predicting physical equations of soil detachment by simulated concentrated flow in ultisols subtropical china soil detachment in concentrated flow is due to the dislodging of soil particles from the soil matrix by surface runoff both aggregate stability and shear strength of the topsoil reflect the erosion resistance of soil to concentrated runoff and are important input parameters in predicting soil detachment models this study was conducted to develop a formula to predict soil detachment rate in concentrated flow by using the aggregate stability index as root density rd and saturated soil strength s in the subtropical ultisols region of china the detachment rates of undisturbed topsoil samples collected from eight cultivated soil plots were measured in a 3 8 m long 0 2 m wide hydraulic flume under five different flow shear stresses 4 54 9 38 15 01 17 49 and 22 54 pa the results indicated that the stability index as was well related with soil detachment rate particularly for results obtained with high flow shear stress 22 54 pa and the stability index as has a good linear relationship with concentrated flow erodibility factors kc there was a positive linear relationship between saturated soil strength s and critical flow shear stress c for different soils a significant negative exponential relationship between erodibility factors kc and root density rd was detected this study yielded two prediction equations that allowed comparison of their efficiency in assessing soil detachment rate in concentrated flow the equation including the root density rd may have a better correlation coefficient r2 0 95 it was concluded that the formula based on the stability index as saturated soil strength s and root density rd has the potential to improve methodology for assessing soil detachment rate in concentrated flow for the subtropical chinese ultisols copyright 2012 john wiley sons ltd

the process of formation of ocean crust ocean crust is the outermost layer of earth under the oceans it is separated from the underlying mantle by a seismic transition zone called the moho a widely held view is that the moho represents a petrologic change from basaltic type rocks to a mantle composed mostly of olivine and pyroxene according to this view crust is formed by a steady segregation of basaltic melt derived from partial melting of the mantle into a crustal magma chamber wherein cooling and crystallization bring about steady state accretion to the continuously spreading plates there is sufficient disagreement between the predictions of this hypothesis and marine geophysical data to cause one to doubt the validity of this formation process at least two other processes are more compatible with the geophysical data in one the crust is formed from the episodic injection of basaltic dikes from a mantle reservoir and the moho is a primary petrologic boundary in the other the crust is treated as a mechanical boundary layer in which thermal contraction results in cracking by comparison in the mantle thermal contraction is accommodated by flow the upper part of the crust is formed from episodic extrusion and intrusion of basaltic melt the lower crust is formed by rapid hydrothermal alteration of mantle that may be continuously or episodically injected by viscous flow at temperatures below the melting temperature

quantitative assessment of the piping erosion susceptibility of loess derived soil horizons using the pinhole test abstract the pinhole test is an empirical test based on the qualitative evaluation of the dispersivity colloidal erodibility of compacted fined grained soils this paper evaluates the pinhole test device for the quantitative assessment of the susceptibility of soil horizons to piping assesses the effects of hydraulic head water quality antecedent soil moisture content and soil disturbance on the hydrological and erosion response during pipeflow and formulates recommendations for the application of the pinhole test to study piping erosion processes in total 255 pinhole test experiments were conducted the results obtained from disturbed soil samples ap horizon indicate i a significant positive linear increase in pipeflow discharge qw and sediment discharge qs with increasing hydraulic head ii no relationship between qw and antecedent soil moisture content asm but significant and negative relationship between qs and asm and iii significantly higher qs for distilled water than for tap water the use of disturbed soil samples resulted in significantly more rapid flow responses and lower sediment discharges than for undisturbed soil samples the pinhole test experiments performed on different loess derived soil horizons show that the plowed topsoil ap and the underlying clay enriched horizon bt are at least two times less susceptible to piping than the decalcified c1 and the calcareous horizons c2 this confirms field observations indicating that loess derived soil horizons are very susceptible for piping erosion especially when the c horizon crops out this study demonstrates that the pinhole test is suitable for assessing the susceptibility of soil horizons to piping in a quantitative way i e qw qs the time to outflow and the cross section of the pipe after the test we recommend the use of i two different hydraulic heads i e 180 and 1020 mm ii distilled water iii contrasting antecedent soil moisture contents of the soil samples to be tested and iv different soil sampling methods to investigate the effect of soil disturbance and particularly to obtain a better understanding of the sediment response during pipeflow

mechanisms of lithospheric extension at mid ocean ridges summary we examine the extensional deformation of oceanic plates at mid ocean ridges especially within an axial yield zone where pervasive faulting occurs thermal models of ridges are developed which include the effects of lithospheric thickening on the mantle flow the heat of magmatic crustal accretion at the ridge axis and the hydrothermal cooling due to seawater circulation in the crust when hydrothermal circulation occurs the brittle lithospheric plate at slow spreading ridges could be as thick as 8 9 km thicker than the crust while the plate at fast spreading ridges is only 1 2 km for a typical slow spreading ridge several kilometres of plate thickening are expected within a distance of 15 km from the ridge axis when subjected to the extensional force due to horizontal stretching shear failure by normal faulting will occur pervasively in an axial zone where the lithospheric plate is the thinnest adopting perfectly plastic rheology as a continuum description of deformation on the distributed faults we obtain approximate solutions for the stress distribution in the yield zone within this yield zone sea floor topography increases significantly away from the ridge axis so that the resuiting gravity sliding force balances the differential horizontal extensional force due to the thickening of the lithospheric plate basal stresses induced by the viscously deforming asthenosphere could significantly influence the stresses inside the lithospheric plate only if the mantle viscosity beneath the ridge is on the order of 10 pa s significantly higher than generally accepted values of 10 pa s model calculations reveal that although the seafloor topography at the mid atlantic ridge at 13 15 on is regionally compensated it is locally supported by stresses in the lithospheric plate the deviation of the lithospheric plate from its thermal isostatic equilibrium position can be explained by necking due to plastic stretching in the axial yield zone and elastic deflection of strong plates outside the yield zone the best fit models require the yield zones to have a half width of 10 15 km we find systematic variations in the gravity and topography of the east pacific rise which indicate strong influence of plate spreading rate on the ridge thermal and mechanical structure at the 16 17 on area where the half spreading rate is 4 3 cm yr a prominent axial topographic high and an axial mantle bouguer gravity low exist implying a crustal or sub crustal low density body the gravity low disappears at the 20 21 n area where the half spreading rate is 3 6 cm yr as the plate spreading rate decreases from 4 3 cm yr at the 16 17 on area to 2 7 cm yr at the 22 23 on area axial ridge topography changes from higher than the thermal isostatic equilibrium position to lower than the isostatic position the low axial topography at the 22 23 n area can be explained by the existence of a low amplitude median valley

geomorphic threshold conditions for ephemeral gully incision abstract ephemeral gully erosion in cultivated land is an important source of sediment that is frequently being overlooked and not accounted for in soil erosion studies furthermore little is known about the factors controlling ephemeral gully erosion in this paper the available information on the initiation and location of ephemeral gullies is summarised focusing on the relationship between the upslope drainage area a and the critical slope gradient s cr for ephemeral gully initiation by plotting the non linear relationship between critical slope gradient measured immediately upstream of the incision head and drainage area at the incision head for gullied sites it was possible to draw a straight line on log log paper through the lower most points for each of the available datasets representing a critical slope area relationship for incision consequently below this line no incision occurs this line or critical relation can also be written as a power function between critical slope and area although many factors vary between the different datasets the exponent of the drainage area relationship 0 4 showed very little variation the observed critical slope area relationship can be related to a simple model of channel initiation by overland flow furthermore this relationship can be used to identify potentially unstable sites where control measures should be undertaken

prediction of concentrated flow width in ephemeral gully channels empirical prediction equations of the form w d aq b have been reported for rills and rivers but not for ephemeral gullies in this study six experimental data sets are used to establish a relationship between channel width w m and flow discharge q m 3 s 1 for ephemeral gullies formed on cropland the resulting regression equation w d 2 51 q 0 412 r 2 d 0 72 n d 67 predicts observed channel width reasonably well owing to logistic limitations related to the respective experimental set ups only relatively small runoff discharges i e q 0 02 m 3 s 1 were covered using field data where measured ephemeral gully channel width was attributed to a calculated peak runoff discharge on sealed cropland the application field of the regression equation was extended towards larger discharges i e 5 10 4 m 3 s 1 q 0 1m 3 s 1 comparing w q relationships for concentrated flow channels revealed that the discharge exponent b varies from 0 3 for rills over 0 4 for gullies to 0 5 for rivers this shift in b may be the result of i differences in flow shear stress distribution over the wetted perimeter between rills gullies and rivers ii a decrease in probability of a channel formed in soil material with uniform erosion resistance from rills over gullies to rivers and iii a decrease in average surface slope from rills over gullies to rivers the proposed w q equation for ephemeral gullies is valid for sealed cropland with no significant change in erosion resistance with depth two examples illustrate limitations of the w q approach in a first example vertical erosion is hindered by a frozen subsoil the second example relates to a typical summer situation where the soil moisture profile of an agricultural field makes the top 0 02 m five times more erodible than the underlying soil material for both cases observed w values are larger than those predicted by the established channel width equation for concentrated flow on cropland for the frozen soils the equation w d 3 17 q 0 368 r 2 d 0 78 n d 617 was established but for the summer soils no equation could be established copyright 2002 john wiley sons ltd

catchment scale analysis of the effect of topography tillage direction and unpaved roads on ephemeral gully incision high resolution airphotographs and digital elevation models dems were used to study areas prone to gully incision in a small agricultural catchment in northern israel data are analyzed to understand better the integrated effect of environmental and human factors on gully incision the effect of flow accumulation slope unpaved roads density and tillage direction on gully width and length is found discontinuous and topography in the form of the topographic threshold was found to be the most dominant factor on gully incision in the catchment scale the topographic threshold coefficients observed in yehezkel catchment were lower than those found in previous studies due to the effect of large rainfall events during the early winter season when the soil in fields is still unprotected by crops in addition a survey of previous studies shows that in cultivated areas there is relationship between the topographic threshold coefficients a multiplier and b exponent the influence of unpaved roads and tillage direction on gully incision was also found significant and consistent during the two seasons observed while the non parametric test showed that it added to the prediction in 15 of the gullies incision points density plot show that of the two parameters the unpaved roads were found to have greater influence on gully head location than tillage direction studying the phenomenon on the catchment scale allows better understanding of the effect of spatial variation on gully head location copyright 2009 john wiley sons ltd

