drainage basin evolution in the rainfall erosion facility dependence on initial conditions four experiments in alluvial drainage basin evolution were carried out in the rainfall erosion facility ref at colorado state university to investigate the dependence of basin evolution on initial topography basins were initially undissected each experiment began with a unique initial condition representing various end members of relief and hypsometry drainage network development hillslope processes basin denudation and basin response to base level lowering all depended strongly on the initial topography no classic model of drainage network evolution was found to be generally applicable initially planar slopes first developed subparallel channels that extended headward dendritically during an early phase of extension channel incision occurred first in the interior of the basin where saturation overland flow was greatest not at the basin outlet as assumed in most classic models of network development channels widened over time initiating lateral migration and drainage capture in the downslope portion of the watershed before transferring lateral migration upslope planar basins of larger initial gradient grew headward more quickly and become more deeply entrenched inhibiting late stage lateral migration an experiment with initial relief concentrated at a plateau edge evolved in several unique ways a high ratio of subsurface to surface flow gave rise to mass movements at the plateau edge and outlet channels deep channels were quickly cut initially but did not extend far upslope because slope instability undermined channel head migration leaving the plateau undissected and hence very slow to erode these results suggest that the distribution of relief within a basin exerts an important control on drainage network pattern and basin denudation in addition erosional basins may evolve in several distinct modes characterized by particular combinations of hypsometry hillslope processes and mean denudation rate

flow characteristics in different densities of submerged flexible vegetation from an open channel flume study of artificial plants abstract the effect of submerged flexible vegetation on flow structure e g flow velocity reynolds shear stress turbulence intensity and manning coefficient was experimentally studied with a 3d acoustic doppler velocimeter adv in an open channel flume the results from flow observations over artificial plants designed to simulate natural vegetation showed that flow structure was affected markedly by the presence of submerged flexible vegetation the study provides understanding of flow patterns variation in velocity profile and turbulence structures that are affected by plant stem density the study also reveals how the flow patterns return to stability at the downstream end of the vegetated area which is critical in determining the length of the vegetated areas for restoration cases also new mathematical expressions equations have been formulated to clearly express variations in velocity profile manning coefficient and flow discharge ratio with vegetation density vertically the velocity profile could be roughly divided into three layers including the upper non vegetated layer the middle canopy layer and the lower sheath layer in the upper non vegetated layer velocity profiles followed the logarithmic law and a corresponding empirical equation was developed based on the observed data the flow is from left to right in this study and the velocity profile followed a left round bracket with the minimum point located at the canopy area 0 7hv where hv denotes vegetation height within the middle canopy layer however the velocity profile followed a right round bracket in the lower sheath section layer with the maximum point located at the sheath section 0 2hv with increasing vegetation density the velocity and corresponding flow rate increased in the upper non vegetated layer and decreased within the middle canopy layer and the lower sheath layer the ratio of average flow discharge in the non vegetated and vegetated layers followed the exponential function law with increasing vegetation density this analysis revealed the effect of vegetation on flood potential and flow bottom scour reynolds stresses peaked above the canopy top z hv 1 0 1 2 here z denotes vertical coordinate and the turbulence intensities reached their maximum peak at two locations including the sheath section z hv 0 1 0 4 and the canopy top z hv 1 0 1 6 for all vegetation densities manning coefficient was highly correlated to vegetation density and inflow rate with new empirical equations being proposed

functional relationships of resistance in wide flood plains with rigid unsubmerged vegetation the effect of rigid unsubmerged vegetation on flow resistance in wide flood plains was examined in this study a series of experiments was run using rigid rods inserted into the flow field of a wide shallow flume several configurations of rod diameters with various lateral and longitudinal spacing were tested over a range of discharge values head losses across the rods were measured and a resistance to momentum absorbing area concept was employed to relate density parameters to the resistance coefficient the effects of flow depth velocity rod diameter d lateral spacing r and longitudinal spacing l were estimated the results demonstrated that flow resistance is greatly impacted by both depth and velocity but that the effects are opposite in sign the friction factor increased in only a slightly nonlinear fashion with depth but it decreased in a highly nonlinear manner with increasing velocity and the velocity effects were very sensitive to vegetation density parameters of the density pa

