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1.
Alluvial fans and debris cones link two zones of the fluvial system (e.g. hillslope gully systems to stream channels; mountain catchment sediment source areas to main river systems or to sedimentary basins) and therefore have important coupling or buffering roles. These roles may be both functional and preservational. The functional role includes debris‐cone coupling, which controls sediment supply from hillslope gully systems to stream channels, influencing channel morphology. Coupling through larger alluvial fans, expressed by fanhead trenching, causes a distal shift in sedimentation zones, or when expressed by through‐fan trenching, causes complete sediment by‐pass. The preservational role stems from the fact that fans and cones are temporary sediment storage zones, and may preserve a record of source–area environmental change more sensitively than would sediments preserved further downsystem. Fan coupling mechanisms include distally‐induced coupling (basal scour, ‘toe cutting’, marginal incision) and proximally‐induced coupling (fanhead and midfan trenching). These mechanisms lead initially to partial coupling, either extending the immediate sediment source area to the stream system or shifting the focus of sedimentation distally. Complete coupling involves transmission of sediment from the feeder catchment through the fan environment into the downstream drainage or a sedimentary basin. The implications of coupling relate to downstream channel response, fan morphology, sedimentation patterns and vertical sedimentary sequences. Temporal and spatial scales of coupling are related, and with increasing scales the dominant controls shift from storm events to land cover to climatic and base‐level change and ultimately to the relationships between tectonics and accommodation space. Finally, future research challenges are identified. Modern dating techniques and sophisticated analysis of remotely sensed data can greatly improve our understanding of fan dynamics, and should lead to better cross‐scale integration between short‐term process‐based approaches and long‐term sedimentological applications, while maintaining high quality field‐based observations. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

2.
The response of 12 fluvial fans near Sydney, Australia to a large storm between 2 and 4 February 1990 was determined by repeating previously surveyed longitudinal profiles and by undertaking detailed field observations of erosion and deposition. Peak rainfall intensities occurred on 3 and 4 February when between 173 and 193·8 mm were recorded. Return periods for 24 h duration peak rainfall ranged between 5·7 and 11·0 years on the annual maximum series at six stations within the study area and return periods for 48 h peak rainfall ranged between 13·5 and 29·4 years. Of the 12 fans, seven were trenched and five untrenched. The most significant geomorphic effects of the storm were recorded on the proximal region of the fans. However, fan response was highly variable, with one fan exhibiting no detectable change, three fans localized deposition, two fans spatially disjunct erosion and deposition, two fans channel avulsions, and seven fans fanhead trench reworking. Some fans exhibited more than one type of response. A four-stage, tentative cyclical model of fanhead development was constructed from the field data. Stage 1 refers to the episodic aggradation of the fanhead by localized deposition, spatially disjunct erosion and deposition and/or channel avulsions. Stage 2 represents the initiation of a fanhead trench when progressive aggradation locally exceeds a threshold slope leading to localized erosion. This erosion initially creates one or more discontinuous flow-aligned scour pools. Over time, the scour pools widen, deepen and extend both up- and downfan. Stage 3 refers to the coalescence of discontinuous scour pools into a continuous trench by the removal of intervening log and boulder steps. Stage 4 represents the backfilling phase of the trench once it has been overwidened and/or slope reduced. Aggradation then continues as for stage one.  相似文献   

