Debris Fans Research Articles

Neotectonic Control on the Geomorphic Evolution of the Gangotri Glacier Valley, Garhwal Himalaya

The paper records evidences of neotectonic activities in the Gangotri glacier valley that are found to be responsible for the present-day geomorphic set-up of the area since the last phase of major glaciation. Geomorphological features indicate the presence of a large glacier in the valley in the geological past. Prominent planar structures present in the rocks were later on modified into sets of normal faults in the present-day Himalayan tectonic set-up giving rise to graben structures. The block nearest the snout is traversed by the NW-SE trending Gaumukh fault. A number of terraces mark the entrenchment of Bhagirathi River in this part. The contrasting drainage morphometric parameters of two sides of the valley and asymmetric recessional patterns of the tributary glaciers further document movement along the fault. The distribution and orientation of debris fans also seem to be controlled by neotectonic activity. The neotectonic activity that followed the process of deglaciation has brought the glacially carved, wide U- shaped valley in contact with the present-day fluvially incised narrow and relatively deep valley. The wider segments have become sites of active deposition of glacially eroded debris. The low gradient and excessive filling has resulted in the river attaining a braided nature in these segments.

Geomorphic changes in a complex gully system measured from sequential digital elevation models, and implications for management

AbstractHigh‐resolution digital elevation models (DEMs) were derived from sequential aerial photography of an active fluvio‐mass movement (gully) complex in New Zealand's North Island East Coast region, to measure geomorphic changes over approximately one year. The gully showed a complex behaviour, combining fluvial and mass movement erosion, deposition, and reworking of materials stored in an active debris fan. During the measurement period 5200 ± 1700 m3 of material were eroded from the 8·7 ha gully complex and 670 ± 180 m3 from the 0·8 ha depositional fan, giving a total of 5870 ± 1710 m3 for the entire gully complex–fan system.The results provide a high‐resolution description of gully behaviour over a short time period, and also demonstrate that mass movement (slumping and debris flows) accounted for almost 90 per cent (4660 ± 200 m3) of the sediment generated. This erosional response is described in terms of gully evolution by comparing the gully complex to other systems in the region in various stages of development. The effect of gully evolution on geomorphic coupling between the gully complex and channel system is described, and coupling is also shown to vary with the magnitude and frequency of rainfall events.From a land management perspective the success of strategies, such as tree planting, to mitigate against gully erosion depends on the stage of gully development – particularly on whether or not mass movement erosion has begun. In contrast to gully rehabilitation efforts elsewhere, basin‐wide afforestation in the early stages of gully incision is favoured over riparian planting, given that mass movement assisted by excessive groundwater pressure is the main process leading to uncontrollable gully expansion.To protect land effectively against continuing gully erosion of headwater catchments and resulting downstream aggradation, it is necessary for land managers to understand the spatial and temporal variability of gully development fully so that mitigation efforts can be targeted appropriately. Copyright © 2003 John Wiley & Sons, Ltd.

Human impact on the environment in the Ethiopian and Eritrean highlands—a state of the art

