discovery of lake effect clouds on titan images from instruments on cassini as well as from telescopes on the ground reveal the presence of sporadic small scale cloud activity in the cold late winter north polar region of saturn s large moon titan these clouds lie underneath the previously discovered uniform polar cloud attributed to a quiescent ethane cloud at similar to 40 km and appear confined to the same latitudes as those of the largest known hydrocarbon lakes at the north pole of titan the physical properties of these clouds suggest that they are due to methane convection and condensation such convection could be caused by a process in some ways analogous to terrestrial lake effect clouds the lakes on titan could be a key connection between the surface and the meteorological cycle

solar system not just a storm in a teacup the cloud that emerged above the south pole of saturn s moon titan in 2012 has been found to consist of hydrogen cyanide particles this unexpected result prompts fresh thinking about the atmosphere of this satellite see letter p 65 in may 2012 the imaging science subsystem on board the cassini spacecraft identified a large cloud like structure above the dark southern pole of saturn s largest moon titan since then the cloud has been seen at very high altitudes 300 km and high southern latitudes in regions of the atmosphere where temperatures were too high to allow the condensation of any of titan s known trace gases this mysterious cloud has now been characterized using near infrared spectra from cassini s visual and infrared mapping spectrometer the data reveal a cloud composed of micrometre sized hydrogen cyanide hcn ice particles and are consistent with dramatic cooling of titan s atmosphere inside the winter polar vortex in early 2012 such a cooling to temperatures a hundred degrees colder than predicted by circulation models contrasts with the high altitude warming observed previously in the polar vortex

a new look at the saturn system the voyager 2 images voyager 2 photography has complemented that of voyager i in revealing many additional characteristics of saturn and its satellites and rings saturn s atmosphere contains persistent oval cloud features reminiscent of features on jupiter smaller irregular features track out a pattern of zonal winds that is symmetric about saturn s equator and appears to extend to great depth winds are predominantly eastward and reach 500 meters per second at the equator titan has several haze layers with significantly varying optical properties and a northern polar collar that is dark at short wavelengths several satellites have been photographed at substantially improved resolution enceladus surface ranges from old densely cratered terrain to relatively young uncratered plains crossed by grooves and faults tethys has a crater 400 kilometers in diameter whose floor has domed to match tethys surface curvature and a deep trench that extends at least 270 around tethys circumference hyperion is cratered and irregular in shape iapetus bright trailing hemisphere includes several dark floored craters and phoebe has a very low albedo and rotates in the direction opposite to that of its orbital revolution with a period of 9 hours within saturn s rings the birth of a spoke has been observed and surprising azimuthal and time variability is found in the ringlet structure of the outer b ring these observations lead to speculations about saturn s internal structure and about the collisional and thermal history of the rings and satellites

the influences of row grade ridge height and field slope on the seepage hydraulics of row sideslopes in contour ridge systems abstract seepage plays an important role in soil erosion in contour ridge systems seepage generated from subsurface flow causes hillslope instability by reducing the soil shear strength and mobilizing soil particles and can produce cave like features called seepage undercuts that can lead to contour failure as the main threat to contour plowing seepage results in large amounts of soil erosion due to row grade models used to predict seepage over a time series will provide a basis for modeling soil erosion resulting from seepage in contour ridging systems understanding seepage and its effects will advance our knowledge regarding seepage erosion mechanisms in contour ridge systems in this study 23 treatments were arranged using an orthogonal rotatable central composite design to model a seepage time series build a simple seepage prediction model and investigate the effects of row grade field slope and ridge height on seepage discharge most of the seepage discharge time series followed an s shaped curve the seepage discharge processes were fit by an exponential model with a determination coefficients r2 greater than 0 995 furthermore the physical meaning of the exponential model was consistent with the experimental results the seepage discharge continuously increased before the inflection point and then decreased finally the seepage discharge approached a steady value the maximum seepage discharge growth rate was achieved within 14 min and the seepage discharge became steady within 106 min second order polynomial regression models were used to determine the total and predicted steady seepage discharge using independent variables of row grade field slope and ridge height which produced r2 values of 0 66 p

cassini imaging of titan s high latitude lakes clouds and south polar surface changes 1 cassini s imaging science subsystem iss has been observing titan since april 2004 compiling a nearly global surface map and monitoring the surface and atmosphere for activity early images of the south polar region revealed numerous dark surface features and contemporaneous convective cloud systems suggesting the presence of hydrocarbon lakes similar to those later detected at titan s north pole intriguingly repeated south polar imaging by iss revealed differences consistent with ponding of hydrocarbon liquids on the surface due to precipitation from a large storm more recent iss images of high northern latitudes illustrate the full extents 500 000 km2 of hydrocarbon seas sections of which have been observed by cassini s radar these observations demonstrate dynamic processes at work on titan and that the poles harbor liquid hydrocarbon reservoirs the extents of which differ from pole to pole and which may be coupled to seasonally varying circulation

shoreline features of titan s ontario lacus from cassini vims observations we analyze observations of titan s south polar lake ontario lacus obtained by cassini s visual and infrared mapping spectrometer during the 38th flyby of titan t38 2007 december 5 these near closest approach observations have the highest signal to noise the finest spatial resolution and the least atmospheric influence of any near infrared lake observation to date we use the large spatially flat and low albedo interior of ontario lacus as a calibration target allowing us to derive an analytical atmospheric correction for emission angle the dark lake interior is surrounded by two separate annuli that follow the lake interior s contours the inner annulus is uniformly dark but not so much as the interior lake and is generally 5 10 kilometers wide at the lake s southeastern margin we propose that it represents wet lakebed sediments exposed by either tidal sloshing of the lake or seasonal methane loss leading to lower lake volume the exterior annulus is bright and shows a spectrum consistent with a relatively low water ice content relative to the rest of titan it may represent fine grained condensate deposits from a past era of higher lake level together the annuli seem to indicate that the lake level for ontario lacus has changed over time this hypothesis can be tested with observations scheduled for future titan flybys

is titan s shape caused by its meteorology and carbon cycle 1 titan s shape is characterized by a difference between the long equatorial radius and the polar radius that is several hundred meters larger than that predicted by the flattening due to its spin rate the north polar region is covered by large mare filled with hydrocarbons including ethane moreover global circulation models predict ethane precipitation on the polar areas this study shows that the shape of titan can be explained by the subsidence associated with the substitution of methane with ethane rich liquids percolating into the crust which as suggested by evolution models may be composed of methane clathrate hydrates such substitutions have been observed in laboratory experiments this process would provide an additional methane source as required for sustaining the presence of this constituent in titan s atmosphere through its history a 270 m subsidence of the polar caps is explained by the circulation of 1 5 to 6 1018kg of ethane in the top three kilometers of an initially methane clathrate crust this process would have operated during the last 300 1200 myr at the present ethane production rate

titan s haze structure in 1999 from spatially resolved narrowband imaging surrounding the 0 94 μm methane window 1 we use narrowband images of titan obtained in november 1999 to explore the haze vertical structure in titan s lower atmosphere the images were taken with the mount wilson 2 54 m telescope using the nasa gsfc built acousto optic imaging spectrometer these images were recorded at five wavelengths surrounding the 0 94 m methane window and are sensitive to a range of altitudes in titan s lower atmosphere we characterize titan s limb darkening by fitting the minnaert function limb darkening coefficient k to our data at 0 latitude and compare our observed limb darkening to that predicted from three radiative transfer models the vertical haze profiles suggest a gap in haze below 75 km with an increase in haze abundance near the surface

evidence for the exposure of water ice on titan s surface the smoggy stratosphere of saturn9s largest moon titan veils its surface from view except at narrow wavelengths centered at 0 83 0 94 1 07 1 28 1 58 2 0 2 9 and 5 0 micrometers we derived a spectrum of titan9s surface within these windows and detected features characteristic of water ice therefore despite the hundreds of meters of organic liquids and solids hypothesized to exist on titan9s surface its icy bedrock lies extensively exposed

observations of titan s northern lakes at 5 μm implications for the organic cycle and geology abstract since titan entered northern spring in august 2009 the north pole has been illuminated allowing observations at optical wavelengths on june 5 2010 the visual and infrared mapping spectrometer vims onboard the cassini spacecraft observed the northern pole area with a pixel size from 3 to 7 km since as we demonstrate little of the solar flux at 5 m is scattered by the atmosphere these observations were obtained at relatively large incidence angles and allowed us to build a mosaic covering an area of more than 500 000 km 2 that overlaps and complements observations made by the synthetic aperture radar sar in 2007 we find that there is an excellent correlation between the shape of the radar dark area known as ligeia mare and the vims 5 m dark unit matching most of the radar shoreline the 2010 vims observations suggest that the 125 000 km 2 surface area of ligeia mare measured by radar in 2007 has not significantly changed the vims observations complement the radar observations to the west of ligeia mare and suggest that ligeia mare is connected to kraken mare by either a diffuse network similar to a swamp area or by well defined sub pixel rivers considering the results of recent evaporation models of methane our preferred interpretation of the relative constancy in surface area of ligeia is that it is principally composed of ethane although we cannot rule out the possibility that methane evaporation is balanced with replenishment by either precipitation or underground seepage there is also strong correlation between the location of the small radar lakes and the small vims 5 m dark patches the geographic location of the small lakes are within a vims pixel of the sar location suggesting that the non synchronous component of titan s spin rate if it exists was less than 2 3 10 4 deg day between 2007 and 2010 in agreement with the recent t64 radar observations these observations question the existence of non synchronous rotation two radar bright features appear dark at 5 m the simplest interpretation is that these are very shallow lakes less than one meter deep three new small lakes named freeman cardiel and towada by the iau are found outside of the area mapped with the sar a single scattering model describing reflection of sunlight at 5 m suggests that the lake surface is mirror like and that the albedo of the solid surfaces surrounding the lakes is about 8 these observations together with information of the haze aerosols allow us to show that titan s lakes atmospheric ethane and aerosol haze are smaller carbon reservoirs than titan s sand dunes and atmospheric methane a simple model involving an outburst of methane a few hundreds of myr ago followed by the dissociation of methane in the atmosphere leading to the formation of the haze particles that constitute the dune fields would be consistent with both the present observations and recent measurements of isotopic ratios in atmospheric methane mandt k e et al 2012 astrophys j 749 160 14

stability of streambanks formed in partially saturated soils and effects of negative pore water pressures the sieve river italy abstract streambanks of alluvial channels are usually composed of loose materials which are unsaturated in ambient conditions unsaturated soils are subject to negative pore water pressures which cause an apparent cohesion the latter is the main factor in allowing the stability of near vertical banks even during moderate in bank flow events the apparent cohesion can be strongly reduced as the material approaches full saturation therefore during the drawdown phase as the confining pressure of the water in the channel disappears a bank failure is likely to occur channel bed level lowering along the sieve river central italy has caused widespread bank instability a geomorphological reconnaissance of forms and processes was followed by in situ tests to determine the shear strength of the banks interpretation of the tests and a streambank stability analysis were based on concepts of soil mechanics for unsaturated soils in order to obtain relations between bank angle and height in limit equilibrium conditions a stability chart was obtained with curves for different apparent cohesion values and a stability analysis was performed taking into account the effects of flow events in order to investigate the pore pressure effects a series of piezo tensiometers were installed in a streambank of the sieve river data from a 1 year monitoring period show variations in pore water pressure and matric suction as a consequence of rainfall evapotranspiration and water stage variations a planar failure with a tension crack occurred in the upper cohesive part of the bank during december 1996 the safety factor has been expressed as a function of the geometry of the bank and of the shear strength of the material safety factor variations through time are therefore shown as a function of seasonal variations in matric suction

composite trough evolution of selected west pacific extratropical cyclones abstract the observed vertical structures of the trough axes for 27 extratropical cyclones are presented this study is motivated by results from a simple theoretical model two observing times during the cyclone life cycle are shown prior to development and during the mature but still amplifying stage prior to development upper and lower troughs are present and separate each has little or no tilt the upper one is typically prominent down to 4 km elevation and the separation between the lower and the upper features varies depending on where the approaching upper trough happens to be at the observing time at the mature stage upper and lower features are connected a uniform tilt typically develops through the entire troposphere the tilt is typically due west with height and the tilt may have a preferred slope an empirical orthogonal function eof analysis finds that two modes account for more than 97 of the variance the equivalent barotropic eof has the most variance by far though the frac

texture and mineralogy of sediments from the ganges brahmaputra meghna river system in the bengal basin bangladesh and their environmental implications the bengal basin bangladesh represents one of the most densely populated recent floodplains of the world the sediment flux through the basin is one of the highest on a global scale a significant portion of this sediment load find its sink in the basin itself because of its lower elevation and frequent flooding the textural mineralogical and chemical nature of the sediments thus have an important bearing on the environmental quality of the basin as well as for the bay of bengal

clouds on titan during the cassini prime mission a complete analysis of the vims data we use data from the vims instrument on board the cassini spacecraft to construct high sensitivity and high spatial resolution maps of the locations of tropospheric clouds on titan in the late northern winter season during which the cassini prime mission took place these observations show that in this season clouds on titan are strongly hemispherically asymmetric mid latitude clouds in particular occur only in the southern hemisphere and have not ever been observed in the north such an asymmetry is in general agreement with circulation models where sub solar surface heating controls the locations of clouds and appears in conflict with models where perennial polar hazes prevent significant summertime polar heating from affecting the circulation the southern mid latitude clouds appear to be distributed uniformly in longitude in contrast to some previous observations southern high latitude clouds exhibit a significant concentration however between about 180 and 270 e longitude a spatially and temporally uniform cloud always appears northward of 50 n latitude this cloud appears unchanged over the course of the observations consistent with the interpretation that it is caused by continuous ethane condensation as air subsides and radiatively cools through the tropopause the location of this cloud likely provides a direct tracer of elements of north polar atmospheric circulation potentially allowing continuous monitoring of circulation changes as titan passes through equinox into north polar spring and summer we show that a similar analysis of this dataset by rodriguez et al 2009 contains substantial errors and should not be used

modelling of the retreat process of composite riverbank in the jingjiang reach using the improved bstem bank retreat in the jingjiang reach is closely related not only to the near bank intensity of fluvial erosion but also to the composition and mechanical properties of bank soils therefore it is necessary to correctly simulate bank retreat to determine the characteristics of fluvial processes in the jingjiang reach the current version of bstem 5 4 was improved to predict riverbank retreat by inputting a dynamic water table and calculating the approximation of the distribution of dynamic pore water pressure in the soil near the river bank face and considering the depositional form of the failed blocks which is assumedly based on a triangular distribution with the slope approximately equalling the stable submerged bank slope and half of collapsed volume deposited in the bank toe region the degrees of riverbank stability at jing34 were calculated using the improved bstem the results indicate the following trends i the degrees of riverbank stability were high during the dry season and the rising stage which led to minimal bank failure and ii the stability degrees were low during the flood season and the recession stage with the events of bank collapse occurring frequently which belonged to a stage of intensive bank erosion considering the effects of bank toe erosion water table lag and the depositional form of the collapsed bank soil the bank retreat process was simulated at the right riverbank of jing34 the model predicted results exhibit close agreement with the measured data including the total bank retreat width and the collapsed bank profile a sensitivity analysis was conducted to determine the quantitative effects of toe erosion and water table lag on the degree of bank stability the calculated results for toe erosion indicate that the amount of toe erosion was largest during the flood season which was a primary reason for bank failure the influence of water table lag on the degree of stability demonstrates that water table lag was an important cause of bank failure during the recession stage

