Planform evolution of deltas with graded alluvial topsets: Insights from three-dimensional tank experiments, geometric considerations and field applications.

The profile of a river that conveys sediment without net deposition and net erosion is referred to as 'graded' with respect to vertical aggradation of the river segment. Three experimental series, designed in terms of the autostratigraphic view of alluvial grade, were conducted to clarify the diagnostic spatial behaviour of graded alluvial-deltaic rivers: an 'R series', which utilized a moving boundary setting with a stationary base level; an 'F series' in a fixed boundary setting with a stationary base level to produce 'forced grade'; and an 'M series' in a moving boundary setting with constant base-level fall to produce 'autogenic grade.' The results of the three experimental series, combined with geometrical modelling of the effects of basin water depth and other experimental data, suggest the following: (i) in a graded alluvial-deltaic system, lateral shifting and avulsing of active distributary channels are suppressed regardless of whether the downstream boundary of the deltaic system is fixed; (ii) in a delta with a downstream-fixed boundary, the graded streams are stabilized within a valley that is incised in the axial part of the delta plain, whereby the alluvial plain outside the valley is abandoned and terraced; (iii) in moving boundary settings, the graded river simply extends basinward as a linearly elongated channel and lobe system without cutting a valley; and (iv) a modern forced-graded alluvial river is most likely to be found in a valley incised into a fan delta in front of very deep water, and the stratigraphic signal of fossil autogenic-graded rivers will be found in deltaic successions that accumulated in the outer to marginal areas of deltaic continental shelves during sea-level falls. This renewed autostratigraphic view of alluvial grade suggests a thorough reconsideration of the conventional understanding that an alluvial river feeding a progradational delta is graded with a stationary base level. [ABSTRACT FROM AUTHOR]