HR: 0800h
AN: H51E-0788    [Abstracts]
TI: Decadal to Centennial Variability of Erosion Rates in a Rapidly Degrading Channel Network
AU: * McElroy, B
EM: bmcelroy@geo.utexas.edu
AF: University of Texas, Jackson School of Geoscience 1 University Station, C1100, Austin, TX 78712-0254,
AU: Willenbring, J
EM: staig001@umn.edu
AF: University of Minnesota, Dept. of Geology and Geophysics 310 Pillsbury Dr. SE, Minneapolis, MN 55455,
AU: Mohrig, D
EM: mohrig@mail.utexas.edu
AF: University of Texas, Jackson School of Geoscience 1 University Station, C1100, Austin, TX 78712-0254,
AB: Quantifying short-term spatial variation of rates of vertical land surface change is a challenge in net degradational systems, but this information has the potential to add significantly to our understanding of transport-limited erosional processes acting over long timescales. We have undertaken a dendrogeomorphic approach to assessing denudation rates in a young channel network on the Florida panhandle where unlithified sands and muds are being incised by water supplied from groundwater seepage. The measured exposure of roots for over 500 trees, combined with tree ages, supply surface change rates along a 2 km reach of channel. This number of samples represents all trees greater than 2" diameter from ~1/3 of the valley bottom area within the study section. Tree rings were counted for cores from 33 trees ranging up to ~200 years in age. From these trees an age- diameter relation was determined to estimate ages for the remaining ~500 trees with ~12% standard error. Mean and maximal lengths of root exposure covaried with tree age resulting in characteristic erosion rates valid over decadal to centennial timescales. Three types of surface change rates were measured: vertical motions of the channel bottoms, vertical motions of the valley bottoms, and lateral motions of the channel sidewalls. Their means are 2.8, 1.7, and 7.3 mm/yr, respectively, and support field observations that suggest the valley bottom is denuding by a process independent of the channel lowering. The vertical erosion rates also covary with tree ages in such a way that mean rates decrease by half an order of magnitude for an order of magnitude increase of the averaging timescale. This result is direct evidence for the unsteadiness of sediment transport within the network. Based upon Central-Limit theory, it also suggests that the decorrelation timescale of the sediment transport process is within the range of decades to centuries. The timing of inception for this network is not well constrained. A rough estimate can be made using the total channel length and the erosion rates measured here by assuming that the steephead propagation rate is related to the valley lowering rate through the streamwise valley slope. For a mean valley lowering rate of 2 mm/yr and a slope of 25 m/km, the whole network could have been created in approximately ~100,000 years. This age can be refined with the development of a local erosion rate-erosion duration relationship.
DE: 1815 Erosion
DE: 1824 Geomorphology: general (1625)
DE: 1862 Sediment transport (4558)
SC: Hydrology [H]
MN: 2007 Fall Meeting