Glacial bedforms in the Northwind Abyssal Plain,Chukchi Borderland

A series of sub-parallel linear glacial scours are identified on the crest of the Baoshi Seamount in the Northwind Abyssal Plain by compiling new multibeam data acquired during the 9 th Chinese Arctic Research Expedition(CHINARE-Arc9)in 2018 and previously published data.The new data reveal scours that developed at water depths of 850–1030 m with an orientation of about 75°/255°.The maximum water depth occurs in the southernmost scour and is deeper than that from previous investigations,which showed a maximum scouring depth of about 900 m on the seamount.The topographic and geomorphological characteristics suggest that these scours resulted from erosion by the ice shelf extending from the Chukchi margin and/or Laurentide Ice Sheet that grounded on the crest of the seamount and moved in a NE–SW direction.Other possibilities of their genesis include armadas of large icebergs/multi-keel icebergs calved from the Chukchi Shelf or the Laurentide Ice Sheet.The new data provide new constraints for assessing the extent and volume of the ice sheet in the Chukchi area during glacial maxima.

Impacts of vegetation over bedforms on flow characteristics in gravel-bed rivers

In the present study,among six gravel-bed river reaches,two natural gravel-bed river reaches with approximately similar bedforms and flow conditions have been found,one river reach with vegetation on bedforms and the other without vegetation on bedforms.Based on field measurements,the impacts of vegetation over bedforms on flow characteristics have been investigated.Flow velocity,Reynolds stress and turbulence intensity distributions over bedforms with vegetation have been compared with those over gravel bedforms without vegetation.By using the quadrant analysis,the dominant events of turbulent flow structures have been assessed for flows over gravel bedforms with vegetation and without vegetation.Results show that the effect of vegetation enhances the anisotropy in turbulence and generates strong secondary currents,which lead to the occurrence of dip phenomenon at the stoss section and the crest of vegetated bedforms.The change of bed roughness from gravel bed to vegetated bed causes a development of a new boundary layer.This resulted in the different shape of velocity distributions over bedforms with vegetation from those over bedforms without vegetation,which can affect the lives of aquatic animals.Reynolds shear stress at downstream section of gravel bedform without vegetation is much higher than that of vegetated bedform,which shows that the erosion takes place from downstream of vegetated bedforms,while in the bare beds;the deposition is at downstream of bedforms.It is found that the presence of vegetation causes a chaotic distribution pattern of Reynolds shear stress,while it has more orderly pattern at the downstream section of gravel bedform without vegetation.Besides,the distribution of turbulence intensity in the lee side of vegetated bedform has a concave shape and in the stoss section,turbulence is generated in ejections and sweeps near the vegetation cover.These findings are significant to the practice of river ecological restoration.

Hydraulic Characteristics and Suspended Sediment Loads during Spring Breakup in Several Streams Located on the National Petroleum Reserve in Alaska, USA

This article presents results from a broad field campaign involving discharge and surface-water slope measurements, water sampling, and longitudinal river-bed profile surveys. During the spring breakup of 2011, fieldwork was carried out in several pristine streams located in the National Petroleum Reserve in Alaska;the studied streams cover two main regions: 1) foothills (Ikpikpuk River, Seabee Creek, Prince Creek, and Otuk Creek);2) coastal plain (Fish Creek, Judy Creek, and Ublutuoch River). Reported data includes basic geometric and hydraulic characteristics such as channel width and depth, cross-sectional area, average velocity, friction factor, shear stress, suspended sediment concentrations from autosamplers and grab samples, and dune dimensions and steepness ratios. The measured discharge in different streams ranged from 2 to 853 m3/s, which corresponded to post-breakup and near peak conditions, respectively. The temporal variation of Manning’s n was in phase with measured discharge, with high values of n associated with the presence of floating ice during the measurements. Calculations indicate that sediment particle sizes ≤2 mmmoved during the measurements. In general, variations in discharge were accompanied by changes in suspended sediment concentrations.

