The role of the sediment budget in understanding debris flow susceptibility

This study proposes a sediment‐budget model to predict the temporal variation of debris volume stored in a debris‐flow prone watershed. The sediment‐budget is dominated by shallow landslides and debris outflow. The basin topography and the debris volume stored in the source area of the debris‐flow prone watershed help evaluating its debris‐flow susceptibility. The susceptibility model is applied to the Tungshih area of central western Taiwan. The importance of the debris volume in predicting debris‐flow susceptibility is reflected in the standardized coefficients of the proposed statistical discriminant model. The high prediction rate (0·874) for the occurrence of debris flows justifies the capability of the proposed susceptibility models to predict the occurrence of debris flows. This model is then used to evaluate the temporal evolution of the debris‐flow susceptibility index. The analysis results show that the numbers of watershed which are classified as a debris‐flow group correspond well to storage of sediment at different time periods. These numbers are 10 before the occurrence of Chi‐Chi earthquake, 13 after the occurrence of Chi‐Chi earthquake, 16 after the occurrence of landslides induced by Typhoon Mindulle (Typhoon M), and 14 after the occurrence of debris flows induced by Typhoon M. It indicates that the occurrence of 7·6 Chi‐Chi earthquake had significant impact on the debris flow occurrence during subsequent typhoons. Copyright © 2009 John Wiley & Sons, Ltd.     

Assessing entrainment of bed material in a debris‐flow event: a theoretical approach incorporating Monte Carlo method

Entrainment of underlying bed sediment by a debris flow can significantly increase the debris‐flow magnitude. To study this phenomenon, a theoretical approach to assessing bed‐sediment entrainment is presented. The approach is based on a static approximation that bed‐sediment entrainment occurs when the shearing stress of the flow is sufficiently high to overcome the basal resistance of the bed sediment. In order to delineate erodible zones in a channel, we analyze the critical condition of this static equilibrium model, and subsequently propose a new concept of a critical line to detect the entrainment reaches in a channel. Considering the spatial and temporal uncertainties of the input parameter, the approach is further incorporated within a Monte Carlo method, and the distribution of entrainment zones and post‐entrainment volumes can be analyzed. This approach is illustrated by back‐analysis of the 2010 Yohutagawa debris‐flow event, Japan. Results from 10 000 trials of Monte Carlo simulation are compared with the in situ surveys. It is shown that the present approach can be satisfactorily used to delineate erodible zones and estimate possible entrainment volume of the event. Discussion regarding the sensitivities and limitations of the approach concludes the paper. Copyright © 2015 John Wiley & Sons, Ltd.     

Empirical prediction of debris‐flow mobility and deposition on fans

A new method to predict the runout of debris flows is presented. A data base of documented sediment‐transporting events in torrent catchments of Austria, Switzerland and northern Italy has been compiled, using common classification techniques. With this data we test an empirical approach between planimetric deposition area and event volume, and compare it with results from other studies. We introduce a new empirical relation to determine the mobility coefficient as a function of geomorphologic catchment parameters. The mobility coefficient is thought to reflect some of the flow properties during the depositional part of the debris‐flow event. The empirical equations are implemented in a geographical information system (GIS) based simulation program and combined with a simple flow routing algorithm, to determine the potential runout area covered by debris‐flow deposits. For a given volume and starting point of the deposits, a Monte‐Carlo technique is used to produce flow paths that simulate the spreading effect of a debris flow. The runout zone is delineated by confining the simulated potential spreading area in the down slope direction with the empirically determined planimetric deposition area. The debris‐flow volume is then distributed over the predicted area according to the calculated outflow probability of each cell. The simulation uses the ARC‐Objects environment of ESRI© and is adapted to run with high resolution (2·5 m × 2·5 m) digital elevation models, generated for example from LiDAR data. The simulation program called TopRunDF is tested with debris‐flow events of 1987 and 2005 in Switzerland. Copyright © 2009 John Wiley & Sons, Ltd.     

Runoff-generated debris flows: Observation of initiation conditions and erosion–deposition dynamics along the channel at Cancia (eastern Italian Alps)

