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1.
The influence of climate on mountain denudation has been the topic of an intense debate for two decades. This debate partly arises from the covariation of rainfall and topography during the growth of mountain ranges, both of which influence denudation. However, the denudational response of this co-evolution is poorly understood. Here, we use a landscape evolution model where the rainfall evolves according to a prescribed rainfall–elevation relationship. This relationship is a bell curve defined by a rainfall base level, a rainfall maximum and a width around the rainfall peak elevation. This is a first-order model that fits a large range of orographic rainfall data at the ca. 1-km spatial scale. We carried out simulations of an uplifting block with an alluvial apron, starting from an initially horizontal surface, and testing different rainfall–elevation relationships. We find that the denudation history is different from that with constant rainfall models. The results essentially depend on the ratio between the final steady-state summit elevation Hss and the prescribed rainfall peak elevation Hp. This ratio is hard to predict because it depends on the transient coupling of rainfall and elevation. We identified three types of results according to Hss/Hp. If Hss/Hp > 4 (Type I), the denudation rates peak when the summits reach values close to Hp. If Hss/Hp > 1.5 and < 4 (Type II), the denudation is strongly accelerated when the elevation of the summits approaches Hp, and then the denudation increases slowly towards the uplift rate. If Hss/Hp < 1.5 (Type III), the denudation evolution is similar to situations with constant and homogeneous rainfall. In the Type I and II experiments, the mountain top is subjected to aridification once the summits have passed through Hp. To adapt to this reduced rainfall, the slopes increase. This can lead to a paradoxical situation where the mountain relief increases faster, whereas the denudation increases more slowly. The development of orographic precipitation may thus favour the stability of the mean denudation rate in a rising mountain. Despite the model limitations, including a constant rainfall–elevation relationship, our study suggests that the “classical” exponential increase in the denudation rate predicted by constant rainfall models is not the common case. Instead, the common case involves pulses and acceleration of the denudation even in the absence of uplift or global climate variations.  相似文献   

2.
Field surveys, location-for-time reasoning, and computer modeling were used to study the evolution of slopes on valley walls of abandoned bedrock meanders on the Eastern Highland Rim, Tennessee. Hillslopes on the undercut slopes of cutoff incised meanders were ordered as to relative age by the height of their meander floors above the modern stream level. The assumption is that the undercut slope is actively eroded by the stream until abandonment of the meander, at which time the slope begins to evolve to a different form. More-advanced stages of evolution occur on walls of higher meanders that were abandoned earlier. The most rapid change in this initial form is the elimination of a free face, which occurs soon after the meander is abandoned. In addition, the hillslopes associated with even the lowest (youngest) cutoff meanders show somewhat gentler overall gradients than the actively undercut slopes. Hillslopes associated with meanders 3 to about 20 m above modern stream level maintain straight segments with angles showing only a slight decrease from the 36–38° associated with the lowest cutoffs; overall angles decrease, however, as the straight segment becomes shorter. The oldest slopes, those on cutoffs 30 m or greater above modern stream level, have developed into convex–concave slopes with maximum slopes of 15°.A hillslope evolution model based on previously published algorithms was used to simulate the transition of actively undercut hillslopes into hillslopes on abandoned meanders. Hillslope modeling is particularly useful in this setting. If the valley incision rate is known, an age can be estimated for the cutoff and hence for the hillslope. Alternatively, if hillslope process rates are known, a model age obtained for the hillslope can be used to estimate an incision rate. Even where both incision rates and hillslope process rates are poorly constrained, as in the present setting, modeling allows assumptions about specific rates to be evaluated by determining their implications for other rates. For example, for three cutoff meanders along one stream, best-fit criteria were used to select process rates for the model. Model ages of hillslopes were then obtained and compared with those calculated from a valley-incision rate measured elsewhere in the same physiographic province. For two of the hillslopes, model ages were found to be much younger than those calculated from the incision rate. In order to make the two ages agree, unreasonably low process rates had to be used in the model, thus implying that the incision rate probably underestimates the actual incision rate in this valley.Experimentation with heights of initial profiles, again using best-fit criteria, suggests that since abandonment of the highest cutoff, the plateau has been downwasting at a rate about one-fourth that of the valley incision rate, a finding in agreement with published rates of chemical denudation in the area.  相似文献   