3.
Long‐term aggradation of the Waiho River, South Westland, New Zealand, has now raised the head of its alluvial fan to unprecedented elevations. In its natural state the river would, like all other major rivers in the area, be somewhat incised into its fanhead. The only relevant factor able to account for the aggradation is the presence of control banks (‘stopbanks’ in local parlance) that restrict the ability of the river to move over the whole of its natural fanhead. A 1 : 3333 scale physical hydraulic model (a ‘microscale’ model) was used to study this situation. An alluvial fan was generated in the model and allowed to develop to equilibrium with steady inputs of water and sediment within boundaries geometrically similar to those of the natural unrestricted Waiho River. The boundaries were then altered to represent the presence of the stopbanks, and the fan allowed to continue evolving under the same water and sediment inputs. The model fanhead aggraded in a spatial pattern similar to that recorded on the Waiho. Taking into consideration the limitations of microscale modelling, these results indicate that the aggradation in the Waiho is a result of the lateral restriction of the river by stopbanks. This poses fundamental questions about the variables that control the behaviour of alluvial fans. The results also suggest that microscale modelling can be used to make reliable quantitative predictions of the effects of engineering works on rivers, in spite of the low level of dynamic similarity with the prototype compared to that in larger‐scale models. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

4.
The relative importance of tectonics, climate, base level and source lithology as primary factors on alluvial‐fan evolution, fan morphology and sedimentary style remain in question. This study examines the role of catchment lithology on development and evolution of alluvial megafans (>30 km in length), along the flanks of the Kohrud Mountain range, NE Esfahan, central Iran. These fans toe out at axial basin river and playa‐fringe sediments towards the centre of basin and tectonics, climatic change and base‐level fluctuations, were consistent for their development. They formed in a tectonically active basin, under arid to semiarid climate and a long term (Plio‐Pleistocene to Recent) change from wetter to drier conditions. The key differences between two of these fans, Soh and Zefreh fans, along the west and south flanks of this mountain range, is that their catchments are underlain by dissimilar bedrock types. The source‐area lithologies of the Soh and Zefreh fans are in sedimentary and igneous terrains, respectively, and these fans developed their geometry mainly in response to different weathering intensities of their catchment bedrock lithologies. Fan surface mapping (based on 1/50000 topographic maps, satellite images, and fieldwork), reveals that the geomorphic evolution of these fans differs in that the relatively large‐scale incision and through trenching of the Soh fan is absent in the Zefreh fan. Whereas the limited sediment supply of the Soh fan has resulted in a deep incised channel, the Zefreh fan has remained aggradational with little or no trenching into proximal to medial fan surface due to its catchment bedrock geology, composed mainly by physically weathered volcaniclastic lithology and characterized by high sediment supply for delivery during episodic flash floods. Sediment supply, which is mainly a function of climate and source lithology, is a dominant driver behind the development of fan sequences in alluvial megafans. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

5.
The degree of glacial modification in small catchments along the eastern Sierra Nevada, California, controls the timing and pattern of sediment flux to the adjacent fans. There is a close relationship between the depth of fan‐head incision and the pattern and degree of Late Pleistocene catchment erosion by valley glaciers; catchments with significant glacial activity are associated with deeply incised fan heads, whereas fans emerging from glacially unmodified catchments are unincised. We suggest that the depth of fan‐head incision is controlled by the potential for sediment storage during relatively dry ice‐free periods, which in turn is related to the downstream length of the glacially modified valley and creation of accommodation through valley floor slope lowering and glacial valley overdeepening and widening. Significant storage in glacially modified basins during ice‐free periods leads to sediment supply‐limited conditions at the fan head and causes deep incision. In contrast, a lack of sediment trapping allows quasi‐continuous sediment supply to the fan and prevents incision of the fan head. Sediment evacuation rates should thus show large variations in glacially modified basins, with major peaks during glacial and lows during interglacial or ice‐free periods, respectively. In contrast, sediment removal from glacially unmodified catchments in this type of setting should be free of this effect, and will be dominated instead by short‐term variations, modulated for example by changes in vegetation cover or storm frequency. This distinction may help improve our understanding of long‐term sediment yields as a measure of erosional efficiency. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

