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Grande Riviere River, Mother nature at work
The village of Grande Riviere located on the north coast of Trinidad, between Toco and Matelot, has been at the forefront of national and international concern since July 2012.
Grande Riviere’s importance lies in the fact that it is a prime nesting site for the endangered leatherback turtle (The local community benefitted from the advent of ecotourism which created numerous jobs ranging from tour guides, souvenir merchandisers and hospitality business as a result of nesting leatherback turtles). State-funded environmental preservation programmes also account for a substantial proportion of village employment.
As such, Grande Riviere Bay presents an interrelated dynamic of socio-economic and ecological facets. The bay is approximately 970 metres long and arcuate in shape with varying beach widths. The beach at the eastern section is much wider than at the western region with a well-defined berm and low backshore topography.
The beach gradient at the eastern section has a relatively gentle backshore with a moderate to steep beach face. The central to western regions of the bay has a moderate sloping beach with the eastern section backed by steep cliffs.
The Institute of Marine Affairs (IMA) has been monitoring this beach since 2000 as part of a project to investigate the beach dynamics and risk posed to the leatherback egg clutches during the nesting season. The study showed that this small pocket beach was efficiently used by turtles during the nesting season.
Results indicate that at the start of the nesting season in March, the majority of leatherback nests were deposited on the eastern section of the beach. After May, there was a continued westward shift in nest distribution that coincided with sand accretion (deposition of sand) in a westerly direction as the season progressed until August.
As such, the seasonal erosional and accretionary cycles on this beach supported the distribution of nests as the nesting season progressed.
Damage to turtle nest, coastal infrastructure
Beaches on the north coast of Trinidad usually build up sand from April to October and erode in the other months, particularly during the North American winter period from December to February, with associated increased swell wave activity and high intensity waves.
As waves deposit sand on the beach during the accretionary months, this creates a berm. While the berm functions as a buffer to protect the backshore from the battering of waves, it unfortunately sometimes prevents the river from draining directly out to sea, and forces the river to find a new outlet, such as in Grande Riviere.
In April 2012, it was observed that the course of the Grande Riviere River had been shifting westward—deviating from its normal path through the berm in a northerly direction to the sea. The shifting mouth of the Grande Riviere River started to flow in a westward direction eroding the beach in the backshore area parallel to the coastline, following the path of least resistance.
The river continued to flow westward crossing the berm and entering the bay approximately 100 metres west of its usual position. The diverted flow of the river, however, created an erosion channel which effected damage to turtle nests and coastal infrastructure.
Sand dam failed
The backshore erosion channel persisted until the intervention of the Drainage Division (Ministry of Works and Infrastructure) in early July 2012, where a sand dam was constructed to arrest the westward flow of the Grande Riviere River. This intervention addressed a need by the beach-front owners to protect their properties from further erosion, as well as loss to the existing turtle nests.
The sand dam forced the river to return to its original course, but also left a section of the erosion channel abandoned and in a state of disrepair. Surface runoff from the village added to the collection of trapped water in this channel, which now posed a threat to public health, and the upcoming nesting season.
The IMA subsequently set up additional beach profiling stations to monitor the recovery of the beach. By November 2012, the sand dam had failed as the marine and hydrological regimes sought to regain balance in the affected areas.
However, the erosion channel remained. Calls by several stakeholders have insisted that the backshore erosion channel be filled in by the relevant authority. Some of their concerns raised included public access to the beach as well as threats to public health and turtles as the nesting season draws near.
As an advisory to the Government on matters related to coastal conservation and sustainability of our coastal resources, the IMA’s position on the matter was that no alteration by in-filling the erosion channel should take place based on existing hindcast and other data. Data collected by the IMA has shown that this river shifting event is not a new phenomenon and that a similar event occurred in 2003.
Decision not to in-fill the erosion channel
Cyclical patterns observed at this IMA beach profiling station indicated a high probability for regeneration of the beach, which will allow for equilibrium status to be achieved over time. The decision not to in-fill the erosion channel has evaded several major concerns related to beach stability and sustainability of this ecologically sensitive beach system.
Some of these concerns include:
• Sand mining is not recommended in small pocket beaches especially where the sediment budget is uncertain as this can lead to severe coastal erosion in the mined areas.
• The beach at Grande Riviere has been in a state of dynamic equilibrium whereby the net-loss of sand during erosive periods is balanced by net-gain during periods of accretion and beach nourishment. The river shifting erosion event of 2012 created a state of dis-equilibrium in the beach system, which would be balanced over time.
• Anthropogenic sand relocation cannot replicate the work of waves in sorting and spreading sediment across and along the littoral zone of a beach. Modifying sediment grain-size characteristics has the potential to alter the hydrology and geomorphology of the beach. This in turn can affect the suitability of the affected area for turtle nesting and hatching success.
• Sand mining and relocation would have involved the use of heavy machinery on the beach. This would have led to compaction of the beach sand, which could have negative implications for successful turtle nesting.
Understanding the science of beach systems
As of January 13, 2013, the backshore erosion channel persisted and the calls for intervention heightened. However, based on an extensive data set, the IMA advised that the winter swells during the months of December to February would superimpose higher energy waves on the beach system which would return the eroded beach sediment from offshore back onto the beach.
While an exact time-frame for beach recovery was not given (since these marine conditions are based on meteorological conditions), it was noted that the beach will recover naturally over time.
On January 14, high swell waves were experienced on the north coast of Trinidad emanating from a large frontal system positioned off the North American continent heading east. This meteorological system produced the swell waves needed by Grande Riviere beach to facilitate its recovery.
By the morning of January 15, 2013, the beach had completely recovered, and the erosion channel in-filled naturally, as sand was brought from offshore and deposited on the beach and backshore areas.
Sand was deposited as far landwards as on the bottom floors of the beach front hotels that had to remain closed for clean-up operations. This highlights the issue of set-back distances for beach-front property, as these infrastructure are clearly built within the limits of the beach.
Before we attempt to intervene or alter beach systems, we need to fully appreciate and understand the science. We need to embrace sound science as the basis for decision making. Winter swell activity though not continuous, will continue through to February.
While this initial set of swells aided in beach recovery, continued swell activity also has the potential to remove sand from the beach. What is required is a joint holistic approach towards monitoring and investigation into this dynamic beach system.
Junior Darsan PhD
Institute of Marine Affairs
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