titan s 2 μm surface albedo and haze optical depth in 1996 2004 1 we observed titan in 1996 2004 with high resolution 2 m speckle and adaptive optics imaging at the w m keck observatory by observing in a 2 m broadband filter we obtain images that have contributions from both titan s surface and atmosphere we have modeled titan s atmosphere using a plane parallel radiative transfer code that has been corrected to agree with 3 d monte carlo predictions we find that titan s surface albedo ranges from 0 02 in the darkest equatorial region of the trailing hemisphere to 0 1 in the brightest areas of the leading hemisphere over the past quarter of a saturnian year haze optical depth in titan s southern hemisphere has decreased substantially from a value of 0 48 in 1996 down to 0 18 in 2004 while the northern haze has been increasing over the past few years as a result of these changes in 2004 the north south haze asymmetry at k band has disappeared

titan evidence for seasonal change a comparison of hubble space telescope and voyager images abstract images of titan were obtained by the hubble space telescope hst on 26 august 1990 comparison with voyager 1 and voyager 2 images obtained 10 and 9 years earlier shows that the seasonal hemispheric brightness asymmetry has reversed near 440 and 550 nm wavelengths with the northern hemisphere now being brighter an additional noisy hst image at 889 nm wavelength for which there are no analogous voyager data suggests that the southern hemisphere may have been brighter than the northern at that wavelength in 1990

titan s cloud seasonal activity from winter to spring with cassini vims abstract since saturn orbital insertion in july 2004 the cassini orbiter has been observing titan throughout most of the northern winter season october 2002 august 2009 and the beginning of spring allowing a detailed monitoring of titan s cloud coverage at high spatial resolution with close flybys on a monthly basis this study reports on the analysis of all the near infrared images of titan s clouds acquired by the visual and infrared mapping spectrometer vims during 67 targeted flybys of titan between july 2004 and april 2010 the vims observations show numerous sporadic clouds at southern high and mid latitudes rare clouds in the equatorial region and reveal a long lived cloud cap above the north pole ubiquitous poleward of 60 n these observations allow us to follow the evolution of the cloud coverage during almost a 6 year period including the equinox and greatly help to further constrain global circulation models gcms after 4 years of regular outbursts observed by cassini between 2004 and 2008 southern polar cloud activity started declining and completely ceased 1 year before spring equinox the extensive cloud system over the north pole stable between 2004 and 2008 progressively fractionated and vanished as titan entered into northern spring at southern mid latitudes clouds were continuously observed throughout the vims observing period even after equinox in a latitude band between 30 s and 60 s during the whole period of observation only a dozen clouds were observed closer to the equator though they were slightly more frequent as equinox approached we also investigated the distribution of clouds with longitude we found that southern polar clouds before disappearing in mid 2008 were systematically concentrated in the leading hemisphere of titan in particular above and to the east of ontario lacus the largest reservoir of hydrocarbons in the area clouds are also non homogeneously distributed with longitude at southern mid latitudes the n 2 mode wave pattern of the distribution observed since 2003 by earth based telescopes and confirmed by our cassini observations may be attributed to saturn s tides although the latitudinal distribution of clouds is now relatively well reproduced and understood by the gcms the non homogeneous longitudinal distributions and the evolution of the cloud coverage with seasons still need investigation if the observation of a few single clouds at the tropics and at northern mid latitudes late in winter and at the start of spring cannot be further interpreted for the moment the obvious shutdown of the cloud activity at titan s poles provides clear signs of the onset of the general circulation turnover that is expected to accompany the beginning of titan s northern spring according to our gcm the persistence of clouds at certain latitudes rather suggests a sudden shift in near future of the meteorology into the more illuminated hemisphere finally the observed seasonal change in cloud activity occurred with a significant time lag that is not predicted by our model this may be due to an overall methane humidity at titan s surface higher than previously expected

the latitudinal distribution of clouds on titan clouds have been observed recently on titan through the thick haze using near infrared spectroscopy and images near the south pole and in temperate regions near 40 s recent telescope and cassini orbiter observations are now providing an insight into cloud climatology to study clouds we have developed a general circulation model of titan that includes cloud microphysics we identify and explain the formation of several types of ethane and methane clouds including south polar clouds and sporadic clouds in temperate regions and especially at 40 in the summer hemisphere the locations frequencies and composition of these cloud types are essentially explained by the large scale circulation

measuring streambank erosion due to ground water seepage correlation to bank pore water pressure precipitation and stream stage limited information exists on one of the mechanisms governing sediment input to streams streambank erosion by ground water seepage the objective of this research was to demonstrate the importance of streambank composition and stratigraphy in controlling seepage flow and to quantify correlation of seepage flow erosion with precipitation stream stage and soil pore water pressure the streambank site was located in northern mississippi in the goodwin creek watershed soil samples from layers on the streambank face suggested less than an order of magnitude difference in vertical hydraulic conductivity ks with depth but differences between lateral ks of a concretion layer and the vertical ks of the underlying layers contributed to the propensity for lateral flow goodwin creek seeps were not similar to other seeps reported in the literature in that eroded sediment originated from layers underneath the primary seepage layer subsurface flow and sediment load quantified using 50 cm wide collection pans were dependent on the type of seep intermittent low flow lf seeps flow rates typically less than 0 05 l min 1 persistent high flow hf seeps average flow rate of 0 39 l min 1 and buried seeps which eroded unconsolidated bank material from previous bank failures the timing of lf seeps correlated to river stage and precipitation the hf seeps at goodwin creek began after rainfall events resulted in the adjacent streambank reaching near saturation i e soil pore water pressures greater than 5 kpa seep discharge from hf seeps reached a maximum of 1 0 l min 1 and sediment concentrations commonly approached 100 g l 1 buried seeps were intermittent but exhibited the most significant erosion rates 738 g min 1 and sediment concentrations 989 g l 1 in cases where perched water table conditions exist and persistent hf seeps occur seepage erosion and bank collapse of streambank sediment may be significant copyright 2007 john wiley sons ltd

evidence of titan s climate history from evaporite distribution abstract water ice poor 5 m bright material on saturn s moon titan has previously been geomorphologically identified as evaporitic here we present a global distribution of the occurrences of the 5 m bright spectral unit identified with cassini s visual infrared mapping spectrometer vims and examined with radar when possible we explore the possibility that each of these occurrences are evaporite deposits the 5 m bright material covers 1 of titan s surface and is not limited to the poles the only regions with extensive long lived surface liquid we find the greatest areal concentration to be in the equatorial basins tui regio and hotei regio our interpretations based on the correlation between 5 m bright material and lakebeds imply that there was enough liquid present at some time to create the observed 5 m bright material we address the climate implications surrounding a lack of evaporitic material at the south polar basins if the south pole basins were filled at some point in the past then where is the evaporite

a sediment transport model for incision of gullies on steep topography 1 we have conducted surveys of gullies that developed in a small steep watershed in the idaho batholith after a severe wildfire followed by intense precipitation we measured gully length and cross sections to estimate the volumes of sediment loss due to gully formation these volume estimates are assumed to provide an estimate of sediment transport capacity at each survey cross section from the single gully forming thunderstorm sediment transport models commonly relate transport capacity to overland flow shear stress which is related to runoff rate slope and drainage area we have estimated the runoff rate and duration associated with the gully forming event and used the sediment volume measurements to calibrate a general physically based sediment transport equation in this steep high shear stress environment we find that a shear stress exponent of 3 corresponding to drainage area and slope exponents of m 2 1 and n 2 25 match our data this shear stress exponent of 3 is approximately 2 times higher than those for bed load transport in alluvial rivers but is in the range of shear stress exponents derived from flume experiments on steep slopes and with total load equations the concavity index of the gully profiles obtained theoretically from the area and slope exponents of the sediment transport equation c m 1 n agrees well with the observed profile concavity of the gullies our results although preliminary because of the uncertainty associated with the sediment volume estimates suggest that for steep hillslopes such as those in our study area a greater nonlinearity in the sediment transport function exists than that assumed in some existing hillslope erosion models which calculate sediment transport capacity using the bed load equations developed for rivers

