ABSTRACT
Coastal areas of the Pacific Islands are among the most vulnerable areas to climate change, with sea-level rise, increased cyclone activity and ocean acidification posing an immediate threat to rich ecosystems and their biodiversity. These threats can be amplified by some land use practices (crop production, logging), which generate considerable amounts of sediment and pollutants reaching in river deltas and coastal lagoons.
This contribution focuses on current and future sediment impacts of a tropical river that discharges on a coastal ecosystem in one of Fiji Islands, the mangrove wetlands of the Dreketi delta. The river drains a catchment with a land use dominated by forest and grasses, but with an important proportion of agriculture. Cyclones are an important contributor to the river sediment output into the wetlands, with up to 40% the annual sediment discharge directly associated with a cyclone event.
In order to identify the impact of cyclones on the river annual sediment yield we used a physically-based hydro-sedimentological model. We continuously simulated flow and sediment discharges at the outlet (1970-2017) and identified the proportion of time influenced by cyclones. Cyclone data was obtained from the Southwest Pacific Enhanced Archive of Tropical Cyclones. The analysis allowed us to determine the per-centage of the annual sediment budget that can be associated with cyclones. We then looked at the impacts of a projected increase in cyclone activity due to climate change on the sediment delivery of the catchment for different land use scenarios and assessed the impact on the mangrove wetlands.