This review analyses the environmental evolution of the Ethiopian highlands in the late Quaternary. The late Pleistocene (20,000–12,000 14C years BP) was cold and dry, with (1) low lake levels in the Rift Valley, (2) large debris fans on the flanks of Lake Abhé basin, and (3) the Blue Nile transporting coarse bedload. Then, a period with abundant and less seasonal rains existed between 11,500 and 4800 14C years BP, as suggested by increased arboreal pollen, high river and lake levels, low river turbidities and soil formation. Around 5000–4800 14C years BP, there was a shift to more arid conditions and more soil erosion. Many phenomena that were previously interpreted as climate-driven might, however, well be of anthropic origin. Thick sediment deposits on pediments as well as an increase of secondary forest, scrub and ruderal species in pollen diagrams are witnesses of this human impact. One important aspect of the late Quaternary palaeoenvironment is unclear: changes in Nile flow discharges and Rift Valley lake levels have been linked to changes in precipitation depth. Most authors do not take into account changes in land use in the highlands, nor changes in the seasonality of rain, both of which can lead to a change in runoff coefficients. Tufa and speleothem deposition around 14,000 years ago tend to indicate that at the end of the Last Glacial Maximum (LGM), conditions might have been wetter than generally accepted. The most important present-day geomorphic processes are sheet and rill erosion throughout the country, gullying in the highlands, and wind erosion in the Rift Valley and the peripheral lowlands. Based on existing sediment yield data for catchments draining the central and northern Ethiopian highlands, an equation was developed allowing to assess area-specific sediment yields: SY=2595A −0.29 (n=20; r 2=0.59) where SY=area-specific sediment yield (t km −2 year −1), and A=drainage area (km 2). With respect to recent environmental changes, temporal rain patterns, apart from the catastrophic impact of dry years on the degraded environment, cannot explain the current desertification in the driest parts of the country and the accompanying land degradation elsewhere. Causes are changing land use and land cover, which are expressions of human impact on the environment. Deforestation over the last 2000–3000 years was probably not a linear process in Ethiopia. Studies on land use and land cover change show, however, a tendency over the last decades of increasing removal of remnant vegetation, which is slowed down or reversed in northern Ethiopia by a set-aside policy. Ongoing land degradation requires urgent action at different levels of society. Soil and water conservation (SWC) structures are now widely implemented. Local knowledge and farmer's initiatives are integrated with introduced SWC techniques at various degrees. Impact assessments show clear benefits of the soil conservation measures in controlling runoff and soil erosion. In high rain areas, runoff management requires greater emphasis during the design of soil conservation structures. In such areas, investment in SWC might not be profitable at farm level, although benefits for society are positive. This pleads in favour of public support. The present land degradation in the Ethiopian highlands has a particular origin, which includes poverty and lack of agricultural intensification. Causes of these are to be found in the nature of past and present regional social relations as well as in international unequal development. This review strengthens our belief that, under improved socio-economic conditions, land husbandry can be made sustainable, leading to a reversal of the present desertification and land degradation of the Ethiopian highlands.

Multiple pathways for woody plant establishment on floodplains at local to regional scales

Summary The structure and functioning of riverine ecosystems is dependent upon regional setting and the interplay of hydrologic regime and geomorphologic processes. We used a retrospective analysis to study recruitment along broad, alluvial valley segments (parks) and canyon segments of the unregulated Yampa River and the regulated Green River in the upper Colorado River basin, USA. We precisely aged 811 individuals of Populus deltoides ssp. wislizenii (native) and Tamarix ramosissima (exotic) from 182 wooded patches and determined the elevation and character of the germination surface for each. We used logistic regression to relate recruitment events (presence or absence of cohort) to five flow and two weather parameters. Woody plant establishment occurred via multiple pathways at patch, reach and segment scales. Recruitment occurred through establishment on (1) vertically accreting bars in the unregulated alluvial valley, (2) high alluvial floodplain surfaces during rare large flood events, (3) vertically accreting channel margin deposits in canyon pools and eddies, (4) vertically accreting intermittent/abandoned channels, (5) low elevation gravel bars and debris fans in canyons during multi‐year droughts, and (6) bars and channels formed prior to flow regulation on the dammed river during controlled flood events. The Yampa River's peak flow was rarely included in models estimating the likelihood that recruitment would occur in any year. Flow variability and the interannual pattern of flows, rather than individual large floods, control most establishment. Regulation of the Green River flow since 1962 has had different effects on woody vegetation recruitment in canyons and valleys. The current regime mimics drought in a canyon setting, accelerating Tamarix invasion whereas in valleys the ongoing geomorphic adjustment of the channel, combined with reduced flow variability, has nearly eliminated Populus establishment. A single year's flow or a particular pattern of flows over a sequence of years, whether natural or man‐made, produces different recruitment opportunities in alluvial and canyon reaches, in diverse landforms within a particular river reach, and for Populus and Tamarix. The design of flows to restore riparian ecosystems must consider these multiple pathways and adjust the seasonal timing, magnitude and interannual frequency of flows to match the desired outcome.

Quaternary deposits and landscape evolution of the central Blue Ridge of Virginia

A catastrophic storm that struck the central Virginia Blue Ridge Mountains in June 1995 delivered over 775 mm (30.5 in) of rain in 16 h. The deluge triggered more than 1000 slope failures; and stream channels and debris fans were deeply incised, exposing the stratigraphy of earlier mass movement and fluvial deposits. The synthesis of data obtained from detailed pollen studies and 39 radiometrically dated surficial deposits in the Rapidan basin gives new insights into Quaternary climatic change and landscape evolution of the central Blue Ridge Mountains. The oldest depositional landforms in the study area are fluvial terraces. Their deposits have weathering characteristics similar to both early Pleistocene and late Tertiary terrace surfaces located near the Fall Zone of Virginia. Terraces of similar ages are also present in nearby basins and suggest regional incision of streams in the area since early Pleistocene–late Tertiary time. The oldest debris-flow deposits in the study area are much older than Wisconsinan glaciation as indicated by 2.5YR colors, thick argillic horizons, and fully disintegrated granitic cobbles. Radiocarbon dating indicates that debris flow activity since 25,000 YBP has recurred, on average, at least every 2500 years. The presence of stratified slope deposits, emplaced from 27,410 through 15,800 YBP, indicates hillslope stripping and reduced vegetation cover on upland slopes during the Wisconsinan glacial maximum. Regolith generated from mechanical weathering during the Pleistocene collected in low-order stream channels and was episodically delivered to the valley floor by debris flows. Debris fans prograded onto flood plains during the late Pleistocene but have been incised by Holocene stream entrenchment. The fan incision allows Holocene debris flows to largely bypass many of the higher elevation debris fan surfaces and deposit onto the topographically lower surfaces. These episodic, high-magnitude storm events are responsible for transporting approximately half of the sediment from high gradient, low-order drainage basins to debris fans and flood plains.

Debris flows as agents of morphological heterogeneity at low-order confluences, Olympic Mountains, Washington

Research Article| September 01, 2003 Debris flows as agents of morphological heterogeneity at low-order confluences, Olympic Mountains, Washington Lee Benda; Lee Benda 1Earth Systems Institute, 310 North Mount Shasta Boulevard, Suite 6, Mount Shasta, California 96067, USA Search for other works by this author on: GSW Google Scholar Curt Veldhuisen; Curt Veldhuisen 2Skagit System Cooperative, 25944 Community Plaza Way, Sedro Woolley, Washington 98284, USA Search for other works by this author on: GSW Google Scholar Jenelle Black Jenelle Black 3Hart Crowser Incorporated, 1257 Main Street, Fortuna, California 95540, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Lee Benda 1Earth Systems Institute, 310 North Mount Shasta Boulevard, Suite 6, Mount Shasta, California 96067, USA Curt Veldhuisen 2Skagit System Cooperative, 25944 Community Plaza Way, Sedro Woolley, Washington 98284, USA Jenelle Black 3Hart Crowser Incorporated, 1257 Main Street, Fortuna, California 95540, USA Publisher: Geological Society of America Received: 26 Sep 2002 Revision Received: 10 Mar 2003 Accepted: 18 Mar 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2003) 115 (9): 1110–1121. https://doi.org/10.1130/B25265.1 Article history Received: 26 Sep 2002 Revision Received: 10 Mar 2003 Accepted: 18 Mar 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Lee Benda, Curt Veldhuisen, Jenelle Black; Debris flows as agents of morphological heterogeneity at low-order confluences, Olympic Mountains, Washington. GSA Bulletin 2003;; 115 (9): 1110–1121. doi: https://doi.org/10.1130/B25265.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Effects of tributary junctions on longitudinal patterns of riverine heterogeneity are relevant to both fluvial geomorphology and riverine ecology. We surveyed 10 km of small- to moderate-sized mountain channels in the Olympic Peninsula, Washington, to investigate how low-order confluences prone to debris flow deposition directly and indirectly influenced channel and valley morphology. In the Olympic Mountains, debris flows scour sediment and organic material from steep first- and second-order channels and create deposits (debris fans) at tributary junctions in higher-order streams. In lower-energy depositional environments there were statistically significant relationships among debris fans at low-order confluences and gravel substrate, wide channels, and numbers of logs and large pools. Effects of debris fans on channel morphology extended upstream and downstream of fan perimeters, indicating the importance of indirect (offsite) effects of debris flows. Consequently, certain aspects of channel morphology (e.g., pool density, substrate texture, and channel widths) were nonuniformly distributed, reflecting the role of network topology and disturbance history on the spatial scale of morphological heterogeneity. Moreover, heterogeneity of channel morphology increased in proximity to low-order confluences prone to debris flows. In contrast, confluence effects in higher-energy depositional environments were limited. Our field data and information from seven other studies indicate how variation in debris flow volume and composition, stream energy, and valley width at the point of deposition influence the relationship between low-order confluences and channel morphology. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

3D NUMERICAL MOVABLE BED FOR FORMATION PROCESS OF DEBRIS FAN

In the formation process of debris fan, the interaction between sediment particles is one of the most important factors. In this study, the formation process of debris fan is simulated by using 3D movable bed simulator which is able to express the interaction between particles with tracking indivisual particles. The mechanism of the formation process of the debris fan due to the sudden decrease of bottom slope and the expansion of the width of channel is investigated through the simulation results. The deposition process found in the simulation data reproduces well the characteristics of previous experimental data.

Landscape evolution within a retreating volcanic arc, Costa Rica, Central America

Subduction of hotspot-thickened seafloor profoundly affects convergent margin tectonics, strongly affecting upper plate structure, volcanism, and landscape evolution. In southern Central America, low-angle subduction of the Cocos Ridge and seamount domain largely controls landscape evolution in the volcanic arc. Field mapping, stratigraphic correlation, and 4 0 Ar/ 3 9 Ar geochronology for late Cenozoic volcanic rocks of central Costa Rica provide new insights into the geomorphic response of volcanic arc landscapes to changes in subduction parameters (slab thickness, roughness, dip). Late Neogene volcanism was focused primarily along the now-extinct Cordillera de Aguacate. Quaternary migration of the magmatic front shifted volcanism northeastward to the Caribbean slope, creating a new topographic divide and forming the Valle Central basin. Stream capture across the paleo-Aguacate divide led to drainage reversal toward the Pacific slope and deep incision of reorganized fluvial networks. Pleistocene caldera activity generated silicic ash flows that buried the Valle Central and descended the Tarcoles gorge to the Orotina debris fan at the coast. Growth of the modern Cordillera Central accentuated relief along the new divide, establishing the Valle Central as a Pacific slope drainage basin. Arc migration, relocation of the Pacific-Caribbean drainage divide, and formation of the Valle Central basin resulted from slab shallowing as irregular, hotspot-thickened crust entered the subduction zone. The geomorphic evolution of volcanic arc landscapes is thus highly sensitive to changes in subducting plate character.

Towards an integration of process measurements, archive analysis and modelling in geomorphology — the kärkevagge experimental site, abisko area, northern sweden

The analysis of Holocene geomorphic process activity demands long–term data sets, which are available for the Kärkevagge catchment due to 50 years of intensive geomorphologic field studies. This data set is used in combination with additional field measurements, remote sensing and digital elevation model (DEM) analysis to provide input data for modelling Holocene valley development. On the basis of this information, geomorphic process units (GPUs) are defined by means of GIS modelling. These units represent areas of homogeneous process composition that transfer sediments. Since the data base enables the quantification of single processes, the interaction of processes within the units can also be quantified. Applying this concept permits calculation of recent sediment transfer rates and hence leads to a better understanding of actual geomorphic landscape development activity. To extrapolate these data in time and space the process–related sediments in the valley are analysed for depth and total volume, primarily using geophysical methods. In this fashion the validity of measured process rates is evaluated for the Holocene time scale. Results from this analysis are exemplified in a cross–profile showing some of the principal sediment units in the valley. For example, the measured modern rates on a slush torrent debris fan seem to represent the Holocene mean rate. This approach should also be suitable for revealing Holocene geomorphic landscape development in terms of climate change.

Simulation of fan formation using a debris mass model

Based on the particle-in-cell computing method, a debris mass model has been established to simulate debris flow fan formation over large downstream areas. Under the assumption that the debris medium is an assembly of many small, identical debris particle masses, the overall flow behavior is obtained by averaging the flow parameters of neighboring debris masses at fixed grid points. The equation of motion for each debris mass is based on the depthaveraged Navier-Stokes equation in two horizontal dimensions. The friction slope of debris flow is modeled by combining the effects of both the liquid phase (slurry composed of water and fine particles) modeled as a Bingham fluid and solid phase (coarse particles) in the debris mixture. The rheological parameters are evaluated according to the density and particle size distribution of the debris material. Convergence of the method is demonstrated by repeatedly doubling the number of debris masses employed in the computation until insignificant change is observed. The debris mass model is demonstrated through a prototype application to a documented 1991 debris flow deposited in the lower reach of the Shawan Ravine in Yunnan Province. China. The final alluvial fan was formed by eight consecutive debris flow events, each lasting about 2000 seconds with a discharge rate of 250 m3/s. The simulation results are in good agreement with field observations. The general features of debris fan development and configuration are well predicted.

The Librilla “rambla”, an example of morphogenetic crisis in the Holocene (Murcia, Spain)

The Librilla basin is situated in the southeast of Spain. It is a tributary of the Guadalentı́n River, which drains an active rift bounded by the Alhama fault. The longitudinal profile of the Librilla rambla is uneven. Rock bars, such as at Fuente–Librilla, have trapped the deposits conveyed by the Librilla torrent (ramblas) and the Guadalentı́n River. Geomorphic processes have remodelled the sediments into polygenetic debris fans in several stages between 6500 and 2500 BP. The Holocene fills contain archaeological artefacts, charcoals, pollen and remains of human settlements that allow reconstruction of a protohistoric morphogenic crisis in this area, which is characterised today by a Mediterranean semi-arid climate.

Analyse d'une lave torrentielle dans le torrent de Faucon (bassin de Barcelonnette, Alpes-de-Haute-Provence). Détermination des zones de déclenchement et de contribution / Study of a debris flow event in the Faucon stream (Barcelonnette basin, Alpes-de-Haute-Provence). Determination of triggering areas and contribution areas

Abstract A geomorphic study was carried out for the first time on the Faucon torrent (Barcelonnette basin, Alpes-de-Haute-Provence), where a debris flow occurred on August 19th 1996. This flow was triggered by a severe thunderstorm in the upper part of the catchment. The rupturing of a natural dam in the stream channel caused the debris-flow. Grain-size distribution and pétrographie analysis of the debris-flow deposit bring out the granular character of the flow during the first hectometre, and its cohesive character beyond that point and as far as the debris fan. Geomorphic observations and laboratory tests show the existence of two source areas: a triggering area and several contributing areas. These contributing areas seem to have supplied the bulk of the flow material.

Streamflow regulation and multi-level flood plain formation: channel narrowing on the aggrading Green River in the eastern Uinta Mountains, Colorado and Utah

The style and degree of channel narrowing in aggrading reaches downstream from large dams is dependent upon the dominant geomorphic processes of the affected river, the magnitude of streamflow regulation, and the post-dam sediment transport regime. We measured different magnitudes of channel adjustment on the Green River downstream from Flaming Gorge Dam, UT, USA, that are related to these three factors. Bankfull channel width decreased by an average of about 20% in the study area. In reaches with abundant debris fans and eddy deposited sand bars, the amount of channel narrowing was proportional to the decrease in specific stream power. The fan–eddy-dominated reach with the greatest decrease in stream power narrowed by 22% while the reach with the least decrease in stream power narrowed by 11%. In reaches with the same magnitude of peak flow reduction, meandering reaches narrowed by 15% to 22% and fan–eddy-dominated reaches narrowed by 11% to 12%. Specific stream power was not significantly affected by flow regulation in the meandering reaches. In the diverse array of reach characteristics and deposit types found in the study area, all pre- and post-dam deposits are part of a suite of topographic surfaces that includes a terrace that was inundated by rare pre-dam floods, an intermediate bench that was inundated by rare post-dam floods, and a post-dam floodplain that was inundated by the post-dam mean annual flood. Analysis of historical photographs and tree-ring dating of Tamarix sp. shows that the intermediate bench and post-dam floodplain are post-dam landforms in each reach type. Although these two surfaces occur at different levels, they are forming simultaneously during flows of different magnitude. And while the relative elevation and sedimentologic characteristics of the deposits differ between meandering reaches and reaches with abundant debris fans and eddies, both reach types contain deposits at all of these topographic levels. The process of channel narrowing varied between fan–eddy-dominated and meandering reaches. In the meandering reaches, where stream power has not changed, narrowing was accomplished by essentially the same depositional processes that operated prior to regulation. In fan–eddy-dominated reaches, where significant reductions in stream power have occurred, channel narrowing has been accompanied by a change in dominant depositional processes. Mid-channel sand deposits are aggrading on deposits that, in the pre-dam era, were active gravel bars. These deposits are creating new islands and decreasing the presence of open-framework gravel bars. In eddies, bare sand bars are replaced with vegetated bars that have a simpler topography than the pre-dam deposits.

