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1.
Cliff stability within the Pembrokeshire Coast National Park was evaluated using a numerical model applied at four sites representative of rock mass failure phenomena and major sedimentary geological sequences. The sites were: Mill Bay, (Old Red Sandstone); St Govan's Head, Carboniferous Limestone (Dinantian); Druidston, Millstone Grit (Namurian) and Lower Coal Measures (Westphalian); and Wiseman's Bridge, Lower Coal Measures (Westphalian). The study integrated a range of geotechnical parameters, measured in the field and laboratory, into a model to predict the likely failure mechanisms. The model is based on the existence of rock prisms delineated by structural parameters, i.e. joints, bedding planes and critical tension fractures behind the cliff face. An iterative approach is used to define the dip of the most probable, stepped failure surface at the base of any potentially unstable multiblock system and to calculate the sliding and toppling forces for each block in the cliff mass. Prediction compared favourably with field observations at three of the four selected sites, i.e. Druidston, St Govan's Head and Wiseman's Bridge. At Druidston sliding is predicted and dominates in the basal blocks, whilst toppling is confined to the upper cliff and is dependent on movement of the lower structural units. St Govan-s Head is shown to have a low risk of toppling and sliding and this was predicted except where basal undercutting reaches a depth of 1·0 m or lateral forces exceed 100 kN m−2 when failure could occur. Copyright © 1998 John Wiley & Sons, Ltd. 相似文献
2.
Bank undercutting and tension failure by groundwater seepage: predicting failure mechanisms 
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下载免费PDF全文 Groundwater seepage can lead to the erosion and failure of streambanks and hillslopes. Two groundwater instability mechanisms include (i) tension failure due to the seepage force exceeding the soil shear strength or (ii) undercutting by seepage erosion and eventual mass failure. Previous research on these mechanisms has been limited to non‐cohesive and low cohesion soils. This study utilized a constant‐head, seepage soil box packed with more cohesive (6% and 15% clay) sandy loam soils at prescribed bulk densities (1.30 to 1.70 Mg m?3) and with a bank angle of 90° to investigate the controls on failure mechanisms due to seepage forces. A dimensionless seepage mechanism (SM) number was derived and evaluated based on the ratio of resistive cohesion forces to the driving forces leading to instability including seepage gradients with an assumed steady‐state seepage angle. Tension failures and undercutting were both observed dependent primarily on the saturated hydraulic conductivity, effective cohesion, and seepage gradient. Also, shapes of seepage undercuts for these more cohesive soils were wider and less deep compared to undercuts in sand and loamy sand soils. Direct shear tests were used to quantify the geotechnical properties of the soils packed at the various bulk densities. The SM number reasonably predicted the seepage failure mechanism (tension failure versus undercutting) based on the geotechnical properties and assumed steady‐state seepage gradients of the physical‐scale laboratory experiments, with some uncertainty due to measurement of geotechnical parameters, assumed seepage gradient direction, and the expected width of the failure block. It is hypothesized that the SM number can be used to evaluate seepage failure mechanisms when a streambank or hillslope experiences steady‐state seepage forces. When prevalent, seepage gradient forces should be considered when analyzing bank stability, and therefore should be incorporated into commonly used stability models. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
3.
Development of a notch at the base of a cliff reduces cliff stability and often induces a collapse. Pleistocene limestone coastal cliffs of elevation 5?m in Kuro‐shima, Ryukyu Islands, have a prominent notch with a depth of 3–4?m at their bases. Around these coastal cliffs, collapses different from previous studies of cliff collapses in the Ryukyu Islands were found; collapses in Kuro‐shima have a horizontal failure surface. The horizontal failure surface, situated at the height of the failure surface corresponding to the retreat point of the notch, is bounded by vertical joints cutting the whole cliff and the reef flat in front of the cliff. Two types of horizontal failure surface were found, triangular and quadrangular; the distinction appears to depend on the angle between the vertical joints and the front face of the cliff. Prior to collapse, these cliffs appear to have been separated from the adjacent cliffs by the development of vertical joints. Consequently, a cliff that will collapse can be identified in advance; cliff instability is strongly dependent on the development of a notch. To study the effect of notch development on cliff collapse, the notch depth at which collapse occurs was calculated using stability analysis. Instability of a cliff increases with notch depth; collapse occurs at the horizontal failure surface when the ratio of the notch depth to the seaward length of the cliff is approximately 0·5–0·7 for a triangular failure surface, and 0·7–0·9 for a quadrangular failure surface. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
4.
