Geometry, Topography and Flow Expansion in Depositional Lobes

Paola, Chris¹, Luca Sittoni¹, Vaughan Voller¹ (1) University of Minnesota, Minneapolis, MN

A key element of both fluvial and marine distributary systems is lobes characterized by expanding flows. In both environments, the lobes are generally thought to be depositional. It has also been suggested that some or all depositional lobes may be associated with jet flows. We compare the spatial distribution and geometry of depositional lobes and associated sheet deposits in submarine and subaerial experimental depositional systems. These include numerous cases with expansion angles too large to be created from jet flow alone. We then use data from subaerial experimental systems to examine the geometry of the lobes as they evolve, emphasizing how the bed topography evolves to allow for large expansion angles. The radial flow pattern from which the lobe develops is forced by a depositional arc at the downstream end of the feeder channel. As the lobe topography evolves, the initial depositional phase is followed by a bypass phase in which the flow remains expansional but streamwise and transverse gradients in sediment flux cancel one another. Self-channelization occurs during this bypass phase. Transverse surface curvature nondimensionalized by width decreases downstream consistently. Hence measured curvature values may be useful in predicting relative downstream position in lobe deposits.