geophysics finding chaos in abyssal hills the abyssal hills which occur all over the floors of the world s oceans are generated at faults at mid ocean ridge spreading centres there are two principal but largely incompatible ways of considering the genesis and subsequent pattern of these hills one largely taken by physical modellers the other by observationalists a new numerical physical model of faulting at mid ocean ridges produces a satisfyingly realistic abyssal hill morphology and will help to reconcile the two different viewpoints

effective discharge in small formerly glaciated mountain streams of british columbia limitations and implications episodic sediment supply past glaciation and slow responses to disturbance make small mountain streams transitional alluvial regimes in which nonequilibrium conditions are common bed load effective discharge in these streams is on average a low magnitude high frequency event but is highly variable using a two phase sediment transport model and long term discharge records we distinguish between three types of streams streams in which gravel sediment 8 mm diameter moves frequently and effective discharge occurs during gravel transport frequently mobile gravel fmg streams in which gravel moves infrequently but effective discharge nonetheless occurs during gravel transport infrequently mobile gravel img and streams in which sand sediment 8 mm diameter moves over largely immobile gravel and effective discharge occurs frequently during sand phase transport sand over immobile gravel sg using only effective discharge frequency or magnitude to characterize a stream without information on mobile sediment type is insufficient to distinguish between fmg and sg streams only the img streams have large rare effective discharges that approximate the bankfull discharge in fmg and sg streams the effective discharge is much more frequent and smaller than the bankfull only in the img streams does the effective discharge approximate a channel forming discharge in fmg and sg streams the effective discharge bears little relation to the size or dimensions of the channel and is at best a channel maintaining flow at worst it is geomorphically meaningless effective discharge should not therefore be used in isolation as a proxy for channel forming discharge for mountain stream channel design or management

effect of grade control structures at various stages of their destruction on bed sediments and local channel parameters abstract grade control structures gcss represent the typical management of torrential streams preventing massive bed erosion and bedload transport the original and present geometric and sedimentary parameters of 18 gcss at various stages of their destruction since the 1970s were evaluated to determine the relationship between the former and present day components of the managed mohelnice river the western carpathians czech republic the latest changes in the gcs geometry related scour holes and bed surface grain size of sedimentary wedges were caused by the 2010 flood event of 20 50 r i discharge no relationship exists between the bed surface grain sizes and the present water drop or the present equilibrium channel slope of the sedimentary wedge a significant downstream coarsening of the largest grain size percentile represented by d 95 is detected through the sequence of gcss also statistically insignificant trends in downstream coarsening were observed for d 16 d 50 and d 84 grain sizes however the investigated sequence is still passable for grain diameters up to 200 mm during high magnitude floods similar to the 2010 event as documented by the development of a confluent gravel bar downstream of the sequence bedload transport simulations provide the highest bedload transport rates for the initial stage of the uppermost studied channel reach without the presence of gcss 30 000 kg min 1 for q 50 grade control structures reconstruction in the 1970s significantly decreased transport rates 2000 kg min 1 for q 50 owing to the erosion of gcs crests and an increase in related equilibrium channel slope damage on gcss can lead to an increase in bedload transport intensity 13 000 kg min 1 for q 50 significant linear relationships exist among the present parameters of the scour holes length of scour hole maximum scour depth and horizontal distance between the point of maximum depth and the gcs crest a statistical significant power relationship exists between the parameters of maximum scour hole depth and the present drop height showing adjustments of maximum scours to the present stage of gcss and the last flood event

abyssal hills influence of topography on benthic foraminiferal assemblages abyssal plains often thought of as vast flat areas encompass a variety of terrains including abyssal hills features that constitute the single largest landscape type on earth the potential influence on deep sea benthic faunas of mesoscale habitat complexity arising from the presence of abyssal hills is still poorly understood to address this issue we focus on benthic foraminifera testate protists in the 150 m fraction of megacorer samples 0 1 cm layer collected at five different sites in the area of the porcupine abyssal plain sustained observatory ne atlantic 4850 m water depth three sites are located on the tops of small abyssal hills 200 500 m elevation and two on the adjacent abyssal plain we examined benthic foraminiferal assemblage characteristics standing stock diversity composition in relation to seafloor topography hills vs plain density and rarefied diversity were not significantly different between the hills and the plain nevertheless hills do support a higher species density i e species per unit area a distinct fauna and act to increase the regional species pool topographically enhanced bottom water flows that influence food availability and sediment type are suggested as the most likely mechanisms responsible for these differences our findings highlight the potential importance of mesoscale heterogeneity introduced by relatively modest topography in regulating abyssal foraminiferal diversity given the predominance of abyssal hill terrain in the global ocean we suggest the need to include faunal data from abyssal hills in assessments of abyssal ecology

a model of channel response in disturbed alluvial channels dredging and straightening of alluvial channels between 1959 and 1978 in west tennessee caused a series of morphologic changes along modified reaches and tributary streams degradation occurred for 10 to 15 years at sites upstream of the area of maximum disturbance and lowered bed levels by as much as 6 1 m following degradation reaches upstream of the area of maximum disturbance experienced a secondary aggradation phase in response to excessive incision and gradient reduction aggradation downstream of the area of maximum disturbance reached 0 12 m per year with the greatest rates occurring near the stream mouths the adjustment of channel geometry and phases of channel evolution are characterized by six process oriented stages of morphologic development premodified constructed degradation threshold aggradation and restabilization down cutting and toe removal during the degradation stage causes bank failure by mass wasting when the critical height and angle of the bank material is exceeded threshold stage channel widening continues through the aggradation stage as the slough line develops as an initial site of lower bank stability the bank profile develops three dynamic elements 1 vertical face 70 to 90 2 upper bank 25 to 50 and 3 slough line 20 to 25 alternate channel bars form during the restabilization stage and represent incipient meandering of the channel

wood recruitment and retention the fate of eroded trees on a braided river explored using a combination of field and remotely sensed data sources abstract this paper investigates wood recruitment and deposition dynamics in a large gravel bed braided river tagliamento river italy we used a combination of field measurements automatically repeated ground images and remotely sensed surveys to quantify wood input through bank erosion and associated downstream deposition two sites were investigated where floods caused the erosion of vegetated island edges a lidar survey preceding the erosion events provided data on the morphology and vegetation structure of the eroded areas allowing estimation of the number of trees that were uprooted sequences of ground based images acquired automatically and supported by field measurements showed the time number and location of deposited trees results show that the complex morphology of braided rivers induces specific deposition patterns we observed wide dispersal of wood on gravel bars with jams characterised by a small number of logs on average 2 3 and in many cases only a single log a large proportion of the eroded trees up to 40 were deposited on the nearest downstream bar this illustrates significant wood retention close to the recruitment site with the remaining wood dispersed widely downstream differences in the observed level of local wood retention were associated with the proximity of the erosion site to the main channel and differences were also observed in retention between the peak and the falling limb of flood events confirming that water depth and probably flow velocity are the crucial parameters controlling wood deposition

effective discharge in rocky mountain headwater streams summary whereas effective discharge q eff in mountain streams is commonly associated with a moderate flow such as bankfull discharge q bf this study found that the maximum discharge q max and not bankfull discharge is the channel forming or effective flow for gravel transport in plane bed streams where partial bed mobility causes steep gravel transport rating curves q eff may approach bankfull flow in some step pool channels where gravel moves over a static cobble boulder bed our conclusions are based on magnitude frequency analyses conducted at 41 gauged rocky mountain headwater streams because these gauged streams lacked gravel transport data as is typical comparable streams with measured transport rates were used to develop scaling relations for rating curve exponents with stream and watershed characteristics those scaling relations were then used to estimate the steepness of gravel rating curves at the 41 gauged but unsampled sites the measured flow frequency distributions were characterized by two fitted power functions the steepness of the flow frequency distributions and the estimated steepness of gravel transport relations were combined in magnitude frequency analyses to compute q eff

impact of channel incision on the hydraulics of flood flows examples from polish carpathian rivers abstract channel deepening may result from channel incision or river metamorphosis changing a wide and shallow channel to a narrow and deep one as only the first type of channel change leads to increased flow capacity of the channel a lowering of water stage associated with a given discharge rather than a lowering of river bed should be used to identify channel incision a lowering of minimum annual stage at gauging stations is typically used to assess the relative importance of channel incision along a river or within a particular region rivers of the polish carpathians incised by 0 5 3 8 m over the twentieth century with the amount of incision being greater in their lower and middle courses than in the upper ones variability in the hydraulic importance of channel incision with increasing river size is analysed by comparing changes in the frequency of valley floor inundation at gauging stations located along the seventh order dunajec river despite a lower absolute amount of channel incision in the upper river course here incision has increased channel conveyance and reduced the frequency of valley floor inundation considerably more than in the lower course hydraulic effects of channel incision depend also on lateral stability of an incising river low energy rivers from the eastern part of the polish carpathians remained laterally stable during channel incision this has resulted in substantial lowering of stages for low flood discharges and markedly smaller one for high magnitude floods whereas velocity of the flows conveyed over the highly elevated floodplains has decreased considerably in high energy rivers from the western part of the polish carpathians alternation of incision and lateral channel migration has led to the formation of incised meander belts with substantially lowered stages for all flood discharges and increased velocity of the flows conveyed over the newly formed low lying floodplains

chronology of processes in high gradient channels of medium high mountains and their influence on the properties of alluvial fans abstract high gradient channels are the locations of the greatest geomorphological activity in medium high mountains the channels frequency and character influence the contemporary morphology and morphometry of alluvial fans there is currently no detailed information regarding the frequency of these processes in high gradient channels and the evolution of alluvial fans in medium high mountains in central europe this study in the moravskoslezske beskydy mts analysed 22 alluvial fans 10 debris flow fans and 12 fluvial fans the processes occurring on the fans were dated using dendrogeomorphological methods a total of 748 increment cores were taken from 374 trees to reconstruct 153 geomorphological process events 60 debris flow and 93 floods the frequency of the processes has been considerably increasing in the last four decades which can be related to extensive tree cutting since the 1970s processes in high gradient channels in the region affecting the alluvial fans across the mountain range are predominantly controlled by cyclonal activity during the warm periods of the year probable triggers of local events are heavy downpours in the summer in addition spring snowmelt has been identified as occasionally important this study of the relations affecting the type and frequency of the processes and their effect on the properties of alluvial fans led to the creation of a universal framework for the medium high flysch mountains of central europe the framework particularly reflects the influence of the character of hydrometeorological extremes on the frequency and type of processes and their reflection in the properties of alluvial fans

twentieth century channel change on the dunajec river southern poland patterns causes and controls abstract spatial and temporal patterns of the twentieth century channel changes of the dunajec the second largest river of the polish carpathians are analysed using data from six historical maps hand auger drillings in paleochannels and hydrometric data from 10 gauging stations in its lower course the river was considerably narrowed shortened and embanked between the 1880s and the 1920s here bed degradation commenced in the late 19th century and has so far resulted in 3 1 m of channel incision in the middle river course little change occurred in mountain reaches where the dunajec is confined by valley sides and where bedrock exposures prevent channel incision in the reaches within intramontane basins channelization works carried out in the 1950s 1970s considerably narrowed the river and transformed its multi thread channel into a single thread straight channel rapid bed degradation induced by the works resulted in up to 2 m of channel incision over the second half of the century upstream progression of bed degradation into the adjacent mountain reach was hindered by bedrock exposures at its downstream end in the upper river course reaches with the deeply incised channels up to 3 5 m prevail over a few past decades in response to gravel exploitation river channelization and a reduction in catchment sediment supply more scarce are vertically stable reaches which owe their stability to low unit stream power in wide unmanaged reaches and to fixing of the channel bed by drop structures in channelized reaches this study shows that despite the overall tendency to incision over the 20th century considerable differences in the timing and course of channel changes occurred between particular river reaches due to variable human impact and local geological and geomorphological conditions the main phase of channel incision took place progressively later in the upstream direction however upstream progression of bed degradation from the already incised downstream reaches was precluded by the roznow dam and bedrock controlled reaches where bed material flushed out from an incising upstream reach had been introduced into an artificially narrowed channel of high transport capacity it was easily transferred downstream and could not restore the downstream reach to its pre channelization vertical position

three dimensional resistivity tomography of vulcan s forge vulcano island southern italy 1 9 525 dc resistivity measurements were taken along 9 profiles crossing the volcanic edifice of la fossa di vulcano the forge of god vulcan in ancient roman mythology vulcano island italy using a total of 958 electrode locations this unique data set has been inverted in 3d by minimizing the l2 norm of the data misfit using a gauss newton approach the true 3d inversion was performed using parallel processing on an unstructured tetrahedral mesh containing 75 549 finite element nodes and 398 208 elements to accurately model the topography of the volcanic edifice the 3d tomogram shows a very conductive body 0 1 s m comprised inside the pietre cotte crater with conductive volumes that are consistent with the position of temperature and co2 anomalies at the ground surface this conductive body is interpreted as the main hydrothermal body it is overlaid by a resistive and cold cap in the bottom of the crater the position of the conductive body is consistent with the deformation source responsible for the observed 1990 1996 deflation of the volcano associated with a decrease of hydrothermal activity

clastic and core lava components of a silicic lava dome the formation of a lava dome involves fractionation of the lava into core and clastic components we show that for three separate successive andesitic lava domes that grew at soufriere hills volcano montserrat between 1999 and 2007 the volumetric proportion of the lava converted to talus or pyroclastic flow deposits was 50 90 of the lava extruded currently only 8 of the total magma extruded during the 1995 2007 eruption remains as core lava the equivalent representation in the geological record will probably be even lower most of the lava extruded at the surface flowed no further than 150 300 m from the vent before disaggregation resulting in a lava core whose shape tends to a cylinder moderate to high extrusion rates at the soufriere hills domes may have contributed to the large clastic fraction observed creating talus dissipates much of the energy that would otherwise be stored in the core lava of domes the extreme hazards from large pyroclastic flows and blasts posed by wholesale collapse of a lava dome depend largely on the size of the lava core and hence on the aggregate history of the partitioning process not on the size of the dome

use of high water marks and effective discharge calculation to optimize the height of bank revetments in an incised river channel abstract in deeply incised rivers bankfull discharge i e the flow filling the channel to the top of the banks does not represent channel forming flow and increasingly large flows are associated with increasingly large boundary shear stress in such rivers solid bank revetments rip rap gabions retaining wall are usually constructed to the top of the banks similarly as in vertically stable rivers despite the fact that the upper parts of the banks may never be flooded to optimize the height of solid bank revetments in deeply incised channels it is thus important to determine whether a flood magnitude can be identified for which the combination of flow duration and bedload transport rate results in the highest river efficiency to transport bedload and perform geomorphic work this question was explored in the moravka river czech carpathians which deeply incised into non resistant flysch bedrock over the past few decades observations of high water marks e g trash lines wash lines after a flood in 2014 enabled reconstruction of the peak flood stage in the deeply incised reach and the adjacent vertically stable reach these observations together with post flood measurements of cross sectional channel geometry distances between consecutive cross sections and estimates of channel roughness were used in one dimensional hydraulic modelling aimed to determine a peak discharge of the flood in a number of cross sections in both reaches despite the close proximity of both reaches markedly higher discharge values were obtained for the incised reach and the discrepancy was used to calibrate roughness coefficients for the incised reach a flow duration curve determined on the basis of a 25 year series of daily discharges in the upstream gauging station together with data about channel geometry and roughness in the incised cross sections were used to simulate bedload transport at successive discharges with the bags sediment transport model the calculations indicated the effective discharge for bedload transport in the incised reach to have the recurrence interval of 7 12 years the upper limit of the reach average effective discharge is associated with the stage slightly higher than a half of the bankfull depth this highlights the disparity between effective and bankfull discharges and indicates that in incised channels channel forming flow should be linked with the former the vertical extent of the reach average effective discharge is proposed as an indicator of the optimal cost effective height of solid bank protection structures in the deeply incised channel and the suitability of this approach is discussed

