channel response to extreme floods insights on controlling factors from six mountain rivers in northern apennines italy abstract this work addresses the geomorphic response of mountain rivers to extreme floods exploring the relationships between morphological changes and controlling factors the research was conducted on six tributaries of the magra river northern apennines italy whose catchments were affected by an extreme flood estimated recurrence interval 100 years in most of the basins on 25 october 2011 an integrated approach was deployed to study this flood including i analysis of channel width changes by comparing aerial photographs taken before and after the flood ii estimate of peak discharges in ungauged streams iii detailed mapping of landslides and analysis of their connectivity with the channel network channel widening occurred in 35 reaches out of 39 in reaches with channel slope results indicate that hydraulic variables alone are not sufficient to satisfactorily explain the channel response to extreme floods and inclusion of other factors such as lateral confinement is needed to increase explanatory capability of regression models concerning hydraulic variables this study showed that the degree of channel widening is more strongly related to unit stream power calculated based on pre flood channel width than to cross sectional stream power and to unit stream power calculated with post flood channel width this could suggest that most width changes occurred after the flood peak finally in terms of hazard it is crucial to document the type and magnitude of channel changes to identify controlling factors and most importantly to develop tools enabling us to predict where major geomorphic changes occur during an extreme flood

evaluating channel response to an extreme sedimentation event in the context of historical range of variability upper colorado river usa in may 2003 a breach in a large irrigation ditch within rocky mountain national park rmnp initiated a debris flow that entered lulu creek and the colorado river where 36 000 m3 of sediment substantially altered channel forms and processes we present a proof of concept to understand whether the 2003 disturbance is within the historical range of variability hrv and whether the recovery potential of the system is sufficient to adapt to the disturbance flow and sediment regimes and channel morphology and stability were monitored on lulu creek and the colorado river from 2004 to 2011 dominant channel response following the debris flow within lulu creek included step development bed armoring and channel widening step height to length ratios h l for three reaches on lulu creek are outside the hrv of reference channels with one reach approaching reference conditions erosion of approximately 23 of the debris fan volume occurred as a result of the long duration 2011 peak flow sediment within the lulu creek fan will persist for 30 190 years assuming current maximum and mean removal rates planform changes on the colorado river since the debris flow include an increase in single thread geometries with braided reaches where bar deposition occurred bedload transport and grain size analysis of bedload indicate translational spreading of a sand wave front with a dispersive component in steeper reaches lulu creek is returning to a condition of natural variability but the colorado river is outside the hrv expected for steep gradient pool riffle channels applying hrv to a situation where management questions require a longer term perspective and pre disturbance baseline data are limited is a useful approach the hrv analysis facilitates a better understanding of site variability and delineates the range of possibilities of channel form and process to achieve management goals copyright 2012 john wiley sons ltd

channel hydraulics and geomorphic effects of an extreme flood event on the sabie river south africa the effects of the february 2000 extreme flood 6000 7000 m3 s 1 200 year return period flow on channel morphology are examined over 108 km of the semiarid bedrock influenced sabie river south africa the river is incised into bedrock forming a macrochannel with a planform that is fixed over the timescale of change being investigated within the confines of the macrochannel the river may be zoned into distinct channel types pool rapid bedrock anastomosing etc between 1 and 20 km long each characterised by a distinctive assemblage of smaller morphologic units ranging from large lateral bars filling most of the macrochannel to metre scale lee bars and sand sheets changes in morphologic unit composition and spatial coverage are presented for each channel type along with reconstructed flood hydraulics this appraisal is based on 1 10 000 scale aerial photographs taken 5 months before and 7 months after the event the latter verified by post flood field observations some channel types e g cohesive mixed anastomosing displayed widespread sedimentary stripping while others e g bedrock anastomosing remained essentially unchanged at the morphologic unit scale large cohesive lateral deposits were largely unaffected by the flood whereas smaller active channel unconsolidated and consolidated features associated with perennially flowing sub channels were stripped at the scale of channel type change appears to be controlled by a complex combination of factors including i spatial and temporal flow energy level change affecting sediment transport and ii tributary location susceptibility to erosion at the scale of the morphologic unit appears to be linked to elevation above the low flow active channel with features at lower elevations in the channel being stripped field observations suggest that unconsolidated active channel units are re forming at stripped locations

geomorphological impacts of a flood event on ephemeral channels in se spain a flood event occurred on 30 september 1997 in three catchments torrealvilla salada and nogalte in se spain which were being monitored for morphological change peak discharges were high in the torrealvilla catchment moderate in the salada and low in the nogalte detailed topographic surveys were made before and after the flood and from these dems of difference were constructed the two sites with highest flow both underwent significant incision of the channel at other sites both scour and aggradation of the channel took place with scour holes of up to 1 m depth eroded elsewhere within the channels major erosion took place downstream of tracks and structures such as check dams that impeded sediment movement the pattern and amounts of change show little relationship to independent factors other than a scale effect of peak discharge behaviour of channels is closely related to local sediment supply sediment flux calculated from changes in volumes at the site is comparable with the other figures produced for ephemeral channels and much higher than for perennially flowing streams

stream power framework for predicting geomorphic change the 2013 colorado front range flood abstract the colorado front range flood of september 2013 induced a diverse range of geomorphic changes along numerous stream corridors providing an opportunity to assess responses to a large flood in a semiarid landscape we defined six classes of geomorphic change related to peak unit stream power and valley confinement for 531 stream reaches over 226 km spanning a gradient of channel scales and slope geomorphic change was generally driven by erosion of channel margins in confined reaches and by a combination of deposition and erosion in unconfined reaches the magnitude of geomorphic change typically increased with unit stream power with greater responses observed in unconfined channels cumulative logit modeling indicated that total stream power or unit stream power unit stream power gradient and valley confinement are significant predictors of geomorphic response for this flood event based on this dataset thresholds for geomorphic adjustment were defined for channel slopes 230 w m 2 16 lb ft s at least 10 of the investigated sites experienced substantial channel widening and a credible potential for avulsions braiding and loss of adjacent road embankments associated with 480 w m 2 33 lb ft s at least 10 of the investigated sites experienced such geomorphic change infrequent to numerous eroded banks were very likely with 700 w m 2 48 lb ft s with substantial channel widening or major geomorphic change shifting from credible to likely importantly in reaches where there were large reductions in as the valley form shifted from confined to relatively unconfined large amounts of deposition induced reach scale geomorphic change occurred in some locations at relatively low additionally alluvial channels with slopes 3 had greater resistance to geomorphic change likely caused by armoring by larger bed material and increased flow resistance from enhanced bedforms finally we describe how these results can potentially be used by practitioners for assessing the risk of geomorphic change when evaluating current or planned conditions

morphodynamics of a pseudomeandering gravel bar reach abstract a large number of rivers in tuscany have channel planforms which are neither straight nor what is usually understood as meandering in the typical case they consist of an almost straight slightly incised main channel fringed with large lateral bars and lunate shaped embayments eroded into the former flood plain in the past these rivers have not been recognised as an individual category and have often been considered to be either braided or meandering it is suggested here that this type of river planform be termed pseudomeandering a typical pseudomeandering river the cecina river is described and analysed to investigate the main factors responsible for producing this channel pattern a study reach 100 300 m was surveyed in detail and related to data on discharge channel changes after floods and grain size distribution of bed sediments during 18 months of topographic monitoring the inner lateral bar in the study reach expanded and migrated towards the concave outer bank which concurrently retreated by as much as 25 m a sediment balance was constructed to analyse bar growth and bank retreat in relation to sediment supply and channel morphology the conditions necessary to maintain the pseudomeandering morphology of these rivers by preventing them from developing a meandering planform are discussed and interpreted as a combination of a few main factors such as the flashy character of floods sediment supply influenced by both natural processes and human impact the morphological effects of discharges with contrasting return intervals and the short duration of flood events finally the channel response to floods with variable sediment transport capacity represented by bed shear stress is analysed using a simple model it is demonstrated that bend migration is associated with moderate floods while major floods are responsible for the development of chute channels which act to suppress bend growth and maintain the low sinuosity configuration of the river