GEOMORPHIC EVOLUTION OF DEBRIS FANS IN THE DU TOIT'S KLOOF, WESTERN CAPE MOUNTAINS

ABSTRACT Two debris fans in Du Toit's Kloof are investigated by means of geomorphological mapping and relative-age dating (Schmidt hammer, lichen cover and weathering rinds) of their deposits. Both fans mostly consist of debris flow deposits, with minor indications of flooding. Volume estimation and relative-age dating show four phases of debris flow activity, with a progressive decrease in magnitude of flow events. Relative-age dating techniques, however, are restricted in the maximum ages that can be determined. A high proportion of very large events (>104m) is associated with late Pleistocene periglacial conditions, but requires direct evidence by reliable dating. Fan stabilisation and incision occurred in response to reduced sediment storage in the catchment and increased slope stability during the Holocene. The data provide important new insights into footslope dynamics of the Western Cape Mountains for the Late Quaternary.

A Silurian reefal succession in the Gotland area, Baltic Sea

Seismic reflection profiling east of Gotland has given information on a succession of four reefal units in the lower Ludlow stratal sequence ranging from the Klinteberg Formation to the Hemse Group in the Gotland sequence. In this paper they are named the Klinte Reef (Klinteberg Formation unit f), the Hammarudden Reef (Hemse Group ‘younger’ units a-c), the Östergarn Reef (Hemse Group unit d) and the Millklint Reef (Hemse Group unit e). The reef units form a mainly regressive succession with younger reefs resting on the seaward slopes of older reefs. The reef succession is exposed on Gotland and in the Baltic Sea east of Gotland, whereas towards the East Baltic coast it is overlain by Pridoli sedimentary bedrock. Each reef unit corresponds to a well-defined seismic unit with distinct facies zonality ranging from lagoon via reef barrier and biohermal slope to basin facies. The ‘fore-reef’ facies is in this paper termed biohermal slope to express its characteristic reefal features. Reconstruction of the post-depositional tilting of the sequence indicates lagoonal depths of up to 10 m. The reef barriers developed at depths of up to 20 m and the biohermal slopes at between 10 and 55 m. The reef barrier and the biohermal slope are generally separated by a debris fan of waste products from the reef, 1-2 km wide. The reef barriers are generally 1-3 km wide, which is about the same width as their lagoons. The biohermal slopes become successively wider towards the upper part of reef succession, ranging from 5-9 km at the Klinte Reef to 15-18 km at the Millklint Reef. Comparisons with the Gotland sequence show that the reef barriers are biostromal stromatoporoid reefs in a matrix of crinoid debris. The biostromes are of the Kuppen type. The bioherms are of the Axelro and Hoburgen types. The bioherms occur randomly on the seaward slope of the barriers.

