The Buccoo Reef: Numerical Modelling of The Hydrodynamic and Wave Characteristics

The Nylon pool, Tobago

What do we know of the Buccoo Reef Tobago?

Now, of course the reef provides socio-economic benefits such as recreation, tourism, livelihood generation and jobs as well as various ecosystem services such as fisheries production, biogeochemical processes such as nitrogen fixation and CO2/Ca control. However, one of the most important functions of the Buccoo Reef (and the least known) is that of shoreline protection.

The Buccoo Reef, Tobago provides shoreline protection through wave attenuation and dissipation, creating low wave energy conditions landward of the reef. Coral reefs dissipate wave energy mainly by depth induced wave breaking along the reef profile as well as frictional dissipation due to coral reef roughness. In fact, an economic valuation of the shoreline protection services has shown that the coral reefs in Tobago contribute between US$ 18 and 33 million per year in terms of potentially avoided damages, with an increase in the importance of the shoreline protection services with climate change and sea level rise.

Over the past couple decades, however, the Buccoo Reef has experienced extreme coral reef degradation and mortality due to climate change and anthropogenic (human) stresses. Studies on the 2005 mass coral bleaching event showed that the mean overall bleaching of hard corals in Tobago was found to be 66% with some areas having more than 85% bleaching. What does this mean for the ability of the Buccoo Reef to protect the shoreline?

This increased coral reef degradation combined with sea level rise would result in smoother corals and an increase in depth on the reef. This means that there would be a decrease in frictional dissipation provided by the reefs, increasing wave energy approaching the shore and a decrease in the effectiveness of the Buccoo Reef at shoreline protection.

Despite the importance of the Buccoo Reef at shoreline protection, little was known about the hydrodynamic and wave conditions on the Buccoo Reef, its effectiveness at coastal protection and the impact of coral reef degradation and sea level rise on future wave conditions. Given this lack of data, it is important to bridge the gap in knowledge as it relates to the hydrodynamic and wave conditions of the Buccoo Reef.

The Buccoo Reef, Tobago

Numerical models, to put it quite simply, are algebraic representations of a physical system. Delft 3D 4.04.02 was used to investigate the hydrodynamic and wave conditions of the Buccoo Reef. Using bathymetric data, tidal data, wind and wave data and representative equations, a numerical model of the Buccoo Reef was created to better understand how the reef affects characteristics such as water levels, wave heights, wave frequency, wave energy dissipation as well as how this may be affected due to sea level rise and continued coral reef degradation.

The figures above show the water levels on the reef during the high tide level. There are areas where the water level increases, as seen by the bright red spots. This increased water level is a result of wave setup due to depth induced wave breaking. When the water depth becomes around 78% of the offshore wave height, it becomes unstable and ‘breaks’. When this happens, there is an increase in mean water level known as wave setup that causes water to flow across the reef.

Similar to previous studies, wave setup was higher during the low tide level. This wave setup is important because it can allow the passage of higher waves into the reef.

The figure on the left shows the wave heights on the reef where the brighter colours represent the higher wave heights and the darker colours represent the lower wave heights. As the waves move towards the shallower forereef, wave height increases due to shoaling, they become steeper, break along the reef and move shoreward. Wave energy is lost by breaking but wave height attenuation also takes place on the reef crest and within the reef lagoon as a result of frictional dissipation provided by the coral reef roughness. This wave breaking on the reef crest is a phenomena that is seen in real life!

The graph on the right shows that almost all of the incoming wave energy is dissipated at the low tide level but there was a reduction in the incoming significant wave height by 92% at high tide.

One of my favourite maps out of the entire study is the wave energy dissipation map seen above. Wave energy is dissipated on the reef crest by the same depth induced wave breaking mentioned previously. What I find personally exciting i that you can clearly identify the shape of the reef because the waves are breaking along the forereef of the Buccoo Reef; a phenomenon that we see happening everyday!

Now that we understand a bit about how the reef affects the wave characteristics and how effective it is at wave attenuation and dissipation, how will this service be reduced by sea level rise and continued coral reef degradation? To investigate this, I reduced the friction factors to represent essentially dead and smooth corals and I increased the mean sea level based on sea level rise projections.

The figure above shows the wave heights on the reef as sea level rises and you can clearly see a steady increase in the wave heights on the reef.

Influence of Sea Level Rise on wave heights

For the +0.25m SLR scenario, depth induced wave breaking accounts for a 77% reduction in significant wave heights. Within the reef lagoon and nearshore area, there is an 80% increase in the significant wave heights from the base scenario due to the lack of frictional dissipation provided by the coral reefs.

For the +0.50 m sea level rise scenario, the location of wave breaking along the reef profile remains constant. Wave breaking accounts for a reduction in incoming wave heights by 47% which is 45% less than the base scenario. The wave heights within the reef lagoon along cross-section N=70 range between 0.3-0.45m, increasing towards the shore. There is a 135% increase in the significant wave heights from the base scenario approaching the shore.

The +1.00 m increase in sea level scenario represents the projected sea level by the year 2021. With the 1m increase in mean sea level, the location of depth induced wave breaking moves slightly landward. Wave breaking on the forereef results in a reduction of the wave heights by 12%, with a large proportion of the waves passing over the reef crest and into the reef lagoon unbroken with wave heights ranging from 0.4-0.65m. The nearshore wave heights range from 0.5-0.7 m representing a 234% increase in the wave heights near the shore from the base scenario.

The results show that if the Buccoo Reef is unable to keep up with Sea Level Rise due to continued degradation as a result of climate change and anthropogenic stresses, there will be a decrease in the wave attenuation and dissipation by the reef and a substantial increase in the nearshore wave heights. These higher energy waves will result in increasing intensity and frequency of coastal hazards such as flooding and erosion, reducing the socio-economic benefits of the Buccoo Reef and area landward of the reef and exacerbating environmental issues.

How do we move on from here? How do we avoid the future scenarios?

Despite the obvious importance of the Buccoo Reef with respect to shoreline protection, there is little that has been done to reduce the anthropogenic stresses on the reef which contribute to the reef degradation. The Buccoo Reef was designated a Marine Protected Area (MPA) in 1973 , a Ramsar site in 2006 and is in the process of being recognised as an Environmentally Sensitive Area under the Environmental Management Act (2000) but little has been implemented to address the coral reef degradation due to anthropogenic influence and adaptations to climate change and sea level rise.

If you are interested in the detailed, technical study feel free to contact me via the contact page.

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