Triggering Debris Flows Research Articles

Piezometric response in shallow bedrock at CB1: Implications for runoff generation and landsliding

Experimental observations comparing two steep unchanneled valleys in the Oregon Coast Range, one intensively instrumented (CB1) and the other monitored for runoff but which produced a debris flow (CB2), shed light on the mechanisms of shallow flow in bedrock, its interaction with the vadose zone, and its role in generating landslides. Previous work at CB1 led to the proposal that during storms pulses of rainfall transmit pressure waves through the vadose zone and down to the saturated zone to create rapid pore pressure response and runoff [Torres et al., 1998]. Here, we document the associated rapid pore pressure response in the shallow fractured bedrock that underlies these colluvium‐mantled sites and examine its influence on the generation of storm flow, seasonal variations in base flow, and slope stability in the overlying colluvial soil. Our observations document rapid piezometric response in the shallow bedrock and a substantial contribution of shallow fracture flow to both storm flow and seasonal variations in base flow. Saturated hydraulic conductivity in the colluvial soil decreases with depth below the ground surface, but the conductivity of the near‐surface bedrock displays no depth dependence and varies over five orders of magnitude. Analysis of runoff intensity and duration in a series of storms that did and did not trigger debris flows in the surrounding area shows that the landslide inducing storms had the greatest intensity over durations similar to those predicted by a simple model of piezometric response. During a monitored storm in February 1992, the channel head at the base of the neighboring CB2 site failed as a debris flow. Automated piezometric measurements document that the CB2 debris flow initiated several hours after peak discharge, coincident with localized development of upward spikes of pressure head from near‐surface bedrock into the overlying colluvial soil in CB1. Artesian flow observed exfiltrating from bedrock fractures on the failure surfaces at CB2 further implicates bedrock fracture flow in both runoff generation by subsurface storm flow and suggests a connection to landslide initiation. Our observations show that the timing of shallow landslide initiation can be delayed relative to both peak rainfall and peak runoff and support the argument that the influence of fracture flow on shallow landsliding helps explain the wide variability in the occurrence of slope instability in topographically analogous locations.

Influences of hydrological and hydrogeological conditions on debris flows in peri-vesuvian hillslopes

Abstract. The paper illustrates some results of research carried out to assess factors triggering debris flows which involve the pyroclastic overburdens covering carbonate mountains around Vesuvius. The aims of the research were to reconstruct a relationship between rainfall and debris flow occurrence and to highlight empirical hydrological thresholds through rainfall pattern analysis. The research was also aimed at investigating hydrogeological features of a pyroclastic cover-carbonate bedrock system to analyse factors inducing temporary hydraulic flow, critical for pyroclastic soil stability. The results of research are the following: i) rainfall pattern highlights empirical hydrological thresholds that differentiate the Lattari and Salerno Mountains from the Sarno Mountains; ii) in some sample areas of the Sarno Mountains close to the trigger zones of the landslides of May 1998 strong variation in hydraulic conductivity has been found in the first few meters below the surface; iii) these permeability variations would seem to justify temporary perched water tables that might affect the stability of the pyroclastic mantle.

Meteorological and earth observation remote sensing data for mass movement preparedness

One of the major problems of the efficient synergetic use of remote sensing data for natural disaster mitigation is the fusion of various meteo- and geodata sets of significantly different spatial resolution. On the other hand, different morphological types of mass movements are based on alternate concepts i.e. of generation which, again has to be initially reflected in differing methodological approaches. The unifying idea and stronghold, however, of the presented approach is the precipitation parameters which trigger the debris flows. Albeit, frequently there are no relevant precipitation climatic data available. Despite significant drawbacks in the integration of Landsat and/or SPOT data sound hazard zonation maps can be generated. For a test area in the French Alps it has been shown that remote sensing data can be used to predict potential debris flow events. High temporal frequency remote sensing data from the Meteosat series of satellites allow the identification of cloud clusters most likely to result in intense rainfall which are, in turn, likely to initiate debris flow activity. Video evidence, field observations and an empirical debris flow model linked to an instantaneous rain gauge were used to ascertain the exact times of debris flow initiation. Due to the high spatial and temporal variability of rainfall in mountainous regions and the large areas vulnerable to debris flows compared to the coverages of these observations, there are, however, restrictions on the use of these data for large regions to provide early warning of debris flow events operationally. Additionally, the possible timeliness of warnings using such observations is restricted to the relatively short interval between the onset of the triggering phenomena and the hazard event. The remote sensing techniques developed in this study, allow warning of potential debris flow events to be derived before the triggering phenomenon occurs, by attempting to recognise the evolution if intense rain-bearing clouds. In this role the meteorological remote sensing data are not used to retrieve rainfall amounts but, instead, to derive rain cloud properties that produce debris flow triggering conditions.

