Hart, Marjolein de

Summary The Government of Bonaire has requested Wageningen Marine Research (WMR) to research the composition of the sand used to construct the artificial beach of the Chogogo Beach and Dive Resort. The major concern regarding the artificial sand was to evaluate whether the sand used could harm the marine park corals. The sand of the beach of Chogogo was sampled on the 10th of May 2022 and analyzed at the Netherlands Institute of Sea Research (NIOZ) for grain size and organic matter content. Additionally, the natural sand in front of Chogogo and several other places was sampled to better compare the sand that naturally occurs around Bonaire to the artificial sand. The results of the analysis and expert evaluation have provided the following responses to specific questions that have been raised.  

What are the potential effects on marine life, specifically corals, in relation to the constructed beach?  There is already sand being transported into the adjacent sea. Corals may already be affected. If so, then stress on corals will be increased. This will affect their metabolism, influencing coral growth and health. Furthermore, a higher sediment load will lead to a higher cover of the bottom by sand and prevent coral recruits from settling. In the longer term, corals more sensitive to sediment will disappear from the location which may be exchanged for more stress-tolerant corals. Generally, these species are less important for maintaining the reef. However, if the sediment load is high (permanently or incidentally), the sea floor may become fully covered by sand, and all corals may disappear. Thus, it is important to monitor artificial beaches for breaks in the retention walls, accumulation of sand along the walls, and make adjustments when necessary. It is important to note that since corals are long-lived organisms, a short-term assessment, as presented within this report, can barely unravel the longterm quantitative impacts on the health of adjacent corals. Thus, long-term monitoring of the coral communities in front of and up-current from (as control) the artificial beach should be favored.  

What are the potential effects on the health of the corals under normal weather conditions? Currently, the sand is already moving towards the sea, and it cannot be excluded that there are no negative effects. Potentially, visibility is already lower, and sedimentation has increased. In addition to the recommendations under question 3, we recommend installing (natural) windbreakers to decrease the wind-funneling effect between the resort's buildings in order to decrease wind effects on the sand.

What are the potential effects on the health of the corals during a storm or wind reversal? During a wind reversal or a storm, there is a high chance that waves will reach the retention wall and wash over it. Consequently, large amounts of sand could suddenly be transported onto the reef and cause massive mortality of corals by burial. The fine sand is likely to be transported downstream (generally in a northerly direction) and may lead to decreased light levels, increased sedimentation, and possibly mortality of corals. The exact transport of the sand is very difficult to predict without extensive measurements of current patterns at the location. We recommend that the retention wall be raised to the prescribed height. It may also be an option to put additional structures in place that can be deployed during wind reversals or storms to prevent sand from being washed away.

How important is the origin of the sand, the kind of sand (river or carbonate sand), the quantity of sand, the layout of the beach, and the constructed wall for the effects? The origin of the sand is very important. Since the sand is not carbonated, it will probably not be processed similarly to carbonated sand. It may be less likely to be cemented into the reef. Compared with natural sand in front of the artificial beach, the grain size distribution is much finer, indicating that the sand, once it travels over the retaining wall, is likely to be washed away by waves and currents. Once the sand washes away, all the negative effects of increased sediment load and sedimentation may occur. The exact location of the beach and the layout, as well as the buildings around the beach, influence the wind erosion of the beach. In this case, local wind conditions transport the sand toward the sea. Even if the retention wall is raised, the sand will continue to be piled up at the downwind side of the beach and will probably need to be redistributed regularly to not end up in the sea. We recommend periodical reinspection and redistribution to ensure that sand is not transported over the retention wall.

Overall, local wind, waves, and current conditions play a major role in the fate of artificial and natural beaches. Since all artificial beaches are on the island’s leeward side, the prevailing wind always blows the sand seaward. Increasing wall height and constructing wind blockers upwind of the beach may provide extra protection against the sand being blown away. Very high waves can accompany wind reversals. This may lead to massive sand transport onto the reef, consequently smothering and killing coral colonies. A general recommendation is to re-evaluate all artificial beaches given the expected consequences of climate change, such as sea level rise and an increase in the frequency of tropical storms, as this may have strong negative consequences for the reef. Current requirements for artificial beaches may have to be reconsidered.