assessment of vegetation effects on hydraulics and of feedbacks on plant survival and zonation in ephemeral channels the interaction of vegetation and flow in channels is important for understanding the influences of forces in channels and effects on erosion sediment flux and deposition it has implications for channel habitats channel instability and restoration schemes methods are needed for calculating forces on plants and data are required on thresholds for plant destruction and survival a simple method of calculating the effect of hydraulics on vegetation and its zonation within ephemeral channels is described detailed cross section surveys of channel morphology vegetation and estimates of manning s n are input into the software program winxspro to calculate the hydraulics of flows across the channel for a given event or flow level incorporating subdivision into zones of differing morphology and vegetation across the section this was applied to a number of cross sections on ephemeral channels in se spain and typical roughness values for mediterranean vegetation types in channels were assessed the method is demonstrated with reference to two well documented floods in se spain in september 1997 on the torrealvilla and in october 2003 along the salada these flows led to the mortality of herbs reed and smaller shrub species some damage to larger shrubs and trees occurred but trees such as tamarisk tamarix canariensis were shown to withstand high forces some grasses were highly resistant to removal and induced sedimentation significant erosion was limited to areas with little vegetation covering the channel floor further quantification of resistance of vegetation to flows and upper threshold values for removal is continuing by relating calculated hydraulic conditions using the methods outlined to measurements of vegetation responses in events at monitoring sites copyright 2009 john wiley sons ltd

the impact of single natural vegetation elements on flow characteristics in this experimental study measurements were conducted to explore the impacts of different forms of individual natural vegetative elements within the flow domain on velocity and turbulence characteristics all the experiments were performed in a flume measuring 26 m in length 0 98 m in width and 0 85 m in depth and real tree saplings were utilized to represent the vegetation element in order to analyse this commonly observed nature phenomenon in floodplains trees with wide trunks were classified into three groups on the basis of their volume versus height relation throughout the velocity measurements three acoustic doppler velocimeters were employed time averaged velocity streamwise and vertical turbulence intensities and turbulent kinetic energy parameters were examined additionally a formulation that gives the velocity profile at a certain distance downstream of vegetation was introduced and the validity of the proposed formulation was checked with experimental data it is seen that despite their porous structures the presence of vegetation considerably disturbs the flow field and dissipates a remarkable amount of energy by turbulence copyright 2008 john wiley sons ltd

multiscalar model for the determination of spatially explicit riparian vegetation roughness 1 improved understanding of the connection between riparian vegetation and channel change requires evaluating how fine scale interactions among stems water and sediment affect larger scale flow and sediment transport fields we propose a spatially explicit model that resolves patch scale submeter patterns of hydraulic roughness over the reach scale caused by stands of shrubby riparian vegetation we worked in tamarisk dominated stands on the yampa and green rivers in dinosaur national monument northwestern colorado usa where questions remain regarding the role of vegetation in inducing or exacerbating documented channel changes hydraulic roughness patterns were derived from patch scale measurements made with detailed terrestrial laser scan tls data that were extrapolated to reach scales based on correlation with light detection and ranging lidar als data two dimensional patch scale hydraulic models were used to parameterize the stage dependence of hydraulic roughness of typical patch types i e sparse moderate and dense patches we illustrate the value of using this approach to characterize vegetation roughness by applying our results to a two dimensional hydraulic model of flow for one of our study sites results from this work predict that the roughness of vegetated floodplains increases with flow depth and is dependent on patch scale stem organization geomorphically relevant patterns i e areas of low or high shear stress that are likely to scour or fill during high flows become apparent with the detail introduced by spatially explicit depth dependent roughness to our knowledge the multiscalar analysis presented here is the first to mechanistically account for shrubby riparian vegetation stand structure and associated hydraulic roughness of vegetation patches at the reach scale

nonrigid nonsubmerged vegetative roughness on floodplains individual pine and cedar tree saplings and branches were used to model the resistance to flow in a water flume for nonsubmerged and nonrigid vegetation to determine the amount that streamlining decreases the drag coefficient and reduces the momentum absorbing area currently vegetation on floodplains is commonly assumed to behave as rigid roughness that can lead to large errors in the relationships between velocity and drag force this presents a basic fluid mechanics problem an extreme variation of roughness with depth of flow can result due to a large increase in the momentum absorbing area in nonsubmerged vegetation as depth is increased this deems all the available roughness equations which generally are based on relative roughness approach useless for this application in this paper a dimensional analysis supported by experimental results is developed to obtain a relationship between roughness conditions i e density and flexural rigidity and flow conditions i e velocity and depth for floodplains and vegetative zones of natural waterways