geomorphic effects flood power and channel competence of a catastrophic flood in confined and unconfined reaches of the upper lockyer valley southeast queensland australia flooding is a persistent natural hazard and even modest changes in future climate are believed to lead to large increases in flood magnitude previous studies of extreme floods have reported a range of geomorphic responses from negligible change to catastrophic channel change this paper provides an assessment of the geomorphic effects of a rare high magnitude event that occurred in the lockyer valley southeast queensland in january 2011 the average return interval of the resulting flood was similar to 2000 years in the upper catchment and decreased to similar to 30 years downstream a multitemporal lidar derived dem of difference dod is used to quantify morphological change in two study reaches with contrasting valley settings confined and unconfined differences in geomorphic response between reaches are examined in the context of changes in flood power channel competence and degree of valley confinement using a combination of one dimensional 1 d and two dimensional 2 d hydraulic modelling flood power peaked at 9800 w m 2 along the confined reach and was 2 3 times lower along the unconfined reach results from the dod confirm that the confined reach was net erosional exporting similar to 287 000 m 3 of sediment whilst the unconfined reach was net depositional gaining similar to 209 000 m 3 of sediment 70 of the amount exported from the upstream confined reach the major sources of eroded sediment in the confined reach were within channel benches and macrochannel banks resulting in a significant increase of channel width in the unconfined reach the benches and floodplains were the major loci for deposition whilst the inner channel exhibited minor width increases the presence of high stream power values and resultant high erosion rates within the confined reach is a function of the higher energy gradient of the steeper channel that is associated with knickpoint development dramatic differences in geomorphic responses were observed between the two adjacent reaches of contrasting valley configuration the confined reach experienced large scale erosion and reorganisation of the channel morphology that resulted in significantly different areal representations of the five geomorphic features classified in this study c 2013 elsevier b v all rights reserved

quantifying different riverbank erosion processes during an extreme flood event riverbank erosion is a major contributor to catchment sediment budgets at large spatial scales data is often restricted to planform channel change with little information on process distributions and their sediment contribution this study demonstrates how multi temporal lidar and high resolution aerial imagery can be used to determine processes and volumes of riverbank erosion at a catchment scale remotely sensed data captured before and after an extreme flood event enabled a digital elevation model of difference dod to be constructed for the channel and floodplain this meant that the spatial area that could be assessed was extensive three dimensional forms of bank failures could be mapped at a resolution that enabled process inference and the volume and rates of different bank erosion processes over time could be assessed a classification of riverbank mass failures integrating form and process identified a total of 437 mass failure polygons throughout the study area these were interpreted as wet flow mass failures based on the presence of a well defined scarp wall and the absence of failed blocks on the failure floor the failures appeared to be the result of bank exfiltration antecedent moisture conditions preceding the event and the historic development of the channel using one dimensional hydraulic modelling to delineate geomorphic features within the main boundary of the macrochannel an estimated 1 466 322m2 of erosion was interpreted as fluvial entrainment occurring across catchment areas from 30 to 1668 km2 only 8 of the whole riverbank planform area was occupied by mass failures whilst fluvial entrainment covered 33 a third of the volume of material eroded came from mass failures even though they occupied 19 of the eroded bank area the availability of repeat lidar surveys combined with high resolution aerial photography was very effective in erosion process determination and quantification at a large spatial scale

how natural are alpine mountain rivers evidence from the italian alps historical human induced channel adjustments in lowland gravel bed rivers have been documented in several geographical contexts worldwide in particular it is now widely accepted that the vast majority of european rivers are far from any natural reference state prior to anthropic disturbances and a complete restoration is hardly achievable however few investigations have addressed changes that have occurred in mountain rivers of the alps and these channels are commonly reckoned quite natural by society this paper intends to describe how human pressure on italian alpine basins has been quite relevant for several centuries in terms of land use variations in channel structures timber transport splash damming and riparian vegetation management such that nowadays reference conditions cannot be found even in small mountain creeks in addition recent natural climatic variations e g the little ice age are superimposed on human disturbances thus defying the definition of any equilibrium morphological conditions even under human free states a summary of published as well as unpublished works on historical channel adjustments in rivers of the italian alps is presented in order to document the impacts deriving from human pressure at different basin scales and for different river morphologies from steep confined streams to large unconfined rivers general options for river management and restoration actions aiming to combine geomorphological functionality and flood hazard mitigation are discussed in the light of the current european legislative context copyright 2011 john wiley sons ltd

geoinformatics in morphological study of river paglia tiber river basin central italy this work reports the results of a geomorphological study on the river paglia tiber basin central italy to analyse the historical evolution of the riverbed floodplain system in its lower valley reaches upstream the confluence of river tiber a morphological sedimentological approach has been used starting with an historical analysis and a survey of the forms and sediments in the riverbed alluvial plain system in order to reconstruct the current evolutionary trend and to identify the causes and processes leading to changes in natural trends this approach is not in contrast to the hydrologic hydraulic one normally applied in engineering fields but it is integrated with it in a complementary and parallel manner in order to achieve the complete knowledge of the river system result of an investigation multidisciplinary aimed to define its physical state historical data cartographic documents digital terrain models dtm topographic surveys aerial photographs etc indicate clear cut narrowing and deepening of the active channel manifested after the second world war and due to the considerable changes which affected the entire fluvial system from the recovery of land for agriculture near the river to sediments taken from its bed over the past few decades the result is a state of great disequilibrium which is shown in the passage from a braided morphology observed in the 1950s to a single channel with low sinuosity wandering type this type of morphology is certainly more unstable in fact the narrowing and deepening of fluvial sections provide a better hydraulic efficiency for the upstream so the peak discharge does not overflow such peak discharge arrives downstream next to the confluence with the tiber river not laminated causing serious flooding effects an increased vulnerability is observed in the last reach in the areas pertaining to the river where most of the industrial economic activities are concentrated and where transportation infrastructures of nationwide importance italy s most important motorway the a1 and the rome florence milan high speed rail links are located the evolution of the paglia which is not substantially different from that of many streams in italy and in general throughout europe is affected by severe anthropic constraints and intense exploitation of resources which have produced a state of disequilibrium approaching irreversibility in an already vulnerable system finally some suggestions for the correct management of the river system are pointed out with the aim of mitigating the flood risk in the lower paglia valley

uas based remote sensing of fluvial change following an extreme flood event the effects of large floods on river morphology are variable and poorly understood in this study we apply multi temporal datasets collected with small unmanned aircraft systems uass to analyze three dimensional morphodynamic changes associated with an extreme flood event that occurred from 19 to 23 june 2013 on the elbow river alberta we documented reach scale spatial patterns of erosion and deposition using high resolution 4 5 cm pixel orthoimagery and digital elevation models dems produced from photogrammetry significant bank erosion and channel widening occurred with an average elevation change of 0 24 m the channel pattern was reorganized and overall elevation variation increased as the channel adjusted to full mobilization of most of the bed surface sediments to test the extent to which geomorphic changes can be predicted from initial conditions we compared shear stresses from a two dimensional hydrodynamic model of peak discharge to critical shear stresses for bed surface sediment sizes we found no relation between modeled normalized shear stresses and patterns of scour and fill confirming the complex nature of sediment mobilization and flux in high magnitude events however comparing modeled peak flows through the pre and post flood topography showed that the flood resulted in an adjustment that contributes to overall stability with lower percentages of bed area below thresholds for full mobility in the post flood geomorphic configuration overall this work highlights the potential of uas based remote sensing for measuring three dimensional changes in fluvial settings and provides a detailed analysis of potential relationships between flood forces and geomorphic change copyright 2015 john wiley sons ltd

