Instantaneous Channel Topography Versus Final Shape of Preserved Channel Bodies

Benjamin A. Sheets¹, Chris Paola², Alessandro Cantelli³, and John Martin^2
1 ExxonMobil Upstream Research Company, Houston, TX
² University of Minnesota, Minneapolis, MN
³ University of Illinois, Urbana, IL

We have investigated the relationship between two-dimensional (flow-perpendicular) preserved channel body shape and instantaneous channel topography using high-resolution (space and time) scanline data from an experimental fan delta in which deposition was induced by steady base-level rise. Channel body shape is constructed by repeated abandonment and reoccupation of channel pathways until deposition makes the site unfavorable to flow. Reoccupation leads to measureable channel clustering in the deposit. Incisional events are individually narrower than the preserved channel bodies, so that depositional depth is relatively unbiased but depositional width and aspect ratio are biased to large values. Part of this is due to lateral variation in flow location and direction, but most is channel widening associated with reoccupation and deposition, a fundamental shape-forming process that seems to occur with similar evolution at scales up to those of incised valleys: the channel base is formed during scour events associated with local spatial acceleration and hence channel narrowing, which is followed by deposition that rapidly widens the channel by eroding the banks and transferring sediment to the center. The widening produces a preserved channel body that does not represent the shape of any channel that ever existed. A new analysis of channels with fully dynamic width shows how this shape evolution can be modeled theoretically.