HR: 11:20h
AN: OS51D-05    [PDF]
TI: Critical Shear Stress and Erosion Rates on the Western Adriatic Shelf
AU: * Wiberg, P
EM: pw3c@virginia.edu
AF: University of Virginia, Department of Environmental Sciences PO Box 400123, Charlottesville, VA 22904=4123 United States
AB: One of the fundamental limitations on our ability to predict sediment transport rates in fine-grained marine settings is our poor understanding of spatial and temporal changes in critical shear stress and erosion rates. Temporal variations in these quantities can be the result of consolidation as well as seasonal and storm-related changes in deposition (e.g., degree of flocculation of settling particles) and bed surface properties (e.g., roughness, biogenic coatings, organic content). Spatial variations may be related to changes in sediment composition, grain size, water depth, distance from sediment sources and changes in hydrodynamic conditions. The western Adriatic shelf provides a low-to-moderate energy, fine-grained shelf setting with a large sediment source (the Po River) and a number of smaller sources along the Apennine coast. Critical shear stress and erosion rates were measured along the 20-m isobath between the Po River mouth and the Gargano Promontory during the winter (February) and spring/summer (June) of 2003 to investigate spatial and temporal effects on their values. Measurements were made using a Gust erosion chamber fitted onto the top of 10-cm-diameter core collected using a hydraulically damped short (~1 m) piston corer, which minimized disturbance to the bed surface. The erosion chamber applied a range of known stresses (0.01 - 0.4 Pa) to the sediment surface of the core, each of which was maintained for 20-30 min. Water in the chamber was replaced by pumping seawater into the chamber; water leaving the chambers passed through a turbidity sensor that recorded turbidity levels every 0.5s. The water was then collected in bottles, filtered, dried and weighed to obtain the mass of eroded sediment. The results were used to calibrate the turbidity measurements. Filtered material was combusted to obtain organic fraction. Two to four replicates were made at each site. The measurements show a clear seasonal signal with higher erosion rates during winter when organic content was lower. Erosion rates vary spatially, but not with any consistent alongshelf trend. Erosion rates are higher near several river mouths and toward the southern end of the sampling line. Parameterizations of erosion rates based on the measurements are being used in a shelf-sediment transport model and compared to near-bed tripod measurements of suspended-sediment concentration.
DE: 3022 Marine sediments--processes and transport
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting