conceptual framework for assessing the response of delta channel networks to holocene sea level rise abstract recent research has identified two fundamental unit processes that build delta distributary channels the first is mouth bar deposition at the shoreline and subsequent channel bifurcation which is driven by progradation of the shoreline the second is avulsion to a new channel a result of aggradation of the delta topset the former creates relatively small branching networks such as wax lake delta the latter generates relatively few long distributaries such as the mississippi and atchafalaya channels on the mississippi delta the relative rate of progradation to aggradation and hence the creation of accommodation space emerges as a controlling parameter on channel network form field and experimental research has identified sea level as the dominant control on holocene delta growth worldwide and has empirically linked channel network changes to changes in the rate of sea level rise here i outline a simple modeling framework for distributary network evolution and use this to explore large scale changes in holocene channel pattern that have been observed in deltas such as the rhine meuse and mississippi rapid early to mid holocene sea level rise forced many deltas into an aggradational mode where i hypothesize that avulsion and the generation of large scale branches should dominate slowing of sea level rise in the last 6000 yr allowed partitioning of sediment into progradation facilitating the growth of smaller scale distributary trees at the shorelines of some deltas and a reduction in the number of large scale branches significant antecedent topography modulates delta response the filling of large incised valleys for example caused many deltas to bypass the aggradational phase human effects on deltas can be cast in terms of geologic controls affecting accommodation constriction of channels forces rapid local progradation and mouth bar bifurcation while accelerated sea level rise increases aggradation and induces more frequent channel avulsion

the structure and evolution of cold surges east of the rocky mountains abstract northerly surges of cold air often move southward along the eastern side of the rockies from southern canada into mexico the strongest surges which generally develop in midwinter are associated with temperature decreases and pressure rises of 20 30 c and 15 30 mb respectively within 24 h surges are usually accompanied by a meridionally elongated pressure ridge and strong low level ageostrophic winds that parallel the terrain the width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters mexico this paper provides a detailed description of a northerly surge to the east of the rocky mountains that occurred on 12 14 november 1986 using both observational and model data the structural evolution of the surge is analyzed in addition the dynamics of the event is explored by diagnosing the momentum thermodynamic energy and vorticity equations to determine the typical synoptic scale evolution of these

finescale vertical structure and dynamics of some dryline boundaries observed in ihop abstract several radar fine lines all with a humidity contrast were sampled in the central great plains during the 2002 international h2o project ihop this study primarily uses aircraft and airborne millimeter wave radar observations to dynamically interpret the presence and vertical structure of these fine lines as they formed within the well developed convective boundary layer in all cases the fine line represents a boundary layer convergence zone this convergence sustains a sharp contrast in humidity and usually in potential temperature across the fine line the key question addressed herein is whether at the scale examined here 10 km the airmass contrast itself in particular the horizontal density virtual potential temperature difference and resulting solenoidal circulation is responsible for the sustained convergence and the radar fine line for the 10 cases examined herein the answer is affirmative

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

simplified process modeling of river avulsion and alluvial architecture connecting models and field data abstract modeling is an invaluable tool for studying sedimentary basin filling and for understanding depositional processes with long recurrence intervals including channel avulsion simplified modeling approaches such as cellular models and process analogue experiments are particularly useful for efficiently exploring alternative hypotheses and evaluating first order controls on river avulsion and alluvial architecture here we review the history and current state of the art in simplified avulsion and alluvial architecture models with a particular focus on how results and insights from these models can be incorporated into field and subsurface studies and vice versa simplified avulsion and alluvial architecture models have proliferated in the past decade providing a wide variety of models to serve as a basis for future coupled field modeling studies we compare features of leading models and discuss avenues for effectively pairing model capabilities with hypotheses and field data outstanding questions highlighted by recent modeling efforts include 1 what thresholds control avulsion initiation in different systems 2 how do floodplain processes and topography influence avulsion dynamics and alluvial architecture 3 what factors determine where avulsion channels stabilize answering these questions will require targeted modeling efforts coupled to data from ancient systems hence our model comparison emphasizes features that can be used to choose or design fit for purpose models and we outline how quantitative data useful for model selection and validation can be obtained from modern systems and ancient deposits matching model goals with targeted questions and model parameters and predictions with quantitative field data will help tighten communication between field and model oriented sedimentary geologists facilitating advances in our understanding of river avulsion and alluvial architecture

