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1.
Sediment dynamics below retreating cliffs   总被引：1，自引：0，他引：1  
The retreat of cliffs may constitute the dominant erosional response to base‐level fall in arid settings underlain by horizontally‐bedded sedimentary rock. These vertical cliffs typically loom above a relatively straight bedrock slope (‘plinth’) that is mantled with a thin layer of sediment and perched near the angle of repose. In detail, a plinth consists of a system of quasi‐parallel ridges and channels. We ask how the sediment supplied from a retreating cliff influences the erosion of the plinth hillslopes and channels, and how this affects the rate of cliff retreat. Motivated by field observations and high‐resolution topographic data from two sites in western Colorado, we develop a two‐dimensional (2D), rules‐based numerical model to simulate the erosion of channels draining a plinth and diffusive erosion of the intervening interfluves. In this model, retreat of a cliffband occurs when the height of the vertical cliff exceeds a threshold due to incision by channels on the plinth below. Debris derived from cliff retreat is distributed over the model plinth according to the local topography and distance from the source. This debris then weathers in place, and importantly can act to reduce local bedrock erosion rates, protecting both the plinth and ultimately the cliff from erosion. In this paper, we focus on two sets of numerical model experiments. In one suite, we regulate the rate of rockfall to limit the cliff retreat rate; in most cases, this results in complete loss of the plinth by erosion. In a second suite, we do not impose a limit on the cliff retreat rate, but instead vary the weathering rate of the rockfall debris. These runs result in temporally steady cliff‐plinth forms and retreat rates; both depend on the weathering rate of the debris. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

2.
Sea stacks are common and striking coastal landforms, but few details are known about how, how quickly, and under what conditions they form. We present numerical and analytical models of sea stack formation due to preferential erosion along a pre‐existing headland to address these basic questions. On sediment‐rich rocky coasts, as sea cliffs erode and retreat, they produce beach sediment that is distributed by alongshore sediment transport and controls future sea cliff retreat rates. Depending on their width, beaches can encourage or discourage sea cliff erosion by acting either as an abrasive tool or a protective cover that dissipates wave energy seaward of the cliff. Along the flanks of rocky headlands where pocket beaches are often curved and narrow due to wave field variability, abrasion can accelerate alongshore‐directed sea cliff erosion. Eventually, abrasion‐induced preferential erosion can cut a channel through a headland, separating it from the mainland to become a sea stack. Under a symmetrical wave climate (i.e. equal influence of waves approaching the coastline from the right and from the left), numerical and analytical model results suggest that sea stack formation time and plan‐view size are proportional to preferential erosion intensity (caused by, for example, abrasion and/or local rock weakness from joints, faults, or fractures) and initial headland aspect ratio, and that sea stack formation is discouraged when the sediment input from sea cliff retreat is too high (i.e. sea cliffs retreat quickly or are sand‐rich). When initial headland aspect ratio is too small, and the headland is ‘rounded’ (much wider in the alongshore direction at its base than at its seaward apex), the headland is less conducive to sea stack formation. On top of these geomorphic and morphologic controls, a highly asymmetrical wave climate decreases sea stack size and discourages stack formation through rock–sediment interactions. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

3.
Riverbank retreat along a bend of the Cecina River, Tuscany (central Italy) was monitored across a near annual cycle (autumn 2003 to summer 2004) with the aim of better understanding the factors influencing bank changes and processes at a seasonal scale. Seven flow events occurred during the period of investigation, with the largest having an estimated return period of about 1·5 years. Bank simulations were performed by linking hydrodynamic, fluvial erosion, groundwater flow and bank stability models, for the seven flow events, which are representative of the typical range of hydrographs that normally occur during an annual cycle. The simulations allowed identification of (i) the time of onset and cessation of mass failure and fluvial erosion episodes, (ii) the contributions to total bank retreat made by specific fluvial erosion and mass‐wasting processes, and (iii) the causes of retreat. The results show that the occurrence of bank erosion processes (fluvial erosion, slide failure, cantilever failure) and their relative dominance differ significantly for each event, depending on seasonal hydrological conditions and initial bank geometry. Due to the specific planimetric configuration of the study bend, which steers the core of high velocity fluid away from the bank at higher flow discharges, fluvial erosion tends to occur during particular phases of the hydrograph. As a result fluvial erosion is ineffective at higher peak discharges, and depends more on the duration of more moderate discharges. Slide failures appear to be closely related to the magnitude of peak river stages, typically occurring in close proximity to the peak phase (preferentially during the falling limb, but in some cases even before the peak), while cantilever failures more typically occur in the late phase of the flow hydrograph, when they may be induced by the cumulative effects of any fluvial erosion. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