3.
Isolated, laterally extensive, gently sloping surfaces known as summit flats are present at high elevations in many Laramide ranges, and are particularly well developed in the Uinta Mountains of northeastern Utah. To investigate the spatial distribution of these surfaces, and to consider possible controls on this pattern, a map of summit flats in the Uintas was developed from digital elevation data. Summit flats were identified as unglaciated areas of the landscape above an elevation of 3400 m, having a slope of less than 0.3 m m− 1, and an area greater than 5 × 10− 2 km2. As defined, summit flats comprise 43% of the unglaciated land area above 3400 m in the Uintas, with the largest individual flat covering nearly 34 km2. To quantitatively evaluate the distribution of summit flats in the Uintas, the area of summit flats was normalized to the total unglaciated area above 3400 m in 10-km-wide swaths oriented normal to the range axis. Values of percent summit flats obtained by this method decrease dramatically westward, from a high of more than 60% at the eastern end of the Uintas, to 0% at the western end. Given that individual summit flats can be diminished through lateral erosion by surrounding valley glaciers, and that the summit flats themselves were apparently never glaciated, this result suggests that glacial erosion has been more effective in the western Uintas over the course of the Quaternary. Focused glacial erosion at the upwind end of the range is consistent with the hypothesis that the proximity of Lake Bonneville enhanced precipitation over the western Uintas during the Last Glacial Maximum [Munroe, J.S., and, Mickelson, D.M., 2002. Last Glacial Maximum equilibrium-line altitudes and paleoclimate, northern Uinta Mountains, Utah, U.S.A. Journal of Glaciology, 48, 257–266].  相似文献   

4.
This paper explores the effects of hillslope mobility on the evolution of a 10-km2 drainage basin located at the northern border of the Swiss Alps. It uses geomorphologic maps and the results of numerical models that are based on the shear stress formulation for fluvial erosion and linear diffusion for hillslope processes. The geomorphic data suggest the presence of landscapes with specific cross-sectional geometries reflecting variations in the relationships between processes in channels and on hillslopes. In the headwaters, the landscape displays parabolic cross-sectional geometries indicating that mass delivered to channels by hillslope processes is efficiently removed. In the trunk stream portion, the landscape is (i) V-shaped if the downslope flux of mass is balanced by erosion in channels (i.e. if mass delivered to channels by hillslope processes is efficiently removed) and (ii) U-shaped if in-channel accumulation of hillslope-derived material occurs. This latter situation indicates a non-balanced mass flux between processes in channels and on hillslopes.Information about the spatial pattern of the postglacial depth of erosion allows comparative estimates to be made about the erosional efficiency for the various landscapes that were mapped in the study area. The data suggest that the erosional potential and sediment discharge are reduced for the situation of a non-balanced mass flux between processes in channels and on hillslopes. These findings are also supported by the numerical model. Indeed, the model results show that high hillslope mobility tends to reduce the hillslope relief and to inhibit dissection and formation of channels. In contrast, stable hillslopes tend to promote fluvial incision, and the hillslope relief increases. The model results also show that very low erosional resistance of bedrock promotes backward erosion and steepening of channel profiles in headwaters. Beyond that, the model reveals that sediment discharge generally increases with decreasing erosional resistance of bedrock, but that this increase decays exponentially with increasing magnitudes of fluvial and hillslope mobilities. Very high hillslope diffusivities even tend to reduce the erosional potential of the whole watershed. It appears that besides rates of base-level lowering, factors limiting sediment discharge might be the nonlinear relationships between processes in channels and on hillslopes.  相似文献   