impact of a large flood on mountain river habitats channel morphology and valley infrastructure abstract the biala river polish carpathians was considerably modified by channelization and channel incision in the twentieth century to restore the biala establishing an erodible corridor was proposed in two river sections located in its mountain and foothill course in these sections longer unmanaged channel reaches alternate with short channelized reaches and channel narrowing and incision increases in the downstream direction in june 2010 an 80 year flood occurred on the river and this study aims at determining its effects on physical habitat conditions for river biota channel morphology and valley floor infrastructure surveys of 10 pairs of closely located unmanaged and channelized cross sections performed in 2009 and in the late summer 2010 allowed us to assess the flood induced changes to physical habitat conditions a comparison of channel planforms determined before 2009 and after 2012 the flood provided information on the degree of channel widening as well as changes in the width of particular elements of the river s active zone in eight stretches of the biala the impact of the flood on valley floor infrastructure was confronted with the degree of river widening in unmanaged and channelized river reaches before the flood unmanaged cross sections were typified by finer bed material and greater lateral variability in depth averaged and near bed flow velocity than channelized cross sections the flood tended to equalize habitat conditions in both types of river cross sections obliterating differences in particular physical habitat parameters between channelized and unmanaged channel reaches river widening mostly reflected an increase in the area of channel bars whereas the widening of low flow channels was less pronounced a comparison of channel planform from 2009 and 2012 indicated that intense channel incision typical of downstream sections limited river widening by the flood active channel width increased by half in the unmanaged cross sections and by one third in the channelized cross sections however damage to the valley floor infrastructure was practically limited to the channelized river reaches with reinforced channel banks this indicates incompetent management of riparian areas rather than the degree of river widening as a principal reason for the economic losses during the flood

crumbling of dacite dome lava and generation of pyroclastic flows at unzen volcano recent modelling of volcanic eruptions has shown that the efficiency of subsurface degassing of magmas determines whether magma erupts explosively or effuses quietly1 2 slow uprise of magma is often accompanied by effective degassing leading to the extrusion of lava flows and domes although lava dome extrusion is one of the less explosive modes of eruption it is often accompanied by explosive pyroclastic activities3 5 the 1991 eruption of unzen volcano provided an opportunity to observe at close range several types of small scale pyroclastic flow glowing avalanches originating from lava domes most of the pyroclastic flows are of merapi type caused by blocks falling from a collapsing dome others are of pelean type originating in an explosion from the side of a dome the lavas apparently show variable degrees of self explosivity we suggest that variable degrees of degassing of the magma produced a wide range of excess pore pressures in the extruded lava domes resulting in both merapi type and pelean type pyroclastic flows from the domes

deep seated gravitational slope deformations in the highest parts of the czech flysch carpathians evolutionary model based on kinematic analysis electrical imaging and trenching deep seated gravitational slope deformations dsgsds played an important role during the quaternary evolution of mountain ridges formed by anisotropic flysch rocks of the outer western carpathians owc four dsgsd affected ridges in the highest part of the czech sector of the owc have been subjected to multidisciplinary investigation involving mapping structural and kinematic analysis geoelectrical imaging trenching and dating all the studied dsgsds are strongly predisposed by regional structural fabric given by the intersection of bedding planes joint sets and faults performed research suggests high diversity of mechanisms leading to the origin of individual dsgsds the area of a single dsgsd can be affected by various types of movements such as sackung lateral spreading toppling or incipient sliding electrical imaging and trenching revealed that besides these mechanisms anisotropic flysch massifs formed by thick bedded sandstones are strongly exposed to subsurface opening of air filled voids related to widened crevices such a process is driven by deep seated creep and translational sliding of sandstone beds upon plastic claystones whose strength has been reduced to residual values of the angle of internal friction performed radiocarbon and osl dating revealed that some of the studied dsgsds originated in the holocene period particularly in the atlantic and subboreal chronozones

geomorphic diversity in the braided wandering bela river slovak carpathians as a response to flood variability and environmental changes abstract changes in the morphological pattern of the gravel bed bela river slovak carpathians were investigated to infer about the impact of flood events and environmental changes on the evolutionary trend of the river since the mid twentieth century the post flood period serial geomorphic analysis popsega approach coupled with gis analysis was used to identify morphological changes that occurred during particular flood periods the study focused on assessing how six flood periods fp with differing flood characteristics have influenced the evolutionary trend of the river several sets of aerial photos 1949 1961 1973 1986 1992 2003 2009 taken after the periods with the occurrence or lack of major floods provided data about channel landform changes in 227 river segments of 100 m length a decreasing trend of the area of the river s active zone from 2 5 km2 in 1949 to 1 4 km2 in 2009 was identified the first fp 1949 1961 with a very high magnitude flood in 1958 ri50 was characterized by increasing total channel length island number area and the number density of confluence diffluence pairs nodes and by decreasing bar area during this fp the frequency of cores of channel dynamics as well as the shannon s index of landforms diversity reached the highest values the following fp 1962 1973 with four 5 to 10 year floods exhibited the inverse pattern of changes in channel form parameters although the core frequency of channel dynamics and shannon s diversity index were still high the river system seems to have been relatively stable between 1974 and 1986 third fp with some decrease in the area of channel forms and in pattern metrics values during the fourth fp 1987 1992 several 1 5 to 3 year floods occurred and were associated with a decrease in some channel forms and a further decrease of the pattern metrics values an increase in all analysed channel parameters especially bar area and pattern metrics values occurred in the years 1993 2002 fifth fp as a consequence of two floods of moderate magnitude in 1997 ri7 and 2001 ri3 5 the lowest values of bar area and pattern metrics during the whole study period were recorded during the last fp 2002 2008 whereas island area increased at that time the study shows that the occurrence of floods during the last six decades has not returned the bela to its former state because of the overall decrease in flood magnitudes changes in catchment sediment supply induced by the increasing forest cover in the catchment from 22 7 in 1957 to 34 8 in 2008 and changes in channel boundary conditions channelization channel incision small hydropower plant constructions