4.
To quantify the changes in flow energy, sediment yield and surface landform impacted by headcut height during bank gully erosion, five experimental platforms were constructed with different headcut heights ranging from 25 to 125 cm within an in situ active bank gully head. A series of scouring experiments were conducted under concentrated flow and the changes in flow energy, sediment yield and surface landform were observed. The results showed that great energy consumption occurred at gully head compared to the upstream area and gully bed. The flow energy consumption at gully heads and their contribution rates increased significantly with headcut height. Gully headcuts also contributed more sediment yield than the upstream area. The mean sediment concentrations at the outlet of plots were 2.3 to 7.3 times greater than those at the end of upstream area. Soil loss volume at gully heads and their contribution rates also increased with headcut height significantly. Furthermore, as headcut height increased, the retreat distance of gully heads increased, which was 1.7 to 8.9 times and 1.1 to 3.2 times greater than the incision depth of upstream area and gully beds. Positive correlations were found between energy consumption and soil loss, indicating that energy consumption could be used to estimate soil loss of headcut erosion. Headcut height had a significant impact on flow energy consumption, and thus influenced the changes in sediment yield and landform during the process of gully headcut erosion. Headcut height was one of the important factors for gully erosion control in this region. Further studies are needed to identify the role of headcut height under a wide condition. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

5.
This study provides fundamental examination of mass fluvial erosion along a stream bank by identifying event timing, quantifying retreat lengths, and providing ranges of incipient shear stress for hydraulically driven erosion. Mass fluvial erosion is defined here as the detachment of thin soil layers or conglomerates from the bank face under higher hydraulic shear stresses relative to surface fluvial erosion, or the entrainment of individual grains or aggregates under lower hydraulic shear stresses. We explore the relationship between the two regimes in a representative, US Midwestern stream with semi‐cohesive bank soils, namely Clear Creek, IA. Photo‐Electronic Erosion Pins (PEEPs) provide, for the first time, in situ measurements of mass fluvial erosion retreat lengths during a season. The PEEPs were installed at identical locations where surface fluvial erosion measurements exist for identifying the transition point between the two regimes. This transition is postulated to occur when the applied shear stress surpasses a second threshold, namely the critical shear stress for mass fluvial erosion. We hypothesize that the regimes are intricately related and surface fluvial erosion can facilitate mass fluvial erosion. Selective entrainment of unbound/exposed, mostly silt‐sized particles at low shear stresses over sand‐sized sediment can armor the bank surface, limiting the removal of the underlying soil. The armoring here is enhanced by cementation from the presence of optimal levels of sand and clay. Select studies show that fluvial erosion strength can increase several‐fold when appropriate amounts of sand and clay are mixed and cement together. Hence, soil layers or conglomerates are entrained with higher flows. The critical shear stress for mass fluvial erosion was found to be an order of magnitude higher than that of surface fluvial erosion, and proceeded with higher (approximately 2–4 times) erodibility. The results were well represented by a mechanistic detachment model that captures the two regimes. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

6.
Gully erosion is a major cause of soil loss and severe land degradation in sub-humid Ethiopia. The objective of this study was to investigate the role and the effect of subsurface water level change on gully headcut retreat, gully formation and expansion in high rainfall tropical regions in the Ethiopian highlands. During the rainy seasons of 2017–2019, the expansion rate of 16 fixed gullies was measured and subsurface water levels were measured by piezometers installed near gully heads. During the study period, headcut retreats ranged from 0.70 to 2.35 m, with a mean value of 1.49 ± 0.56 m year−1, and average depth of the surface water level varied between 1.12 and 2.82 m, with a mean value of 2.62 m. Gully cross-section areas ranged from 2.90 to 20.90 m2, with an average of 9.31 ± 4.80 m2. Volumetric retreat of gully headcuts ranged from 4.49 to 40.55 m3 and averaged 13.34 ± 9.10 m3. Soil loss from individual gullies ranged from 5.79 to 52.31 t year−1 and averaged 17.21 ± 11.74 t year−1. The headcut retreat rate and sediment yield were closely related over the three study seasons. Elevated subsurface water levels facilitated the slumping of gully banks and heads, causing high sediment yield. When the soil was saturated, bank collapse and headcut retreat were favoured by the combination of elevated subsurface water and high rainfall. This study indicates that area exclosures are effective in controlling subsurface water level, thus reducing gully headcut retreat and associated soil loss.  相似文献   

