HR: 11:35h
AN: H42B-06    [Abstracts]
TI: Estimating Net Bank Erosion Rates From the Floodplains of Meandering Rivers
AU: * Lauer, J W
EM: laue0050@umn.edu
AF: University of Minnesota, Saint Anthony Falls Laboratory 2 3rd Avenue S.E., Minneapolis, MN 55414 United States
AU: Parker, G
EM: parke002@umn.edu
AF: University of Minnesota, Saint Anthony Falls Laboratory 2 3rd Avenue S.E., Minneapolis, MN 55414 United States
AB: Eroding streambanks are sometimes cited as net sources of sediment for rivers. If generally true, this presents an obvious mass balance problem: If rivers continually remove material from active floodplains without replacing all of it, then the floodplains should eventually disappear. For graded river/floodplain systems, then, deposition on the floodplain must, over the long term, balance what is eroded. However, there are at least two reasons why the net erosion due to bank migration, defined as the volume eroded from cut banks minus the volume deposited on point bars, should usually be positive. First, rivers generally migrate into natural levees that are somewhat higher in elevation than the rest of the floodplain. Since point bars are not built as high as natural levees, this represents a net loss of material from the floodplain. Second, river bends tend to migrate outwards, expanding over time. Since the eroding bank is invariably longer than the depositional bank, more material is eroded than deposited, even if the elevation at the top of both banks is constant. This leads to a steady increase in channel sinuosity over time until a cutoff occurs. For a floodplain that is in equilibrium, the erosion caused by natural levee recycling should be balanced primarily by overbank deposition, while the erosion caused by the systematic sinuosity increase should be balanced primarily by depositional processes in abandoned stream courses or oxbow lakes. Until now, it has not been clear which of the two processes is generally more important. This study presents a comparison of their relative importance, as well as system-wide net erosion rates, for portions of three U.S. rivers: a 91 km reach of the Pearl River in Louisiana, a 62 km reach of the Bogue Chitto River in Louisiana, and a 35 km reach of the Neuse River in North Carolina. The study is made possible by high resolution LIDAR datasets along these systems that represent the topography of the natural levees and newly constructed point bars in some detail. Channel migration rates taken from sequences of historic aerial photographs, together with the detailed topography, allow the levee recycling rate to be approximated. The sequential photos also allow the rate of channel sinuosity increase to be computed, which, combined with a characteristic bankfull geometry, allows the flux associated with the sinuosity increase to be estimated. For the systems studied here, the flux due to the sinuosity increase is generally greater than the flux due to levee recycling.
DE: 9350 North America
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1640 Remote sensing
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting