local scale variability of seepage and hydraulic conductivity in a shallow gravel bed river seepage rate and direction measured with a seepage metre modified for use in flowing water were greatly variable along a 300 m reach of a shallow gravel bed river and depended primarily on the local scale bed topography the median value of seepage measured at 24 locations was 24 cm day but seepage measured at specific sites ranged from 340 to 237 cm day seepage also varied substantially over periods of hours to days and occasionally reversed direction in response to evolution of the sediment bed vertical hydraulic conductivity was related to seepage direction and was larger during upward seepage than during downward seepage with differences ranging from 4 to 40 in areas of active sediment transport to more than an order of magnitude in areas where current was too slow to mobilize bed sediment seepage was poorly related to hydraulic gradient measured over vertical distances of 0 3 m and appeared to be opposite the hydraulic gradient at 18 of the locations where both parameters were measured results demonstrate the scale dependence of these measurements in coarse grained hyporheic settings and indicate that hydraulic gradients should be determined over a much shorter vertical increment if used to indicate exchange across the sediment water interface published in 2009 by john wiley sons ltd

wind tunnel study of turbulent flow structure in the convective boundary layer capped by a temperature inversion abstract experiments on simulating the atmospheric convective boundary layer cbl capped by a temperature inversion and affected by surface shear were carried out in the thermally stratified wind tunnel of the institute of hydrology and water resources university of karlsruhe the tunnel is of the closed circuit type with a test section 10 m long 1 5 m wide and 1 5 m high the return section of the tunnel is subdivided into 10 layers each driven by its own fan and heating system by this means velocity and temperature profiles can be preshaped at the inlet of the test section which allows for the reproduction of developed cbl over comparatively short fetches the bottom heating is controlled to produce the constant heat flux through the floor of the test section the flow velocity components in the tunnel are measured with a laser doppler system for temperature measurements the resistance wire technique is employed a quasi stationary horizontally evolving cbl was reproduced in the tunnel wi

the contribution of shear to the evolution of a convective boundary layer the role of shear in the development and maintenance of a convective boundary layer is studied by means of observations and large eddy simulations less particular emphasis is given to the growth of the boundary layer and to the way in which this growth is affected by surface fluxes of heat and moisture and entrainment fluxes this paper analyzes the processes that drive the latter mechanism which accounts for approximately 30 of the growth of the mixing layer typically it is found that under pure convective conditions without shear the entrainment buoyancy flux at the inversion is about 220 of the surface buoyancy flux this value is widely used for entrainment rate closures in general circulation models the data collected during the atmospheric radiation measurement campaign allow one to introduce realistic vertical profiles and surface fluxes into the les runs and to compare the simulation results with the observed evolution of the boundary layer height during a convective situation with high entrainment rates and high geostrophic winds the analysis of the turbulent kinetic energy tke budget shows that the inclusion of geostrophic winds which produce shear at the surface and in the entrainment zone modifies the vertical profile of the various terms in the tke budget as a consequence the entrainment flux is enhanced resulting in increased growth of the boundary layer the numerical experiments and the observations enable one to validate the efficiency of earlier representations based on the tke equation which describe the evolution of the ratio between entrainment and surface buoyancy fluxes the proposed parameterization for the entrainment and surface buoyancy flux ratio b which includes the main buoyancy and shear contributions is in good agreement with the les results some aspects of the parameterization of b for instance the absence of entrainment flux and its behavior during the transition between convective to neutral conditions are discussed

estimating streambed parameters for a disconnected river evaluation of stream aquifer interaction and water balance for a catchment often requires specific information on streambed parameters such as streambed hydraulic conductivity seepage flux across the streambed and so on this paper describes a simple inexpensive instrument that is used to measure these streambed parameters under the condition of a stream disconnected from groundwater our method includes a seepage cylinder for simulation of river water depth the proposed method was applied to estimate the vertical hydraulic conductivity of a streambed and the changes in vertical seepage rate from stream to groundwater with varied stream water depth in the manasi river of xinjiang uygur autonomous region china the vertical hydraulic conductivities of the streambed determined from 12 sites along the manasi river vary from 1 01 to 29 m day where the stream disconnects from the groundwater the experimental results suggest that there are two kinds of relations between the vertical seepage rate and the simulated stream water depth one is a linear relation between the two variables with low reynolds numbers less than 10 the other is a nonlinear relation exponential relation between the two variables with larger reynolds numbers greater than 10 this second relationship is quite different from the traditional model that usually calculates the vertical seepage rate from stream to groundwater under the condition of disconnection using a linear relation darcy s law our results suggest that a linear relation can only be used for a limited range of river water depth this method gives a convenient tool for rapidly estimating the streambed hydraulic conductivity and the changes in the vertical seepage rate across streambed with varied stream water depths for the case of a stream disconnected from groundwater copyright 2013 john wiley sons ltd