Depositional processes, triggering mechanisms and sediment composition of carbonate gravity flow deposits: examples from the Late Cretaceous of the south-central Pyrenees, Spain

Cenomanian through Coniacian strata near the town of Sopeira in the south-central Pyrenees (northern Spain) are composed of a variety of autochthonous and allochthonous carbonate slope lithologies that are divided into six depositional sequences based on facies distribution patterns and stratal relationships. The sequences record three major phases of platform margin evolution: rifting, burial, and exhumation. During the first phase (sequences UK-1, UK-2, UK-3, UK-4, and lower UK-5), deposition occurred on the edge of a wrench basin, and a normal fault located beneath the platform margin strongly influenced slope evolution. Background hemipelagic sediments on the slope were commonly redeposited by submarine slumps and slides. More intense reworking resulted in matrix-supported, slope-derived megaconglomerates (debrites). During the Cenomanian and Turonian, seismically triggered debris flows originated at the platform margin, bypassed the upper slope, and were deposited on the lower slope as polymictic, clast-supported, matrix-rich megabreccias. The megabreccias form channelized and sheet-like bodies with erosional basal surfaces. Shallow carbonate environments backstepped during the Late Turonian and Coniacian, but displacement along the fault at this time resulted in the development of a steep submarine scarp and the exposure of Cenomanian and Lower Turonian strata to submarine erosion. Matrix-poor, margin-derived megabreccias form a thick talus pile at the base of the scarp. Some of the breccias were transported into the basin as debris falls, forming sheet-like beds. Marl eventually buried the Coniacian scarp in sequence UK-5, resulting in the second major phase of platform slope evolution. The slope profile at this time was relatively gentle, and redeposited material is less common. In the third phase (sequence UK-6), tectonically induced bankward erosion during the Santonian resulted in a high (greater than 800 m) erosional scarp with a regional east–west trend that was subsequently onlapped by siliciclastic turbidites. Rejuvenation of erosion in the same vicinity suggests that long-term tectonism controlled the position of the slope, rates of erosion, and sediment type on the slope. Sediment gravity flow processes are laterally and temporally related. Submarine slide and slump deposits commonly grade laterally downslope into slope-derived megaconglomerates. Debris flows that originated at the platform margin appear to have initiated slumps, slides, and other debris flows on the slope. Debris fall deposits are commonly capped by coarse, graded, lithoclastic packstones that may represent turbidites generated by the debris falls. Sediment fabric exerted a profound impact on depositional processes, distribution of facies, and morphology of the slope. Fine-grained, mud-rich, lower slope deposits were unstable at even moderate slope angles, and have been extensively redeposited. Redeposition of grain-rich, upper slope facies was triggered by syndepositional seismic activity and upslope migration of instability and erosion. In the presence of mud, the transport mechanisms are typically cohesive debris flows, which were able to carry material onto the lower slope and into the basin. When no mud was available, rock falls and debris falls were the dominant sediment gravity flows, and their deposits are restricted to a position on the hanging wall proximal to the fault.

Paleoseismic signature in late Holocene sediment cores from Saanich Inlet, British Columbia