paleoflood hydrology and extraordinary flood events paleoflood hydrology is the study of past or ancient flood events the most accurate technique involves the analysis of slackwater deposits and paleostage indicators swd psi slackwater deposits are sand and silt emplaced from suspension in exceptionally deep high velocity floods that characterize narrow deep canyons in resistant geologic materials these and other paleostage indicators are used to establish hydraulic grade lines for the generating flow events favorable sites for slackwater deposition include channel margin areas where stresses and velocities are reduced below critical values necessary to maintain fine grained bedload material sand and silt in suspension computerized procedures for hydraulic flow modeling are used to tie the elevations of the highest slackwater deposits to surveyed river cross sections the flood discharges calculated by this method can be calibrated through the study of modern floods on gaged rivers correlation of multiple swd psi sites along a river reach is used to identify the maximum paleostage achieved by a given flood advances in the dating of flood deposits permit estimates of flood frequency to be made extending over a data base of thousands of years the major geochronologic tool is radiocarbon dating of various kinds of organic matter intercalated with the slackwater deposits an important development is the use of the tandem accelerator mass spectrometer for direct measurement of 14c tiny blebs of charcoal seeds and other organics can be analyzed in order to date ancient flood deposits of hydrologic significance since swd psi studies yield very accurate determinations of paleoflood ages and magnitudes there is a pressing need for new statistical procedures that make optimum use of the information content in paleoflood records for flood frequency analysis nevertheless swd psi paleoflood hydrology has moved beyond the research phase its use should be encouraged in evaluating past experience of extraordinary floods at appropriate hazardous sites

modelling the flooding capacity of a polish carpathian river a comparison of constrained and free channel conditions abstract the gravel bed biala river polish carpathians was heavily affected by channelization and channel incision in the twentieth century not only were these impacts detrimental to the ecological state of the river but they also adversely modified the conditions of floodwater retention and flood wave passage therefore a few years ago an erodible corridor was delimited in two sections of the biala to enable restoration of the river in these sections short channelized reaches located in the vicinity of bridges alternate with longer unmanaged channel reaches which either avoided channelization or in which the channel has widened after the channelization scheme ceased to be maintained effects of these alternating channel morphologies on the conditions for flood flows were investigated in a study of 10 pairs of neighbouring river cross sections with constrained and freely developed morphology discharges of particular recurrence intervals were determined for each cross section using an empirical formula the morphology of the cross sections together with data about channel slope and roughness of particular parts of the cross sections were used as input data to the hydraulic modelling performed with the one dimensional steady flow hec ras software the results indicated that freely developed cross sections usually with multithread morphology are typified by significantly lower water depth but larger width and cross sectional flow area at particular discharges than single thread channelized cross sections they also exhibit significantly lower average flow velocity unit stream power and bed shear stress the pattern of differences in the hydraulic parameters of flood flows apparent between the two types of river cross sections varies with the discharges of different frequency and the contrasts in hydraulic parameters between unmanaged and channelized cross sections are most pronounced at low frequency high magnitude floods however because of the deep incision of the river both cross section types are typified by a similar low potential for the retention of floodwater in floodplain areas the study indicated that even though river restoration has only begun here it already brings beneficial effects for flood risk management reducing flow energy and shear forces exerted on the bed and banks of the channel in unmanaged river reaches only within wide unmanaged channel reaches can the flows of low frequency high magnitude floods be conveyed with relatively low shear forces exerted on the channel boundary in contrast in channelized reaches flow velocity and shear forces are substantially higher inevitably causing bank erosion and channel incision

river response to channel regulation case study of the raba river carpathians poland processes induced by the channelization of the raba river in the present century are examined to illustrate the response of a gravel bed stream following narrowing and straightening of its channel up to 3 m of incision has occurred the change from a slow and relatively steady degradation in the lower reaches to separate degradation events in the higher reaches is attributed to the differential rate of headcut retreat and to the control exerted by mid channel bars upon the rate of river energy dissipation progressive outwashing of finer grains from bed material has followed the diminishing sediment yield of the basin and the increase in stream power the ensuing growth in mean grain size and changes in sediment fabric have increased boundary resistance to flow and reduced particle susceptibility to entrainment downstream magnification of peak discharges has become increasingly pronounced with the advancing incision the decrease in flood plain storage and self acceleration of flows passing the relatively deep and straight channel has caused flood waves to become progressively more flashy in nature an increase in channel depth and reduction in gradient caused by downward and backward erosion as well as bed material coarsening has promoted the re establishment of an equilibrium conversely flow velocity increases due to flow concentration in the deepened channel reduction in grain mobility allows the river to attain a new equilibrium at flow velocity and stream power levels higher than before the channelization numerous disadvantages of the applied regulation scheme and its failure to reduce flood hazard raise the question of its maintenance to be successful any regulation design must take into account changes in sediment supply and flood hydrographs resulting from the simultaneous alterations in basin management

complex transformation of the geomorphic regime of channels in the forefield of the moravskoslezske beskydy mts case study of the moravka river czech republic abstract this paper presents a complex analysis of both the contemporary and the historic development of the geomorphic regime of the transformed reach of the moravka river in the czech carpathians the assessment concentrates on the conditions and causes of the channel development in the last c 200 years compared with the state of european channels especially those of the carpathian zone the moravka r pattern has undergone a rapid change in the last 50 years particularly in connection with the active channel narrowing and massive incision the original anabranching river pattern has gradually been replaced by a simple narrowed channel incised into the bedrock the average width of the moravka r active channel changed from 153 m in 1836 1852 and 165 m in 1876 1878 to 44 m in the year 2010 at some parts the original river bed has lowered as much as 8 m in the last 40 years which indicates an incision rate of 12 24 cm year these changes have been caused by strong anthropogenic impacts in the form of the river channel control bank stabilisation and weir and valley dam construction other reasons are related to land cover and land use changes a great influence on the contemporary processes is also exerted by the geological predisposition of the carpathian flysch lithology in channel bedrock particularly the occurrence of claystone layers that present little resistance to water erosion currently the deficit of transportable sedimentary material along with the increased transport capacity of the incised river bed has generated conditions for constantly intensifying erosion processes in the moravka r channel