effect of viscosity on the equilibrium scour depth at single cylindrical piers abstractmost studies on scouring seem to have overlooked the effect of viscosity the assumption being that the flow is free of viscous effects inside the scour hole because of the presence of highly turbulent flow structures such as down flow horseshoe vortex and wake vortices irrespective of the approach flow regime the present work reports the results of experiments conceived to revisit the effect of viscosity on scouring the experiments were performed by changing the reynolds like numbers for two flow shallowness values and keeping constant the remaining dimensionless parameters that control the scour process the values of the reynolds number corresponded to transitional approach flows the experimental results indicate that scouring does depend on viscosity in usual laboratory conditions the viscous effect leading to scour depths in the safety side

1d unsteady state flow simulation of a section of the upper rhine abstract a 1d steady state flow simulation model was developed a module evaluating the roughness parameters due to the surface roughness of the main channel and floodplains riparian forests on the floodplains and the momentum exchange between the main channel and floodplains was introduced into the model in the module the roughness parameters were evaluated by the formulas proposed by the dvwk hydraulische methoden zur erfassung von rauheiten 1990 hydraulische berechnung von fliesgewassern 1991 for steady and uniform flow conditions the model was calibrated with data obtained from three flood events roughness parameters of the main channel and floodplains of both sides were identified in the situation before may 1999 the morphological situation changed slightly several sensitivity analyses were carried out for the following cases a the morphological situation before may 1999 with the original parameters b the situation of a with various parameters to the realistic extent c the situation being so that the floodplain on the german side were imaginarily enlarged up to utmost 90 m with the original vegetation and d the situation of c with various parameters to the realistic extent many combinations of the roughness parameters due to the bed roughness and density of the vegetation were examined the results showed that the most significant parameter is the bed roughness of the floodplains and the next is that of the main channel a modification of parameters of the riparian vegetation is of minor importance the model was extended to a 1d unsteady state flow simulation model and applied to two flood data obtained in the reach between rheinweiler 186 156 km from lake constance and hartheim 214 244 km in germany in 1998 and 1999 both measured and calculated hydrographs at hartheim had their peak discharge at the same time and the discrepancy of the discharge between the measured and calculated hydrographs was less than 10

two dimensional hydraulic flood modelling using a finite element mesh decomposed according to vegetation and topographic features derived from airborne scanning laser altimetry airborne scanning laser altimetry lidar is an important new data source that can provide two dimensional river flood models with spatially distributed floodplain topography for model bathymetry together with vegetation heights for parameterization of model friction methods are described for improving such models by decomposing the model s finite element mesh to reflect floodplain vegetation features such as hedges and trees having different frictional properties to their surroundings and significant floodplain topographic features having high height curvatures the decomposition is achieved using an image segmentation system that converts the lidar height image into separate images of surface topography and vegetation height at each point the vegetation height map is used to estimate a friction factor at each mesh node the spatially distributed friction model has the advantage that it is physically based and removes the need for a model calibration exercise in which free parameters specifying friction in the channel and floodplain are adjusted to achieve best fit between modelled and observed flood extents the scheme was tested in a modelling study of a flood that occurred on the river severn uk in 1998 a satellite synthetic aperture radar image of flood extent was used to validate the model predictions the simulated hydraulics using the decomposed mesh gave a better representation of the observed flood extent than the more simplistic but computationally efficient approach of sampling topography and vegetation friction factors on to larger floodplain elements in an undecomposed mesh as well as the traditional approach using no lidar derived data but simply using a constant floodplain friction factor use of the decomposed mesh also allowed velocity variations to be predicted in the neighbourhood of vegetation features such as hedges these variations could be of use in predicting localized erosion and deposition patterns that might result in the event of a flood copyright 2003 john wiley sons ltd

