Point bar sedimentation and erosion produced by an extreme flood in a sand and gravel-bed meandering river

Abstract

In fluvial environments, extreme floods are often catalysts for substantial geomorphologic planform change. Despite the significant sediment erosion and deposition associated with high-magnitude, low-frequency floods, their sedimentological products often lack distinctive characteristics. However, the sedimentological expression of extreme floods in sand and gravel-bed meandering rivers has yet to be fully described. The sedimentological characteristics of a 1 in 100 year flood on a point bar of the South Saskatchewan River in Alberta, Canada, are investigated. Digital elevation models from multitemporal LiDAR surveys, field sediment sampling, measured sections, shallow boreholes, and aerial photographs were used to identify regions of sediment deposition and erosion, quantify the amount of vertical elevation change, and describe the sedimentology, stratigraphy and grain-size distribution of the flood deposit. The upstream portion of the point bar was dominated by high-velocity flow and erosion. The majority of sedimentation occurred at the meander-bend apex in lee and scroll bars composed of medium- to coarse-grained sand. Downstream of the bend apex, very fine- to fine-grained sand beds accumulated on ridges and lee bars. Silt, <10 cm thick, was deposited in the point bar interior. No single grain size or sedimentary structure typifies the flood deposit. Sedimentological evidence of extreme floods on point bars are likely best preserved in floodplain strata (rather than in-channel strata) and consist of beds with a coarser texture than what is normally dominant at that position on the point bar. In particular, the presence of gravel beds at high stratigraphic elevations appears indicative of an extreme-flood event. These criteria have been used to identify extreme-flood sediments in other coarse-grained point-bar deposits and may be applicable to the deposits of braided and wandering rivers, or lower-competence meandering rivers.