geomorphological impacts of an extreme flood in se spain abstract long term field studies in semiarid ephemeral streams are rare these geomorphic data are essential for understanding the nature of the processes in order to develop modelling for risk assessments and management an extreme flood event on 28 september 2012 affected the murcia region of se spain including long tem monitoring sites on two fluvial systems in the guadalentin basin the nogalte and torrealvilla detailed morphological data were collected before and immediately after the event and the amount of morphological change erosion and deposition have been related to peak flow conditions at the sites on the nogalte channel peak flow reached 2500 m3 s 1 at the downstream end of the catchment in less than 1 h the event had a recurrence interval of 50 years based on rainfall records and damage to old irrigation structures the major effect in the braided gravel channel of the nogalte was net aggradation with massive deposition in large flat bars the measured changes in bankfull capacity were highly correlated with most hydraulic variables net changes in cut and fill in cross sections on the nogalte were highly related to peak discharge and stream power but much less so to measures of hydraulic force velocity shear stress unit stream power relationships of amount of erosion to hydraulic variables were much weaker than for amount of deposition which was largely scaled to channel size and flow energy changes on the torrealvilla were much less than on the nogalte and net erosion occurred at all sites sites on the nogalte channel in schist exhibited higher deposition than those of the torrealvilla sites on marl for the same hydraulic values overall less morphological change took place in the extreme event on the nogalte than predicted from some published hydraulic relations probably reflecting the high sediment supply and the hydrological characteristics of the event the results demonstrate the high degree of adjustment of these channels to the occasional high magnitude flash flood events and that such events need to be allowed for in management the detailed quantitative evidence produced by these long term monitoring sites provide valuable rare data for modelling morphological response to flood events in ephemeral channels

channel patterns braided anabranching and single thread a new channel pattern classification is presented based on theoretically derived channel pattern discriminant functions the thresholds are formulated as power laws that relate the critical slope associated with a change in channel pattern to dimensionless discharge and relative bank strength one threshold demarcates the boundary between stable single thread channels both straight and meandering and stable multiple thread channels anabranching another threshold separates anabranching from braided channels where braided channels are considered to be fundamentally unstable the exponents of the thresholds are nearly identical to that in the threshold equation originally proposed by leopold and wolman 1957 and the coefficients are quite similar as well an analysis of their data set using our dimensionless approach reveals that no fundamental difference exists between meandering and straight patterns and thus data from both types are grouped together under the more general heading of single thread channels furthermore we demonstrate that over a limited range of conditions an unstable single thread channel can form stable multiple threads but that for systems far from the threshold bounding the single thread channels the number of divisions required to produce stable anabranches grows geometrically this motivates a separation of multiple thread channels into anabranching and braided types our theoretical thresholds are then compared against several large data sets of field data and the results broadly confirm our proposed thresholds

spatial disaggregation and aggregation procedures for characterizing fluvial features at the network scale application to the rhone basin france network scale heterogeneity in fluvial forms and processes has been analyzed for decades through the field based characterization of a reduced number of sites spatially continuous data sets are increasingly available at a national scale e g digital elevation models or aerial orthophotographs and open up new possibilities to investigate spatial structures in stream networks the paper introduces a gis methodological framework to support and automate the large scale characterization of the fluvial system based on spatial disaggregation and aggregation procedures the spatial disaggregation consists in the longitudinal discretisation of geographical objects into relatively high resolution spatial units to characterize fluvial features continuously along stream networks because rivers are heterogeneous and hierarchically organized the spatial aggregation aims to delineate statistically pattern or process based spatial units to answer questions at appropriate scales the paper develops the gis implementation of the disaggregation and aggregation procedures for characterizing stream networks in a geomorphic perspective we illustrate the approach on the rhone network focusing on the characterization of three geographical objects commonly used in classification schemes the streamline the valley bottom and the active channel the latter on a limited area the spatial aggregation is automated using the threshold test of pettitt 1979 to detect change points in univariate data series and delineate homogeneous spatial units throughout the stream network we provide three simple application examples to illustrate the potentialities of the spatial aggregation to investigate spatial heterogeneity in stream networks finally insights limitations and future challenging issues emerging from the application of the methodological framework are discussed

recent channel adjustments in alluvial rivers of tuscany central italy drastic channel adjustments have affected the main alluvial rivers of tuscany central italy during the 20th century bed level adjustments were identified both by comparing available topographic longitudinal profiles of different years and through field observations changes in channel width were investigated by comparing available aerial photographs 1954 and 1993 98 bed incision represents the dominant type of vertical adjustment and is generalized along all the fluvial systems investigated the arno river system is the most affected by bed level lowering up to 9 m whereas lower incision generally less than 2 m is observed along the rivers of the southern part of the region human disturbances appear to be the dominant factors of adjustments the main phase of vertical change occurred during the period 1945 80 in concomitance with the phase of maximum sediment mining activity at the regional scale the second dominant type of adjustment that involved most of the rivers in the region consists of a narrowing of the active channel based on measurements of channel width conducted on aerial photographs 38 of the reaches analysed experienced a narrowing greater than 50 of the initial channel width the largest values of channel narrowing were observed along initially braided or sinuous with alternate bars morphologies in the southern portion of the region a regional scheme of channel adjustments is derived based on initial channel morphology and on the amounts of incision and narrowing different styles of channel adjustments are described rivers that were originally sinuous with alternate bars to braided generally became adjusted by a moderate incision and a moderate to intense narrowing in contrast sinuous meandering channels mainly adjusted vertically with a minor amount of narrowing copyright 2003 john wiley sons ltd

response of the ha ha river to the flood of july 1996 in the saguenay region of quebec large scale avulsion in a glaciated valley modifications to valley form due to extreme flooding in the lower 34 km of the ha ha river were analyzed heavy regional precipitation in july 1996 triggered extreme runoff and the catastrophic drainage of ha ha lake producing discharges of 1100 m3 s 8 times the 100 year flood dominant valley modifications revealed by comparing preflood and postflood topographies derived photogrammetrically were related to two large scale avulsions in particular a deep retrogressive incision which bypassed the 30 m high perron falls exporting 6 106 m3 of glacial stratified drift from a 2 km section of valley and producing massive sedimentation in the reach downstream reconstructed maximum flow power values support the existence of a 300 w m2 threshold for major scouring of the alluvial valley bottom the evidence highlights the potential for massive scour and fill and reorganization of the long profile with potentially catastrophic effects on infrastructure during extreme floods in glaciated valley settings

a rational sediment transport scaling relation based on dimensionless stream power the concept of stream channel grade according to which a stream channel reach will adjust its gradient s in order to transport the imposed sediment load having magnitude qb and characteristic grain size db with the available discharge q mackin 1948 geological society of america bulletin59 463 512 lane 1955 american society of civil engineers proceedings81 1 17 is one of the most influential ideas in fluvial geomorphology herein we derive a scaling relation that describes how externally imposed changes in either qb or q can be accommodated by changes in the channel configuration described by the energy gradient mean flow depth characteristic grain size and a parameter describing the effect of bed surface structures on grain entrainment one version of this scaling relation is based on the dimensionless bed material transport parameter w presented by parker and klingeman 1982 water resources research18 1409 1423 an equivalent version is based on a new dimensionless transport parameter e using dimensionless unit stream power this version is nearly identical to the relation based on w except that it is independent of flow resistance both versions of the scaling relation are directly comparable to lane s original relation in order to generate this stream power based scaling relation we derived an empirical transport function relation relating e to dimensionless stream power using data from a wide range of stable bed load dominated channels the form of that transport function is based on the understanding that while grain entrainment is related to the forces acting on the bed described by dimensionless shear stress sediment transport rate is related to the transfer of momentum from the fluid to the bed material described by dimensionless stream power copyright 2010 john wiley sons ltd

medium and short term channel planform changes on the rivers tay and tummel scotland abstract channel planform change was analysed using a variety of data sources for the medium term 25 years and

flow variability and hillslope hydrology examination of spatial variability of streamflow in headwater areas can provide important insight about factors that influence hillslope hydrology detailed observations of variations in stream channel input based on a tracer experiment indicate that topography alone cannot explain flow variability however determination of changes in channel input on a small spatial scale can provide valuable clues to factors such as structural geology that control subsurface flows