5.
Studies on denudation rates can provide insight into the influence of climate change, tectonics, and human activities on landscape evolution. Research performed in Central Italy has shown considerable spatial variability of denudation rates in the major river basins. These studies have focused mainly on the Tyrrhenian side of the Italian peninsula, where Plio-Pleistocene marine deposits filling NW–SE elongated sedimentary basins have been uplifted during the Quaternary up to several hundreds of meters above present sea level. Small sub-catchments developed on clays are affected by sharp- and/or rounded-edged badlands (i.e. calanchi and biancane), representing denudation “hot spots” in the present-day morphoclimatic framework.In this paper, we analyze the relationships between indirectly estimated denudation rates at the catchment scale and field monitoring data at the hillslope scale. We attempt to better understand and quantify all hillslope processes that contribute to seasonal variability of denudation, to help with predicting the net input from “hot spots” to the overall estimated sediment yield at the basin outlets. At the hillslope scale, we discuss, in particular, the variability of denudation rates at calanchi and biancane badlands as a function of their different morphoevolution.  相似文献   

6.
Transient landscape disequilibrium is a common response to climatic fluctuations between glacial and interglacial conditions. Such landscapes are best suited to the investigation of catchment-wide response to changes in incision. The geomorphology of the Trub and Grosse Fontanne, adjacent stream systems in the Napf region of the Swiss Molasse, was analyzed using a 2-m LIDAR DEM. The two catchments were impacted by the Last Glacial Maximum, LGM, even though the glaciers never overrode this region. They did, however, cause base levels to drop by as much as 80 m. Despite their similar tectonic, lithologic and climatic settings, these two basins show very different responses to the changing boundary conditions. Stream profiles in the Trub tend to be smooth, while in the Fontanne, numerous knickzones are visible. Similarly, cut-and-fill terraces are abundant in the Trub watershed, but absent in the Fontanne, where deep valleys have been incised. The Trub appears to be a coupled hillslope–channel system because the morphometrics throughout the basin are uniform. The morphology of hillslopes upstream of the knickzones in the Fontanne is identical to that of the Trub basin, but different downstream of the knickzones, suggesting that the lower reaches of the Fontanne have been decoupled from the hillslopes. However, the rapid incision of the Fontanne is having little effect on the adjacent upper hillslopes.We tested this interpretation using cosmogenic 10Be-derived basin-averaged denudation rates and terrace dating. The coupled nature of the Trub basin is supported by the similarity of denudation rates, 350 ± 50 mm ky− 1, at a variety of spatial scales. Upstream of the knickzones, rates in the Fontanne, 380 ± 50 mm ky− 1, match those of the Trub. Downstream of the knickzones, denudation rates increase to 540 ± 100 mm ky− 1. The elevated rates in the downstream areas of the Fontanne are due to rapid incision causing a decoupling of the hillslope from the channel. Basin response time and the magnitude of base level drop exert the principal control over the difference in geomorphic response between the two basins. The timing of the filling of the Trub valley, 17 ± 2 ka, and the initial incision of the Fontanne, 16 ± 3 ka, were calculated, verifying that these are responses to late glacial perturbations. Unique lithologic controls allow for one of the fastest regolith production rates yet to be reported,  380 mm ky− 1.  相似文献   

7.
During the last decade, slope failures were reported in a 500 km2 study area in the Geba–Werei catchment, northern Ethiopia, a region where landslides were not considered an important hazard before. Field observations, however, revealed that many of the failures were actually reactivations of old deep-seated landslides after land use changes. Therefore, this study was conducted (1) to explore the importance of environmental factors controlling landslide occurrence and (2) to estimate future landslide susceptibility. A landslide inventory map of the study area derived from aerial photograph interpretation and field checks shows the location of 57 landslides and six zones with multiple landslides, mainly complex slides and debris flows. In total 14.8% of the area is affected by an old landslide. For the landslide susceptibility modelling, weights of evidence (WofE), was applied and five different models were produced. After comparison of the models and spatial validation using Receiver Operating Characteristic curves and Kappa values, a model combining data on elevation, hillslope gradient, aspect, geology and distance to faults was selected. This model confirmed our hypothesis that deep-seated landslides are located on hillslopes with a moderate slope gradient (i.e. 5°–13°). The depletion areas are expected on and along the border of plateaus where weathered basalts rich in smectite clays are found, and the landslide debris is expected to accumulate on the Amba Aradam sandstone and upper Antalo limestone. As future landslides are believed to occur on inherently unstable hillslopes similar to those where deep-seated landslides occurred, the classified landslide susceptibility map allows delineating zones where human interventions decreasing slope stability might cause slope failures. The results obtained demonstrate that the applied methodology could be used in similar areas where information on the location of landslides is essential for present-day hazard analysis.  相似文献   