evolution of the morphology of the river dragonja sw slovenia due to land use changes the effects of increasing agricultural land use on fluvial morphology have received much attention in fluvial research however in several regions in europe a reversing trend of decreasing agricultural activity and land abandonment followed by reforestation is observed the response of fluvial morphology deserves attention because of its large impacts on landscape and riverine habitats with the help of geomorphological mapping multi date aerial photography and a range of dating techniques we reconstructed the evolution of the morphology of the riverbed and the floodplain of the dragonja river in southwestern slovenia the results of this study show that the fluvial morphology in this mediterranean catchment has changed considerably as a result of shifts in agricultural land use in particular large scale land abandonment in the second half of the 20th century until the first half of the 19th century floodplain aggradation prevailed probably around 1870 a large erosion event occurred from which the floodplain did not fully recover a terrace standing 2 5 m above the present floodplain was formed natural reforestation due to depopulation since world war ii caused a reduction in discharge and sediment supply to the river the decreased intensity and frequency of floods allowed invasion of the riverbed by vegetation causing narrowing and incision of the riverbed this resulted in the formation of a terrace which now stands 1 5 m above the present day river this terrace is about 60 years old however the largest increase in forest area occurred since 1975 which intensified this process of riverbed narrowing and incision creating a local terrace at 0 5 m at 0 5 m above the presently meandering river

sediment supply and the development of the coarse surface layer in gravel bedded rivers the bed surface of most gravel rivers is considerably coarser than the sub surface or the gravel load transported over it a phenomenon affecting river dynamics morphology and ecology the coarse surface layer often called an armour or pavement has been attributed to an inherent tendency for small grains to settle between larger ones during active transport of all sizes1 2 and to selective erosion or trapping of finer particles when the coarse grains are relatively immobile3 4 where bedload supply is cut off below dams selective erosion causes coarsening5 6 using a simple quantitative model we propose that surface coarsening develops in gravel bedded rivers when local bedload supply from upstream is less than the ability of the flow to transport that load we present laboratory results which support this hypothesis and show that supply reduction causes changes in bedforms and progressive confinement of active bedload transport to a narrow band of finer bed surface bordered by a coarse less active bed it may therefore be possible to relate the degree of river bed surface coarsening to sediment supply resulting from land use

bank and near bank processes in an incised channel abstract gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion the interaction of these forces control streambank mechanics hydraulic forces exerted by flowing water on in situ bank toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower bank profiles seepage forces exerted on in situ bank material by groundwater downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength data from a study site on goodwin creek ms usa clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore water pressures negative pore water pressures matric suction have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear a stable bank can be transformed into an unstable bank during periods of prolonged rainfall through 1 increase in soil bulk unit specific weight 2 decrease or complete loss of matric suction and therefore apparent cohesion 3 generation of positive pore water pressures and therefore reduction or loss of frictional strength 4 entrainment of in situ and failed material at the bank toe and 5 loss of confining pressure during recession of stormflow hydrographs relatively small frequent flows during the winter have the ability to erode failed bank materials maintain oversteepened unstable bank surfaces and promote prolonged periods of bank retreat channel migration and high yields of fine grained sediment confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe which steepens the bank for example more than 2 m of bank retreat occurred during the study period at the research site on goodwin creek northern mississippi the loss of matric suction negative pore pressures due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability apparent cohesion friction angle soil bulk unit weight and moisture content were measured in situ matric suction was measured continuously in situ with a series of five pressure transducer tensiometers a bank failure algorithm which combines the mohr coulomb approach for saturated conditions and the fredlund modification for unsaturated conditions was developed for layered cohesive streambanks the resulting equation has been used successfully to investigate the role of matric suction positive pore water pressures and confining pressure for layered streambanks composed of cohesive materials

combined palaeoflood and rainfall runoff assessment of mountain floods spanish pyrenees abstract this paper reports a comparative study of the use of palaeohydrological and hydrological techniques to estimate magnitude and frequency of floods applied to two high gradient small catchments located in the spanish central pyrenees the aras basin 18 6 km2 the montardit basin 15 2 km2 palaeohydrological palaeoflood techniques based on palaeostage indicators and the application of both manning s equation and critical flow equation were used to estimate discharge values corresponding to seven cross sections of the montardit stream this method yielded discharges ranging from 7 to 112 m3 s 1 palaeoflood and historical flood analyses indicated a greater frequency of high magnitude events for both the aras and montardit basins than suggested by the application of two different methodologies based on precipitation data the rational and the unit hydrograph methods the 1996 aras basin flood was modelled using the unit hydrograph method with most reliable results 217 m3 s 1 obtained using a semidistributed basin model for small ungauged mountain basins palaeoflood reconstruction provides an excellent tool to obtain past flood discharges on which to base the calibration of parameters e g runoff threshold used in rainfall runoff methods it is proposed that the combined results of long term flood record analysis based on palaeoflood and historical data and of standard hydrometeorological methods will aid in the assessment of flood risks for application to the design of emergency measures and floodplain management in ungauged mountain streams

