SPATIAL PATTERNS OF EROSION ON DOMINICA, A VOLCANIC TROPICAL ISLAND

We fingerprinted sediment sources and measured erosion rates on the volcanically active island of Dominica, located in the Lesser Antilles. Because of the steep volcanic topography and frequent tropical storms, landslides are a major erosional process on this island. Although natural hazards are a concern in Dominica, little prior research has been conducted on the effects of landslides on erosional patterns in Dominica. Our work seeks to fill this gap. We spatially characterized 20 river basins (>10 km2) using detrital fallout radionuclides, in situ 10Be-derived erosion rates, and river incision rates.

In these watersheds mean slope = 19.6, mean annual rainfall = 1981.41 mm/yr, and the land is 89% forested; in addition, 89% of the study area is underlain by block-and-ash flows and ignimbrites. We used topographic data and published ignimbrite ages to calculate river incision rates, which ranged from 0.448 - 113.9 mm/yr in the north and 0.86 - 44 mm/yr in the south. We mapped 1615 recent landslides in Google Earth. We hypothesize that the slope, rainfall, land use, and geology of the different basins correlate with landslide frequency and erosional characteristics of each watershed.

Fluvial sediment collected from active channels will be used to compare geologic erosion rates with calculated incision rates and to fingerprint sediment sources. Basin average erosion rates will be compared to local parameters to determine primary controls on erosion. Fallout radionuclides 7Be, 210Pbex, and 137Cs; XRD analysis of clay minerals; and the concentration of acid-extractable grain coatings will be used to characterize the sediment sources for watersheds. Comparing two grain sizes (< 63 and 250-850 μm) allows us to distinguish between sources of fine and medium grained sediment. Differences in the formation, rate of deposition, and half-life in each radionuclide will allow us to interpret the rate and depth of erosion. Clay minerals and grain coating composition and concentration will provide information on the degree of weathering of eroded sediment. Finally, we will compare these properties with basin average statistics in order to better understand tropical sediment source dynamics, which can further landslide mitigation in Dominica.