6.
We present laboratory and field evidence that in mountainous catchment‐fan systems persistent alluvial fanhead aggradation and trenching may result from infrequent, large sediment inputs. We suggest that the river‐fan systems along the fault‐bounded range front of the western Southern Alps, New Zealand, are likely to be in a dynamic equilibrium on ≥103‐yr timescales, superimposed on which their fanheads undergo long‐term cumulative episodic aggradation. These fanheads are active only in rare events, do not take part in the usual behaviour of the catchment‐fan system and require much longer to exhibit dynamic equilibrium than the rest of the fan. These findings (1) increase our knowledge of the effects of extreme events on alluvial fan morphodynamics in humid climates, (2) question the general applicability of inferring past climatic or tectonic regimes from alluvial‐fan morphology and stratigraphy and (3) provide a conceptual basis for hazard zonation on alluvial fans. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

7.
An alluvial bajada has been recognized along a part of the active Narmada-Son Fault(NSF) and confined by the Karjan River on the eastern side and by the Madhumati River on western side.The bajada sequence exposed along the incised cliffs of various north flowing parallel streams has been studied in their proximal,medial and distal part. The sediments are characterized on the basis of grain size,fabric,sedimentary structures,bed geometry and sorting, and lithologs prepared from the mapped cliff sections.Detailed study of sedimentary characteristics,lithofacies analysis and facies associations indicate that sediments are mainly of three types-sediments that were deposited by debris flows and sediment gravity flows,and as extensive bar deposits.Three major aggredational sequences are recognized.Each sequence is composed of coarsening-upward sequence of proximal facies overlain by fining-upward sequence of distal facies.Coarsening upward sequence record periods of tectonic activity related to uplift along the NSF and fan progradation,whereas fining-upward sequence results from tectonic quiescence periods.The presence of rhizocretions,calcium carbonate nodules and calcite sheets within the basal debris flow and sediment gravity flow indicate semi-arid climate,whereas the formation of pedoginized paleosol indicate relatively wetter climate prevailing in the study area.Tectonic activity along the NSF has played dominant role by controlling the geometry and volume of bajada sediments.Climate is found to be responsible for compositional and temporal distribution of bajada sediments.OSL dating suggests that the bajada sediments were deposited during the later part of late Pleistocene.The incision of the sediments is attributed to uplift due to inversion of the lower Narmada basin during the early Holocene.  相似文献   

8.
Two distinct types of alluvial fans occur in the Bow River Valley, Alberta, Canada: fluvially dominated and debris flow dominated. Large, gently sloping fans dominated by fluvial processes are associated with large and less rugged drainage basins, and small rugged basins have produced small, steep fans dominated by debris flow processes. Quantitative analysis demonstrates that strong fan-basin morphometric relationships occur despite a short fan history. Statistical analysis of fan area-basin area relationships indicate that debris flow fan areas do not increase in size as quickly as contributing basins. The relationship of fluvial fan area to basin area is not statistically significant. However, this relationship is probably affected by fan erosion. Examination of fan slope to basin ruggedness relationships indicates that fan slope increases more rapidly than basin ruggedness for both fan types. This is likely related to non-linear discharge and sediment size effects on fluvial fans, and reworking of larger fan surfaces by fluvial processes on debris flow fans.  相似文献   

9.
Desert pavements are widely used as a relative surface‐dating tool because they are progressively better developed on surfaces ranging from thousands to hundreds of thousands of years in age. Recent work, however, has highlighted the dynamic nature of pavements and undermined their use as surface‐age indicators. Quade (2001) proposed that latest Pleistocene vegetation advances destroyed all Mojave Desert pavements above 400 m elevation, making all such pavements Holocene in age. In an effort to reconcile young‐pavement evidence with their widespread use as Pleistocene surface‐age indicators, we developed a numerical model based on the classic conceptual model in which pavements co‐evolve with their underlying eolian epipedons over millennial timescales. In this co‐evolutionary process, fine‐grained eolian deposition and Av‐horizon development within the eolian epipedon promotes surface clast motion and pavement development, enhancing the eolian‐sediment‐trapping ability of the pavement in a positive feedback. Model results illustrate the multi‐scale nature of pavement dynamics: pavements may require tens of thousands of years to fully develop from a newly abandoned alluvial surface, but may heal over timescales of decades to centuries if a mature eolian epipedon is present. As such, there is no inconsistency between rapid pavement healing and a Pleistocene age for the underlying alluvial surface. To calibrate the model, we conducted surficial geologic mapping and pavement‐sedimentological analysis on two desert piedmonts. Our study areas include both proximal and distal fan environments, illustrating the role of parent‐material texture in controlling the mode of pavement formation. Using available geochronology, our work provides a rigorous calibration of pavement formation rates in our study areas and provides evidence supporting the use of pavements as local relative surface‐age indicators over Holocene to late Pleistocene timescales. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