evidence for a polar ethane cloud on titan spectra from cassini s visual and infrared mapping spectrometer reveal the presence of a vast tropospheric cloud on titan at latitudes 51 to 68 north and all longitudes observed 10 to 190 west the derived characteristics indicate that this cloud is composed of ethane and forms as a result of stratospheric subsidence and the particularly cool conditions near the moon s north pole preferential condensation of ethane perhaps as ice at titan s poles during the winters may partially explain the lack of liquid ethane oceans on titan s surface at middle and lower latitudes

cassini radar views the surface of titan the cassini titan radar mapper imaged about 1 of titan s surface at a resolution of 0 5 kilometer and larger areas of the globe in lower resolution modes the images reveal a complex surface with areas of low relief and a variety of geologic features suggestive of dome like volcanic constructs flows and sinuous channels the surface appears to be young with few impact craters scattering and dielectric properties are consistent with porous ice or organics dark patches in the radar images show high brightness temperatures and high emissivity and are consistent with frozen hydrocarbons

mapping polar atmospheric features on titan with vims from the dissipation of the northern cloud to the onset of a southern polar vortex abstract we have analyzed the complete archive of the visual and infrared mapping spectrometer vims data in order to monitor and analyze the evolution of the clouds and haze coverage at both poles of titan during the entire cassini mission our objective is to give a cartographic synopsis from a vims perspective to provide a global view of the seasonal evolution of titan s atmosphere over the poles we leave the detailed comparison with the imaging science subsystem iss and the composite infrared spectrometer cirs data sets to further studies we have computed global hyperspectral mosaics for each of the 127 targeted flybys of titan to produce synthetic color maps emphasizing the main atmospheric features the north pole appears fully covered by a huge cloud as soon as the first observations in 2004 and up to the equinox in 2009 le mouelic et al 2012 the northern skies then became progressively clearer after the circulation turnover in 2009 revealing the underlying lakes and seas to the optical instruments up to 2017 the reverse situation is observed over the south pole which was mostly clear of such a high obscuring cloud during the first years of the mission but started to develop a polar cloud in 2012 this feature grew up month after month until the end of the mission in 2017 with a poleward latitudinal extent of 75 s in 2013 up to 58 s in april 2017 thanks to the spectral capabilities of vims we have detected hcn spectral signatures over the north pole in almost all flybys between 2004 and 2008 these hcn signatures started then to show up over the south pole in almost all flybys between 2012 and 2017 so perfectly matching the timing and spatial extent of the northern and southern polar atmospheric features

seepage erosion in layered stream bank material current stream restoration practices often require anthropogenic manipulation of natural field soils to reconstruct stream banks in the absence of stabilizing vegetation for this study researchers conducted laboratory experiments on reconstructed non vegetated stream banks with layered soils experiencing seepage the objective of the study was to determine the effect of seepage pore water pressure and bank geometry on erosion and bank stability of layered streambanks the experimental design consisted of an intermediate size soil lysimeter packed with a sandy clay loam top soil and an underlying fine sand layer at three bank slopes 90 45 and 26 shallow groundwater flow and seepage resulted in bank failure of geometrically stable banks pop out failures liquid deformation and piping were all observed failure mechanisms in the underlying sand material dependent on the bank angle groundwater seepage processes created small scale failures of the underlying sand leading to larger scale failures of the overlying sandy clay loam the underlying sand layer eroded according to the initial bank angle and change in overburden loading the overlying loam layer failed along linear failure planes the gradually sloped bank i e 26 slope failed faster hypothesized to be due to less confining pressure and greater vertical seepage forces researchers analyzed the laboratory experiments using the bank stability and toe erosion model version 4 1 the model calculated an accurate shear surface angle similar to the failure angle observed in the lysimeter tests the model predicted failure only for the undercut 90 bank slope and indicated stable conditions for the other geometries steeper initial bank slopes and undercut banks decreased the bank factor of safety the observed failure mechanisms and measured saturation data indicated an interaction between overburden pressure seepage forces and bank slope on bank stability future bank stability modeling would benefit by incorporating lateral seepage erosion and soil liquefaction prediction calculations copyright 2009 john wiley sons ltd

modeling seepage erosion and bank retreat in a composite river bank in large alluvial river courses where bank failure is prominent seepage erosion may be a significant contributor that is often overlooked seepage erosion typically occurs simultaneously with fluvial erosion and other mass failure processes during the recession period of hydrographs in this study a functional relationship between seepage erosion rate and its controlling variables was developed using lysimeter experiments this experimental relationship was used to formulate a mathematical seepage model that predicts the daily seepage erosion rate for a composite river bank soils collected from the brahmaputra river banks were repacked in lysimeters with horizontal stratification similar to the actual bank stratification a total of 71 lysimeter experiments were conducted with various combinations of the controlling variables the results indicate that the seepage gradient has a dominating effect on the time taken in developing undercuts that lead to the bank collapse increase in the number of silt layers in the bank profile increased the time to collapse significantly in situ bank erosion measurements were taken to validate the performance of a bank erosion model for the middle reaches of the brahmaputra river in india the model predictions of seepage erosion rate showed a close agreement with the in situ bank retreat measurements

the structure and dynamics of titan s middle atmosphere titan s middle atmosphere is characterized by cyclostrophic winds and strong seasonal modulation cassini cirs observations obtained in northern winter indicate that the stratosphere near 1mbar i

channel morphometry sediment transport and implications for tectonic activity and surficial ages of titan basins abstract fluvial features on titan and drainage basins on earth are remarkably similar despite differences in gravity and surface composition we determined network bifurcation r b ratios for five titan and three terrestrial analog basins tectonically modified earth basins have r b values greater than the expected range 3 0 5 0 for dendritic networks comparisons with r b values determined for titan basins in conjunction with similarities in network patterns suggest that portions of titan s north polar region are modified by tectonic forces sufficient elevation data existed to calculate bed slope and potential fluvial sediment transport rates in at least one titan basin indicating that 75 mm water ice grains observed at the huygens landing site should be readily entrained given sufficient flow depths of liquid hydrocarbons volumetric sediment transport estimates suggest that 6700 10 000 titan years 2 0 3 0 10 5 earth years are required to erode this basin to its minimum relief assuming constant 1 m and 1 5 m flows these lowering rates increase to 27 000 41 000 titan years 8 0 12 0 10 5 earth years when flows in the north polar region are restricted to summer months

dunes on titan observed by cassini radar thousands of longitudinal dunes have recently been discovered by the titan radar mapper on the surface of titan these are found mainly within 30 of the equator in optically near infrared and radar dark regions indicating a strong proportion of organics and cover well over 5 of titan s surface their longitudinal duneform interactions with topography and correlation with other aeolian forms indicate a single dominant wind direction aligned with the dune axis plus lesser off axis or seasonally alternating winds global compilations of dune orientations reveal the mean wind direction is dominantly eastwards with regional and local variations where winds are diverted around topographically high features such as mountain blocks or broad landforms global winds may carry sediments from high latitude regions to equatorial regions where relatively drier conditions prevail and the particles are reworked into dunes perhaps on timescales of thousands to tens of thousands of years on titan adequate sediment supply sufficient wind and the absence of sediment carriage and trapping by fluids are the dominant factors in the presence of dunes

observations of a stationary mid latitude cloud system on titan abstract we report the observation of a cloud system on titan that remained localized near 40 s latitude and 60 w longitude for at least 34 h ground based observations obtained with the sinfoni imaging spectrograph at the very large telescope over four consecutive nights recorded the lifetime and altitude of the unresolved cloud system concomitant measurements made by cassini vims over 3 h resolved changes in the altitude and opacity of individual regions within the system during this time clouds are measured from 13 to 37 km altitude with optical depths per pixel ranging from 0 13 to 7 short timescale rise times are consistent with previous measurements of the evolution of mid latitude clouds however the long timescale localization of the cloud structure is unexplained we speculate about the role of mesoscale circulation in relation to cloud formation