8.
Soil profiles, colluvial stratigraphy, and detailed hillslope morphology are key elements used for geomorphic interpretations of the form and long-term evolution of triangular facets on a 1200 m high, tectonically active mountain front. The facets are developed on Precambrian gneisses and Tertiary volcanic and plutonic rocks along a complexly segmented, active normal-fault zone in the Rio Grande rift of northern New Mexico. The detailed morphologies of 20− to 350 m high facets are defined by statistical and time-series analyses of 40 field transects that were keyed to observations of colluvium, bedrock, microtopography, and vegetation. The undissected parts of most facets are transport-limited hillslopes mantled with varying thicknesses (0.1 to > 1 m thick) of sand and gravel colluvium between generally sparse (≤10–30%) bedrock outcrops. Facet soils range from (a) thin (≤ 0.2 m) weakly developed soils with cumulic silty A or transitional A/B epipedons above Cox horizons in bedrock or colluvium, to (b) deep (≥0.5–1 m) moderately to strongly developed profiles containing thick cambic (Bw) and/or argillic (Bt) horizons that commonly extend into highly weathered saprolitic bedrock. The presence of strongly weathered profiles and thick colluvium suggests that rates of colluvial transport and hillslope erosion are less than or equal to rates of soil development over at least a large part of the Holocene.The catenary variation of soils and colluvium on selected facet transects indicate that the degree of soil development generally increases and the thickness of colluvium decreases upslope on most facets. This overall pattern is commonly disrupted on large facet hillslopes by irregular secondary soil variations linked to intermediate-scale (20–60 + m long) concave slope elements. These features are interpreted to reflect discontinuous transport and erosion of colluvium down-slope below bedrock outcrops. The degree of weathering in subsurface bedrock commonly increases more systematically upslope on most facets than colluvial soils. This pattern is consistent with an increase in age with height on these fault-generated facet hillslopes.The characteristic range of internal variation in soils and colluvial deposits on a given facet also varies greatly among facets with differing overall morphologies and external environments. Deep cumulic soils and thick colluvium occur consistently on steep (≥ 30°), high, and relatively undissected facets above the narrow central sections of fault segments. Much thinner and less weathered colluvium and soils overlie saprolitic bedrock at shallow depths on low, highly dissected, gently sloping (≤ 20°) facets above complex fault segment boundaries. Parametric and nonparametric analyses of variance indicate that these large-scale contrasts in facet morphology correlate primarily with a few facet subgroups related, in decreasing importance, to variations in range-front faulting, bedrock lithology, and piedmont dissection or aggradation. These factors are related to facet morphology, drainage evolution, and hillslope-soil stratigraphy in a general geomorphic model for fault-generated facets. In this model, segmentation-related changes in the geometry and/or rates of faulting most strongly affect facet size, slope gradient, the thickness of colluvium and soil development, and drainage patterns. Facets of varying heights have similar hillslope forms at the same position on the range front; these characteristic morphologies are established under prevailing tectonic and nontectonic conditions on facets as bedrock is initially exposed from beneath alluvial-covered fault scarps above a height threshold of 15–35 m.  相似文献   

9.
In this paper we discuss the large-scale geomorphological characteristics of the Swiss Alps based on numerical analysis of a digital elevation model and compare these to an erodibility map constructed from a geotechnical map of Switzerland and regional geomorphological studies. Comparing the erodibility map with the large-scale morphometry shows an intimate relationship between mountain-scale erodibility and topography. On average, higher mean elevations and steeper mean slopes correlate with regions where rocks of low erodibility prevail. Areas with high peaks as well as the main water divides are controlled by the presence of bedrock with low to very low detachability. The drainage network of the Swiss Alps shows a close relationship to the lithological differences as well. Major longitudinal valleys follow easily erodible units. In the eastern and western part of the Swiss Alps, the highest values of local relief are located to the south of the main water divide, whereas in the central part, local relief is higher to the north of the main water divide. The large-scale geomorphic characteristics regarded in the framework of the geological history of uplift and denudation suggest that low and very low erodibilities lead to the development of areas of high elevations which are likely to persist over periods of 10–15 Ma. As the analysis of the Lepontine area shows, 20 Ma after cessation of exhumation, such high elevations are likely to be worn down and to manifest themselves as high relief only.  相似文献   