convective boundary layer structure in the presence of wind following swell the marine boundary layer is known to be influenced by fast long ocean swell waves travelling away from their generation area where they were initiated by momentum transferred to the ocean wave field during storms the atmospheric boundary layer during wind following swell and various stability states has been investigated using large eddy simulation les data the dominant energy containing motions in the near neutral atmospheric boundary layer over flat terrain are known to be dominated by near ground shear induced regions of high and low speed flow wind fields and momentum fluxes from les for swell dominated situations have been used to interpret field measurements suggesting that these motions are disrupted by effects related to the underlying wave field in the presence of swell waves statistical analysis and visualization are used to further describe the effects of stratification during swell for convective boundary layer winds and fluxes a mechanism for transport of momentum to the upper levels of the boundary layer is suggested from interpretation of les data coherent detached eddies from the directly wave induced motions near the surface are found to maintain an upward momentum transfer this mechanism is found to strengthen during stronger swell conditions and also during slightly convective conditions in this way it is argued that processes related to both the wave field and surface convection can have a significant influence on the global structure of neutral and convective boundary layers during swell this has implication for the turbulence length scales during wind following swell copyright 2012 royal meteorological society

blueprint for a coupled model of sedimentology hydrology and hydrogeology in streambeds the streambed constitutes the physical interface between the surface and the subsurface of a stream across all spatial scales surface water groundwater interactions are controlled by the physical properties of the streambed streambed properties such as topography or hydraulic conductivity are continuously altered through erosion and sedimentation processes recent studies from the fields of ecology hydrogeology and sedimentology provide field evidence that sedimentological processes themselves can be heavily influenced by surface water groundwater interactions giving rise to complex feedback mechanisms between sedimentology hydrology and hydrogeology more explicitly surface water groundwater exchanges play a significant role in the deposition of fine sediments which in turn modify the hydraulic properties of the streambed we explore these feedback mechanisms and critically review the extent of current interaction between the different disciplines we identify opportunities to improve current modeling practices for example hydrogeological models treat the streambed as a static rather than a dynamic entity while sedimentological models do not account for critical catchment processes such as surface water groundwater exchange a blueprint for a new modeling framework is proposed that bridges the conceptual gaps between sedimentology hydrogeology and hydrology specifically this blueprint 1 fully integrates surface subsurface flows with erosion transport and deposition of sediments and 2 accounts for the dynamic changes in surface elevation and hydraulic conductivity of the streambed the opportunities for new research within the coupled framework are discussed

the effect of streambed topography on surface subsurface water exchange in mountain catchments a numerical hydrological simulation suggested that water exchange between stream channels and adjacent aquifers is enhanced by convexities and concavities in streambed topography at st kevin gulch an effluent stream in the rocky mountains of colorado subsurface hydraulic gradients and movement of ionic tracers indicated that stream water was locally recharged into well defined flow paths through the alluvium stream water filled flow paths in the alluvium referred to as substream flow paths returned to the stream a short distance downstream 1 to 10 m recharge to the substream flow paths occurred where stream water slope increased at the transition from pools 1 to steeper channel units 5 20 return of substream flow paths to the stream occurred where stream water slope decreased at the transition from steeper channel units to pools a net water flux calculation is typically used to characterize water and solute fluxes between surface and subsurface zones of catchments along our study reach at st kevin gulch the net inflow of water from subsurface to stream 1 6 ml s 1 m 1 underestimated the gross inflow 2 7 ml s 1 m 1 by 40 the influence of streambed topography is to enhance hydrological fluxes between stream water and subsurface zones and to prolong water sediment contact times these effects could have important consequences for solute transport retention and transformation in catchments

