SPATIAL DISTRIBUTION OF AVULSIONS ON MODERN DELTAS

Avulsion is a primary mechanism for creating distributary channels in deltaic settings. Modern deltas provide information about the spatiotemporal distribution of avulsions in the form of relict-channel scars and bifurcations (partial avulsions) of active channels. Floodplain deposition acts to destroy this information by progressively erasing channel scars. Channel features on the surface of a modern delta thus preserve a relatively short history of avulsion processes. Avulsion mechanics is poorly understood, but most models treat the process as an instability arising from super-elevation of a channel relative to its floodplain, suggesting that, in principle, avulsion could operate anywhere on the delta surface. Cursory inspection of modern deltas suggests that avulsions are distributed non-uniformly. For a given delta, some of this non-uniformity simply reflects the instantaneous channel arrangement; the remainder, particularly the radial or ‘dip’ component of the avulsion distribution, may provide insight into the workings of avulsion processes on deltas. To address this possibility, we mapped avulsions (local, nodal, random, and partial) on a large set of modern deltas that encompass a broad range of delta geometries (size and opening angle) and river and wave parameters. Our analysis focuses on how the density of avulsion sites varies with normalized distance from the delta apex for subsets of deltas with similar parameters. The spatial distributions of avulsions show considerable variability across the set of deltas we analyzed. Using number density of mapped avulsion sites as a proxy for avulsion frequency, our preliminary results suggest that the frequency of the largest-scale nodal avulsions decreases with increasing opening angle and, to a lesser extent, with delta length. Furthermore, as delta length increases, the overall frequency of random and partial avulsions increases; the distribution of random and partial avulsion sites becomes more uniform; the number of local avulsions increases; and the peak in the spatial distribution of local avulsions shifts away from the delta apex. While the relationships are less clear, our observations suggest that an increase in wave energy reduces the frequency of all types of avulsions.