an objective study of 500 hpa moving troughs in the southern hemisphere abstract the location and movement of 500 hpa troughs using an automated method are studied with data from a 24 yr period with the objective of determining the trough formation and dissipation regions in the southern hemisphere to identify the 500 hpa mobile troughs an objective method that uses the eulerian centripetal acceleration eca is developed on average 868 troughs per year were identified by the method with an increase in trend during the period studied the troughs have an average lifetime of 4 3 days being longer shorter in subtropical high latitudes the average calculated phase velocity was 13 6 m s 1 being higher lower in middle high latitudes the troughs are normally found in the 60 40 s latitudinal band with maximum occurrence at 50 s the longitudinal distribution of trough genesis has three maximum regions over the drake strait and the south atlantic ocean over the indian ocean around 50 s and over the southwestern pacific ocean between 150 e and 150 w the trough d

the structure of titan s atmosphere from cassini radio occultations occultations from the prime and equinox missions abstract we present the results of six soundings of the atmosphere of titan by the radio occultation technique using the cassini spacecraft currently in orbit around saturn these occultations occurred during four separate targeted titan encounters in both the prime and equinox missions of cassini over 3 years they cover a wide range of latitude from 75 s to 79 n split so that three soundings are in the northern hemisphere and three are in the southern hemisphere techniques and error analysis are similar to schinder et al 2011 the six temperature altitude profiles presented here are compared to those earlier results of special interest is the sudden cooling observed at altitudes of 80 100 km in the two high northern winter soundings at 74 n and 80 n where the temperature drops by about 10 k over the course of 20 km the northern profiles also exhibit a transition between the troposphere and stratosphere that is much more abrupt than in the south and the northern tropopause temperatures are much cooler

seepage caused tension failures and erosion undercutting of hillslopes summary seepage has been suggested as an important factor in gully and river bank erosion this study investigated the underlying mechanisms of instability by seepage in laboratory studies a 25 cm tall 50 cm wide and 20 cm long soil block with a focused inflow reservoir was constructed to investigate seepage gradient forces and the three dimensional nature of seepage particle mobilization i e seepage erosion and undercutting experiments included sand and loamy sand soil blocks packed at prescribed bulk densities 1 30 1 70 mg m 3 and with an outflow face at various angles 90 75 and 60 constant heads of 15 25 and 35 cm were imposed on the soil to induce flow a laser scanner was utilized to obtain the three dimensional coordinates of the bank and undercut surfaces at approximately 15 30 s intervals the bulk density of the two different soil types controlled which seepage failure mechanism occurred 1 tension or pop out failures due to the seepage force exceeding the soil shear strength which was being concurrently reduced by increased soil pore water pressure or 2 particle entrainment in the seepage flow particle mobilization bank undercutting and bank collapse when the initial seepage force gradient was less than the resistance of the soil block for cases experiencing particle mobilization and undercutting seepage erosion initiated as unimodal i e concentrated at one point or as multimodal i e initiating at several locations across the bank face and this result was largely controlled by the bank angle a five parameter gaussian function was fitted to the measured three dimensional undercut shapes to derive parameters for the maximum depth of undercutting position of the center of the peak and the vertical and lateral spreads of the undercut the parameters of this distribution can be useful in the development of improved sediment transport functions and the incorporation of this failure mechanism into hillslope stability models

titan s north south asymmetry from hst and voyager imaging comparison with models and ground based photometry abstract new measurements of titan s hemispheric brightness asymmetry from hst images from 260 to 1040 nm show that the contrast is strongest near 450 nm blue and with the opposite sign at 889 nm methane band comparison with a full titan year of disk integrated albedo data indicates that the seasonal cycle in asymmetry is smooth but is insufficient to explain the variation in albedo and a twice per year global albedo enhancement 50 larger than the hemispheric asymmetry amplitude is required as noted by other workers we also report measurements of limb darkening strongest at red wavelengths and note that the transition zone between the hemispheres lies in the northern brighter hemisphere at present comparison of models to the hst data set indicates that a change in aerosol number density above 70 km and largely below 120 km is the likely mechanism and is probably driven by aerosol transport by meridonal and vertical winds

the evolution of titan s mid latitude clouds spectra from cassini s visual and infrared mapping spectrometer reveal that the horizontal structure height and optical depth of titan s clouds are highly dynamic vigorous cloud centers are seen to rise from the middle to the upper troposphere within 30 minutes and dissipate within the next hour their development indicates that titan s clouds evolve convectively dissipate through rain and over the next several hours waft downwind to achieve their great longitude extents these and other characteristics suggest that temperate clouds originate from circulation induced convergence in addition to a forcing at the surface associated with saturn s tides geology and or surface composition

a study of frontal cyclone surface and 300 hpa geostrophic kinetic energy distribution and scale change abstract an earlier article deduced a doubling of the scale of the sea level pressure pattern for lows as they developed in the north pacific scale here refers to horizontal extent of the low this study uses a different technique to estimate scale change in the upper troposphere the prior study used wavelets here circular averaging is used on several fields with primary emphasis on the geostrophic kinetic energy gke field the technique herein confirms the earlier result that sea level pressure slp scale increases when applied to the 300 hpa level the trough extent does not change scale significantly the average scale has radius of about 1200 km at sea level and 1700 km at 300 hpa during development the average radius of maximum gke changes little at the surface but decreases at upper levels the maximum gke is typically located 600 1100 km from the 300 hpa low center 450 650 km from the slp low center composite maps of gke are shown during different stages in cyclone development at both le

seasonal photometric variability of titan 1972 2006 abstract measurements at lowell observatory of titan in the b 472 nm and y 551 nm filters of the stromgren photometric system at thirty four consecutive apparitions 282 nights from 1971 72 to 2006 show a 10 sinusoidal variation that lags seasonal extremes by about 1 8 of a titan year the seasonal variations are asymmetric the autumn lightcurve maxima of the northern and southern hemispheres differ significantly as do the spring lightcurve minima changes also occur from one titan year to the next titan was 3 fainter in b and 1 fainter in y following the 2002 southern summer solstice than it was one titan year earlier in 1973 these changes appear to be intrinsic to titan s atmosphere and cannot be explained by instrumental effects and changing geometries orbital variations visible in recent hubble space telescope images at 673 nm and voyager orange images 590 640 nm may have a small 0 002 0 001 mag counterpart in the b y photometric record eastern elongation brighter consistent with the cassini near infrared albedo map

the geology of hotei regio titan correlation of cassini vims and radar abstract joint cassini vims and radar sar data of 700 km wide hotei regio reveal a rich collection of geological features that correlate between the two sets of images the degree of correlation is greater than anywhere else seen on titan central to hotei regio is a basin filled with cryovolcanic flows that are anomalously bright in vims data in particular at 5 m and quite variable in roughness in sar the edges of the flows are dark in sar data and appear to overrun a vims bright substrate sar stereo topography shows the flows to be viscous 100 200 m thick on its southern edge the basin is ringed by higher 1 km mountainous terrain the mountains show mixed texture in sar data some regions are extremely rough exhibit low and spectrally neutral albedo in vims data and may be partly coated with darker hydrocarbons around the southern margin of hotei regio the sar image shows several large dendritic radar bright channels that flow down from the mountainous terrain and terminate in dark blue patches seen in vims images whose infrared color is consistent with enrichment in water ice the patches are in depressions that we interpret to be filled with fluvial deposits eroded and transported by liquid methane in the channels in the vims images the dark blue patches are encased in a latticework of lighter bands that we suggest to demark a set of circumferential and radial fault systems bounding structural depressions conceivably the circular features are tectonic structures that are remnant from an ancient impact structure we suggest that impact generated structures may have simply served as zones of weakness no direct causal connection such as impact induced volcanism is implied we also speculate that two large dark features lying on the northern margin of hotei regio could be calderas in summary the preservation of such a broad suite of vims infrared color variations and the detailed correlation with features in the sar image and sar topography evidence a complex set of geological processes pluvial fluvial tectonic cryovolcanic impact that have likely remained active up to very recent geological time