7.
This study investigates how medium‐term gully‐development data differ from short‐term data, and which factors influence their spatial and temporal variability at nine selected actively retreating bank gullies situated in four Spanish basin landscapes. Small‐format aerial photographs using unmanned, remote‐controlled platforms were taken at the gully sites in short‐term intervals of one to two years over medium‐term periods of seven to 13 years and gully change during each period was determined using stereophotogrammetry and a geographic information system. Results show a high variability of annual gully retreat rates both between gullies and between observation periods. The mean linear headcut retreat rates range between 0·02 and 0·26 m a–1. Gully area loss was between 0·8 and 22 m² a–1 and gully volume loss between 0·5 to 100 m³ a–1, of which sidewall erosion may play a considerable part. A non‐linear relationship between catchment area and medium‐term gully headcut volume change was found for these gullies. The short‐term changes observed at the individual gullies show very high variability: on average, the maximum headcut volume change observed in 7–13 years was 14·3 times larger than the minimum change. Dependency on precipitation varies but is clearly higher for headcuts than sidewalls, especially in smaller and less disturbed catchments. The varying influences of land use and human activities with their positive or negative effects on runoff production and connectivity play a dominant role in these study areas, both for short‐term variability and medium‐term difference in gully development. The study proves the value of capturing spatially continuous, high‐resolution three‐dimensional data using small‐format aerial photography for detailed gully monitoring. Results confirm that short‐term data are not representative of longer‐term gully development and demonstrate the necessity for medium‐ to long‐term monitoring. However, short‐term data are still required to understand the processes – particularly human activity at varying time scales – causing fluctuations in gully erosion rates. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

8.
First discovered in Poland, glacial curvilineations (GCLs) are enigmatic landforms comprising parallel sets of sinuous ridges and troughs of metres amplitude and around 150 m wavelength, found within kilometres‐wide valleys interpreted as being produced by meltwater flowing subglacially. Their morphological and sedimentary characteristics and association with tunnel valleys has been described for some prominent Polish examples. From these observations the existing hypothesis is that they form as a consequence of erosion by longitudinal vortices that develop in subglacial floods. Here we report, for the first time, GCLs found along the southern sector of the Laurentide Ice Sheet in three northern states of the USA. Using mapping and topographic analysis from high resolution digital elevation models we report observations on their morphological properties and landform associations. We find aspects of their context and morphology difficult to explain using the existing hypothesis. We instead suggest that these glacial curvilineations are produced by subglacial bank and slope failures that locally widen tunnel valleys, or that occur near subglacial lake shorelines. Further investigation is required to test this hypothesis and to ascertain the mechanisms of proposed mass movements, which may have occurred by rotational or translational slope failure or by creep deformation. Our preferred mechanism is that such movements occurred where subglacial water was emplaced over previously perma‐frozen ground. Under such circumstances, sediment blocks thawed by the water may then easily glide over a frozen décollement at low slope angles; analogous to subaerial active‐layer glides in permafrost environments. Permafrost spring sapping may have provided lines of weakness for slope failure. If the requirement for permafrost is found to hold, then GCLs may become an important indicator of the palaeo‐distribution of permafrost. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