delta progradation and chenier formation in the huanghe yellow river delta china abstract to clarify holocene development of the huanghe yellow river delta and the relationship between delta progradation and chenier formation detailed sediment analyses and high resolution radiocarbon dating were done on borehole samples taken from two sites on the present huanghe h9601 latitude 37 40 5 n and longitude 118 28 7 e with an altitude of 5 5 m and h9602 lat 37 47 8 n and long 118 54 3 e 4 8 m downcore changes of sediment facies and accumulation rate show that delta progradation occurred at least twice during ca 2 6 1 2 c 14 yrbp and 1855 present at the two borehole sites these phases of progradation correlate with superlobes 6 and 7 and superlobe 10 respectively of ten superlobes composing the holocene huanghe delta shown by c xue historical changes in the yellow river delta china marine geology 113 321 329 1993 the boundary of these sediments at h9602 is sharp and erosional and correlates with cheniers located between borehole sites on the delta plain the diastem period between these phases of progradation almost coincides with the periods when the river mouth of the huanghe was located in the northwestern part of the bohai sea during 1048 1128 ad superlobe 8 and the jiangsu region facing the yellow sea during 1128 1855 superlobe 9 this suggests that chenier formation and delta progradation are linked and controlled by sediment supply and river course shifts of the lower reaches of the huanghe

the evolution of an oklahoma dryline part i a meso and subsynoptic scale analysis abstract a study is made of the 8 june 1974 oklahoma dryline and tornado outbreak case using data synthesis 1 to fit existing concepts on dryline structure and behavior to this case and 2 to identify processes contributing to moisture convergence along the dryline the dryline undergoes a major transformation in structure from sloped to slopeless during the day as implied from mesoscale 10 100 km and subsynoptic scale 100 1000 km analysis of virtual potential temperature fields mesoscale examination of dryline movement reveals the presence of wavelike perturbations which propagate along the dryline irregardless of its slope and contribute more to its eastward progression than does the downward slope of the terrain all but one of 22 tornadoes reported in oklahoma on this date were associated with thunderstorms that formed within a subsynoptic moisture convergence region at the dryline in central oklahoma results indicate a downward transport of southwesterly momentum through a well develope

testing morphodynamic controls on the location and frequency of river avulsions on fans versus deltas huanghe yellow river china a mechanistic understanding of river avulsion location and frequency is needed to predict the growth of alluvial fans and deltas the huanghe china provides a rare opportunity to test emerging theories because its high sediment load produces regular avulsions at two distinct nodes where the river debouches from the loess plateau avulsions occur at an abrupt decrease in bed slope and reoccur at a time interval 607 yrs consistent with a channel filling timescale set by the superelevation height of the levees downstream natural deltaic avulsions reoccur at a timescale that is fast 7 yrs compared to channel filling timescale due to large stage height variability during floods unlike the upstream node deltaic avulsions cluster at a location influenced by backwater hydrodynamics and show evidence for episodic downstream migration in concert with progradation of the shoreline providing new expectations for the interplay between avulsion location frequency shoreline rugosity and delta morphology