titan s smile and collar hst observations of seasonal change 1994 2000 we observed titan with the hubble space telescope in november 2000 using the wide field plane tary camera wfpc2 and the space telescope imaging spectrograph stis considerable change is apparent be tween these and observations in 1994 1997 in particular the north south asymmetry at visible wavelengths has re versed with the southern hemisphere brighter as during the voyager epoch the asymmetry shows considerable varia tion with wavelength in the near infrared in the 889nm methane band the asymmetry which formerly resembled a smile has reversed in the last 3 years while at 953nm the southern limb is still bright a south polar collar dark at ultraviolet wavelengths is apparent some five years after equinox

titan s surface revealed by hst imaging we present for the first time relative albedo maps of titan s surface the maps were made from images taken by the hubble space telescope s planetary camera 295 km per pixel through atmospheric windows at 940 and 1080 nm coverage at all longitudes and between 45 s and 60 n was obtained with 14 orbits from 4 18 october 1994 each image is characterized by haze with both limb effects brightening and a large north south contrast subtracting an averaged image from each frame removes haze effects and reveals surface features of 10 maximum contrast many features are clearly visible in multiple images and in both filters but the surface map is dominated by a large bright roughly rectangular feature centered at 110 w 10 s and elongated in an east west direction 4000 2500 km2 this feature and others indicate the diversity of titan s surface we consider the origin of the bright feature in an otherwise dark landscape and suggest that highland washing by methane rainfall is a likely mechanism

the sand seas of titan cassini radar observations of longitudinal dunes the most recent cassini radar images of titan show widespread regions up to 1500 kilometers by 200 kilometers of near parallel radar dark linear features that appear to be seas of longitudinal dunes similar to those seen in the namib desert on earth the ku band 2 17 centimeter wavelength images show 100 meter ridges consistent with duneforms and reveal flow interactions with underlying hills the distribution and orientation of the dunes support a model of fluctuating surface winds of 0 5 meter per second resulting from the combination of an eastward flow with a variable tidal wind the existence of dunes also requires geological processes that create sand sized 100 to 300 micrometer particulates and a lack of persistent equatorial surface liquids to act as sand traps

runoff and soil loss characteristics on loess slopes covered with aeolian sand layers of different thicknesses under simulated rainfall abstract in the wind water erosion crisscross region of the northern loess plateau parts of loess slopes have been covered by layers of aeolian sand of different thicknesses knowledge of soil erosion processes and magnitudes on these slopes is essential to understanding the coupled water wind erosion processes and to address the resulting downstream coarse sediment problems in the yellow river simulated rainfall intensity 90 mm h 1 was performed to explore the effects of sand layer thickness on runoff and soil loss from loess slopes covered with different sand layer thicknesses 0 0 5 2 5 10 15 20 and 25 cm initial runoff time increased with increasing sand layer thickness with greater changes occurring for the increases in the thinner 0 5 cm than for the thicker layers 5 25 cm total runoff yield from the sand covered loess slopes was 18 55 lower than from the uncovered loess slope and decreased with increasing sand layer thickness in contrast total sediment yield was up to 14 times greater from the sand covered loess slopes than from the uncovered loess slope and rapidly increased with increasing sand layer thickness during the rainstorm runoff and soil loss rates exhibited unimodal distributions and they were related by a positive linear function both before and after the maximum soil loss rate that had a high determination coefficient r2 0 8 p

coupled modeling of bed deformation and bank erosion in the jingjiang reach of the middle yangtze river abstract recent significant channel evolution in the jingjiang reach has raised much attention particularly the remarkable bank erosion a coupled model for simulating bed deformation and bank erosion has been proposed in this study which focuses on the erosion of the bank with a composite structure in the lower jingjiang reach in order to cover three contributing processes that may interact with each other the proposed model integrates a one dimensional morphodynamic module with a two dimensional module of ground water flow and a bank erosion module for the cantilever failure of a composite riverbank model performance was evaluated through a detailed simulation of channel evolution along a 150 8 km subreach in the jingjiang reach over the 2005 hydrological year satisfying results were obtained from the simulation showing relatively close agreement between the calculations and measurements in terms of hydrological data at the outlet section bank erosion sites longitudinal channel profile and typical cross sectional profiles in addition investigations into temporal changes in bank soil properties and critical overhanging width at cantilever failure demonstrate that there was a seasonal variation in the volumetric water content of bank soil which increased during the rising and flood periods and then decreased during the recession period showing an impact on the occurrence timing of cantilever failures the tensile strength and critical overhanging width had an inverse relationship with the water content whereas the critical width sharply increased and then decreased during high flows affected by a rapid change in river stage the temporal distribution of cantilever failure events indicates that cantilever failure primarily occurred in the flood and recession periods the effects of bed roughness water content variation and secondary flow on bank erosion were also discussed

an analytical solution of groundwater response to tidal fluctuation in a leaky confined aquifer an analytical solution is derived to investigate the influence of leakage on tidal response in a coastal leaky confined aquifer system the analytical solution developed here is more general than the traditional solution obtained by ferris 1951 which can be regarded as a special case of the solution presented in this paper this solution is based on a conceptual model under the assumption that the groundwater level in the confined aquifer fluctuates in response to sea tide while that of the overlying unconfined aquifer remains constant this conceptual model is supported by numerous field studies by previous researchers which have demonstrated that the tidal response in an unconfined aquifer may be negligible compared to that in a confined aquifer the leakage has a significant impact on the tidal behavior of the confined aquifer hypothetical studies indicate that both tidal amplitude of groundwater head in the aquifer and the distance over which the aquifer can be disturbed by the sea tide will be considerably reduced because of the existence of leakage this analytical solution is used to investigate the tidal and piezometer data at the chek lap kok airport hong kong special administrative region people s republic of china

numerically predicting seepage gradient forces and erosion sensitivity to soil hydraulic properties summary research has suggested that streambank seepage can be an important mechanism of bank instability however limited information is available on the level of soil characterization necessary to accurately predict seepage gradient forces and erosion the objective of this research was to quantify the expected range of predicted seepage gradients for various degrees of site characterization uncertainty analysis on seepage gradient predictions was performed relative to variability in soil hydraulic properties a two dimensional unsaturated saturated groundwater flow model was used to simulate a homogeneous soil layer for sand and loamy sand soils packed at various bulk densities qb a pedotransfer function rosetta designed to estimate unsaturated hydraulic properties from surrogate soil data i e texture and bulk density was used to derive the saturated hydraulic conductivity ks and water retention parameters for various levels of site information i e only textural class percent sand silt and clay ssc ssc and qb and ssc qb and ks statistical distributions were derived for each soil hydraulic parameter and monte carlo simulations were performed to generate distributions of maximum seepage gradient the deviation in predicted seepage gradient was calculated using assumed baseline conditions ranges in predicted soil hydraulic parameters and maximum seepage gradients were considerably reduced when using ssc as compared to soil texture therefore at a minimum soil samples should be taken for particle size analysis for qb between 1450 and 1500 kg m 3

the influence of groundwater on surface flow erosion processes during a rainstorm surface erosion rates on a disturbed natural soil in a 10 m indoor flume increased by an order of magnitude when a water table developed at a 10 cm depth during simulated rainstorms erosion rate increases did not correlate well with surface hydraulic flow conditions and all significant erosion increases began before the full soil depth was saturated before the water table reached the soil surface and before seepage was possible groundwater influenced erosion processes primarily by increasing unsaturated pore water pressures and decreasing soil shear strength in surface rainflow rather than through the direct entrainment of soil particles by seepage flow there was no unique morphologic expression of the influence of groundwater during a rainstorm subsurface processes influencing surface erosion were detected only by appropriate subsurface instrumentation which included micropiezometers tensiometers and time domain reflectometry erosion rate increases occurred all along the slope and were not concentrated at the base of slope due to a seepage zone soil depth was crucial to determining surface erosion increase it is likely that confusing trends in surface flow erosion rates in past studies have occurred due to unrecorded groundwater development or an emphasis on seepage effects groundwater must be monitored along hillslopes under all moisture and soil conditions in order to avoid misleading and inconsistent conclusions derived solely from surface flow or seepage data copyright 2002 john wiley sons ltd