a case study of convective boundary layer development during ihop_2002 numerical simulations compared to observations abstract results are presented from a combined numerical and observational study of the convective boundary layer cbl diurnal evolution on a day of the international h2o project ihop 2002 experiment that was marked by the passage of a dryline across part of the oklahoma and texas panhandles the initial numerical setup was based on observational data obtained from ihop 2002 measurement platforms and supplementary datasets from surrounding locations the initial goals of the study were as follows i numerical investigation of the structure and evolution of the relatively shallow and homogeneous cbl east of the dryline by means of large eddy simulation les ii evaluation of les predictions of the sheared cbl growth against lidar observations of the cbl depth evolution and iii comparison of the simulated turbulence structures with those observed by lidar and vertically pointing radar during the cbl evolution in the process of meeting these goals complications associated with comparisons betwee

coriolis effects in homogeneous and inhomogeneous katabatic flows katabatic flows along a planar slope in a viscous stably stratified fluid are investigated analytically and numerically with an emphasis on flow features arising from the action of the coriolis force two idealized flow types are considered turbulent flow along a uniformly cooled slope and two dimensional laminar flow induced by a cold strip of finite width running down the slope in the case of turbulent flow along a uniformly cooled slope the downslope velocity exhibits a boundary layer structure but the cross slope velocity and buoyancy fields spread inexorably upward this behaviour is qualitatively similar to that found in laminar flows along a uniformly cooled slope in contrast in the two dimensional strip flow a steady state is reached in which the cross slope wind and buoyancy fields vanish far above the slope but the downslope and slope normal velocity fields do not vanish these latter two flow components comprise two purely horizontal along isentrope counter flowing currents an upslope current entering the top of the boundary layer on one side of the strip and a downslope current flowing out of the boundary layer into the environment on the other side of the strip the slope normal vorticity associated with these currents originates in the stretching of planetary vorticity in a broad zone of convergent flow over the cold strip copyright 2008 royal meteorological society

effects of sediment transport and seepage direction on hydraulic properties at the sediment water interface of hyporheic settings summary relations between seepage flux and hydraulic properties are difficult to quantify in fluvial settings because of the difficulty in measuring these variables in situ tests conducted in a 1 5 m diameter by 1 5 m tall sediment filled tank indicate that hydraulic gradient increased and hydraulic conductivity k decreased following the onset of downward seepage but both parameters were little changed following the onset of upward seepage reductions in k during downward seepage were more pronounced when surface water current was sufficient to mobilize sediment on the bed averaged ratios of k determined during upward seepage to k determined during downward seepage k up k down through a sand and gravel bed increased from 1 4 to 1 7 with increasing surface water velocity and decreased to slightly greater than 1 when the sediment bed became fully mobile k up k down for tests conducted with a silt veneer on the bed surface was greater than 2 for all but the fastest surface water velocities substantial reductions in k also were associated with a silt floc that formed on the bed surface during and following test runs although the silt floc was typically less than 0 5 mm in thickness most of the hydraulic gradient was distributed across this thin layer k of the thin silt floc was reduced by two to three orders of magnitude relative to the underlying sediment directional bias in k and relation between k and surface water velocity require the presence or absence of a layer of lower k sediment at or near the bed surface without which no reduction in k and corresponding increase in hydraulic gradient can occur at the bed surface the lack of prior observation of the consistent bias in k associated with seepage direction is somewhat surprising given the numerous studies where k has been measured in fluvial settings but may be explained by the small value of the bias relative to the typical uncertainty associated with field determinations of k if shown to exist in field settings this bias and its relation to fluvial processes will be relevant to many studies conducted in hyporheic settings that require determination of fluxes across the sediment water interface

investigation of flow upstream of orifices a physical model study is conducted to investigate the flow field upstream of orifices in particular new experimental data for the upstream flow pattern resulting from multiple orifices an orifice near a free surface and a large orifice where the pressure gradient across the orifice cannot be ignored are collected and presented a new potential flow solution for flow behind orifices is developed to include pressure gradient effects as well as to accurately superpose the solution due to multiple orifices and determine a solution close to orifices the proposed solution compares well with the measured data for multiple orifices and for an orifice near a free surface for a large orifice the skew in the velocity profile in the vertical direction due to the pressure gradient is accurately predicted