10.
Ireneusz Malik   《Geomorphology》2008,93(3-4):421-436
Small gullies occur in forested gully systems on the undulating loess plateau in southern Poland. The old gully hillslopes are mainly covered with 200-year old beech trees in contrast with the surface of the summit plateau, which is cultivated agricultural land. Beech roots are exposed in the gullies through erosion. Wood vessels in the root tree rings divide into early wood and late wood and, after the roots are exposed, start to make fewer vessels. These anatomical changes in root tree rings allow us to date erosion episodes.Small gullies form in a different manner on the valley floor and on hillslopes. In valley bottoms, erosion features are often formed at some distance from one another, and in time small gullies combine to form a single, longer one. Depending on local conditions, such as the hillslope profile, hillslopes may exhibit headward erosion or may be eroded downwards. Hilllope gullies may be transformed into side valleys as a result of gradual widening and deepening.Dating the exposure of roots indicates that small gullies had already formed in the valley system by 1949. Intensive gully erosion was recorded between 1984 and 2002, during intense precipitation in 1984 and, of particular note, during the extraordinary flood of 1997 which affected all of Central Europe. The mean rate of small gully erosion in the old gully system studied is 0.63 m/year. On hillslopes the mean gully erosion rate is 0.21–0.52 m/year, and on the valley bottoms 0.18–1.98 m/year. High bottom erosion rates resulted from the emergence of long gullies during the erosion episodes in 1984 and 1997. Sheet flow in valley floors intensifies at times of heavy rainfall which causes long gullies to form.Taking into account the fact that conditions favoured erosion, the rate at which the old gullies under forest were transformed should be considered slow. New side gullies form slowly within the valley and it appears that if erosion progressed at the rate observed, new side valleys would take a few hundred years to form.  相似文献   

11.
This study examines the development of a conceptual model of sediment processes in the upper Yuba River watershed; and we hypothesize how components of the conceptual model may be spatially distributed using a geographical information system (GIS). The conceptual model illustrates key processes controlling sediment dynamics in the upper Yuba River watershed and was tested and revised using field measurements, aerial photography, and low elevation videography. Field reconnaissance included mass wasting and channel storage inventories, assessment of annual channel change in upland tributaries, and evaluation of the relative importance of sediment sources and transport processes. Hillslope erosion rates throughout the study area are relatively low when compared to more rapidly eroding landscapes such as the Pacific Northwest and notable hillslope sediment sources include highly erodible andesitic mudflows, serpentinized ultramafics, and unvegetated hydraulic mine pits. Mass wasting dominates surface erosion on the hillslopes; however, erosion of stored channel sediment is the primary contributor to annual sediment yield. We used GIS to spatially distribute the components of the conceptual model and created hillslope erosion potential and channel storage models. The GIS models exemplify the conceptual model in that landscapes with low potential evapotranspiration, sparse vegetation, steep slopes, erodible geology and soils, and high road densities display the greatest hillslope erosion potential and channel storage increases with increasing stream order. In-channel storage in upland tributaries impacted by hydraulic mining is an exception. Reworking of stored hydraulic mining sediment in low-order tributaries continues to elevate upper Yuba River sediment yields. Finally, we propose that spatially distributing the components of a conceptual model in a GIS framework provides a guide for developing more detailed sediment budgets or numerical models making it an inexpensive way to develop a roadmap for understanding sediment dynamics at a watershed scale.  相似文献   