a long flume study of the dynamic factors affecting the resistance of a loamy soil to concentrated flow erosion experiments were carried out in a 20 m long flume to assess the variation of the runoff erosion resistance of a loamy soil as a function of initial moisture content and compaction the results of seven experimental runs show that the runoff erosion resistance of a loamy material is extremely sensitive to variations in initial moisture content and to a somewhat lesser extent in bulk density the very low resistance to runoff erosion of initially dry material is explained by structural changes which take place at the time of wetting slaking and microfissuration cause a considerable decrease of the soil s shear strength and therefore of its resistance to runoff erosion a first analysis shows that during a given rainfall event initially dry soils may well show significantly more erosion than initially wet soils despite the increase of infiltration with decreasing initial moisture content

an ocean bottom gravity study of the southern juan de fuca ridge we use seafloor and sea surface gravity data to model density structure along the southern juan de fuca ridge we determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models of 63 seafloor gravity measurements obtained 42 observations were distributed over the 3 km left lateral overlapping rift zone orz located at latitude 45 03 n and separating the juan de fuca ridge into the cleft and vance segments a 21 measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44 52 n a region of linear ridge geometry on the cleft segment these seafloor gravity measurements which are sensitive to shallow crustal density variations were augmented by roughly 800 km of sea surface gravity measurements using a bathymetry bouguer anomaly analysis of the seafloor gravity data we determined the average density of the shallow roughly upper 2 km oceanic crust at these two locations to be 2630 kg m 3 50 kg m 3 within the uncertainties of the density determinations there is no difference between the average shallow oceanic crustal density at the linear cleft segment and at the cleft vance orz using the seafloor measurements we modeled the study area s underlying density to provide constraints on its fine scale structure localized porosity of up to 17 within the upper 500 m of oceanic crust layer 2a provide a possible explanation for the observed gravity anomalies at the cleft segment two dimensional density models of the crust underlying the linear cleft segment 44 52 n show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis for the cleft vance orz three dimensional modeling suggests low density material between the rifts in agreement with thickened layer 2a from seismic observations in contrast layer 2a variations do not explain the low density inferred at the northern end of the cleft segment since seismic observations suggest a thin layer 2a the magmatic activity recently observed in this region may have a distinct deeper source

interaction between bed roughness and flow hydraulics in eroding rills govers 1992 showed that the flow velocity in rills eroding loose nonlayered materials could be predicted from knowledge of discharge only without significant slope or soil effect the objective of this paper is to investigate to what extent the observed slope independence of flow velocity in eroding rills can be explained by the interaction between rill bed roughness and flow hydraulics in a laboratory study two situations were compared 1 rills which can freely erode a uniform soil layer and 2 rills with a fixed bed geometry during the experiments rill discharge and flow velocity were recorded after each experiment a detailed topographic survey of the rill bed was carried out using a laser scanner from these data the main hydraulic variables mean values of flow depth wetted perimeter and hydraulic radius were estimated the experiments confirmed the slope independence of rill flow velocities on mobile beds when the bed is fixed the flow velocity in rills is clearly slope dependent the slope independence of flow velocity on mobile beds is due to a feedback between rill bed morphology and flow conditions the roughness amplitude which was assessed from the standard deviation of corrected height values and the frequency of macroroughness elements which was assessed by counting the number of peaks per unit length in a previously simplified longitudinal profiles both increase and tend to counteract the effect of the increase in slope gradient on rill flow velocity the final rill flow velocity appears to be characterized by a constant average froude number

a seafloor and sea surface gravity survey of axial volcano seafloor and sea surface gravity measurements are used to model the internal density structure of axial volcano seafloor measurements made at 53 sites within and adjacent to the axial volcano summit caldera provide constraints on the fine scale density structure shipboard gravity measurements made along 540 km of track line above axial volcano and adjacent portions of the juan de fuca ridge provide constraints on the density over a broader region and on the isostatic compensation the seafloor gravity anomalies give an average density of 2 7 g cm 3 for the uppermost portion of axial volcano the sea surface gravity anomalies yield a local compensation parameter of 23 significantly less than expected for a volcanic edifice built on zero age lithosphere three dimensional ideal body models of the seafloor gravity measurements suggest that low density material with a density contrast of at least 0 15 g cm 3 may be located underneath the summit caldera the data are consistent with low density material at shallow depths near the southern portion of the caldera dipping downward to the north the correlation of shallow low density material and surface expressions of recent volcanic activity fresh lavas and high temperature hydrothermal venting suggests a zone of highly porous crust seminorm minimization modeling of the surface gravity measurements also suggest a low density region under the central portion of axial volcano the presence of low density material beneath axial caldera suggests a partially molten magma chamber at depth

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

characteristics of sediment deposits formed by intense rainfall events in small catchments in the belgian loam belt erosion and deposition patterns within two agricultural catchments in the belgian loam belt were mapped and their volumes measured after an intense rainfall event from these data the total erosion and deposition budget was calculated the surveys clearly indicated that deposits could be differentiated according to the type of process that caused deposition for most deposits topography was the controlling factor however important deposits were also found at field borders where a vegetation barrier caused deposition vegetation controlled deposition occurs at significantly higher slope gradients than slope controlled sediment deposition this implies that vegetation controlled deposition has an important effect on the spatial distribution of deposited sediment and on the sediment delivery ratio at the catchment outlet the undispersed aggregate size distribution of sediment deposits in front of vegetation barriers is finer than the sediment deposited under topographically controlled conditions however the dispersed particle size distributions of both types of sediment are very similar and only slightly coarser than the dispersed particle size distribution of the source material during these extreme rainfall events sediment is eroded transported and deposited in aggregated form the aggregates themselves have a particle size distribution closely resembling the source material consequently considerable quantities of fine material and associated pollutants which are expected to be exported to the river system are trapped within the catchment

geophysical evidence for the absence of a crustal magma chamber under the northern juan de fuca ridge a contrast with rose results deep tow reflection data seismic refraction data to ocean bottom seismometers and gravity interpretations of the northern juan de fuca ridge all suggest a very narrow or absent crustal magma chamber under this portion of the rise axis the seismic features that have been suggested as indicating crustal magma chambers on other parts of the east pacific rise such as travel time delays and reflections from the top of a shallow magma reservoir are not found on the northern part of this ridge these results indicate that at spreading rates of 3 cm yr crustal cooling probably by water penetration is sufficiently rapid to prevent the existence of a steady state crustal magma chamber and therefore crustal accretion may be episodic

laboratory experimental study of soil crusting relation between aggregate breakdown mechanisms and crust stucture abstract crusts formation on cores of air dry and prewetted aggregates was studied using a laboratory rainfall simulator samples were taken out during the experiment in order to study changes in water content and aggregates size distribution inside the crust and under the crust pore space geometry of the crusts is described using light optical microscopy and mercury porosimetry in the case of air dry aggregates aggregates become micro cracked macropores at the surface are quickly closed and ponding occurs rapidly with prewetted aggregates microcracking does not occur and only a very slow abrasion of the aggregates is observed porosity remains high and no ponding excess occurs the discussion of these results shows that it is the way of the wetting and the initial water content which determine the breakdown mechanism and therefore the behaviour of the aggregates submitted to rainfall

rivera ocean seismic experiment rose overview the rivera ocean seismic experiment rose was designed as a combined sea and land seismic program to utilize both explosive and earthquakes to study a number of features of the structure and evolution of a mid ocean ridge a major oceanic fracture zone and the transition region between ocean and continent the primary region selected for the experiment included the rivera fracture zone the crest and eastern flank of the east pacific north of the rivera and adjacent areas of baja california and mainland mexico the experiment included 1 study of the east pacific rise south of the orozco fracture zone primarily using ocean bottom recording and explosive sources 2 a seismicity program at the orozco and 3 a land based program of recording natural events along the coastal region of mexico a considerable amount of useful data was obtained in each of the three subprograms in the marine parts of the experiment we were able to address a variety of problems including structure and evolution of young oceanic crust and mantle structure and dynamics of the east pacific rise seismicity of the orozco fracture zone and partitioning of energy transmission between the ocean volume and the crust lithosphere on land the fortuitousmore occurrence of the petatlan m7 6 earthquake of march 14 1979 permitted the acquisition of an excellent data set of foreshocks and aftershocks of this large event which provide new insight into the filling of a major seismic gap in the region this overview describes the scientific rationale and the design of the experiments along with some general results less

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

runoff and interrill erosion on sandy soils under cultivation in the western paris basin mechanisms and an attempt at measurement measurements made on different scales such as rainfall simulations on 1 m2 and 20 m2 experimental plots and water sampling at the outlet of a watershed enable the analysis of the mechanisms of pluvial erosion and therefore the importance of runoffs and soil losses in the hilly and sandy parts of the western paris basin interrill erosion accounts for slow transfer of materials towards the lower part of plots and slopes the overland flow caused by restructuring of the surface tertiary and cenomanian soils erodes some 150 to 200 kg ha 1 during the month following sowing only a part of these deposits reach the river which carries away less than 50 kg ha 1 each month interrill erosion makes slopes still more uneven fills in valley bottoms and so paves the way to catastrophic erosion which scoops out rises in the ground and colluvial deposits in the lower part of slopes and valley bottoms copyright 2001 john wiley sons ltd

soil erodibility and processes of water erosion on hillslope abstract the importance of the inherent resistance of soil to erosional processes or soil erodibility is generally recognized in hillslope and fluvial geomorphology but the full implications of the dynamic soil properties that affect erodibility are seldom considered in canada a wide spectrum of soils and erosional processes has stimulated much research related to soil erodibility this paper aims to place this work in an international framework of research on water erosion processes and to identify critical emerging research questions it focuses particularly on experimental research on rill and interrill erosion using simulated rainfall and recently developed techniques that provide data at appropriate temporal and spatial scales essential for event based soil erosion prediction results show that many components of erosional response such as partitioning between rill and interrill or surface and subsurface processes threshold hydraulic conditions for rill incision rill network configuration and hillslope sediment delivery are strongly affected by spatially variable and temporally dynamic soil properties this agrees with other recent studies but contrasts markedly with long held concepts of soil credibility as an essentially constant property for any soil type properties that determine erodibility such as soil aggregation and shear strength are strongly affected by climatic factors such as rainfall distribution and frost action and show systematic seasonal variation they can also change significantly over much shorter time scales with subtle variations in soil water conditions organic composition microbiological activity age hardening and the structural effect of applied stresses property changes between and during rainstorms can dramatically affect the incidence and intensity of rill and interrill erosion and therefore both short and long term hillslope erosional response similar property changes linked to climatic conditions may also significantly influence the stability and resilience of plant species and vegetation systems full understanding of such changes is essential if current event based soil erosion models such as wepp and eurosem are to attain their full potential predictive precision the complexity of the interacting processes involved may however ultimately make stochastic modelling more effective than physically based modelling in predicting hillslope response to erodibility dynamics