assimilation of historical head data to estimate spatial distributions of stream bed and aquifer hydraulic conductivity fields management of water resources in alluvial aquifers relies mainly on understanding interactions between hydraulically connected streams and aquifers numerical models that simulate this interaction often are used as decision support tools in water resource management however the accuracy of numerical predictions relies heavily on the unknown system parameters i e stream bed conductivity and aquifer hydraulic conductivity which are spatially heterogeneous and difficult to measure directly this paper employs an ensemble smoother to invert groundwater level measurements to jointly estimate spatially varying streambed and alluvial aquifer hydraulic conductivity along a 35 6 km segment of the south platte river in northeastern colorado the accuracy of the inversion procedure is evaluated using a synthetic experiment and historical groundwater level measurements with the latter constituting the novelty of this study in the inversion and validation of high resolution fields of streambed and aquifer conductivities results show that the estimated streambed conductivity field and aquifer conductivity field produce an acceptable agreement between observed and simulated groundwater levels and stream flow rates the estimated parameter fields are also used to simulate the spatially varying flow exchange between the alluvial aquifer and the stream which exhibit high spatial variability along the river reach with a maximum average monthly aquifer gain of about 2 3 m3 day and a maximum average monthly aquifer loss of 2 8 m3 day per unit area of streambed m2 these results demonstrate that data assimilation inversion provides a reliable and computationally affordable tool to estimate the spatial variability of streambed and aquifer conductivities at high resolution in real world systems

physical modeling on sand erosion around defective sewer pipes under the influence of groundwater abstractthis paper studies soil erosion problem around cracked sewer pipes which is usually associated with groundwater infiltration in sewer systems four factors soil particle size defect size water head and soil height that are associated with the erosion process were investigated experimentally two experimental setups were used a cylindrical bin with a circular outlet at the bottom center to simulate the erosion process and a rectangular box with the outlet on the side wall to show the internal erosion process the quantitative influences of the four factors on the erosion void shape volume and length were analyzed it is found that the geometry of the erosion void is determined by water head and soil height relationships were developed for calculating the water discharge rate sand discharge rate and erosion void diameter

numerical and laboratory study of a horizontally evolving convective boundary layer part i transition regimes and development of the mixed layer results are presented from a large eddy simulation les and wind tunnel study of the turbulence regime in a horizontally evolving sheared atmospheric convective boundary layer cbl capped by a temperature inversion the wind tunnel part of the study has been conducted in the thermally stratified tunnel of the university of karlsruhe for the numerical part a modified les procedure that was originally designed for simulation of the horizontally homogeneous atmospheric cbl has been employed the study focuses on the transition between the neutrally buoyant boundary layer in the initial portion of the wind tunnel flow and a quasi homogeneous convectively mixed layer developing downwind the character of the transition between the two boundary layers and the associated changes in the turbulence structure are found to be strongly dependent on the magnitude and distribution of disturbances in the flow at the entrance of the wind tunnel test section for all simulated inflow conditions the transition is preceded by accumulation of potential energy in the premixed cbl the eventual energy release in the transition zone leads to turbulence enhancement that has a form of turbulence outbreak for particular flow configurations the numerically simulated cbl case with temperature fluctuations introduced in the lower portion of the incoming flow appears to be the closest to the basic cbl flow case studied in the wind tunnel second order turbulence statistics derived from the les are shown to be in good agreement with the wind tunnel measurements main features of transition including the turbulence enhancement within the transition zone are successfully reproduced by the les

flow field upstream of an orifice under fixed bed and equilibrium scour conditions abstractflow behavior upstream of a circular orifice with the invert at the bed level initially is investigated under fixed bed and equilibrium scour mobile bed conditions the experiments are performed under three different constant heads and sediment sizes longitudinal and vertical components of velocity are measured in the vertical and horizontal planes passing through the center of the orifice the velocity decay upstream of the orifice along the centerline of the orifice is slower for the fixed bed case than for the unbounded orifice under equilibrium scour condition the velocity decay transitions from the velocity profile for the unbounded orifice to the fixed bed condition the longitudinal velocity profiles in the horizontal plane at different locations upstream of the orifice are found to be similar for both fixed and mobile bed cases the longitudinal velocity profiles in the vertical plane are also found to be similar for each case the maximum velocity is found to occur below the cent