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

causes of 20th century channel narrowing in mountain and piedmont rivers of southeastern france extensive channel narrowing in southeastern france provides an illustration of geomorphic response to land use changes the study region comprises a range of environments from large piedmont and intramountain gravel bed rivers to small mountain streams field measurements and analysis of historical data demonstrate two distinct periods of channel change from 1850 to 1950 channel narrowing is interpreted to be the result of a recovery process in response to widespread channel destabilization induced by major floods during the second half of the 19th century at the time the largely deforested basins were highly responsive to flooding whereas the recovery process was accelerated by floodplain and basin scale land use changes afforestation and torrential control works which in turn reduced sediment delivery and enabled vegetation development in channels from 1950 to 1970 channel narrowing accelerated in most of the studied rivers this recent phenomenon is considered as a human induced fluvial adjustment directly related to forest development on river margins and human abandonment of intensive floodplain land uses at the same time long profile degradation occurred as a result of long term bedload supply decrease in small mountain streams channel narrowing is mainly explained by channel incision which seems to progress from upstream where sediment sources are progressively stabilized by afforestation copyright 2002 john wiley sons ltd

reach scale channel sensitivity to multiple human activities and natural events lower santa clara river california usa abstract understanding the cumulative impact of natural and human influences on the sensitivity of channel morphodynamics a relative measure between the drivers for change and the magnitude of channel response requires an approach that accommodates spatial and temporal variability in the suite of primary stressors multiple historical data sources were assembled to provide a reach scale analysis of the lower santa clara river lscr in ventura county california usa sediment supply is naturally high due to tectonic activity earthquake generated landslides wildfires and high magnitude flow events during el nino years somewhat typically for the region the catchment has been subject to four reasonably distinct land use and resource management combinations since european american settlement when combined with analysis of channel morphological response quantifiable since ca 1930 reach scale and temporal differences in channel sensitivity become apparent downstream reaches have incised on average 2 4 m and become narrower by almost 50 with changes focused in a period of highly sensitive response after about 1950 followed by forced insensitivity caused by structural flood embankments and a significant grade control structure in contrast the middle reaches have been responsive but are morphologically resilient and the upstream reaches show a mildly sensitive aggradational trend superimposing the natural and human drivers for change reveals that large scale stressors related to ranching and irrigation have been replaced over time by a suite of stressors operating at multiple spatial scales lower reaches have been sensitive primarily to local scale impacts urban growth flood control and aggregate mining whereas upstream catchment scale influences still prevail including flow regulation and climate driven sediment supply factors these factors illustrate the complexity inherent to cumulative impact assessment in fluvial systems provide evidence for a distinct anthropocene fluvial response and underpin the enormity of the challenge faced in trying to sustainably manage and restore rivers

episodes of vertical accretion and catastrophic stripping a model of disequilibrium flood plain development this paper describes flood plains formed episodically by vertical accretion along high energy laterally stable channels in southeastern australia overbank deposition gradually builds a flood plain of fine textured alluvium over a period of hundreds or thousands of years following which catastrophic erosion by a single large flood or a series of more moderate floods strips the flood plain to a basal lag deposit from which it slowly reforms this periodic destruction appears due to the progressive development of large levee banks and flood plain surfaces of highly variable relief as the levees and flood plain grow overbank flow is gradually displaced from the broad flood plain into the main channel and flood plain backchannels with a resulting concentration of erosional energy eventually high flows greatly exceed erosional thresholds and wholesale scour of the channel boundary and flood plain occurs vertical accretion flood plains at different stages of development result in a wide range of bankfull recurrence intervals even along the same river some of these flood plains are so infrequently flooded that they can be mistaken for terraces formed under a prior flow regime the almost random but catastrophic nature of this flood plain erosion means that sediment supply and transport are highly variable and probably impossible to predict this model of flood plain formation is seen as only part of a continuum of alluvial environments ranging from vestigial coarsegrained traction load flood plains along high energy rivers in narrow gorges to extensive low gradient flood plains in which alluvial stratigraphy is dominated by fine grained overbank deposition

controls on bedrock channel incision along nahal paran israel nahal paran drains 3600 km2 of egypt s sinai peninsula and israel s negev desert much of the channel is alluvial but a canyon 10 5 km long has been incised into late cretaceous chert and dolomite in the lower portion of the basin slackwater deposits and paleostage indicators preserved within the canyon record approximately 10 floods of 200 to 2500 m3 s 1 over a period of at least 350 years step backwater simulations of flood flow hydraulics indicate extreme variations in stream power per unit area along the length of the canyon and associated variability in energy expenditure and sediment transport these variations reflect channel cross sectional morphology the greatest values of stream power occur along the lower half of the study reach in association with three pronounced knickpoints and an inner channel the locations of these features reflect the exposure of thick resistant chert layers along the channel the presence of several similar but buried and inactive knickpoints along the upper study reach indicates that the locus of most active channel incision has shifted with time probably in response to baselevel changes associated with tectonic activity along the dead sea rift thus the rate and manner of channel incision along the canyon of nahal paran are controlled by lithologic variability and tectonic uplift as they influence channel morphology and gradient which in turn influence hydraulics and sediment transport

spatial dynamics of overbank sedimentation in floodplain systems abstract floodplains provide valuable social and ecological functions and understanding the rates and patterns of overbank sedimentation is critical for river basin management and rehabilitation channelization of alluvial systems throughout the world has altered hydrological and sedimentation processes within floodplain ecosystems in the loess belt region of the lower mississippi alluvial valley of the united states channelization the geology of the region and past land use practices have resulted in the formation of dozens of valley plugs in stream channels and the formation of shoals at the confluence of stream systems valley plugs completely block stream channels with sediment and debris and can result in greater deposition rates on floodplain surfaces presently however information is lacking on the rates and variability of overbank sedimentation associated with valley plugs and shoals we quantified deposition rates and textures in floodplains along channelized streams that contained valley plugs and shoals in addition to floodplains occurring along an unchannelized stream to improve our understanding of overbank sedimentation associated with channelized streams feldspar clay marker horizons and marker poles were used to measure floodplain deposition from 2002 to 2005 and data were analyzed with geospatial statistics to determine the spatial dynamics of sedimentation within the floodplains mean sediment deposition rates ranged from 0 09 to 0 67 cm y at unchannelized sites 0 16 to 2 27 cm y at shoal sites and 3 44 to 6 20 cm y at valley plug sites valley plug sites had greater rates of deposition and the deposited sediments contained more coarse sand material than either shoal or unchannelized sites a total of 59 of 183 valley plug study plots had mean deposition rates 5 cm y the geospatial analyses showed that the spatial dynamics of sedimentation can be influenced by the formation of valley plugs and shoals on channelized streams however responses can vary restoration efforts in the region need to have basinwide collaboration with landowners and address catchment scale processes including the geomorphic instability of the region to be successful

large wood recruitment and transport during large floods a review large wood lw elements transported during large floods are long known to have the capacity to induce dangerous obstructions along the channel network mostly at bridges and at hydraulic structures such as weirs however our current knowledge of wood transport dynamics during high magnitude flood events is still very scarce mostly because these are locally rare and thus unlikely to be directly monitored therefore post event surveys are invaluable ways to get insights although indirectly on lw recruitment processes transport distance and factors inducing lw deposition all aspects that are crucial for the proper management of river basins related to flood hazard mitigation this paper presents a review of the quite limited literature available on lw transport during large floods drawing extensively on the authors own experience in mountain and piedmont rivers published and unpublished the overall picture emerging from these studies points to a high catchment specific variability in all the different processes affecting lw dynamics during floods specifically in the lw recruitment phase the relative floodplain bank erosion vs hillslope landslide and debris flows contribution in mountain rivers varies substantially as it relates to the extent of channel widening which depends on many variables itself but also to the hillslope channel connectivity of lw mobilized on the slopes as to the lw transport phase within the channel network it appears to be widely characterized by supply limited conditions whereby lw transport rates and thus volumes are ultimately constrained by the amount of lw that is made available to the flow indeed lw deposition during floods was mostly in terms of volume observed at artificial structures bridges in all the documented events this implies that the estimation of lw recruitment and the assessment of clogging probabilities for each structure for a flood event of given magnitude are the most important aspects for the prediction of lw transport magnitude at any cross section along the river network finally the review discusses the optimal strategies to manage lw related hazard which should consider riparian vegetation and in channel dead wood as key components of river ecosystems and thus should interfere with lw as well as with sediment transport dynamics only for limited spatial and temporal scales

