--> Abstract: Downstream Control from Delta Distributary Channel Avulsions, by Douglas A. Edmonds, David Hoyal, Benjamin A. Sheets, and Rudy L. Slingerland; #90078 (2008)

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Downstream Control from Delta Distributary Channel Avulsions

Douglas A. Edmonds1, David Hoyal2, Benjamin A. Sheets2, and Rudy L. Slingerland1
1Department of Geosciences, Pennsylvania State University, State College, PA
2ExxonMobil Upstream Research, Houston, TX

In distributary deltas, the locations of suitable reservoir sands are a function of the distributary channel network. Channel geometry and topology is determined by two primary channel formation processes: channel bifurcation around a river mouth bar and full or partial channel avulsion. While previous work has focused on how channel bifurcation affects network geometry, the role of channel avulsion has been difficult to assess. To understand the processes and length scales associated with channel avulsion in deltas we use physical experiments consisting of a river carrying a cohesive sediment mixture entering a still body of water under constant inlet and boundary conditions. We collected data on the dynamic evolution of five separate distributary channel and river mouth bar complexes from inception to avulsion. For each complex, we recorded millimeter-scale changes in the bed and water surface topography and the cross-sectional-average channelized flow velocity at fifteen minute intervals.

Results confirm that a class of distributary channel avulsions is associated with the growth of a downstream river mouth bar. In response to the growing river mouth bar, the upstream water surface elevation and slope increase. As this ‘morphodynamic backwater effect’ propagates upstream it causes increased overbank flow and channel bed aggradation, which triggers an avulsion. The location of the avulsion is a function of preexisting topography and the magnitude of the backwater effect. We hypothesize that the magnitude of the backwater effect is controlled by the channel Froude number and river mouth bar geometry. Because these are a function of distributary network topology at any time, the location of avulsions and therefore the location of distributary reservoir sands can be predicted.

 

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas