HR: 14:45h
AN: H23C-06    [Abstracts]
TI: How Much Water and Suspended Sediment Does a Large Tropical Island Shed During a Major Hurricane? Hydrologic and Geomorphologic Effects of Hurricane Georges, September 1998, Puerto Rico
AU: * Larsen, M C
EM: mclarsen@usgs.gov
AF: Matthew C. Larsen, U S Geological Survey 436 National Center, Reston, VA 20190 United States
AU: Webb, R M
EM: rmwebb@usgs.gov
AF: Richard M. T. Webb, U S Geological Survey Denver Federal Center, Denver, CO 80225 United States
AU: Warne, A G
EM: deceased
AF: Andrew G. Warne, U S Geological Survey 651 Federal Drive, Guaynabo, PR 00965 United States
AB: On September 21-22 1998, Hurricane Georges, a category-3 hurricane on the Saffir-Simpson scale, produced heavy rainfall, flooding, and landslides in the mountains and coastal plains of Puerto Rico. In general, rainfall, runoff, and sediment yield vary across the 8,711 square kilometer island of Puerto Rico because of an orographic barrier, the Cordillera Central. Mean annual runoff for the island is estimated to be 910 mm (equal to 8 billion cubic meters), which is about 57 percent of mean annual precipitation: 1,600 mm (14 billion cubic meters). Mean annual suspended-sediment discharge from Puerto Rico into surrounding coastal waters is estimated at 5.9 (+/- 3.2) million metric tonnes. The largely mountainous watersheds of the island are small (tens to hundreds of square kilometers), channel gradients are steep, and most stream valleys tend to be well-incised and narrow. Major rainstorms are intense but brief. As a consequence, flood waters rise rapidly (minutes to tens of minutes) with peak discharges several orders of magnitude above base discharge, and flood waters recede quickly (hours). Major storms transport a substantial portion of suspended sediment from uplands to the coast, based on data from a set of nine suspended-sediment monitoring stations representative of typical conditions in Puerto Rico. During Hurricane Georges, U. S. Geological Survey and National Weather Service rain-gage networks recorded 2-day rainfalls that ranged from about 100 mm to 630 mm (average was 300 mm, equal to about 2.6 billion cubic meters of water). Many streams rose more than 5 meters, resulting in severe flooding in northern, southwestern, and western watersheds. Landslides dissected roads and isolated communities on both the northern and southern slopes of the Cordillera Central. More than twice the mean annual discharge flowed from some watersheds (approximately 1 billion cubic meters of water for the entire island) carrying with it one to six times the mean annual load of sediment from some watersheds. A total of approximately 2.4 million metric tonnes of suspended sediment were discharged from the island to the coastal areas and insular shelf, equal to an average suspended-sediment yield of 280 metric tonnes per square kilometer. Suspended-sediment concentrations for streams draining the steeper, drier, and less vegetated southern watersheds were four to five times greater than the concentrations for streams draining the lower relief, wetter, and more vegetated northern watersheds. The runoff from this single storm was about 13 percent of mean annual runoff and was responsible for about 40 percent of mean annual suspended-sediment discharge from the island.
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
SC: Hydrology [H]
MN: 2005 Joint Assembly