9.
Bank retreat in the Jingjiang Reach is closely related not only to the near‐bank intensity of fluvial erosion but also to the composition and mechanical properties of bank soils. Therefore, it is necessary to correctly simulate bank retreat to determine the characteristics of fluvial processes in the Jingjiang Reach. The current version of bank stability and toe erosion model (5.4) was improved to predict riverbank retreat, by inputting a dynamic water table, and calculating the approximation of the distribution of dynamic pore water pressure in the soil near the river bank face, and considering the depositional form of the failed blocks, which is assumedly based on a triangular distribution, with the slope approximately equalling the stable submerged bank slope and half of collapsed volume deposited in the bank‐toe region. The degrees of riverbank stability at Jing34 were calculated using the improved bank stability and toe erosion model. The results indicate the following trends: (a) the degrees of riverbank stability were high during the dry season and the rising stage, which led to minimal bank failure, and (b) the stability degrees were low during the flood season and the recession stage, with the events of bank collapse occurring frequently, which belonged to a stage of intensive bank erosion. Considering the effects of bank‐toe erosion, water table lag, and the depositional form of the collapsed bank soil, the bank‐retreat process was simulated at the right riverbank of Jing34. The model‐predicted results exhibit close agreement with the measured data, including the total bank‐retreat width and the collapsed bank profile. A sensitivity analysis was conducted to determine the quantitative effects of toe erosion and water table lag on the degree of bank stability. The calculated results for toe erosion indicate that the amount of toe erosion was largest during the flood season, which was a primary reason for bank failure. The influence of water table lag on the degree of stability demonstrates that water table lag was an important cause of bank failure during the recession stage.  相似文献   

10.
Traditionally gully erosion has been identified with the dissection of the landscape in agricultural settings but it is also recognized as a prevalent erosion feature in earthen dam auxiliary spillways and embankments. Flows through earthen spillways and over dam embankments, due to large rainfall events, have the potential to erode and breach the dam or spillway and result in catastrophic releases from the reservoir. The gully erosion process in an earthen spillway or on an embankment can be characterized by stages of initiation, development, and migration of a headcut. A headcut is defmed as a near vertical drop at the upstream end of a gully. The rate of headcut migration is important in determining the breach potential of an earthen spillway and dam embankment. A research program is being conducted to examine the gully erosion processes of earthen dam auxiliary spillways and embankments. This paper describes： l ） the unique test facilities constructed to examine the dominant factors affecting the erosion of earthen spillways and embankments; 2） the observations of the erosion processes and results to date; and 3） the predictive relationships that have been developed for dam gully erosion research at the ARS Hydraulic Engineering Research Unit laboratory in Stillwater, OK.  相似文献   

11.
Those factors controlling the weathering and erosion of sandstone on the field scale are still not well understood. In this study, a specific sandstone overhang (and its surroundings) with artificially induced and extremely high erosion rates was subjected to a complex investigation. Contrast between the erosion rate of the wet and dry portions of the same cliff enabled isolation of the factors responsible for rapid sandstone retreat. Erosion rates, moisture, and salt content, as well as suction were monitored in the field. Mineral phases and water chemistry were analyzed. The measurement of tensile strength, laboratory frost weathering tests, and numerical modeling of stress were performed. The acquired data show that an increase of moisture content in pores in the area of the studied overhang decreased tensile strength of the sandstone to 14% of its dry value, and increases the sandstone weathering and erosion rate, by nearly four orders of magnitude, compared to the same sandstone under natural moisture conditions outside of the cliff area. Consequently, frost weathering, in combination with wetting weakening was found to play a major role in weathering/erosion of the sandstone cliff and overhang. Frost weathering rate in both the laboratory and field increases up to 15 times with decreasing gravity‐induced stress. The results also indicate that sandstone landforms in temperate climates may potentially develop very rapidly if the pore space is nearly saturated with water, and will later remain relatively stable when the moisture content decreases. As a general implication, it is suggested that overhangs in Central Europe (and elsewhere) might be the result of rapid frost weathering of nearly saturated sandstone during the Last Glacial. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

