocean of Mauritius, gives a wonderful contrast to the verdant trees of the back land. The beach of St Félix is very lonely and offers an excellent opportunity to chill out and relax. However, a small disadvantage from the beach of St Felix in Mauritius is that bathing is problematic. First the ground of the lagoon is covered with corals in which one or the other fish from the family of the highly toxic stone fishes can be founded. Secondly the current can be quite strong and should not be sneezed. http://www.mauritius-guide.net/beaches-mauritius/
AIMS AND OBJECTIVES
The aim of this study is to give an account of the spatial and temporal changes in beach profile data for the area of St Felix, Gris Gris and Blue Bay and to serve as a baseline for further research on coastal erosion for these 3 beaches.
This independent study has two main objectives:
To prove what changes in beach profile shape has occurred on a spatial scale , along the length of St Felix, Gris Gris and Blue Bay and to produce a description report of these changes.
To ascertain what changes in St Felix, Gris Gris and Blue Bay beach profile's shape have occurred on a temporal scale and to provide a longitudinal and descriptive report of these changes.
The "Emery board" method which was developed by the famous coastal scientist, K.O Ernery, is the simplest technique to measure a beach profile. The apparatus consists of two stakes which are connected by a rope of known length (5 M or 10 M) as shown in figure 1.4 This length initiates the measurements interval along the profile for individual data points. Each stake has a measurement scale running from 0 at the top, down to the bottom. When the beach is sloping downwards towards the sea, the observer views across the top of the stake facing the sea to the level of the horizon and calculates the distance (a) from the top of the board lying towards the land to the view afforded. On the contrary, if the beach is sloping upward away from the shore, then (a) is determined on the seaward board and the viewing is with the horizon of the inland board. The calculated distance (a) must be equal to the distance (b) even if the beach has either risen or dropped within the horizontal distance between the stakes (Komar, 1998). This approach has the advantages of having inexpensive and light equipment which can be easily transported to long survey sites for rapid surveys, and provides very accurate data for beach profiles.
Figure 1.4 Illustration of the Emery Board technique.
After having selected the site for the beach profile measurement when arriving at the beach, the first work is to mark a baseline (a line in the sand which runs parallel to the shoreline). Measurement of beach profile (elevations) will be carried out along the baseline at specific intervals. These intervals will be labeled as the horizontal distance (along the coastline) (x).
A landward surveyor, a seaward surveyor and a data recorder will be required to carry out the survey. The landward surveyor is appointed for holding the landward board, viewing over the seaward board towards the horizon and crying out the measurement (cm) to the data recorder. The latter must note down these measurements in an organized manner which includes the horizontal distance (x) of elevation (a) and a cumulative change in all elevations for one profile measurement. The seaward board and make sure that the rope is level between the 2 stakes when extended by moving the loop up or down.
Cross-shore data points of elevations are measured at the specific sampling intervals determined by the length of the rope when fully extended, starting at the landward extent of the base. If the beach is wide , more than 5 cross shore data points should be collected.
A cumulative vertical elevations (y-axis) v/s the horizontal position (x-axis) must be plotted using the beach profiles data recorded. The actual beach profile will be revealed. If the horizontal and vertical scales are equal, the beach profile may not appear to be thrilling (almost like a horizontal line). If that is the case, a technique called 'vertical exaggeration' is used, (to alter vertical scale by a known factor e.g a factor of 5 or 10). This technique is mainly used by geographers, geologists and cartographers.
A huge amount of data concerning several aspects of beach profile data exists around the world. Regarding measurement of beach profiles, Delgado and Lloyd (2004) elaborated one of the simplest technique of measuring beach profiles in their paper; explaining the method, and its pros and cons. The set-up is light and simple, can be conducted by one person alone, does not require sighting of the horizon and can operate in different weather conditions. The only equipment needed is a horizontal and vertical rod attached perpendicularly to each other and, a calibrated standing pole from which measurements can be taken. Also systematic errors (slightly bent rods) can be reduced by calibration on a flat surface. This technique has been used in various locations and results have shown that errors associated with the Delgado and Lloyd method have an average of 0.024m with regards to 50 m long profiles and a precision of 0.014 m when used by a single profiler. When compared to the use of electronic methods (theodolites) to carry out the same survey, the DL method shows a precision of 99.76%.
In spite of pointing out the accuracy of this method with respect to the sources of instrumented errors, Emery (1961) did not take into account the influence of the properties of different sediment surfaces on his method. Krause G (2004) showed that this effect cannot be disregarded as it can improve the overall accuracy of the data. Moreover it was proved that Emery method is sensitive to systematic errors introduced (small deviations between the scales of the 2 measurement poles). In order to achieve 90% accuracy with errors introduced, the minimum beach slope should not drop below 125 cm for a 100 m long profile. Increased sampling spacing from 2 m to 10 m in order to accelerate the survey and reduce errors was deemed inacceptable if sampling intervals is in the order of weeks or months. Cooper et al (2000) laid emphasis on the fact that coastal monitoring data sets should be up-to-date and accurate because these data will enable a better identification and understanding of changes occurring at the shoreline. As shown above, the Emery method holds both because it is accurate on a dynamic beach. With added modifications to the original method, this technique is best suited for a good beach profile measurement.