12.
拱王山风化壳的发育特征及其环境和构造意义   总被引：6，自引：0，他引：6  
风化壳的化学、粘粒矿物及磁化率等实验分析表明 ,云南东川拱王山地区碳酸盐岩风化壳的红土化程度较低 ,处于Ca、Mg淋失阶段的后期或SiO2 淋失、Fe、Mn富积的早期。该地区风化壳在不同程度上存在与现代环境的不相容性。它们是在古风化壳基础上发育的继承性风化壳。在不同海拔高度的山前剥蚀面上 ,风化壳红土化程度的差异 ,是不同时期风化作用的产物。除山顶残留古夷平面上的风化壳之外 ,随海拔高度的增加 ,风化壳的风化程度存在逐渐增强的趋势 ,这一现象反映了该区新构造运动的间歇性抬升特征。  相似文献   

13.
A single shrub in a widely spaced shrubby area acts as a roughness element and, therefore, it can affect the distribution of the rainfall received on the ground surface surrounding it. This paper focuses on such rainfall distribution on arid hillslopes and its eco-geomorphic implications. Relatively simple methods were used to measure the rainfall around shrubs growing in a small basin that is associated with a prevailing wind direction during rain events. There was a trend toward significantly reduced rainfall onto bare soil areas located on the leeward side, compared with that on other bare soil areas along the hillslope. This may alert to another potential cause for patchiness in the hydrological response of arid hillslope systems.  相似文献   

14.
The landscape of the Canadian Rockies in southern Alberta is not a direct result of constructional processes; that is, the ridges and peaks have not been pushed into the positions in which we see them today. Tectonic activity provided original elevation but not mountains: at the end of Laramide time, what are now the front ranges and foothills of the Rockies comprised a high-elevation upland of relatively low relief. The present mountain physiography is the result of 55–60 million years of post-orogenic differential erosion, in which more resistant rocks have been left at higher elevations than less-resistant rocks.The Canadian Rockies and the foothills are developed in a thin-skinned, thrust-and-fold belt created during the Laramide Orogeny; the adjacent Interior Plains cut across foreland basin sediments derived from the mountains. The mountains currently consist of large parts of ridges of well-indurated Paleozoic and, locally, Proterozoic rock alternating with valleys developed in soft Mesozoic clastic rock. In the foothills, where the soft Mesozoic rock is at the surface, relief is subdued, but ridges of more-resistant sandstone rise above shaley lowlands. The plains are relatively flat but also contain erosional outliers of higher paleo-plains-surfaces.Numerous lines of evidence suggest that the mountains and foothills have lost several kilometers of overburden since the end of the Laramide Orogeny, while the western plains have lost at least 2 km, requiring that the local relief of the mountains and foothills that we see is erosional in origin. Local physiography is adjusted to lithology: the mountains have high relief because the exposed sub-Mesozoic rocks can hold up high, steep slopes, whereas the foothills have low relief because the underlying Cretaceous rocks cannot hold up high, steep slopes. The east-facing escarpment at the mountain front is a fault-line scarp along a low-angle thrust.Mesozoic rocks involved in the deformation originally extended all the way across the thrust and fold belt, and physiography of the belt at the end of Laramide time (60–55 Ma) depended mainly on whether Mesozoic or Paleozoic/Proterozoic rocks were exposed at the surface at that time. A reconstruction using critical-taper theory generally agrees with reconstructions from earlier stratigraphic and paleothermometry studies: what are now the front ranges at the eastern edge of the Rocky Mountains were mostly or perhaps entirely covered with Mesozoic rocks and despite that high elevation had a hilly, not mountainous, character. The main ranges, in the central Rocky Mountains, were in part stripped of Mesozoic cover by then and more mountainous. Treeline was higher then, and the thrust belt may have been largely or entirely vegetated. Generation of modern relief in the front ranges, including the escarpment at the mountain front, had to await stripping of Mesozoic rocks and incision of rivers into harder substrates in post-Laramide time.The Interior Plains are an erosional surface that was cut 1 to 3 km below the aggradational top of the foreland basin sediments. Although some of the present low local relief of the plains results from weakness of underlying Cretaceous/Tertiary rocks, the low relief is probably largely related to the process of denudation.  相似文献   