Abstract This paper explores the paleoseismic record potentially preserved in the upper 40 m of hydraulic piston cores collected in 1996 at two sites in Saanich Inlet, British Columbia, during ocean drilling program (ODP) Leg 169S. The ODP cores are missing 1–2 m of water-rich sediment directly underlying the seafloor, but this sediment is preserved in shorter piston cores collected in 1989 and 1991. The upper part of the ODP cores consists of rhythmically laminated (varved) marine mud with intercalated massive beds, interpreted to be debris flow deposits. Some of the debris flow deposits are linked to past earthquakes, including the 1946 Vancouver Island earthquake (M7.2), a great (M8-9) plate-boundary earthquake at the Cascadia subduction zone in January 1700, and a large crustal or plate-boundary earthquake about 1000 yr ago. Earthquakes may also be responsible for debris flows in about ad 1600, 1500, 1250, 1150, 850, 450, 350, 180, and bc 200, 220, 500, 900, and 1050. If so, the average recurrence interval for moderate to large earthquakes, which trigger debris flows in Saanich Inlet, is about 150 yr. This recurrence interval is broadly consistent with the frequency of moderate to large earthquakes in the region during the historical period. Debris flows, however, can also be triggered by non-seismic processes, making it difficult to assemble a complete earthquake record from the Saanich Inlet cores. We propose that extensive debris flow deposits, emplaced by single large failures or many smaller coincident failures, probably have a seismic origin.

Debris-flow hazards in the Blue Ridge of central Virginia

Abstract The June 27, 1995, storm in Madison County, Virginia produced debris flows and floods that devastated a small (130 km 2 ) area of the Blue Ridge in the eastern United States. Although similar debris-flow inducing storm events may return only approximately once every two thousand years to the same given locale, these events affecting a similar small-sized area occur about every three years somewhere in the central and southern Appalachian Mountains. From physical examinations and mapping of debris-flow sources, paths, and deposits in Madison County, we develop methods for identifying areas subject to debris flows using Geographic Information Systems (GIS) technology. We examined the rainfall intensity and duration characteristics of the June 27, 1995, and other storms, in the Blue Ridge of central Virginia, and have defined a minimum threshold necessary to trigger debris flows in granitic rocks. In comparison with thresholds elsewhere, longer and more intense rainfall is necessary to trigger debris flows in the Blue Ridge.

Experimental evidence from the triggering of debris flow along a granular slope

The instability of a granular slope, with seepage flow and low tail downstream water level, has been experimentally investigated to point out the conditions associated to the formation of a debris flow, excluding the case of the debris flow triggered by a failure due to the toe erosion. The materials employed in the experiments are uniform glass spheres (d = 0.003 m) and two different gravel (d50 = 0.007 m and d50 = 0.0022 m). Experimental runs show that the triggering of debris flow is due to the overland flow.

The development of a remote sensing based technique to predict debris flow triggering conditions in the French Alps

The effects of mass movements, including debris flows, on the inhabitants of mountainous regions can often be catastrophic, causing serious casualties and property damage. These impacts could potentially be reduced with the development of early warning systems. Debris flows are generally initiated by either heavy rainfall or snow melt. In the past, prediction of debris flow events has been limited to the moment of the flow onset and dependant on the accurate description of free flowing water conditions at the time of initiation. This has remained problematic not least because of the high spatial and temporal variabilities of the triggering phenomena, making their accurate measurement by conventional means, such as by raingauges, difficult. Remote sensing data offers an ideal opportunity to provide information on debris flow triggering conditions, including details of the evolution of triggering rainfall conditions before they initiate a debris flow event. In this paper we outline the development of a remote sensing technique to provide early warning of debris flow triggering conditions using infrared data measured from the Meteosat satellite series, for the Bachelard Valley in the French Alps. The relatively simple relationship and short time interval between the onset of heavy rainfall, and the initiation, movement and deposition of a debris flow allows information on the triggering conditions to be considered as early warning of the actual debris flow event itself, in locations of known debris flow hazard. Predictive information of triggering conditions of a particular hazard is of vital importance to the development of an effective early warning system. The technique outlined in this paper was developed using the debris flow initiation model, of Blijenberg et al ., linked to automated raingauges over a four year period from 1991 to 1994. Of the six case studies identified warning times of 1-12 hours were given in five of these. A false alarm test over a month period for the region revealed false alarms on two days, only. This paper shows that high temporal resolution remote sensing data can be used to provide early warning of atmospheric conditions likely to initiate debris flow events. This information is of importance to the development of a debris flow hazard early warning system.