12.
The complete natural drainage in 2008, 2011, and 2012 of Mountain Lake in Giles County, Virginia, allowed detailed observations of the only natural lake basin in the southern Appalachian Mountains. Here we use these observations to support geomorphic analysis and develop a model of basin evolution, which may advance the understanding of rare flow‐through lakes with subsurface drainage elsewhere. Key features included (a) an angle‐of‐repose slope with a smoothly concave planform across the entire 260 m width of the north end of the basin, (b) an arc of steep‐sided depressions along the deep northern margin of the basin floor, and (c) an abrupt transition between colluvial and finer‐grained sedimentary deposits on the floor. Our geomorphic analysis suggests that subsurface erosion has enabled long‐term northward scarp retreat in the basin by removing water and sediment. Mountain Lake formed on the northern limb of a breached anticline along the Eastern Continental Divide, where strong‐over‐weak stratigraphy and a small watershed have enabled the basin to evolve generally as follows. (1) Pond Drain, a first‐order tributary of the New River, incised north‐dipping sandstones and underlying shales on the northern limb of the anticline. The valley floor subsequently accumulated meters to tens of meters of mostly late Pleistocene colluvial fill. (2) Subsurface drainage developed likely along the contact between the sandstones and shales, facilitated by pre‐existing fractures. (3) Ongoing subsurface erosion has progressively undermined the sandstone, causing scarp retreat along the northern margin of the basin while a surface stream intermittently incised the shallow southern end. Sedimentary deposits indicate that only the deeper northern portion of the basin is usually flooded under Holocene conditions. Our basin evolution model suggests slow development of the basin over hundreds of thousands of years rather than sudden damming by a catastrophic landslide. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

13.
Channel bars and banks strongly affect the morphology of both braided and meandering rivers. Accordingly, bar formation and bank erosion processes have been greatly explored. There is, however, a lack of investigations addressing the interactions between bed and bank morphodynamics, especially over short timescales. One major implication of this gap is that the processes leading to the repeated accretion of mid‐channel bars and associated widenings remain unsolved. In a restored section of the Drau River, a gravel‐bed river in Austria, mid‐channel bars have developed in a widening channel. During mean flow conditions, the bars divert the flow towards the banks. One channel section exhibited both an actively retreating bank and an expanding mid‐channel bar, and was selected to investigate the morphodynamic processes involved in bar accretion and channel widening at the intra‐event timescale. We repeatedly surveyed riverbed and riverbank topography, monitored riverbank hydrology and mounted a time‐lapse camera for continuous observation of riverbank erosion processes during four flow events. The mid‐channel bar was shown to accrete when it was submerged during flood events, which at the subsequent flow diversion during lower discharges narrowed the branch along the bank and increased the water surface elevation upstream from the riffle, which constituted the inlet into the branch. These changes of bed topography accelerated the flow along the bank and triggered bank failures up to 20 days after the flood events. Four analysed flow events exhibited a total bar expansion from initially 126 m2 to 295 m2, while bank retreat was 6 m at the apex of the branch. The results revealed the forcing role of bar accretion in channel widening and highlighted the importance of intra‐event scale bed morphodynamics for bank erosion, which were summarized in a conceptual model of the observed bar–bank interactions. Copyright © 2015 John Wiley & Sons, Ltd.  相似文献   

14.
Gully erosion is a major environmental problem, posing significant threats to sustainable development. However, insights on techniques to prevent and control gullying are scattered and incomplete, especially regarding failure rates and effectiveness. This review aims to address these issues and contribute to more successful gully prevention and control strategies by synthesizing the data from earlier studies. Preventing gully formation can be done through land use change, applying soil and water conservation techniques or by targeted measures in concentrated flow zones. The latter include measures that increase topsoil resistance and vegetation barriers. Vegetation barriers made of plant residues have the advantage of being immediately effective in protecting against erosion, but have a short life expectancy as compared to barriers made of living vegetation. Once deeply incised, the development of gullies may be controlled by diverting runoff away from the channel, but this comes at the risk of relocating the problem. Additional measures such as headcut filling, channel reshaping and headcut armouring can also be applied. To control gully channels, multiple studies report on the use of check dams and/or vegetation. Reasons for failures of these techniques depend on runoff and sediment characteristics and cross-sectional stability and micro-environment of the gully. In turn, these are controlled by external forcing factors that can be grouped into (i) geomorphology and topography, (ii) climate and (iii) the bio-physical environment. The impact of gully prevention and control techniques is addressed, especially regarding their effect on headcut retreat and network development, the trapping of sediment by check dams and reduction of catchment sediment yield. Overall, vegetation establishment in gully channels and catchments plays a key role in gully prevention and control. Once stabilized, gullies may turn into rehabilitated sites of lush vegetation or cropland, making the return on investment to prevent and control gullies high. © 2020 John Wiley & Sons, Ltd.  相似文献   

