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1.
Groundwater in the wetlands of the Okavango Delta, Botswana, and its contribution to the structure and function of the ecosystem 总被引：5，自引：0，他引：5
The Okavango Delta of northern Botswana is a large (40,000 km2) alluvial fan located at the terminus of the Okavango River. The river discharges about 10 km3 of water onto the fan each year, augmented by about 6 km3 of rainfall, which sustains about 2500 km2 of permanent wetland and up to 8000 km2 of seasonal wetland. Interaction between this surface water and the groundwater strongly influences the structure and function of the wetland ecosystem. The climate is semi-arid, and only 2% of the water leaves as surface flow and probably very little as groundwater flow. The bulk of the water is lost to the atmosphere. The Okavango River also delivers about 170,000 tonnes of bedload sediment and about 360,000 tonnes of solutes to the Delta each year, most of which are deposited on the fan. Bedload is deposited in the proximal, permanent wetland, whilst much of the solute load is deposited in the seasonal wetland. Notwithstanding the high evapotranspirational loss, saline surface water is rare. Between 80 and 90% of the seasonal flood water infiltrates the ground, recharging the groundwater beneath the flood plains and the many islands on the flood plains. The remainder is lost by evaporation. This groundwater reservoir is transpired into the atmosphere by both aquatic vegetation on the flood plains and terrestrial vegetation on the islands, and the water table is steadily lowered following passage of the seasonal flood. Trees, which are almost exclusively confined to islands, are particularly important, as they lower the water table beneath islands relative to the surrounding wetlands. There is therefore a net flow of groundwater towards islands. Accumulation of dissolved salts in this groundwater leads to precipitation of solutes (mainly of silica and calcite) in the soils beneath island fringes and the islands grow by vertical expansion. Islands are thus an expression of the chemical sedimentation taking place on the fan. Sodium bicarbonate accumulates in the groundwater beneath island centres, and this impacts on the vegetation, leading ultimately to barren island interiors. Dense saline brine thus produced subsides under density-driven flow. This cycling of seasonal flood water through the groundwater reservoir thus plays a key role in creating and maintaining the biological and habitat diversity of the wetland, and inhibits the formation of saline surface water. 相似文献
2.
This study investigated the local‐scale generation and movement of dust in the seasonal swamps of the Okavango Delta, Botswana, with a view to examining possible transfer of material between ?ood plains and islands. It was found that most of the dust load was carried in the lowest 3 m of the air column, and consisted mainly of amorphous silica, indicating that dust was generated largely on the ?ood plains. Dust loads were found to be highest above the ?ood plains and lowest over the interiors of islands, probably due to the baf?ing effect of the island trees on wind velocity. The contrast in dust loads between islands and ?ood plains suggests that there is a net transfer of dust from ?ood plains to islands, but it was not possible to quantify this transfer. It is evident, however, that ?ood plains experience net erosion and islands net aggradation. A strong seasonality in dust loads was observed, with the maximum dust loads coinciding with maximum wind velocity in October. This also coincides with peak seasonal ?ooding in the delta, and only non‐inundated ?ood plains are capable of generating dust. Years of low ?ood therefore appear to be more dusty. There may also be transfer of material from higher‐lying to lower‐lying ?ood plains, which may reduce the topographic contrast on the ?ood plains. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
3.
The Okavango wetland in northern Botswana is one of the world's largest inland deltas. The delta is a dynamic environment with shifting channel routes, causing growth and decay of ?anking wetlands, and giving birth to islands. Primary island nuclei are formed by ?uvial processes and bioengineering, and subsequently grow into secondary larger islands of irregular shape by clastic and chemical sedimentation, and later by coalescence. This article presents classi?cations and quantitative estimations of channels, wetlands and islands of the Okavango Delta. Islands were classi?ed dependent on composition, pattern of composition, shape and juxtaposition. 90 per cent of all islands in the entire wetland were identi?ed, with a classi?cation accuracy of 60 to 85 per cent. Smaller islands of the nucleus types dominate the upper parts of the delta, whereas larger secondary islands are more common in the distal part, a re?ection of the age of the islands. Islands in the entry valley of the delta, the Panhandle, are larger in the top end – the primary region of recent clastic sedimentation. The overall size distribution of islands in the delta, however, shows no clumps, indicating that island growth is a uniform process over time and space. The total area ?ooded at least every decade is approximately 14 000 km2, of which 9000 km2 is classi?ed as actual wetland. Channel meandering decreases from the Panhandle to the distal part of the delta, with the abandoned Thaoge channel as an exception. Occurrence of ?uvially formed islands in the distal delta indicates that the water ?ow and area of inundation must once have been much larger. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
4.
