A barrier-beach system under threat; Cocos Bay (Manzanilla), Trinidad. Junior Darsan University of the West Indies

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1 A barrier-beach system under threat; Cocos Bay (Manzanilla), Trinidad. Junior Darsan University of the West Indies Trinidad and Tobago, the most southerly of the Caribbean islands, is situated between 10º 02 to 10º 50 N latitude and 60º 55 to 61º 56 W longitudes. Trinidad and Tobago has a tropical maritime climate with two distinct seasons; a wet (May to December) and dry season (December to May). The tidal regime experienced is a function of the tide waves from both the Caribbean Sea and the Atlantic Ocean (Figure 1). The tidal regime is semi-diurnal with a maximum spring tidal range of 1.2 metres (Kenny, 2008). During the months of October to April, mid-latitude storms in the North Atlantic Ocean produce swell waves that attack the coast, causing enhanced erosion (Cambers, 1998). Figure 1: Map of Trinidad Showing the Location of the Study Area. Source:

2 Manzanilla beach is a barrier beach system about 20 km long located in Cocos Bay, embraced by two prominent headlands at Manzanilla Point and Radix Point (Figure 2). Cocos Bay has been classified as an open sea beach (Georges 1983) with a zeta-form (Hsu et al. 1987) or fish-hook beach shape, where the headland at Manzanilla Point partially blocks longshore transport. In the sheltered area behind the headland, the beach is swash aligned, but further down-drift, the through-put of sediment transport reasserts itself and the beach becomes drift-aligned (Silvester 1960; Swift 1976; Davies 1980). Figure 2: Map of the Manzanilla area The Manzanilla beach is a typical barrier beach whereby east flowing rivers (L Ebranche, Nariva and Ortoire) have ponded the extensive Nariva freshwater swamp; which became an international wetland reserve in 1993 under the Ramsar Convention (1988). This swamp is the largest in the country and home to a wide variety of flora and fauna (including the endangered West Indian Manatee) forming a distinct wetland

3 ecosystem. The swamp also provides a livelihood for many people who farm rice and other food crops at its margins. The Nariva River also carries large quantities of particulates and nutrients to Cocos Bay which has implications for marine biota and productivity (Bacon et.al. 1979; Bacon 1996). The viability and longevity of this wetland depends on the maintenance of the Manzanilla barrier beach (Figure 2). The coastal geomorphology of Manzanilla is constantly changing being exposed to the dynamic destructive marine processes of the Atlantic. Cocos bay s underlying geology is mainly of sedimentary origin, and forms a wide gentle plain (of recent alluvium) with small folded hills in the Central Range. The Central Range is mainly of sedimentary origin and forms a line of northeast-southwest trending hills formed by an asymmetrical anticlinal structure with a Cretaceous to Paleogene core (Donovan and Jackson 1994). The sedimentary rocks at Manzanilla Point and Radix Point, though susceptible to marine erosion, offer more resistance than the low-lying alluvium found between the headlands, upon which the barrier beach is built (Plate 1). The inherent geology as well as the high energy marine environment of the Atlantic has created conditions which promote coastal erosion. Coconut Estate Undercutting by waves lead to collapse Recent Alluvium highly susceptible to erosion Manzanilla Beach Plate 1: Showing the vulnerable nature of the Manzanilla coastline at south Cocos Bay

4 Owing to the severe coastal erosion occurring at this bay, there are several aspects of concern. First, coastal erosion threatens the Manzanilla-Mayaro main road which provides a major transportation link between north Trinidad and the south eastern communities. The coconut estates are also at risk since the coastal erosion claims dozens of coconut trees annually. Beach front properties especially to the northern section of the bay are also in danger of being eroded by the sea as coastline erosion brings the sea into their backyards (Plate 2). There is also economic decline in these developed areas that suffer significant coastal erosion, as land and property prices fall. Finally, there is the environmental issue of maintaining the sensitive balance in salinity levels in the Nariva Swamp. If this beach is breached, the swamp area may be drained, or an increase in salinity levels can occur as sea water enters, which can upset the delicate ecosystem balance that has been established over the years. Coconut Estates Old Tyres being used as a Rip-Rap Guest Houses under threat Felled Coconut trees by erosion Plate 2: Sowing visible signs of coastal erosion at north Cocos Bay Because of the fairly low topography of the Manzanilla sand bar, with some sections below sea level (Williams 2000), there is a threat that coastal erosion could breach the barrier beach. There is also significant recorded erosion along several parts of the Manzanilla beach (Singh 1997; Darsan 2005a, 2005b, 2012). This erosion is more pronounced particularly south of the L Ebranche (north Cocos), Nariva (central Cocos),

