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Alessandro Ielpi Mathieu G.A. Lapôtre Alvise Finotello Massimiliano Ghinassi 《地球表面变化过程与地形》2021,46(2):357-370
River bends occasionally meander to the point of cutoff, whereby a river shortcuts itself and isolates a portion of its course. This fundamental process fingerprints a river's long-term planform geometry, its stratigraphic record, and biogeochemical fluxes in the floodplain. Although meander cutoffs are common in fast-migrating channels, timelapse imagery of the Earth surface typically does not offer a long enough baseline for statistically robust analyses of these processes. We seek to bridge this gap by quantifying cutoff kinematics along the Humboldt River (Nevada) – a stream that, from 1994 to 2019, hosted an exceptionally high number of cutoffs (specifically, 174 of the chute type and 53 of the neck type). A coincidence between major floods and cutoff incidence is first suggestive of hydrographic modulation. Moreover, not just higher sinuosity but also upstream planform skewness is associated with higher cutoff incidence and channel widening for a sub-population of chute cutoffs. We propose a conceptual model to explain our results in terms of channel-flow structure and then examine the distances between adjacent cutoffs to understand the mechanisms governing their clustering. We find that both local and nonlocal perturbations together trigger the clustering of new cutoffs, over distances capped by the backwater length and over yearly to decadal timescales. Our research suggests that planform geometry and backwater controls might sway the occurrence of cutoff clusters – both local and nonlocal – thereby offering new testable hypotheses to explore the evolution of meandering-river landscapes that have significant implications for river engineering and stratigraphic modelling. © 2020 John Wiley & Sons, Ltd. 相似文献
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Massimiliano Ghinassi Andrea D'Alpaos Laura Tommasini Lara Brivio Alvise Finotello Cristina Stefani 《Sedimentology》2019,66(7):2926-2949
The present study aims to improve current understanding of the sedimentation of subtidal point bars, analyzing interaction between tidal currents and waves in shaping a submerged meander bend of the microtidal Venice Lagoon (Italy), and it is based on coupling of sedimentological studies, geophysical analyses and numerical modelling. The Venice Lagoon is characterized by an average depth of about 1·5 m over subtidal platforms and a mean tidal range of about 1·0 m. The morphodynamic evolution of the lagoon is strongly affected by intense seasonal windstorms, which promote the formation of wind waves triggering sediment resuspension and bottom erosion. The study channel is 70 to 100 m wide, it has a radius of curvature of about 260 m and cuts through a permanently submerged subtidal platform. Water depth ranges from 1·0 to 5·0 m below mean sea level on the subtidal platform and channel thalweg, respectively. Different from classical architectural models, the study point‐bar beds do not show sigmoidal geometries, but consist of horizontally‐bedded deposits abruptly overlying clinostratified beds. Sedimentation in the study bar is hypothesized to stem from the interaction between the in‐channel secondary helical flow, as for most meander bends, and wave winnowing of the subaqueous overbank areas. Laterally accreting point‐bar deposits point out that the curvature‐induced helical flow redistributed sediment from the channel thalweg to the bar top and contributed to the development of the ‘classical’ fining‐upward grain size trend. The marked truncation surface, separating clinostratified bar deposits from overlying horizontally‐bedded platform sediments is interpreted here as due to bar top wave‐winnowing, which also possibly promoted bank collapses. In the proposed model, sediments remobilized from bar top and subaqueous overbank areas were transported into the channel, forming peculiar ‘apron‐like’ accumulations, where sand accumulated through avalanching processes and mud settled down from suspension. 相似文献
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Francesco Barbariol Jose-Henrique G. M. Alves Alvise Benetazzo Filippo Bergamasco Luciana Bertotti Sandro Carniel Luigi Cavaleri Yung Y. Chao Arun Chawla Antonio Ricchi Mauro Sclavo Hendrik Tolman 《Ocean Dynamics》2017,67(3-4):535-549
