Filgueira-Rivera, Manuel1, Norman D. Smith1
(1) University of Nebraska-Lincoln, Lincoln, NE
ABSTRACT: Controls on Natural Levee Development in the Columbia River, British Columbia, Canada
Natural levees, although ubiquitous features of alluvial floodplains, remain poorly characterized and understood. This study investigates levees of the Columbia River near Golden, British Columbia, to identify the mechanisms that control levee development and morphology. Topographic profiles of 24 levees were surveyed, and measurements of water-surface elevation, flow velocity and direction, and turbidity were obtained during an average magnitude flood. Sedimentation rates and grain size distributions were measured with sediment traps along levee-to-floodbasin transects. Results show that channel-to-floodbasin mass exchange is predominantly advective, and despite expectations based on previous works, diffusive transfer was not observed over mature levees. During flooding, the floodbasins behave as efficient water pathways characterized by relatively competent flows capable of limiting basinward levee growth. Levee shape results from two independently operating factors: maximum channel stage, which controls height, and floodbasin hydraulics, which controls width. The competence of floodbasin flows, enhanced by confining valley walls, results in relatively narrow levees (compared to many alluvial rivers) by impeding deposition in their distal portions. Levees grow under two conditions of deposition as governed by flood magnitude: frontal loading and back loading. During large floods when levee crests are inundated, frontal loading preferentially aggrades the proximal levee, increasing levee slope. During lower magnitude floods when most levees are only partially inundated, back loading preferentially aggrades the floodplain and distal levee, reducing slope. In the study area, a balance between frontal and back loading sustains these narrow and steep levee shapes for long time periods.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.