titan s surface and atmosphere from cassini vims data with updated methane opacity abstract we present an analysis of titan data acquired by the cassini visual and infrared mapping spectrometer vims making use of recent improvements in methane spectroscopic parameters in the region 1 3 5 2 m we first analyzed vims spectra covering a 8 10 km2 area near the huygens landing site in order to constrain the single scattering albedo 0 of the aerosols over all of the vims spectral range our aerosol model agrees with that derived from huygens probe descent imager spectral radiometer disr in situ measurements below 1 6 m at longer wavelengths 0 steadily decreases from 0 92 at 1 6 m to about 0 70 at 2 5 m and abruptly drops to about 0 50 near 2 6 m a spectral variation that differs from that of khare et al s khare b n sagan c arakawa e t suits f callcott t a williams m w 1984 icarus 60 127 137 laboratory tholins our analysis shows that the far wings of the strong methane bands on both sides of the transparency windows provide a significant source of opacity in these windows and that their unknown sub lorentzian behavior limits our ability to determine precisely the surface albedos below 1 6 m the retrieved surface albedos agree with those derived from huygens disr the vims spectrum at 2 0 m indicates a surface albedo of 0 11 0 01 larger than derived in previous studies and inconsistent with the signature of water ice a series of vims data taken from 2004 to 2010 between 40 s and 40 n were then analyzed to monitor the latitudinal and temporal evolution of the atmospheric aerosol content in the 2004 2008 period the haze extinction is larger at northern mid latitudes by 20 with respect to the huygens site whereas southern mid latitudes are depleted by 15 20 in 2009 2010 a progressive decline of the haze content in the northern hemisphere is observed but no reversal of the north to south asymmetry is seen till mid 2010 finally data from five regions in tui regio and fensal that show markedly different spectral behaviors and morphologies were analyzed to investigate the wavelength dependence of their surface albedo the difference between bright and dark regions can be explained by different contents of small sized tholins at the surface brighter regions being more tholin rich than dark regions including the huygens landing site on the other hand the albedo spectrum of the so called blue regions either dark or bright can be explained by an excess of water ice particles compared with the huygens landing site the spectrum of a 5 m bright region in tui regio indicates a large excess of small sized tholins relative to the huygens site but does not point to any particular surface composition

soil properties controlling seepage erosion contributions to streambank failure the majority of sediment leaving catchments may be from streambank failure seepage erosion of unconsolidated sand above a restrictive layer is an important erosion process in incised streams that leads to streambank failure by undercutting banks the objective of this study was to determine the impact of soil properties on seepage erosion and the resulting streambank failure seepage flow and sediment concentrations were measured in situ at eight locations along the banks of a deeply incised stream in northern mississippi using field observations as a guide the soil profile conditions of a shallow 45 cm streambank consisting of 30 cm of topsoil a 10 cm conductive layer and a 5 cm restrictive layer were mimicked in laboratory lysimeter experiments to quantify the hydrologic properties controlling seepage erosion and bank failure under a 40 cm head the time to flow initiation and the flow rate were linearly related to the slope of the restrictive layer seepage erosion began within minutes of flow initiation and resulted in substantial 3 to 34 cm undercutting of the bank sediment concentrations of seeps were as high as 660 g l 1in situ and 4500 g l 1 in the lysimeters sediment concentrations were related to the layer slope thereby indicating the importance of detailed site characterization the usda ars streambank stability model demonstrated the increase in instability of banks due to undercutting by seepage erosion but failed to account for the sediment loss due to sapping for stable banks and overestimated the sediment loads for failed banks published in 2006 by john wiley sons ltd

stochastic erosion of composite banks in alluvial river bends the erosion of composite river banks is a complex process involving a number of factors including fluvial erosion seepage erosion and cantilever mass failure to predict the rate of bank erosion with these complexities a stochastic bank erosion model is suitable to define the probability distribution of the controlling variables in this study a bank erosion model in a river bend is developed by coupling several bank erosion processes with an existing hydrodynamic and morphological model the soil erodibility of cohesive bank layers was measured using a submerged jet test apparatus seasonal bank erosion rates for four consecutive years at a bend in the brahmaputra river india were measured by repeated bankline surveys the ability of the model to predict erosion was evaluated in the river bend that displayed active bank erosion in this study different monsoon conditions and the distribution functions of two variables were considered in estimating the stochastic bank erosion rate the probability of the soil erodibility and stochastic stage hydrographs for the nth return period river stage additionally the influences of the deflection angle of the streamflow longitudinal slope of river channel and bed material size on bank erosion rate were also investigated the obtained stochastic erosion predictions were compared with the observed distribution of the annual average bank erosion rate of 45 river bends in the brahmaputra river the developed model appropriately predicted the short term morphological dynamics of sand bed river bends with composite banks copyright 2014 john wiley sons ltd

numerical simulation of widening and bed deformation of straight sand bed rivers ii model evaluation in this paper the numerical model presented in the companion paper is tested and applied assessment of model accuracy was based on two approaches first predictions of evolution of a 13 5 km reach of the south fork of the forked deer river in west tennessee were compared to observations over a 24 yr period results suggest that although the model was able to qualitatively predict trends of widening and deepening quantitative predictions were not reliable simulated widths and depths were within 15 of the corresponding observed values but observed change in these parameters at the study sites were also close to these values simulated rates of depth adjustment were within 15 of observed rates but observed rates of channel widening at the study sites were approximately three times those simulated by the model in the second approach the model was used to generate relationships between stable channel width and bank full discharge the model was able to successfully replicate the form of empirically

speckle imaging of titan at 2 microns surface albedo haze optical depth and tropospheric clouds 1996 1998 we present results from 14 nights of observations of titan in 1996 1998 using near infrared centered at 2 1 microns speckle imaging at the 10 meter w m keck telescope the observations have a spatial resolution of 0 06 arcseconds we detect bright clouds on three days in october 1998 with a brightness about 0 5 of the brightness of titan using a 16 stream radiative transfer model disort to model the central equatorial longitude of each image we construct a suite of surface albedo models parameterized by the optical depth of titan s hydrocarbon haze layer from this we conclude that titan s equatorial surface albedo has plausible values in the range of 0 0 20 titan s minimum haze optical depth cannot be constrained from this modeling but an upper limit of 0 3 at this wavelength range is found more accurate determination of titan s surface albedo and haze optical depth especially at higher latitudes will require a model that fully considers the 3 dimensional nature of titan s atmosphere

coupled two dimensional modeling of bed evolution and bank erosion in the upper jingjiang reach of middle yangtze river abstract at the initial stage of dam operation in an alluvial river significant vertical bed incision and transverse bank erosion processes always occur in the river reach downstream of the dam with the latter being usually an integrated product of several interacting processes a coupled two dimensional 2d model for the processes of bed evolution and bank erosion has been proposed in this study which integrates a 2d morphodynamic module with a bank erosion module and a groundwater flow module in addition to the action of fluvial erosion the effect of groundwater flow on bank erosion was considered with the changes in pore water pressure and matrix suction being calculated the proposed model was calibrated and validated through simulating channel evolution processes in a 30 54 km sub reach of the upper jingjiang reach of the middle yangtze river with the effects of infiltration recharge on bank erosion and incoming flow on the central bar evolution being investigated the results show that i the proposed model had a good performance in reproducing flow and sediment factors in terms of water depth depth averaged velocity and suspended sediment concentration daily averaged discharge and river stage and flow and sediment diversion ratios in local anabranching subreaches ii the proposed model correctly calculated the bank erosion processes in the study reach with the calculated bank retreat width being close to the measured value however it was not enough to fully reproduce the evolution of central bars indicating that a specialized and improved module may be necessary for central bar evolution and its interaction with bank erosion iii the groundwater level change lagged behind the river stage variation and it showed a more obvious phase lag and a higher value at the same river stage corresponding to a higher pore water pressure when encountering with a more rapidly changing river stage and iv the rate of infiltration recharge caused by rainfall process would generally increase the groundwater level and thus the bank erosion degree in the study reach in addition the shifting of incoming main flow might take an obvious impact on the calculation of nearby central bar evolution

