首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The landscape evolution in Neogene intramontane basins is a result of the interaction of climatic, lithologic, and tectonic factors. When sedimentation ceases and a basin enters an erosional stage, estimating erosion rates across the entire basin can offer a good view of landscape evolution. In this work, the erosion rates in the Guadix–Baza basin have been calculated based on a volumetric estimate of sediment loss by river erosion since the Late Pleistocene. To do so, the distribution of a glacis surface at ca. 43 kyr, characterised by a calcrete layer that caps the basin infilling, has been reconstructed. To support this age, new radiometric data of the glacis are presented. The volume of sediment loss by water erosion has been calculated for the entire basin by comparing the reconstructed geomorphic surface and the present-day topography. The resulting erosion rates vary between 4.28 and 6.57 m3 ha− 1 yr− 1, and are the consequence of the interaction of climatic, lithologic, topographic, and tectonic factors. Individual erosion rates for the Guadix and Baza sub-basins (11.80 m3 ha− 1 yr− 1 and 1.77 m3 ha− 1 yr− 1 respectively) suggest different stages of drainage pattern evolution in the two sub-basins. We attribute the lower values obtained in the Baza sub-basin to the down-throw of this sub-basin caused by very recent activity along the Baza fault.  相似文献   

2.
A new and simple method is developed to efficiently quantify erosion and deposition rates based on stock unearthing measurements. This is applicable to spatial scales ranging from plot to hillslopes, and to time scales ranging from single hydrologic events to centennial scales. The method is applied to a plot area on vineyard hillslopes in Burgundy (Monthélie, France), with measurement of 4328 vine plants. A sediment budget established at the plot scale shows a mean soil lowering of 3.44 ± 1 cm over 20 years, involving a minimal erosion rate of 1.7 ± 0.5 mm yr− 1. Locally, erosion rates can reach up to 8.2 ± 0.5 mm yr− 1.This approach allows the sediment redistribution to be mapped and analyzed at 1-m resolution. It provides novel insights into the characterization of erosion patterns on pluri-decennial scales and into the analysis of spatial distribution of erosion processes on cultivated hillslopes.  相似文献   

3.
Muddy floods, i.e. runoff from cultivated areas carrying large quantities of soil, are frequent and widespread in the European loess belt. They are mainly generated in dry zero-order valleys and are nowadays considered as the most likely process transferring material eroded from cultivated hillslopes during the Holocene to the flood plain. The huge costs of muddy flood damages justify the urgent installation of control measures. In the framework of the ‘Soil Erosion Decree’ of the Belgian Flemish region, a 12 ha-grassed waterway and three earthen dams have been installed between 2002–2004 in the thalweg of a 300-ha cultivated dry valley in the Belgian loess belt. The measures served their purpose by preventing any muddy flood in the downstream village, despite the occurrence of several extreme rainfall events (with a maximum return period of 150 years). The catchment has been intensively monitored from 2005–2007 and 39 runoff events were recorded in that period. Peak discharge (per ha) was reduced by 69% between the upstream and the downstream extremities of the grassed waterway (GWW). Furthermore, runoff was buffered for 5–12 h behind the dams, and the lag time at the outlet of the catchment was thereby increased by 75%. Reinfiltration was also observed within the waterway, runoff coefficients decreasing by a mean of 50% between both extremities of the GWW. Sediment discharge was also reduced by 93% between the GWW's inflow and the outlet. Before the installation of the control measures, specific sediment yield (SSY) of the catchment reached 3.5 t ha− 1 yr− 1 and an ephemeral gully was observed nearly each year in the catchment. Since the control measures have been installed, no (ephemeral) gully has developed and the SSY of the catchment dropped to a mean of 0.5 t ha− 1 yr− 1. Hence, sediment transfer from the cultivated dry valley to the alluvial plain should dramatically decrease. Total cost of the control measures that are built for a 20 year-period is very low (126 € ha− 1) compared to the mean damage cost associated with muddy floods in the study area (54 € ha− 1 yr− 1). Similar measures should therefore be installed to protect other flooded villages of the Belgian loess belt and comparable environments.  相似文献   

4.
In catchments adjacent to the Great Barrier Reef World Heritage Area in Queensland, Australia, there is a growing concern that sediments and nutrients being exported from the land are having a detrimental effect on coral reef communities. There is a need to determine the processes and rates of erosion from the major land use types, so that management intervention can be initiated to reduce sediment yields where required. This paper presents a sediment budget for Weany Creek, a 13.5 km2 grazed semi-arid sub-catchment of the Burdekin River catchment, Australia. A range of field methods was used to measure erosion from hillslopes, gullies and stream banks, as well as identify the amount of sediment being deposited and remobilised on the bed of gullies and the stream network. The data suggests that at least during drought conditions, the primary erosion source in this catchment is gully erosion. However, the largest source of sediment in the budget is actually associated with the remobilisation of in-channel sediment stores. Overall, the sediment budget is comprised of  81% coarse material and 19% fine sediment and an agreement between the fine sediment yield estimated in the sediment budget and the yield measured at the catchment outlet is within 10%. The total sediment yield estimated for this catchment is  4205 t yr− 1 and is much lower than expected for a catchment of this size. This may reflect the drought conditions during the measurement period; however, there is also the possibility that the primary erosion sources have been exhausted, and the rates of sediment loss may be much lower now than they may have been in the past. Nonetheless, the results show that stored sediment, which may have been deposited in the channel many decades ago, is an important contributor to end of catchment sediment yields and warrants further investigation.  相似文献   

5.
Rates of sheet and rill erosion in Germany — A meta-analysis   总被引：2，自引：0，他引：2  
K. Auerswald  P. Fiener  R. Dikau   《Geomorphology》2009,111(3-4):182-193
Knowledge of erosion rates under real conditions is of great concern regarding sustainability of landuse and off-site effects on water bodies and settlements. Experimentally derived rates of sheet and rill erosion are often biased by experimental settings, which deviate considerably from typical landuse, by short measuring periods and by small spatial extensions, which do not account for the pronounced spatio-temporal variability of erosion events. We compiled data from 27 studies covering 1076 plot years to account for this variability. Modelling was used to correct for deficiencies in the experimental settings, which overrepresented arable land and used steeper and shorter slopes as well as higher erosivity than typically found in reality. For example, the average slope gradient was 5.9° for all arable plot experiments while it is only 2.6° on total arable land in Germany. The expected soil loss by sheet and rill erosion in Germany after taking real slopes, landuse and erosivity into account averaged 2.7 t ha− 1 yr− 1. Annual crops contributed the largest proportion (90%) but hops despite its negligible contribution to landuse (0.06%) still contribute 1.0% due to its extraordinary rapid erosion, which was even faster than the measured bare fallow soil loss standardized to otherwise identical conditions. Bare fallow soil loss, which is often used as baseline, was 80 t ha− 1 yr− 1 when standardized to 5.1° slope gradient, 200 m flow path length, and average German erosivity.  相似文献   

6.
The construction of multiple dams and barrages in many Indian River basins over the last few decades significantly reduced river flow to the sea and affected the sediment regime. More reservoir construction is planned through the proposed National River Linking Project (NRLP), which will transfer massive amounts of water from the North to the South of India. The impacts of these developments on fertile and ecologically sensitive deltaic environments are poorly understood and quantified at present. In this paper an attempt is made to identify, locate and quantify coastal erosion and deposition processes in one of the major river basins in India—the Krishna—using a time series of Landsat images for 1977, 1990 and 2001 with a spatial resolution ranging from 57.0 m to 28.5 m. The dynamics of these processes are analyzed together with the time series of river flow, sediment discharge and sediment storage in the basin. Comparisons are made with similar processes identified and quantified earlier in the delta of a neighboring similarly large river basin—the Godavari. The results suggest that coastal erosion in the Krishna Delta progressed over the last 25 years at the average rate of 77.6 ha yr− 1, dominating the entire delta coastline and exceeding the deposition rate threefold. The retreat of the Krishna Delta may be explained primarily by the reduced river inflow to the delta (which is three times less at present than 50 years ago) and the associated reduction of sediment load. Both are invariably related to upstream reservoir storage development.  相似文献   

7.
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 L1.29; R2=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.  相似文献   

8.
Although in the last decades gully erosion has been a thriving research field, few studies have specifically addressed the contribution and location of sidewall erosion processes in gullies. In this paper, sidewall erosion in some large gullies in a Mediterranean area (Anoia-Penedès, NE Spain) is mapped and assessed for two time intervals (1975–1995 and 1995–2002), using detailed digital elevation models derived from aerial photographs at a scale of 1:5000 to 1:7000. Logistic regression analysis is applied to compute the probability of occurrence of gully sidewall erosion from terrain variables. The results confirm the complex nature of sidewall processes, whose intensity is most probably related to rainfall characteristics. Prolonged wet soil conditions in the period 1995–2002, together with the large and high-intensity rainfall of an extreme event occurred on 10th June 2000, help to explain the different sediment production rates: 16±0.4 Mg ha−1 year−1 in 1975–1995 and 83±6.3 Mg ha−1 year−1 in 1995–2002. The logistic regression analysis revealed that gully-wall slope angle was the main factor controlling gully sidewall failure. In gully walls with high slope angles, tension crack development is the main process promoting wall collapse. The application of the logistic regression model showed a high overall accuracy (87%) but over 50% of commission and omission errors for the class of interest (sidewall erosion), in agreement with the variance explained by the model.  相似文献   

9.
T.C. Hales  J.J. Roering 《Geomorphology》2009,107(3-4):241-253
In the Southern Alps, New Zealand, large gradients in precipitation (< 1 to 12 m year− 1) and rock uplift (< 1 to 10 mm year− 1) produce distinct post-glacial geomorphic domains in which landslide-driven sediment production dominates in the wet, rapid-uplift western region, and rockfall controls erosion in the drier, low-uplift eastern region. Because the western region accounts for < 25% of the active orogen, the dynamics of erosion in the extensive eastern region are of equal importance in estimating the relative balance of uplift and erosion across the Southern Alps. Here, we assess the efficacy of frost cracking as the primary rockfall mechanism in the eastern Southern Alps using air photo and topographic analysis of scree slopes, cosmogenic radionuclide dating of headwalls, paleo-climate data, and a numerical model of headwall temperature. Currently, active scree slopes occur at a relatively uniform mean elevation ( 1450 m) and their distribution is independent of hillslope aspect and rock type, consistent with the notion that frost cracking (which is maximized between − 3 and − 8 °C) may control rockfall erosion. Headwall erosion rates of 0.3 to 0.9 mm year− 1, measured using in-situ 10Be and 26Al in the Cragieburn Range, confirm that rockfall erosion is active in the late Holocene at rates that roughly balance rock uplift. Models of the predicted depth of frost activity are consistent with the scale of fractures and scree blocks in our field sites. Also, vegetated, paleo-scree slopes are ubiquitous at elevations lower than active scree slopes, consistent with the notion that lower temperatures during the last glacial advance induced pervasive rockfall erosion due to frost cracking. Our modeling suggests temporally-averaged peak frost cracking intensity occurs at 2300 m a.s.l., the approximate elevation of the highest peaks in the central Southern Alps, suggesting that the height of these peaks may be limited by a “frost buzzsaw.”  相似文献   

10.
Using the USPED (Unit Stream Power Erosion Deposition) model, three land use scenarios were analysed for an Italian small catchment (15 km2) of high landscape value. The upper Orme stream catchment, located in the Chianti area, 30 km south of Florence, has a long historical agriculture record. Information on land use and soil conservation practices date back to 1821, hence offering an opportunity to model impacts of land use change on erosion and deposition. For this study, a procedure that takes into account soil conservation practices and potential sediment storage is proposed. The approach was to calculate and model the flow accumulation considering rural and logging roads, location of urban areas, drainage ditches, streams, gullies and permanent sediment sinks. This calculation attempts to assess the spatial variability, especially the impact of support practices (P factor). Weather data from 1980–2003 were taken into account to calculate the R factor. However, to consider the intense pluviometric conditions in terms of the erosivity factor, the 0.75th quantile was used, while the lowest erosivity was modelled using the 0.25th quantile. Results of the USPED model simulation show that in 1821 the mean annual net erosion for the watershed was 2.8 Mg ha− 1 y− 1; in 1954 it was 4.2 Mg ha− 1 y− 1; and in 2004 it was 5.3 Mg ha− 1 y− 1. Conservation practices can reduce erosion processes by ≥ 20 Mg ha− 1 y− 1 when the 1821 practices are introduced in the present management. On the other hand, if the support practices are not considered in the model, soil erosion risk is overestimated. Field observation for the present-day simulation confirmed that erosion and associated sediment deposition predicted by the model depend, as expected, on geomorphology and land use. The model shows limitations that are mainly due to the input data. A high resolution DEM is essential for the delineation of reliable topographic potential to predict erosion and deposition especially in vineyards.  相似文献   

11.
Modification of Lateglacial and Holocene talus sheets by debris flows and gully incision on Mynydd Du, Wales, has resulted in a convergence of upper slope form characterised by an upper rectilinear slope gradient of 36°±3° and a range of concavities of c. 0.1–0.2. In most cases, gully incision and accumulation of debris cones have led to an increase in slope concavity. Evidence for talus erosion, reworking and redeposition on the upper slope emphasises secondary reworking processes, as well as primary talus accumulation on the upper slope, and permits construction of a model of talus development at Mynydd Du. On the basis of talus volume, calculation of the first rockwall retreat data set for southern Britain suggests that c. 7.1 m (84%) of overall rockwall retreat (8.5 m) took place during the Lateglacial, and only c. 1.4 m (16%) occurred during the Holocene. These figures imply that Lateglacial retreat rates ranged from 1.01 to 2.44 m ka−1, with an overall mean rate of 1.23 m ka−1. In contrast, Holocene rockwall retreat rates range from 0.10 to 0.17 m ka−1, with a mean rate of 0.12 m ka−1. Approximately 27% of cliff retreat is attributed to microgelivation. While similar to Holocene and present-day alpine environments, these Lateglacial retreat rates are one order of magnitude higher than most equivalent values for arctic sites. This reflects both ‘alpine-style’ diurnal freeze–thaw activity on Mynydd Du during the Younger Dryas and paraglacial rock-mass instability following deglaciation. Assuming an exponential decline in rockwall sediment release, it is estimated that approximately half the talus had accumulated within c. 1 ka of deglaciation. At one site, paraglacial talus accumulation appears to have contributed significantly to the glacial sediment transport system of a subsequent ice advance. Present-day rates of rockwall retreat and talus accumulation by rockfall are estimated to be 0.014 and 0.022 mm yr−1 (m ka−1), respectively, similar to values for other British sites and markedly lower than Holocene rates of cliff recession due to microgelivation. By implication, the geomorphic significance of microgelivation may have been greatly underestimated in studies of inland rock-slope evolution in temperate, mid-latitude environments.  相似文献   

12.
Abstract

Erosion rates in residual limestone soils in a humid climate were measured for 10 years at one site, and for 4 years at another site, using erosion pins. Erosion pins were placed in gully floors and on convex divides between adjacent gullies, on abandoned land where vegetation had been removed. We measured an average erosion rate of 20 mm yr?1 over 10 years at one site and only 5 mm yr?1 over 4 years at another site where chert gravel was common on the surface. The 10-year average erosion rate of divides (26 mm yr?1) was significantly greater than the average erosion rate of gullies (14 mm yr?1), suggesting control by different processes, some of which may be seasonal. In winter, it was observed that frost action produced a thin layer of loose soil on the surface of divides. In summer, a hardpan developed on divides, as the soil loosened by winter frosts was transported to gullies, likely by rainsplash or dry ravel. The diffusive processes of frost action, rainsplash, and dry ravel appear to shape the convex divides in this study. Down-cutting of gullies requires channelized flow produced by intense rainfall, which is more common in summer for this location.  相似文献   