Cosmogenic 10Be concentrations in exposed bedrock surfaces and alluvial sediment in the northern Flinders Ranges reveal surprisingly high erosion rates for a supposedly ancient and stable landscape. Bedrock erosion rates increase with decreasing elevation in the Yudnamutana Catchment, from summit surfaces (13·96 ± 1·29 and 14·38 ± 1·40 m Myr?1), to hillslopes (17·61 ± 2·21 to 29·24 ± 4·38 m Myr?1), to valley bottoms (53·19 ± 7·26 to 227·95 ± 21·39 m Myr?1), indicating late Quaternary increases to topographic relief. Minimum cliff retreat rates (9·30 ± 3·60 to 24·54 ± 8·53 m Myr?1) indicate that even the most resistant parts of cliff faces have undergone significant late Quaternary erosion. However, erosion rates from visibly weathered and varnished tors protruding from steep bedrock hillslopes (4·17 ± 0·42 to 14·00 ± 1·97 m Myr?1) indicate that bedrock may locally weather at rates equivalent to, or even slower than, summit surfaces. 10Be concentrations in contemporary alluvial sediment indicate catchment‐averaged erosion at a rate dominated by more rapid erosion (22·79 ± 2·78 m Myr?1), consistent with an average rate from individual hillslope point measurements. Late Cenozoic relief production in the Yudnamutana Catchment resulted from (1) tectonic uplift at rates of 30–160 m Myr?1 due to range‐front reverse faulting, which maintained steep river gradients and uplifted summit surfaces, and (2) climate change, which episodically increased both in situ bedrock weathering rates and frequency–magnitude distributions of large magnitude floods, leading to increased incision rates. These results provide quantitative evidence that the Australian landscape is, in places, considerably more dynamic than commonly perceived. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
5.
Instability investigation of cantilevered seacliffs 总被引：1，自引：0，他引：1
Wave action is a fundamental mechanism in seacliff erosion, whereby wave undercutting creates an unstable cantilevered seacliff profile and can lead to large catastrophic cliff failures, thus threatening coastal infrastructure. This study investigated the instability of two such failures that occurred in Solana Beach, California, by combining terrestrial LIDAR scanning, cantilever beam theory and finite element analysis. Each landslide was detected by evaluating the surface change between subsequent high resolution digital terrain models derived from terrestrial LIDAR data. The dimensions of failed cantilever masses were determined using the surface change measurements and then incorporated into failure stress analysis. Superimposing stress distributions computed from elastic cantilever beam theory and finite element modeling provided a method to back‐calculate the maximum developed tensile and shear stresses along each failure plane. The results of the stress superposition revealed that the bending stresses caused by the cantilevered load contributed the majority of stress leading to collapse. Both shear and tensile failure modes were investigated as potential cliff failure mechanisms by using a comparison of the back‐calculated failure stresses to material strengths found in laboratory testing. Based on the results of this research, the tensile strength of the cliff material was exceeded at both locations, thus causing the cliffs to collapse in tension. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
6.