15.
Application of a simple headcut advance model for gullies   总被引：1，自引：0，他引：1       下载免费PDF全文
Gully erosion begins in streambanks and uplands as a consequence of adjustments in driving forces on the landscape imposed by changes in land use or climate. The deleterious effects of gullies worldwide have led to many site‐specific studies of gully form and function. In the continental United States, gully erosion in agricultural land has destroyed valuable farmland yet, prediction of gully processes remains problematic on a national scale. This research has proposed a simple method to predict gully headcut advance. When combined with SWAT hydrologic flow routines, the model predicted gully headcut advance with reasonable accuracy on a daily time step for time periods exceeding two decades. The model was tested in two distinct land resource areas of the United States with differing climate, soils, cover and drainage. The inputs for the headcut model have been kept simple as the model will be applied over large areas. Model inputs consist of headcut height, headcut resistance (based on soil erodibility and a root‐cover factor), and daily flow. The model is compared with an annual time step model used in assessment of headcut advance and appears to offer a better way to assess gully headcut advance. Copyright © 2017 John Wiley & Sons, Ltd.  相似文献   

16.
We quantify erosion rates in the higher sectors of the Huasco Valley, in the Main Cordillera of the semi‐arid Andes of Chile, using elevation differences between three successive geomorphic markers (pediments and paleo‐valleys) and the present day valley. Available Ar‐Ar ages of Neogene pediments are used to estimate mean erosion rates for the three periods (16 to 13 My, 13 to 8 My, and following 8 My). The landscape of the Huasco Valley is in a transient state, as indicated by well‐preserved pediment surfaces in interfluves, valleys deeply incised by fluvial and glacial erosion and scarped head‐valleys that represent the current knickzones. Higher erosion rates (45–75 m/My) are calculated for the more recent period (< 8 My) during which deep incision developed compared to previous periods (6–31 m/My). Quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the Main Cordillera. Comparison with erosion rates calculated in other drainage basins of the Chilean Andes suggests that the variability of erosion rates depends on the landscape's transient erosive state. The landscape's geomorphologic response to the uplift of the Main Cordillera results in the retreat of a knickzone, for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

17.
A three year monitoring programme of gully‐head retreat was established to assess the significance of sediment production in a drainage network that expanded rapidly by gully‐head erosion on the low‐angled alluvio‐lacustrine Njemps Flats in semi‐arid Baringo District, Kenya. This paper discusses the factors controlling the large observed spatial and temporal variation in gully‐head retreat rates, ranging from 0 to 15 m a?1. The selected gullies differed in planform and in runoff‐contributing catchment area but soil material and land use were similar. The data were analysed at event and annual timescales. The results show that at annual timescale rainfall amount appears to be a good indicator of gully‐head retreat, while at storm‐event timescale rainfall distribution has to be taken into account. A model is proposed, including only rainfall (P) and the number of dry days (DD) between storms: which explains 56 per cent of the variation in retreat rate of the single‐headed gully of Lam1. A detailed sediment budget has been established for Lam1 and its runoff‐contributing area (RCA). By measuring sediment input from the RCA, the sediment output by channelized flow and linear retreat of the gully head for nine storms, it can be seen that erosion shifts between different components of the budget depending on the duration of the dry period (DD) between storms. Sediment input from the RCA was usually the largest component for the smaller storms. The erosion of the gully head occurred as a direct effect of runoff falling over the edge (GHwaterfall) and of the indirect destabilization of the adjacent walls by the waterfall erosion and by saturation (GHmass/storage). The latter component (GHmass/storage) was usually much larger that the former (GHwaterfall). The sediment output from the gully was strongly related to the runoff volume while the linear retreat, because of its complex behaviour, was not. Overall, the results show that the annual retreat is the optimal timescale to predict retreat patterns. More detailed knowledge about relevant processes and interactions is necessary if gully‐head erosion is to be included in event‐based soil erosion models. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献   

