Catalogue Search | MBRL
Are you sure you want to remove the book from the shelf?
{{itemTitle}}
307
result(s) for
"Paola, Chris"
Sort by:
Shredding of environmental signals by sediment transport
Landscapes respond to climate, tectonic motions and sea level, but this response is mediated by sediment transport. Understanding transmission of environmental signals is crucial for predicting landscape response to climate change, and interpreting paleo‐climate and tectonics from stratigraphy. Here we propose that sediment transport can act as a nonlinear filter that completely destroys (“shreds”) environmental signals. This results from ubiquitous thresholds in sediment transport systems; e.g., landsliding, bed load transport, and river avulsion. This “morphodynamic turbulence” is analogous to turbulence in fluid flows, where energy injected at one frequency is smeared across a range of scales. We show with a numerical model that external signals are shredded when their time and amplitude scales fall within the ranges of morphodynamic turbulence. As signal frequency increases, signal preservation becomes the exception rather than the rule, suggesting a critical re‐examination of purported sedimentary signals of external forcing.
Journal Article
Influence of steady base-level rise on channel mobility, shoreline migration, and scaling properties of a cohesive experimental delta
Here we describe results from an alluvial delta physical experiment with and without steady base‐level rise. Using a unique cohesive sediment mixture that promotes the development of persistent channels and a rugose shoreline, we quantitatively characterize channel network properties as a function of base‐level rise in a distributary system that reproduces many aspects of the geometry of natural deltas. Analysis of the experimental data shows clear dynamical differences between the predominantly progadational and aggradational phases of the experiment. The experiment was conducted in two phases: a first in which the delta prograded into standing water of constant depth in the absence of base‐level rise and a second during which steady base‐level rise was imposed on the system, forcing a twofold increase in topset aggradation due to greater sediment retention in the fluvial reach. The shift in sediment mass balance to enhanced fluvial deposition in the second phase caused channel network mobility to increase, reducing the autogenic channel time scale from 23.5 to 12.5 h and supporting a positive correlation between deposition and channel avulsion frequency. An independent shoreline time scale that characterizes the dominant time over which shoreline regression is persistent closely correlates with measurements of the channel network (28.3 h during fan progradation and 9.5 h during fan aggradation). These metrics suggest a strong coupling between channel network and shoreline kinematics and a more active fluvial surface during fan aggradation by a factor of 2 to 3, similar to the increase in aggradation rate. Spatial scaling of shoreline roughness reveals that maximum shoreline variability is set by the scale of distributary lobes. Strong coupling exists between delta growth and shoreline geometry during progradation. During aggradation, however, shoreline variability is not solely due to distributary lobe growth but is also set by shoreline transgression over inactive portions of the delta, illustrating a decoupling between fan kinematics and fan geometry. This decoupling, together with the matched increase in channel network mobility and fluvial aggradation, suggest the stratigraphic architecture may not be a strong geometric signal of different fluvial surface conditions either.
Journal Article
Exploring the role of organic matter accumulation on delta evolution
We explore the role of plant matter accumulation in the sediment column in determining the response of fluvial‐deltas to base‐level rise and simple subsidence profiles. Making the assumption that delta building processes operate to preserve the geometry of the delta plain, we model organic sedimentation in terms of the plant matter accumulation and accommodation (space made for sediment deposition) rates. A spatial integration of the organic sedimentation, added to the known river sediment input, leads to a model of delta evolution that estimates the fraction of organic sediments preserved in the delta. The model predicts that the maximum organic fraction occurs when the organic matter accumulation rate matches the accommodation rate, a result consistent with field observations. The model also recovers the upper limit for coal accumulation previously reported in the coal literature. Further, when the model is extended to account for differences in plant matter accumulation between fresh and saline environments (i.e., methanogenesis versus sulfate reduction) we show that an abrupt shift in the location of the fresh‐salt boundary can amplify the speed of shoreline retreat. Key Points First land building model coupling physical and biogeochemical processes A quantitative prediction of field observed patterns of organic fraction Demonstration of the role of fresh water supply in delta dynamics
Journal Article
Prevalence of exponential bed thickness distributions in the stratigraphic record: Experiments and theory
