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 “ocean basin.” 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 transport‐limited erosion model, while other authors suggest that a detachment‐limited 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 end‐member fluvial erosion models to reproduce the well‐documented 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 transport‐limited model is unable to account for the catchments’response to the increase in uplift rate, consistent with the observed low rates of sediment supply to the channels. Instead, a detachment‐limited model with a threshold corresponding to the field‐derived median grain size of the sediment plus a slope‐dependent 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 slope‐ 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

Experimental study on coarse grain saltation dynamics in bedrock channels

Saltation of bed load particles on bedrock surfaces is important for landscape evolution and bedrock incision in steep landscapes. However, few studies have investigated saltation in bedrock channels where, unlike alluvial channels, the bed roughness height and the sediment size may be independent. To address this data gap, we measured the saltation hop height, hop length, and velocity of gravel saltating over a planar bed using 80–160 readings from high-speed photography and direct measurements. Two separate dimensional analyses are used: one leading to a bed shear stress scaling and another leading to a Froude number (Fr) scaling. Our new saltation data coupled with numerous data from previous studies suggest that both shear stress and Fr-scaling analyses are valid in characterizing bed load saltation dynamics with bed roughness ranging from smooth to alluvial beds. However, the Fr approach has the advantages that (1) there is no need to estimate a critical Shields stress (τ*_c), which alone can vary up to 2 orders of magnitude (e.g., 0.001–0.1) due to changes in relative bed roughness and slope and (2) the Fr-based scaling fits the saltation data set better in a least squares sense. Results show that the saltation velocity of bed load is independent of grain density and grain size and is linearly proportional to flow velocity. Saltation height has a nonlinear dependence on grain size. Saltation length increases primarily with flow velocity, and it is inversely proportional to submerged specific density. Our results suggest that either (τ*_c) or bed roughness coefficient must be properly estimated to yield accurate results in saltation-abrasion models

Flow resistance and hydraulic geometry in contrasting reaches of a bedrock channel

Assumptions about flow resistance in bedrock channels have to be made for mechanistic modeling of river incision, paleoflood estimation, flood routing, and river engineering. Field data on bedrock flow resistance are very limited and calculations generally use standard alluvial-river assumptions such as a fixed value of Manning's n. To help inform future work we measured how depth, velocity and flow resistance vary with discharge in four short reaches of a small bedrock channel, one with an entirely rock bed and the others with 20%-70% sediment cover, and in the alluvial channel immediately upstream. As discharge and submergence increase in each of the partly or fully alluvial reaches there is a rapid increase in velocity and a strong decline in both n and the Darcy-Weisbach friction factor f. The bare-rock reach follows a similar trend from low to medium discharge but has increasing resistance at higher discharges because of the macro-roughness of its rock walls. Flow resistance at a given discharge differs considerably between reaches and is highest where the partial sediment cover is coarsest and most extensive. Apart from the effect of rough rock walls, the flow resistance trends are qualitatively consistent with logarithmic and variable-power equations and with non-dimensional hydraulic geometry, but quantitative agreement using sediment D84 as the roughness height is imperfect