Effects of tropical cyclones on catchment sediment delivery to coastal ecosystems

Coastal areas in the tropics are remarkably vulnerable to extreme weather-related events (e.g. floods, storm surge, erosion), which are expected to intensify in the future. Erosion of catchments is affected by changes in climate and vegetation, which alters the production of sediment transported into coastal areas and can lead to the loss of important ecosystem services. In many areas in the tropics, cyclones and depressions (TCs) produce the largest floods and massive discharges of sediment, which can affect coastal habitats in wetlands, lagoons and reefs. Here we focus on the Pacific Islands, but the methodology can be used in any tropical catchment affected by cyclones. A hydro-sedimentological model was used to analyse the sediment outputs of the Dreketi River (Fiji) and their relationship with cyclone activity over the last 45 years. The model was developed based on satellite and ground information on topography, soils, land use, and climatic data. The sediment export was related to TCs in the area, which were identified using the Southwest Pacific Enhanced Archive of Tropical Cyclones (SPEArTC) database. TC-related precipitation events were found to be three times more intense than non-TC events, and the average annual contribution of TCs to sediment export was about 25 %, with years contributing more than 50 %. TC-related events produced six times the sediment export of non-TC events, indicating a strong relation with precipitation intensity. Relations between precipitation rate and sediment export were developed for both TC and non-TC events, which can be used to predict the effects of increase of cyclone activity in the future.