18.
Between a.d. 2006 and 2008, we completed annual surveys of two mercury‐contaminated eroding banks, one forested and the other grass covered, along the gravel‐bed, bedrock South River in Virginia. Gridded digital terrain models with a resolution of 0·05 m were created from bank topography data collected using a terrestrial laser scanner. Model comparisons indicate that the forested bank retreated nearly 1 m around two leaning trees, while elsewhere the extent of bank retreat was negligible. On the grassy bank, retreat was controlled by the creation of small overhanging clumps of turf at the top of the bank, their occasional failure, and the ultimate removal of failed debris from the bank toe. Partial autocorrelation analysis of vertically integrated bank retreat demonstrates that bank profile erosion is virtually uncorrelated at horizontal distances greater than about 1 m on both banks, a length scale of approximately half the bank height. This extensive streamwise variability suggests that widely spaced profile data cannot adequately represent bank erosion at these sites. Additional analysis of our comprehensive spatial data also indicates that traditional bank profile surveys with any spacing greater than 1 m would result in measurement errors exceeding 10%, an important conclusion for assessing annual rates of mercury loading into the South River from bank erosion. Our results suggest that three‐dimensional gridded bare‐earth models of bank topography may be required to accurately measure annual bank retreat in similar river systems. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

19.
Hydrogeomorphic processes influencing alluvial gully erosion were evaluated at multiple spatial and temporal scales across the Mitchell River fluvial megafan in tropical Queensland, Australia. Longitudinal changes in floodplain inundation were quantified using river gauge data, local stage recorders and HEC‐RAS modelling based on LiDAR topographic data. Intra‐ and interannual gully scarp retreat rates were measured using daily time‐lapse photographs and annual GPS surveys. Erosion was analysed in response to different water sources and associated erosion processes across the floodplain perirheic zone, including direct rainfall, infiltration‐excess runoff, soil‐water seepage, river backwater and overbank flood inundation. The frequency of river flood inundation of alluvial gullies changed longitudinally according to river incision and confinement. Near the top of the megafan, flood water was contained within the macrochannel up to the 100‐year recurrence interval, but river backwater still partially inundated adjacent gullies eroding into Pleistocene alluvium. In downstream Holocene floodplains, inundation of alluvial gullies occurred beyond the 2‐ to 5‐year recurrence interval and contributed significantly to total annual erosion. However, most gully scarp retreat at all sites was driven by direct rainfall and infiltration‐excess runoff, with the 24‐h rainfall total being the most predictive variable. The remaining variability can be explained by seasonal vegetative conditions, complex cycles of soil wetting and drying, tension crack development, near‐surface pore‐water pressure, soil block undermining from spalling and overland flow, and soil property heterogeneity. Implications for grazing management impacts on soil surface and perennial grass conditions include effects on direct rainfall erosion, water infiltration, runoff volume, water concentration along tracks, and the resistance of highly dispersible soils to gully initiation or propagation under intense tropical rainfall. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

20.
Streambank erosion is a pathway for sediment and nutrient loading to streams, but insufficient data exist on the magnitude of this source. Riparian protection can significantly decrease streambank erosion in some locations, but estimates of actual sediment load reductions are limited. The objective of this research was to quantify watershed‐scale streambank erosion and estimate the benefits of riparian protection. The research focused on Spavinaw Creek within the Eucha‐Spavinaw watershed in eastern Oklahoma, where composite streambanks consist of a small cohesive topsoil layer underlain by non‐cohesive gravel. Fine sediment erosion from 2003 to 2013 was derived using aerial photography and processed in ArcMap to quantify eroded area. ArcMap was also utilized in determining the bank retreat rate at various locations in relation to the riparian vegetation buffer width. Box and whisker plots clearly showed that sites with riparian vegetation had on average three times less bank retreat than unprotected banks, statistically significant based on non‐parametric t‐tests. The total soil mass eroded from 2003 to 2013 was estimated at 7.27 × 107 kg yr.?1, and the average bank retreat was 2.5 m yr.?1. Many current erosion models assume that fluvial erosion is the dominant stream erosion process. Bank retreat was positively correlated with stream discharge and/or stream power, but with considerable variability, suggesting that mass wasting plays an important role in streambank erosion within this watershed. Finally, watershed monitoring programs commonly characterize erosion at only a few sites and may scale results to the entire watershed. Selection of random sites and scaling to the watershed scale greatly underestimated the actual erosion and loading rates. Copyright © 2016 John Wiley & Sons, Ltd.  相似文献   

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