10.
Geomorphological response to an extreme flood: A case from southeast Spain   总被引：1，自引：0，他引：1  
On 28–29 September 1980 an extreme storm of 150 mm of rain in c. 5 hours caused a flash flood near Tabernas in a semi-arid area in southern Spain. With the rainfall total approximating the mean annual rainfall the recurrence interval is estimated to be 25–100 years. Flood peak discharges have been estimated from a mountain source area across an alluvial fan and the geomorphological response to the storm described. Channel adjustment differed between channels in the fanhead trench and those unconfined channels on the fan surface. The implications of the spatial pattern of erosional an depositional feaures for landform adjustment in semi-arid regions are discussed.  相似文献   

11.
A mathematical model which estimates the scale-independent sediment surface profile of alluvial fans has been developed. This model utilizes a diffusive sediment transport model and an unsteady, radial flow, conservation relationship. These equations are approximately solved assuming a quasi-steady-state closure with appropriate modelling assumptions for two end member fan types: (1) fans where most of the fan surface is depositionally active (denoted here as ‘homogeneous’) and (2) fans characterized by channelling and sediment sorting processes. The fundamental result for these two fan types is a dimensionless sediment profile relationship which approximates most fan surfaces. The model suggests that the overall dimensionless morphology of alluvial fans is governed more by fundamental diffusion principles in sediment deposition than by individual environmental or basin characteristics. Additionally, this work potentially can be extended to model temporal variation in fan development. Preliminary comparison with alluvial fan profiles is reasonable, indicating that this model provides useful qualitative and quantitative information relating to alluvial fan process and morphology. Copyright © 1999 John Wiley & Sons, Ltd.  相似文献   

12.
The flooding susceptibility of alluvial fans in the Southern Apennines has long been neglected. To partly address this oversight, we focus on the region of Campania which contains highly urbanized piedmont areas particularly vulnerable to flooding. Our findings are based on stratigraphic analysis of the fans and morphometric analysis of the basin‐fan systems. Using geomorphological analysis we recognized active alluvial fans while stratigraphic analysis together with statistical analysis of the morphometric variables was used to classify the fans in terms of the transport process involved. The results indicate that in the geological context examined, the best discrimination between debris flow (Df) and water flood (Wf) processes is achieved by means of two related variables, one for the basin (feeder channel inclination, Cg) and one for the fan (fan length, Fl). The probability that an unclassified fan belongs to group Wf is computed by applying a logistic function in which a P value exceeding 0.5 indicates that a basin/fan system belongs to group Wf. This important result led to the classification of the entire basin/fan system data. As regards process intensity, debris flow‐dominated fans are susceptible to the occurrence of flows with high viscosity and hence subject to more severe events than water flood‐dominated fans. Bearing this in mind, the data gathered in this study allow us to detect where alluvial fan flooding might occur and give information on the different degrees of susceptibility at a regional scale. Regrettably, urban development in recent decades has failed to take the presence of such alluvial fans into account due to the long recurrence time (50–100 years) between floods. This paper outlines the distribution of such susceptibility scenarios throughout the region, thereby constituting an initial step to implementing alluvial fan flooding control and mitigation. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