geomorphology and endangered fish habitats of the upper colorado river 2 linking sediment transport to habitat maintenance alluvial reaches of the colorado river near grand junction colorado provide important habitat for the endangered colorado squawfish this paper examines recent changes in the geomorphology of the colorado river and addresses the question of what can be done to improve existing fish habitats observations of channel change during periods of above average runoff from 1993 through 1995 indicate minor scour and fill of the main channel but more significant scour and enlargement of side channel and backwater habitats and flushing of fine sediment from the bed modeled relations between discharge and dimensionless shear stress indicate that the threshold of bed load transport 0 03 occurs at about half the bank full discharge these flows are exceeded about 26 days per year on average the bank full discharge produces an average of 0 047 and is exceeded about 8 days per year the width and depth of the channel thus appear to be set by a discharge that produces an average boundary shear stress of 1 5 times greater than the critical shear stress the effective discharge is slightly less than the bank full discharge and approximately 80 of the total sediment load is carried by the highest 10 of all discharges

flow upstream of orifices and sluice gates potential flow solutions of a point line sink are extended to study the velocity field upstream of a finite size orifice and sluice gate it is found that in the near field zones the iso velocity surfaces appear to be semiellipsoidal while in the far field zones they become hemispheres the shape and size of the orifice sluice gate were found to be of no effect on the flow behavior beyond a certain distance the development of velocity profile away from the orifice and sluice gate is examined and the effects of water depth are studied the results of this study compare very well with other numerical and experimental studies and provide a general understanding of the flow field upstream of orifices and sluice gates

dynamics of sheared convective boundary layer entrainment part i methodological background and large eddy simulations abstract the reported study examines the dynamics of entrainment and its effects on the evolution of the dry atmospheric convective boundary layer cbl when wind shear is present the sheared cbl can be studied by means of direct measurements in the atmosphere laboratory studies and numerical techniques the advantages and disadvantages of each technique are discussed in the present paper which also describes the methodological background for studying the dynamics of entrainment in sheared cbls for the reported study large eddy simulation les was chosen as the primary method of convective entrainment investigation twenty four les runs were conducted for cbls growing under varying conditions of surface buoyancy flux free atmospheric stratification and wind shear the simulations were divided into three categories cbl with no mean wind ns cbl with a height constant geostrophic wind of 20 m s 1 gc and cbl with geostrophic wind shear gs in the simulated cases the sheared cbls grew fastes

spatial variability in streambed hydraulic conductivity of contrasting stream morphologies channel bend and straight channel streambed hydraulic conductivity is one of the main factors controlling variability in surface water groundwater interactions but only few studies aim at quantifying its spatial and temporal variability in different stream morphologies streambed horizontal hydraulic conductivities kh were therefore determined from in stream slug tests vertical hydraulic conductivities kv were calculated with in stream permeameter tests and hydraulic heads were measured to obtain vertical head gradients at eight transects each comprising five test locations in a groundwater dominated stream seasonal small scale measurements were taken in december 2011 and august 2012 both in a straight stream channel with homogeneous elevation and downstream of a channel meander with heterogeneous elevation all streambed attributes showed large spatial variability kh values were the highest at the depositional inner bend of the stream whereas high kv values were observed at the erosional outer bend and near the middle of the channel calculated kv values were related to the thickness of the organic streambed sediment layer and also showed higher temporal variability than kh because of sedimentation and scouring processes affecting the upper layers of the streambed test locations at the channel bend showed a more heterogeneous distribution of streambed properties than test locations in the straight channel whereas within the channel bend higher spatial variability in streambed attributes was observed across the stream than along the stream channel copyright 2014 john wiley sons ltd

