An official website of the United States government
Across varied environments, meandering channels evolve through a common morphodynamic feedback: the sinuous channel shape causes spatial variations in boundary shear stress, which cause lateral migration rates to vary along a meander bend and change the shape of the channel. This feedback is embedded in all conceptual models of meandering channel migration, and in numerical models, it occurs over an explicit timescale (i.e., the model time step). However, the sensitivity of modeled channel trajectory to the time step is unknown. In numerical experiments using a curvature‐driven model of channel migration, we find that channel trajectories are consistent over time if the channel migrates ≤10% of the channel width over the feedback timescale. In contrast, channel trajectories diverge if the time step causes migration to exceed this threshold, due to the instability in the co‐evolution of channel curvature and migration rate. The divergence of channel trajectories accumulates with the total run time. Application to hindcasting of channel migration for 10 natural rivers from the continental US and the Amazon River basin shows that the sensitivity of modeled channel trajectories to the time step is greatest at low (near‐unity) channel sinuosity. A time step exceeding the criterion causes over‐prediction of the width of the channel belt developed over millennial timescales. These findings establish a geometric constraint for predicting channel migration in landscape evolution models for lowland alluvial rivers, upland channels coupled to hillslopes and submarine channels shaped by turbidity currents, over timescales from years to millennia.
more »
« less
Glacial Isostatic Adjustment Modulates Lateral Migration Rate and Morphology of the Red River (North Dakota, USA and Manitoba, Canada)
Abstract The lateral migration of a river meander is driven by erosion on the outer bank and deposition on the inner bank, both of which are affected by shear stress (and therefore channel slope) through complex morphodynamic feedbacks. To test the sensitivity of lateral migration to channel slope, we quantify slope change induced by glacial isostatic adjustment along the Red River (North Dakota, USA and Manitoba, Canada) and two of its tributaries over the past 8.5 ka. We demonstrate a statistically significant, positive relationship between normalized cutoff count, which we interpret as a proxy for channel lateral migration rate, and slope change. We interpret this relationship as the signature of slope change modulating the magnitude of shear stress on riverbanks, suggesting that slope changes that occur over thousands of years are recorded in river floodplain morphology.
more »
« less
- Award ID(s):
- 2120574
- PAR ID:
- 10530506
- Publisher / Repository:
- AGU
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 50
- Issue:
- 15
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Mobile river channels endanger human life and property and over centuries shape ecosystems, landscapes, and stratigraphy. Quantifying channel movements from remote sensing is difficult, in part due to the diversity of river mobility processes (e.g., channel migration, cutoffs, avulsion) and planform morphologies (e.g., meandering, braided). Here, we present a framework for quantifying riverbank migration from remote sensing that upscales recent methodological advances from laboratory flume studies utilizing particle image velocimetry (PIV). We apply PIV to image time series of 21 rivers worldwide, showing PIV ignores cutoff and avulsion processes by design and is well suited for tracking riverbank migration regardless of planform morphology. We show that PIV‐derived results for riverbank migration are consistent with published results from centerline‐ and bank‐based Lagrangian methods. Unlike existing methods, PIV offers a grid‐based Eulerian framework where defining channel centerlines is unnecessary and quantified uncertainty in riverbank positions is propagated into uncertainty in migration rates. PIV offers means to efficiently extract global patterns in riverbank migration from decades of satellite data, as well as investigate river response to climate change and human activities in our rapidly changing world.more » « less
-
Flow regime, sediment supply and base level control geometry and evolution of alluvial channels and floodplains. Single thread rivers subject to constant forcing can reach equi-librium conditions in which the amount of sediment deposited on the floodplain through point bar deposition and overbank sedimentation is balanced by erosion of floodplain sed-iment through channel migration. At equilibrium, floodplain slope and sediment size dis-tribution, reach-averaged channel geometry (width and depth) and channel migration rates do not change in time. In response to changes in sediment supply and floodplain width, channel geometry and migration rate, floodplain slope and size distribution are expected to evolve in space and time. Predicting this response remains an open problem for geoscien-tists and engineers. Here we use an equilibrium solution of a 1D morphodynamic frame-work of channel-floodplain evolution to investigate how equilibrium conditions change as a function of sediment supply and floodplain width. Sediment is modeled here as a mix-ture of two grain sizes, sand and mud. Channel migration rate and width are functions of near-bank flow properties and floodplain characteristics. We zero the model using input parameters based on the pre-1930 ~ reach of the Minnesota River from Mankato to Jordan, USA, where data is available for proper field scale model verification. We then use the validated model to quantify the long-term (equilibrium) response of the schematic reach to changes in sediment supply magnitude and size distribution, as well as to changes in floodplain width.more » « less
-
Flow regime, sediment supply and base level control geometry and evolution of alluvial channels and floodplains. Single thread rivers subject to constant forcing can reach equi-librium conditions in which the amount of sediment deposited on the floodplain through point bar deposition and overbank sedimentation is balanced by erosion of floodplain sed-iment through channel migration. At equilibrium, floodplain slope and sediment size dis-tribution, reach-averaged channel geometry (width and depth) and channel migration rates do not change in time. In response to changes in sediment supply and floodplain width, channel geometry and migration rate, floodplain slope and size distribution are expected to evolve in space and time. Predicting this response remains an open problem for geoscien-tists and engineers. Here we use an equilibrium solution of a 1D morphodynamic frame-work of channel-floodplain evolution to investigate how equilibrium conditions change as a function of sediment supply and floodplain width. Sediment is modeled here as a mix-ture of two grain sizes, sand and mud. Channel migration rate and width are functions of near-bank flow properties and floodplain characteristics. We zero the model using input parameters based on the pre-1930 ~ reach of the Minnesota River from Mankato to Jordan, USA, where data is available for proper field scale model verification. We then use the validated model to quantify the long-term (equilibrium) response of the schematic reach to changes in sediment supply magnitude and size distribution, as well as to changes in floodplain width.more » « less
-
Abstract. Lateral migration of meandering rivers poses erosional risks to human settlements, roads, and infrastructure in alluvial floodplains. While there is a large body of scientific literature on the dominant mechanisms driving river migration, it is still not possible to accurately predict river meander evolution over multiple years. This is in part because we do not fully understand the relative contribution of each mechanism and because deterministic mathematical models are not equipped to account for stochasticity in the system. Besides, uncertainty due to model structure deficits and unknown parameter values remains. For a more reliable assessment of risks, we therefore need probabilistic forecasts. Here, we present a workflow to generate geomorphic risk maps for river migration using probabilistic modeling. We start with a simple geometric model for river migration, where nominal migration rates increase with local and upstream curvature. We then account for model structure deficits using smooth random functions. Probabilistic forecasts for river channel position over time are generated by Monte Carlo runs using a distribution of model parameter values inferred from satellite data. We provide a recipe for parameter inference within the Bayesian framework. We demonstrate that such risk maps are relatively more informative in avoiding false negatives, which can be both detrimental and costly, in the context of assessing erosional hazards due to river migration. Our results show that with longer prediction time horizons, the spatial uncertainty of erosional hazard within the entire channel belt increases – with more geographical area falling within 25 % < probability < 75 %. However, forecasts also become more confident about erosion for regions immediately in the vicinity of the river, especially on its cut-bank side. Probabilistic modeling thus allows us to quantify our degree of confidence – which is spatially and temporally variable – in river migration forecasts. We also note that to increase the reliability of these risk maps, we need to describe the first-order dynamics in our model to a reasonable degree of accuracy, and simple geometric models do not always possess such accuracy.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1029/2023GL103995