hydraulic conditions for rill incision under simulated rainfall a laboratory experiment a series of controlled laboratory experiments were conducted in order to obtain precise data on the hydraulic and sediment transport conditions during rill formation tests were carried out using a crusting prone binary mixed soil in a 15 m long flume at an average slope of 0 087 under simulated rainfall rainfall intensities varied from 30 35 mm h 1 and developed about 70 per cent of the kinetic energy of natural rainfall of similar intensity runoff and sediment discharge measured at the downstream weir were strongly influenced by rill forming processes essentially rill incision reduced runoff discharge as a result of increased percolation in rill channels but increased sediment discharge secondary entrainment processes such as bank collapse also increased sediment discharge at the weir knickpoint bifurcation and colluvial deposition however decreased sediment discharge rills always developed through the formation of a knickpoint the critical condition for knickpoint initiation was the development of supercritical flow and waves which mould and incise the bed prior smoothing of the soil surface by entrainment and redistribution of sediment facilitated supercritical flow statistical analysis showed that hydraulic and sediment transport conditions differed significantly in rilled and unrilled flows the relationship between sediment discharge rill erosion and flow hydraulics was found to be nonlinear conforming to a standard power function in the form y axb rills were also associated with significantly increased sediment transport capacities however rill initiation was not clearly defined by any specific hydraulic threshold instead rilled and unrilled flows were separated by zones of transition within which both types of flow occur

a possible explanation of gravity anomalies over mid ocean ridges gravity fields over the crests of slow spreading mid ocean ridges are characterized by small amplitude free air gravity anomaly lows 30 70 mgal with wavelengths of 40 60 km flanked by smaller gravity highs in general the amplitude and wavelength of these gravity lows decrease with increasing spreading rate until at fast spreading ridges 5 cm yr free air gravity highs 10 20 mgal are observed previous explanations of these anomalies which involve uncompensated ridges thermally expanded ridges and elastic plate compensation models are generally not consistent with the geological evidence regarding the nature of the crust and upper mantle beneath the ridge crest in particular the postulated presence of wide zones of partial melt in the uppermost mantle is inconsistent with petrologic data from mid ocean ridge basalts which indicates that primary melts are in equilibrium with and extracted from residual mantle at depths of 30 km or more in addition similar gravity lows observed over extinct spreading ridges suggest that the anomalies are not mainly due to any dynamic aspect of the accretion process taking place at active spreading centers geometrical models of mid ocean ridges derived from detailed structural and petrologic studies of the bay of islands ophiolite complex have been used to generate gravity anomalies over ridges with various spreading rates modeling results show that four main sources serve to define the amplitude and wavelength of the gravity anomaly the ridge topography the crustal level magma chamber the configuration of the isotherms near the ridge crest and a low density partially gabbroic cumulate root extending vertically into the mantle beneath the ridge systematic variations in the contributions from these sources with increasing spreading rate control the changes in character of the gravity anomaly from slow to fast spreading centers major contributions are found to come from the topography and from the lowdensity root which broadens with slower spreading rates resulting in both increasing amplitude and increasing wavelength with decreasing spreading rate at extinct spreading centers contributions from the magma chamber isotherms and topography gradually diminish with time once the ridge ceases to spread whereas the low density root remains a permanent part of the oceanic lithosphere and continues its contribution to the gravity field seismic evidence from slow spreading centers appears to support the existence of the lowdensity gabbroic root zone inferred from the study of the bay of islands ophiolite and the gravity analysis presented here

detachment of soil particles by shallow flow sampling methodology and observations abstract although several scientists have recently studied overland flow detachment the methodology of measurement has not yet been properly discussed this study compares several different methods a flume was constructed to measure soil detachment remoulded soils were tested in order to minimize the effects of sediment load and deposition on the soil sample surface by way of a suitable methodology the soil sample sizes were established on the basis that erosion must scour the entire surface of the soil target incipient detachment rate determined through the use of several methods was compared with the condition for incipient entrainment and the method which showed the best agreement with shields criterion was selected two data sets collected following the selected procedure and a different one were compared while the general trends deducible from the two data sets were similar the difference in intensity of the detachment rate was close to a factor 10 showing that measurements of flow detachment rate are sensitive to the procedure applied the sensitivity of the proposed methodology to various factors that can influence detachment was examined using remoulded and undisturbed soil samples particularly differences in initial soil moisture conditions and soil surface characteristics due to the combined effects of various types of crops and applied raindrop kinetic energy caused differences in soil detachability by overland flow furthermore soil sampling in different periods of the year resulted in variations in soil detachment indicating a seasonal trend in soil detachability

empirical models for predicting the erosion reducing effects of plant roots during concentrated flow erosion recent research indicates that plant roots can reduce soil erosion rates during concentrated flow significantly earlier studies revealed the erosion reducing effects of plant roots under controlled laboratory conditions and presented some equations applicable to the tested soil and flow conditions only although an attempt was made to unravel the impact of different environmental plant and flow properties on the erosion reducing potential of plant roots during concentrated runoff significant effects were hard to demonstrate because of the small number of data therefore the objective of this study is to pool different datasets 384 data collected under standardized experimental conditions in total together for constructing empirically based models predicting 1 soil detachment rates for both bare and root permeated topsoils and 2 the erosion reducing potential of plant roots during concentrated flow erosion the model that best predicts absolute soil detachment rates from bare and root permeated silt or sandy loam topsoils asd requires information on root density topsoil moisture content prior to concentrated flow erosion bulk density and mean bottom flow shear stress although all these variables contribute significantly to the prediction of asd the validation results indicate that there is still a large scatter on the data especially for small detachment rates the model tends to overestimate the observed values another model predicting the reduction in detachment rates of root permeated topsoils compared to bare ones rsd does not show good validation results either however the model for fibrous root systems was found to be very promising this model explains 79 of the variation using only root density and soil texture information and the validation results show that 69 of the variation in the validation dataset is accounted for by the model for topsoils permeated with tap root systems however the model results were not satisfactory only 10 of the variation in the validation dataset could be explained by a model using root density and mean root diameter as input variables we conclude that the erosion reducing effect of topsoils permeated with fibrous roots can be predicted very well whereas relative erosion rates for tap root permeated topsoils still remain difficult to predict with the studied variables only more process based knowledge is needed to distinguish erosion reducing effects from erosion accelerating effects for tap root permeated topsoils

the east pacific rise and the thermal model although the thermal mode of lithosphere cooling can be used to predict the east pacific rise topography reasonably well it does not adequately predict the observed gravity when the zero temperature boundary condition is at the seafloor the misfit arises primarily because this model predicts thermally lowered densities in the crust at the rise axis which counteract the gravitational effect of the topography generating a theoretical gravity anomaly smaller than that observed it is shown using data at 12 n as an example that improved agreement with the thermal model is obtained if to first order the crust is treated as an isothermal constant thickness constant density layer with variable depth this result is consistent with seismic data and observations of convective cooling of the crust by seawater circulation it implies that convective cooling of the whole crust is very rapid even at the axis of fast spreading ridges the oceanic moho may represent the depth to which cracking and convective cooling extends

rainfall induced consolidation and sealing effects on soil erodibility during concentrated runoff for loess derived topsoils flume experiments simulating concentrated runoff were carried out on remolded silt loam soil samples 0 36 0 09 0 09 m3 to measure the effect of rainfall induced soil consolidation and soil surface sealing on soil erosion by concentrated flow for loess derived soils and to establish a relationship between soil erodibility and soil bulk density soil consolidation and sealing were simulated by successive simulated rainfall events 0 600 mm of cumulative rainfall alternated by periods of drying soil detachment measurements were repeated for four different soil moisture contents 0 04 0 14 0 20 and 0 31 g g 1 whereas no effect of soil consolidation and sealing is observed for critical flow shear stress cr soil erodibility kc decreases exponentially with increasing cumulative rainfall depth the erosion reducing effect of soil consolidation and sealing decreases with a decreasing soil moisture content prior to erosion due to slaking effects occurring during rapid wetting of the dry topsoil after about 100 mm of rainfall kc attains its minimum value for all moisture conditions corresponding to a reduction of about 70 compared with the initial kc value for the moist soil samples and only a 10 reduction for the driest soil samples the relationship estimating relative kc values from soil moisture content and cumulative rainfall depth predicts kc values measured on a gradually consolidating cropland field in the belgian loess belt reasonably well mef 0 54 kc is also shown to decrease linearly with increasing soil bulk density for all moisture treatments suggesting that the compaction of thalwegs where concentrated flow erosion often occurs might be an alternative soil erosion control measure in addition to grassed waterways and double drilling copyright 2007 john wiley sons ltd

sediment availability on burned hillslopes 1 erodibility describes the inherent resistance of soil to erosion hillslope erosion models typically consider erodibility to be constant with depth this may not be the case after wildfire because erodibility is partly determined by the availability of noncohesive soil and ash at the surface this study quantifies erodibility of burned soils using methods that explicitly capture variations in soil properties with depth flume experiments on intact cores from three sites in western united states showed that erodibility of fire affected soil was highest at the soil surface and declined exponentially within the top 20 mm of the soil profile with root density and soil depth accounting for 62 of the variation variation in erodibility with depth resulted in transient sediment flux during erosion experiments on bounded field plots material that contributed to transient flux was conceptualized as a layer of noncohesive material of variable depth dnc this depth was related to shear strength measurements and sampled spatially to obtain the probability distribution of noncohesive material as a function of depth below the surface after wildfire in southeast australia the initial dnc ranged from 7 5 to 9 1 mm which equated to 97 117 mg ha 1 of noncohesive material the depth decreased exponentially with time since wildfire to 0 4 mm or 5 mg ha 1 after 3 years of recovery the results are organized into a framework for modeling fire effects on erodibility as a function of the production and depletion of the noncohesive layer overlying a cohesive layer

