Controls on natural levee development in the Columbia river, British columbia, Canada

摘要

Natural levees of the Columbia River near Golden, British Columbia, were investigated to identify the mechanisms that control levee development and morphology. Topographic profiles of 12 levee pairs were surveyed, and measurements of water-surface elevation, flow velocity, flow direction and turbidity were obtained during an average magnitude flood (1 center dot 2 years recurrence interval). Sedimentation rates and grain-size distributions were measured from sediment traps placed along levee-to-floodbasin transects. Results show that water and sediment exchange between the channel and floodbasin was mainly by advection. During flooding, local floodbasins behave more as efficient water pathways than water storage features, resulting in down-valley floodbasin flows capable of limiting basinward growth of levees. Levee shape results primarily from two independent factors: (1) maximum channel water stage, which limits levee height; and (2) floodbasin hydraulics, which control width. In the Columbia River, the competence of floodbasin flows results in relatively narrow and steep levees. Natural levees grow under two general conditions of deposition as governed by flood-stage elevation relative to levee-crest elevation: front loading and back loading. During large floods when crests are inundated, front loading preferentially aggrades the proximal portions of levees with sediment directly from the channel, thus increasing levee slope. During average or below-average floods when many levee crests are not overtopped, back loading preferentially aggrades the distal levee areas and floodbasin floor, reducing levee slope. In the study area, a balance between front and back loading sustains these narrow and steep levee shapes for long periods, reflecting an equilibrium between hydraulic regime, floodplain morphology and deposition.