surface based fractional transport rates mobilization thresholds and partial transport of a sand gravel sediment twenty eight coupled observations of flow transport and bed surface grain size distribution were made in a laboratory flume using a wide range of flows and a sediment with a very poorly sorted bimodal grain size distribution these observations permit the transport rates of individual size fractions to be scaled by the proportion of each size immediately available for transport on the bed surface the key to our observations is the use of a sediment in which each size fraction has been painted a different color which permits reliable repeatable and nondestructive measurement of the bed surface grain size distribution from photographs of the bed surface at a given flow the fractional transport rates may be divided into two parts a finer grained portion within which fractional transport rates are a function only of their proportion on the bed surface and total transport rate and a coarser grained portion for which fractional transport rates also depend on the proportion of individual grains within a fraction that remain essentially immobile throughout the experimental run we define the latter condition as one of partial transport and observe that the grain size separating partial and fully mobilized transport consistently increases with flow strength complete mobilization of a size fraction occurs at roughly twice the shear stress necessary for incipient motion of that fraction zones of partial and full mobility are quite distinct when fractional transport rates are scaled by the bed surface grain size distribution although a region of partial transport is evident when these data and other experimental and field observations are scaled by the bulk grain size distribution of the sediment bed critical shear stresses for the incipient motion of individual fractions in our experimental sediment vary over an order of magnitude a result strongly in contrast to many earlier observations but consistent with our observations of incipient motion in sediments with bimodal grain size distributions

interplay of stream subsurface exchange clay particle deposition and streambed evolution 1 hyporheic exchange mixing of stream water with pore water beneath the stream is responsible for the transport of many ecologically relevant substances across the stream subsurface interface this process is dependent on both the streamflow conditions and sedimentary properties but the complex nature of fluvial systems has presented a barrier to the development of mechanistic understanding of the dynamics of stream subsurface interactions this work presents the results of a controlled study to examine in detail the relationship between stream subsurface exchange fluxes delivery of suspended sediments to the hyporheic region fine particle accumulation in the streambed and alteration of sedimentary properties laboratory flume experiments were used to observe kaolinite clay deposition in a sand bed and the resulting alteration of hyporheic exchange fluxes solute and suspended sediment exchange with clean sand beds is predicted well by a fundamental model for bed form driven advective pumping exchange however substantial accumulation of clay in the bed causes an alteration of the pore water flow environment which reduces both water flux across the stream subsurface interface and subsequent particle deposition measurement of bulk solute exchange and direct observation of clay accumulation in the bed both indicate that transported fine particles are preferentially removed near the stream subsurface interface clogging of inflow regions produces heterogeneous subsurface clay deposits even when the bed is initially homogeneous this behavior is also predicted by the fundamental colloid pumping model these results contradict the accepted view that suspended sediments will generally deposit in the deepest regions of the bed and thus clog the bed from the bottom upward our results indicate that clogging of the streambed surface will often isolate deeper regions of the bed from the streamflow so that even relatively low amounts of suspended sediments can substantially degrade streambed habitat

3d numerical flow model for fish diversion studies at wanapum dam a numerical flow model is developed for evaluating different fish bypass systems at the wanapum dam on the columbia river wash the model solves the three dimensional reynolds averaged navier stokes equations combined with the k e turbulence equations the governing equations are formulated in generalized curvilinear coordinates and the bed roughness is introduced through wall functions the paper discusses the calibration of the numerical model and its application in the design of a bypass system at wanapum dam on the columbia river wash the numerical model is validated with data taken in a 1 16 scale hydraulic model of a powerhouse intake bay the model is found to reproduce accurately all of the flow characteristics that are believed to be important for the evaluation of a fish diversion system

upstream hydraulics of a run of the river hydropower facility for fish entrainment risk assessment abstractrun of the river hydropower is a typical form of power generation in mountainous regions this study investigates the velocity field upstream of aberfeldie dam a run of the river facility on the bull river near wardner british columbia canada as is typical for these types of run of the river facilities the headpond of this dam is very shallow approximately 1 m deep because of long term sediment deposition near the facility s power intakes a large scour hole has been created because of increased velocities induced by the intakes this study conducted field measurements and developed a computational fluid dynamic cfd model to investigate the velocity field under various flow conditions the cfd solver shows in a simplistic way that flow induced bed shear stress likely causes a fluctuating dynamic bed form that varies by season and the distance upstream so that a consistent velocity pattern is seen across the entire headpond the velocity field generated by the cfd solver is used to eval