constraints on the structure of the east pacific rise from gravity gravity data across the east pacific rise at 9 n and 12 n the rose area are characterized by a high of 12 20 mgal with a width of about 20 km it is shown that this axial gravity high is not adequately explained by uncompensated constant density topography or an isostatically compensated thermally expanded ridge crest it can be adequately explained by airy compensation with a depth of compensation of at least 20 km or by elastic plate compensation with a flexural rigidity between 1018 and 1020 n m the significance of the depth and flexural rigidity parameters is that in both cases they result in a gravity field from the compensation whose amplitude is below the data resolution for wavelengths less than about 50 km that is the gravity data do not require compensation of the rise axis for wavelengths less than about 50 km this suggests that these wavelengths are either supported mechanically or by other forces dynamic the mechanical interpretation is not consistent with partial melt under the axis indicating that dynamic forces may be important in supporting the rise axis the same conclusion is arrived at from the thermal model the thermal model can be made to fit the gravity data if we allow departures from isostasy and the addition of a dike like body about 2 km wide in the crust having a positive density contrast of about 0 25 mg m 3 this geometry is consistent with a seismically determined low velocity zone in the crust in the rose area in this model the nonhydrostatic pressures must be supported dynamically a general conclusion of this study for the portions of the east pacific rise studied here is that the axial topography is not buoyantly supported by low density material in the crust therefore either the crust under the rise axis mechanically supports the axial topography or a mass excess at the thermally expanded rise axis is supported by other forces such as plate driving forces the latter option is favored on the basis of heat flux and seismic data

constraints on the structure of the east pacific rise from seismic refraction data university of washington seismic data taken during rose phase 1 have been used to study the structure of the east pacific rise in the vicinity of 12 n off axis but within 50 km of the axis we find that the oceanic crust can be modeled with a rapid change in velocity in the top 2 5 km a zone of low but uniform velocity gradient from 2 5 to about 5 5 km and a transitional zone about 1 km thick to mantle material there is also evidence for substantial anisotropy in mantle velocity gradients with a positive velocity gradient in the spreading direction and a negative velocity gradient parallel to the rise under the rise axis the data allow at most a very narrow zone of low velocity less than about 2 km wide in the crust

regional variation of mechanical and chemical denudation upper colorado river basin u s a the variation of mechanical and chemical denudation is investigated using discharge and sediment yield data from the upper colorado river system annual precipitation ranges from approximately 150 mm to 1500 mm mean specific yield ranges from 0 2 1 s km2 6 mm p a to 151 s km2 475 mm p a the hydrological geomorphological system adjusts itself to these varying climatic conditions in some areas however the effects of lithology or land use seem to override the climatic controls it is demonstrated that the increase in the absolute and particularly the relative amount of suspended sediment is closely related to a decrease in annual runoff and to an increase in the importance of high magnitude low frequency events this indicates that in areas of low annual runoff and high runoff variability soluble rocks are more resistant than in more humid areas during high magnitude low frequency events suspended sediment concentrations and loads are very high in semiarid areas due to sparse vegetation cover and dominance of direct runoff events of moderate magnitude and frequency which in more humid areas transport most of the dissolved load seldom occur the trend towards increasing mechanical denudation is even observed in areas of very low runoff 0 221 s km2 7 mm p a the peak of sediment yield in dry areas seems to approximate the point of no runoff very closely mechanical and chemical denudation are of equal importance at a runoff of about 300 mm per year

the value of a physically based model versus an empirical approach in the prediction of ephemeral gully erosion for loess derived soils abstract a data set on soil losses and controlling factors for 58 ephemeral gullies has been collected in the belgian loess belt from march 1997 to march 1999 of the observed ephemeral gullies 32 developed at the end of winter or in early spring winter gullies and 26 ephemeral gullies developed during summer summer gullies the assessed data have been used to test the physically based ephemeral gully erosion model egem and to compare its performance with the value of simple topographical and morphological indices in the prediction of ephemeral gully erosion analysis shows that egem is not capable of predicting ephemeral gully cross sections well although conditions for input parameter assessment were ideal some parameters such as channel erodibility critical flow shear stress and local rainfall depth showed great uncertainty rather than revealing egem s inability of predicting ephemeral gully erosion this analysis stresses the problematic nature of physically based models since they often require input parameters that are not available or can hardly be obtained with respect to the value of simple topographical and morphological indices in predicting ephemeral gully erosion this study shows that for winter gullies and summer gullies respectively over 80 and about 75 of the variation in ephemeral gully volume can be explained when ephemeral gully length is known moreover when previously collected data for ephemeral gullies in two mediterranean study areas and the data for summer gullies formed in the belgian loess belt are pooled it appears that one single length l volume v relation exists v 0 048 l 1 29 r 2 0 91 these findings imply that predicting ephemeral gully length is a valuable alternative for the prediction of ephemeral gully volume a simple procedure to predict ephemeral gully length based on topographical thresholds is presented here secondly the empirical length volume relation can also be used to convert ephemeral gully length data extracted from aerial photos into ephemeral gully volumes

cohesive material erosion by unidirectional current the initiation of motion of consolidated cohesive sediments under a unidirectional flow of clear water was studied experiments were performed in a flume tunnel capable of providing a bed shear stress up to 26 pa and a velocity of 3 5 m s 3 mm above the bed samples were prepared in a specially designed press using a carefully controlled consolidation procedure critical shear stress and velocity were found to increase with compressive strength vane shear strength plasticity index clay content and consolidation pressure

migration modification and merging in aeolian systems and the significance of the depositional mechanisms in permian and triassic dune sands of europe and north america abstract the main depositional mechanisms acting in aeolian systems can be assessed in two different and independent ways first by the internal anatomical approach with analysis of the cross stratification fabric and second by the external morphological approach with study of shape and surface features of dunes anatomical investigations of various ancient aeolian systems in the permian and triassic of europe and north america and an extensive literature review concerning both internal anatomy and external morphology of ancient and modern dunes reveal three main depositional mechanisms operating in aeolian systems migration modification and merging migration comprises downwind movement of isolated dunes or trains of aeolian bedforms under predominantly constant wind direction the orientation of the interset surfaces partially depends on the angular relationship between the path of bedform climb and the depositional surface and also on subsidence simple systems are characterized by successive dunes of similar size whereas compound systems originate by superimposition of smaller aeolian bedforms on larger dunes or draas compound migration gives rise to the hierarchy of diastems modification comprises overprinting of existing dunes by winds coming from directions other than those that govern migration of the original aeolian bedforms the anatomical elements representing modification include both intrasets and extensive intersets depending on duration and on the scale of the changes of the winds simple modification comprises occasional origin of either only cross dipping erosional boundaries or both diastems and intraset laminae oriented in different directions a special case of modification is the creation of intrasets dipping in the same direction as the foreset inclination comprising either reactivation or relaxation surfaces depending on whether the foreset slope is steepened or lowered in angle complex modification takes place under repeatedly alternating winds of changing effectiveness small scale modification comprises minor alterations of existing dune types and medium scale modification is characterized by conversion of primary aeolian bedforms into secondary dune types small scale and medium scale modification take place during short term to smaller scale long term changes of effective wind direction large scale modification originates by large scale long term changes accompanied by a certain subsidence which result in burial of the older dunes by a younger train of aeolian bedforms within the same sand sea migrating obliquely to perpendicularly to the former wind direction very large scale modification is induced by very large scale long term changes which result int deflationary erosion of older dune fields and subsequent superimposition of a new sand sea on top of the eroded remnants of the former erg merging comprises coalescing of bedforms with only negligible erosion during burial of the first bedform by subsequent dunes merging is induced mainly when migration of dunes is stopped by a morphological barrier or ceases as a consequence of weakening winds merging further occurs during coalescence of numerous isolated barchans in rows to parallel wavy barchanoid ridges as a consequence of increasing availability of sand the main depositional mechanism in compound systems is migration of hierarchically superimposed bedforms with subordinate modification under predominantly unidirectional winds in complex systems migration modification and merging take place in different amounts under unidirectional bidirectional and multidirectional winds depending on direction and strength of both earlier and subsequent winds complex systems are either charcterized by mainly migration with subordinate modification or by considerable modification with either conversion or dissection of older dunes the effects of migration modification and merging are discussed in the light of both internal anatomy of ancient cross stratification fabrics and external morphology of recent dunes a classification of dune types based on these mechanisms divides the aeolian bedforms into primary secondary and tertiary types primary dunes originate by simple or compound migration with minor modification and comprise mainly transverse dunes secondary dunes originate by complex interaction of migration and modification or even predominantly by modification with the best examples being longitudinal dunes tertiary dunes form by merging of transverse and longitudinal elements with varying amounts of migration and modification and are mainly represented by star dunes the results of various anatomical case studies of ancient aeolianites indicate that simple migration was the dominant mechanism for construction of ergs in the northern hemisphere in permian and triassic times

sediment export by water from an agricultural catchment in the loam belt of central belgium abstract an agricultural drainage basin of 250 ha was selected in the belgian loam belt to evaluate sediment export by water water discharge was continuously measured at the outlet of the catchment and suspended sediment samples were taken proportional with discharge during rainfall events from these samples supended sediment concentration and grain size distribution were determined within the catchment regular surveys were conducted in order to obtain detailed data on vegetation cover soil surface parameters and erosion and sedimentation features important variations in sediment yield were detected at various time scales ranging from a year over a season to one event these variations could be linked with processes occurring within the catchment due to the development of a vegetation cover the suspended sediment concentrations were lower during the summer period than during the winter period for the same discharge the evolution in both suspended sediment concentration and grain size distribution during the winter period is explained by variations in the soil surface state of the fields as well as the development of a rill and gully network the positive hysteresis observed within a single event is explained by taking into account the distance between the sediment source and the catchment outlet and not by sediment flushing our analysis shows that the role of the main factors controlling the geomorphic response of such a catchment can be identified only if measurements at the outlet are complemented by detailed data on the conditions within the catchment catchment model calibration and validation are therefore impossible if this information is lacking