13.
A wide range of sedimentological and geomorphological field research depends on the availability of accurate and detailed depositional age models. Although exposure dating techniques such as cosmogenic nuclide and luminescence dating are now widely available, they remain expensive and time‐consuming, and this frequently limits the density of age constraints and the resolutions of age models for many study areas. We present a simple and effective, field‐based approach for extending and correlating existing age models to un‐dated surfaces. In Owens Valley, California, we make use of detailed beryllium‐10 (10Be) chronologies reported for four different alluvial fan systems, to precisely calibrate the rate at which weathering fractures are enlarged in granitic surface boulders. We show that these fractures have widened at a time‐integrated rate of 1.05 ± 0.03 mm ka?1 for at least 140 ka at this location, and this relationship can be represented by a linear regression that makes them ideal chronometers for surface dating. Our analysis offers a new approach to refining the uncertainties of both surface erosion rate and cosmogenic age estimates at this location. Ultimately, we integrate our observations to devise a robust age calibration for clast fracture widths in Owens Valley, and we demonstrate its application by estimating the ages of 27 additional local fan surfaces. We present an updated and extended stratigraphy for eight Sierra Nevada fan systems in total, with exceptional age control. This novel approach to dating sedimentary surfaces is inexpensive and easily applied in the field, and has the potential to significantly increase the temporal and spatial density of age constraints available for a particular study area. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

14.
Deltas on planet Mars record past climate, but so far a wide range of hypotheses for their formation have been proposed. The objective of this paper is to understand martian fan deltas, their formative conditions, evolution and formative duration, and implications for the past climate. As an introduction to Mars, physiographic provinces are described and unambiguous proof is listed for the presence of flowing water in the past, such as certain minerals, groundwater, catastrophic outflow channels, alluvial fans and fan deltas, distributary networks and glaciers. The climate history of Mars differs from that of Earth by having had much drier conditions than on Earth, extreme intermittency and extreme events, most of them billions of years ago. Tens of fan deltas, unchannelized fan deltas and stepped fans have been found in impact crater and other lakes. The stepped fans were likely formed by backstepping under fast rising lake levels and have no known terrestrial equivalent. The fan deltas were formed once the lake overflowed. Alluvial fans are much more numerous and formed with less water. The delta studies illustrate how major challenges of martian morphology and reconstruction of past conditions can be taken up most effectively by combinations of the available high‐resolution images and digital elevation models, terrestrial analogues, laboratory experiments and physics‐based models gleaned from geomorphology. To resolve formative mechanisms and time scale of martian fans and deltas, morphological distinctions between dense debris flows and dilute fluvial flows must be identified for both source and sink areas. Furthermore, the properties of the martian surface material are very poorly constrained but can be explored by modelling various mass wasting, fluvial and glacial phenomena and hydrology, and by experimentation with slightly cohesive sediment. Finally, the highly debated role of groundwater sapping in valley and delta formation on Mars should be explored experimentally. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

15.
Arid alluvial fan and fluvial dry wash surfaces in Stonewall Flat, Nevada, USA, are characterized using surface geomorphic surveys, soil pits, botanical line surveys, and varnish microlamination dating techniques. Active and abandoned washes, and active fan surfaces are dominated by primary geomorphic processes of high‐energy sedimentation from flash floods. These surfaces are characterized by bar and swale topography, a lack of stone pavements, soil horizons, and rock varnish. Younger terraces and slightly older intermediate fan surfaces are in transition from primary sedimentation processes to lower energy secondary surface‐modifying processes of sheet wash and eolian transport and deposition. These surfaces are characterized by faint to no bar and swale topography, incipient to moderately well‐developed pavements and soil horizons, and abundant coppices. Old and stable fan surfaces are dominated by lower energy secondary processes and manifest well‐developed pavements, soils, and sparse coppices around widely distributed shrubs. Varnish microlamination dating yields ages of 13·15 ka for intermediate fan surfaces and 25·55 to 86·75 ka for stable fan surfaces. Plant communities co‐developing with these surfaces affect and are affected by both primary and secondary geomorphic fan processes. Relatively active surfaces contain few woody species. Co‐dominance of shrubs and annuals with abundant annuals between the shrubs is characteristic of surfaces transitional from primary processes to secondary processes. Stable surfaces dominated by secondary processes are characterized by woody perennials, with long‐lived woody species inhabiting the oldest surfaces. Feedback mechanisms between early botanical communities and eolian deposition affect coppice and pavement development. In turn, these surface features control both the composition and distribution of botanical communities on older, more stable surfaces. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.  相似文献   