Large rock slope failures from near‐vertical cliffs are an important geomorphic process driving the evolution of mountainous landscapes, particularly glacially steepened cliffs. The morphology and age of a 2·19 × 106 m3 rock avalanche deposit beneath El Capitan in Yosemite Valley indicates a massive prehistoric failure of a large expanse of the southeast face. Geologic mapping of the deposit and the cliff face constrains the rock avalanche source to an area near the summit of ～8·5 × 104 m2. The rock mass free fell ～650 m, reaching a maximum velocity of 100 m s?1, impacted the talus slope and spread across the valley floor, extending 670 m from the base of the cliff. Cosmogenic beryllium‐10 exposure ages from boulders in the deposit yield a mean age of 3·6 ± 0·2 ka. The ～13 kyr time lag between deglaciation and failure suggests that the rock avalanche did not occur as a direct result of glacial debuttressing. The ～3·6 ka age for the rock avalanche does coincide with estimated late Holocene rupture of the Owens Valley fault and/or White Mountain fault between 3·3 and 3·8 ka. The coincidence of ages, combined with the fact that the most recent (AD 1872) Owens Valley fault rupture triggered numerous large rock falls in Yosemite Valley, suggest that a large magnitude earthquake (≥M7.0) centered in the south‐eastern Sierra Nevada may have triggered the rock avalanche. If correct, the extreme hazard posed by rock avalanches in Yosemite Valley remains present and depends on local earthquake recurrence intervals. Published in 2010 by John Wiley & Sons, Ltd. 相似文献
7.
We evaluated controls on locations of channel incision, variation in channel evolution pathways and the time required to reconnect incised channels to their historical floodplains in the Walla Walla and Tucannon River basins, northwestern USA. Controls on incision locations are hierarchically nested. A first‐order geological control defines locations of channels prone to incision, and a second‐order control determines which of these channels are incised. Channels prone to incision are reaches with silt‐dominated valley fills, which have sediment source areas dominated by loess deposits and channel slopes less than 0·1(area)?0·45. Among channels prone to incision, channels below a second slope–area threshold (slope = 0·15(area)?0·8) did not incise. Once incised, channels follow two different evolution models. Small, deeply incised channels follow Model I, which is characterized by the absence of a significant widening phase following incision. Widening is limited by accumulation of bank failure deposits at the base of banks, which reduces lateral channel migration. Larger channels follow Model II, in which widening is followed by development of an inset floodplain and aggradation. In contrast to patterns observed elsewhere, we found the widest incised channels upstream of narrower reaches, which reflects a downstream decrease in bed load supply. Based on literature values of floodplain aggradation rates, we estimate recovery times for incised channels (the time required to reconnect to the historical floodplain) between 60 and 275 years. Restoration actions such as allowing modest beaver recolonization can decrease recovery time by 17–33 per cent. Published in 2007 by John Wiley & Sons, Ltd. 相似文献
8.
G. V. Wilson R. K. Periketi G. A. Fox S. M. Dabney F. D. Shields R. F. Cullum 《地球表面变化过程与地形》2007,32(3):447-459
The majority of sediment leaving catchments may be from streambank failure. Seepage erosion of unconsolidated sand above a restrictive layer is an important erosion process in incised streams that leads to streambank failure by undercutting banks. The objective of this study was to determine the impact of soil properties on seepage erosion and the resulting streambank failure. Seepage flow and sediment concentrations were measured in situ at eight locations along the banks of a deeply incised stream in northern Mississippi. Using field observations as a guide, the soil profile conditions of a shallow (45 cm) streambank, consisting of 30 cm of topsoil, a 10 cm conductive layer, and a 5 cm restrictive layer, were mimicked in laboratory lysimeter experiments to quantify the hydrologic properties controlling seepage erosion and bank failure under a 40 cm head. The time to flow initiation and the flow rate were linearly related to the slope of the restrictive layer. Seepage erosion began within minutes of flow initiation and resulted in substantial (3 to 34 cm) undercutting of the bank. Sediment concentrations of seeps were as high as 660 g l?1 in situ and 4500 g l?1 in the lysimeters. Sediment concentrations were related to the layer slope, thereby indicating the importance of detailed site characterization. The USDA‐ARS Streambank Stability model demonstrated the increase in instability of banks due to undercutting by seepage erosion, but failed to account for the sediment loss due to sapping for stable banks and overestimated the sediment loads for failed banks. Published in 2006 by John Wiley & Sons, Ltd. 相似文献
9.