Extraction of Morphometric Bedform Characteristics from Profiling Sonar Datasets Recorded in Shallow Coastal Waters of China

The morphological characteristics of small-scale bedforms were measured by means of an acoustic profiling sonar on the Dafeng tidal flat, Jiangsu, in 2009, and in the Jiulong Estuary, Xiamen, in 2010, respectively. The ＂multi-threshold value＂ method was utilized to reveal the morphological undulations along which bedforms were present. Analyses of the datasets obtained show that： （1） sand ripples can have irregular shapes, and （2） changes in bedform morphology are small within a single tidal cycle but may be significant over several tidal cycles. Fractal and variogram analyses of the seabed roughness revealed the existence of a significant relationship between current speed and the fractal dimension of the seabed roughness. As current speed increases, seabed roughness increases with a trend of smaller-scale bottom structures being replaced by larger-scale structures. Furthermore, the surface of the larger-scale bottom structures can either become smooth due to the absence of smaller-scale features or become rougher due to the presence of superimposed smaller-scale structures.

Flow resistance in the channel-bar landscape of large alluvial rivers

Accurate approaches for estimating flow resistance in large alluvial rivers are fundamental for simulating discharge,sediment transport,and flood routing.However,methods for estimating riverbed resistance and additional resistance in the channel-bar landscapes remain poorly investigated.In this study,we used in situ river bathymetry,sediment,and hydraulic data from the Shashi Reach in the Yangtze River to develop a semi-empirical approach for calculating flow resistance.Our method quantitatively separates flow resistance into riverbed resistance and additional resistance and shows high accuracy in terms of deviation ratio(~20%),root-mean-square error(~0.008),and geometric standard deviation(~3).Additional resistance plays a dominant role under low-flow conditions but a secondary role under high flows,primarily due to the reduction in momentum exchange in channel-bar regions as discharge increases.Riverbed resistance first decreases and then increases,which might be attributed to bedform changes in the lower and transitional flow regimes as flow velocity increases.Overall,our findings further the understanding of dynamic changes in flow resistance in the channel-bar landscapes of large river systems and have important implications for riverine ecology and flood management.

Sediment bed destabilization by an isolated vortex:an experimental approach

The destabilization of an erodible sediment bed by a reproducible impulsive phenomenon is studied experimentally.For this,a specific setup is designed to produce a well-controlled isolated vortex,advected over a uniform sand bed.Particle image velocimetry(PIV)measurements are performed to get fluid flow information;suspended particles images are also recorded.A new measurement technique,based on a stereo-correlation method,provides a precise reconstruction of the bedform morphology over time.Strong turbulent structures(namely sweep events)are evidenced in a near-wall region,where particles are mainly dislodged.The sediment plume follows the vortex:in particular,the upward velocity inside maintains the suspension phenomenon.However,the dispersion process seems to be not dependent on the vortex parameters.Then,the bed morphology is investigated:a scour hole is dug as the vortex hits the granular layer.Its dynamics is described by a typical scour law,even if the initial perturbation is transient.Another area,where the particles settled,can be observed:it behaves like a ripple,with a logarithmic growth of its crest.This deposition region has a specific geometry,studied here by a Fourier contour approach.Finally,a pickup rate is also derived from the morphological data:it appears independent of the vortex and bedform features.

Self-organization Mechanism during the Bedform Process

This paper investigates the functional and morphological self organization phenomena that occur during the bedform process in river systems. The fluvial process has architecturally functional self organization actions that serve to self adjust the river regime. The bedform (or sand waves) process is part of the functional self organization at the middle level of the geometrical scale. By increasing the resistance of the mobile bed and simultaneously decreasing the capacity carrying sediment, the bedform serves to self adjust the river system. The morphological self organization of the bedform process is the basis for the functional self organization. The concept of the water sand interaction region is suggested, and a nonlinear model is constructed to describe the complex interaction among water flow, bed load transport, and local bed deformation, i e , the sand waves. A numerical simulation was developed based upon this model. The primary results show that the model is able to repeat many important phenomena in the bedform process, especially the critical phase transition.

Free Boundary Problem in Bedform Evolution of Alluvial River System

The bedform evolution in an alluvial river system is studied. A mathematical model is designed to simulate the flow in the water sand region and the bedform processes.The model results in a free boundary problem. Features of the free boundary problem are given.Explicit solutions as well as their physical implications for a few special cases of the free boundary problem are presented.