3d electrical imaging of the inner structure of a complex lava dome puy de dome volcano french massif central france abstract lava domes result from extrusion of massive lava frequent explosions and collapses this contribution focuses on a complex trachytic lava dome the puy de dome volcano located in the chaine des puys volcanic field french massif central france we performed electrical resistivity tomography ert acquisitions on the entire edifice in order to investigate its overall inner structure as well as to detail its summit area the resulting large ert dataset integrated a recently developed 3d inversion code based on an unstructured discretization of the geometrical model the 3d inversion models obtained refine the existing geological model of the puy de dome s inner structure obtained by previous geophysical studies these results also highlight the strong fracturing and fumarolic alteration that affect the summit part of the volcano

distribution of buried hydrothermal alteration deduced from high resolution magnetic surveys in yellowstone national park yellowstone national park ynp displays numerous and extensive hydrothermal features although hydrothermal alteration in ynp has been extensively studied the volume geometry and type of rock alteration at depth remain poorly constrained in this study we use high resolution airborne and ground magnetic surveys and measurements of remanent and induced magnetization of field and drill core samples to provide constraints on the geometry of hydrothermal alteration within the subsurface of three thermal areas in ynp firehole river smoke jumper hot springs and norris geyser basin we observe that hydrothermal zones from both liquid and vapor dominated systems coincide with magnetic lows observed in aeromagnetic surveys and with a decrease of the amplitude of short wavelength anomalies seen in ground magnetic surveys this suggests a strong demagnetization of both the shallow and deep substratum within these areas associated with the removal of magnetic minerals by hydrothermal alteration processes such demagnetization is confirmed by measurements of rock samples from hydrothermal areas which display significantly decreased total magnetization a pronounced negative anomaly is observed over the lone star geyser and suggests a significant demagnetization of the substratum associated with areas displaying large scale fluid flow the ground and airborne magnetic surveys are used to evaluate the distribution of magnetization in the subsurface this study shows that significant demagnetization occurs over a thickness of at least a few hundred meters in hydrothermal areas at ynp and that the maximum degree or maximum thickness of demagnetization correlates closely with the location of hydrothermal activity and mapped alteration

abyssal hills formed by stretching oceanic lithosphere tectonic plates are formed and move apart at mid ocean ridges some portion of this plate separation process can occur by stretching of the crust resulting in a complex pattern of extensional faults abyssal hills the most ubiquitous topographic features on earth1 are thought to be a product of this faulting2 3 here we report the results of a self consistent numerical model of lithospheric formation and stretching that includes spontaneous fault creation in this model an axial valley develops where the fault activity is most concentrated the frozen fault generated topography rafted out of the axial valley is visually and statistically similar to observed abyssal hills formed at many slower spreading ridges faults appear to be replaced by new faults because their offset changes the local stress field we accordingly need no temporal variation in magmatism as required by some previous models4 5 6 to control the spacing or offset of faults our model results suggest instead that the irregularity of abyssal hill relief may result from a self organized critical stress state at spreading centres

river channel adjustments downstream from channelization works in england and wales relatively little attention has been given to river channel adjustments that occur downstream from channelization works this study is concerned with the nature of channel adjustments downstream from a total of 46 channelization works located in low and high energy environments in england and wales channel changes are identified principally by the method of field survey and by reconstructing the original positions of eroded beds and banks use is also made of maps aerial photographs and engineering drawings of different dates and the technique of space for time substitution is applied enlargement of channel cross sections through erosion had occurred downstream from a variety of types sizes and dates of channelization works the maximum increase of channel size was 153 per cent out of a total of 14 sites with enlarged channel cross sections seven had undergone a change of width only at a further three width increased rather than depth and at the remaining four sites depth increases were dominant these sites all have relatively high stream powers factors causing spatial variation of erosion included tree roots locally binding bank sediments and the occurrence of bends planform change had taken place at only one site a further three high stream power sites had downstream reaches incised into bedrock and therefore did not exhibit adjustment channel enlargement is explained in terms of increased flood flows downstream from channelization works causing higher stream velocities which in turn cause erosion thereby increasing channel width and or depth examination of flow records for 35 stations revealed flood events which would formerly have spread overbank but are now confined by the channelization works and are therefore likely to alter downstream flows at sites with downstream change it is proposed that the energy of increased flows was sufficient to exceed a threshold required for erosion of perimeter sediments by contrast the absence of change at a majority of sites in low energy lowland areas could be a reflection of both the incompetence of increased flows to erode and resistance provided by perimeter sediments sites with erosion features appear not to have yet attained new equilibrium conditions

catchment scale cumulative impact of human activities on river channels in the late anthropocene implications limitations prospect abstract evidence for the proposed anthropocene epoch in fluvial geomorphology hinges on the influence of human activities relative to natural forcing however research on cause effect understanding in river channel evolution has rarely focused on the cumulative impact of multiple drivers for change limiting insights systematic review of 25 recent studies professing to explain reach scale channel responses to cumulative impacts of human activities and natural forcing over the recent past ca 1880 2005 reveals some consistencies in spatio temporal response across various catchment sizes median 3000 km2 in mostly industrialized nations common drivers for change include changing flood and flow regimes dam construction changing land uses and forest cover bank protection and instream aggregate mining recent channel evolution has predominantly involved narrowing incision and terrace development reduced bed sediment storage lower activity rates and simplified channel geometries rates of channel change frequently peaked 1955 1990 providing some support for the anthropocene great acceleration evidence here suggests that many river systems are now in morphologically novel configurations coinciding temporally with dramatic recent declines in global freshwater aquatic biodiversity sustainable approaches to freshwater management must acknowledge these configurations placing emphasis on process based approaches to river ecosystem health in which sediment cascades are reconceived to reflect altered longitudinal and lateral connectivity however the reviewed studies are driven largely by expert judgment depicting cause effect associations through summary conceptual models based on spatial proximity and temporal synchronicity providing insufficient scientific proof for the anthropocene based on the overwhelming impact of human factors more conclusive cause effect statements will require hypothetic deductive approaches explicit functional criteria and best practice environmental model building geomorphologists should now move beyond the phase of discovery and develop rigorous proofs for cause effect relationships of cumulative impact this may be enhanced by developing an avowedly anthropocene perspective in which rivers are viewed critically as socio biophysical systems co evolving with human activity