Progressive rock‐fall failures in natural rock slopes are common in many environments, but often elude detailed quantitative documentation and analysis. Here we present high‐resolution photography, video, and laser scanning data that document spatial and temporal patterns of a 15‐month‐long sequence of at least 14 rock falls from the Rhombus Wall, a sheeted granitic cliff in Yosemite Valley, California. The rock‐fall sequence began on 26 August 2009 with a small failure at the tip of an overhanging rock slab. Several hours later, a series of five rock falls totaling 736 m3 progressed upward along a sheeting joint behind the overhanging slab. Over the next 3 weeks, audible cracking occurred on the Rhombus Wall, suggesting crack propagation, while visual monitoring revealed opening of a sheeting joint adjacent to the previous failure surface. On 14 September 2009 a 110 m3 slab detached along this sheeting joint. Additional rock falls between 30 August and 20 November 2010, totaling 187 m3, radiated outward from the initial failure area along cliff (sub)parallel sheeting joints. We suggest that these progressive failures might have been related to stress redistributions accompanying propagation of sheeting joints behind the cliff face. Mechanical analyses indicate that tensile stresses should occur perpendicular to the cliff face and open sheeting joints, and that sheeting joints should propagate parallel to a cliff face from areas of stress concentrations. The analyses also account for how sheeting joints can propagate to lengths many times greater than their depths behind cliff faces. We posit that as a region of failure spreads across a cliff face, stress concentrations along its margin will spread with it, promoting further crack propagation and rock falls. Published in 2012. This article is a US Government work and is in the public domain in the USA. 相似文献
10.
Adam P. Young Robert T. Guza William C. O'Reilly Olivier Burvingt Reinhard E. Flick 《地球表面变化过程与地形》2016,41(11):1564-1573
Concurrent observations of waves at the base of a southern California coastal cliff and seismic cliff motion were used to explore wave–cliff interaction and test proxies for wave forcing on coastal cliffs. Time series of waves and sand levels at the cliff base were extracted from pressure sensor observations programmatically and used to compute various wave impact metrics (e.g. significant cliff base wave height). Wave–cliff interaction was controlled by tide, incident waves, and beach sand levels, and varied from low tides with no wave–cliff impacts, to high tides with continuous wave–cliff interaction. Observed cliff base wave heights differed from standard Normal and Rayleigh distributions. Cliff base wave spectra levels were elevated at sea swell and infragravity frequencies. Coastal cliff top response to wave impacts was characterized using microseismic shaking in a frequency band (20–45 Hz) sensitive to wave breaking and cliff impacts. Response in the 20–45 Hz band was well correlated with wave–cliff impact metrics including cliff base significant wave height and hourly maximum water depth at the cliff base (r2 = 0.75). With site‐specific calibration relating wave impacts and shaking, and acceptable anthropogenic (traffic) noise levels, cliff top seismic observations are a viable proxy for cliff base wave conditions. The methods presented here are applicable to other coastal settings and can provide coastal managers with real time coastal conditions. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
11.