assessment of the interrill and rill contributions to total soil loss from an upland field plot abstract govers g and poesen j 1988 assessment of the interrill and rill contributions to total soil loss from an upland field plot geomorphology 1 343 354 erosion measurements were carried out on an upland field plot in the loam region of belgium the sediment being detached by splash on interrill areas is transported to the channel system mainly by interrill wash over the entire 7500 m field plot rill and gully erosion is more important than interrill erosion however the relative importance of interrill erosion varies in time and in space due to changes of the interrill surface characteristics and the activation of sidewall and gullying processes in the channel network these factors should be taken into account in a realistic rill interrill erosion model introduction at present few available field data permit the separation of rill and or gully and interrill contributions to sediment removal from arable land the scarcity of this type of field data is a major factor inhibiting the validation and fur ther development of recently developed physi cally based models describing upland erosion by water e g foster 1982 rose et al 1983 knowledge of the contribution of rill and inter rill processes to total soil loss has also some practical interest if the ratio between rill and interrill erosion were known even approxi mately it would be possible to obtain a quick estimate of total erosion by making a volumet ric recording of rill patterns plot measure

canadian geomorphology 2000 geomorphology has a distinguished history in canada as old as the country itself initially the science of geomorphology of canada drew on rich british and french scientific traditions but later it developed its own character strongly influenced by the richness and diversity of the canadian landscape the strong imprint of glaciation and the importance of periglacial processes permafrost and sea ice canada is the second largest country in the world its coastline is longer and more varied than that of any other country nearly one third of canada is underlain by permafrost some up to 700 m thick the country s physiography ranges from nearly flat coastal and interior plains to the glacierised mountains of baffin and ellesmere islands british columbia and the yukon its climate and vegetation are equally diverse ranging from the cold semiarid tundra of the high arctic to the temperate rainforest of the pacific coast given this diversity it is not surprising that canada is active in many areas of geomorphic research an incredible spectrum of landforms and geomorphic processes has fostered world class research in canada both by canadians and by scientists from other countries much of this research is field based and empirical rooted in field observation in recent decades however there have been important advances in inductive laboratory and

in situ experimental research on water scouring of loess slopes frequent occurrence of landslides in the chinese loess plateau has been influenced by the changes of water conditions this paper based on in situ water scouring experiments under various slope gradients cross sectional discharge shapes and flow quantities analyzed the rill flow information characteristics on a loess slope and its driving factors for anti scourability of the loess stratum the results show that the critical erosion slope gradient for the loess stratum q3 is about 24 and the mean erosion rate under half round cross sectional discharge is smaller than that under rectangular discharge the relationship between flow quantity and mean erosion rate is linear and is positively correlated mechanical subsurface erosion was the primary condition responsible for water scouring on loess slopes because of stronger runoff channels such as vertical joints and large amounts of macro void caused by plants and animals loess anti scourability is a significant issue to advance the research for soil and water conservation in the loess plateau of china

assessing the susceptibility to water induced soil erosion using a geomorphological bivariate statistics based approach the aim of this study was the assessment at basin scale of the susceptibility to water induced soil erosion processes i e gully erosion and sheet rill erosion using geomorphological analysis geographical information systems gis and bivariate statistics the study was carried out in a watershed located in southern italy a detailed analysis of the pre existing literature led to select lithology land use slope angle and slope aspect as soil erosion determining factors dfs as they are non redundant and affect both the soil forming processes that control soil erodibility and the erosive power of running waters water produced erosional landforms such as gullies and areas severely affected by sheet rill erosion were surveyed and mapped using classical techniques of geomorphological analysis the gis processing of the geomorphological data allowed calculating the areal density of these landforms in each df class weighting values wi corresponding to the susceptibility level of each df class were calculated using bivariate statistics finally gis overlay procedures of the thematic maps previously reclassified on the basis of the calculated wi allowed to produce two susceptibility maps i e gully erosion and sheet and or rill erosion susceptibility map the soil geomorphological coherence of the produced results has been checked and widely discussed in the framework of the pre existing literature both the geomorphological coherence of the calculated wi and the results of the validation procedure suggested a good reliability of the method which is also relatively easy to apply and update

drying soil surface condition and interrill erosion on two ontario soils abstract interrill erodibility summarizes the resistance of soil to rainsplash sheetwash and rainflow erosion and incorporates changes in infiltration roughness and resistance to detachment and transportation resistance to interrill erosion varies during storms reflecting the effect of wetting sealing crusting and erosion on surface conditions significant changes in soil conditions also occur as a result of progressive drying between storms producing considerable uncertainty in event based erosion prediction however a clear link between drying primary soil properties and soil condition as well as interrill erosion during subsequent storms has not been established in this study soil condition and interrill erosion of soils from southern ontario were examined in detail during a sequence of simulated rainstorms and linkages between primary soil properties drying surface soil structure and interrill erosion were established interrill erosion after drying differed depending on the persistence of the changes in soil structure caused by drying pronounced stabilization of aggregates on drying led to an increase of erodibility indicating a transition from a coherent to granular structure requiring lower stresses for particle detachment the sensitivity of soil erodibility to drying appears to be determined by the tendency of a soil to form aggregates the results show that a better understanding of the linkages between primary soil properties rainfall and drying history soil condition and interrill erosion is required for effective event based erodibility assessment however the wide range of possible initial soil conditions and the complex nature of soil climate interaction are likely to limit the effectiveness of erodibility assessment

analytical method for determining rill detachment rate of purple soil as compared with that of loess soil abstract rill detachment is an important process in rill erosion the rill detachment rate is the fundamental basis for determination of the parameters of a rill erosion model in this paper an analytical method was proposed to estimate the rill detachment rate the method is based on the exact analytical solution of rill erosion to the differential equation of rill detachment the rill sediment concentration distribution as a function of rill length was identified through laboratory experiments under different slope gradients and flow rates the sediment concentration processes from experiments on loess and purple rills were considered to estimate the rill detachment rates of both soils analytically they were respectively used as a function of rill length and sediment concentration the analytical detachment rates were compared with the numerically determined values to verify the analytical methods the rill detachment rates of the two soils under different flow rates and slope gradients estimated by the analytical method were further compared on the basis of detachment sediment function and detachment rill length function results indicated that the analytically estimated values were very close to the numerically estimated values numerical and analytical methods were equally useful for rill detachment rate estimation therefore the analytical method was verified to be rational and applicable for the rapid determination of the rill detachment rates based on either sediment concentration or rill length the analytical detachment values of purple and loess soils suggested that the detachment rates of loess soil were significantly and considerably higher than those of purple soil the erosion potentials of loess soil were also significant higher than those of purple soil the differences in the erosion of the two soils decreased as the slope gradient and flow rate increased these observations implied that the degree of loess soil erosion was greater than that of purple soil erosion at low slope gradients and flow rates the factors causing these differences may be determined in terms of the properties of the two soils therefore the proposed analytical method in addition to the concepts and direct relationship to erosion process could be applied to obtain the rill detachment rate for the determination of rill erosion model parameters

a two and three dimensional analysis of gravity anomalies associated with the east pacific rise at 9 n and 13 n we have used a unique data set collected during the 1985 east pacific rise multichannel seismic experiment to reevaluate the constraints that gravity data can place on the crustal structure of the east pacific rise epr the close spacing of track lines within the two main survey areas 8 45 n 9 55 n and 12 20 13 30 n allowed us to perform three dimensional analyses of high quality gravity data 1 mgal uncertainity obtained with the bell aerospace bgm 3 gravity meter our gravity modeling was enhanced by the availability of high resolution sea beam bathymetric data and by independent structural constraints provided by seismic reflection and refraction data to model the crustal and upper mantle density structure at the ridge axis we first calculated the gravity anomalies due to the density contrasts at the water crust and crust mantle boundaries and the changes in density caused by the cooling of the lithosphere with age and subtracted these predictable signals from the observed free air anomaly the residual anomalies were then used to place constraints on the magnitude and distribution of anomalous mass at the epr our results show that over 90 of the power in the observed free air anomaly can be modeled by these predictable components of the gravity signal the amplitude and wavelength of the small residual anomaly can be modeled by a broad region 20 km wide centered on the rise axis of slightly lower than normal crustal and or upper mantle densities 0 03 mg m 3 this region of anomalous mass and density variations in the underlying mantle provide the principle isostatic support for the axial topographic high the anomalies are consistent with but do not require the presence of a low density crustal magma chamber at the rise axis if a largely molten magma chamber exists then the gravity data require that it be a narrow volumetrieally small body there are first order differences in the distribution of anomalous mass between the two survey areas with the 9 n region generally being characterized by lower densities in the axial region along axis variations in anomalous mass within each of the survey areas correlate with first order changes in the depth and cross sectional shape of the epr axial high we attribute these differences in axial morphology and the amplitude of the associated gravity anomalies to along strike changes in the size of the crustal magma chamber the width of the surrounding zone of cooling crustal rocks and densities in the uppermost mantle beneath the rise axis

an interpretation of trends in grain size measures abstract the mean grain size sorting and skewness of a sedimentary deposit are dependent on the sediment grain size distribution of its source and the sedimentary processes of i winnowing erosion ii selective deposition of the grain size distribution in transport and iii total deposition of the sediment in transport if a source sediment undergoes erosion and the resultant sediment in transport is deposited completely the deposit must be finer better sorted and more negatively skewed than the source this trend is referred to as case i the lag remaining after erosion on the other hand must be coarser better sorted and more positively skewed case ii if sediment in transport undergoes selective deposition the resultant deposit can either be finer case iiia or coarser cas iiib than the source but the sorting will be better and the skew more positive although exceptions to these trends may occur they suggest that comparison of one sediment must be made with another for the proper identification of the sedimentary process and therefore it is not possible for a single grain size distribution to identify the depositional environment the trends also suggest that the skewness of a grain size distribution has been widely misinterpreted and implies neither the truncation of one of the tails nor the mixing of more than one mode rather a skewed sediment is the natural result of the sedimentary process in a system of related environments these trends can be used to identify both the probable source and the probable deposit and by inference the net sediment transport paths among sedimentary deposits such an analysis provides a rapid understanding of the sedimentary processes identifies patterns of erosion and accretion and may suggest transport processes