hydromorphological processes at submerged solitary boulder obstacles in streams abstract a series of 51 experiments was conducted in a laboratory flume to investigate hydromorphological processes at submerged solitary boulder like obstacles as part of a programme to validate the experimental results five sites with obstacle marks at large boulders were surveyed in two gravel bed ephemeral streams located in eastern spain three hydromorphological system states i iii were observed each exhibiting characteristic dynamic interactions between hydraulic and sedimentary processes i wake vortex domination and absent scouring ii wake vortex domination and downstream scouring iii horseshoe vortex domination frontal scouring and ridge like sediment accumulation at the lee side the latter system state iii comprises typical fluvial obstacle marks in mobile alluvial beds obstacles may tilt upstream into the scour hole when frontal scour depths exceed 0 6 times obstacle length favouring the rapid achievement of steady state conditions state iii field and experimentally modelled obstacle marks are related to each other when frontal scour depth width and length as well as ridge width are used as morphometric variables an analytical model was adapted and successfully applied for determination of these variables at the steady state condition the model was also used to assess the critical threshold for the formation of obstacle marks it integrates mean flow velocity water depth obstacle height and width obstacle shape median grain size relative to obstacle width and grain size distribution dynamic interactions between hydraulic processes and sediment exchange around obstacles together with well defined boundaries between evolutionary states critical thresholds suggest a self organised scale invariant mechanism driving the generation of obstacle marks

hack s law sinuosity convexity elongation hack s law an empirical power law relationship between drainage basin area and the length of the main stream channel has long been taken to imply that drainage basins become more elongate relatively longer and narrower with increasing basin size a study of the geometry of 38 basins from three distinct geomorphic settings shows that this geometric interpretation of hack s law is only occasionally true even though hack s power law relationship holds between basin area and main channel length these basins do not necessarily become more elongate with increasing size rather hack s law is an expression of a balance between changes in basin shape and changes in channel planform geometry for the basins in this study changes in channel sinuosity play the most important role in this balance changes in basin shape are far less regular local conditions appear to determine the partitioning of importance between changes in basin shape and channel sinuosity

scouring at bed sills as a response to flash floods the temporal development of clear water local scour depth at bed sills in uniform gravel beds is considered experiments are presented on the development of scour holes under unsteady hydraulic conditions with the triangular shaped hydrographs tested being of different durations and different rates of flow variation based on the experimental results and a theoretical framework a method is given for the definition and prediction of the scouring process under unsteady flows in terms of a dimensionless temporal parameter a flash flood is here defined as an event for which the scour doesn t attain its potential magnitude i e the equilibrium value for the peak hydrograph flow rate this flood nature is dependent on both the characteristics of the flood event itself and the characteristics of the stream a quantitative measure of what constitutes a flash flood is given in terms of the identified temporal parameter results show that the ratio between the final scour depth and the potential scour depth

estimating final scour depth under clear water flood waves abstractin this paper we experimentally investigate the final scour depth around cylindrical piers under flood waves only clear water conditions are considered an approach to estimate the final scour depth under a flood wave is proposed based on the local scour depth calculated with steady flow equations under peak flow conditions the new experimental results are also used to investigate the performance of existing expressions or approaches to predict time evolution under steady and unsteady flow conditions

hydraulic resistance in overland flow during partial and marginal surface inundation experimental observations and modeling hydraulic resistance as a function of surface roughness inundation was evaluated in a set of experiments designed to simulate overland flow on a rough granular surface the data are compared with an additive drag model based on the contribution of individual elements to flow resistance and a mixing length model for estimating bulk flow resistance during partial inundation of the surface roughness the observed coefficient of drag per element is much higher than for an isolated element and a model based on element form drag alone underestimates the observed friction these high resistance values are strongly correlated with the hydrostatic wave drag estimated from the free surface deformation around elements the mixing length model incorporating a multiplier setting the hydraulically effective roughness height reliably reproduces the trends in resistance during marginal inundation this multiplier is shown to have a value similar to the root mean square of the surface height

open channel flow through different forms of submerged flexible vegetation laboratory experiments are used to explore the effect of two forms of flexible vegetation on the turbulence structure within a submerged canopy and in the surface flow region above the two simulated plant forms involve flexible rods stipes of constant height and the same rods with a frond foliage attached these plant forms were arranged in a regular staggered configuration set at the same stipe density the plant geometry and its mechanical properties have been scaled from a real aquatic plant using froudian similarity and the methods used for quantifying the bending stiffness flexural rigidity and drag force velocity relationship of the vegetation are outlined experimental results reveal that within the plant layer the velocity profile no longer follows the logarithmic law profile and the mean velocity for the rod frond canopy is less than half of that observed for the simple rod array in addition to the mean flow field the turbulence intensities indicate that the additional superficial are