frequency predisposition and triggers of floods in flysch carpathians regional study using dendrogeomorphic methods abstract dangerous overland flood events in the foothills of the flysch carpathians often result from a cumulative effect of floods in high gradient channels detailed understanding of the origin of floods in these catchments is only possible if the occurrence of these floods in the past has thoroughly been studied yet no gauging stations can be found in the local catchments the reconstruction of floods in ungauged catchments has so far been performed using dendrogeomorphic methods stems or branches floating in the floodwater can affect stems or roots of living trees and injure them trees are able to record these signals in their tree ring series within the flysch carpathians the floods have been reconstructed based on the analysis of 446 cross sections from scarred tree roots and 192 increment cores from the stems of affected trees in the studied area of 10 catchments surrounding the highest peak of the moravskoslezske beskydy mts the lysa hora mt the dating comprised 64 floods in different catchments in 28 flood years for the maximal period of 1883 2012 most catchments nine out of ten were affected by floods in the year 1997 above average frequency of floods has also been found for the last two decades namely thanks to numerous samples taken from young tree roots that revealed more flood impacts by contrast although tree ring series enabled the reconstruction of a longer time series they only recorded the major floods the most significant factor affecting the frequency of floods is the orientation of catchments toward the prevailing wind direction positive influence of catchment gradient on flood frequency and higher occurrence of floods in the period of intensive slope deforestation show that the floods of the 1950s to 1970s could have a character of flash floods this assumption is also supported by the character of probable triggering precipitation high magnitude short duration precipitation of this period generally the most frequent probable triggering factor is however an above average seven day precipitation amount which indicates along with the positive relation between the catchment extent and flood frequency that a majority of floods hardly ever occur as an immediate response to one day precipitation extremes fast spring melting of thick snow cover plays an insignificant role in floods in the studied medium high mountains

variation in surface bed material along a mountain river modified by gravel extraction and channelization the czarny dunajec polish carpathians abstract longitudinal changes in the grain size of bar sediments of the mountain czarny dunajec southern poland were investigated to determine the impact of human activity in the river on depositional conditions in its channel the grain size of surface bed material was established on 47 gravel bars along an 18 km long river reach without tributaries in which some sections were modified over the past few decades by channelization or gravel extraction and the resultant channel incision a downstream fining trend of bar sediments was determined from samples taken at the sites with close to average river width and a vertically stable channel and used as reference for the other samples in the deeply incised upper part of the reach bar gravels are markedly coarser than the reference grain size in the narrow channelized section in the middle part of the reach bar sediments exhibit better than average sorting and change from coarser gradation to one similar to that of the reference grain size along the section in the wide multithread channel in the lower part of the reach bar gravels are distinctly finer than the reference grain size a similar pattern of downstream variation in the study reach was determined for the river competence on the basis of critical bed shear stress required to entrain d95 particles of the bar gravels the extraction of larger particles from the channel bed in the upper part of the study reach facilitated entrainment of exposed finer grains hence inducing rapid bed degradation at the same time the concentration of flood flows in an increasingly narrow and deep channel must have increased their competence enabling a delivery of the coarse particles previously typical of the upstream reach the middle section has been channelized to prevent sediment delivery to a downstream located dam reservoir however it actually operates as a conveyor belt transferring downstream the bed material flushed out from the upstream incising river section as low unit stream power and high channel form roughness facilitate sediment deposition in the unmanaged lower part of the reach it functions as a sediment sink and may reduce bed material delivery to the dam reservoir

effective discharge for suspended sediment transport of the ganga river and its geomorphic implication abstract effective discharge q e for suspended sediment transport in the alluvial reaches of the ganga river in the western ganga plains wgp has been computed using analytical and an alternative magnitude frequency approach thirty years of mean monthly discharge data from various sites of the ganga river have been assessed and the abundance of discharge occurrence has been determined our analysis shows that less than 40 of the flow causes effective sediment transport in the ganga and this can be considered as the effective discharge for suspended sediment transport alternatively 50 of the sediment load for all studied sites is moved by a discharge varying between 14 and 40 of the total discharge effective discharges calculated over the period of record are well below the bankfull discharges q b a few events are close to the bankfull level but with a high return period ri 40 years and therefore not effective to transport most of the available sediments our computation shows that the mean annual discharge ri 2 33 yrs can transport only 0 to 10 of the total available sediments the computation of effective discharge also provided important insights to understand the linkage between hydrology and channel morphology sediment storage and removal processes which are reflected in sediment budget cause changes in cross sectional area channel bathymetry at various sites but the channel margins are not affected a high ratio of bankfull to effective discharge q b q e forces the flow lines to be concentrated to the thalweg position and channels are incised our study also implies that incision and aggradation of the river valley during a relatively long period are caused by changes in effective discharge we argue that the valley incision and filling episodes in the western ganga plains at late quaternary timescales in response to monsoonal fluctuations were primarily affected by changes in the effective discharges of the rivers

fossil and recent debris flows in medium high mountains moravskoslezske beskydy mts czech republic abstract the significance of debris flows for the morphogenesis of the medium high mountains of central europe has long been neglected herein we provide evidence from the moravskoslezske beskydy mountains flysch western carpathians czech republic where fossil pleistocene debris flow events in spite of their low recent activity were important erosion and accumulation agents in shaping the mountainous terrain geomorphological and sedimentological analyses helped to differentiate between two classes of debris flow deposits deposits of the 1st class are relatively voluminous contemporary terraced landforms which originated in the pleistocene transport limited conditions recent activity of debris flows presented by the 2nd class accumulations is characterized by the presence of low magnitude events mostly originated during the reworking of older diamictons the inheritance of large volumes of old debris flow diamictons in steep mountainous terrain is the evidence of slow erosional transformation of the last glacial landforms pointing to the fact that recent topography of the mountains is not in full equilibrium with contemporary climatic and environmental conditions