HR: 16:15h
AN: H44D-02 INVITED     [Abstracts]
TI: The Morphodynamics of Sub-aerial Natural Levees
AU: * Slingerland, R
EM: sling@geosc.psu.edu
AF: Department of Geosciences, Penn State University, 513A Deike Bldg, University Park, PA 16802 United States
AU: Smith, N D
EM: nsmith3@unl.edu
AF: Department of Geosciences, University of Nebraska, 217 Bessey Hall, P.O. Box 880340, Lincoln, NE 68588-0340 United States
AB: As important as natural levees are to floodplain conveyance systems, it is surprising that they have yet to be quantitatively characterized and related to channel and floodplain hydraulics. We do not yet know what factors determine their presence or absence, nor can we predict their basic morphometry as a function of the relevant hydraulic factors. A survey of natural levees from the literature and particularly from the anastomosed reach of the Columbia River in British Columbia, and the Cumberland reach of the Saskatchewan River, Saskatchewan, indicates that the morphology of natural sub-aerial levees depends upon levee age, channel size, and the maximum height to which waters are ponded during floods or storm surges. Vegetation, grain size, floodbasin width, channel stability, and rate of channel alluviation exert secondary controls. Slopes of levees along young channels tend to be markedly steeper than those along mature channels, suggesting an asymptotic approach to a dynamic equilibrium height. Levee slopes also decline exponentially with increasing floodbasin width. Levee widths are highly variable, ranging from fractions of channel width to over 10 for alluvial and deltaic cases. Although data are sparse, levee cross-sectional area appears to scale linearly with channel cross-sectional area. In alluvial and deltaic levees, one levee of a pair is often significantly higher, wider, or steeper than the other, with no systematic variation along a channel. We conjecture that the origin of these relationships lies in the relative volumes of sediment transferred from the channel to the floodbasin via two mechanisms: diffusion and advection. Diffusive transfer occurs when turbid turbulent eddies along the channel boundary during flooding spin off onto the floodbasin and decelerate, allowing grains to settle at distances determined by channel geometry, floodplain roughness, particle size distribution, and flow character. Advective transfer occurs when turbid flows leave the channel as channel water overtops the banks or exits through crevasses to deposit sediment on the distal areas of levees. Although hard data are lacking, we suspect that both transfer mechanisms are significantly influenced by the baffling effects of levee vegetation. Dynamic equilibrium levee height is set by a channel-shaping bankfull flow that has just enough power to lift bed material to the levee top. Levee width is set by the dynamics of floodbasin filling.
DE: 1820 Floodplain dynamics
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: Fall Meeting 2005