A group of islands of varying size on the floodplain of the Okavango alluvial fan, were studied to establish the processes which lead to the initiation and growth of islands. It was found that islands are initiated by the mound-building activities of the termite Macrotermes michaelseni. These termites import fine grained materials to use as a mortar for the construction of epigeal mounds. Their activities create a topographic feature, raised above the level of seasonal flooding, and also change the physical properties and nutrient status of the mound soil. Shrubs and trees are able to colonize these mounds, which results in increased transpiration. As a result, precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds, resulting in vertical and especially lateral growth, causing island expansion. © 1998 John Wiley & Sons, Ltd. 相似文献
5.
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
6.
The Whangaehu fan is the youngest sedimentary component on the eastern ring plain surrounding Ruapehu volcano. Fan history comprises constructional (830–200 years bp) and dissectional (<200 years bp) phases. The constructional phase includes four aggradational periods associated with both syneruptive and inter-eruptive behavior. All four aggradational periods began when deposition by large lahars changed flow conditions on the fan from channelized to unchannelized. Subsequent behavior was a function of the rate of sediment influx to the fan. The rate of sediment influx, in turn, was controlled by frequency and magnitude of volcanic eruptions, short-term climate change, and the amount of sediment stored on the volcano flanks. Fanwide aggradation occurred when rates of sediment influx and deposition on the fan were high enough to maintaìn unchannelized flow conditions on the fan surface. Maintenance of an undissected surface required sedimentation from frequent and large lahars that prevented major dissection between events. These conditions were best met during major eruptive episodes when high frequency and magnitude eruptions blanketed the volcano flanks with tephra and rates of lahar initiation were high. During major eruptive episodes, volcanism is the primary control on sedimentation. Climatic variations do not influence sediment accumulation. Local aggradation occurred when lahars were too small to maintain unchannelized flow across the entire fan. In this case, only the major channel system received much sediment following the deposition from the initial lahar. This localized aggradation occurred if (1) the sediment reservoir on the flank was large enough for floods to bulk into debris flows and (2) sedimentation events were frequent enough to maintain sediment supply to only some parts of the fan. These conditions were met during both minor eruptive and inter-eruptive episodes. In both cases, a large sediment reservoir remained on the volcano flanks from previous major eruptive intervals. Periods of increased storm activity produced floods that bulked to relatively small debris flows. When the sediment reservoir was depleted, the fan entered the present dissectional phase. Syneruptive and noneruptive lahars are mostly channelized and sediment bypasses the fan. Fan deposits are rapidly reworked. This is the present case at Ruapehu, even though the volcano is in a minor eruptive episode and the climate favors generation of intense storm floods. 相似文献
7.
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
8.
Marc S. Humphries Claudia R. Benitez-Nelson Michael Bizimis Timothy J. Ralph Zacchary T. Larkin Terence S. McCarthy 《地球表面变化过程与地形》2020,45(8):1705-1716
Dust plays a globally important role in supplying biologically essential elements to landscapes underlain by nutrient-poor substrates. Here we show that dust may play a significant role in sustaining productivity in the vast wetlands of the Okavango Delta in southern Africa, one of the world's richest biodiversity hotspots. Dust accumulates preferentially on tree-covered islands in the seasonal swamps of the Delta, creating pockets of fine-grained, nutrient-rich material within the semi-arid landscape of the Kalahari Desert. Strontium and neodymium isotopes reveal that this dust likely originates predominantly from the Makgadikgadi salt pans, located 300 km away, and contributes 10–80% of the fine-grained material present in Okavango island soils. Surface material sourced from the Makgadikgadi Pans contains relatively high amounts of bioavailable phosphorus and iron, potentially influencing Okavango Delta biological productivity. We propose that long-term ecosystem productivity and nutrient availability in the Okavango may be strongly mediated by regional dust inputs. Understanding the influence of dust deposition on nutrient loads and biogeochemical cycling is thus critical for predicting the response of the Okavango Delta to future changes in climate. We suggest that dust inputs may play a significant role in the supply of nutrients to other large, global wetland systems located in dryland environments. © 2020 John Wiley & Sons, Ltd 相似文献
9.
