HR: 1330h
AN: OS23B-06    [Abstracts]
TI: A Refined Numerical Model for Sorted Bedform Formation and Evolution
AU: * Murray, A B
EM: abmurray@duke.edu
AF: Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences/Center for Nonlinear and Complex Systems, Duke University, Box 90230 , Durham, NC 27708-0230 United States
AU: Coco, G
EM: c.coco@niwa.co.nz
AF: National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, New Zealand
AU: Green, M O
EM: m.green@niwa.co.nz
AF: National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, New Zealand
AU: Hume, T M
EM: thume@niwa.co.nz
AF: National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, New Zealand
AU: Thieler, E R
EM: rthieler@usgs.gov
AF: U. S. Geological Survey, Coastal and Marine Geology Program, 384 Woods Hole Road, Woods Hole, MA 02543-1598 United States
AB: Murray and Thieler (2004) hypothesized that a feedback operating on a poorly sorted seabed, and subsequent emergent interactions, lead to development of large-scale grain-size sorted patterns (`sorted bedforms') on inner continental shelves that have traditionally been referred to as `rippled scour depressions.' Unlike familiar bedforms that grow because of an interaction between bed topography and sediment flux, the initiation and evolution of these sorted bedforms results from a coupling between bed composition and sediment flux. In essence, larger wave ripples form in coarser patches, and these inhibit fine-sediment accumulation and enhance fine sediment flux over the patch by enhancing vertical mixing. As an initial test of the hypothesis, Murray and Thieler developed an exploratory numerical model that produced sorted bedforms exhibiting the main characteristics of the patterns observed in nature. Murray and Thieler's initial model parametrically treated the coupling between bed composition and sediment flux. We further developed the model, incorporating more detailed parameterizations, including: 1) empirical predictions of ripple dimensions as a function of bed composition and near-bed wave conditions; 2) near-bed wave-orbital motions that do not involve a shallow-water assumption; 3) calculations of the vertical profiles of suspended-sediment concentration and mean-current velocities that result from wave and current motions interacting with the ripples; and 4) bedload transport. We tested the sensitivity of model behavior to the parameterizations adopted by conducting numerical experiments using wave, depth, and current conditions measured off the coastline of Tairua (New Zealand). We performed two tests: a comparison of predicted ripple characteristics and sediment concentration profiles with tripod observations; and a comparison of the large-scale geometry emerging from model simulations with some of the features observed during bathymetric surveys. Further numerical experiments were aimed at determining which combinations of forcing conditions promote and which suppress the development of sorted bedforms, and how the seabed patterns develop under time-varying forcing. Unsteady wave and current conditions appear to substantially affect the growth of sorted bedforms, and can lead to the development of more complex patterns. Adding net sediment accretion to the model can, under some conditions, lead to the burial of sorted bedforms, which is in agreement with stratigraphic field observations from Tairua. The Andrew W. Mellon Foundation and the Foundation for Research, Science and Technology (contract C01X0401) supported this work.
DE: 3210 Modeling
DE: 3220 Nonlinear dynamics
DE: 4546 Nearshore processes
DE: 4558 Sediment transport
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly