HR: 11:50h
AN: H51F-07    [PDF]
TI: Measuring and Modeling the Effects of Development on Sediment Production and Delivery, St. John, U.S. Virgin Islands
AU: * Ramos-Scharron, C E
EM: cramos@cnr.colostate.edu
AF: Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523 United States
AU: MacDonald, L H
EM: leemac@cnr.colostate.edu
AF: Department of Forest, Rangeland, and Watershed Stewardship, Colorado State University, Fort Collins, CO 80523 United States
AB: Increases in sediment delivery rates into tropical marine environments may pose a serious threat to nearshore coral reef communities. The island of St. John in the U.S. Virgin Islands is at particular risk because the areas outside of the Virgin Islands National Park are being subjected to rapid development, 80% of its land surface has slopes greater than 30%, and the average annual erosivity is 13,500 MJ mm ha$^{-1}$ hr$^{-1}$. The objectives of this study were to: (1) measure erosion rates from a variety of anthropogenic and natural sources; and (2) develop a GIS-based model to estimate basin-scale increases in sediment delivery. Sediment production rates were measured from unsurfaced roads, road cutslopes, undisturbed hillslopes, streambanks, and treethrow. Unsurfaced roads had the highest erosion rates with values of up to 38 kg m$^{-2}$ yr$^{-1}$, while road cutslopes generated only about 0.8 kg m$^{-2}$ yr$^{-1}$. Since undisturbed areas generated only 0.001 kg m$^{-2}$ yr$^{-1}$, unsurfaced roads can increase hillslope-scale sediment production rates by up to four orders of magnitude. Sediment production rates from erodible streambanks were estimated to be nearly 13 kg m$^{-2}$ yr$^{-1}$, while uprooting of trees along stream margins was estimated to deliver 0.17 kg yr$^{-1}$ of sediment per meter of stream length. The St. John Erosion model (STJ-EROS) uses a series of empirical sediment production models and sediment delivery ratios to estimate watershed-scale sediment yields in a GIS-based system. Using this model, the estimated sediment yields for the highly-developed 6.0 km$^{2}$ Fish Bay basin was 41 tons km$^{-2}$ yr$^{-1}$, while the estimated sediment yield for the less-developed 4.3 km$^{2}$ Lameshur Bay basin was only 11 tons km$^{-2}$ yr$^{-1}$. The model simulations suggest that current sediment yields are seven and four times above undisturbed conditions in the Fish Bay and Lameshur Bay basins, respectively. The predicted sediment yields are consistent with measured sediment yields and sedimentation rates in both Fish Bay and Lameshur Bay. In both cases actively used unsurfaced roads were identified as the dominant source of sediment. Cutslopes and streambanks played a secondary role in total sediment yields, while undisturbed hillslopes and treethrow contributed only minimal amounts.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
SC: Hydrology [H]
MN: 2003 Fall Meeting