5 and north of Ortoire river mouths (south Cocos); attributable to fresh water outflow and tidal inflow dynamics (Darsan 2012). At several points along Manzanilla beach, the sand bar has been eroded from fresh water outflow and sea water inflow; creating points where salt water is able to directly penetrate and alter salinity in the Nariva Swamp (Environmental Management Authority 2001). At south Cocos, the intertidal beach position has receded progressively and dramatically from 1990 to the present. Between 1990 and 1996, the beach retreated by an average of about 10 m; a rate of 1.7 m/yr. Although some accretion occurred in 1993 especially during the wet (summer) season, this accretion was at an insignificant rate compared to the erosion rate (Singh 1997). A similar study by Mahabir and Nurse (2007) reported that the average accretion rate north of the Nariva inlet was 0.17m/yr. On average, the erosion rate for the period 1990 to 1999 was approximately 0.55m/yr. Upper beach erosion was almost zero for the period ; however the rate increased as time proceeded. The Institute of Marine affairs (IMA) (2004) suggested that the most intense erosion occurs 3 km north of the Ortoire River mouth. According to Mahabir and Nurse (2007), there was rapid erosion of approximately 1.7m/yr during the period ; which was similar to the rate reported by Singh (1997) for an earlier period ( ). More recently, Darsan (2012) noted that erosion rates varied along the bay, with higher rates near the outflow of the L Ebranche (north Cocos), Nariva (central Cocos), and north of Ortoire river mouths (south Cocos). During the period , north, central and south Cocos recorded erosion rates of 1.55m/yr, 1.15m/yr and 1.21m/yr respectively. Cocos bay does not behave as a typical zeta-form beach since the northern section of the beach behind Manzanilla Point undergoes severe coastal erosion in an area which is supposed to be sheltered. Coastal erosion here is caused by wave refraction which concentrates wave energy into the sheltered area and by the outflow of L Ebrance River during times of flood discharge. While higher erosion rates are being experienced at north Cocos, its relative distance from the Manzanilla-Mayaro main road as well as the boundary of the Nariva swamp did not classify it as high priority. As such, this erosion issue at the northern end of the bay has received little attention from the responsible government agencies (Plate 3).

6 December 2005 September 2006 Before Erosion Area of Coastline Eroded Plate 3: Significant Erosion at north Cocos Bay from December 2005 to September 2006 Attempts by the government to arrest the erosion at south Cocos Bay has resulted in the construction of a limestone revetment (Plate 4) in 2007 to buffer the energy of oncoming wave attack and preserve the integrity of the Manzanilla-Mayaro main road. This hard engineering structure has addressed the erosion problem to the south of the bay, however there remains similar issues at north Cocos and south of Nariva River mouth (central Cocos) that need addressing. Radix Point Coconut Estates Limestone Revetment Plate 4: Showing the limestone revetment at south Cocos Bay

7 While this attempt at protecting the main road is commendable, the bigger threat is to the Nariva Swamp, an internationally protected wetland reserve. Measures to protect this coastline should therefore focus on the areas along the Manzanilla sand bar that are being threatened by coastal erosion, which might upset the salinity balance in the swamp. References Bacon, P. R., J. Kenny, M. Alkins, S. Mootoosingh, E.Ramcharan, and G. Seeberan Nariva Swamp Development Project: Studies on the biological resources of Nariva Swamp, Trinidad. Occasional Papers No. 4. St. Augustine, Trinidad: University of the West Indies. Bacon, P Water management for sustainable development in the Nariva Swamp. Trinidad. Maraval, Port of Spain: Caribbean Forest Conservation Association. Cambers, G Coping with beach erosion: With case studies from the Caribbean. In Coastal management sourcebooks 1, Paris: UNESCO Publishing. Darsan, J. 2005a. A comparative study of the coastal geomorphology of Cocos Bay and Las Cuevas Bay, Trinidad. Caribbean Geography 14(2): Darsan, J. 2005b. A comparative study of the coastal geomorphology of Manzanilla and Las Cuevas Bays, along the eastern and north-western coasts of Trinidad. Unpublished BA Thesis., Department of Geography and Geology, University of the West Indies, Mona Campus, Kingston, Jamaica. Darsan, J An analysis of the coastal geomorphology and evolution of Cocos Bay, Trinidad. Unpublished PhD Thesis (submitted)., Department of Geography and Geology, University of the West Indies, Mona Campus, Kingston, Jamaica. Davies, J.L Geographical variation in coastal development. London: Longman. Donovan, S.K., and T.A. Jackson., eds Caribbean geology: An introduction. Kingston: UWI Press. Environmental Management Authority Background paper: Climate change vulnerability assessment for Nariva Swamp. Port of Spain: Environmental Management Authority. Georges, C A coastal classification for Trinidad. Hilltop Lane, Chaguaramas: Institute of Marine Affairs.

8 Hsu, J.R.C., R. Silvester, and Y.M. Xia New characteristics of equilibrium shaped bays. In Proceedings of the 8 th International Conference on Coastal Engineering, 1962, Mexico City, Mexico: American Society of Civil Engineers. Institute of Marine Affairs A Guide to Beaches and Bays of Trinidad and Tobago. Hilltop Lane, Chaguaramas: Institute of Marine Affairs. Kenny, J. S The Biological Diversity of Trinidad and Tobago: A Naturalist s Notes. NIHERST: Prospect Press. Map of Trinidad showing the location of the study area (accessed January 6, 2009). Ramsar The Ramsar Convention., (accessed August 18, 2007). Silvester, R Stabilisation of sedimentary coastlines. Nature 188: Singh, B Climate-related global changes in the southern Caribbean: Trinidad and Tobago. Global and Planetery Change 15: Swift, D.J.P Coastal Sedimentation. In Marine sediment transport and environmental management, ed. D.J. Stanley, and D.J.P. Swift, New York: John Wiley & Sons Ltd. Williams, P Station descriptions for cross-sections along the east coast of Trinidad. In NEDECO and Delft Hydraulics (2000): Feasibility study and detailed design for coastal protection works, Trinidad. Trinidad: Government of Trinidad and Tobago Ministry of Works and Transportation Drainage Division.