gravity anomalies over extinct spreading centers a test of gravity models of active centers gravity fields over active mid ocean ridges display long wavelength free air gravity highs with superimposed short wavelength anomalies over the spreading axis these axial anomalies are negative over slow spreading active ridges with amplitudes of typically 30 70 mgal and wavelengths of 40 60 km but decrease in amplitude and wavelength with increasing spreading rate until at fast spreading centers the anomalies are positive 10 20 mgal with wavelengths of about 20 30 km similar free air anomalies are observed over extinct spreading ridges and typically show the same paleo spreading rate dependency suggesting that a large component of the anomaly is not dynamically maintained calculation and removal of gravitational effects from basement relief results in significant residual gravity lows at the extinct ridges analyzed here even though the thermal effects associated with active spreading have diminished and no longer contribute to such short wavelength gravity lows the amplitudes of the residual lows at slow paleo spreading rates are 25 45 mgal with half widths of about 20 45 km and decrease with increasing spreading rate to about 5 mgal amplitude and 10 15 km half width over fast paleo spreading ridges the amplitudes and wavelengths of these residual anomalies are nearly identical to those of the residual anomalies observed over active spreading centers most previously proposed models of mid ocean ridge structure are unable to explain these anomalies and many are inconsistent with a least some aspect of the large body of knowledge currently available for mid ocean ridges this research provides a further constraint on geometric models of accretion as these models predict vastly different gravity fields results indicate that a frozen in low density accretionary structure is probably confined to within about 10 km of either side of the extinct axis this conclusion is consistent with the presence of a low density gabbroic root situated at shallow mantle depths beneath the ridge axis this root tends to narrow with increasing spreading rate producing the required residual gravity field variation

the mid ocean ridge axial valley as a steady state neck abstract in this paper the mid ocean ridge axial valley is modelled as a steady state lithospheric neck in which lithospheric stretching balances lithospheric accretion conversely the axial high is a steady state lithospheric bulge the lithosphere is modelled as a thin plate with a newtonian rheology it is shown that an axial valley will occur if the rate of viscosity increase away from the ridge axis is faster than the rate at which accretion decreases an axial high will occur if the opposite condition holds this is consistent with the observation that axial valleys occur at low spreading rates and axial highs at high spreading rates by fitting our model to profiles across the mid atlantic ridge and the east pacific rise and assuming the lithospheric thickness at the ridge axis to be 5 km we find accretion widths of 6 8 km we find the width over which there is a significant increase in lithospheric viscosity to be also 6 8 km

marine tectonics processes at mid ocean ridges the use of multibeam echo sounders and various side looking sonar systems has greatly increased our knowledge and understanding of mid ocean ridge mor processes since the last iugg review on mid ocean ridge research macdonald 1983 the extensive use of swath mapping tools such as sea beam deep tow seamarc i and ii the application of multichannel seismic reflection techniques to the detection of axial magma chambers and the use of satellite data in the study of oceanic lithosphere have rejuvenated marine tectonics and have generated and stimulated a large number of important contributions this review will focus on what we feel were the major developments in marine tectonics with an emphasis on mid ocean ridges in the last four years certain topics such as marine seismicity and satellite studies are only briefly mentionned for reference and will be covered in greater detail in other reviews because of the exponential growth in the number of publications related to mors during 1982 1986 only a few references pertinent to each of the major research topics will be quoted in the text as examples but the reader will find an extensive bibliography at the end of the text references published prior to 1982 were added only where appropriate

gravity anomalies isostasy and mantle flow at the east pacific rise crest bathymetry and gravity data obtained during a detailed hydrosweep survey of the southern east pacific rise from 7 s to 9 s are used to investigate isostasy at the axis of a fast spreading mid ocean ridge in particular we examine the manner in which the bathymetric crestal high is supported and how this support varies along the axis within the 160 km long 7 12 s 8 38 s ridge segment the crestal high stands about 400 m above the adjacent ridge flanks and has a nearly constant minimum axial depth for a distance of 140 km the summit is broad and flat and an axial summit caldera is present for the entire length of the ridge segment however the width and cross sectional shape of the crestal high vary systematically along the axis it is broad with gentle slopes in the center of the segment but becomes progressively narrower and steeper toward the ends of the ridge segment the ridge crest is marked by a free air gravity anomaly high about 15 20 km wide with an amplitude of 10 15 mgal relative to the ridge flanks mantle bouguer anomalies vary systematically along the axis with minimum values found near the center of the segment the axial mantle bouguer anomalies thus do not reflect the axial depth but are correlated with changes in the cross sectional area of the crestal bathymetric high the effects of cooling and subsidence away from the axis were removed from the bathymetry and free air anomalies to isolate residual topographic and gravity anomalies associated with the ridge crest the residual crestal bathymetric high was modeled as a flexural feature resulting from the upward buoyant load of a region of low density material centered beneath the axis the lithosphere was treated as a broken plate either with a constant flexural rigidity or an effective elastic thickness te which grows at a rate proportional to the square root of distance from the axis the best fitting values of te for the constant rigidity case are in the range of 0 3 0 6 km for the growing plate model te increases at a rate of 0 2 0 3 km the gravity constrains the mass deficiency to extend to a depth of 20 30 km for both lithospheric models we interpret this low density material as a region of partial melt feeding magma to the ridge axis the best fitting density anomalies imply that a 4 9 melt fraction is present beneath the crestal high upwelling of melt to the axis is thus confined to a narrow zone within about 10 km of the axis the mass deficiency and thus the upwelling partial melt are not distributed evenly along the ridge axis but rather are concentrated in the central portion of the ridge segment it thus appears that differences between the along axis gravity and depth patterns observed at slow spreading and at fast spreading ridges are not the result of a change from three dimensional focused upwelling at slow spreading ridges to two dimensional sheetlike upwelling at fast spreading ridges rather the differences in axial gravity and depth between fast and slow spreading ridges reflect differences in the efficiency of the shallow along axis magma distribution

effects of grass roots on the erodibility of topsoils during concentrated flow abstract traditional vegetative techniques to control gully development rely mainly on the effects of above ground biomass whereas little attention has been given to the role of below ground biomass yet in a context where above ground biomass may temporarily or spatially disappear e g due to fire or grazing roots can play an important role in protecting soil against erosion few studies have investigated the impacts of roots of natural vegetation such as grass on the resistance of topsoils in concentrated flow erosion zones although grasses grow in many environments therefore the objective of this study is to investigate the impact of root density and root length density of grass on the erodibility of root permeated saturated topsoils three plots were established on a sandy loam their treatments were 1 bare 2 low density drilled grass and 3 high density drilled grass simulating different root densities after one month topsoil samples were taken and subjected to concentrated flow using a hydraulic flume in the laboratory slope flow discharge mean velocity water temperature and sediment concentration were measured root density and root length density values were assessed relative soil detachment rates and mean flow shear stresses were calculated the results indicate a negative exponential relation between the relative soil detachment rate and root density as well as root length density independent of the applied flow shear stresses however the best relationship fitting the data is the hill curve indicating that relative soil detachment rates decrease to very low values 0 05 with an increase in root density from 0 to 4 kg m 3 or root length density from 0 to 400 km m 3 a comparison between the effects of vegetation cover on sheet and rill erosion rates and those of the root area ratio of grass roots on relative soil detachment rates reveals that grass roots are very effective in reducing soil detachment rates the equations obtained can be used to predict the effect of grass roots on soil erosion rates during concentrated runoff and to evaluate the ability of roots to increase topsoil resistance against erosion by concentrated flow calculations of relative erosion rates using the equations from the rusle and wepp models indicate that the observed trend is better predicted with the rusle model and the wepp model for croplands than with the wepp model for rangelands

the influence of subsurface moisture on rill system evolution rill development studies have focused almost exclusively on surface erosion processes and critical threshold hydraulic conditions characteristic rill features such as arcuate headcuts and knickpoints are morphologically similar to the theatre headed valleys which have been associated with sapping processes at various scales this paper reports on laboratory experiments designed to identify linkages between surface flow hydraulics subsurface moisture conditions and rill development experiments were carried out in a 16 57 m2 flume under simulated rainfall with soil samples up to 0 15 m depth in which moisture conditions were monitored by miniature time domain reflectometer probes tests showed complex responses in which some rill incision reflected surface flow conditions but major rill system development with markedly enhanced sediment yield was closely associated with high soil moisture contents it was not possible to measure seepage forces directly but calculation and observation indicate that these were less important than reduction in soil strength with saturation which resulted in increased effective runoff erosivity this caused concentrated undercutting along the water table at rill walls while slightly stronger surface layers above the water table formed microscarps these retreated along the water table into interrill surfaces producing residual pediment transport slopes the microscarps eventually disappeared when the water table reached the surface eliminating differential soil strength the experiments showed complex relationships between surface and subsurface erosional processes in evolving rill systems strongly influenced by soil moisture dynamics the very small topographic and hydraulic head amplitudes indicate that seepage forces and sapping were minimal the dominant effect of soil moisture was reduction of soil strength with saturation and increased runoff entrainment experimental conditions were not unusual either for agricultural fields or natural hillslopes and the intricate interrelationship of surface and subsurface erosion processes observed is probably not uncommon attempts to link specific morphologic features at rill scale to dominance of surface or subsurface processes alone are therefore unlikely to be successful or reliable 1998 john wiley sons ltd