16.
This study investigated a series of dammed lakes and downstream-adjacent alluvial fans in the upstream to middle reaches of the Golmud River in the eastern Kunlun Mountain, on the north-eastern Qinghai-Tibetan Plateau (QTP). An optically stimulated luminescence (OSL) chronology shows the sediments of five dammed lakes developed from c. 45–40, 30–25, 18–14, and 12–8 ka, corresponding to MIS 3b, late MIS 3a, Last Deglaciation, and early Holocene, respectively. The remote sensing data show these dammed lakes have a total area of 109.4 km2, with the lake volume of more than 4.0 km3. Symmetric alluvial fans from north–south tributary valleys produced OSL ages of c. 61–52, 42–31, 26–20, and 16–10 ka, corresponding to glaciation periods: the MIS 3c and MIS 3a, MIS 2, and the Last Deglaciation. This suggests that glacial activity is responsible for the alluvial fan development, where dammed rivers occurred first, but lake formation did not take place synchronously until later periods of strong hydrologic activity, resulting from northward intrusions of the Indian summer monsoon (ISM) or glacier melt. Thus, the blocking pattern is that river valleys were dammed during periods of glacial activity and lakes formed during wet periods. The lake formation and subsequent drainage may have resulted in: (i) impeded headwater incision and strengthening of downstream dissection; (ii) enriched the halite and potash in the distal Qarhan Salt Lake through hydrologic and hydrochemical processes of abundant water input, the salt lake expansion, salt redissolution from playa and final resedimentation during later dry periods. The alluvial-dammed lake pattern in the mountain-basin systems of eastern Kunlun Mountain offers a model for assessing the linkages between monsoon dynamics, geomorphic processes and distal salt lake evolutions in other arid regions.  相似文献   