15.
This study is based on the data from Zizhou and Wangjiagou experimental stations on the Loess Plateau in the major sediment‐producing areas of the middle Yellow River drainage basin. It deals with characteristics of hyperconcentrated flows in the slope‐channel systems in the gullied hilly areas on the Loess Plateau. The results show that the formation of hyperconcentrated flows is closely related to the vertical differentiation of landforms. Based on data from 21 rainfall events in the period 1963–1970, event‐averaged suspended sediment concentration for hilltop, upper hillslope, lower hillslope and gully slope was calculated as 36 kg/m3, 89 kg/m3, 304 kg/m3 and 505 kg/m3, and the frequency of hyperconcentrated flows was 0.0, 0.17, 0.74 and 1.0, respectively. Thus, hyperconcentrated flows form on the lower part of hillslopes and on the gully slopes, and develope well in gully channels of various orders. There exists a sediment storing‐releasing mechanism, resulting from different behaviours of sediment transport by non‐hyperconcentrated and hyperconcentrated flows. When water flows are nonhyperconcentrated, the relatively coarse fractions of sediment from the slopes are deposited in the channel. When hyperconcentrated flows occur, the previously deposited coarse sediment may be eroded and released from the channel. A close relationship is found between rainstorms and the formation of hyperconcentrated flows, and some thresholds of rainfall and runoff for the occurrence of hyperconcentrated flows have been identified.  相似文献   

16.
Various researchers have studied the spatial pattern of soil surface components such as vegetation, rock fragments, bare soil, litter and surface crusts, as a key factor of hydrological behaviour in Mediterranean settings with heterogeneous patches of vegetation cover and strong human impact. The studies indicate that there is a mosaic of patches that generate run-off or infiltrate overland flow, distributed in various ways along hillslopes. Few of these studies, however, have looked at areas underlain by metamorphic rocks such as phyllites or schists. This study analysed the temporal and spatial variability of the effects of soil surface components on hydrological processes in a small dry Mediterranean catchment underlain by metamorphic rocks. A systematic sampling of multiple sites throughout a hydrological year was carried out.We related the hydrological behaviour of soil surface components to 1) their position along the hillslope, 2) the distance of existing vegetation tussocks from the line of run-off, 3) rainfall intensity and 4) the main physical/chemical soil properties affecting infiltration processes. Statistical analysis was used to check the validity of the relationships. The results show that soil surface components have highly variable effects, in both space and time, on soil hydrological behaviour. These effects particularly depend on the location along the line of maximum slope and the intensity of preceding rainfall, whose interaction defines soil hydrological status. These results are similar to those for other Mediterranean settings with different lithology, in that the succession of contributing patches are hydrologically interconnected along a hillslope. The variables used and the grouping of explanatory variables through principal component analyses were found to be suitable for discussing the spatial distribution of soil surface components in the hydrologically dynamic environment of the study area.  相似文献   

17.
鄂西山区存在广阔的峰顶面,具体表现为许多中低山顶部起伏平缀,“一望如平湖”．过去,一直把它视力被构造运动抬升的“准平原”——“夷平面”．经十多年来的调查研究,发现该山区自早中生代以来一直保持为区域性分水地带,山顶上存在较强烈风化剥蚀均夷的地貌发育过程,并使山顶面上的风化剥蚀残留物质不断地更新．因此认为该山区的峰顶面力构造运动缓慢抬升基础上,不断地遭受强烈风化剥蚀的产物．  相似文献   