quantification of tertiary exhumation in the united kingdom southern north sea using sonic velocity data sonic velocities from the upper and middle chalk upper cretaceous the bunter sandstone and the bunter shale both lower triassic were used to independently quantify apparent exhumation height above maximum burial depth in the united kingdom uk southern north sea apparent exhumation is the displacement on the depth axis of a given velocity depth trend from the normal unaffected by exhumation trend apparent exhumation results derived from the upper and middle chalk the bunter sandstone and the bunter shale are statistically similar the consistency of results from carbonate and clastic units suggests that at a formational and regional scale overcompaction i e anomalously high sonic velocity in all three units analyzed reflects previously greater buria depth rather than sedimentological and or diagenetic processes and validates the use of lithologies other than shale in maximum burial depth studies the consistency of results from units of early triassic to late cretaceous age suggests that tertiary exhumation was of sufficiently great magnitude to mask any earlier mesozoic periods of exhumation and that maximum mesozoic cenozoic burial depth in the southern north sea was attained prior to tertiary exhumation the proposed magnitudes of exhumation are generally greater than those previously published for the southern north sea but they are consistent with recent estimates from apatite fission track analysis the amount of exhumation of the rock column is greatest on the recognized inversion axes i e sole pit cleveland where it reaches approximately 2 5 km however there was a regional component of 1 0 1 5 km exhumation during the tertiary that affected structurally uninverted areas and on which the more localized inversion related exhumation was superimposed cretaceous tertiary burial prior to exhumation must have been of greater magnitude and more rapid than suggested by the preserved stratigraphy the effect of this extra burial and subsequent exhumation on sedimentary rock decompaction procedure and thermal maturation modeling is illustrated for the cleethorpes 1 and 44 7 1 wells and must also be incorporated in modeling reservoir diagenesis the regional tertiary tectonic uplift associated with exhumation must have had a thick skinned origin

on the anomalous counterclockwise turning of the surface wind with time in the plains of the united states abstractvertical shear in the boundary layer affects the mode of convective storms that can exist if they are triggered in western portions of the southern plains of the u s vertical shear in the absence of any transient features changes diurnally in a systematic way thus leading to a preferred time of day for the more intense modes of convection when the shear particularly at low levels is greatest in this study yearly and seasonally averaged wind observations for each time of day are used to document the diurnal variations in wind at the surface and in the boundary layer with synoptic and mesoscale features effectively filtered out data from surface mesonets in oklahoma and texas doppler wind profilers instrumented tower data and also seasonally averaged wind data for each time of day from convection allowing numerical model forecasts are used it is shown through analysis of observations and model data that the perturbation wind above anemometer level turns in a clockwise manner wit

conditions for branching in depositional rivers it is often taken for granted that rivers organize transport into a single active channel in some net depositional environments however flow of water and sediment is distributed in several stable channels such branching rivers may be confined in valleys anabranching or anastomosed or unconfined on deltas distributaries and their existence confronts us with the very basic question of what governs the spatial organization of channel patterns in sedimentary landscapes current models for equilibrium channel morphology cannot predict the occurrence of branching rivers because they do not consider dynamical processes such as avulsion i e the rapid abandonment of a channel in favor of a new path at lower elevation the requisite conditions for avulsion have been the subject of ongoing debate here we resolve the conditions leading to channel avulsion and show that branching rivers occur when avulsion is the dominant mechanism of lateral channel motion a compilation of field and laboratory data demonstrates that avulsion frequency scales with the time required for sedimentation on channel beds to produce a deposit equal to one channel depth from the relative rates of bank erosion and channel sedimentation we derive a dimensionless mobility number that accurately predicts the conditions under which anabranching and distributary channels occur results may be directly applied to modeling landscape evolution over human and geologic time scales and for inverting formative environmental conditions from channel deposits on earth and other planetary surfaces

predicting delta avulsions implications for coastal wetland restoration river deltas create new wetlands through a continuous cycle of delta lobe extension avulsion and abandonment but the mechanics and timing of this cycle are poorly understood here we use physical experiments to quantitatively define one type of cycle for river dominated deltas the cycle begins as a distributary channel and its river mouth bar prograde basinward eventually the mouth bar reaches a critical size and stops prograding the stagnated mouth bar triggers a wave of bed aggradation that moves upstream and increases overbank flows and bed shear stresses on the levees an avulsion occurs as a time dependent failure of the levee where the largest average bed shear stress has been applied for the longest time r 2 0 93 these results provide a guide for predicting the growth of intradelta lobes which can be used to engineer the creation of new wetlands within the delta channel network and improve stratigraphic models of deltas

drowning of the mississippi delta due to insufficient sediment supply and global sea level rise global sea level rise reduced sediment supply and subsidence threaten the stability of the mississippi delta calculations of riverine sediment load and storage indicate that 10 000 13 500 km2 of the delta could be submerged by ad 2100

