On the Connections Between Surficial Processes and Stratigraphy in River Deltas
📝 Abstract
We explore connections between surficial deltaic processes (e.g. avulsion, deposition) and the stratigraphic record using a simple numerical model of delta-plain evolution, with the aim of constraining these connections and thus improving prediction of subsurface features. The model represents channel dynamics using a simple but flexible cellular approach, and is unique in that it explicitly includes backwater effects that are known to be important in low-gradient channel networks. The patterns of channel deposits in the stratigraphic record vary spatially due to variation in avulsion statistics with radial distance from the delta’s source of water and sediment. We introduce channel residence time as an important statistical measure of the surface channel kinematics. The model suggests that the mean channel residence time anywhere within the delta is nicely described by a power law distribution showing a cutoff that depends on radial distance. Thicknesses of channel deposits are not uniquely determined by the residence time of channelization. The channel residence time distributions at given radial distances from the source are found to be approximately lognormally distributed, a finding consistent with the scale-dependent radial structure of channel deposits in the stratigraphic record.
💡 Analysis
We explore connections between surficial deltaic processes (e.g. avulsion, deposition) and the stratigraphic record using a simple numerical model of delta-plain evolution, with the aim of constraining these connections and thus improving prediction of subsurface features. The model represents channel dynamics using a simple but flexible cellular approach, and is unique in that it explicitly includes backwater effects that are known to be important in low-gradient channel networks. The patterns of channel deposits in the stratigraphic record vary spatially due to variation in avulsion statistics with radial distance from the delta’s source of water and sediment. We introduce channel residence time as an important statistical measure of the surface channel kinematics. The model suggests that the mean channel residence time anywhere within the delta is nicely described by a power law distribution showing a cutoff that depends on radial distance. Thicknesses of channel deposits are not uniquely determined by the residence time of channelization. The channel residence time distributions at given radial distances from the source are found to be approximately lognormally distributed, a finding consistent with the scale-dependent radial structure of channel deposits in the stratigraphic record.
📄 Content
1 ON THE CONNECTIONS BETWEEN SURFICIAL PROCESSES AND STRATIGRAPHY IN RIVER DELTAS
MICHAEL J. PUMA1,6, RACHATA MUNEEPEERAKUL1, CHRIS PAOLA2,3,
ANDREA RINALDO4,5, IGNACIO RODRIGUEZ-ITURBE1
1Department of Civil & Environmental Engineering, Princeton University, Princeton, NJ, 08544, USA 2Department of Geology & Geophysics, St. Anthony Falls Laboratory, University of Minnesota, Minneapolis, MN, 55414, USA 3National Center for Earth-surface Dynamics, University of Minnesota, Minneapolis, MN, 55414, USA 4Laboratory of Ecohydrology, Faculté ENAC, École Polytechnique Fédérale, CH-1015 Lausanne, Switzerland 5Department IMAGE & International Centre for Hydrology “Dino Tonini”, Universitá di Padova, via Loredan 20, I-35131, Padova, Italy 6Center for Climate Systems Research, Columbia University, NASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025,USA
e-mail: mpuma@giss.nasa.gov
In Review with the Journal of Sedimentary Research (August 2008) Abstract: We explore connections between surficial deltaic processes (e.g. avulsion, deposition) and the stratigraphic record using a simple numerical model of delta-plain evolution, with the aim of constraining these connections and thus improving prediction of subsurface features. The model represents channel dynamics using a simple but flexible cellular approach, and is unique in that it explicitly includes backwater effects that are known to be important in low-gradient channel networks. The patterns of channel deposits in the stratigraphic record vary spatially due to variation in avulsion statistics with radial distance from the delta’s source of water and sediment. We introduce channel residence time as an important statistical measure of the surface channel kinematics. The model
2 suggests that the mean channel residence time anywhere within the delta is nicely described by a power law distribution showing a cutoff that depends on radial distance. Thicknesses of channel deposits are not uniquely determined by the residence time of channelization. The channel residence time distributions at given radial distances from the source are found to be approximately lognormally distributed, a finding consistent with the scale-dependent radial structure of channel deposits in the stratigraphic record. Keywords: River delta, avulsion, stratigraphy, scale invariance, power law
INTRODUCTION Deltas are fragile geomorphic systems controlled by complex interactions among sediment supply, geomorphological fluvial processes, relative sea-level change, and marine energy conditions (Giosan and Bhattacharya 2005; Orton and Reading 1993; Overeem et al. 2005; Rinaldo et al. 1999). The sensitivity of deltaic systems to these processes and conditions is particularly important given the expected effects of climate change as predicted by the Intergovernmental Panel on Climate Change (Parry et al. 2007). Unfortunately, the intrinsic complexity of deltas has hindered progress on quantification of the connections among surficial processes and depositional architecture and, consequently, on our ability to model and manage them (Giosan and Bhattacharya 2005).
For a delta model, it is convenient to identify processes in three sections: the delta plain, the river mouth/shoreline, and the subaqueous delta slope (e.g. Orton and Reading 1993). The focus of this paper is on processes and deposition in the delta plain, which is often a region that is of great importance to society (highly populated and/or valuable
3 agricultural regions). The morphology of the delta plain is controlled by fluvial dynamics, flood plain sedimentation, base level, and tectonics. Avulsion, which is the diversion of flow out of an established channel to a new channel on the existing floodplain, is a particularly important process (Slingerland and Smith 2004). Early modeling efforts that focused on avulsion include the work of Leeder, Allen, Bridge, and coworkers (Allen 1978; Bridge and Leeder 1979; Leeder 1978; Mackey and Bridge 1992; Mackey and Bridge 1995). These models simulate channel avulsions in alluvial systems to predict channel and floodplain deposition. The general strategy of these models is to estimate channel and floodplain deposition using empirical relationships.
Other quantitative analyses of deltas have focused on physically-based modeling (Syvitski and Daughney 1992; Syvitski and Hutton 2001), which is one of the most promising approaches to quantify the connection among the surficial processes and depositional architecture of deltas. Overeem et al. (2005) reviewed current models that use physically-based approaches. The processes of sediment transport and avulsion on the delta plain are represented in a variety of ways. For example, the 3D-Sedflux model (Overeem et al. 2005) uses the linear diffusion model originally described by Paola et al. (1992). However, Jerolmack and Paola (2007)
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