13.
Understanding and quantifying sediment load is important in catchments draining highly erodible materials that eventually contribute to siltation of downstream reservoirs. Within this context, the suspended sediment transport and its temporal dynamics have been studied in the River Isábena (445 km2, south-central Pyrenees, Ebro basin) by means of direct sampling and turbidity recording during a 3-year dry period. The average flood-suspended sediment concentration was 8 g l− 1, with maximum instantaneous values above 350 g l− 1. The high scatter between discharge and suspended sediment concentrations (up to five orders of magnitude) has not permitted the use of rating curve methods to estimate the total load. Interpolation techniques yielded a mean annual sediment load of 184,253 t y− 1 for the study period, with a specific yield of 414 t km− 2 y− 1. This value resembles those reported for small torrents in nearby mountainous environments and is the result of the high connectivity between the badland source areas and stream courses, a fact that maximises sediment conveyance through the catchment. Floods dominated the sediment transport and yield. However, sediment transport was more constant through time than that observed in Mediterranean counterparts; this can be attributed to the role of base flows that entrain fine sediment temporarily stored in the channel and force the river to carry high sediment concentrations (i.e., generally in the order of 0.5 g l− 1), even under minimum flow conditions.  相似文献   

14.
In the Mediterranean area, forest fires have become a first-order environmental problem. Increased fire frequency progressively reduces ecosystem recovery periods. The fire season, usually followed by torrential rains in autumn, intensifies erosion processes and increases desertification risk. In this work, the effect of repeated experimental fires on soil response to water erosion is studied in the Permanent Field Station of La Concordia, Valencia, Spain. In nine 80 m2 plots (20 m long × 4 m wide), all runoff and sediment produced were measured after each rainfall event. In 1995, two fire treatments with the addition of different biomass amounts were applied. Three plots were burned with high fire intensity, three with moderate intensity, and three were unburned to be used as control. In 2003, the plots with the fire treatments were burned again with low fire intensities. During the 8-year interval between fires, plots remained undisturbed, allowing regeneration of the vegetation–soil system. Results obtained during the first 5 months after both fire experiments show the high vulnerability of the soil to erosion after a repeated fire. For the burned plots, runoff rates increased three times more than those of 1995, and soil losses increased almost twice. The highest sediment yield (514 g m− 2) was measured in 2003, in the plots of the moderate fire intensity treatment, which yielded only 231 g m− 2 of sediment during the corresponding period in 1995. Runoff yield from the control plots did not show significant temporal changes, while soil losses decreased from 5 g m− 2 in the first post-fire period to 0.7 g m− 2 in the second one.  相似文献   

15.
Sediment supply provides a fundamental control on the morphology of river deltas, and humans have significantly modified these supplies for centuries. Here we examine the effects of almost a century of sediment supply reduction from the damming of the Elwha River in Washington on shoreline position and beach morphology of its wave-dominated delta. The mean rate of shoreline erosion during 1939–2006 is ~ 0.6 m/yr, which is equivalent to ~ 24,000 m3/yr of sediment divergence in the littoral cell, a rate approximately equal to 25–50% of the littoral-grade sediment trapped by the dams. Semi-annual surveys between 2004 and 2007 show that most erosion occurs during the winter with lower rates of change in the summer. Shoreline change and morphology also differ spatially. Negligible shoreline change has occurred updrift (west) of the river mouth, where the beach is mixed sand to cobble, cuspate, and reflective. The beach downdrift (east) of the river mouth has had significant and persistent erosion, but this beach differs in that it has a reflective foreshore with a dissipative low-tide terrace. Downdrift beach erosion results from foreshore retreat, which broadens the low-tide terrace with time, and the rate of this kind of erosion has increased significantly from ~ 0.8 m/yr during 1939–1990 to ~ 1.4 m/yr during 1990–2006. Erosion rates for the downdrift beach derived from the 2004–2007 topographic surveys vary between 0 and 13 m/yr, with an average of 3.8 m/yr. We note that the low-tide terrace is significantly coarser (mean grain size ~ 100 mm) than the foreshore (mean grain size ~ 30 mm), a pattern contrary to the typical observation of fining low-tide terraces in the region and worldwide. Because this cobble low-tide terrace is created by foreshore erosion, has been steady over intervals of at least years, is predicted to have negligible longshore transport compared to the foreshore portion of the beach, and is inconsistent with oral history of abundant shellfish collections from the low-tide beach, we suggest that it is an armored layer of cobble clasts that are not generally competent in the physical setting of the delta. Thus, the cobble low-tide terrace is very likely a geomorphological feature caused by coastal erosion of a coastal plain and delta, which in turn is related to the impacts of the dams on the Elwha River to sediment fluxes to the coast.  相似文献   