18.
Landslides pose serious hazards in the Mercantour Massif and the French Riviera in southeastern France. The context for landslide development is a particularly favourable one, both in terms of the geomorphic and structural setting of this Alpine region, and of the climatic, hydrologic and seismic factors that trigger such failures. High mountain relief and steep slopes constitute a very favourable setting for failures affecting massive basement rocks and a very heterogeneous sedimentary cover whose resistance has been weakened by weathering, tectonic stresses, and cambering due to gravity. Among trigger factors, the important appears to be the precipitation regime. Rainfalls are commonly concentrated into short high-intensity downpours or into bursts of sustained falls over periods of several days, leading to soil saturation and lubrication of potential failure planes. Snowmelt also contributes to these lubrication processes. Earthquakes affecting this region are also a potentially important landslide trigger. However, while a lot of work has been done on the relationship between extreme climatic events and landslide activity, much less is known of the trigger effects of earthquakes.Both the background factors that promote landslide development and the factors that trigger such failures are discussed within a time frame of landslide development. Progressive changes in soil strength due to weathering, rock cambering and shattering processes lead to long-term reduction in resistance. Superimposed on these progressive changes are episodic triggerings related to rainfall and snowmelt episodes or earthquakes. Landslides may occur as shallow, low-volume “one-time” events or may be part of a progressive long-term failure. The former generally affect unconsolidated or clay-rich sedimentary rocks, especially on the coastal hillslopes of the French Riviera. A notable exception of a major, voluminous “one-time” event was the submarine landslide of the Var Delta in 1979. This landslide, like numerous other smaller subaerial landslides onland, was largely a result of human activities. This landslide occurred following extensive modification of the Var Delta and, notably, reclamation of the steep, fine-grained delta front. Deforestation, quarrying, urbanisation and road network developments are various ways in which human activity has destabilized the coastal hillslopes, favouring the development of numerous shallow landslides following each episode of heavy rainfall.Progressive landslides on the upper hillslopes of the Mercantour Massif have developed over long time spans (order of 101 to 105 yrs) and have involved more complex interactions between lithological controls, slope characteristics and trigger factors. The Collelongue and Bois de Malbosc landslides have evolved into now stable failures buttressed by resistant migmatitic diorites or amphibolites. The more voluminous and well monitored Clapière landslide is a relatively simple rotational landslide of the toe-failure type. This active landslide poses a serious to inhabitants and infrastructure in the Tinée Valley. The importance of continued field monitoring, modelling and mapping of landslides and their hazards is emphasised.  相似文献   

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20.
In the Solway Firth — Morecambe Bay region of Great Britain there is evidence for heightened hillslope instability during the late Holocene (after 3000 cal. BP). Little or no hillslope geomorphic activity has been identified occurring during the early Holocene, but there is abundant evidence for late Holocene hillslope erosion (gullying) and associated alluvial fan and valley floor deposition. Interpretation of the regional radiocarbon chronology available from organic matter buried beneath alluvial fan units suggests much of this geomorphic activity can be attributed to four phases of more extensive gullying identified after 2500–2200, 1300–1000, 1000–800 and 500 cal. BP. Both climate and human impact models can be evoked to explain the crossing of geomorphic thresholds: and palaeoecological data on climatic change (bog surface wetness) and human impact (pollen), together with archaeological and documentary evidence of landscape history, provide a context for addressing the causes of late Holocene geomorphic instability. High magnitude storm events are the primary agent responsible for gully incision, but neither such events nor cooler/wetter climatic episodes appear to have produced gully systems in the region before 3000 cal. BP. Increased gullying after 2500–2200 cal. BP coincides with population expansion during Iron Age and Romano-British times. The widespread and extensive gullying after 1300–1000 cal. BP and after 1000–800 cal. BP coincides with periods of population expansion and a growing rural economy identified during Norse times, 9–10th centuries AD, and during the Medieval Period, 12–13th centuries AD. These periods were separated by a downturn associated with the ‘harrying of the north’ AD 1069 to 1070. The gullying episode after 500 cal. BP also coincides with increased anthropogenic pressure on the uplands, with population growth and agricultural expansion after AD 1500 following 150 years of malaise caused by livestock and human (the Black Death) plagues, poor harvests and conflicts on the Scottish/English border. The increased susceptibility to erosion of gullies is a response to increased anthropogenic pressure on upland hillslopes during the late Holocene, and the role of this pressure appears crucial in priming hillslopes before subsequent major storm events. In particular, the cycles of expansion and contraction in both population and agriculture appear to have affected the susceptibility of the upland landscape to erosion, and the hillslope gullying record in the region, therefore, contributes to understanding of the timing and spatial pattern of human exploitation of the upland landscape.  相似文献   

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