Sedimentology and flow behavior of a rain-triggered lahar, Mangatoetoenui Stream, Ruapehu volcano, New Zealand

On October 28, 1995, heavy rain triggered failure of approximately 3.3 × 10 5 m 3 of fall deposits on the Mangatoetoenui Glacier and generated a lahar event that flowed east-northeast of Ruapehu volcano, down the Mangatoetoenui Stream, and eventually entered the Tongariro River 19 km downstream. The lahar left multiple stacked deposits of at least three major flow surges, or pulses, which traveled down slope at calculated velocities of as much as 27 m s −1 and calculated maximum peak discharges of as much as 2900 m 3 s −1 . The source area of the Mangatoetoenui lahar is delineated by a thin furrowed lag of unsorted ash and lapilli margined by steep headwall scarps marking the edges of undisturbed fall deposits. Failure of fall deposits in the source area triggered debris flows down a north and south branch of the Mangatoetoenui Stream. These two debris flows coalesced and ponded, then flowed across seasonal snowpack covering the stream. In the proximal zone, to 5 km downstream of the source area, the deposits of the individual constituent debris flows cannot be distinguished. However, in the medial zone, between 5 and 16 km downstream of the source area, and beyond the limit of the snowpack and the junction with perennial streamflow, the depositional record of this lahar shows clear evidence for downstream transformation of individual peak flows from debris flow to hyperconcentrated flow. Transformation began at 5 km from source due to dilution (1) by each peak flow incorporating normal streamflow in the perennial Mangatoetoenui Stream and tributaries, and (2) through deposition of part of each peak flow9s sediment load. The hyperconcentrated-flow deposits frequently may be subdivided into multiple units, which is evidence that the lahar flowed as a series of surges or waves. We consider that these were caused by episodic, but contemporaneous, inputs from a divergent flow from a single source area or by ponding and episodic release of debris within the channel. Estimated Froude numbers for the Mangatoetoenui lahar are greater than 3, implying supercritical flow and the development of fluid mechanical instabilities that enhanced these flow surges.

Analysis of rainstorm-induced slide-debris flows on natural terrain of Lantau Island, Hong Kong

Lantau Island, the largest outlying island of the territory of Hong Kong, experienced a severe rainstorm on 4–5 November 1993, which induced >800 slope failures on natural terrain there. Detailed field investigations were carried out to study the failure modes, in relation with various influencing factors. It was found that the occurrence of slide-debris flows has a close relationship with bedrock geology, slope gradient, vegetation cover and micro landform. The failure modes of slide-debris flows may be classified into translational slides and rotational slides, and the former are predominant. Analysis of the hydrological response of colluvial slopes during the rainstorm indicated that the majority of the failures were caused by the development of a perched water table in the thin surface layer of colluvium of volcanic origin due to infiltration during the heavy rain. Undisturbed soil samples from south Lantau have been subjected to anisotropically consolidated undrained compression tests at comparatively low stress levels. Constant deviatoric stress path tests (CQD) simulating the stress path in the field at in situ stress levels have been performed to investigate soil behavior. The CQD test results indicate that the material of slopes at undisturbed state is brought to dilation because of the increase in pore water pressure caused by infiltration of rain water. For a translational slide, the displacement, resulting from dilation, may destroy cohesion along the failure surface and locally within the interior of the slide. The surplus water during the intense rainstorm was able to equilibrate the reduction in pore pressure caused by dilation, and the dilation and displacement may be further increased. The strain-softening after significant strains triggered debris flow mobilization. However, for a rotational slide, the increase in pore water pressure caused by surplus water infiltration during the intense rainstorm could not equilibrate the reduction in pore pressure caused by dilation, much or even all of the sliding block could not mobilize into a debris flow.

A Sequence of the 1997 Sumikawa Landslides and its Indused Debris Flows Ocurred on May 11 at Hachimantai, Kazuno City, Akita Prefecture, Japan

In this paper we describe the reasons related to the landslide and debris flow events that occurred in May 11 th of 1997 in Sumikawa area located Hachimantai, Kazuno city, Akita Prefecture, Japan.The environmental factors for these events can be explained on the fact that the area is located in a geothermal field where hydrothermally alteredrocks can be found together with the development of a cap rock conforming the geothermal structure. The topogoraphy of the area give also indications that landslide events occurred several times in the past.The main reasons that triggerd the landslide are considered to be due to a considerable amount of snow that rapidly melted in May of 1997 together with heavy rainfalls with a precipitation of about 110 mm during 24 hours from around 8 pm of May 7 th. These two factors caused an unusual increase in the underground water level with in turn originated the landslide.It is remarkable to mention here that the landslide event that occurred about 8 AM in May 11 th of 1997, triggered debris flow and steam oruptions within several minutes.