effectiveness of exceptional rainstorms on a small mediterranean basin abstract a comprehensive investigation of rainstorms and their consequent impacts on landscape evolution is geomorphologically important but only scant information may be available on exceptional events because parameters on synoptic conditions rainstorm landforms and hydrology for such events may be incomparable with previous knowledge we studied an exceptional storm on april 2 2006 in the ramot menashe region israel our investigation of rainfall landslides debris flows and channel suggests the effectiveness of such an event on the development of basin scale morphology the storm caused damage and casualties although it covered relatively narrow strips neither direct rainfall nor runoff measurements exist for the most severely affected area of ramot menashe but the geomorphologic evidence combined with high resolution meteorological radar data provides the basic understanding of the processes and hazardous conditions which prevailed at the time in the storm core based on estimation from meteorological radar data 263 mm of rain fell within 3 h with a maximum intensity of 220 mm h 1 for 10 min triggering both sporadic landslides at the soil bedrock contact on the upper slopes and widespread landslides at the fractured massive bedrock contact on the lower slopes the 1st order channels on the alternation of chalk and marl also underwent erosion and the produced sediment deposited on alluvial fans at the confluence with the main channel the specific peak discharges for catchment size of 0 3 10 km2 were 11 to 73 m3 s 1 km 2 higher than any recorded floods in the mediterranean climatic region of israel the effectiveness of the flood for geomorphic work represented by shear stress and stream power per unit boundary area reached 87 398 n m 2 and 212 2134 w m 2 respectively this kind of analysis can be applied to hazard prediction in other areas under similar geomorphological conditions

spatio temporal patterns of channel changes in response to a major flood event the case of the magra river central northern italy on october 25th 2011 the magra river central northern italy was affected by a significant flood event with an estimated return interval ranging from 30 to 200 years this study investigates the morphological responses of the magra river to this major flood event with the aims of i documenting the channel changes which occurred in response to the flood ii analysing this response within the context of the historical trend of width changes iii investigating the possible factors controlling the spatial pattern and the intensity of the observed changes significant channel widening was documented along the entire length studied about 34 km ranging from 3 to 90 of the pre flood channel width braided reaches were characterized by the most severe planimetric changes although characterized by high rates of change these variations are however amply included in the historical range of channel width variability over approximately the last 150 years from 1877 to present day and are comparable with though generally lower than the channel width of the 1950s statistical analyses simple and multiple regressions between the observed changes in channel width and a series of selected morphological and hydraulic controlling factors showed no robust correlations to explain the spatial pattern and the variability of channel adjustments instead major adjustments occurred along reaches characterized by the confluence of those streams draining the sub catchments where the storm was concentrated copyright 2014 john wiley sons ltd

thresholds and the spatial variability of flood power during extreme floods abstract analysis of the effects of extreme events on overall landscape form has shifted geomorphic attention away from a focus on the magnitude and frequency relationship of discharge to a focus on flood power this paper analyzes the variation in flood power both at a station and downstream in a basin of differing lithologies and discusses the geologic and geomorphic controls on flood power design floods ranging from the 2 year discharge tothe 500 year discharge are routed trough a drainage basin and channel boundary shear stress and unit stream power are calculated using hec 2 along 21 cross valley transects because of the strong lithologic control on valley width and channel slope an irregular downstream pattern of flood power emerges which reflects primarily the pattern in valley width narrow valleys correspond to a greater percent of channel conveyance during large magnitude floods and increasing rates of depth increases thus maximizing flood power in these zones the opposite occurs in anomalously wide valleys where flood power is minimized a downstream spatial pattern emerges reflecting both local and general controls for a given probability event flood power varies at least threefold within the basin with a maximum occuring in a specific downstream section holding flood power constant produces an extremely wide range in flow probabilities in order to establish a link between critical flood power magnitude and flow frequency minimum flood powers are developed from an analysis of previous work on catastrophic flooding for humid alluvial channels these minimum thresholds tentatively correspond to 100 n m2 or 300 w m2 for shear stress and unit stream power respectively discharges required to attain these critical flood powers vary from events roughly twice the 100 year flood to a maximum of roughly 18 times the discharge of the 100 year flood

recent large magnitude floods and their impact on valley floor environments of northeastern yellowstone abstract the lamar river watershed of northeastern yellowstone contains some of the most diverse and important habitat in the national park broad glacial valley floors feature grassland winter range for ungulates riparian vegetation that provides food and cover for a variety of species and alluvial channels that are requisite habitat for native fish rapid neogene uplift and quaternary climatic change have created a dynamic modern environment in which catastrophic processes exert a major influence on riverine riparian ecosystems uplift and glacial erosion have generated high local relief and extensive cliffs of friable volcaniclastic bedrock as a result steep tributary basins produce voluminous runoff and sediment during intense precipitation and rapid snowmelt recent major floods on trunk streams deposited extensive overbank gravels that replaced loamy soils on flood plains and allowed conifers to colonize valley floor meadows tree ring dating identifies major floods in 1918 ca 1873 and possibly ca 1790 in 1996 and 1997 discharge during snowmelt runoff on soda butte creek approached the 100 year flood estimated by regional techniques with substantial local bank erosion and channel widening indirect estimates show that peak discharges in 1918 were approximately three times greater than in 1996 with similar duration and much greater flood plain impact nonetheless 1918 peak discharge reconstructions fall well within the range of maximum recorded discharges in relation to basin area in the upper yellowstone region the 1873 and 1918 floods produced lasting impacts on the channel form and flood plain of soda butte creek channels may still be locally enlarged from flood erosion and net downcutting has occurred in some reaches leaving the pre 1790 flood plain abandoned as a terrace gravelly overbank deposits raise flood plain surfaces above levels of frequent inundation and are well drained therefore flood plain soils are drier noncohesive gravels also reduce bank stability and may have persistent effects on channel form overall floods are part of a suite of catastrophic geomorphic processes that exert a very strong influence on landscape patterns and valley floor ecosystems in northeastern yellowstone

geomorphological impacts of a glacier lake outburst flood in the high arctic zackenberg river ne greenland abstract glacier lake outburst floods glofs especially those in the arctic can deliver exceptionally high volumes of sediment and solutes to fjords and shallow marine settings in comparison to typical seasonal river flows these sediments and solutes strongly affect coastal geomorphology and aquatic ecosystems yet are rarely observed in this study we have quantified the short term geomorphological response of the most distal part of the zackenberg river northeast greenland where it enters young sund to a glacier lake outburst flood that occurred on august 6th 2017 the main aims were to 1 quantify riverbank and floodplain geomorphology changes that occurred as a consequence of the flood 2 analyse the spatial patterns of those geomorphological changes and suggest the key controls on them we used a time series of very high resolution uav generated images taken on the 5th 6th and 8th of august 2017 which enabled us to compare pre and post flood fluvial geomorphology the glof induced intense and widespread geomorphological changes which was surprising because several floods of a similar magnitude have occurred along this river approximately 30 of the area of interest experienced changes that were larger than the minimum level of detection 0 15 m lateral erosion reached almost 10 m in some places the total volume loss from bank erosion was at least 26 561 m3 14 whereas the deposition was at least 7745 m3 39 such an intensive geomorphological response resulted from a combination of factors namely 1 bank geometry 2 composition of bank material 3 time of occurrence of the event 4 presence of permafrost 6 channel geometry and 7 multitude and diversity of geomorphological processes we speculate the severity of the geomorphological impact relative to that from previous floods could have been due to warming air temperatures that provided sediment from thawed permafrost and to an aggrading delta that raised the river base level overall we contend that climate warming will not only make outburst floods from glaciers more likely but that those floods will achieve more geomorphological work with mechanical erosion of permafrost erosion and gravitational failures during future flood events will perhaps become even more widespread and intense

variation in monsoonal rainfall sources arabian sea and bay of bengal during the late quaternary implications for regional vegetation and fluvial systems abstract indian summer monsoon ism rainfall is contributed by two moisture sources arabian sea as and bay of bengal bob while monsoonal rainfall in eastern and central india is dominantly contributed by the vapor derived from bob the source of rainfall in western india is mostly from as vapor meteorological conditions in these regions also differ in contrast to the bob component of monsoon in central india temporal variations in the as sourced rainfall from western india are poorly constrained due to lack of paleohydrological records towards this pedogenic carbonates were collected from two chronologically constrained cliff sections in the gujarat alluvial plain western india oxygen isotopic ratio of carbonate 18ocarbonate was used to reconstruct variation in as derived rainfall and its influence on vegetation and fluvial systems in western india for the last 75 ka a negative correlation between the 18o value of modern rainwater and its amount known as amount effect in the study site is used as a framework for interpreting variations in 18ocarbonate values the amount effect based monsoon reconstruction showed that two distinct dry phases at 75 ka to 60 ka and 30 ka to 10 ka were punctuated by a wet phase from 60 ka to 30 ka these hydroclimatic variations in western india during the late quaternary were responsible for the changes in regional fluvial architecture comparison with published records showed that fluctuations in the as sourced rainfall were more pronounced compared to bob component carbon isotopic ratio 13c of carbonate and co existing organic matter were used to reconstruct vegetation composition significant and strong positive correlation between 18ocarbonate and 13ccarbonate values indicated that rainfall amount controlled the relative abundance of c3 c4 plants in western india during the late quaternary period furthermore c3 plants were more abundant in western india for the last 75 ka relative to central india