large scale geomorphology classical concepts reconciled and integrated with contemporary ideas via a surface processes model linear systems analysis is used to investigate the response of a surface processes model spm to tectonic forcing the spm calculates subcontinental scale denudational landscape evolution on geological timescales 1 to hundreds of million years as the result of simultaneous hillslope transport modeled by diffusion and fluvial transport modeled by advection and reaction the tectonically forced spm accommodates the large scale behavior envisaged in classical and contemporary conceptual geomorphic models and provides a framework for their integration and unification the following three model scales are considered micro meso and macroscale the concepts of dynamic equilibrium and grade are quantified at the microscale for segments of uniform gradient subject to tectonic uplift at the larger meso and macroscales which represent individual interfluves and landscapes including a number of drainage basins respectively the system response to tectonic forcing is linear for uplift geometries that are symmetric with respect to baselevel and which impose a fully integrated drainage to baselevel for these linear models the response time and the transfer function as a function of scale characterize the model behavior numerical experiments show that the styles of landscape evolution depend critically on the timescales of the tectonic processes in relation to the response time of the landscape when tectonic timescales are much longer than the landscape response time the resulting dynamic equilibrium landscapes correspond to those envisaged by hack 1960 when tectonic timescales are of the same order as the landscape response time and when tectonic variations take the form of pulses much shorter than the response time evolving landscapes conform to the penck type 1972 and to the davis 1889 1899 and king 1953 1962 type frameworks respectively the behavior of the spm highlights the importance of phase shifts or delays of the landform response and sediment yield in relation to the tectonic forcing finally nonlinear behavior resulting from more general uplift geometries is discussed a number of model experiments illustrate the importance of fundamental form which is an expression of the conformity of antecedent topography with the current tectonic regime lack of conformity leads to models that exhibit internal thresholds and a complex response

valley shape influence on deformation mechanisms of rock slopes stress distribution in mountainous areas is influenced by local morphology valley morphology and the relationship between main and tributary valleys strongly depend on geological characteristics and evolution they may control the evolution of slope instabilities especially when interacting with pervasive structural features we performed parametric three dimensional 3d numerical modelling of simplified slope geometries with variable slope angle from 218 to 358 length combining different orientations for different slope sectors and changing attitude of pervasive planes of anisotropy foliation schistosity bedding data used in the 3d models are the initial slope geometry rock mass properties and internal anisotropy we assumed mohr coulomb behaviour with the presence of ubiquitous joints and different piezometric levels the model results show that plastic deformation initiates near the highest ridge just after deglaciation commences a shear zone develops and propagates toward the toe of the slope and its shape is strongly controlled by slope geometry anisotropy and in situ stresses the thickness of the failing mass for model slope reliefs up to 3200 m increases from 50 mto some hundreds of metres during glacier retreat and it depends on geometry of slopes anisotropy and in situ stresses results are compared to examples of deep seated slope deformations from the alps which helps in the interpretation of such phenomena and in the understanding of their influence on valley evolution

clear water scour at comparatively large cylindrical piers abstractnew long duration clear water scour data were collected at single cylindrical piers with the objective of investigating the effect of sediment coarseness dp d50 dp pier diameter d50 median grain size on the equilibrium scour depth and improving the scour depth time evolution modeling by making use of the exponential function suggested in the literature experiments were carried out for the flow intensity close to the threshold condition of initiation of sediment motion imposing wide changes of sediment coarseness and flow shallowness d dp d approach flow depth the effect of sediment coarseness on the equilibrium scour depth was identified existing predictors were modified to incorporate this effect for u uc 1 0 dp d50 60 and d dp 0 5 the complete characterization of a known scour depth time evolution model was achieved for u uc 1 0 60 dp d50 500 and 0 5 d dp 5 0

floodplain friction parameterization in two dimensional river flood models using vegetation heights derived from airborne scanning laser altimetry two dimensional 2 d hydraulic models are currently at the forefront of research into river flood inundation prediction airborne scanning laser altimetry is an important new data source that can provide such models with spatially distributed floodplain topography together with vegetation heights for parameterization of model friction the paper investigates how vegetation height data can be used to realize the currently unexploited potential of 2 d flood models to specify a friction factor at each node of the finite element model mesh the only vegetation attribute required in the estimation of floodplain node friction factors is vegetation height different sets of flow resistance equations are used to model channel sediment short vegetation and tall and intermediate vegetation the scheme was tested in a modelling study of a flood event that occurred on the river severn uk in october 1998 a synthetic aperture radar image acquired during the flood provided an observed flood extent against which to validate the predicted extent the modelled flood extent using variable friction was found to agree with the observed extent almost everywhere within the model domain the variable friction model has the considerable advantage that it makes unnecessary the unphysical fitting of floodplain and channel friction factors required in the traditional approach to model calibration copyright 2003 john wiley sons ltd