piping sapping erosion ii identification diagnosis although erosion of streambanks and other shorelines by emergent seepage is widespread this erosion mechanism termed piping or sapping has not been recognized as important to the overall erosion process the mechanism is complex and interactions with other bank and shore processes tend to mask the effects of piping sapping direct evidence water emerging from a soil face and carrying away soil particles is rarely encountered several types of indirect evidence are presented and illustrated in this paper including cavities formed by piping deposits of dislocated particles below piping zones blind gullies staining produced by persistent seepage outflow and particular types of localized failures slab toppling block shearing and tensile falls caused by undercutting due to piping sapping the interactions of this erosion mechanism with other erosion deposition processes are described the purpose of this paper is to facilitate identification and evaluation of piping sapping erosion particularly

the methane cycle on titan despite titan s cold temperatures about 93 7 k at the equator fluvial and atmospheric processes are active on this moon of saturn with methane playing a similar role to water on earth however titan lacks a global methane ocean and rainfall appears to be episodic saturn s moon titan is the second largest natural satellite in the solar system and the only one that possesses a substantial atmosphere with a surface temperature of 93 7 k at the equator titan s water is almost completely frozen out of the atmosphere water ice comprises between 35 and 45 of the mass of titan depending on the interior model but methane seems to play many of the roles on titan that water does on earth clouds have been observed fluvial and dendritic features have been imaged suggesting episodic heavy rainfall and there is compelling but circumstantial evidence for near polar lakes or seas of methane and its atmospheric photochemical product ethane however whereas earth possesses a massive global ocean of water titan lacks a global methane ocean and on titan low latitude rainfall appears to be an occasional process limited by the small amount of available solar energy compared with that of earth titan is therefore distinct from the earth but is also different from venus in retaining an active cycle of precipitation and evaporation and from mars in the preponderance of active fluvial and pluvial processes in the present day

an experimental investigation on mass failures occurring in a riverbank composed of sandy gravel abstract this paper reports the results and interpretation of laboratory experiments carried out on a model of a relatively coarse sandy gravel riverbank with the aim of investigating the basic processes and possible factors of instability after a series of initial tests three main experiments were carried out in a glass walled tank where a bank model was built with bank angles varying from 75 to 90 bank height of 70 cm and same sediment mixture 60 gravel 40 sand but with the addition of 1 of cement in the third experiment only during the experiments the bank was subject to a given hydrograph associated with a static oscillation of the water level and corresponding variations in pore water pressures were measured results show the occurrence of a large variety of processes including erosion and failures because of loss of matric suction cantilever slab and slide failures and granular flows during the ascending phase of the hydrograph bank instability was related to a superimposition of initial geometric factors and progressive reduction of apparent cohesion during the rising water stage apparent cohesion was sufficient to maintain a stable bank in loose material but only for low bank height and or slopes whereas a limited percentage of cement is able to explain a markedly different response in terms of stability and mechanisms of failure

on slope instability induced by seepage erosion this paper is focused on seepage erosion causing liquefaction processes and rapid slope failures one case history which occurred along an old fluvial terrace in the po plain northern italy is

effect of clay fraction on turbulence characteristics of flow near an eroded bank abstract stream bank erosion in the alluvial channel is a matter of great concern particularly for the irretrievable loss of fertile land this in effect has direct impact on the agriculture environment economy and social issues at large a series of laboratory experiments were carry out to study the turbulence structure over eroded bank topography modulation of mean turbulent flow characteristics and the benefaction of turbulent bursting structures to the total reynolds shear stress due to the changed geometry of the eroded bank wall are described for three different clay fraction variations a side looking micro acoustic doppler velocimeter adv was used to record the detailed high frequency velocity data near the bank wall to understand the coupled dynamics of flow and erosion on sediment water interface the information on clay and fine sand content of natural river bank was obtained from field observation results from the present study indicate that higher turbulence intensities and reynolds shear stress have prominent role in the removal of bank materials from the bank wall and continues the dislodgement of material from the bank face until mass failure quadrant analysis to the random velocity fluctuation in the vicinity of eroded stream bank shows that downstream bank material movement is strongly correlated to the presence of sweep events probability density function pdf kinetic energy fluxes and coefficients of skewness and kurtosis of velocity fluctuations show significant changes with the change in clay fraction of the bank material composition though the velocity spectral densities are in general observed to be similar for all the three different bank undercut depths for different clay fraction of bank however spectral energy shows signature of higher energy peak for lower clay fraction of the bank material

tidal winds on titan caused by saturn abstract the influence of saturn s gravitational tide on the atmosphere of titan is investigated by means of a three dimensional general circulation model titan s orbital eccentricity of 0 0292 gives rise to time dependent radial and librational tide whose potential circles eastward on titan unlike atmospheric tides on terrestrial planets saturn s tide on titan has a large impact on the dynamic meteorology down to the surface the surface pressure oscillates by up to 1 5 hpa through the orbit near the surface the tidal wind dominates the atmospheric flow and exhibits strong temporal and spatial variation the superposition of the annually present thermally forced latitudinal pressure gradient and tidally caused pressure variation produces a unique wind pattern near the surface characterized by equatorward flow and high latitude whirls at higher levels the tidal wind manifests itself as eastward traveling planetary scale wave of wavenumber 2 superposed on the background wind in general tidal winds are more significant in the troposphere where other forcing mechanisms are weak meridional tidal winds become as fast as 5 m s 1 in the troposphere and change direction periodically through the orbit and along the parallel of latitude except in the lower troposphere zonal winds always remain prograde because the tidal wind amplitude is usually smaller than the mean zonal wind the tide also has a large impact on the mean zonal circulation in the stratosphere a meridional drift of the descending huygens probe in the troposphere would be the easiest way to verify the tidal wind on titan but more complete observations of tropospheric wind and surface pressure by a future mission would be required to unveil the complete details of the tidal wind

on type b cyclogenesis in a quasi geostrophic model a quasi geostrophic qg model is used to approximate some aspects of type b cyclogenesis as described in an observational paper that appeared several decades earlier in this journal though often cited that earlier work has some ambiguity that has propagated into subsequent analyses the novel aspects examined here include allowing advective nonlinearity to distort and amplify structures that are quasi coherent and nearly stable in a linear form of the model also separate upper and lower structures are localized in space cases are studied separately where the upper trough tracks across different low level features an enhanced baroclinic zone stronger horizontal temperature gradient or a region of augmented temperature growth by superposition of lower and upper features is excluded by experimental design the dynamics are evaluated with the vertical motion equation the qg vorticity equation the qg perturbation energy equation and potential vorticity thinking results are compared against control cases having no additional low level features nonlinearity is examined relative to a corresponding linear calculation and is generally positive the results are perhaps richer than the seminal article might imply because growth is enhanced not only when properties of the lower feature reinforce growth but also when the lower feature opposes decay of the upper feature for example growth is enhanced where low level warm advection introduces rising warm air to oppose the rising cold air ahead of the upper trough such growth is magnified when adjacent warm and cold anomalies have a strong baroclinic zone between them the enhanced growth triggers an upstream tilt in the solution whose properties further accelerate the growth copyright 2005 royal meteorological society