CLIMATIC CHANGE AND DEBRIS FLOWS IN HIGH MOUNTAIN REGIONS: THE CASE STUDY OF THE RITIGRABEN TORRENT (SWISS ALPS)

Debris flows in the region of Ritigraben (Valais, Swiss Alps), which generally occur in the months of August and September, have been analyzed in relation to meteorological and climatic factors. The principal trigger mechanisms for such debris flows are abundant rain on the one hand, and snow-melt and runoff on the other hand, or a combination of both. Debris flows linked to rain are likely to be triggered when total rainfall amount over a three-day period exceeds four standard deviations, i.e., a significant extreme precipitation event. An analysis of climatological data for the last three decades in the region of Ritigraben has highlighted the fact that the number of extreme rainfall events capable of triggering debris flows in August and September has increased. Similar trends are observed for the 20th Century in all regions of Switzerland. The general rise in temperature in a region of permafrost may also play a role in the response of slope stability to extreme precipitation. At the foot of the Ritigraben, warming trends of both minimum and maximum temperatures have been particularly marked in the last two decades.

Quaternary eruptive history and hazard-zone model at Nevado del Tolima and Cerro Machin volcanoes, Colombia

The ice-clad and fumarolic Nevado del Tourna volcano (4 ° 39′N, 75 ° 20′W) south of Nevado del Ruiz, is offset toward the southeast from the axis of the volcanic Ruiz-Tolima massif with respect to the major NE-trending strike-slip Palestina fault. It is composed of four units: (1) a pre-Tolima plateau-like basement of basaltic andesite lava flows of early Quaternary age; (2) a dissected, ancestral Tourna stratovolcano, cut by a presumed collapse caldera of middle Pleistocene age; (3) an older Tolima stratovolcano of late Pleistocene age, partly destroyed by a summit caldera; and (4) composite domes of the cone-shaped young and present Tolima. Young Tolima volcano is an andesitic and dacitic composite cone formed over the past 40,000 years within a 3-km-wide caldera that opened around 0.14 Ma. Deposits of welded and nonwelded pumice- and scoria-flows were emplaced toward the southeast (Rio Combeima) and northeast (Rio Totare). Repeated growth of lava domes over the past 16,000 years is witnessed by thick block-lava flows on the southern and eastern flanks and by block-and-ash or scoria-rich pyroclastic-flow deposits. This activity occurred during at least six eruptives stages as follows: El Placer, ca. 16,200-14,000 yr B.P.; Romerales, ca. 13,000–12,300 yr B.P.; Canalones, ca. 11,500–9750 yr B.P.; Mesetas, ca. 7200 – 4600 yr. B.P.; Encanto, ca. 3600 – 1700 yr B.P., and Nieves, historical. Interactions with the ice cap probably triggered debris flows that partly filled the Combeima and Totare valleys and formed the Holocene terraces on the upper Pleistocene volcaniclastic fans of Ibaguéand Venadillo as much as 60 km from the source. The latest major activity was a plinian eruption, which deposited a pumice-fall layer ca. 3600 yr B.P. (0.5 km 3 actual volume) mainly toward the west and northwest. Minor tephra-falls and debris flows occurred during the historical period before the reported 1918 and 1943 small (phreatic ?) events. A general hazard-zone map shows areas potentially affected by future eruptions both at Nevado del Tolima and at active Cerro Machin 12 km southward. The extent of areas likely to be affected by tephra-falls, debris flows, pyroclastic flows or surges, debris avalanches and lava flows is shown. Subplinian and plinian eruptions of Nevado del Tolima were used to represent the moderate and large events to be expected. 300,000 people live within a 35-km distance from those volcanoes, which have exhibited a behaviour more explosive than Nevado del Ruiz. Despite the small-sized ice cap, debris flows are the most probable hazard for even a minor eruption, because of the very steep slope gradient, and because of probable interactions of hot eruptive products with ice and snow. Additionally, scoria flows and debris avalanches can be directed toward the southeast and could be transformed into debris flows that would devastate the Combeima valley and suburbs of Ibaguécity, where about 50,000 people live