16.
东北漫岗黑土区切沟侵蚀发育特征   总被引：14，自引：0，他引：14  
随着人们对土壤侵蚀空间尺度认识的加深, 发育在更大空间尺度上的切沟愈来愈受到重视。切沟是土壤侵蚀的重要组成部分, 但现有的土壤侵蚀模型尚未包括切沟侵蚀部分。本研究利用高精度差分GPS, 在对东北典型漫岗黑土区切沟监测的基础上, 借助GIS 平台生成DEM, 通过DEM 的叠加分析, 探讨了沟内蚀积变化特征。并在此基础上, 提出了东北切沟侵蚀的概念模型, 认为冬春季冻融侵蚀产生沟内堆积-雨季径流产生侵蚀的过程可能是该区切沟发育的一种重要模式。  相似文献   

17.
To determine for how long a landslide affects sediment discharge, the sediment yields of 15 check-dam basins were compared with the time series of landslide distributions in a mountain basin in the Tanzawa region, central Japan. The distribution of sediment yield was quantitatively estimated from deposition in the sediment pools of check dams. The relationship between the landslide history and sediment discharge in the Nakagawa River basin was examined for an approximately 80-year period. Two major landslide events occurred during this period: the 1923 Kanto Earthquake and the 1972 disaster caused by heavy rainfall. The resulting trend in sediment discharge of the whole basin, estimated using reservoir sedimentation in the Miho Dam at its base, was nearly constant, with high sediment discharge (2897 m3 km− 2 yr− 1) in the intervening quarter-century, despite the recovery of vegetation on landslide areas in this period. Comparisons of the landslide distributions resulting from the two disasters, the sediment yields of check-dam basins, and the sediment discharge of the whole basin indicate that recent sediment discharge contains landslide debris that was originated by the Kanto Earthquake that occurred over 80 years ago. Thus, to understand high sediment discharge, it is essential to investigate not only the current basin condition and recent events, but also the landslide history of the basin for at least the previous 100 years.  相似文献   

18.
The “La Clapière” area (Tinée valley, Alpes Maritimes, France) is a typical large, complex, unstable rock slope affected by Deep Seated Gravitational Slope Deformations (DGSD) with tension cracks, scarps, and a 60 × 106 m3 rock slide at the slope foot that is currently active. The slope surface displacements since 10 ka were estimated from 10Be ages of slope gravitational features and from morpho-structural analyses. It appears that tensile cracks with a strike perpendicular to the main orientation of the slope were first triggered by the gravitational reactivation of pre-existing tectonic faults in the slope. A progressive shearing of the cracks then occurred until the failure of a large rock mass at the foot of the slope. By comparing apertures, variations and changes in direction between cracks of different ages, three phases of slope surface displacement were identified: 1) an initial slow slope deformation, spreading from the foot to the top, characterized by an average displacement rate of 4 mm yr− 1, from 10–5.6 ka BP; 2) an increase in the average displacement rate from 13 to 30 mm yr− 1 from the foot to the middle of the slope, until 3.6 ka BP; and 3) development of a large failure at the foot of the slope with fast displacement rates exceeding 80 mm yr− 1 for the last 50 years. The main finding of this study is that such a large fractured slope destabilization had a very slow displacement rate for thousands of years but was followed by a recent acceleration. The results obtained agree with several previous studies, indicating that in-situ monitoring of creep of a fractured rock slope may be useful for predicting the time and place of a rapid failure.  相似文献   

