Flooding, flow path selection and growth of alluvial fans and deltas

by Douglas J. Jerolmack

Tuesday 11 May 2010, 16:00 → 18:00 Europe/Rome

Aula Conversi (Dip. di Fisica - Edificio G. Marconi)

The surfaces of alluvial fans and river deltas are often dissected by a small number of channels. On long timescales, channels migrate via processes of deposition and abandonment (i.e., avulsion) that often result in catastrophic flooding and loss of life on densely populated fans. What governs the selection of new flow paths is poorly understood. The experimental system we present strongly channelizes in one location until localized shoreline progradation diminishes transport capacity of the channel, resulting in backfilling and subsequent widespread flooding; avulsion is completed when a new channel path is selected. This cycle occurs with a periodicity that is predictable from conservation of mass, and results in fluctuations around an equilibrium slope. Selection of a new flow path is inherently stochastic; we observe, however, that previously abandoned channels act as significant attractors for the flow. We demonstrate that a directed random walk model with memory quantitatively reproduces these limiting dynamics and is consistent with natural fans. Thus, channel migration behaves as a history-dependent, threshold-driven, stochastic process. Experiments also provide another example of realistic morphodynamics arising in (near-) laminar flows, indicating that macroscopic fan evolution does not depend on the details of within-channel sediment transport.