geomorphic tectonic control of sediment discharge to the ocean the importance of small mountainous rivers analysis of data from 280 rivers discharging to the ocean indicates that sediment loads yields are a log linear function of basin area and maximum elevation of the river basin other factors controlling sediment discharge e g climate runoff appear to have secondary importance a notable exception is the influence of human activity climate and geology on the rivers draining southern asia and oceania sediment fluxes from small mountainous rivers many of which discharge directly onto active margins e g western south and north america and most high standing oceanic islands have been greatly underestimated in previous global sediment budgets perhaps by as much as a factor of three in contrast sediment fluxes to the ocean from large rivers nearly all of which discharge onto passive margins or marginal seas have been overestimated as some of the sediment load is subaerially sequestered in subsiding deltas before the proliferation of dam construction in the latter half of this century rivers

mesoscale dynamics of the near dryline environment analysis of data from cops 91 abstract a technique is introduced using surface data from triangular networks of adjacent mesonet stations to estimate the terms in the horizontal equation of motion at anemometer level 10 m on either side of the dryline in the southern plains of the united states data from an instrumented surface mesonetwork in the texas panhandle and western and central oklahoma during the cooperative oklahoma profiler studies cops 91 field program were employed for this purpose east of the dryline and surface pressure trough the vertical mixing term had a component normal and to the right of the surface wind in accord with ekman theory west of the dryline and surface pressure trough the vertical mixing term had a component normal and to the left of the surface wind in disagreement with ekman theory it is suggested that disagreements with ekman theory may be due to baroclinic effects in the boundary layer it is also shown that during the day both the westward component of the pressure gradient force and the ea

the evolution of an oklahoma dryline part ii boundary layer forcing of mesoconvective systems abstract data from the national severe storms laboratory surface mesonetwork are objectively analyzed to give insight into processes that contributed to the development of three tornadic mesoconvective systems near the 8 june 1974 oklahoma dryline storm cells constituting each of the systems form over recurring zones of convergence within 20 km of the dryline different mechanisms appear to force the individual convergence zones storms of the first system appear simultaneously only after the establishment of a pressure trough just cast of a zone of convergence 15 km east of the dryline the convergence zone intensifies and progresses eastward with the storms meanwhile a second convergence zone appears at the dryline in response to apparent storm induced pressure systems trailing the storms the fact that deep convection did not occur over the second zone is attributed to static stabilization caused by mesoscale unsaturated downdrafts in the upper troposphere storms of the second system develop in a c

the influence of topography and lower tropospheric winds on dryline morphology abstract the effects of topography wind shear and zonal wind speed on dryline formation and evolution are investigated using a three dimensional nonhydrostatic mesoscale model rather than conduct a case study a parameter study was performed to examine factors that control the depth and strength of the dryline circulation this study reveals that the potential for convective storm formation is greatest in those environments in which the cross dryline flow is weak above the dryline location this results in boundary layer flow nearly parallel to the north south oriented dryline boundary under these conditions the subsiding westerly flow and elevated residual layer formation do not strengthen the capping inversion above the eastern convective boundary layer in addition moist air parcels from near the surface are able to reside within the dryline updraft resulting in higher midtropospheric relative humidity subsequently increasing the likelihood of convective storm initiation as downslope westerly flo

appalachian cold air damming abstract appalachian cold air damming is investigated by means of 1 a 50 yr monthly climatology 2 a synoptic case study of the event of 21 23 march 1985 and 3 an investigation of the flow structure and force balance within the cold dome the climatology reveals cold air damming is a year round phenomenon in the southern appalachians with the most frequent prolonged and intense events occurring in winter particularly december and march when three five events per month can be expected cold air damming is least frequent and intense in july the synoptic case study reveals that cold air damming is critically dependent upon the configuration of the synoptic scale flow the cold dome can be identified by a u shaped ridge trough in the sea level isobar thermal patterns and the 930 mb height temperature fields representative of conditions at the base of the inversion overlying the cold dome differential horizontal and vertical thermal advection as well as adiabatic and evaporative cooling are r

