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Natural channel design (NCD) and analytical channel design (ACD) are two competing approaches to stable channel design that share fundamental similarities in accounting for sediment transport processes with designs based on hybrid fluvial geomorphology and hydraulic engineering methods. In this paper, we highlight the linkage between ACD's capacity/supply ratio (CSR) and NCD's sediment capacity models (FLOWSED/POWERSED), illustrating how ACD and NCD have reached a point of convergent evolution within the stream restoration toolbox. We modified an existing CSR analytical spreadsheet tool which enabled us to predict relative channel stability using both conventional bed load transport equations and regional sediment regression curves. The stable channel design solutions based on measured data most closely matched the Parker (ACD) and/or Pagosa good/fair (NCD) relationships, which also showed the greatest CSR sensitivity in response to channel alterations. We found that CSR differences among the transport relationships became more extreme the further the design width deviated from the supply reach, suggesting that a stable upstream supply reach may serve as the best design analog. With this paper, we take a step toward resolving lingering controversy in the field of stream restoration, advancing the science and practice by reconciling key differences between ACD and NCD in the context of reach scale morphodynamics. [ABSTRACT FROM AUTHOR]

Modeling network‐scale sediment (dis)connectivity and its response to anthropic pressures provides a baseline understanding of river processes and sediment dynamics that can be used to forecast future hydro‐morphological changes in river basins. However, this requires a solid understanding of how a system is currently operating, and how it operated in the past. We present the basin‐scale, dynamic sediment connectivity model D‐CASCADE, which combines concepts of network modeling with empirical sediment transport formulas to quantify spatiotemporal sediment (dis)connectivity in river networks. D‐CASCADE accounts for multiple factors affecting sediment transport, such as spatiotemporal variations in hydrological regime, different sediment grain sizes, sediment entrainment and deposition. Add‐ons are included in D‐CASCADE to model local changes in river geomorphology driven by sediment‐induced variations in features such as channel width. We apply D‐CASCADE to the well‐documented Bega River catchment, NSW, Australia, where significant geomorphic changes to rivers have occurred post European colonization (after 1850s), including widespread channel erosion and sediment mobilization. The Bega catchment provides a useful case study to test D‐CASCADE, as original source data on the historical sediment budget are available. By introducing historic drivers of change in the correct chronological sequence, the D‐CASCADE model successfully reproduced the timing and magnitude of major phases of sediment transport and associated channel adjustments over the last two centuries. With this confidence, we then ran the model to test how well it performs at estimating future trajectories of basin‐scale sediment transport and sediment budgets at the river reach scale. Key Points: We present the new dynamic D‐CASCADE model for network‐scale river sediment (dis)connectivity modelingD‐CASCADE successfully reproduced major historic geomorphological changes observed in the Bega river network after European settlementD‐CASCADE is used to test future trajectories of basin‐scale sediment transport and reach‐scale sediment budget in the Bega river system [ABSTRACT FROM AUTHOR]

The battering of bedrock by bedload collisions is the primary mechanism by which bedrock rivers erode and landscapes evolve. The energy imparted via impacts acts to detach bedrock via the growth and intersection of surface fractures. We present impact experiments designed to test the influence of particle impact energy on bedrock erosion rates. We found that erosional efficiency increased with increasing impact energy. Notably, these increases in efficiency are not captured by a widely‐used mechanistic bedrock erosion model. Observed increases in erosional efficiency were linked with enhanced elastic energy dissipation captured by differences in the coefficient of restitution. We suggest that this increase in energy dissipation is indicative of enhanced crack extension for high velocity impacts. Our experiments indicate a clear energy‐dependence for bedrock detachment processes that is not yet captured by bedrock incision models but may be integrated into long‐term erosion rates and landscape evolution. Plain Language Summary: Bouncing sediment grains batter the bedrock beds of rivers during floods. As these impacts collide with the bed, they slowly chip away at the riverbed bedrock. It's perhaps intuitive that a grain with more energy when it collides with the riverbed is able to chip away more bedrock. But, how much more destructive are these high energy impacts? We designed an experiment where we hit a bedrock sample with a steel ball at a range of impact energies, over and over again, up to 30,000 times, and measured how much material was chipped away. High energy impacts eroded more material, but this increased efficiency of erosion is not fully captured by impact‐based erosion models. We hypothesize that for higher energy impacts, a larger fraction of the incoming energy is used to crack the bedrock, more easily eroding material. We liken the relative influence of these high energy impacts to a battering ram, where only a few strikes can easily break down a door, whereas lower energy impacts, similar to a simple knock, are unlikely to achieve the same result. We suggest that this effect may be important in determining the influence of extreme events on bedrock erosion over long timescales. Key Points: Impact experiments demonstrate that bedrock erosional efficiency is enhanced with increasing impact energyExperimentally‐derived coefficients of restitution and rock resistance coefficients indicate enhanced elastic energy dissipation with increasing impact energyResults suggest that more energetic impacts enhance crack extension, increasing erosional efficiency [ABSTRACT FROM AUTHOR]