In the Dolomitic region, abundant coarse hillslope sediment is commonly found at the toe of rocky cliffs. Ephemeral channels originate where lower permeability bedrock surfaces concentrate surface runoff. Debris flows initiate along such channels following intense rainfall and determine the progressive erosion and deepening of the channels. Sediment recharge mechanisms include rock fall, dry ravel processes and channel-bank failures. Here we document debris flow activity that took place in an active debris flow basin during the year 2015. The Cancia basin is located on the southwestern slope of Mount Antelao (3264 m a.s.l.) in the dolomitic region of the eastern Italian Alps. The 2.5 km² basin is incised in dolomitic limestone rocks. The data consist of repeated topographic surveys, distributed rainfall measurements, time-lapse (2 s) videos of two events and pore pressure measurements in the channel bed. During July and August 2015, two debris flow events occurred, following similarly intense rainstorms. We compared rainfall data to existing rainfall triggering thresholds and simulated the hydrological response of the headwater catchment with a distributed model in order to estimate the total and peak water discharge. Our data clearly illustrate how debris entrainment along the channel is the main contributor to the overall mobilized volume and that erosion is dominant when the channel slope exceeds 16°. Further downstream, sediment accumulation and depletion occurred alternately for the two successive events, indicating that sediment availability along the channel also influences the flow behaviour along the prevailing-transport reach. The comparison between monitoring data, topographical analysis and hydrological simulation allows the estimation of the average solid concentration of the two events and suggests that debris availability has a significant influence on the debris flow volume. © 2020 John Wiley & Sons, Ltd.     

Geomorphic effects of large debris flows on channel morphology at North Fork Mountain,eastern West Virginia,USA

Extreme rainfall in June 1949 and November 1985 triggered numerous large debris flows on the steep slopes of North Fork Mountain, eastern West Virginia. Detailed mapping at four sites and field observations of several others indicate that the debris flows began in steep hillslope hollows, propagated downslope through the channel system, eroded channel sediment, produced complex distributions of deposits in lower gradient channels, and delivered sediment to floodwaters beyond the debris-flow termini. Based on the distribution of deposits and eroded surfaces, up to four zones were identified with each debris flow: an upper failure zone, a middle transport/erosion zone, a lower deposition zone, and a sediment-laden floodwater zone immediately downstream from the debris-flow terminus. Geomorphic effects of the debris flows in these zones are spatially variable. The initiation of debris flows in the failure zones and passage through the transport/erosion zones are characterized by degradation; 2300 to 17 000 m³ of sediment was eroded from these zones. The total volume of channel erosion in the transport/erosion zones was 1·3 to 1·5 times greater than the total volume of sediment that initially failed, indicating that the debris flows were effective erosion agents as they travelled through the transport/erosion zones. The overall response in the deposition zones was aggradation. However, up to 43 per cent of the sediment delivered to these zones was eroded by floodwaters from joining tributaries immediately after debris-flow deposition. This sediment was incorporated into floodwaters downstream from the debris-flow termini causing considerable erosion and deposition in these channels. © 1998 John Wiley & Sons, Ltd.     

A depth‐averaged two‐phase model for debris flows over erodible beds

Mass exchange between debris flow and the bed plays a vital role in debris flow dynamics. Here a depth‐averaged two‐phase model is proposed for debris flows over erodible beds. Compared to previous depth‐averaged two‐phase models, the present model features a physical step forward by explicitly incorporating the mass exchange between the flow and the bed. A widely used closure model in fluvial hydraulics is employed to estimate the mass exchange between the debris flow and the bed, and an existing relationship for bed entrainment rate is introduced for comparison. Also, two distinct closure models for the bed shear stresses are evaluated. One uses the Coulomb friction law and Manning's equation to determine the solid and fluid resistances respectively, while the other employs an analytically derived formula for the solid phase and the mixing length approach for the fluid phase. A well‐balanced numerical algorithm is applied to solve the governing equations of the model. The present model is first shown to reproduce average sediment concentrations in steady and uniform debris flows over saturated bed as compared to an existing formula underpinned by experimental datasets. Then, it is demonstrated to perform rather well as compared to the full set of USGS large‐scale experimental debris flows over erodible beds, in producing debris flow depth, front location and bed deformation. The effects of initial conditions on debris flow mass and momentum gain are resolved by the present model, which explicitly demonstrates the roles of the wetness, porosity and volume of bed sediments in affecting the flow. By virtue of extended modeling cases, the present model produces debris flow efficiency that, as revealed by existing observations and empirical relations, increases with initial volume, which is enhanced by mass gain from the bed. Copyright © 2017 John Wiley & Sons, Ltd.     