Scott G. Leibowitz Parker J. Wigington Jr. Randy L. Comeleo Joseph L. Ebersole 《水文研究》2012,26(5):741-759
High‐resolution, spatially extensive climate grids can be useful in regional hydrologic applications. However, in regions where precipitation is dominated by snow, snowmelt models are often used to account for timing and magnitude of water delivery. We developed an empirical, nonlinear model to estimate 30‐year means of monthly snowpack and snowmelt throughout Oregon. Precipitation and temperature for the period 1971–2000, derived from 400‐m resolution PRISM data, and potential evapotranspiration (estimated from temperature and day length) drive the model. The model was calibrated using mean monthly data from 45 SNOTEL sites and accurately estimated snowpack at 25 validation sites: R2 = 0·76, Nash‐Sutcliffe Efficiency (NSE) = 0·80. Calibrating it with data from all 70 SNOTEL sites gave somewhat better results (R2 = 0·84, NSE = 0·85). We separately applied the model to SNOTEL stations located < 200 and ≥ 200 km from the Oregon coast, since they have different climatic conditions. The model performed equally well for both areas. We used the model to modify moisture surplus (precipitation minus potential evapotranspiration) to account for snowpack accumulation and snowmelt. The resulting values accurately reflect the shape and magnitude of runoff at a snow‐dominated basin, with low winter values and a June peak. Our findings suggest that the model is robust with respect to different climatic conditions, and that it can be used to estimate potential runoff in snow‐dominated basins. The model may allow high‐resolution, regional hydrologic comparisons to be made across basins that are differentially affected by snowpack, and may prove useful for investigating regional hydrologic response to climate change. Published in 2011 by John Wiley & Sons, Ltd. 相似文献
12.
Excessive terrestrial nutrient loadings adversely impact coral reefs by primarily enhancing growth of macroalgae, potentially leading to a phase‐shift phenomenon. Hydrological processes and other spatial and temporal factors affecting nutrient discharge must be examined to be able to formulate effective measures for reducing nutrient export to adjacent reefs. During storm events and baseflow periods, water samples were obtained from the tropical Todoroki River, which drains an intensively agricultural watershed into Shiraho coral reef. In situ nutrient analyzers were deployed for 6 months to hourly measure dissolved nutrient (NO3−‐N and PO43−‐P) concentrations. Total phosphorus (TP) and suspended solid concentration (TSS) were increased by higher rainfall intensity (r = 0·94, p < 0·01) and river discharge Q (r = 0·88, p < 0·01). In contrast, NO3−‐N concentration tends to decrease drastically (e.g. from 3 to 1 mg l−1) during flood events. When base flow starts to dominate afterwards, NO3−‐N manifested an increasing trend, but decreases when baseflow discharge becomes low. This counter‐clockwise hysteresis for NO3−‐N highlights the significant influence of groundwater discharge. N delivery can therefore be considered a persistent process compared to sediment and P discharge, which are highly episodic in nature. Based on GIS analysis, nutrient concentration along the Todoroki River was largely affected by the percentage of sugarcane/bare areas and bedrock type. The spatial distribution of N concentration in the river reflects the considerable influence of subsurface geology—higher N levels in limestone‐dominated areas. P concentrations were directly related to the total length of artificial drainage, which enhances sediment transport. The use of high‐resolution monitoring data coupled with GIS‐based spatial analysis therefore enabled the clarification of control factors and the difference in the spatio‐temporal discharge characteristics between N and P. Thus, although erosion‐reduction schemes would reduce P discharge, other approaches (e.g. minimize fertilizer) are needed to reduce N discharge. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
13.
Nel Caine 《地球表面变化过程与地形》1982,7(2):133-152
Large cliff failures involving forward toppling over a basal hinge have occurred on more than half of the plateau edge of Ben Lomond, northeastern Tasmania. This mode of failure, which is readily identified from the columnar structure of the dolerite involved, has affected up to 107 m3 of rock at a time and a total of more than 50 × 106 m3 in all the cases which can still be identified. It represents perhaps the most important form of cliff retreat, amounting to a rate of 0.2 mm yr?1 over the last 100,000 years. Topographic evidence and joint surveys suggest that two different mechanisms have produced the topples on Ben Lomond. One has involved failure in the sediments underlying the dolerite with consequent foundering and cambering of large sections of the plateau edge. This mechanism accounts for relatively few of the Ben Lomond topples, though it includes the largest individual cases. The second mechanism, dominant in most of the topples, involved slab failure in the cliffs. Both modes of failure have been facilitated by vertical weaknesses within the bedrock and both require an initially steep cliff profile. Because of the latter requirement, which is not met on the other mountains of northeastern Tasmania, large-scale topples are found only on Ben Lomond, and only there where glacial steepening of the cliff has been possible. Following the initial failure, topples of both types have migrated downslope by block sliding for distances up to 2 km. 相似文献
14.