Remote sensing plays a central role in the assessment of environmental phenomena and has increasingly become a powerful tool for monitoring shorelines, river morphology, flood-wave delineation and flood assessment. Optical-based monitoring and the characterization of river evolution at long time scales is a key tool in fluvial geomorphology. However, the evolution occurring during extreme events is crucial for the understanding of the river dynamics under severe flow conditions and requires the processing of data from active sensors to overcome cloud obstructions. This work proposes a cloud-based unsupervised algorithm for the intra-event monitoring of river dynamics during extreme flow conditions based on the time series of Sentinel-1 SAR data. The method allows the extraction of multi-temporal series of spatially explicit geometric parameters at high temporal and spatial resolutions, linking them to the hydrometric levels acquired by reference gauge stations. The intra-event reconstruction of inundation dynamics has led to (1) the estimation of the relationship between hydrometric level and wet area extension and (2) the assessment of bank erosion phenomena. In the first case, the behavior exhibits a change when the hydrometric level exceeds 1 m. In the second case, the erosion rate and cumulative lateral erosion were evaluated. The maximum erosion velocity was greater than 1 m/h, while the cumulative lateral erosion reached 130 m. Time series of SAR acquisitions, provided by Sentinel-1 satellites, were analyzed to quantify changes in the wet area of a reach of the Tagliamento river under different flow conditions. The algorithm, developed within the Python-API of GEE, can support many types of analyses of river dynamics, including morphological changes, floods monitoring, and bio-physical habitat dynamics. The results encourage future advancements and applications of the algorithm, specifically exploring SAR data from ICEYE and Capella Space constellations, which offer significantly higher spatial and temporal resolutions compared to Sentinel-1 data. [ABSTRACT FROM AUTHOR]

Bed irregularities of water bodies play a significant role in many hydraulic and river engineering experiments and models. Accurate measurement of river geomorphology requires great fieldwork effort. Optimizing the dataset size of measured points will reduce the time and costs involved. In this study, the geomorphology of a gravel bed river reach was measured using different spatial acquisition methods. Digital elevation models were created for each measurement method and the volumes of under/overestimation were calculated. The results show that the sampling methods had more effect on the accuracy of the interpolated geomorphology than the density of the measured points. By choosing an optimized sampling method, the measurement efforts decreased to less than 50%, with negligible errors of around 15 m3 and 10 m3 over and underestimation, respectively, in a water body area of around 2200 m2. These findings help to provide more accurate geomorphological data with less effort as inputs for experimental and numerical models to derive better results. [ABSTRACT FROM AUTHOR]

This article addresses a major theme in fluvial geomorphology on which Professor Ken Gregory contributed much pioneering research, that of quantitative relations of morphological change to discharge variations. It examines the morphodynamics and processes of adjustment of river channels to flow characteristics on event to decadal timescales, using field and remote sensing data collected over a 40‐year period for a very active meandering reach of the River Bollin, northwest England. The free meandering and rapid rates of changes provide insight into response timescales and processes. Morphological variations and rates of change are analysed in relation to peak flow parameters. The results show that the sinuosity has continuously increased since a resetting of the planform by multiple cutoffs in 2001, this autogenic trend underlying the effects of flow events. Width of the channel has varied by 50% between map dates, corresponding to discharge characteristics of each 6‐year period and integrating the impacts of individual events. A trend of decrease in intensity of erosion and deposition and of narrowing for the period 2001–2019 is apparent and may be an indication of the recovery and adjustment time to the major morphological changes in 2001. Other possible influences include decline in sediment supply but no obvious external cause is identified. A feedback effect of lower process rates producing less sediment from banks which decreases rates of channel movement may be occurring. A major contributor to the process rate decline and narrowing could be an increase in riparian vegetation cover. Complex sequences of change between peak flow events emerge, unrelated to discharge magnitude or numbers of peak events, and a process of basal bank sedimentation followed by upper bank deposition is evident. Channel capacity varies by up to 30% year to year, which has major implications for variability of flood risk and floodplain inundation. [ABSTRACT FROM AUTHOR]

