flow patterns and morphology of a prograding river delta the transition of flow between laterally confined channels and the unchannelized delta front controls the morphodynamic evolution of river deltas but has rarely been measured at the field scale we quantify flow patterns and bathymetry that define the evolution of the subaqueous delta front on the wax lake delta a rapidly prograding delta in coastal louisiana a significant portion of flow 59 departs the channel network over lateral channel margins as opposed to the downstream channel tips bathymetric surveys and remotely sensed estimates of flow direction allow spatial changes in flow velocity to be quantified and patterns of erosion and deposition to be estimated shallowing along channel margins produces spatial acceleration and erosion lateral spreading deceleration and deposition occur within three to eight channel widths outside of the channel margins in interdistributary bays the shape of each flow path is constrained by nourishment boundaries that separate the outflows from neighboring channels deposit elevation decreases with a basinward slope of 2 4 10 4 with distance from a channel margin along any flow path regardless of the channel or location that flow departed the network bathymetric depressions called interdistributary troughs form along nourishment boundaries where flow paths are the longest and deposit elevation is correspondingly low we conclude that the deposit morphology exerts a strong control on bathymetric evolution and that interaction between neighboring channels and even neighboring deltas can influence delta front morphology

landslide mobility analysis for design of multiple debris resisting barriers multiple debris resisting barriers have been commonly used worldwide to mitigate debris flows in drainage lines however a well developed methodology to assess the mobility of debris flows with co

interaction between dry granular flow and rigid barrier with basal clearance analytical and physical modelling some types of barriers are designed with a clearance between the bottom of the barrier and the channel bed this feature allows small discharges to pass thereby reducing the maintenance required o

shock waves dead zones and particle free regions in rapid granular free surface flows shock waves dead zones and particle free regions form when a thin surface avalanche of granular material flows around an obstacle or over a change in the bed topography understanding and modelling these flows is of considerable practical interest for industrial processes as well as for the design of defences to protect buildings structures and people from snow avalanches debris flows and rockfalls these flow phenomena also yield useful constitutive information that can be used to improve existing avalanche models in this paper a simple hydraulic theory first suggested in the russian literature is generalized to model quasi two dimensional flows around obstacles exact and numerical solutions are then compared with laboratory experiments these indicate that the theory is adequate to quantitatively describe the formation of normal shocks oblique shocks dead zones and granular vacua such features are generated by the flow around a pyramidal obstacle which is typical of some of the defensive structures in use today

formation of coastline features by large scale instabilities induced by high angle waves along shore sediment transport that is driven by waves is generally assumed to smooth a coastline this assumption is valid for small angles between the wave crest lines and the shore as has been demonstrated in shoreline models but when the angle between the waves and the shoreline is sufficiently large small perturbations to a straight shoreline will grow here we use a numerical model to investigate the implications of this instability mechanism for large scale morphology over long timescales our simulations show growth of coastline perturbations that interact with each other to produce large scale features that resemble various kinds of natural landforms including the capes and cuspate forelands observed along the carolina coast of southeastern north america wind and wave data from this area support our hypothesis that such an instability mechanism could be responsible for the formation of shoreline features at spatial scales up to hundreds of kilometres and temporal scales up to millennia

the multi decadal morphodynamic changes of the mouth bar in a mixed fluvial tidal estuarine channel abstract inner channel mouth bars imb are common and vital geomorphological structures in estuaries that can efficiently promote the progradation of a fluvial delta however these significant structures face serious interferences by natural and human forces this study mainly focuses on imb deposited in the mixed fluvial tidal dominated south passage sp of the changjiang estuary between 1959 and 2018 to reveal how multidecadal morphodynamic variability in a mouth bar responds to natural forcing and human interferences the results show that the volume of the imb in the sp increased from 1128 106 m3 in 1959 to 1636 106 m3 in 1989 and then decreased to 462 106 m3 in 2018 meanwhile the evolution of the imb could be divided into five stages the growing phase 1959 to 1979 which showed a v shape the partial adjustment phase 1999 2003 which showed a crest dent shape the stable phase which showed a flat shape 2004 2010 the partial adjustment phase 2011 2017 when jiangyanan shola inserted into the sp and finally evolved into a flat shape in 2018 fluvial water discharge and suspended sediment discharge ssd did not control the variation in the mouth bar enso events were normally responsible for not only the periodic variations in the landward slope and the water depth of the crest of the imb but also occasional extreme changes in the imb moreover local sandbar insertion sediment from the seaside induced by strong tidal power and intensified engineering projects resulted in the continual shrinkage of the mouth bar by depositing more sediment at the mouth bar our work implies that understanding and governing this imb will bring additional economic benefits to this fast developing society but the protection of this delicate estuarine geomorphology system should still receive great attention