Late Quaternary stratigraphy of a 50 km2 catchment on the south-eastern highlands of Australia reveals processes and history of denudation, and helps resolve a long-standing debate about factors controlling episodic valley aggradation and degradation during Holocene times. Valley sedimentation occurred when swampy vegetation fully colonized valley floors and obliterated all channels, promoting aggradation for periods of several thousand years, with most incoming sediment being trapped in swampy meadows. Much of the sediment was reworked from late Pleistocene alluvial fan and valley fill deposits, and primary hillslope erosion was minor during the Holocene. Differing sedimentation patterns between the Late Pleistocene, Holocene and Post-European settlement periods reflect regional changes in sediment supply and transport capacity as a result of major environmental change. Within the Holocene, however, valley fill stratigraphy is controlled by massive, episodic gully erosion terminating aggradation. Gully initiation appears to be controlled more by thresholds of incision into vegetated valley floors than by changes to sediment supply. Whether the thresholds are exceeded because of climatic change, autonomous change or extreme events cannot yet be determined. Overall, the Holocene history represents continuing complex response to events of the Late Pleistocene, and does not support the K-cycle concept, which has strongly influenced late Quaternary geomorphology in Australia. 相似文献
10.
HAN Long-fei ZENG Jing YUAN Zhao-de SHAO Yan-xiu WANG Wei YAO Wen-qian WANG Peng LIANG Ou-bo XU Xin-yue 《地震地质》2019,41(2):251-265
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
11.
Co‐evolution of riparian vegetation and channel dynamics in an aggrading braided river system,Mount Pinatubo,Philippines 
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下载免费PDF全文 Increased bank stability by riparian vegetation can have profound impacts on channel morphology and dynamics in low‐energy systems, but the effects are less clear in high‐energy environments. Here we investigate the role of vegetation in active, aggrading braided systems at Mount Pinatubo, Philippines, and compare results with numerical modeling results. Gradual reductions in post‐eruption sediment loads have reduced bed reworking rates, allowing vegetation to finally persist year‐round on the Pasig‐Potrero and Sacobia Rivers. From 2009–2011 we collected data detailing vegetation extent, type, density, and root strength. Incorporating these data into the RipRoot model and BSTEM (Bank Stability and Toe Erosion Model) shows cohesion due to roots increases from zero in unvegetated conditions to > 10·2 kPa in densely‐growing grasses. Field‐based parameters were incorporated into a cellular model comparing vegetation strength and sediment mobility effects on braided channel dynamics. The model shows both low sediment mobility and high vegetation strength lead to less active systems, reflecting trends observed in the field. The competing influence of vegetation strength versus channel dynamics is a concept encapsulated in a dimensionless ratio between timescales for vegetation growth and channel reworking known as T*. An estimated T* between 1·5 and 2·3 for the Pasig‐Potrero River suggests channels are still very mobile and likely to remain braided until aggradation rates decline further. Vegetation does have an important effect on channel dynamics, however, by focusing flow and thus aggradation into the unvegetated fraction of braidplain, leading to an aggradational imbalance and transition to a more avulsive state. The future trajectory of channel–vegetation interactions as sedimentation rates decline is complicated by strong seasonal variability in precipitation and sediment loads, driving incision and armoring in the dry season. By 2011, incision during the dry season was substantial enough to lower the water‐table, weaken existing vegetation, and allow for vegetation removal in future avulsions. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
12.
The dynamics and the surface evolution of a post‐LGM debris‐flow‐dominated alluvial fan (Tartano alluvial fan), which lies on the floor of an alpine valley (Valtellina, Northern Italy), have been investigated by means of an integrated study comprising geomorphological field work, a sedimentological study, photointerpretation, quantitative geomorphology, analysis of ancient to modern cartography and consultation of historical documents and records. The fan catchment meteoclimatic, geological and geomorphological characteristics result in fast rates of geomorphic reorganization of the fan surface (2 km2). The dynamics of the fan are determined by the alternation of low‐return period catastrophic alluvial events dominated by non‐cohesive debris flows triggered by extreme rainstorms which caused aggradation and steepening of the fan and avulsion of its main channel, with periods of low to moderate streamflow discharge punctuated by low‐ to intermediate‐magnitude flood events, causing slower but steady topographic reworking. The most ancient parts of the fan surface date back at least to the first half of the 19th century, but most of the fan surface has been restructured after 1911, mainly during the debris‐flow‐dominated events of 1911 and 1987. Phases of rapid fan toe incision and fan degradation have been recognized; since the 1930s or 1940s, the Tartano fan has been subjected to a state of deep entrenchment and narrowing of the main trunk channel and distributary area. Post‐Little Ice Age climate change and present‐day surface uplift rates have been considered as possible explanations for the observed geomorphic evolution, but tectonic or climatic controls cannot account for the order of magnitude of the erosional pace. Anthropogenic controls plausibly override the natural ones: in particular, the building of a dam in the late 1920s, about 2 km upstream of the fan, seems to have triggered fan dissection, having altered the sediment discharge through sediment retention. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
13.