a new method of tree structure for analysing nested watershed shape abstract a new method based on tree structure is proposed to study the relationship of multi scale watershed shapes considering the nested relationship of watershed characteristics the method uses a tree structure to reflect different watershed scales we investigated the loess plateau of shanxi province china using 30 m resolution digital elevation models dems the study consists of three stages 1 by using geographic information system gis tools different scale watersheds were extracted from the dems and used to build a tree structure model based on spatial inclusion relationships 2 the tree structure was transformed into a table based on the size of watershed area and 3 the shape was quantified by roundness and tightness coefficients to analyse shape relationships among different scales of watersheds the application of the method to 15 types of geomorphological features suggests that the smaller the watersheds are the rounder they tend to be the new tree structure method introduced here provides a good understanding of nested relationships between watershed characteristics

the influence of macrophyte growth typical of eutrophic conditions on river flow velocities and turbulence production the influence of emergent and submerged macrophytes on flow velocity and turbulence production is demonstrated in a 140 m reach of the river blackwater in farnborough hampshire uk macrophyte growth occurs in patches and is dominated by sparganium erectum and sparganium emersum in may 2001 patches of s erectum were already established and occupied 18 of the channel area the flow adjusted to these predominantly lateral patches by being channelled through a narrower cross section the measured velocity profiles showed a logarithmic form with deviations attributable to topographic control the channel bed was the main source of turbulence in september 2001 in stream macrophytes occupied 27 of the channel and overhanging bank vegetation affected 32 of the area overall flow resistance described by manning s n showed a threefold increase that could be attributed to the growth of s emersum in the middle of the channel velocity profiles showed different characteristic forms depending on their position relative to plant stems and leaves the overall velocity field had a three dimensional structure turbulence intensities were generally higher and turbulence profiles tended to mirror the velocity profiles evidence for the generation of coherent eddies was provided by ratios of the root mean square velocities spectral analysis identified deviations from the kolmogorov 5 3 power law and provided statistical evidence for a spectral short cut indicative of additional turbulence production this was most marked for the submerged vegetation and in some instances the overhanging bank vegetation the long strap like leaves of s emersum being aligned approximately parallel to the flow and the highly variable velocity field created by the patch arrangement of macrophytes suggest that the dominant mechanism for turbulence production is vortex shedding along shear zones wake production around individual stems of s emersum close to the bed may also be important locally

fluvial islands first stage of development from nonmigrating forced bars and woody vegetation interactions abstract fluvial islands can develop from the channel bed by interactions between pioneer trees and bars although vegetation recruitment and survival is possible on all bar types it is easier for trees to survive on nonmigrating bars developed from a change in channel geometry or to the presence of a steady perturbation this field study details the first stages of development of a vegetated mid channel nonmigrating or forced bar and its evolution toward an island form over six years analysis of bed topographical changes vegetation density and roughness scour and fill depths sediment grain size and architecture and excess bed shear stress highlighted a specific signature of trees on topography and grain size segregation two depositional processes combining the formation of obstacle marks and upstream shifting deposition of sediments led to the vertical accretion of the vegetated bar during the first stage of the bar accretion bedload sediment supply coming from surrounding channels during floods was identified as a key process modulated by the presence of woody vegetation and a deflection effect induced by the preexisting topography grain size segregation between vegetated and bare areas was also highlighted and interpreted as an important process affecting the development of surrounding channels and the degree of disconnection and hence the speed of development of a growing island the heterogeneity of bedload supply can explain why sediment deposition and density of trees are not strictly related a general conceptual model detailing the first stages of evolution from a bar to an established island is proposed for relatively large lowland rivers