A statistical method to predict debris flow deposited volumes on a debris fan

The present paper aims to give a useful methodology to predict the debris volume Gs associated to a debris flow and deposited on the basin's terminal fan in the alpine region. Predicting the occurrence and the magnitude of a debris flow is often a very difficult task, as it is necessary to consider the variability of the geological, hydraulic and pluviometric factors connected to the beginning, propagation and deposition of a debris flow. These factors are not easily quantifiable, as well as their influence on the debris volumes deposited on the debris fan. Even if some useful mathematical (and deterministic) relations have been proposed by some authors to predict the likely debris volumes, they do not seem to take into account the high variability of the influencing factors. For these reasons a statistical investigation of the relationship between Gs and the basin area A has been undertaken. By means of well known statistical methodologies and by using a sample of about two hundred data (referred to the alpine region), a new criterion is proposed, in the present paper, for the prediction of a probable Gs value. For a given basin, this value is obtained on the basis of a statistical risk degree arbitrarily chosen. A probabilistic methodology is also proposed, which, when results of an accurate survey are not available for a particular basin, may provide a rational basis for a first hazard zonation and the design of mitigative measures.

The Multetta Debris Fan, Eastern Swiss Alps: A 500-year Debris Flow Chronology

The opening up of the Alps for tourism has increased the risk potential from natural hazards, one of the greatest of which is debris flows. Since 1996 we have been studying the debris fan of Multetta above Tschierv in Val Müstair in the Swiss Alps to determine the dates and hence the frequency of debris flows. Abrupt growth changes, impact scars, and the formation of adventitious roots allowed precise dating by dendrochronological methods in cases where tree ring characteristics occurred synchronously in several different trees. Six debris flows since the late 15th century could be recognized, occurring at the end of the 15th century, and in 1573, 1655, 1784, 1958, and 1989. The debris deposited during the past 500 yr has formed a layer about 5 m deep. Two further debris flows were identified through scars formed in 1759 and 1837. A greater frequency of scars between the mid-18th century and the end of the 19th century indicates considerably higher morphological activity during the last peak of the Little Ice Age.

Denudation of Small Alpine Basins, Nanga Parbat Himalaya, Pakistan

Thirty-three debris fans and five small alpine basins on the south side of the rapidly uplifting Nanga Parbat Himalaya of northern Pakistan were assessed to determine how much alpine processes contribute to the overall denudation of the massif. A high-resolution digital elevation model was used to measure the volume of the small alpine fans and a few basins in the Rupal valley. These volumetric estimates, coupled with time-constraining dates from cosmogenic-nuclides of glacially exposed rocks and infrared-stimulated luminescence of sediments, indicate that estimated average denudation rates in these systems have been ~2 mm yr−1 over the past 4600 to 6000 yr since the last major deglaciation. This is similar to other estimates of rates of denudation in the Himalaya.

The Multetta Debris Fan, Eastern Swiss Alps: A 500-Year Debris Flow Chronology

The opening up of the Alps for tourism has increased the risk potential from natural hazards, one of the greatest of which is debris flows. Since 1996 we have been studying the debris fan of Multet...

Cosmogenic [formula omitted] ages and frequency of late Holocene debris flows from Prospect Canyon, Grand Canyon, USA

Lava Falls Rapid, which was created and is maintained by debris flows from Prospect Canyon, is the most formidable reach of whitewater on the Colorado River in Grand Canyon and is one of the most famous rapids in the world. Debris flows enter the Colorado River at tributary junctures, creating rapids. The frequency of debris flows is an important consideration when management of regulated rivers involves maintenance of channel morphology. We used cosmogenic 3 He , 14 C , and historical photographs to date 12 late Holocene and historic debris flows from Prospect Canyon. The highest and oldest deposits from debris flows on the debris fan yielded a 3 He date of about 3 ka, which indicates predominately late Holocene aggradation of one of the largest debris fans in Grand Canyon. The deposit, which has a 25-m escarpment caused by river reworking, crossed the Colorado River and raised its base level by 30 m for an indeterminate although likely short period. We mapped depositional surfaces of 11 debris flows that occurred after 3 ka. Two deposits inset against the highest deposit yielded 3 He ages of about 2.2 ka, and at least two others followed shortly afterwards. At least one of these debris flows also dammed the Colorado River. The most recent prehistoric debris flow occurred no more than 0.5 ka. The largest historic debris flow, which constricted the river by 80%, occurred in 1939. Five other debris flows occurred after 1939; these debris flows constricted the Colorado River by 35–80%. Assuming the depositional volumes of late Holocene debris flows can be modeled using a lognormal distribution, we calculated recurrence intervals of 15 to more than 2000 years for debris flows from Prospect Canyon.