川藏公路四川段泥石流灾害研究与治理

大量现场调查和观测发现,川藏公路四川段的泥石流灾害集中分布在4个区域,即龙胆溪泥石流区、雅江泥石流区、波戈溪泥石流区和巴塘—金沙江泥石流区。波戈溪泥石流区和巴塘—金沙江泥石流区是境内泥石流灾害最严重的地区,流域面积超过20km^2的特大型泥石流有24条。自东向西,川藏公路四川段泥石流由水石流逐渐演变为稀性泥石流;复杂的地质构造环境、高山深谷地貌特征、强烈的地震活动、高强度的降雨和冰雪融水,构成泥石流灾害显著发育的环境条件。将公路沿线泥石流形成机理概括为4种,即降雨冲击机理、强度衰减机理、冲刷切割机理和沟床拖拽机理;2003年以来采用护岸结构、速流结构、抗冲击结构、拦渣坝、消能坎等技术实施了30余个大型泥石流灾害治理工程,治理效果显著。     

Hydrometeorological controls and erosive response of an extreme alpine debris flow

On 29 August, 2003, an intense convective storm system affected the Fella River basin, in the eastern Italian Alps, producing rainfall peaks of approximately 390 mm in 12 h. The storm triggered an unusually large debris flow in the ungauged Rio Cucco basin (0·65 km²), with a volume of approximately 78 000 m³. The analysis of the time evolution of the rainstorm over the basin has been based on rainfall estimates from radar observations and data recorded by a raingauge network. Detailed geomorphological field surveys, carried out both before and after the flood of August 2003, and the application of a distributed hydrological model have enabled assessment of flood response, estimation of erosion volumes and sediment supply to the channel network. The accounts of two eyewitnesses have provided useful elements for reconstructing the time evolution and the flow processes involved in the event. Liquid peak discharge estimates cluster around 20 m³ s^?1 km^?2, placing this event on the flood envelope curve for the eastern Italian Alps. The hydrological analysis has shown that the major controls of the flood response were the exceptional cumulated rainfall amount, required to exceed the large initial losses, and the large rainfall intensities at hourly temporal scales, required to generate high flood response at the considered basin scale. Observations on the deposits accumulated on the alluvial fan indicate that, although the dominant flow process was a debris flow, sheetflood also contributed to fan aggradation and fluvial reworking had an important role in winnowing debris‐flow lobes and redistributing sediment on the fan surface. This points out to the large discharge values during the recession phase of the flood, implying an important role for subsurface flow on runoff generation of this extreme flash flood event. Copyright © 2009 John Wiley & Sons, Ltd.     

Conditions for generation of fire‐related debris flows,Capulin Canyon,New Mexico

Comparison of the responses of three drainage basins burned by the Dome fire of 1996 in New Mexico is used to identify the hillslope, channel and fire characteristics that indicate a susceptibility specifically to wildfire‐related debris flow. Summer thunderstorms generated three distinct erosive responses from each of three basins. The Capulin Canyon basin showed widespread erosive sheetwash and rilling from hillslopes, and severe flooding occurred in the channel; the North Tributary basin exhibited extensive erosion of the mineral soil to a depth of 5 cm and downslope movement of up to boulder‐sized material, and at least one debris flow occurred in the channel; negligible surface runoff was observed in the South Tributary basin. The negligible surface runoff observed in the South Tributary basin is attributed to the limited extent and severity of the fire in that basin. The factors that best distinguish between debris‐flow producing and flood‐producing drainages are drainage basin morphology and lithology. A rugged drainage basin morphology, an average 12 per cent channel gradient, and steep, rough hillslopes coupled with colluvium and soil weathered from volcaniclastic and volcanic rocks promoted the generation of debris flows. A less rugged basin morphology, an average gradient of 5 per cent, and long, smooth slopes mantled with pumice promoted flooding. Flood and debris‐flow responses were produced without the presence of water‐repellent soils. The continuity and severity of the burn mosaic, the condition of the riparian vegetation, the condition of the fibrous root mat, accumulations of dry ravel and colluvial material in the channel and on hillslopes, and past debris‐flow activity, appeared to have little bearing on the distinctive responses of the basins. Published in 2000 by John Wiley & Sons, Ltd.     

The coupling status of alluvial fans and debris cones: a review and synthesis

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

Geomorphological characteristics of small debris flows on Junior's Kop,Marion Island,maritime sub ‐ Antarctic

The geomorphological characteristics of small debris flows in a maritime sub‐Antarctic environment are described. The morphological and sedimentological characteristics of the debris flows are comparable to debris flows documented for other parts of the world; their initiation appears closely linked to the unusual environment in which they are found. Sediment supply is generated by diurnal frost heave of loamy sediment associated with Azorella selago. The debris flows are triggered by sediment mobilization upon saturation of the frost‐heaved surface gravel and overland flow over the low‐permeability and frost‐susceptible slope materials. Morphological effects of the flows are short‐lived due to obliteration by subsequent frost heave activity. Copyright © 2000 John Wiley & Sons, Ltd.     