Research indicates that the aeolianite (Kurkar) cliffs along the Israeli Mediterranean coastline have continuously retreated eastward during the last few decades. There seems to be no dispute among Earth scientists regarding the general trend of cliff retreat. However the majority of papers displaying cliff retreat rates are based upon comparison of aerial photographs. Their lack of advanced geometric measurement methods causes a high margin of error. Public attention is focused upon the Beit‐Yannay coastal cliff since private homes are located along the southern section of the cliff crest. The current research compares the historic location of the cliff crest edge at Beit‐Yannay as observed in a series of aerial photographs taken during the period 1918–2000. Quantitative measurement methods included applications of satellite geodesy and digital photogrammetry and mapping. Research results offer quantitative, consecutive and highly accurate data regarding retreat rates over a relatively long period of 82 years. It is concluded that: 1. Annual average cliff retreat rates of the cliff crest is 20 cm/year. 2. Categorization of the study time span reveals periods displaying varying retreat rates such as 27 cm/year during 1918–1946, 21 cm/year during 1946–1973 and 10 cm/year during 1973–2000. 3. Maximum retreat distances of the cliff crest, over the study period were found to be approximately 25 m along the northern, lowest section of the cliff. Minimum distances of 11 m were identi?ed at the highest, southern section of the cliff. 4. The eolianite (Kurkar) cliffs along the Israeli Mediterranean coast throughout the 20th century have been an important source of sediment, contributing approximately 24 × 106 m3 of sediments to the sediment balance of Israeli beaches. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
15.
B. H. Luckman 《地球表面变化过程与地形》1976,1(3):287-298
The major controls of rockfall activity are divided into two interacting groups. (a)Climatic factors which, through their control of temperatures and the availability and state of water, are primary controls of rockfall trigger mechanisms. (b) Geologic factors which, via cliff form (plan, profile, dissection etc.) and the character and availability of materials, influence the type, spatial distribution and intensity of rockfall activity. Detailed examination of these controls suggest that both seasonal and daily patterns of rockfall activity can vary markedly over a very small area. The implications of this variability for the design and interpretation of rockfall inventories are discussed using sample data from Surprise Valley, Jasper National Park, for the period from May-October 1969. On a seasonal basis rockfall activity showed a major spring peak with secondary maxima in the fall and associated with major summer storms. Two sites of differing aspect and morphology were studied in more detail revealing marked differences in the mean hourly frequency (0·94:0·20), mean hourly probability (0·40:0·16, hours with rockfall/hours observed) and daily pattern of rockfall activity. The west facing site showed equal hourly probability of rockfalls from 1000 to 2000 hr whereas the east facing slope had a greater hourly probability when it was in the sun (1100 1400hr) than in the shade (1400-1900hr). The differences in rockfall frequency and probabilities reflect the physical characteristics of the individual sites whereas the daily pattern of rockfalls is related to microclimate. These results suggest that if future inventory studies are to make a significant contribution, they must be carefully designed field experiments in which. (i) Data are restricted to the study or comparison of single continuously observed sites. (ii) Study sites and/or data periods are carefully selected to isolate and investigate specific controls e.g. the influence of cliff form, aspect, periods of rainfall etc. on rockfall patterns. (iii) Good on-site microclimate data are available. Also comparison of frequency measures based on the arithmetic mean should be avoided since the distribution of rockfalls per hour closely follows a Poisson distribution with occasional high values which unduly influence the value of the arithmetic mean. It is suggested that rockfall probabilities, based on the binary decision of whether or not a rockfall occurs in a given period, are a more useful measure for daily patterns of rockfall activity. 相似文献
16.