relationship between the morphological evolution of the river mouth bar and fluvial input in the modaomen estuary river mouth bars commonly exist in estuaries and are important for flood control estuarine ecology and the navigation of estuarine areas the modaomen estuary is a river dominated estuary in the pearl river delta and has a typical river mouth bar here hydrologic and bathymetric datasets that span 2000 2011 are used to analyze the relationship between the fluvial input of river discharge and sediment load and the evolution of the river mouth bar the results show that the eastern and western channels have notably extended seaward since 2000 the evolution rates exhibit distinct asymmetry with the seaward extension rate of the eastern channel being approximately 1 6 times greater than that of the western channel the upstream bar face and bar back of the center mouth bar have been eroded and intensive human activities have significantly affected the evolution of the center mouth bar the center mouth bar s apex has consistently moved seaward from 2000 to 2011 river discharge especially during extreme river floods rather than sediment load has played a more vital role in the development of the bifurcation channels and the center mouth bar an empirical relationship between the bar apex displacement rates and dominant discharge is obtained based on the observations indicating that greater river discharge and longer duration will move the bar apex further seaward the bar apex s displacement during the extreme river floods of 2005 6 and 2008 6 was 133 6 m and 99 4 m respectively

alongshore sediment bypassing as a control on river mouth morphodynamics river mouths shoreline locations where fluvial and coastal sediments are partitioned via erosion trapping and redistribution are responsible for the ultimate sedimentary architecture of deltas and because of their dynamic nature also pose great management and engineering challenges to investigate the interaction between fluvial and littoral processes at wave dominated river mouths we modeled their morphologic evolution using the coupled hydrodynamic and morphodynamic model delft3d swan model experiments replicate alongshore migration of river mouths river mouth spit development and eventual spit breaching suggesting that these are emergent phenomena that can develop even under constant fluvial and wave conditions furthermore we find that sediment bypassing of a river mouth develops though feedbacks between waves and river mouth morphology resulting in either continuous bypassing pathways or episodic bar bypassing pathways model results demonstrate that waves refracting into the river mouth bar create a zone of low alongshore sediment transport updrift of the river mouth which reduces sediment bypassing sediment bypassing in turn controls the river mouth migration rate and the size of the river mouth spit as a result an intermediate amount of river discharge maximizes river mouth migration the fraction of alongshore sediment bypassing can be predicted from the balance between the jet and the wave momentum flux quantitative comparisons show a match between our modeled predictions of river mouth bypassing and migration rates observed in natural settings

scaling relationships and evolution of distributary networks on wave influenced deltas 1 no genetic model can explain the variability in distributary network pattern on modern deltas here we derive scaling relationships for two processes known to create distributary channels and with these laws construct a simple model for distributary network evolution the first process is mouth bar deposition at the shoreline and subsequent channel bifurcation the second is avulsion the wholesale abandonment of a channel in favor of a new path the former creates relatively small networks with power law distributions of channel length the latter generates relatively few backwater scale distributaries frequency magnitude plots of channel length on natural deltas agree with theoretical predictions and show a clear separation in scale that reflects these processes mouth bar distributary lengths scale with the width of the parent channel and avulsive distributary lengths scale with the backwater length intermediate channel lengths are relatively rare wave energy controls network topology by suppressing mouth bar development thereby preferentially eliminating smaller scale distributaries