in channel woody vegetation controls on sedimentary processes and the sedimentary record within alluvial environments a modern example of an anabranch of the river loire france a multidisciplinary approach was used to analyse the role of pioneer trees populus nigra on bar formation in the modern fluvial system of the river loire france data were collected on seven plots located on a secondary channel chosen for their vegetation cover and their sedimentary context physical features of vegetation topographical evolution sediment grain size and flow velocity were investigated during and after floods between 2001 and 2003 woody vegetation responds morphologically to fast flowing waters and sediment deposition these adaptations influence the role played by trees on local sedimentary processes some morphological types of trees exert a strong influence during flood events by deflecting reducing or increasing the flow energy as a consequence the evolution of bedforms distribution of sediment grain size and slopes are controlled by the flood type the morphological context i e high or low energy zones the morphology of the vegetated bar and the characteristics of vegetation conceptual models are proposed to account for local processes and evolution of tree groves the first model which describes the behaviour of groves during a single flood underlines the sediment deposition downstream of the trees and the trapping of bedload sediments during the falling stage of the flood the reorientation of current streams constrained by both local bed topography and woody vegetation induces transverse sediment fluxes for these water levels the control exerted by trees on small scale sedimentary processes becomes significant the second model shows the evolution of vegetation bands over several flood events in particular it describes the influence of vegetation on the fixing and reworking of bedload sediments during the falling limb of the hydrograph the coalescence of vegetation induced bedforms and the resulting morphological changes increase the deflection power of the vegetated bar inducing the deposition of finer sediments

scour around a monopile induced by directionally spread irregular waves in combination with oblique currents abstract the progressive expansion of offshore wind energy towards greater water depths demands for an optimization of foundation structure designs to a wider range of load conditions in offshore waters wind driven wave irregularity and directionality become important aspects of realistic sea states to further improve the scour prediction in marine environment a novel experimental study was conducted to investigate the influence of directionally spread 3d irregular waves on the scouring process the tests were carried out in the 3d wave basin of the ludwig franzius institute leibniz university hannover germany a monopile structure was simulated by a transparent pile made of acrylic glass the study was set up to progress understanding and explore dissimilarities of scour development and patterns induced by directionally spread 3d and unidirectional 2d waves the model tests were complemented by tests with superimposed oblique currents despite identical total wave energy in terms of m 0 between the directionally spread and unidirectional wave spectra minor but distinct differences in scour depth and rate could be observed which inevitably can only be attributed to the presence and role of the wave spread for wave only conditions final scour depths s d induced by directionally spread waves were on average 33 smaller than those for unidirectional waves furthermore final scour depths decreased with increasing wave spreading and displayed a growing dependency on kc numbers with increasing wave spreading in combined wave and current conditions of up to u c w

constriction effects in clear water scour at abutments the erosion process at a bridge abutment may be affected by the flow constriction when the abutment occupies a significant part of the flume width we devised a specific experimental campaign to investigate the effect of the obstruction ratio i e the ratio between the abutment length and the channel width on the erosion depth we performed homogeneous series of clear water scour experiments in each of which the obstruction ratio was the only parameter that varied the experimental results are presented in light of a dimensionless framework it was found that the effect of the obstruction ratio on the time development of local scour depth may not be large nevertheless significant effect can be observed even for relatively small values of the parameter during the earliest phases the latter are important for example for the step by step modeling of the erosion development under unsteady flow conditions we propose a simple equation to quantify the scour enhancement due to the flow constriction in c

modelling the bulk flow of a bedrock constrained multi channel reach of the mekong river siphandone southern laos the general nature of bulk flow within bedrock single channel reaches has been considered by several studies recently however the flow structure of a bedrock constrained large river with a multiple channel network has not been investigated previously the multiple channel network of the siphandone wetlands in laos a section of the mekong river was modelled using a steady one dimensional hydraulic model the river network is characterized by a spatially varying channel form leading to significant changes in the bulk flow properties between and along the channels the challenge to model the bulk flow in such a remote region was the lack of ideal boundary conditions the flow models considered both low flow high inbank and overbank flows and were calibrated using spot satellite sensor imagery and limited field data concerning water levels the application of the model highlighted flow characteristics of a large multi channel network and also further indicated the field data that would be required to properly characterize the flow field empirically important results included the observation that adjacent channels within the network had different water surface slopes for the same moments in time thus calibration data for modelling similar systems needs to account for these significant local differences further the in channel hydraulic roughness coefficient strongly varied from one cross section to the next manning s n range 0 01 to 0 10 these differences were amplified during low flow but persisted in muted form during high discharges

rate of growth and other features of the temporal development of pool bar complexes in meandering streams despite substantial research on various aspects of bed deformation in meandering streams no systematic effort has yet been made to establish the timescales of the process additionally in past experiments as a rule the bed was not monitored throughout the development which has left undocumented essential features of the growth of meandering pool bar complexes in this paper an effort is made to penetrate some of these aspects more specifically the five laboratory runs presented by the writers in their 2009 paper in this journal are revisited with the following two objectives 1 to reveal the rate of growth of the resulting meandering pool bar complexes and 2 to document the extent if any of upstream or downstream migration of developing pool bar complexes in examples of sufficiently wide streams in which the bed deformation is primarily because of the convective behavior of flow the runs were conducted in a 70 sine generated channel having rigid banks in each run the bed was allowed to