As tides propagate inland, they become distorted by channel geometry and river discharge. Tidal dynamics in fluvial‐marine transitions are commonly observed in high‐energy tidal environments with relatively steady river conditions, leaving the effects of variable river discharge on tides and longitudinal changes poorly understood. To study the effects of variable river discharge on tide‐river interactions, we studied a low‐energy tidal environment where river discharge ranges several orders of magnitude, the diurnal microtidal Tombigbee River‐Mobile Bay fluvial‐marine transition, using water level and velocity observations from 21 stations. Results showed that diurnal tidal attenuation was reduced by the width convergence in seaward reaches and height convergence of the landward backwater reaches, with the channel convergence change location ∼40–50 km inland of the bayhead and seaward of the largest bifurcation. River events amplified tides in seaward regions and attenuated tides in landward regions. This created a region of river‐induced peak amplitude seaward of the flood limit (i.e., bidirectional‐unidirectional current transition), allowing more tidal energy to propagate inland. Tidal currents were attenuated and delayed more by river discharge than water levels, making the phase lag dynamic. The river impacts on the tides were delineated longitudinally and shifted seaward as river discharge increased, ranging up to ∼180 km. Results indicated the longitudinal shifts of river impacts on tides in alluvial systems can be estimated analytically using the ratio of river discharge to tidal discharge and the geometric convergence of the system. Our simple analytical theory provides a pathway for understanding the tide‐river‐geomorphic equilibrium along increasingly dynamic coasts. Plain Language Summary: Coastal cities and ports are affected by tides and rivers, concentrating industry, infrastructure, and economies in regions where tide‐river interactions affect flooding, erosion, and water quality. Tide‐river interactions are commonly studied in stable rivers with large tides, limiting the variability of tide‐river interactions, leaving the effects of river discharge on tides poorly understood, and the risks for coastal communities during large river discharge events difficult to determine. To study the effects of river discharge on tide‐river interactions, observations in Alabama were investigated, spanning the Tombigbee River through Mobile Bay to the Gulf of Mexico. Results show that channel geometry and river discharge affected the tides. As river discharge increased, flood‐ebb tidal currents became seaward only, causing a region up to ∼180 km (110 miles) long to change from estuarine to riverine. The shift was captured in a simple equation using the river‐tidal energy ratio and generalized geometry, suggesting the affected region increases with the range of river discharge and that the greatest flooding and erosion risks to coastal communities can change between the sea and rivers. This simple equation provides a new tool for managers to understand tide‐river interactions now and under future changes from human development and sea level rise. Key Points: Along the fluvial‐marine transition, river discharge attenuated tides in landward regions and amplified tides in seaward regionsRegions of bidirectional tidal flow became unidirectional as river discharge increased, shifting the tidal river ∼180 km seawardPlanform channel geometry (e.g., width convergence) may not be a robust indicator of the transition in tide‐river hydrodynamics [ABSTRACT FROM AUTHOR]

Significant growth in mountain rivers research since 1990 has promoted the concept that canyon‐confined mountain rivers have complex topographic features nested from base‐ to flood‐stages due to canyon structure and abundant large bed elements. Nesting means literally structures inside of structures. Mathematically, nesting means that multiple individual features and repeating patterns exist at different frequency, amplitude, and phasing, and can be added together to obtain the complete structure. Until now, subreach‐scale landform structure, including nesting, has not been quantified sufficiently to understand morphodynamic mechanisms that control and respond to such organization. Geomorphic covariance structure analysis offers a systematic framework for evaluating nested topographic patterns. In this study, a threshold stage in mountain river inundation was hypothesized to exist. Above this stage landform structure is organized to be freely self‐maintaining via flow convergence routing morphodynamics. A 13.2 km segment of the canyon‐confined Yuba River, California, was studied using 2944 cross‐sections. Geomorphic covariance structure analysis was carried out on a meter‐resolution topographic model to test the hypothesis. River width and bed elevation had significantly less variability than previously reported for lower slope, partially confined gravel/cobble river reaches. A critical stage threshold governing flow convergence routing morphodynamics was evident in several metrics. Below this threshold, narrow/high "nozzle" and wide/low "oversized" were the dominant landforms (excluding "normal channel"), while above it wide/high "wide bar" and narrow/low "constricted pool" were dominant. Three‐stage nesting of base‐bankfull‐flood landforms was dictated by canyon confinement, with nozzle–nozzle–nozzle nesting as the top permutation, excluding normal channel. [ABSTRACT FROM AUTHOR]