82 research outputs found
Backwater controls of avulsion location on deltas
River delta complexes are built in part through repeated river-channel avulsions, which often occur about a persistent spatial node creating delta lobes that form a fan-like morphology. Predicting the location of avulsions is poorly understood, but it is essential for wetland restoration, hazard mitigation, reservoir characterization, and delta morphodynamics. Following previous work, we show that the upstream distance from the river mouth where avulsions occur is coincident with the backwater length, i.e., the upstream extent of river flow that is affected by hydrodynamic processes in the receiving basin. To explain this observation we formulate a fluvial morphodynamic model that is coupled to an offshore spreading river plume and subject it to a range of river discharges. Results show that avulsion is less likely in the downstream portion of the backwater zone because, during high-flow events, the water surface is drawn down near the river mouth to match that of the offshore plume, resulting in river-bed scour and a reduced likelihood of overbank flow. Furthermore, during low-discharge events, flow deceleration near the upstream extent of backwater causes enhanced deposition locally and a reduced channel-fill timescale there. Both mechanisms favor preferential avulsion in the upstream part of the backwater zone. These dynamics are fundamentally due to variable river discharges and a coupled offshore river plume, with implications for predicting delta response to climate and sea level change, and fluvio-deltaic stratigraphy
Sediment transport and topographic evolution of a coupled river and river plume system: An experimental and numerical study
Sediment transfer from rivers to the ocean is the fundamental driver of continental sedimentation with implications for carbon burial, land use dynamics, and unraveling global climate change and Earth history from sedimentary strata. Coastal rivers are dynamically coupled to their offshore plumes at the river mouth creating regions of nonuniform flow that can dictate patterns of erosion and deposition both onshore and offshore. However, there are limited experimental and modeling studies on sediment transport and morphodynamics of coupled river and river plume systems and their response to multiple flood events. To address this knowledge gap, we developed a quasi-2-D, morphodynamic numerical model and conducted exploratory flume experiments in a 7.5 m long flume where a 10 cm wide river channel was connected to a 76 cm wide âocean basin.â Both the numerical model and the flume results demonstrate that (1) during low-discharge flows, backwater hydrodynamics cause spatial-flow deceleration and deposition in the river channel and the offshore plume area, and (2) during high flows the water surface is drawn down to sea level, resulting in spatial-flow acceleration and bed scour. During high-discharge flows, we also found that the offshore river plume self-channelized owing to both levee formation and bed scour. Our study suggests that coastal rivers may be in a perpetual state of morphodynamic adjustment and highlights the need to link rivers and river plumes under a suite of flow discharges to accurately predict fluvio-deltaic morphodynamics and connectivity between fluvial sediment sources and marine sinks
Testing fluvial erosion models using the transient response of bedrock rivers to tectonic forcing in the Apennines, Italy
The transient response of bedrock rivers to a drop in base level can be used to
discriminate between competing fluvial erosion models. However, some recent studies of
bedrock erosion conclude that transient river long profiles can be approximately
characterized by a transportâlimited erosion model, while other authors suggest that a
detachmentâlimited model best explains their field data. The difference is thought to be
due to the relative volume of sediment being fluxed through the fluvial system. Using a
pragmatic approach, we address this debate by testing the ability of endâmember fluvial
erosion models to reproduce the wellâdocumented evolution of three catchments in the
central Apennines (Italy) which have been perturbed to various extents by an
independently constrained increase in relative uplift rate. The transportâlimited model is
unable to account for the catchmentsâresponse to the increase in uplift rate, consistent with
the observed low rates of sediment supply to the channels. Instead, a detachmentâlimited
model with a threshold corresponding to the fieldâderived median grain size of the
sediment plus a slopeâdependent channel width satisfactorily reproduces the overall
convex long profiles along the studied rivers. Importantly, we find that the prefactor in the
hydraulic scaling relationship is uplift dependent, leading to landscapes responding faster
the higher the uplift rate, consistent with field observations. We conclude that a slopeâ
dependent channel width and an entrainment/erosion threshold are necessary ingredients
when modeling landscape evolution or mapping the distribution of fluvial erosion rates in
areas where the rate of sediment supply to channels is low
The influence of sediment cover variability on longâterm river incision rates: An example from the Peikang River, central Taiwan
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94660/1/jgrf824.pd
Controls and limits on bedrock channel geometry
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94837/1/jgrf728.pd
Reduction of Long-Term Bedrock Incision Efficiency by Short-Term Alluvial Cover Intermittency
International audienceFluctuations of the sediment volume stored in mountain channels are driven by stochastic variations of discharge and sediment supply and can inhibit bedrock incision if sediment thickness is too large. Here, I study how this shortÆ]term stochasticity propagates into the longÆ]term reduction of bedrock incision efficiency (the cover effect) at geological time scales. I introduce a new numerical model that resolves sediment transport and bedrock incision at daily time scales, and is run for thousands of years. It incorporates (1) a transport threshold and daily stochastic variations in water discharge and sediment supply, (2) a freely evolving channel width and slope, and (3) an explicit treatment of alluvial thickness variations and corresponding bed incision reduction. For typical mountain river conditions the model predicts that alluvial cover oscillates between complete and negligible incision reduction. In this intermittent regime the longÆ]term cover effect is mainly set by the fraction of time spent in full cover, and the presentÆ]day extent of alluvial cover is not representative of longÆ]term dynamics. The longÆ]term dynamics. The longÆterm integrated cover effect law differs strongly from proposed theoretical and experimental models, and it is controlled by sediment supply stochasticity rather than the details of cover development at the hydraulic time scale. Model results also suggest that steady state channel configuration always depends on sediment supply rate, while being never limited by transport capacity or strictly detachment limited. These results point out that discharge and sediment supply stochasticity should not be considered less important than the intricate details of incision laws to model long-term bedrock channel dynamics
Field calibration of sediment flux dependent river incision
Bed erosion and sediment transport are ubiquitous and linked processes in rivers. Erosion can either be modeled as a âdetachment limitedâ function of the shear stress exerted by the flow on the bed, or as a âtransport limitedâ function of the sediment flux capacity of the flow. These two models predict similar channel profiles when erosion rates are constant in space in time, but starkly contrasting behavior in transient settings. Traditionally detachment limited models have been used for bedrock rivers, whereas transport limited models have been used in alluvial settings. In this study we demonstrate that rivers incising into a substrate of loose, but very poorly sorted relict glacial sediment behave in a detachment limited manner. We then develop a methodology by which to both test the appropriate incision model and constrain its form. Specifically we are able to tightly constrain how incision rates vary as a function of the ratio between sediment flux and sediment transport capacity in three rivers responding to deglaciation in the Ladakh Himalaya, northwest India. This represents the first field test of the so-called âtools and coverâ effect along individual rivers
- âŠ