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

a new isostatic model for the east pacific rise crest we have developed a new isostatic model for the east pacific rise epr that explains the origin of both the axial topographic high and the rise crest gravity anomaly our model involves a thin elastic plate with a constant thickness crust that is broken at the ridge crest we assume that a buoyant force beneath the plate bends the free edge of the plate upward at the rise axis forming the topographic high and the rise crest gravity anomaly we have used this model and two conventional isostatic models to examine the isostasy of three segments of the epr at 9 l4 n 6 11 s and 16 21 s rather than modeling individual profiles we have stacked summed the gravity and bathymetry data in each area and computed a single 200 km long mean mirrored profile for each different ridge segment these stacked profiles are a remarkable improvement over even the best individual profiles and clearly show both the axial topographic high and its associated free air gravity anomaly in all three areas the best fitting model parameters are similar and give average compensation depths of about 6 7 km below the seafloor near the base of the oceanic crust and an upper limit on the flexural rigidity of the plate that increases from about 1018 n m at the ridge axis to about 3 1019 n m 5 km from the axis and 1020 n m 25 km or more from the rise crest these results indicate that the epr crest is compensated at much shallower depths than suggested by previous investigators our results are consistent with the presence of a small low density magma chamber in the lower crust although at least part of the compensating root for the axial high must extend into the upper mantle we interpret the low upper mantle densities below the rise crest as a zone of partial melt which accumulates at the base of the oceanic crust between episodic rifting events when the overlying crustal magma chamber is replenished

santa ana river an example of a sandy braided floodplain system showing sediment source area imprintation and selective sediment modification abstract the santa ana river floodplain can be divided into five geomorphic environments recognized by both morphology and sedimentary characteristics braided channels bars vegetated islands sand flats and floodplain terraces braided channels which vary in width from 30 to 80 m and are bordered by sand flats flow round vegetated islands and emergent mid channel bars deep braided channels have linguoid dume bedforms which are reworked during falling flood stages if channel bars emerge in early spring vegetation is rapidly established and they may become vegetated islands bars emerging in minor channels and in areas of increased channel width may develop into extensive sand flats away from the channel margins sand flats are subject to wind erosion as the water table declines in summer and fall this sediment is deposited in sand sheets and vegetated dunes these distal sand flat environments are characterized by a well sorted medium sand sized sediment population and are separated from negatively skewed moderately sorted coarse channel sands by a mixed sediment population along channel margins the drainage basin discharge is dominated by the regional mediterranean climate the coarse channel sediment reflects the large temporal variations in drainage discharge and the resulting greater variability and competence to transport sediment major decreases in channel width during both seasonal summer drought and prolonged periodic drought exposes large areas of the floodplain to selective wind winnowing of fluvially derived sediment this process operates on a small scale today but during arid conditions in the late wisconsin interglacial interval prior to 22 000 yrs b p extensive sand flats along the santa ana river distal alluvial fans and sediment from minor tributaries were the sediment source for a major riverine dune field adjacent to the floodplain this dune field was stabilized during the late wisconsin glacial interval subsequent channel entrenchment has isolated this dune field from its source region santa ana river sediments originate from two distinct geologic provinces the san gabriel san bernardino granitic and metamorphic complex to the north and the granitic perris block to the south settling tube analysis of sediments from tributary streams draining the perris block shows a distinctive bimodal distribution characterized by minimal sediment in the very coarse sand size 1 to 0 phi size inherited from the granitic grus parent material this is imprinted onto all sediment from this source transportation results in sediment erosion sediments are better sorted and finer grained with increasing distance from their source the sediment in these intermittent tributary streams frequently represents a single sedimentary event minor flash flood and all the sediment including the fines is left behind this produces a negatively skewed sediment distribution when the fines are added to an existing deposit within the limits of a predetermined size range older alluvium is strongly negatively skewed and may indicate in situ weathering coarse channel sands from the santa ana river maintain this characteristic sediment distribution but selective deposition and winnowing by the wind on the sand flats creates a distinctive floodplain sub environment analysis of these deposits and material from tributary streams illustrates that a particle population is dependent upon grain size distribution at the initial source sediment populations are modified by subsequent erosion selective grain size transport deposition and total deposition and mixing of several source areas

soil detachment by overland flow on hillslopes with permanent gullies in the granite area of southeast china abstract permanent gullies are often initiated from rill erosion followed by the formation of abrupt deep and wide gullies when topsoil and subsoil are carried away by fast flowing surface water few studies have been performed to evaluate soil detachment processes on hillslopes with permanent gullies which may significantly affect the further development of gully erosion soil detachment was investigated by using natural undisturbed soil samples collected in different spatial locations upper catchment middle slope lower slope and scour channel of hillslopes with permanent gullies the relationships of soil detachment capacity with soil property root system and hydraulic parameters were quantified selected samples were tested in a hydraulic flume 3 8 m length 0 2 m width under five different shear stresses 3 67 7 81 10 50 13 97 and 17 28 pa soil detachment capacity was found to be significantly greater in the scour channel than in the other three hillslope positions for the initial and stable gullies the soil detachment capacity was notably reduced in the middle slope relative to the upper catchment and lower slope while a different trend was observed in the active gully the variability of soil detachment capacity at different spatial locations showed a negative correlation to bulk density soil cohesion clay organic matter and root density p

relationship between discharge velocity and flow area for rills eroding loose non layered materials a relationship between discharge flow velocity and flow area in rills is established using data from four field and laboratory studies the proposed relationship is shown to predict successfully flow velocities measured in six other studies although slopes range from 0 035 to 0 45 and soil materials range from stony sands over silt loams to vertisols mean flow velocity can be well predicted from discharge alone thus there is no important influence of slope and or soil material characteristics on flow velocities in rills the proposed relationship may be used to improve performance of deterministic flow routing models when applied to rilled catchments furthermore it allows the calculation of unit stream power which has been shown to be related to the transporting capacity of overland flow in terms of slope and discharge

isostasy magma chambers and plate driving forces on the east pacific rise the density distribution within a cooling plate is calculated which incorporates temperature and pressure effects from this density distribution the pressure field within the plate and the gravity field at sea level are computed for various degrees of isostatic compensation in this model the pressure field within the plate has a horizontal gradient at shallow depths away from zero age and a horizontal gradient toward zero age at greater depths caused by the loading of the ocean isostatic equilibrium is approached if one allows the loading due to the water to depress the seafloor and at the same time allows mass conservation by flow at depth toward zero age a viscosity model based on a newtonian rheology which included temperature and pressure effects has a high gradient close to the plane separating positive and negative pressure gradients which would facilitate the return flow and decouple the lithosphere from the asthenosphere addition of a crust to the homogeneous model does not substantially change these conclusions comparison of this model with examples of east pacific rise data suggests that some areas may not be in complete isostatic equilibrium implying the existence of horizontal pressure gradients toward zero age in the asthenosphere this model can be made to fit the general features of the east pacific rise but not the detailed gravity and topography near zero age if one allows convective cooling of the crust by water partial melting of the upper mantle and intrusion of this partial melt into the crust the water depths increase more rapidly near zero age and an increased positive gravity anomaly is produced over the rise axis both of which produce a better fit to east pacific rise data at 12 n this study suggests that crustal magma chambers on the east pacific rise may be associated with anomalous positive gravity anomalies caused by a positive density contrast between magma and fractured porous crustal rocks

gloria survey of the east pacific rise near 3 5 s tectonic and volcanic characteristics of a fast spreading mid ocean rise abstract a long range sidescan sonar gloria survey of an approximately 100 km square area of the east pacific rise crest between 3 s and 4 s extends results obtained in the same area by lonsdale 1977 using deep tow the axial linear volcano was found to be continuous over a distance of 75 km the presence of major inward and outward facing fault scarps was confirmed but the gloria data show several distinct differences between the two fault sets the inward dipping faults are more numerous more closely spaced and longer than the outward dipping ones and their dip slopes backscatter sound more extensively than those of the outward dipping faults moreover most of them appear to be formed within 2 km of the axis whereas the majority of the outward dipping faults begin to develop between 5 and 8 km from the axis these differences suggest that the two sets of faults have different origins the horizontal pattern of inward dipping faults is similar to those observed on other mid ocean rises at all spreading rates though the lengths and separations of the scarps are slightly and their throws considerably less than on slower spreading rises this common horizontal pattern suggests that inward dipping faults on all rises have a common mode of formation regardless of spreading rate horizontal tension is probably the dominant factor but an additional mechanism is needed to explain the polarization of fault dips that occurs in the region 2 8 km from the axis the similarity of major fault spacings on the east pacific rise to those on slower spreading rises suggests that faulting is invariant in space rather than time and that the lithosphere where these faults are formed about 2 km from the spreading axis has a similar small thickness for all spreading rates this is attributed to the regulating effect of hydrothermal circulation and plate cooling

assessment of soil erosion sensitivity and post timber harvesting erosion response in a mountain environment of central italy article i nfo this study aimed to assess the effects of forest management on the occurrence of accelerated soil erosion by water the study site is located in a mountainous area of the italian central apennines here forest harvesting is a widespread forestry activity and is mainly performed on the moderate to steep slopes of the highlands through modeling operations based on data on soil properties and direct monitoring of changes in the post forest harvestingsoilsurfacelevelatthehillslope scale we show thattheobserved site became pronetosoilero sion after human intervention indeed the measured mean soil erosion rate of 49 t ha 1 yr 1 for the harvested watershed is about 21 times higher than the rate measured in its neighboring undisturbed forested watershed 2 3 t ha 1 yr 1 the erosive response is greatly aggravated by exposing the just harvested forest with very limited herbaceous plant cover to the aggressive attack of the heaviest annual rainfall without adopting any conservation practices the erosivity of the storms during the first four months of field measurements was 1571 mj mm h 1 ha 1

a bathymetry and altimetry profile across the southwest indian ridge crest at 31 s latitude abstract during march 1983 the m v marion dufresne obtained a bathymetry profile along more than 1500 km of a seasat altimeter sub track across the southwest indian ridge crest at 31 s latitude we have used these data to estimate the thermal and elastic thickness of the oceanic lithosphere in the vicinity of the ridge and to evaluate the performance of the seasat altimeter the thermal thickness appears to be in the range 72 109 km while the elastic thickness is about 2 5 km there is a good correlation between the geoid calculated from the bathymetry and its compensation and the observed geoid derived from seasat data the correlation is strongest for wavelengths greater than 45 km and poorest for shorter wavelengths we attribute the poor resolution at short wavelengths to instrument and oceanographic noise in the altimeter data combined with the reduced sensitivity of the geoid to bathymetric relief at very short wavelengths