a methodological framework for hydromorphological assessment analysis and monitoring idraim aimed at promoting integrated river management a methodological framework for hydromorphological assessment analysis and monitoring named idraim has been developed with the specific aim of supporting the management of river processes by integrating the objectives of ecological quality and flood risk mitigation the framework builds on existing and up to date geomorphological concepts and approaches and has been tested on several italian streams the framework includes the following four phases 1 catchment wide characterization of the fluvial system 2 evolutionary trajectory reconstruction and assessment of current river conditions 3 description of future trends of channel evolution and 4 identification of management options the framework provides specific consideration of the temporal context in terms of reconstructing the trajectory of past channel evolution as a basis for interpreting present river conditions and future trends a series of specific tools has been developed for the assessment of river conditions in terms of morphological quality and channel dynamics these include the morphological quality index mqi the morphological dynamics index mdi the event dynamics classification edc and the river morphodynamic corridors mc and emc the monitoring of morphological parameters and indicators alongside the assessment of future scenarios of channel evolution provides knowledge for the identification planning and prioritization of actions for enhancing morphological quality and risk mitigation

integrating hydraulic equivalent sections into a hydraulic geometry study abstract hydraulic geometry hg is an important geomorphic concept that has played an indispensable role in hydrological analyses physical studies of streams ecosystem and aquatic habitat studies and sedimentology research more than 60 years after leopold and maddock 1953 first introduced the concept of hg researchers have still not uncovered the physical principles underlying hg behavior one impediment is the complexity of the natural river cross section the current study presents a new way to simplify the cross section namely the hydraulic equivalent section which is generalized from the cross section in the gradually varied flow of an alluvial river gvfar and features hydrodynamic properties and bed building laws similar to those of the gvfar energy balance was used to derive the stage z discharge q relationship in the gvfar the gvfar in the songhua river and the yangtze river were selected as examples the data including measured discharge river width water stage water depth wet area and cross section were collected from the hydrological yearbooks of typical hydrological stations on the songhua river and the yangtze river from 1955 to 1987 the relationships between stage z discharge q and cross sectional area a stage z at various stations were analyzed and at a station hydraulic geometry ahg relationships were obtained in power law forms based on derived results and observational data analysis the z q and z a relationships of ahg were similar to rectangular weir flows thus the cross section of the gvfar was generalized as a compound rectangular hydraulic equivalent cross section as to bed building characteristics the bankfull discharge method and the stage discharge relation method were used to calculate the dominant variables of the alluvial river this hydraulic equivalent section has the same z q relation z a relation dominant discharge dominant river width and dominant water depth as the cross section in the gvfar with the hydraulic equivalent section the relationships between the discharge and river width river depth and width to depth ratio are easier to depict in dhg analysis replacement of the cross section in the gvfar with the hydraulic equivalent section is expected to promote further development of hg

response and recovery of the eel river california and its tributaries to floods in 1955 1964 and 1997 abstract northwestern california is prone to regional high magnitude winter rainstorms which repeatedly produce catastrophic floods in the basins of the northern coast ranges major floods on the eel river in 1955 and 1964 resulted in substantial geomorphic changes to the channel adjacent terraces and tributaries this study evaluated the changes and the effects of a moderate flood in 1997 through field observations and examination of aerial photographs that spanned from 1954 to 1996 the purpose was to document the nature and magnitude of geomorphic responses to these three floods and assess the rates and controls on the recovery of the eel river and its tributaries channel widening from extensive bank erosion was the dominant geomorphic change along the lower eel river during major floods as a result of the 1964 flood the largest amount of widening was 195 m and represented an 80 change in channel width channel narrowing characterized the periods after the 1955 and 1964 floods more than 30 years after the 1964 flood however the river had not returned to pre flood width which suggests that channel recovery required decades to complete a long recovery time is unusual given that the eel river is located in an area with a superhumid climate and has an exceptionally high sediment yield this long recovery time may reflect highly seasonal precipitation and runoff which are concentrated in 3 5 months each winter in contrast to the main stem of the eel river the dominant effects of floods on the tributaries of the eel river were rapid aggradation of channel bed and valley floor followed by immediate downcutting dendrogeomorphic data aerial photographs and field observations indicate that thick wedges of gravel derived largely from hillslope failures in upper reaches of the tributaries are deposited at and immediately upstream of the mouths of tributaries as the stage of the eel river exceeded that of the tributaries during major floods in the waning stages of the flood the tributaries cut through the gravel at a rate equal to the lowering of the eel and generated unpaired terraces and nickpoints the complete process of deposition and incision can occur within a few days of peak discharge although reworking of some sediment on the valley floor may continue for years after large floods channel morphology in the tributaries appears to be a product of infrequent high magnitude events the morphology of the tributary channel also appears to be greatly influenced by the frequency and magnitude of mass wasting in headwater areas of small basins

geomorphology of the colorado river in the grand canyon sediment supplied to the colorado river within the grand canyon has been sorted into distinct deposits of three grain size ranges the major rapids are formed by boulder deposits from side canyon tributaries as a result of a fourfold reduction in peak discharge when glen canyon dam was closed in 1963 new fan debris may increase the gradient through some of the rapids by a factor of 1 8 cobbles and gravel transported only during flood stages are preferentially deposited in the wider sections of the river as bars and riffles and are for the most part inactive during post dam discharges fine grain largely sandy terraces occur throughout the canyon especially along the banks of the large reverse eddies above and below the rapids the lower terraces are being reworked into beach like shores by diurnally varying post dam discharges a slight net lateral erosion of the terraces has resulted prior to construction of the dam sandy bed deposits underwent scour averaging about 1 m during spring floods

accuracy assessment of georectified aerial photographs implications for measuring lateral channel movement in a gis aerial photographs are commonly used to measure planform river channel change we investigated the sources and implications of georectification error in the measurement of lateral channel movement by testing how the number 6 30 and type human versus natural landscape features of ground control points gcps and the order of the transformation polynomial first second and third order affected the spatial accuracy of a typical georectified aerial photograph error was assessed using the root mean square error rmse of the gcps as well as error in 31 independent test points the rmse and the mean and median values of test point errors were relatively insensitive to the number of gcps above eight but the upper range of test point errors showed marked improvement i e the number of extreme errors was reduced as more gcps were used for georectification using more gcps thus improved overall georectification accuracy but this improvement was not indicated by the rmse suggesting that independent test points located in key areas of interest should be used in addition to rsme to evaluate georectification error the order of the transformation polynomial also influenced test point accuracy the second order polynomial function yielded the best result for the terrain of the study area gcp type exerted a less consistent influence on test point accuracy suggesting that although hard edged points e g roof corners are favored as gcps some soft edged points e g trees may be used without adding significant error based upon these results we believe that aerial photos of a floodplain landscape similar to that of our study can be consistently georectified to an accuracy of approximately f5 m with 10 chance of greater error the implications of georectification error for measuring lateral channel movement are demonstrated with a multiple buffer analysis which documents the inverse relationship between the size of the buffers applied to two channel centerlines and the magnitude of change detected between them this study demonstrates the importance of using an independent test point analysis in addition to the rsme to evaluate and treat locational error in channel change studies d 2005 published by elsevier b v

geomorphic impacts of a 100 year flood kiwitea stream manawatu catchment new zealand abstract the geomorphic impacts of a 100 year flood are assessed in the kiwitea stream 254 km 2 a tributary within the manawatu river catchment new zealand using sequential aerial photographs and reach based morphological sediment budgeting channel expansion and avulsion eroded in excess of one million cubic metres of sediment over 1 km 2 of floodplain along a 30 km long reach of kiwitea stream channel transformation was spatially discontinuous and predominantly associated with large scale bank erosion in response to a flood over 5 times bigger than the mean annual flood annual recurrence interval ari 100 years total energy expenditure of this flood in the kiwitea was 14 900 10 3 j the spatial discontinuity of channel transformation relates to valley floor and channel configurations high stream powers generated in confined channels at bends produced catastrophic channel transformation where flood flows dissipated overbank stream powers and the extent of channel transformation were reduced hydrologic hydraulic and geomorphic variables can be invoked to thus explain the variability of geomorphic impacts encountered during this event