Ada W. Pringle 《地球表面变化过程与地形》1995,20(5):387-405
This paper presents results of investigations (1983–1992) into rates of change, morphology and processes occurring during the current erosional phase in a Morecambe Bay cyclic saltmarsh, in which it has narrowed from c. 1000 m (1975) to c. 150 m (1992). Monthly monitoring of marsh edge erosion and creek knickpoint retreat has revealed temporal and spatial variations. Highest frequency changes of low magnitude coincided with non-storm conditions and overmarsh tides above 5·80 m OD, which submerged the whole marsh. Less frequent changes of greater magnitude were associated with both overmarsh tides and strong onshore winds over 15 ms?1, which generated high energy waves. The lowest frequency change of greatest magnitude occurred during an extreme onshore storm event and surge. Morphologically, during the erosional phase, a low angled landward slope was generated as erosion of the c. 0·5 m high active seaward cliff coincided with vertical accretion of 0·07 ma?1 of relatively coarse sediment on the marsh surface immediately landward. Tidal hydrodynamics strongly influence the saltmarsh, which is confined to the upper 2·5 m of the macrotidal range (maximum c. 10·5 m). During overmarsh spring tides (maximum creek flood flow rate 0·13 ms?1, up to bankfull level), flooding begins over lower landward creek banks before submerging the higher marsh edge. During ebb tides, water trapped by this higher edge can escape seaward only via the creeks (maximum ebb velocities 2·07 ms?1 below bankfull level). Wave erosion also is limited to spring tides. Monthly mapping of the Kent Estuary channel pattern seaward of the saltmarsh showed that medium term higher erosion rates were related to the presence of a large channel, which lowered the adjacent creek base level and allowed larger waves to attack the marsh edge than when a sandbank flanked the marsh. Major River Kent channel shifts appear to initiate accretional or erosional phases of cyclic saltmarsh development. 相似文献
17.
Stable isotope data on humid tropical hydrology are scarce and, at present, no such data exist for Borneo. Delta18O, δ2H and δ13C were analysed on 22 water samples from different parts of the Sungai (river) Niah basin (rain, cave drip, rainforest pool, tributary stream, river, estuary, sea) in north‐central Sarawak, Malaysian Borneo. This was done to improve understanding of the modern stable isotope systematics of the Sungai Niah basin, essential for the palaeoenvironmental interpretation of the Late Quaternary stable isotope proxies preserved in the Great Cave of Niah. The Niah hydrology data are put into a regional context using the meteoric water line for Southeast Asia, as derived from International Atomic Energy Agency/World Meteorological Organization isotopes in precipitation network data. Although the Niah hydrological data‐set is relatively small, spatial isotopic variability was found for the different subenvironments of the Sungai Niah basin. A progressive enrichment occurs towards the South China Sea (δ18O ?4·6‰; δ2H ?29·3‰; δ13C ?4·8‰) from the tributary stream (δ18O ?8·4‰; δ2H ?54·7‰; δ13C ?14·5‰) to up‐river (δ18O c. ?8‰; δ2H c. ?51‰; δ13C c. ?12‰) and down‐river values (δ18O c. ?7·5‰; δ2H c. ?45‰; δ13C c. ?11‰). This is thought to reflect differential evaporation and mixing of different components of the water cycle and a combination of depleted biogenic δ13C (plant respiration and decay) with enriched δ13C values (due to photosynthesis, atmospheric exchange, mixing with limestone and marine waters) downstream. Cave drip waters are relatively enriched in δ13C as compared to the surface waters. This may indicate rapid degassing of the cave drips as they enter the cave atmosphere. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
18.
