Geomorphic and Stratigraphic Legacy of Dynamic Backwater Effects in Coastal Rivers
Ganti, Vamsi and Lamb, Michael P.
Sediment flux from rivers to oceans is the fundamental driver of fluvio-deltaic morphodynamics and recent research has highlighted the importance of modeling the effect of variable discharges in the backwater zone of coastal rivers [e.g., Lamb et al., 2012]. In this study, we investigate the legacy of the dynamic backwater effects (caused by variable discharges) in the morphology of coastal rivers and their signature in the depositional record using a quasi 2-D formulation for conservation of fluid mass and momentum, and conservation of sediment mass. Unlike the normal flow reach of a river where the water discharge variations in a river are accommodated by changes in stage height, the water discharge variations in the backwater reach are accommodated by morphological changes in the long-profile of the river. This prompts us to define the nondimensional curvature κ (curvature of the long-profile in the backwater reach normalized by the backwater length) of the long-profile as a metric to study the response of the river to variable discharges in the backwater zone. Within this framework, under constant base level conditions, a flood event causes water surface drawdown and erosion near the river mouth, resulting in convex-up long-profile (κ<0) that flattens out as the flood event persists (κ→0). In contrast, a low flow, which is spatially decelerating as it approaches the river mouth, results in a concave-up long-profile (κ>0) that flattens out as the flow persists (κ→0). The approach to equilibrium (κ=0) of the river profile is characterized by an exponential function κ=±κmax exp(-t/tR); thus, defining a relaxation timescale tR of the river for a given water discharge. Inasmuch as the river profile in the backwater reach can be interpreted to be a result of the competition between erosional flood events and the depositional low flows, we argue that the relaxation timescale of low flows together with the return period of the floods determine the legacy of flood events in a given river, i.e., how long does a given river “remember” a flood event. Further, we show that the scour events resulting from the river drawdown during flood events are likely preserved in the stratigraphic record as unconformities, which may appear similar to those previously interpreted to be a result of relative sea level changes or other allogenic forcings (“incised valleys”). To the extent that these unconformities are a result of autogenic, dynamic backwater effects, we argue that the nondimensional curvature κ of the unconformity provides us with a tool to estimate the magnitude-duration of the extremes of paleo-discharges and the estimated backwater length provides us with a measure of the delta size. Finally, we discuss the implications of the interplay between the autogenic and allogenic forcings (e.g., sediment supply, relative sea level changes) in setting the geometrical characteristics of unconformities in the fluvio-deltaic stratigraphy.
AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013