17.
Range-front alluvial fan deposition in arid and semiarid environments records vast amounts of climatic and tectonic information. Differentiating and characterizing alluvial fan morphology is an important part in Quaternary alluvial fan research. Traditional method such as field observations is a most important part of deciphering and mapping the alluvial fan. Large-scale automatically mapping of alluvial fan stratigraphy before traditional field observations could provide guidance for mapping alluvial fan morphology, thus improving subsequent field work efficiency. In this research, high-resolution topographic data were used to quantify relief and roughness of alluvial fan within the Laohushan. These data suggest that mean surface roughness plotted against the size of the moving window is characterized by an initial increase in surface roughness with increased window size, but it shows no longer increase as a function of windows size. These data also suggest that alluvial fans in this study site smooth out with time until a threshold is crossed where roughness increases at greater wavelength with age as a result of surface runoff and headward tributary incision into the oldest surfaces which suggests the evolution process of alluvial fan. Researchers usually differentiate alluvial morphology by mapping characteristics of fan surface in the field by describing surface clast size, rock varnish accumulation, and desert pavement development and analysis of aerial photographs or satellite imagery. Recently, the emergence of high-resolution topographic data has renewed interest in the quantitative characterization of alluvial and colluvium landforms. Surface morphology that fan surface initially tends to become smoother with increasing age due to the formation of desert pavement and the degradation of bar-and-swale topography and subsequently, landforms become more dissected due to tectonics and climatic change induced increased erosion and channelization of the surface with time is widely used to distinguish alluvial fan types. Those characteristics would reflect various kinds of morphology metrics extracted from high-resolution topographic data. In the arid and semiarid regions of northwestern China, plenty of alluvial fans are preserved completely for lack of artificial reforming, and there exists sparse surface vegetation. In the meantime, range-front alluvial fan displaced by a number of active faults formed a series of dislocated landforms with different offsets which is a major reference mark in fault activity research. In this research, six map units(Qf6-Qf1), youngest to oldest, were observed in the study area by mapping performed by identifying geomorphic features in the field that are spatially discernible using hill-shade and digital orthophoto map. Alluvial fan relief and roughness were computed across multiple observation scales(2m×2m to 100m×100m)based on the topographic parameters of altitude difference and standard deviation of slope, curvature and aspect. In this research, mean relief keeps increasing with increased window size while mean surface roughness is characterized by a rapid increase over wavelengths of 6~15m, representing the typical length scale of bar-and-swale topography. At longer wavelengths, surface roughness values increase by only minor amounts, suggesting the topographic saturation length is 6~15m for those fan surfaces in which saturation length of standard deviation of curvature is less than 8m. Box and whisker plot of surface roughness averaged over 8m2 for each alluvial fan unit in the study area suggests that the pattern of surfaces smoothing out with age and then starting to become rougher again as age increases further beyond Qf4 or Qf3 unit. The younger alluvial fan is characterized by prominent bar-and-swale while the older alluvial fan is characterized by tributaries headward incision. Cumulative frequency distributions of relief and surface roughness in Figure 8 are determined in an 8m by 8m moving window for the comparison of six alluvial fan units in the northeast piedmont of Laohushan. From these distributions we know that Qf6 and Qf1 reflect the prominent relief which is related to bar-and-swale and tributaries headward incision respectively, while Qf4 and Qf3 reflect the moderate relief which is related to subdued topography. Surface roughness, in addition to facilitating the characterization of individual fan units, lends insight to alluvial landform development. We summarize an alluvial landform evolutionary scheme which evolves four stages depending on characteristics of alluvial fan morphology development and features of relief and roughness. The initial stage in this study site is defined as the active alluvial fan channels with bars of coarse cobbles and boulders and swales consisting of finer-grained pebbles and sand which could be reflected by high mean relief and mean roughness values. As time goes, bar-and-swale topography is still present, but an immature pavement, composed of finer grained clasts, has started to form. In the third stage, the bar-and-swale topography on the fan surface is subdued, yet still observable, with clasts ranging from pebbles to cobbles in size and there exists obvious headward tributary incision. Eventually, tributary channels form from erosion by surface runoff. Headward incision of these tributaries wears down the steep walls of channels that are incised through the stable, planar surface, transforming the oldest alluvial landforms into convex hillslopes, leaving only small remnants of the planar surface intact. Those evolutionary character suggests that alluvial fans in this area smooth out with time, however, relief or roughness would be translated to increase at greater wavelength with age until a threshold is crossed. This research suggests that relief and roughness calculated from high-resolution topographic data of this study site could reflect alluvial fan morphology development and provide constraint data to differentiate alluvial fan unit.  相似文献   