time scale for local scour at bridge piers the temporal development of clear water local scour depth at cylindrical bridge piers in uniform sand beds is considered new data are presented and used to quantify the influence of flow duration on the depth of local scour an equilibrium time scale t is defined the data show that both t and the equilibrium scour depth dse are subject to similar influences of flow and sediment parameters as might be expected because they are inherently interdependent a method is given for determination of the time for development of dse for a given pier sediment and approach flow velocity and the concomitant estimation of the scour depth at any stage during development of the equilibrium scour hole the results show that the scour depth after 10 of the time to equilibrium is between about 50 and 80 of the equilibrium scour depth depending on the approach flow velocity of local scour d se is rapidly attained in live bed conditions but rather more slowly in clear water conditions fig 1 clear water scour occurs for mean flow velocities up to the threshold velocity for bed sediment entrainment i e v vc while live bed scour occurs for v vc the maximum equilibrium scour depth dse max occurs at v vc in armored cobble or cohesive sediment bed streams multiple flood events may be required before the maximum clear water scour is reached this may take many years the equilibrium scour depth in live bed con ditions fluctuates due to the effects of bed form migration the dashed lines in fig 1 represent the temporal average scour depth under live bed conditions the diagram also shows the time taken te for the equilibrium scour depth to develop the equilibrium time te is the focus of this paper it increases rapidly with flow velocity under clear water conditions but then decreases rapidly for live bed scour existing equations for depth of local scour at bridge piers give the equilibrium depth and are therefore conservative re garding temporal effects for the live bed conditions that typ ically pertain in floods equilibrium scour depths are appro priate however where clear water scour conditions exist the equilibrium depth of scour may be overly conservative peak flood flows may last only a number of hours or a few days in the field and short floods have insufficient time to generate equilibrium depths for example bridge piers situated on the floodplain may be wet for periods of less than one day during a flood typically clear water conditions pertain at such sites the actual scour may be only a small fraction of the equilib rium scour depth which could take weeks to fully develop johnson and mccuen 1991 developed an analytical model to simulate the temporal process of local scour at piers the model was applied to a hypothetical bridge pier using a gen erated sequence of flood flows over a 75 year period the in dividual storms being of 24 h duration at the end of the 75 year period the scour depth was still increasing

flow and sediment dynamics in the vegetated secondary channels of an anabranching river the loire river france this study investigates the hydrological and sedimentological mechanisms occurring in the vegetated secondary channels of an anabranching river affected by incision the loire river france during and after flood events that occurred between 2000 and 2003 observations and measurements were performed on a vegetated secondary channel located in the study site of brehemont 790 km downstream the source morphological changes and sediment dynamics were analysed using low elevation airborne photographs topographic and bathymetric surveys and scour chains the hydraulic behaviour of the channel was also analysed by measurements performed on flow velocity and direction during different flood stages in order to quantify the influence of woody vegetation on flow resistance the roughness of bands of trees was determined from measurements performed on the field the impact of the disruption of armour layers on bedload pulses the variation of sedimentary processes during a single flood event and the fixation of bedforms by vegetation are all identified as key processes influencing the behaviour of the study channel topographic surveys demonstrate that sediment dynamics is substantial in the upstream part of the channel and that sediment budgets are different according to the temporal scale considered moreover an asymmetrical behaviour of the secondary channel is demonstrated reduced quantities of sediment deposited and preserved in the vegetated zones contrast with material by passing observed in the third order channels flow velocity and direction measurements indicate that these parameters vary according to the water level and to the morphological units of the channel pools riffles vegetated areas during low flows scouring and export of particles from the secondary channel are a consequence of reduced sediment supply from the main channel of the loire river for these water levels sedimentation occurs in pools where velocity and turbulence decrease whereas third order channels are subjected to erosion during high discharges large quantities of sediment available in the main channel feed the temporary stores formed by riffles and bars in the secondary channel the vegetated area located in the downstream part of the secondary channel deflects current trickles at low discharges and decreases flow velocity during high water levels the sedimentary accretion observed in this area exerts a feedback on flow and sedimentary processes d 2005 elsevier b v all rights reserved