a model for the runout analysis of rapid flow slides debris flows and avalanches runout analyses are used for risk assessment and design of remedial measures against rapid landslides such as debris flows debris avalanches rockslide avalanches large scale liquefaction failure

dry granular flow interaction with dual barrier systems multiple barriers are commonly installed along predicted geophysical flow paths to intercept large flow volumes the main criterion for multiple barrier design is volume retained the velocity of t

significant effect of sediment cohesion on delta morphology delta morphology is thought to be controlled by factors such as river discharge tides and waves numerical modelling shows that sediment cohesion also strongly influences the development of a delta s characteristics the morphologies of the world s deltas are thought to be determined by river discharge tidal range and wave action 1 more recently sea level rise 2 3 and human engineering4 have been shown to shape delta evolution the effects of factors such as sediment type and the overall amount of sediment carried by rivers are considered secondary4 5 6 in particular the role of sediment cohesion which is controlled by sediment size and type of vegetation is unclear here we use a numerical flow and transport model7 8 9 10 to show that sediment cohesiveness also strongly influences the morphology of deltas we find that holding all other factors constant highly cohesive sediments form bird s foot deltas with rugose shorelines and highly complex floodplains whereas less cohesive sediments result in fan like deltas with smooth shorelines and flat floodplains in our simulations sediment cohesiveness also controls the number of channels that form within the deltas and the average angle of bifurcation of those channels as vegetation generally acts as a cohesive agent we suggest that deltas that formed before the expansion of land plants in the devonian period should show fan like characteristics a finding consistent with the limited data from the sedimentological record11

wave angle control of delta evolution 1 wave influenced deltas with large scale arcuate shapes and demarcated beach ridge complexes often display an asymmetrical form about their river channel here we use a numerical model to demonstrate that the angles from which waves approach a delta can have a first order influence upon its plan view morphologic evolution and sedimentary architecture the directional spread of incoming waves plays a dominant role over fluvial sediment discharge in controlling the width of an active delta lobe which in turn affects the characteristic rates of delta progradation oblique wave approach and a consequent net alongshore sediment transport can lead to the development of morphologic asymmetry about the river in a delta s plan view form this plan form asymmetry can include the development of discrete breaks in shoreline orientation and the appearance of self organized features arising from shoreline instability along the downdrift delta flank such as spits and migrating shoreline sand waves features observed on natural deltas somewhat surprisingly waves approaching preferentially from one direction tend to increase sediment deposition updrift of the river this morphodynamic groin effect occurs when the delta s plan form aspect ratio is sufficiently large such that the orientation of the shoreline on the downdrift flank is rotated past the angle of maximum alongshore sediment transport resulting in preferential redirection of fluvial sediment updrift of the river mouth

scaling and design of landslide and debris flow experiments abstract scaling plays a crucial role in designing experiments aimed at understanding the behavior of landslides debris flows and other geomorphic phenomena involving grain fluid mixtures scaling can be addressed by using dimensional analysis or more rigorously by normalizing differential equations that describe the evolving dynamics of the system both of these approaches show that relative to full scale natural events miniaturized landslides and debris flows exhibit disproportionately large effects of viscous shear resistance and cohesion as well as disproportionately small effects of excess pore fluid pressure that is generated by debris dilation or contraction this behavioral divergence grows in proportion to h3 where h is the thickness of a moving mass therefore to maximize geomorphological relevance experiments with wet landslides and debris flows must be conducted at the largest feasible scales another important consideration is that unlike stream flows landslides and debris flows accelerate from statically balanced initial states thus no characteristic macroscopic velocity exists to guide experiment scaling and design on the other hand macroscopic gravity driven motion of landslides and debris flows evolves over a characteristic time scale l g 1 2 where g is the magnitude of gravitational acceleration and l is the characteristic length of the moving mass grain scale stress generation within the mass occurs on a shorter time scale h gl 1 2 which is inversely proportional to the depth averaged material shear rate a separation of these two time scales exists if the criterion h l