Computer simulations of the topographic evolution of the proposed post‐mining rehabilitated landform for the ERA Ranger Mine, showed that for the unvegetated and unripped case, the landform at 1000 years would be dissected by localized erosion valleys (maximum depth = 7·6 m) with fans (maximum depth = 14·8 m) at the outlet of the valleys. Valley form simulated by SIBERIA has been recognized in nature. This indicates that SIBERIA models natural processes efficiently. For the vegetated and ripped case, reduced valley development (maximum 1000 year depth = 2·4m) and deposition (maximum 1000 year depth = 4·8m) occurred in similar locations as for the unvegetated and unripped case (i.e. on steep batter slopes and in the central depression areas of the landform). For the vegetated and ripped condition, simulated maximum valley depth in the capping over the tailings containment structure was c. 2·2 m. By modelling valley incision, decisions can be made on the depth of tailings cover required to prevent tailings from being exposed to the environment within a certain time frame. A reduction in thickness of 1 m of capping material over tailings equates to c. 1 000 000 Mm3 over a 1 km2 tailings dam area. This represents a saving of c. $1 500 000 in earthworks alone. Incorporation of SIBERIA simulations in the design process may result in cost reduction while improving confidence in environmental protection mechanisms. Copyright 2000 © Environmental Research Institute of the Supervising Scientist, Commonwealth of Australia. 相似文献
19.
We explore a new method for documenting the long-term retreat rate of seacliffs based on measurements and modeling of 10Be concentration transects across present-day shore platforms. The proposed forward numerical model relies on a scenario of sea-level rise since the last deglaciation, and predicts the shape of 10Be concentration transects as a function of prescribed cliff recession and vertical coastal platform downwearing rates. Two independent transect features allow fitting the long-term recession rate model to field observations: a sharp 10Be concentration drop predicted at the former stationary location of the cliff during the last glacial period ∼100 ka ago, and a characteristic dome shape whose magnitude is directly related to the recession rate of the cliff. A retreating chalk cliff site from the English Channel coast of France, at Mesnil-Val, where the 7 m tidal range broadly exposes the shore platform, was selected to test the proposed method. Although retreat rates were too high to pinpoint the predicted 10Be concentration drop at the last glacial cliff position, the 10Be concentration of the flints sampled across the shore platform is consistent with the expected dome shape. When modeled using the proper tidal range and proposed Holocene sea-level rise history, the 10Be data yield a cliff retreat rate since the mid-Holocene of 11–13 cm/yr. This is consistent with a 30-year-long measurement record, strongly supporting the utility of the 10Be method. 相似文献
20.
Abstract Damage, destruction and casualties related to landslide and debris flow are common phenomena in the Himalaya, especially during the summer monsoon. This fact was tragically illustrated on 22 July 1996, when Larcha, situated at the 109-km mark of the Arniko Highway, upper Bhotekoshi Valley, central Nepal, experienced a catastrophic debris flow powered by the Bhairab Kunda Stream. Of the 22 houses in Larcha, 16 were swept away, two were partially damaged and 54 people were killed in a matter of a few minutes. The event attracted attention when media linked it to a glacial lake outburst flood as a result of the fact that the source of the stream is a glacial lake. To understand the cause, initiation mechanism and deposition process, the basin area was studied from geological, geomorphologic and engineering geological points of view and the role of precipitation was evaluated. A combination of rainfall, runoff from cliff faces and stream undercutting triggered failure of the bedrock and colluvium, both on the dip and counter-dip slopes, 500 m upstream from the highway. The landslide debris dammed the channel, which was eventually breached, and deposited approximately 104 000 m3 of coarse debris, dominated by the metasediments of the Lesser Himalayan origin, and overwhelmed the village of Larcha. The debris deposit was studied for clast size, composition, texture and dimensions. Lack of sorting and the presence of abundant silt and clay in the source area helped in the initiation of debris flow. The abundance of the Lesser Himalayan metasediment clasts together with the absence of debris traces upstream from the landslide site ruled out the possibility of a glacial lake outburst flood. The disaster was a result of landslide damming triggered by precipitation and stream undercutting and sudden bursting. 相似文献