Shigehiro Fujino Hajime Naruse Dan Matsumoto Norihiko Sakakura Apichart Suphawajruksakul Thanawat Jarupongsakul 《Island Arc》2010,19(3):389-398
Measurements of thickness and grain size along flow‐parallel transects across onshore deposits of the 2004 Indian Ocean tsunami revealed macroscopic horizontal variations and provided new insights into tsunami sedimentation. The tsunami caused severe erosion of beaches, river mouths, and the shallow seafloor along the coast of southwestern Thailand and supplied sufficient sediment to deposit a kilometer‐wide blanket of sand on the land surface. The tsunami deposits generally fine landward with some fluctuations caused by local entrainment and settlement of sediments. Sediments of medium and fine sand are restricted to a few hundreds of meters inland from their source, whereas finer grained sediments were suspended longer and deposited 1 km or more inland. Although the thickness of the tsunami deposits is strongly influenced by local topography, they generally thin landward. In areas of low‐relief topography, the rate of landward thinning is exponential and reflects the dominance of sediment supply to nearshore areas over that to areas farther inland. 相似文献
14.
Gregory R. Brooks 《地球表面变化过程与地形》2005,30(13):1617-1632
Slow earth sliding is pervasive along the concave side of Red River meanders that impinge on Lake Agassiz glaciolacustrine deposits. These failures form elongated, low‐angled (c. 6 to 10°) landslide zones along the valleysides. Silty overbank deposits that accumulated during the 1999 spring freshet extend continuously along the landslide zones over hundreds of metres and aggraded the lower slopes over a distance 50 to 80 m from the channel margin. The aggradation is not obviously related to meander curvature or location within a meander. Along seven slope profiles surveyed in 1999 near Letellier, Manitoba, the deposits locally are up to 21 cm thick and generally thin with increasing distance from, and height above, the river. Local deposit thickness relates to distance from the channel, duration of inundation of the landslide surface, mesotopography, and variations in vegetation cover. Immediately adjacent to the river, accumulated overbank deposits are up to 4 m thick. The 1999 overbank deposits also were present along the moderately sloped (c. 23 to 27°) concave banks eroding into the floodplain, but the deposits are thinner (locally up to c. 7 cm thick) and cover a narrower area (10 to 30 m wide) than the deposits within the landslide zones. Concave overbank deposition is part of a sediment reworking process that consists of overbank aggradation on the landslide zones, subsequent gradual downslope displacement from earth sliding, and eventually reworking by the river at the toe of the landslide. The presence of the deposits dampens the outward migration of the meanders and contributes to a low rate of contemporary lateral channel migration. Concave overbank sedimentation occurs along most Red River meanders between at least Emerson and St. Adolphe, Manitoba. © Her Majesty the Queen in right of Canada. 相似文献
15.
The hydrological and erosive response of the Mediterranean eco‐geomorphologic system has showed a very variable and complex behaviour depending on several factors: topography, geology, vegetation pattern, soil properties, land use management, etc. Climate is a key factor due to the great spatial and temporal variability. This research was carried out over different micro‐environments representative from five hillslopes localized in the Littoral Bethic Mountains in the south of Spain. The results of 20 experiments with rainfall simulations on micro‐plots (0.24 m2) and the differences of the incidence of some biotic and abiotic factors in the eco‐geomorphologic system from semi‐arid, dry‐Mediterranean and sub‐humid sites are exposed. Runoff, soil moisture and sediment were measured before, during and after the experiments. The results have shown rock fragments disposition on soil surface and vegetation seem to be the main factors that control the hydrological and erosive response at the micro‐plot‐scale of the experiments. Embedded rock fragments are the most important soil surface property because they reduce the infiltration. Whilst vegetation increases it what is more influential on the hydrological and erosive response of micro‐environments at more arid sites. We have also observed that there are micro‐environment particularities which play a more important role than the localization in the climatic gradient at micro‐plot scale. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
16.