pressure and buoyancy fields derived from doppler radar data in a tornadic thunderstorm abstract a method for retrieval of pressure and buoyancy distributions in deep convection is applied to doppler radar data collected at two analysis times during the tornadic del city oklahoma thunderstorm of 20 may 1977 change of a previous version of the technique necessitated by application to real data include procedures for handling irregularly bounded volumes and missing data and new assumptions to include reflectivity data and turbulent effects in the equations internal consistency cheeks on the quality of retrieved pressure fields imply that the input data are generally of good quality and point out times and heights within the storm at which greater confidence can be placed in the derived fields in the pretornadic stage the pressure distribution includes at each level a high low couplet across the updraft with the maximum pressure gradient generally oriented along the environmental shear vector at that altitude these results are in agreement with predictions of linear theory locations of

formation and diurnal variation of the dryline abstract the formation and diurnal evolution of the dryline during fair weather have been investigated through the use of a two dimensional mesoscale model that includes condensation evaporation budget equations of surface energy and moisture field as well as turbulence and radiation parameterizations a moderately strong vertical wind shear was introduced on a sloping terrain where the soil is very dry on the west side but moist on the east initially a weak easterly geostrophic wind exists to the east but a weak westerly geostrophic wind to the west of the dryline during daytime deepening of the mixed layer due to vertical mixing especially on the west side forces the dryline to advance eastward the westerly wind on the west side is maintained by downward transport of westerly momentum due to strong vertical mixing the easterly wind on the east side is strengthened due to the inland sea breeze circulation therefore the resulting low level convergence sustains a strong moisture gradient along

observations of the finescale structure of a dryline during vortex 95 abstract the three dimensional finescale structure of a dryline observed over the texas panhandle during the verification of the origins in tornadoes experiment vortex on 6 may 1995 is presented high resolution observations documenting dryline evolution were collected from midafternoon while it moved slowly eastward until late afternoon as it retrogressed back to the west dryline variability in the horizontal along line direction was revealed by airborne doppler radar observations from the national center for atmospheric research electra doppler radar and ground based measurements from the nearby wsr 88d at amarillo texas this variability was created as the dryline interacted with horizontal convective rolls forming west of the dryline the rolls intersected the dryline at periodic locations creating radar reflectivity and vertical velocity maxima more importantly clouds initiated at these intersection points additional dryline variability was generated through the interaction of the low level fl

quantitative models of sedimentary basin filling summary quantitative modelling of the filling of sedimentary basins was begun in earnest in the 1960s dozens of themes and variations have been proposed since then and have yielded an abundance of idealized stratigraphic patterns as functions of both imposed changes and basin properties post plate tectonic modelling began with rigid lid models which show the stratigraphic signature of subsidence variation this work introduced the connection between stratigraphy and the rheology of the lithosphere rigid lid models are the simplest type of geometric model in which the sediment surface is assigned prescribed geometries usually corresponding to different depositional environments these can reproduce many aspects of overall stratal geometry but are formally restricted to relatively long timescales for which quasi steady surface topography can be assumed so called dynamic models attempt to represent the morphodynamics of the sediment surface by abstracting and averaging short term transport processes most of the dynamic models proposed to date can be seen as special cases of a single general morpho dynamic equation the most important result of the first wave of quantitative basin filling models is that even relatively simple models can produce reasonable stratal patterns we now have a wide array of tools for exploring scenarios searching for general behaviours and effects and making initial quantitative predictions we have also learned that basin response to external forcing as recorded in stratigraphy can be as sensitive to the characteristics of the basin as to the forcing the main brake on the development of basin modelling is not computing power but lack of methods and data for testing the models we have already developed physical experiments which are only just beginning are one means of doing this experimental stratigraphy is a bridge to quantitative field tests which will require collaboration among academic researchers from a wide range of areas and between academia and industry on projects of greater scale and degree of integration than we have seen so far the advancement of quantitative sedimentary geology will also require significant changes in the way the subject is taught at all levels