application of a perturbation recycling method in the large eddy simulation of a mesoscale convective internal boundary layer an arrangement of boundary conditions is described and demonstrated that facilitates the large eddy simulation les of inhomogeneous boundary layers such as internal boundary layers in addition to the domain where the internal boundary layer develops the method requires a section of domain over the upwind surface that is of the order of 10 boundary layer thicknesses and thus similar in size to that needed for the les of the upwind boundary layer in addition to periodic lateral closed upstream and open downstream boundary conditions a simple and efficient perturbation recycling technique which follows from one of lund wu and squires is used to generate a steady supply of fully developed turbulence from the inflow boundary the arrangement is used to simulate a convective internal boundary layer during a cold air outbreak over water results show the method consistently produces a solution that is homogeneous over the upwind surface and inhomogeneous over the downwind surface and that the statistics are stationary after spinup the sensitivity to the placement of the outflow boundary is tested and examples of instantaneous and mean fields generated by the simulations are shown

large eddy simulation of the convective atmospheric boundary layer abstract large eddy simulations of a free convective atmospheric boundary layer with an overlying capping inversion are considered attention is given to the dependence of the results upon the various factors influencing the simulation the subgrid model the domain size and the mesh resolution by providing artificial constraints upon the convection the results also provide extra insight into the underlying dynamics the gross features of the boundary layer such as the overall energy budget are not sensitive to the details of the simulations but a number of important factors are revealed it has been found that near the surface the subgrid diffusivity must be larger than is usually supposed in order for the vertical velocity skewness to have the correct sign this region of the flow has a significant subgrid scale heat flux and it seems that the subgrid model requires improvement in such cases a revised model which under statically unstable conditions allows the mixing length of the subgrid scale tu

changes in streambed sediment characteristics and solute transport in the headwaters of valley creek an urbanizing watershed abstract changes in streambed sediments were monitored in conjunction with q series of conservative solute tracer injections over a 2 year period to assess the effects of urbanization on two streams in the valley creek watershed located in chester county pennsylvania approximately 30 km west of philadelphia the modeling package otis was used to analyze the solute transport behavior comparison of the results from the two streams demonstrates that the fine sediment fraction of the streambed controls hydraulic conductivity and transient storage exchange in this gravel and cobble bed piedmont system one site had a narrow 10 40 m riparian corridor of mowed lawn and woody brush at this site the silt clay fraction d d

inflow seepage influence on straight alluvial channels a flume study was conducted to identify and analyze the effects of inflow seepage on flow in straight alluvial open channels the effect of inflow seepage on energy slope water surface depth velocity sediment transport scour bed forms and resistance to flow was investigated the results of this investigation indicated that inflow seepage increases localized mean channel velocity energy slope and stream power in the zone of inflow the water surface depth decreased for subcritical flow and remained nearly constant for supercritical flow during inflow conditions sediment transport was slightly increased when inflow was introduced in the bed inflow seepage did not appear to enhance channel scour inflow seepage significantly enhanced bed form transformation and the bed roughness inflow seepage also caused dunes to become longer flatter and move more erratically in the reach where inflow occurred it was concluded that inflow seepage could significantly influence the channel hydraulics stream po

effects of suspended sediment characteristics and bed sediment transport on streambed clogging fine sediment deposition in streambeds can reduce pore water fluxes and the overall rate of hyporheic exchange producing deleterious effects on benthic and hyporheic ecological communities to increase understanding of the factors that control the reduction of hyporheic exchange by fine sediment deposition we conducted experiments in a laboratory flume to observe changes in the rates of solute exchange and kaolinite clay deposition as substantial amounts of kaolinite accumulated in the streambed two long term experiments were conducted with durations of 14 days and 29 days use of a laboratory flume system allowed steady stream flow conditions to be maintained throughout both experiments and alternating injections of known quantities of kaolinite and a sodium chloride tracer were used to assess the effect of clay accumulation on hyporheic exchange directly in the first experiment there was no bed sediment transport and kaolinite deposition formed a highly clogged near surface layer that greatly reduced hyporheic exchange application of a fundamental model for advective hyporheic exchange indicated that the effective permeability and porosity of the streambed decreased substantially during the course of the experiment in the second experiment the kaolinite was prepared with different surface properties to be more mobile and the experiment was conducted with a small degree of bed sediment transport as a result no distinct clogged layer developed and the rate of hyporheic exchange was found to remain approximately constant throughout the experiment 29 days these results indicate that increasing fine sediment loads e g those that occur from changes in land use can have substantially different impacts on hyporheic exchange and associated ecological processes depending on the stream flow conditions the rate and frequency of bed sediment transport and the extent of interaction of the introduced fines with bed sediments copyright 2004 john wiley sons ltd