19.
Gully erosion is commonly associated with agricultural landscapes where vegetation clearance and farming practices increase runoff, leading to fluvial incision. However, gully erosion can also occur in forests that have undergone some form of disturbance, either natural or resulting from human impacts. This paper reports on recent gully development within areas of undisturbed indigenous forest as a result of a high magnitude rainfall event on the East Coast of New Zealand's North Island. This region, through a combination of crushed and sheared rock types, steep topography, and tectonic and climatic setting, has high natural rates of erosion, exacerbated by European deforestation in the late 19th and early 20th centuries.Sequential air photographs, spanning a 58 year period between 1939 and 1997 were used to classify and document the growth and recovery of gully systems in the 14.1 km2 headwaters of the Mangaoporo catchment. Following a severe cyclone in 1988, with a rainfall of 535 mm, there were 21 active gully systems within the indigenous forest. On photography prior to 1988 only four gully systems were present. During this period there were 8 major rainfall events (150–250 mm). Despite further 5 rainfall events of 150–250 mm between 1988 and 1997 all gully systems showed signs of recovery, with a combined reduction in active area of 37%. The nature and location of these features is strongly influenced by lithology (orientation of jointing and bedding), and to a topographic threshold defined by catchment slope and catchment area.  相似文献   

20.
James C. Knox   《Geomorphology》2006,79(3-4):286
Understanding the time scales and pathways for response and recovery of rivers and floodplains to episodic changes in erosion and sedimentation has been a long standing issue in fluvial geomorphology. Floodplains are an important component of watershed systems because they affect downstream storage and delivery of overbank flood waters, and they also serve as sources and temporary sinks for sediments and toxic substances delivered by river systems. Here, 14C and 137Cs isotopic dating methods are used along with ages of culturally related phenomena associated with mining and agriculture to determine rates of sedimentation and morphologic change for a reach of the upper Mississippi River and adjacent tributaries in southwestern Wisconsin and northwestern Illinois. The most important environmental change that influenced fluvial activity in this region during last 10,000 years involved the conversion of a late Holocene mosaic of prairie and forest to a landscape dominated by cropland and pastureland associated with Euro-American settlement. Results presented herein for the Upper Mississippi Valley (UMV) show that the shift from pre-agriculture, natural land cover to landscape dominance by agricultural land use of the last 175–200 years typically increased rates and magnitudes of floodplain sedimentation by at least an order of magnitude. Accelerated overbank flooding led to increased bank heights on tributary streams and, in turn, contributed to more frequent deep flows of high energy. These high energy flows subsequently promoted bank erosion and lateral channel migration, and the formation of a historical meander belt whose alluvial surface constitutes a new historical floodplain inset against the earlier historical floodplain. The new historical floodplain serves as a “flume-like” channel that provides efficient downstream transport of water and sediment associated with moderate and large magnitude floods. Floodplains on lower tributaries, however, continue to experience rates of overbank sedimentation that are of anomalously high magnitude given improved land cover and land conservation since about 1950. This lower valley anomaly is explained by minimal development of historical (agriculture period) meander belts because of relatively low stream power in these channel and floodplain reaches of relatively low gradient. In general, long-term pre-agriculture rates of vertical accretion between about 10,000 and 200 years ago averaged about 0.2 mm yr− 1 in tributary watersheds smaller than about 700 km2 and about 0.9 mm yr− 1 on the floodplain of the upper Mississippi River where the contributing watershed area increases to about 170,000 km2. On the other hand, rates of historical vertical accretion during the period of agricultural dominance of the last 200 years average between 2 and 20 mm yr− 1, with short episodes of even higher rates during times of particularly poor land conservation practices. Significant hydrologic effects of mining and agricultural started by the 1820s and became widespread in the study region by the mid-19th century. The hydrologic and geomorphic influences of mining were relatively minor compared to those related to agriculture. High resolution dating of floodplain vertical accretion deposits shows that large floods have frequently provided major increments of sedimentation on floodplains of tributaries and the main valley upper Mississippi River. The relative importance of large floods as contributors to floodplain vertical accretion is noteworthy because global atmospheric circulation models indicate that the main channel upper Mississippi River should experience increased frequencies of extreme hydrologic events, including large floods, with anticipated continued global warming. Instrumental and stratigraphic records show that, coincident with global warming, a shift to more frequent large floods occurred since 1950 on the upper Mississippi River, and these floods generally contributed high magnitudes of floodplain sedimentation.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号