18.
Hydrothermal alteration at Mount Rainier waxed and waned over the 500,000-year episodic growth of the edifice. Hydrothermal minerals and their stable-isotope compositions in samples collected from outcrop and as clasts from Holocene debris-flow deposits identify three distinct hypogene argillic/advanced argillic hydrothermal environments: magmatic-hydrothermal, steam-heated, and magmatic steam (fumarolic), with minor superimposed supergene alteration. The 3.8 km3 Osceola Mudflow (5600 y BP) and coeval phreatomagmatic F tephra contain the highest temperature and most deeply formed hydrothermal minerals. Relatively deeply formed magmatic-hydrothermal alteration minerals and associations in clasts include quartz (residual silica), quartz–alunite, quartz–topaz, quartz–pyrophyllite, quartz–dickite/kaolinite, and quartz–illite (all with pyrite). Clasts of smectite–pyrite and steam-heated opal–alunite–kaolinite are also common in the Osceola Mudflow. In contrast, the Paradise lahar, formed by collapse of the summit or near-summit of the edifice at about the same time, contains only smectite–pyrite and near-surface steam-heated and fumarolic alteration minerals. Younger debris-flow deposits on the west side of the volcano (Round Pass and distal Electron Mudflows) contain only low-temperature smectite–pyrite assemblages, whereas the proximal Electron Mudflow and a < 100 y BP rock avalanche on Tahoma Glacier also contain magmatic-hydrothermal alteration minerals that are exposed in the avalanche headwall of Sunset Amphitheater, reflecting progressive incision into deeper near-conduit alteration products that formed at higher temperatures.  相似文献   

19.
When a sediment laden river reaches a flat basin area the coarse fraction of their sediment load is deposited in a cone shaped structure called an alluvial fan. In this article we used the State Space Soil Production and Assessment Model (SSSPAM) coupled landform–soilscape evolution model to simulate the development of alluvial fans in two- and three-dimensional landforms. In SSSPAM the physical processes of erosion and armouring, soil weathering and sediment deposition were modelled using state-space matrices, in both two and three dimensions. The results of the two-dimensional fan showed that the fan grew vertically and laterally keeping a concave up long profile. It also showed a downstream fining of the sediments along the fan profile. Both of these observations are in agreement with available literature concerning natural and experimental fan formations. Simulations with the three-dimensional landform produced a fan with a semicircular shape with concave up long profiles and concave down cross profiles which is typical for fans found in nature and ones developed in laboratory conditions. During the simulation the main channel which brings sediment to the fan structure changed its position constantly leading to the semicircular shape of the fan. This behaviour is similar to the autogenic process of ‘fanhead trenching’ which is the major mechanism of sediment redistribution while the fan is developing. The three-dimensional fan simulation also exhibited the downstream fining of sediments from the fan apex to the peripheries. Further, the simulated fan also developed complex internal sediment stratification which is modelled by SSSPAM. Currently such complex sediment stratification is thought to be a result of allogenic processes. However, this simulation shows that, such complex internal sediment structures can develop through autogenic processes as well. © 2020 John Wiley & Sons, Ltd.  相似文献   

20.
The Dead Sea has been continuously dropping 0·4–1·0 m yr?1 since the middle of the 20th century and thus provides a unique field laboratory for studying in real time the response of drainage systems to a non‐tectonic base‐level fall. The aim of this work is to study the short‐term ongoing erosive response to a rapid base‐level drop in a small, steep‐fronted, erodible fan‐delta setting. The work explores the controls of the steep Qedem fan‐delta, guided by its clinoform structure, on its incision. Longitudinal profiles of the fan‐delta and of its entrenched channel were measured in the field. Sedimentary facies changes – fluviatile, shallow lacustrine and beach – were followed along exposures. The existence of large boulders provided an opportunity to examine the uncertain role of armouring and boulder flux on incision. The field study was combined with digital elevation models (DEMs) that were extracted from pairs of overlapping aerial photos. Maps of erosion and deposition were prepared using a change detection algorithm. The longitudinal profile of the entrenched channel was found to be steep and linear. The outlet temporarily ‘hangs’ elevated and ungraded above the retreating lake level, indicating years without incision flow events, which cause lags in response to the rapid lake level drop. In spite of the large boulders, the small drainage basin and precipitation volume over the basin of the Qedem, the recorded vertical incision rates in the unconsolidated sediments are as high as 0·8 m yr?1, i.e. similar to those of the largest wadis draining to the Dead Sea. The steep front of the fan‐delta is suggested to be a main factor controlling the efficient incision. A unique transport mechanism of rolling boulders, following undercutting, contributed to the entrenchment efficiency. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

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