a modeling study of the dryline abstract results of a modeling study of the 24 may 1989 dryline are presented a nonhydrostatic two dimensional version of the colorado state university regional atmospheric modeling system csu rams is used to deduce the impact of east west variability of soil moisture and vegetation on convective boundary layer evolution and dryline formation the effects of the initial moisture and wind fields and the impact of the coriolis force on the model results are also examined model output is compared with special airborne and sounding observations of the 24 may dryline several findings of an earlier observational study of the 24 may dryline are supported in the present study the modeled drylines are broadly comparable to the observed dryline with respect to the following properties 1 width 10 km 2 strong horizontal moisture and virtual potential temperature gradients 4 g kg 1 10 km and 2 k 10 km 3 strong horizontal convergence updraft w e shear of n s wind component 8 10 4 s 1 1 m s 1 10 m

characteristics of dryline passage during cops 91 abstract the characteristics of dryline passage are documented through an analysis of data from an instrumented surface mesonetwork in the texas panhandle and western and central oklahoma during the cooperative oklahoma profiler studies field program some eastward moving drylines at the surface during the day were characterized by monotonic drops in dewpoint after dryline passage others were marked by a series of rapid drops punctuated by periods of no change after dryline passage which suggests that the dryline often progresses in discrete steps rather than continuously the dryline during the daytime was not always collocated with a pressure trough although the strongest dryline observed was analyses of surface pressure traces indicated that westward moving drylines during the evening did not display behavior characteristic of strong intense density currents as had been found in other studies evidence is presented in one case of 90 min oscillations in water vapor and wind behind the dryline

a case study of severe storm development along a dryline within a synoptically active environment part i dryline motion and an eta model forecast through a case study approach the motion of a dryline on 16 may 1991 within a synoptically active environment in the southern plains along which severe storms ultimately developed is examined in detail observations from research aircraft surface mesonetwork stations mobile ballooning vehicles radar wind profilers and operational surface and upper air networks are examined and combined additionally output from the operational mesoscale eta model is examined to compare predictions of dryline motion with observations and to aid in interpretation of observations the dryline on this day advanced rapidly eastward and included formation of a bulge additionally in at least two instances it exhibited redevelopment loss of definition at one location and gain at another aircraft observations revealed that an eastward redevelopment occurred in the early afternoon and was characterized by a series of four steps along the western edge of the boundary layer moisture the westernmost and easternmost steps coincide with the locations of the dryline before and after redevelopment respectively the retreat of the dryline in the central and southern portion of the analysis domain in the late afternoon included both continuous motion and redevelopment toward the west northwest this dual mode retreat of the dryline was accompanied by gradual backing of the winds and moistening in low levels the eta model forecast initialized at 1200 utc produced dryline features that were qualitatively similar to observed fields the eastward motion of a broad area of enhanced moisture gradient agreed well with observations following an initial spinup period a north south moisture convergence axis preceded the rapid eastward motion of the dryline by several hours lack of subsidence in the air behind the modeled dryline leads to the conclusion that processes other than downward transfer of horizontal momentum by larger scale motions that would support eastward advection produced the rapid dryline motion and observed eastward dryline bulge results of diagnosing physical processes affecting model dryline motion point toward boundary layer vertical mixing coupled with advection of dry air aloft as vital components in rapid advance of the dryline eastward in this synoptically active case

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

overbank and channelfill deposits of the modern yellow river delta abstract the huanghe is noted for its high transport rate of silt and clay which may reach depth averaged values of 200 kg m 3 during peak discharge the sediment load transported through the river on entering the delta plain amounts to 1012 kg per year in contrast to most other large deltas only one distributary channel is active at any one time the high sediment load causes the rivermouth to prograde at a yearly rate of 1 4 km into the shallow less than 20 m deep bohai gulf the vertical aggradation of the channel belt and mouth bar complex is also rapid decimetres per year on average so that after a normal average of twelve years increasing channel instability and avulsion create the start of a new delta lobe a series of satellite images covering the last fifteen years has provided insight in the evolution of the river pattern as well as the progradation of the delta front a newly developed distributary passes from a multichannel to a single straight channel system and ends with the formation of meanders the protruding mature delta lobe shows a radiating pattern of crevasse channels overbank crevasse deposits are made of vertically stacked dm scale waning flow sequences structurally characterized by from bottom to top small scour and fills even parallel lamination and climbing ripple crosslamination accumulation rates on crevasse splays can be predicted on the basis of estimated river sediment discharge it can be concluded that each sequence has been deposited within a few hours and that tidal waterlevel fluctuations may have played a role in the generation of a single sequence

