Monday, May 25, 2015

Coastal erosion

A challenge for small island developing states

Coastal erosion is a major problem experienced in many areas of the world. It occurs when sediment is removed by the action of coastal processes (waves, currents, winds and tides).

Erosion undermines coastal infrastructures including commercial jetties, roads, bridges and buildings. It can remove coastal agricultural land and destroy recreational areas and habitats.

Localised tectonic events and land subsidence may result in accelerated erosion rates especially during hurricane periods. This can be further exacerbated by sea level rise as the waves travel with increased energy and break further inland. The Intergovernmental Panel on Climate Change (IPCC) reported that sea levels may rise as much as 0.06 m/yr in the 21st century. Trinidad and Tobago like other Small Island Developing States (SIDS) have limited land space. Their coasts have been and are subjected to erosion whether natural or human induced.

Beaches are dynamic coastal features found at the interface between land and sea. They respond to storms and coastal processes differently depending on geology. Where the coastal geology is resistant to wave attack, erosion may occur at a reduced rate and where it is more susceptible, it may erode at a faster rate. Areas consisting of loose, unconsolidated sediment will be more susceptible to erosion and re-alignment of the shoreline than those where the beach is backed by more resistant rocks.

Most shorelines naturally align themselves parallel to the predominant wave direction, based on the movement of sediment within the system, although other factors are contributory. As the beach is eroded, sediment is transported either along or across the shoreline. Sediment transported along the shoreline may change the orientation of the bay. Sediment transported across the shoreline may either be deposited over the continental slope where it cannot return to the beach, or form sand bars that may or may not return with seasonal changes of waves.

Wave activity varies seasonally and is greater during the winter months from November to April due to intense mid-latitude storms in the North Atlantic Ocean which generate swell waves. These swell waves have higher energy and affect the north, east and west coasts of the Caribbean islands. During the summer (May–October) the beach undergoes accretion due to the lower wave energy and longer wave periods, but the higher wave energy and shorter wave period present in the winter period increase erosion of the beach. When these cycles of erosion and accretion occur without any long term deleterious effects on the beach, a state of dynamic equilibrium is said to exist. Dynamic Equilibrium or a state of relative stability is also achieved when the shoreline has adjusted (become parallel) to the prevailing pattern of the waves.

Beaches can be classified as being in a state of erosion, accretion or dynamic equilibrium. Erosion can occur either horizontally where the backshore retreats landward or vertically where the sand elevation decreases along the beach face. Accretion however, occurs where there is an increase in sediment on the beach face which extends the beach horizontally increasing its width. Beaches undergo erosion and accretion cycles during the rise and fall of the tides, changes in the moon phases between spring and neap cycles, and during the summer and winter seasons. Stable beaches have no net loss of sediment although their profiles change during the year.

In Trinidad, beaches provide considerable benefits through tourism. Nesting of leatherback turtles (Dermochelys coriacea) occur along the north, north-east and east coast beaches of Trinidad and contribute to the eco-tourism. However, these beaches are impacted naturally or by activities such as trenching and pipe laying which are associated with the oil and gas industry, landing of telecommunication network cables and the construction of coastal protection structures. In some instances coastal protection structures are needed to stabilise the shoreline.

Most of the beaches and bays monitored in Trinidad and Tobago by the Institute of Marine Affairs are in dynamic equilibrium (Figure 1) (Alexis & Darsan, 2014). The north coast beaches are predominantly stable; however, sections of Las Cuevas and Blanchisseuse Bays are showing signs of erosion which has resulted in a lowering of sand elevations. This coastline is backed by resistant metamorphic rocks so recession of cliffs was not observed. At Macqueripe Bay, erosion of the backshore area led to the re-construction of a seawall to support this bay and protect it.

East coast beaches are predominantly in dynamic equilibrium with the exception of South Cocos Bay, Manzanilla. Although this bay is backed by extensive coconut palms and other low shrubs, this section of coastline experienced the most erosion of all bays monitored suggesting that the presence of vegetation was insufficient to combat the erosive force of the waves. Other sections of this coastline were also experiencing erosion, but were stabilised using hard engineering structures.

The south coast of Trinidad is more sheltered than the north and east coast but the geology of the backshore makes is very susceptible to erosion. Unconsolidated sands and clays succumb to weathering, undercutting at the base from wave action and results in erosion. Laying of pipelines and trenching in the near shore zone have resulted in increased rates of erosion and relocation of one village at Guayaguayare Bay. Punta del Arenal is the only beach that experiences accretion. This accretion however was not a seaward extension of the berm but a vertical increase in the beach elevation.

The west coast beaches were predominantly in dynamic equilibrium with the exception of North Chatham, Irois Bay and the western section of Guapo Beach. Erosion in North Chatham is ongoing and has doubled during the period 2004–2008. The monitored site at west Guapo Beach was in dynamic equilibrium between 2004 and 2006 but construction activities led to erosion in 2007. The beach in this region was subsequently stabilised by the construction of an offshore breakwater.

In Tobago, most of the beaches and bays are in a state of dynamic equilibrium where the seasonal changes of erosion and accretion revolve around a state of stability (Figure 2). Tobago is oriented in a northeast, southwest direction. The coastline is generally rocky and rugged. The northeastern part of the island is steeper and more irregular, resulting in a highly indented coastline. The southwestern region in contrast consists of a limestone platform and is less rugged. Tobago’s Leeward coast is rugged and fringed by coral reefs. The beaches here are generally of biogenic origin and some of them are leatherback turtle nesting sites.

The beaches on the leeward coast are less prone to erosion due to the more resilient metamorphic rocks that form these bays. However, changes in sand elevations due to coastal processes do occur. All beaches monitored on the leeward coast were in a state of dynamic equilibrium with the exception of the western region of Pigeon Point, the eastern region of Sheerbird’s Point, the southern section of Buccoo Bay.

All windward coast beaches are in a state of dynamic equilibrium except for Richmond Bay, Goldsborough Bay and the western region of Barbados Bay. Erosion along this part of the coastline threatened to breach the roadway and has prompted the employment of coastal protection measures such as revetments and groins.

The main factors driving erosion are the aspect or configuration of the bay, geology of the backshore, waves, currents, winds, tides and sediment supply. Other natural forces that exacerbate erosion include storm surges, weathering and surface run off. Coastal development, construction and offshore activities such as offshore dredging or construction in the near shore zone also result in severe environmental impacts on the coastline which interfere with the natural coastal processes. Any disturbance to this natural equilibrium can affect wave energy and longshore drift which results in erosion.

Submitted by Christopher Alexis,

research officer, Institute of

Marine Affairs