Stratigraphy preserved in alluvial basins holds important information for reconstructing past environmental conditions via inversion methodologies. In this paper we explore, through the use of physical and numerical experiments, the forward problem, that is, we quantify how the probabilistic structure of the processes that govern the evolution of depositional systems relates to the probability distribution of the preserved bed thicknesses. We demonstrate that the extreme variability, as evidenced by heavy‐tailed distribution of the surface elevation increments, largely cancels itself out in the resulting stratigraphy. Specifically, we show that bed thickness is well described by an exponential distribution even when erosional and depositional increments characterizing the surface evolution exhibit heavy‐tailed statistics, i.e., large, infrequent events have a significant chance of occurrence. We attribute this finding to the symmetric nature of the distribution of elevation increments (both erosional and depositional events) and the additive nature of the stratigraphic filter. We also show that the variability of surface elevation increments, as measured by the interquartile range of their probability distribution, has a robust and well‐defined relationship with the preserved mean bed thickness. Key Points Heavy‐tailed surface statistics can result in thin‐tailed bed thickness stats Predicitive relationship between surface fluctuations stats and strat stats
Journal Article
Fluvial bevelling of topography controlled by lateral channel mobility and uplift rate
Valley morphologies of rivers crossing zones of active uplift range from narrow canyons to broad alluvial surfaces. They provide illuminating examples of the fundamental, but poorly understood, competition between relief creation and landscape flattening. Motivated by a field example of abandoned kilometre-wide, fluvially eroded platforms on active detachment folds in the Tian Shan foreland, we present physical experiments investigating the controls on the area of a growing fold that is reworked by antecedent rivers. These experiments reproduce the range of observed field morphologies, varying from wholesale bevelling of the uplifting fold to the formation of narrow, steep-walled canyons. A log-linear fit to a simple dimensionless parameter shows that the competition between lateral channel mobility and rock-uplift rate explains >95% of the variance in the bevelled fraction of the folds. Our data suggest that lateral bedrock erosion rates of 0.5–40 m yr
−1
are required to explain the formation of extensive platforms in the Tian Shan foreland and imply that varying water and sediment fluxes can cause striking changes in the degree of landscape flattening by influencing the lateral erosion rate.
Rivers crossing zones of active uplift can bevel broad alluvial platforms. Experiments suggest that competition between lateral channel mobility and uplift rate controls the ability of a river to flatten the landscape.
Journal Article
Emergent sculpture
Sandstone arches and other striking landforms are the showpieces of national parks around the globe. Experiments and numerical analyses show that they result from a self-organization process that involves vertical load, wind erosion and grain locking.
Journal Article
Chaos in a simple model of a delta network
The flux partitioning in delta networks controls how deltas build land and generate stratigraphy. Here, we study flux-partitioning dynamics in a delta network using a simple numerical model consisting of two orders of bifurcations. Previous work on single bifurcations has shown periodic behavior arising due to the interplay between channel deepening and downstream deposition. We find that coupling between upstream and downstream bifurcations can lead to chaos; despite its simplicity, our model generates surprisingly complex aperiodic yet bounded dynamics. Our model exhibits sensitive dependence on initial conditions, the hallmark signature of chaos, implying long-term unpredictability of delta networks. However, estimates of the predictability horizon suggest substantial room for improvement in delta-network modeling before fundamental limits on predictability are encountered. We also observe periodic windows, implying that a change in forcing (e.g., due to climate change) could cause a delta to switch from predictable to unpredictable or vice versa. We test our model by using it to generate stratigraphy; converting the temporal Lyapunov exponent to vertical distance using the mean sedimentation rate, we observe qualitatively realistic patterns such as upwards fining and scale-dependent compensation statistics, consistent with ancient and experimental systems. We suggest that chaotic behavior may be common in geomorphic systems and that it implies fundamental bounds on their predictability. We conclude that while delta “weather” (precise configuration) is unpredictable in the long-term, delta “climate” (statistical behavior) is predictable.
Journal Article
Rules of river avulsion change downstream
Avulsing rivers create new pathways on the floodplain and the associated flooding can profoundly affect society
1
–
4
. River avulsions are thought to occur when the water column becomes perched above the floodplain
5
or when the slope down the flanks of the channel provides a steeper descent than the existing river channel
6
,
7
. We test these classical ideas by quantifying the topography around avulsing rivers and show that these mechanisms, historically invoked separately, work together. Near coasts, rivers avulse when the slope away from the channel is steeper, not because they are perched. The opposite is true near mountain fronts; on fans, the alternative paths are similarly steep to the downstream path, so rivers avulse when they are perched above the surrounding landscape. We reconcile these findings and present a new theoretical framework that identifies which rivers are vulnerable to avulsion and predicts the path of an avulsing river. These first-order rules of avulsion suggest that avulsion risks are underestimated in many coastal environments
8
and that probabilistic predictions of avulsion pathfinding can efficiently map hazards with minimal information. Applying these principles for risk assessment could particularly benefit the Global South, which is disproportionately affected by avulsions.
A novel theoretical framework reveals how topography surrounding rivers causes dramatic changes in their courses, with implications for natural hazard prediction, particularly in the Global South.
Journal Article