A novel implementation of parameters estimating the space-time wave extremes within the spectral wave model WAVEWATCH III (WW3) is presented. The new output parameters, available in WW3 version 5.16, rely on the theoretical model of Fedele (J Phys Oceanogr 42(9):1601-1615, 2012) extended by Benetazzo et al. (J Phys Oceanogr 45(9):2261–2275, 2015) to estimate the maximum second-order nonlinear crest height over a given space-time region. In order to assess the wave height associated to the maximum crest height and the maximum wave height (generally different in a broad-band stormy sea state), the linear quasi-determinism theory of Boccotti (2000) is considered. The new WW3 implementation is tested by simulating sea states and space-time extremes over the Mediterranean Sea (forced by the wind fields produced by the COSMO-ME atmospheric model). Model simulations are compared to space-time wave maxima observed on March 10th, 2014, in the northern Adriatic Sea (Italy), by a stereo camera system installed on-board the “Acqua Alta” oceanographic tower. Results show that modeled space-time extremes are in general agreement with observations. Differences are mostly ascribed to the accuracy of the wind forcing and, to a lesser extent, to the approximations introduced in the space-time extremes parameterizations. Model estimates are expected to be even more accurate over areas larger than the mean wavelength (for instance, the model grid size). 相似文献
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Image analysis techniques are used for retrieving water surface elevation fields spatially and temporally from CCD-images and CCD-image-sequences. The technique proposed herein utilizes binocular stereogrammetry to recover topographic information from a sequence of synchronous, overlapping video images. The method used differs from the traditional stereo-photogrammetric analysis of a single stereo-pair because the use of video allows for a continuous sequence of stereo-images to be digitally sampled and analyzed. For data acquisition two synchronized progressive-scan cameras were used.A partially supervised 3-D stereo system (called WASS, Wave Acquisition Stereo System) is shown here. It is used to reconstruct the 3-D shape of water surface waves, acquired at frame rate, with small computational time needed. The stereo method is presented, including the derivation of a relationship relating the geometry of the stereo rig and the expected errors. Finally, the 3-D calculated scattered points give the complete spatio-temporal distribution of the water surface elevations. The measurable length-scales depend on the pixel resolution, the triangulation accuracy, and the acquisition frame rate. Limitations in the stereo measurements are also discussed.Two experiments to test and to demonstrate the system took place: one on the Venice lagoon, north of the city of Venice in September 2004 and the second on the coast of California at San Diego in December 2005. For the second experiment, qualitative and quantitative intercomparisons of the stereo-matching and in-situ sensor measurements are presented. All the measurements of water surface waves indicate that the proposed approach is both accurate and applicable for measuring water surface waves. Moreover, shape estimates are accurately and extremely dense both in space and time, and the remote location of the instruments alleviates some difficulties associated with in situ instrumentation. 相似文献
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Guimarães Pedro Veras Ardhuin Fabrice Perignon Yves Benetazzo Alvise Bouin Marie-Noëlle Garnier Valerie Redelsperger Jean-Luc Accensi Mickael Thomson Jim 《Ocean Dynamics》2022,72(8):621-639
Ocean Dynamics - Short waves growth is characterized by nonlinear and dynamic processes that couple ocean and atmosphere. Ocean surface currents can have a strong impact on short wave steepness and... 相似文献
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Alessandro Ielpi Mathieu G.A. Lapôtre Alvise Finotello Massimiliano Ghinassi Andrea D'Alpaos 《地球表面变化过程与地形》2020,45(8):1717-1731
The links between flood frequency and rates of channel migration are poorly defined in the ephemeral rivers typical of arid regions. Exploring these links in desert fluvial landscapes would augment our understanding of watershed biogeochemistry and river morphogenesis on early Earth (i.e. prior to the greening of landmasses). Accordingly, we analyse the Mojave River (California), one of the largest watercourses in the Great Basin of the western United States. We integrate discharge records with channel-migration rates derived from dynamic time-warping analysis and chronologically calibrated subsidence rates, thereby constraining the river's formative conditions. Our results reveal a slight downstream decrease in bankfull discharge on the Mojave River, rather than the downstream increase typically exhibited by perennial streams. Yet, the number of days per year during which the channel experiences bankfull or higher stages is roughly maintained along the river's length. Analysis of historical peak flood records suggests that the incidence of channel-formative events responds to modulation in watershed runoff due to the precipitation in the river's headwaters over decades to centuries. Our integrated analysis finally suggests that, while maintaining hydraulic geometries that are fully comparable with many other rivers worldwide, ephemeral desert rivers akin to the Mojave are capable of generating a surprisingly wide range of depositional geometries in the stratigraphic record. © 2020 John Wiley & Sons, Ltd. 相似文献
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