mechanics of river mouth bar formation implications for the morphodynamics of delta distributary networks 1 in this paper we use observational data and numerical modeling to present a new explanation for the formation of river dominated delta networks observational data from deltas throughout the world show that distributary channel widths depths and lengths decrease nonlinearly with successive bifurcations trends in width and depth are an outcome of hydraulic geometry scaling the trend in channel length is a consequence of delta growth analyses of serial maps show that the positions of bifurcations are the fossilized locations of river mouth bars also called middle ground and distributary mouth bar in front of old delta channel mouths therefore the trend in channel length can be explained through the mechanics of river mouth bar formation and evolution we use delft3d a coupled hydrodynamic and morphodynamic model to simulate the process of river mouth bar formation within an expanding turbulent jet in front of distributary channel mouths our results describe in detail the formation and evolution of a river mouth bar system and demonstrate that the distance to the river mouth bar is proportional to jet momentum flux and inversely proportional to grain size therefore channel length decreases down delta because with each successive bifurcation the jet momentum flux decreases these results can be used to predict the future evolution of river dominated deltas and can be used to aid in hydrocarbon exploration of these depositional environments

the effect of wind waves on the development of river mouth bars 1 river deltas are among the most important environments on earth housing productive ecosystems and a large fraction of the human population the key process for delta development is the deposition of mouth bars in front of delta distributaries predicting mouth bar formation on marine coastlines is complex because of the interactions between waves tides water and sediment discharge here we study the effects of waves on mouth bar growth with the coupled sediment transport and wave model delft3d swan our results show that wave characteristics height period and direction play an important role in the formation of mouth bars in our numerical experiments waves affect bar development in three ways by modifying the direction of the river jet by increasing bottom shear stresses at the river mouth and by changing bottom friction and hence increasing jet spreading we further show that high waves with long period prevent the formation of mouth bars in particular wave angles between 45 and 60 are the least favorable to bar formation likely producing a deflected river mouth

sediment eddy diffusivity in meandering turbulent jets implications for levee formation at river mouths 1 depositional patterns characteristic of river mouths are controlled by the sediment laden turbulent jet exiting from rivers here we show that jet instability developing at river mouths with small width to depth ratio can affect these patterns numerical simulations indicate that sediment eddy diffusivity which is a measure of the spreading of sediments out of the jet core depends on the interplay between sediment settling and large coherent flow structures which are associated with unstable jets our results suggest that optimal conditions for eddy diffusivity are met when the time scale of sediment settling is equal to the characteristic eddy time scale in this condition sediments are deposited at the farthest distance from the jet centerline thus promoting the formation of lateral deposits settling time scales much greater or much smaller than the eddy time scale is associated with low eddy diffusivity which promotes the formation of a central deposit such eddy mediated sediment transport is more evident in the zone of established flow than in the zone of flow establishment since the former is characterized by fully developed vortices a simplified model is proposed to estimate the settling and eddy time scales and to predict the optimal condition for levee formation

controls on river delta formation insights from numerical modelling abstract primary hydraulic and sedimentary controls on river deltas were previously postulated in classification schemes in the field of sedimentary geology based on many field observations however detailed mechanistic models were restricted to the river dominated delta class analogous to the concept of morphological conditioning here we show that antecedent stratigraphy controls morphometrics of prograding river dominated delta distributary networks under steady sea level further we use coupled hydrodynamic morphodynamic stratigraphic detailed numerical modelling to assess the influence of wind generated waves and tides on clastic river delta formation our synthetic simulations show that deltas forming under mere riverine forcing prograde via sequences of mouth bar induced flow bifurcation and upstream channel shifting windwave action suppresses sequestration of fine sediments on the developing delta plain entailing relatively smooth shorelines perturbed by a limited number of distributary channels in contrast tide influenced river deltas are found to prograde mainly via lengthening of initially formed relatively stable distributaries as well as being characterized by cyclicity in deposits interbedding of sands and silts these results provide a framework for physics based river delta modelling under various environmental conditions particularly our findings suggest that relatively low energetic basin conditions can already significantly impact morphological and stratal patterns of prograding river deltas