The wind‐driven‐rain effect refers to the redistribution of rainfall over micro‐scale topography due to the existence of local perturbed wind‐flow patterns. Rainfall measurements reported in the literature point to the fact that the wind‐driven‐rain distribution can show large variations over micro‐scale topography. These variations should be taken into account in hillslope hydrology, in runoff and erosion studies and in the design of rainfall monitoring networks. In practice, measurements are often not suitable for determining the wind‐driven‐rain distribution. Therefore, a few researchers have employed numerical modelling. In order to provide confidence in using numerical models, experimental verification for a range of different topographic features is imperative. The objective of this study is to investigate the adequacy of a two‐dimensional Computational Fluid Dynamics (CFD) model to predict the wind‐driven‐rain distribution over small‐scale topography. The numerical model is applied to a number of topographic features, including a succession of cliffs, a small isolated hill, a small valley and a field with ridges and furrows. The numerical results are compared with the corresponding measurement results reported in the literature. It is shown that two‐dimensional numerical modelling can provide a good indication of the wind‐driven‐rain distribution over each type of micro‐scale topography that is considered in this study. It is concluded that more detailed verification procedures are currently inhibited due to the lack of available and detailed spatial and temporal rainfall data from field measurements. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
17.
18.
A precipiton‐based approach to model hydro‐sedimentary hazards induced by large sediment supplies in alluvial fans 下载免费PDF全文
下载免费PDF全文 Thomas Croissant Dimitri Lague Philippe Davy Tim Davies Philippe Steer 《地球表面变化过程与地形》2017,42(13):2054-2067
Mountain ranges are frequently subjected to mass wasting events triggered by storms or earthquakes and supply large volumes of sediment into river networks. Besides altering river dynamics, large sediment deliveries to alluvial fans are known to cause hydro‐sedimentary hazards such as flooding and river avulsion. Here we explore how the sediment supply history affects hydro‐sedimentary river and fan hazards, and how well can it be predicted given the uncertainties on boundary conditions. We use the 2D morphodynamic model Eros with a new 2D hydrodynamic model driven by a sequence of flood, a sediment entrainment/transport/deposition model and a bank erosion law. We first evaluate the model against a natural case: the 1999 Mount Adams rock avalanche and subsequent avulsion on the Poerua river fan (West Coast, New Zealand). By adjusting for the unknown sediment supply history, Eros predicts the evolution of the alluvial riverbed during the first post‐landslide stages within 30 cm. The model is subsequently used to infer how the sediment supply volume and rate control the fan aggradation patterns and associated hazards. Our results show that the total injected volume controls the overall levels of aggradation, but supply rates have a major control on the location of preferential deposition, avulsion and increased flooding risk. Fan re‐incision following exhaustion of the landslide‐derived sediment supply leads to sediment transfer and deposition downstream and poses similar, but delayed, hydro‐sedimentary hazards. Our results demonstrate that 2D morphodynamics models are able to capture the full range of hazards occurring in alluvial fans including river avulsion aggradation and floods. However, only ensemble simulations accounting for uncertainties in boundary conditions (e.g., discharge history, initial topography, grain size) as well as model realization (e.g., non‐linearities in hydro‐sedimentary processes) can be used to produce probabilistic hazards maps relevant for decision making. Copyright © 2017 John Wiley & Sons, Ltd. 相似文献
19.
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
20.
The barrier islands that fringe the western shore of the Outer Hebrides are globally unusual in that they are developed on a planated bedrock (strandflat) surface. They also contain the most extensive area of machair (a distinctive vegetated sandy plain) in the British Isles. This paper presents the first investigation of the internal structure and morphology of these barrier islands and investigates the controls on their structure. The barriers form extensive (300-1000 metres wide) but thin (1.5-2 m) surficial deposits typically resting on bedrock. In areas where depressions exist in the bedrock, and where sediment supply permits, transgressive dunes underlie the machair. A distinctive machair facies of sub-horizontal, undulating reflections, which are laterally continuous over tens of metres is the dominant component of the barriers at each site. This reflects episodic deposition of windblown sand up to the level of the water table. Thereafter any additional sand is transported through the system to accumulate in topographic lows as lake fills, or on topographic highs as ‘high machair’. Eight radar facies were identified, the extent and presence of which vary between the study sites. Bedrock topography and sediment supply are interpreted as the dominant controls on variability in barrier structure. © 2019 John Wiley & Sons, Ltd. 相似文献