hydro geomorphic hazards and impact of man made structures during the catastrophic flood of june 2000 in the upper guil catchment queyras southern french alps the guil river valley queyras southern french alps is prone to catastrophic floods as the long historical archives and holocene sedimentary records demonstrate in june 2000 the upper part of this valley was affected by a b30 yearq recurrence interval r i flood although of lower magnitude and somewhat different nature from that of 1957 n100 year r i flood the 2000 event induced serious damage to infrastructure and buildings on the valley floor use of methods including high resolution aerial photography multi date mapping hydraulic calculations and field observations made possible the characterisation of the geomorphic impacts on the guil river and its tributaries the total rainfall 260 mm in four days and maximum hourly intensity 17 3 mm h 1 aggravated by pre existing saturated soils explain the immediate response of the fluvial system and the subsequent destabilisation of slopes abundant water and sediment supply landsliding bank erosion particularly from small catchment basins cut into slaty schist bedrock resulted in destructive pulses of debris flow and hyperconcentrated flows the specific stream power of the guil and its tributaries was greater than the critical stream power thus explaining the abundant sediment transport the guil discharge was estimated as 180 m 3 s 1 at aiguilles compared to the annual mean discharge of 6 m 3 s 1 and a june mean discharge of 18 m 3 s 1 the impacts on the guil valley floor flooding aggradation generalised bank erosion and changes in the river pattern were widespread and locally influenced by variations in the floodplain slope and or channel geometry the stream partially reoccupied former channels abandoned or modified in their geometry by various structures built during the last four decades as exemplified by the aiguilles case study where the worst damage took place a comparative study of the geomorphic consequences of both the 1957 and 2000 floods shows that despite their poor maintenance the flood control structures built after the 1957 event were relatively efficient in contrast to unprotected places the comparison also demonstrates the role of land use changes conversion from traditional agro pastoral life to a ski hikingbased economy construction of various structures in reducing the guil channel capacity and more generally in increasing the vulnerability of the human installations the efficiency of the measures taken after the 2000 flood narrowing and digging out of the channel is also assessed final evaluation suggests that in such high mountainous environments there is a need to keep

logistic analysis of channel pattern thresholds meandering braiding and incising a large and geographically diverse data set consisting of meandering braiding incising and post incision equilibrium streams was used in conjunction with logistic regression analysis to develop a probabilistic approach to predicting thresholds of channel pattern and instability an energy based index was developed for estimating the risk of channel instability associated with specific stream power relative to sedimentary characteristics the strong significance of the 74 statistical models examined suggests that logistic regression analysis is an appropriate and effective technique for associating basic hydraulic data with various channel forms the probabilistic diagrams resulting from these analyses depict a more realistic assessment of the uncertainty associated with previously identified thresholds of channel form and instability and provide a means of gauging channel sensitivity to changes in controlling variables

the use of multi temporal lidar to assess basin scale erosion and deposition following the catastrophic january 2011 lockyer flood se queensland australia advances in remote sensing and digital terrain processing now allow for a sophisticated analysis of spatial and temporal changes in erosion and deposition digital elevation models dems can now be constructed and differenced to produce dems of difference dod which are used to assess net landscape change for morphological budgeting to date this has been most effectively achieved in gravel bed rivers over relatively small spatial scales if the full potential of the technology is to be realised additional studies are required at larger scales and across a wider range of geomorphic features this study presents an assessment of the basin scale spatial patterns of erosion deposition and net morphological change that resulted from a catastrophic flood event in the lockyer creek catchment of se queensland seq in january 2011 multitemporal light detection and ranging lidar dems were used to construct a dod that was then combined with a one dimensional flow hydraulic model hec ras to delineate five major geomorphic landforms including inner channel area within channel benches macrochannel banks and floodplain the lidar uncertainties were quantified and applied together with a probabilistic representation of uncertainty thresholded at a conservative 95 confidence interval the elevation change distribution ecd for the 100 km2 study area indicates a magnitude of elevation change spanning almost 10 m but the mean elevation change of 0 04 m confirms that a large part of the landscape was characterised by relatively low magnitude changes over a large spatial area mean elevation changes varied by geomorphic feature and only two the within channel benches and macrochannel banks were net erosional with an estimated combined loss of 1 815 149 m3 of sediment the floodplain was the zone of major net deposition but mean elevation changes approached the defined critical limit of uncertainty areal and volumetric ecds for this extreme event provide a representative expression of the balance between erosion and deposition and importantly sediment redistribution which is extremely difficult to quantify using more traditional channel planform or cross sectional surveys the ability of lidar to make a rapid and accurate assessment of key geomorphic processes over large spatial scales contributes to our understanding of key processes and as demonstrated here to the assessment of major geomorphological hazards such as extreme flood events

contrast in stream channel response to major storms in two mountainous areas of california the generally catastrophic effects of major storms on the geometry of intermediate and high order channels in northwestern california contrast with localized storm related channel changes found in the santa cruz mountains sediment delivery in northwestern california during storms overwhelms transport capacities throughout the length of most intermediate and high order channels and causes long lasting changes in geometry in contrast sediment delivery during storms in the santa cruz mountains overwhelms transport capacities only in localized reaches this contrast in channel behavior is at least partially caused by contrasts in the style of landsliding found in the two areas

floods in mountain environments a synthesis floods are a crucial agent of geomorphic change in the channels and valley floors of mountains watercourses at the same time they can be highly damaging to property infrastructure and life because of their high energy mountain watercourses are highly vulnerable to environmental changes affecting their catchments and channels many factors have modified and frequently still tend to modify the environmental conditions in mountain areas with impacts on geomorphic processes and the frequency magnitude and timing of floods in mountain watercourses the ongoing climate changes vary between regions but may affect floods in mountain areas in many ways in many mountain regions of europe widespread afforestation took place over the twentieth century considerably increasing the amounts of large wood delivered to the channels and the likelihood of jamming bridges at the same time deforestation continues in other mountain areas accelerating runoff and amplifying the magnitude and frequency of floods in foreland areas in many countries in channel gravel mining has been a common practice during recent decades the resultant deficit of bed material in the affected channels may suddenly manifest during flood events resulting in the failure of scoured bridges or catastrophic channel widening during the past century many rivers in mountain and foreland areas incised deeply the resultant loss of floodplain water storage has decreased attenuation of flood waves hence increasing flood hazard to downstream river reaches on the other hand a large amount of recent river restoration activities worldwide may provide examples of beneficial changes to flood risk attained as a result of increased channel storage or reestablished floodplain water storage relations between geomorphic processes and floods operate in both directions which means that changes in flood probability or the character of floods e g increased wood load may significantly modify the morphology of mountain rivers but morphological changes of rivers can also affect hydrological properties of floods and the associated risk for societies this paper provides a review of research in the field of floods in mountain environments and puts the papers of this special issue dedicated to the same topic into context it also provides insight into innovative studies methods or emerging aspects of the relations between environmental changes geomorphic processes and the occurrence of floods in mountain rivers