effect of natural restoration time of abandoned farmland on soil detachment by overland flow in the loess plateau of china vegetation restoration has significant effects on soil properties and vegetation cover and thus affects soil detachment by overland flow few studies have been conducted to evaluate this effect in the loess plateau where a great green project was implemented in the past decade this study was carried out to quantify the effects of age of abandoned farmland under natural vegetation restoration on soil detachment by overland flow and soil resistance to erosion as reflected by soil erodibility and critical shear stress the undisturbed soil samples were collected from five abandoned farmlands with natural restoration age varying from 3 to 37 years the samples were subjected to flow scouring in a 4 0 m long by 0 35 m wide hydraulic flume under six different shear stresses ranging from 5 60 to 18 15 pa the results showed that the measured soil detachment capacities in currently cultivated farmland were 24 1 to 35 4 times greater than those of the abandoned farmlands for the abandoned farmlands soil detachment capacities fluctuated greatly due to the complex effects of root density and biological crust thickness and could be simulated well by flow shear stress and biological crust thickness with a power function nse 0 851 soil erodibility of abandoned farmlands decreased gradually with restoration age and reached a steady stage when restoration age was greater than 28 years the critical shear stress of the natural abandoned farmlands declined when restoration age was less than 18 years and then increased due to the episodic influences of vegetation recovery and biological crust development more studies in the loess plateau are necessary to quantify the relationship between soil detachment capacity and biological crust thickness for better understanding the mechanism of soil detachment under natural vegetation restoration copyright 2013 john wiley sons ltd

gravity modeling of a volcanically active site on the east pacific rise axis abstract seafloor and shipboard gravity observations are used to study the east pacific rise epr axis at 9 48 n a site of recent volcanic activity a constant crustal density model fits the seafloor and shipboard gravity data within expected errors a layered density model consistent with rise axis seismic constraints also fits the gravity data these constant and layered density models show no evidence for a volumetrically large magma chamber and indicate the importance of the shallow crustal density structure to the marine gravity field density variations in the shallow crust may be explained by porosity and thermal variations the average shallow 0 450 m epr 9 48 n crust has a density of 2420 80 kg m 3 significantly lower than other rise axis sites this density is less than molten basalt suggesting that magma buoyancy will not contribute to ascent through the uppermost crust enhancing the formation of volcanic dikes and sills below the layer 2 a 2 b boundary

deriving transport pathways in a sediment trend analysis sta abstract sediment trend analysis sta is a technique that enables patterns of net sediment transport to be determined by relative changes in grain size distributions of all naturally occurring sediments in addition sta can determine the dynamic behaviour of bottom sediments with respect to erosion accretion or dynamic equilibrium the data requirements for sta are the full grain size distributions taken from sediment grab samples collected at a regular spacing over a particular area of interest two types of methods are presently used in the derivation of the transport pathways a line by line approach in which transport pathways are determined by searching for sample sequences in which the distributions change in a statistically acceptable manner and various vector approaches in which each sample is compared to neighbouring sites from which a vector sum is calculated the basic assumption for sta is that the processes that cause sediment transport will affect the statistics of sediment distributions in a predictable way in reality this type of analysis is complicated through the inclusion of a number of uncertainties or noise the goal of sta is to extract the information the transport pathways from the noisy signal the grain size distributions an approach that is made difficult because neither the nature of the information nor the noise is known because of this obtaining results by simply applying its theory in a black box approach may provide poor solutions the line by line approach to sta draws from communications theory to achieve a solution in some communications systems the information from many sources is combined into one signal the resulting signal is from a statistical viewpoint nothing but noise the extraction of specific information assumes that information is indeed present and determines if that assumption is consistent with the received signal for sta the procedure is to assume a transport direction over an area comprising many sample sites from this assumption the predicted sediment trends are compared with the pathways determined by the actual samples the assumed transport direction is repeatedly modified until the best fit is achieved vector techniques may be very helpful to provide insights and guidance to the line by line approach however the approach is ultimately necessary to finalize a credible solution as well as providing important further information on the dynamic behaviour of the sediments and to delineate specific sediment transport environments

quantifying the surface covering binding and bonding effects of biological soil crusts on soil detachment by overland flow biological soil crusts bscs have impacts on soil detachment process through surface covering and binding and bonding b b mechanisms which might vary with successional stages of bscs this study was conducted to quantify the effects of surface covering binding and bonding of bscs on soil detachment capacity by overland flow in a 4 m long hydraulic flume with fixed bed two dominant bsc types developed well in the loess plateau the early successional cyanobacteria and the later successional moss were tested using natural undisturbed soil samples collected from the abandoned farmlands two treatments of undisturbed crusts and one treatment of removing the above ground tissue of bscs were designed for each bsc type for comparison bare loess soil was used as the baseline the collected soil samples were subjected to flow scouring under six different shear stresses ranging from 6 7 to 21 2 pa the results showed that soil detachment capacity dc and rill erodibility kr decrease with bsc succession and the presence of bscs obviously increased the critical shear stress especially for the later successional moss crust for the early successional cyanobacteria crust dc was reduced by 69 2 compared to the bare loess soil where 37 7 and 31 5 are attributed to the surface covering and b b respectively for the later successional moss crust dc decreased by 89 8 compared to the bare loess soil where 68 9 and 20 9 contributed to the surface covering and b b respectively these results are helpful in understanding the influencing mechanism of bscs on soil erosion and in developing the process based erosion models for grassland and forestland copyright 2017 john wiley sons ltd

the de ploey erosional susceptibility model for catchments es abstract this article is based on an unpublished manuscript by the late prof dr jan de ploey it focusses on the erosional susceptibility of catchments in terms of total energy input by the meteorological factors water and wind e s model the form and the universal character of the basic expression of the erosional susceptibility are explained applications of the model are illustrated for different processes such as landsliding and debris flows gullying creep and rill interrill combinations characteristic e s values exist for these processes also e s values vary in regions with differing pedo botanical characteristics and land use the difference between e s values for single storm events and longer periods are related to the magnitude frequency distribution of erosive activities and allow in many cases an assessment of the age of erosion phenomena and an estimation of the recurrence period of extreme single events

a stochastic model for the creation of abyssal hill topography at a slow spreading center the present study is an attempt to model abyssal hill topography i e the randomly varying topography that is superimposed on the subsidence of the ocean floor due to thermal contraction we assume that abyssal hill topography is created by faulting and that it can be represented as the convolution of a series of stochastic fault offsets with a fault impulse response as a constraint the power spectrum of the model generated topography will have to resemble the power spectrum of a profile of abyssal hill topography across a typical slow spreading ridge such as found in the south atlantic using a series of fault offsets positively correlated in space and fault impulse responses limited to a finite width w profiles that statistically resemble abyssal hill topography have been generated if the fault offsets are constrained to be positive i e to cause uplift the synthetic profiles contain a steady state depression at the axis the fit of the synthetically derived topography to the observed topography is very sensitive to the width w of the fault impulse response our results suggest that the effects of a faulting event must be limited to distances of about 3 5 km and that tectonic activity must take place in the immediate vicinity of the inner wall of the axial valley if faulting is indeed the process that creates abyssal hill topography the best fit flexural impulse response to faulting implies a flexural rigidity of 5 1017 n m if instead constructional volcanism contributes significantly to the topography our study suggests that the emplacement of volcanic constructions must also take place within a narrow width w about the axis

effects of rock fragment covers on erosion and transport of noncohesive sediment by shallow overland flow rock fragments on soil surfaces can have a variety of contrasting effects on the hydraulics of overland flow and soil erosion this makes modeling soil erosion difficult systematic studies are needed to assess the effects of rock fragments on the hydraulics of overland flow and to specifically relate these hydraulics to sediment yield therefore flume experiments were conducted in which overland flow was applied to a highly erodible sediment surface covered by various percentages of rock fragments high local turbulence was caused by horseshoe vortex erosion around individual rock fragments for a given discharge and slope the dimension of the scour was affected by the space between the rock fragments for low cover percentages the scour can develop undisturbed and sediment yield increases with percent of rock fragment cover up to about 20 sediment yield then declines as percent cover increases above 20 beyond a cover threshold value of 20 the larger opportunity for scour hole development is counteracted by limited inter rock fragment space that does not allow scour holes to develop to their full extent as sediment yield varies nonmonotonically with rock fragment cover percent sediment yield cannot be predicted by spatially averaged flow hydraulics

effects of freshly incorporated straw residue on rill erosion and hydraulics abstract rill hydraulics and hence flow detachment are modified by the presence of incorporated vegetation residue typically water flow in the rill is retarded due to the extra shear stress generated by the residue the main objective of this study was to develop an approach to predict soil detachment by rill flow in the presence of freshly incorporated vegetation residue that is compatible with our current understanding of rill hydraulics and requires no additional information on rill geometry laboratory experiments were carried out to collect a dataset on rill flow detachment on surfaces with incorporated straw that was compatible with existing dataset on bare soils gimenez and govers 2001 interaction between bed roughness and flow hydraulics in eroding rills water resources research vol 37 no 3 791 799 we found that effective unit length shear force e is well related to soil detachment when incorporated residue is present the determination of e is based on the recalculation of the wetted cross section area for a given flow velocity and slope a hypothetical wetted cross section area is estimated using empirical relationships defined for rills formed in bare soils this procedure was also successfully applied to data from former field experiments the procedure allows estimating erosion rates when flow characteristics velocity and hydraulic radius are known however the prediction of these flow characteristics remains uncertain

concentrated flow erosion rates as affected by rock fragment cover and initial soil moisture content abstract concentrated flow experiments using a small hydraulic flume and a constant flow discharge and bed slope have been conducted in order to investigate the effects of rock fragment cover rc on sediment yield for an initially wet and an initially air dry loamy topsoil the experimental results indicate that rc reduces concentrated flow erosion rates e in an exponential way i e e e brc which is similar to previously reported relations for other water erosion processes such as interrill erosion and sheet rill erosion measured on runoff plots the decay rate b of this exponential relationship increased throughout the experiments because of scour hole development and bed armouring the concentrated flow erosion rates and b values also depend on the initial moisture content of the topsoil depending on rc mean concentrated flow erosion rates were 20 to 65 less on initially wet compared to initially air dry topsoils the mean value for b was 0 032 for the initially wet but only 0 017 for the initially air dry topsoil indicating that a rock fragment cover is less efficient in reducing concentrated flow erosion rates when the topsoil is initially air dry than when it is initially wet the results help explain the data scatter in reported relationships between rc and interrill rill erosion rates they also indicate that a given surface rock fragment cover will offer more protection to wet topsoils than to dry topsoils which are very common in mediterranean environments event based water erosion models should incorporate effects of antecedent soil moisture content as well as those of rc on concentrated flow erosion rates