Coastal engineering and large-scale mangrove destruction in Guyana, South America: Averting an environmental catastrophe in the making

Study Number

81

Author

Edward J. Anthony, Nicolas Gratiot

Abstract

This short communication highlights potential destabilisation of the muddy coast of Guyana, South America, caused by large-scale mangrove destruction. The stability of the coast of Guyana, which is part of one of the world’s most extensive mangrove coasts, depends on large mud banks migrating alongshore from the mouth of the Amazon River and on mangrove colonisation of these banks. Under the pressures of economic development, the coastal zone of Guyana is progressively being transformed into agricultural land and aquaculture estates, protected by coastal dikes. These hard coastal defence structures, constructed in recent years, are less effective in dissipating wave energy than mud banks. They also hinder the various processes involved in the consolidation and subsequent mangrove colonisation of these banks, notably by enclosing mature mangrove forests and preventing propagule transport from these forests to mud banks. If unchecked, the progressive breakdown in the mud-bank and associated mangrove system that has led to progradation of the coastal plain of Guyana over the last 5000 years will result in large-scale coastal erosion that can only be countered by further engineering structures at prohibitive costs. The only coastal defence strategy, sound and viable over the long term, with regards to both environmental conservation and cost, consists in restoring a dynamic mud-bank and mangrove system on this wave-exposed coast.

Main Results and Conclusions

• Mangroves naturally provide wave dissipation, and serve to positively contribute to coastal defense. The dissipation also fosters a positive feedback loop that facilitates mangrove colony growth.
  • “Mud banks and their dense mangrove forests generate efficient wave-energy dissipation” (270)
  • “Closer inshore, the remaining energy is gently dissipated by mangroves, with a positive effect on flocculation and sediment trapping (Furukawa et al., 1997).” (270)
  • “The capacity to maintain a rapid and large-scale mud-bank colonisation strategy is thus fundamental if mangroves are to continue playing this important role of coastal protection in complement to the mud banks in South America.” (270)
• Coastal engineering has a negative impact on mangroves. Sea walls and other coastal defenses lead to erosion and liquefaction, which make the environment less favorable for mangrove colonies. As mangrove colonies deteriorate, other coastal defenses become less effective.
  • “The mangrove system in Guyana is also being affected by hard-rock coastal defences aimed at protecting and promoting economic activities. These ventures impinge on terrain hitherto occupied by old mangroves that source the regeneration of newer seaward mangrove colonies on new incoming mud banks.” (271)
  • “…waves are rapidly totally dissipated by seawalls. This results in a very localised input of diffusive turbulence, which facilitates floc breakage and bed erosion, and ultimately leads to a net loss of sediment evacuated offshore by downwelling currents.” (271)
  • “…dikes isolate the old mangrove forests from the sea and act as barriers to the dissemination of propagules that could lead to mangrove regeneration seaward.” (271)
• Mangrove restoration is expensive, and the success of mangrove seedling planting is dependent on location and topography. Nonetheless, retaining a robust mangrove system is an important component of a coastal defense in Guyana, because alternatives that ignore mangroves will have long-term deleterious effects on the coastline.
  • “…the cost of coastal protection in Guyana works out at US$ 2000 (€ 1600) for a 100 m-long compacted earth dike, US$ 350,000 (€ 280,000) for a 100 m-long rock armour dike, and US$ 5000-20,000 (€4000-16,000) for a 100 m x 20 m swathe of replanted mangroves.” (271-272)
  • “The cost of mangrove replanting in Guyana on a comparative basis is, thus, relatively high at US$ 25,000-100,00 ha-1.” (272)

Works Cited

Furukawa, K., Wolanski, E., Mueller, H., 1997. Currents and sediment transport in mangrove forests. Estuar. Coast. Shelf Sci. 44, 301-310