environment friendly reduction of flood risk and infrastructure damage in a mountain river case study of the czarny dunajec abstract migration of a mountain river channel may cause erosional risk to infrastructure or settlements on the valley floor following a flood of 2010 a cutbank in one of the bends of the main channel of the czarny dunajec polish carpathians approached a local road by 50 m to arrest the erosion of the laterally migrating channel water authorities planned construction of a ditch cutting the forested neck of the bend reinforcement of the ditch banks and damming the main channel with a boulder groyne in order to avoid channelization of the highly valued multithread river reach that would deteriorate its ecological status and cause increased flood risk to downstream reaches an alternative approach to prevent bank erosion was proposed the new scheme applied in 2011 included opening of the inlets to inactive side braids located by the neck of the bend of the main channel this solution reestablished the flow in the steeper low flow channels allowing us to expect a cutoff and abandonment of the main channel during subsequent floods gravelly deflectors were constructed directly below the inlets to the reactivated side channels to divert the flow into the channels and prevent the water from entering the main channel hydraulic measurements performed before and after the implementation of the scheme confirmed that it enabled shifting the main water current with the highest average velocity and bed shear stress from the braid closest to the road to the most distant braid similar surveys of fish and benthic macroinvertebrate communities indicated that flow reactivation in the side channels was beneficial for these groups of river biota increasing their abundance and taxonomic richness in the reach not only was the implemented solution significantly less expensive but it also enhanced ecological functions of the multithread channel and the variability of physical habitat conditions and maintained the role of the reach as a wood debris trap however avulsion of the main channel in the river bend immediately upstream during the flood in may 2014 again caused erosional risk to the road although at another location this indicates that with the highly unstable multithread channel pattern of the czarny dunajec the best practice of river maintenance in a relatively unmanaged valley reach would be allowing free channel migration within the floodplain area and reinforcing where necessary the boundary between the erodible river corridor and the managed terrace

the geomorphic impact of the catastrophic october 1984 flood on the planform of squamish river southwestern british columbia small format aerial photography of squamish river just prior to and immediately following the october 1984 flood of record forms the basis of a quantitative evaluation of floodplain erosion and construction during this extreme event channel changes in 1984 are compared with those determined from sequential aerial photography at various other times since 1947 and associated with less extreme flooding depending on the type of river planform the degree of channel change during the 1984 flood varied from a relatively minor response to a major reorganization of the channel despite its large size the 1984 flood accomplished little more floodplain modification in the meandering and transitional semibraided reaches than had previous smaller floods of similar duration in general greater erosion was accomplished here by relatively small but longer duration flood events in contrast in the braided reach the 1984 flood caused floodplain erosion and major reorganization of the channel to an extent previously un

energy expenditure and geomorphic work of the cataclysmic missoula flooding in the columbia river ggorge usa cataclysmic releases from the glacially dammed lake missoula producing exceptionally large floods have resulted in significant erosional processes occurring over relatively short time spans erosional landforms produced by the cataclysmic missoula floods appear to follow a temporal sequence in many areas of eastern washington state this study has focused on the sequence observed between celilo and the john day river where the erosional features can be physically quantified in terms of stream power and geomorphic work the step backwater calculations in conjunction with the geologic evidence of maximum flow stages indicate a peak discharge for the largest missoula flood of 10 10 6 m 3 s 1 the analysis of local flow hydraulics and its spatial variation were obtained calculating the hydrodynamic variables within the different segments of a cross section the nature and patterns of erosional features left by the floods are controlled by the local hydraulic variations therefore the association of local hydraulic parameters with erosional and depositional flood features was critical in understanding landform development and geomorphic processes the critical stream power required to initiate erosion varied for the different landforms of the erosional sequence ranging from 500 w m 2 for the streamlined hills up to 4500 w m 2 to initiate processes producing inner channels erosion is possible only during catastrophic floods exceeding those thresholds of stream power below which no work is expended in erosion in fact despite the multiple outbursts which occurred during the late pleistocene only a few of them had the required magnitude to overcome the threshold conditions and accomplish significant geomorphic work 1997 by john wiley sons ltd

geomorphic and sedimentological controls on the effectiveness of an extreme flood the 1993 flood on the upper mississippi river was a rare large magnitude hydrological event field and aerial survey analyses and landsat 5 thematic mapper data were used to appraise the thickness of overbank deposits on leveed and unleveed reaches results indicate that minimal 5 mm overbank sedimentation occurred except in the immediate vicinity of a levee break unleveed sections also lacked overbank sedimentation little geomorphological or sedimentological evidence of this extreme event is likely to be preserved this raises questions about the completeness of the stratigraphic record in situations where wide floodplains with cohesive soils provide effective resistance and dissipate energy so that erosion is minimized and or sediment supply is limited by event timing or sequencing a large flood may leave little or no substantive evidence of its occurrence

evolutionary trajectory of channel morphology and controlling factors in a large gravel bed river abstract interpretation of channel changes is not always straightforward since it requires a detailed reconstruction of the evolutionary trajectory of channel morphology a quantitative analysis of controlling factors and finally identification of links between evolutionary trajectories and controlling factors the aim of this paper is to explain channel adjustments and controlling factors in the tagliamento river a large gravel bed river in northeastern italy traditional methods for studying historical channel changes i e use of aerial photos topographic data were employed but numerical modelling turned out to be very useful for the quantification of bedload transport and for supporting interpretation of past changes and likely future channel evolution river channel underwent three main phases of adjustment over the last 200 years the first two phases from the end of the nineteenth century to the early 1990s were characterized by narrowing channel width decreased from 1250 to 540 m and incision about 1 m and the third phase from the 1990s to present day by widening from 540 to 600 m and slight aggradation about 0 2 m combining evolutionary trajectories of channel morphology and analysis of controlling factors we argue that the long term channel evolution of the tagliamento river was driven mainly by human intervention at the reach scale i e sediment mining and channelization changes in sediment supply in the catchment area had no or minor effects in the study reach the most recent changes i e widening and slight aggradation are explained as a response to past disturbances that have produced a remarkable change of channel geometry and an increase of unit stream power in the reach magnitude and frequency of formative discharges are a key driving factor of the intensity of recent changes while this was not the case during the previous phases of adjustment

upper limits of flash flood stream power in europe abstract flash floods are characterized by strong spatial gradients of rainfall inputs that hit different parts of a river basin with different intensity stream power values associated with flash floods therefore show spatial variations that depend on geological controls on channel geometry and sediment characteristics as well as on the variations of flood intensity this stresses the need for a field approach that takes into account the variability of the controlling factors post flood assessment of peak discharge after major floods makes it possible to analyse stream power in fluvial systems affected by flash floods this study analyses the stream power of seven intense return period of rainfall 100 years at least in some sectors of the river basin flash floods that occurred in mountainous basins of central and southern europe from 2007 to 2014 in most of the analysed cross sections high values of unit stream power were observed this is consistent with the high severity of the studied floods the highest values of cross sectional stream power and unit stream power usually occur in mediterranean regions this is mainly ascribed to the larger peak discharges that characterize flash floods in these regions the variability of unit stream power with catchment area is clearly nonlinear and has been represented by log quadratic relations the values of catchment area at which maximum values of unit stream power occur show relevant differences among the studied floods and are linked to the spatial scale of the events values of stream power are generally consistent with observed geomorphic changes in the studied cross sections bedrock channels show the highest values of unit stream power but no visible erosion whereas major erosion has been observed in alluvial channels exceptions to this general pattern which mostly occur in semi alluvial cross sections urge the recognition of local or event specific conditions that increase the resistance of channel bed and banks to erosion or like short flow duration reduce the geomorphic effectiveness of the flood

geomorphological response to an extreme flood a case from southeast spain on 28 29 september 1980 an extreme storm of 150 mm of rain in c 5 hours caused a flash flood near tabernas in a semi arid area in southern spain with the rainfall total approximating the mean annual rainfall the recurrence interval is estimated to be 25 100 years flood peak discharges have been estimated from a mountain source area across an alluvial fan and the geomorphological response to the storm described channel adjustment differed between channels in the fanhead trench and those unconfined channels on the fan surface the implications of the spatial pattern of erosional an depositional feaures for landform adjustment in semi arid regions are discussed

a method for the assessment and analysis of the hydromorphological condition of italian streams the morphological quality index mqi article i nfo a new index has been developed for the hydromorphological assessment of italian rivers the method was designed to comply with the eu water framework directive requirements but its use can be extended to other applications in river management the evaluation of stream morphological quality is preceded by a phase of river segmentation consisting ofan initial division of the network intoriver reaches with homogeneous morphological characteristics the evaluation procedure consists of a set of 28 indicators which were defined to assess longitudinal and lateral continuity channel pattern cross section configuration bed structure and substrate and vegetation in the riparian corridor these characteristics are analyzed in terms of geomorpholog ical functionality artificiality and channel adjustments indicators classes and the scoring system were defined based on expert judgement the scoringsystemleads tothe definition of the morphologicalquality index mqi application of the method to 102 river reaches covering a wide range of physical conditions and human pres sures of italian streams enabled the testing of the overall methodology and the refinement of the indicators and scores limitations strengths and the applicability of the method are also discussed in the paper