experimental alluvial fan evolution channel dynamics slope controls and shoreline growth 1 river deltas and alluvial fans have channelization and deposition dynamics that are not entirely understood but which dictate the evolution of landscapes of great social economic and ecologic value our lack of a process based understanding of fan dynamics hampers our ability to construct accurate prediction and hazard models leaving these regions vulnerable here we describe the growth of a series of experimental alluvial fans composed of a noncohesive grain mixture bimodal in size and density we impose conditions that simulate a gravel sand fan prograding into a static basin with constant water and sediment influx and the resulting fans display realistic channelization and avulsion dynamics we find that we can describe the dynamics of our fans in terms of a few processes 1 an avulsion sequence with a timescale dictated by mass conservation between incoming flux and deposit volume 2 a tendency for flow to reoccupy former channel paths and 3 bistable slopes corresponding to separate entrainment and deposition conditions for grains several important observations related to these processes are an avulsion timescale that increases with time and decreases with sediment feed rate fan lobes that grow in a self similar quasi radial pattern and channel geometry that is adjusted to the threshold entrainment stress we propose that the formation of well defined channels in noncohesive fans is a transient phenomenon resulting from incision following avulsion and can be directly described with dual transport thresholds we present a fairly complete process based description of the mechanics of avulsion and its resulting timescale on our fans because the relevant dynamics depend only on threshold transport conditions and conservation of mass we show how results may be directly applied to field scale systems

aspects of the baroclinic boundary layer abstract the ekman taylor boundary layer model is solved for the case of a linear variation of the geosptophic wind with height the two layer model couples a monin obukhov similarity layer to an ekman layer with a vertically constant eddy diffusivity the presence of the thermal wind contributes both an along isotherm and a cross isotherm component to the boundary layer flow the along isotherm flow is supergeostrophic and results from the net downward transport of geostrophic momentum by the eddies the cross isotherm flow is toward the warm air and results from the coriolis deflection of the geostrophic momentum rich air aloft that has been mixed downward the effect of the baroclinity i e the thermal wind shear on the wind field is conveniently summarized geometrically the model predicts that the surface vorticity increases in regions of cyclonic thermal vorticity i e the vorticity of the thermal wind however anticyclonic thermal vorticity produces convergence of the low level warmward flow

changing pattern of accretion erosion of the modern yellow river huanghe subaerial delta china based on remote sensing images abstract the yellow river shifted from the diaokou course to the qingshuigou course in 1976 and shifted again at the north bank of the qingshuigou course in 1996 as a result the entire modern yellow river delta has experienced complicated patterns of erosion and accretion multitemporal remote sensing data of landsat mss and tm from 1976 to 2000 totaling twenty scenes were used to examine the changing pattern of accretion and erosion of the modern yellow river subaerial delta with gis tools during the period 1976 to 2000 the maximum erosion occurred at the head of the last two abandoned promontories diaokou and shenxiangou promontories where the coastline retreated landward over 7 and 4 5 km respectively with mean net erosion rates of 0 29 and 0 19 km a respectively in contrast the maximum accretion occurred at the head of the qingshuigou promontory where the coastline advanced seaward over 20 km with a mean net accretion rate of 0 83 km a the coastline at the head of the q 8 promontory also advanced seaward 7 km with a mean net accretion rate of 0 29 km a during the period 1996 to 2000 total area of the diaokou and shenxiangou promontories was reduced by 141 3 km 2 with a mean net erosion rate of 6 03 km 2 a total area of the qingshuigou and q 8 promontories increased by 384 16 and 11 88 km 2 respectively with mean net accretion rates of 16 4 and 3 03 km 2 a respectively relative to the diaokou shenxiangou and qingshuigou promontories the west area of the diaokou promontory and south area of the qingshuigou promontory were relatively stable although minor erosion occurred the general pattern of accretion and erosion of the modern yellow river subaerial delta was divided into three stages rapid accretion stage 1976 1984 accretion erosion adjustment stage 1985 1995 and slow erosion stage 1996 present the factors affecting the pattern of erosion and accretion over the yellow river delta such as structural geology the yellow river sediment discharge coastal processes meteorological conditions human activity sea level rise and land subsidence were summarized the cumulative area of increase over the entire subaerial delta is closely correlated with the cumulative yellow river sediment discharge wave induced longshore current is the major driving force to transport sediment from eroded areas along the last two abandoned promontories diaokou and shenxiangou promontories mostly westerly and partially easterly together with these coastal processes is the geographical shift of the yellow river course associated with its discharge regime after 1976 leading to the accretion along the present qingshuigou promontory and then the q 8 promontory