Rainfall Thresholds for the Initiation of Debris Flows at La Honda, California

In order to study the relation between heavy rainfall, shallow pore pressures, and slope stability in hillslopes susceptible to debris flows, we have been observing debris flows and measuring rainfall and hillslope pore pressures in a 10-km 2 study area in the central Santa Cruz Mountains near La Honda, California. A simple numerical model, based on the physical analogy of a leaky barrel, can simulate significant features of the interaction between rainfall and shallow-hillslope pore pressures. In the model, the barrel is filled at a rate equal to the rainfall intensity and drained at a rate equal to the product of the water level retained in the barrel, Z, times the drainage coefficient, k d . If the retained rainfall exceeds a critical water level, Z c , the slope becomes unstable. Thus, the threshold for the intensity and duration of storm rainfall required to initiate debris flows is determined by the k d and Z c values of the most susceptible slopes in the study area. Although comparisons of leaky-barrel-model simulations with piezometer measurements indicate reasonable agreement, the k d values of individual piezometers in the study area vary widely because of spatial variations in soil properties and slope geometry. The spatial distribution of rainfall, in contrast, is fairly uniform over a scale of a few square kilometers. Thus, rather than attempt to model an actual slope in any detail, the leaky-barrel model may be used to compare the relative hydrologic effects of storms by plotting the maximum response level, Z max , against the drainage coefficient, k d . Values of Z max versus k d can then be compared for storms that triggered debris flows versus storms that failed to trigger debris flows. Using this technique, we can backcalculate k d and Z c values for the most susceptible slopes in the study area. These threshold values are k d = 0.85/hr and Z c = 8.5 mm. This leaky-barrel-model threshold is consistent with, but slightly higher than, an earlier, purely empirical, threshold. We can also relate the number of debris flows triggered by a storm to the time and amount by which the leaky-barrel-model response exceeded the threshold during the storm.

Debris flow triggering by impulsive loading: mechanical modelling and case studies : Bovis, M J; Dagg, B R Can Geotech JV29, N3, June 1992, P345–352

Debris flow triggering by impulsive loading: mechanical modelling and case studies : Bovis, M J; Dagg, B R Can Geotech JV29, N3, June 1992, P345–352

Debris flow triggering by impulsive loading: mechanical modelling and case studies

A mechanism is proposed by which debris flows can be triggered through impulsive loading. Momentum transferred from hillslope failures to steep stream bed materials may be sufficient to initiate a debris flow where one may not otherwise occur. An important parameter in the momentum transfer is the planimetric angle between the slide path axis and the stream channel axis. Preliminary stability equations for both drained and undrained loading are developed from formulae commonly used to assess stream channel stability. Case studies from two basins in the southern Coast Mountains of British Columbia are used to illustrate the mechanisms. Key words : debris flow, triggering, mechanisms, rock slide, debris slide, Coast Mountains.

Debris flows as geomorphic agents in the Huachuca Mountains of southeastern Arizona

Numerous debris flows occurred in the Huachuca Mountains of southeastern Arizona during the summer rainy season of 1988 in areas that were burned by a forest fire earlier in the summer. Debris flows occurred following a major forest fire in 1977 as well, suggesting a causal link between fires and debris flows. Abundant evidence of older debris flows preserved along channels and in mountain front fans indicates that debris flows have occurred repeteadly during the late Quaternary in this environment. Soil development in sequences of debris-flow deposits indicates that debris flows probably recur over time intervals of several hundred to a thousand years in individual drainage basins in the study area. Surface runoff in the steep drainage basins of the Huachuca Mountains is greatly enhanced following forest fires, as the hillslopes are denuded of their vegetative cover. Water and sediment eroded from the hillslope regolith are rapidly introduced into the upper reaches of tributary channels by widespread rilling and slope wash during rainfall events. This influx of water and sediment destabilizes regolith previously accumulated in the channel, triggering debris flows that scour the channel to bedrock in the upper reaches. Following a debris flow, the scoured, trapezoidally-shaped channel gradually assumes a swale shape and the percentage of exposed bedrock declines, as material is introduced from the slopes. Debris flows do a tremendous amount of work in a very short time, however, and are the major channel-forming events. Where the tributary channels enter larger, trunk channels, the debris flows serve as the main source of very coarse sediment. The local slope and coarse particle distribution of the trunk channel depend on the competence of water flows in the channel to transport the material introduced by debris flows. Where the smaller channels drain directly to the mountain front, debris flows create extensive alluvial fans which dominate the morphology of the basin-range boundary. Time intervals between debris flows in the drainage basins of the Huachuca Mountains are probably controlled by complex interactions among climate, forest fires and slope processes. Fires destroy the protective vegetation that stabilizes the upper catchment slopes and inhibits erosion. However, not every fire that burns a catchment causes debris flows, because sufficient weathered material must accumulate in the upper channel reaches to initiate a large debris flow. If such accumulation has not occurred, the material introduced to a channel following a forest fire will move only a short distance down the channel. Thus, the episodic nature of debris flows probably depends on rates of slope weathering and erosion, which are in turn controlled by climate, both directly and through vegetation and forest fires.

