Sediment Deposition Volume Assessment in Tropical Regions

Floods and associated landslides account for a large number of natural disasters and affect many people wherever they occur (Hong et al., 2007). Sediment-free floods are rare, and in most cases, floods carry a notable amount of sediments (Acreman, 2016). Mass movement processes also transport a huge amount of sediments within a short period (Varnes, 1978). The mobilized sediments cause significant costs and damages as soon as they reach urban or rural environments. These damages and costs include (but are not limited to) cleaning or dredging cost, damage to contents of buildings (e.g. furniture, electric appliances), and blockage of drainage and sewer systems which get filled up with sediments (Einstein, 1950; Merz et al., 2010; Rodríguez et al., 2012).

This study aims to achieve a reliable sediment deposition quantification which is useful for assessing the risk of such events. Three methods were implemented for this purpose. First, the sediment deposition height was determined through in-situ investigation and the average height was estimated. Second, the deposition surface was simulated using trend interpolation and DEM was subtracted from that to get deposition height. Third, the deposition height and extent were determined by calculating the difference in elevation using pre- and post-event drone and LiDAR flights.

Dominica has experienced sediment deposition events in the past. It is significantly vulnerable to tropical storms and hurricanes. Dominica is a mountainous island covered by tropical rainforests and located about halfway between the French islands of Guadeloupe and Martinique in the Eastern Caribbean sea (Knutson et al., 2015; Malhotra et al., 2007; Wilkinson, 2018). Hurricane Maria made landfall on this island on September 18^th, 2017 and it heavily impacted the housing, transport infrastructure, tourism, agriculture, and education sectors (Dominica News Online, 2018). The intense rainfall caused flash floods, landslides, and debris flows resulting in a massive amount of sediments being deposited in urban and rural areas. The overall damages and losses are estimated at approximately USD 1.3 billion (The Government of the Commonwealth of Dominica, 2017). Dominica’s Ministry of Public Works reported that the total cost related to deposition of sediments (e.g. dredging rivers, cleaning streets and main roads, and clearing of airports and seaports)  exceeds USD 92 million which is a considerable portion of total damages and costs. This implies the significance of the risk imposed by sediment deposition.

The results of this research were compared with each other and with the findings of in-situ investigations. They indicate similar deposition heights and volumes, however, the pattern and extent of deposition are not the same. The practicality of the third method depends on the availability of data, but when data is available the outcomes provide a reliable assessment of sediment deposition volume. However, this cannot be trusted unless an in-situ investigation is performed.