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

backwater and river plume controls on scour upstream of river mouths implications for fluvio deltaic morphodynamics sediment flux from rivers to oceans is the fundamental driver of fluvio deltaic morphodynamics and continental margin sedimentation yet sediment transport across the river to marine boundary is poorly understood coastal rivers typically are affected by backwater a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume flow deceleration in the backwater zone as well as spreading of the offshore plume should render rivers highly depositional near their mouths leading to sedimentation and eventual elimination of the backwater zone at steady state this reasoning is counter to observations of riverbed scour erosional bed forms and long lived backwater zones near the mouths of some coastal rivers e g mississippi river united states to explain these observations we present a quasi 2 d model of a coupled fluvial backwater and offshore river plume system and apply it to the mississippi river results show that during high discharge events the normal flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface spatial acceleration of flow and erosion of the riverbed as proposed by lane 1957 the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume this points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks

finescale vertical structure and evolution of a preconvective dryline on 19 june 2002 abstract high resolution airborne cloud radar data and other international h2o project datasets are used to describe the vertical structure of an unusual prefrontal dryline this dryline observed in northwestern kansas on 19 june 2002 first progressed eastward and tilted toward the west and later became more stationary and reversed its tilt toward the moist side the convective boundary layer cbl depth difference also reversed only in the later phase did the dry side cbl become deeper than on the moist side echo and single dual doppler velocity data in a vertical transect across the dryline suggest a solenoidal circulation dynamically consistent with the observed horizontal buoyancy gradient both this gradient and the solenoidal circulation reversed in the later phase simultaneously confluence toward the dryline increased resulting in an increasing moisture gradient as well as a deepening cbl in the dryline convergence zone it is speculated that the baroclinically generated horizontal vortici

alluvial fans formed by channelized fluvial and sheet flow i theory alluvial fans and fan deltas are of three basic types those built up primarily by the action of constantly avulsing river and stream channels those constructed by sheet flows and those resulting from the successive deposition of debris flows the present analysis is directed toward the first two types a mechanistic formulation of flow and sediment transport through river channels is combined with a simple quantification of the overall effect of frequent avulsion to derive relations describing the temporal and spatial evolution of mean i e averaged over many avulsions bed slope and elevation in an axially symmetric fan an example of a fan formed predominantly by the deposition of sand is compared to a similar one formed predominantly by the deposition of gravel in each example the case of channelized flow is compared to the case of sheet flow the model is applied to the tailings basin of a mine in the companion paper

the finescale structure of a west texas dryline abstract during spring and early summer a surface confluence zone often referred to as the dryline forms in the midwestern united states between continental and maritime air masses the dewpoint temperature across the dryline can vary in excess of 18 c in a distance of less than 10 km the movement of the dryline varies diurnally with boundary layer growth over sloping terrain leading to an eastward apparent propagation of the dryline during the day and a westward advection or retrogression during the evening in this study we examine the finescale structure of a retrogressing dryline using data taken by a doppler lidar a dual channel radiometer and serial rawinsonde ascents while many previous studies were unable to accurately measure the vertical motions in the vicinity of the dryline our lidar measurements suggest that the convergence at the dryline is intense with maximum vertical motions of 5 m s 1 the winds obtained from the doppler lidar measurements were combined with the equations of m