Debris Flows and Hyperconcentrated Floods Along the Wasatch Front, Utah, 1983 and 1984

The Wasatch Front north of Salt Lake City has a recurrent history of debris flows. Two distinctly different types of climatic events—summer thunderstorms and rapid spring snowmelts—have triggered debris flows. Heavy autumn rains, exceptional winter snowpacks, and cool and extended springs followed by sudden warming characterized weather during 1983 and 1984. As a result of these climatic events, large amounts of water were rapidly added to the landscape, resulting in several hundred landslides, many of which subsequently mobilized into debris flows and (or) hyperconcentrated floods. Reconnaissance during June of 1983 revealed many hillsides with recently active landslides that had moved slightly but had not mobilized, or had only partially mobilized, as debris flows. The presence of these partly-detached landslides prompted an investigation of the potential for future debris flows. Wieczorek and others (1983) developed a technique for evaluating the potential for debris flows and hyperconcentrated floods to reach the canyon mouths, based on three assumptions: 1) debris flows would occur in response to a large influx of water to the soils, either from a thunderstorm or rapid snowmelt; 2) the partly-detached landslides were the most likely sources for future debris flows; and 3) debris-flow material incorporated from channels would be proportional to that from landslide sources. The climatic and debris-flow events of 1984 were sufficiently similar to those of 1983 that we could evaluate these assumptions. Slightly fewer debris flows occurred in 1984 than in 1983, consistent with a slightly lower snow-pack water content and slightly slower rate of melting. Debris flows occurred near the rapidly receding snowline, apparently as a result of the large influx of water into the soils there. On an areal basis, partly-detached landslides identified in 1983 were statistically significant as sources for debris flows in 1984. In the few documented cases, the contribution of material scoured from channels was variable but very high and appeared significantly influenced by the geomorphic condition of the stream channel.

Debris flow during intense rainfall in Snowdonia, North Wales: A preliminary survey

AbstractIntense rainfall after the abnormally dry and warm summer of 1983 triggered debris flows in mountainous terrain in North Wales. This preliminary investigation concentrates on a flow which blocked the A5, requiring £56,000 of remedial work. An estimated 118·4 mm of rain fell over steep, rocky catchments in 5 hrs (peak intensity 39·9 mm hr−1) and water emerging from a rock chute mobilized colluvium on lower slopes, in which pore water pressure was probably already rising fast and bulk properties and other geotechnical conditions, including low shearing resistance, were favourable. Debris flowed in a narrow concave track 585 m long, x slope 27·8°, descending 282 m. The scoured channel, levées and debris lobes typical of documented flows elsewhere suggest that flow was rapid, turbulent, and pulsing.

Residual colluvio-aeolian aprons in the Negev highlands (Israel) as palaeo-climatic indicator

Slope features, treated previously mainly pedogenetically and described as alluvial terraces or as a catenary compound of stony loessial Sierozem, are shown to be remains of former colluvial-loessial aprons. The loess was deposited in the central Negev during the uppermost Pleistocene by dust-laden rainstorms which triggered debris flows under conditions twice as humid as today. During the Holocene the aprons were selectively eroded by slope-runoff, resulting in a wide exposure of the basal slope, truncation of the wadi fill and the final formation of the present-day patchy slope morphology. The dust-laden rainy regime fits the last, more humid period, i.e. 70,000–10,000 yr B.P.