Processes and rates of sediment and wood accumulation in headwater streams of the Oregon Coast Range,USA

Channels that have been scoured to bedrock by debris flows provide unique opportunities to calculate the rate of sediment and wood accumulation in low‐order streams, to understand the temporal succession of channel morphology following disturbance, and to make inferences about processes associated with input and transport of sediment. Dendrochronology was used to estimate the time since the previous debris flow and the time since the last stand‐replacement fire in unlogged basins in the central Coast Range of Oregon. Debris flow activity increased 42 per cent above the background rate in the decades immediately following the last wildfire. Changes in wood and sediment storage were quantified for 13 streams that ranged from 4 to 144 years since the previous debris flow. The volume of wood and sediment in the channel, and the length of channel with exposed bedrock, were strongly correlated with the time since the previous debris flow. Wood increased the storage capacity of the channel and trapped the majority of the sediment in these steep headwater streams. In the absence of wood, channels that have been scoured to bedrock by a debris flow may lack the capacity to store sediment and could persist in a bedrock state for an extended period of time. With an adequate supply of wood, low‐order channels have the potential of storing large volumes of sediment in the interval between debris flows and can function as one of the dominant storage reservoirs for sediment in mountainous terrain. Copyright © 2003 John Wiley & Sons, Ltd.     

Debris-flow-dominated sediment transport through a channel network after wildfire

Field studies that investigate sediment transport between debris-flow-producing headwaters and rivers are uncommon, particularly in forested settings, where debris flows are infrequent and opportunities for collecting data are limited. This study quantifies the volume and composition of sediment deposited in the arterial channel network of a 14-km² catchment (Washington Creek) that connects small, burned and debris-flow-producing headwaters (<1 km²) with the Ovens River in SE Australia. We construct a sediment budget by combining new data on deposition with a sediment delivery model for post-fire debris flows. Data on deposits were plotted alongside the slope–area curve to examine links between processes, catchment morphometry and geomorphic process domains. The results show that large deposits are concentrated in the proximity of three major channel junctions, which correspond to breaks in channel slope. Hyperconcentrated flows are more prominent towards the catchment outlet, where the slope–area curve indicates a transition from debris flow to fluvial domains. This shift corresponds to a change in efficiency of the flow, determined from the ratio of median grain size to channel slope. Our sediment budget suggests a total sediment efflux from Washington Creek catchment of 61 × 10³ m³. There are similar contributions from hillslopes (43 ± 14 × 10³ m³), first to third stream order channel (35 ± 12 × 10³ m³) and the arterial fourth to fifth stream order channel (31 ± 17 × 10³ m³) to the total volume of erosion. Deposition (39 ± 17 × 10³ m³) within the arterial channel was higher than erosion (31 ± 17 × 10³ m³), which means a net sediment gain of about 8 × 10³ m³ in the arterial channel. The ratio of total deposition to total erosion was 0.44. For fines <63 μm, this ratio was much smaller (0.11), which means that fines are preferentially exported. This has important implications for suspended sediment and water quality in downstream rivers. © 2019 John Wiley & Sons, Ltd.     

Morphology and sedimentology of a complex debris flow in a clay‐shale basin

Coupling morphological, sedimentological, and rheological studies to numerical simulations is of primary interest in defining debris‐flow hazard on alluvial fans. In particular, numerical runout models must be carefully calibrated by morphological observations. This is particularly true in clay‐shale basins where hillslopes can provide a large quantity of poorly sorted solid materials to the torrent, and thus change both the mechanics of the debris flow and its runout distance. In this context, a study has been completed on the Faucon stream (southeastern French Alps), with the objectives of (1) defining morphological and sedimentological characteristics of torrential watersheds located in clay‐shales, and (2) evaluating through a case study the scouring potential of debris flows affecting a clay‐shale basin. Morphological surveys, grain‐size distributions and petrographic analyses of the debris‐flow deposits demonstrate the granular character of the flow during the first hectometre, and its muddy character from there to its terminus on the debris fan. These observations and laboratory tests suggest that the contributing areas along the channel have supplied the bulk of the flow material. Copyright © 2005 John Wiley & Sons, Ltd.     

Impact of the magnitude and frequency of debris‐flow events on the evolution of an alpine alluvial fan during the last two centuries: responses to natural and anthropogenic controls

The dynamics and the surface evolution of a post‐LGM debris‐flow‐dominated alluvial fan (Tartano alluvial fan), which lies on the floor of an alpine valley (Valtellina, Northern Italy), have been investigated by means of an integrated study comprising geomorphological field work, a sedimentological study, photointerpretation, quantitative geomorphology, analysis of ancient to modern cartography and consultation of historical documents and records. The fan catchment meteoclimatic, geological and geomorphological characteristics result in fast rates of geomorphic reorganization of the fan surface (2 km²). The dynamics of the fan are determined by the alternation of low‐return period catastrophic alluvial events dominated by non‐cohesive debris flows triggered by extreme rainstorms which caused aggradation and steepening of the fan and avulsion of its main channel, with periods of low to moderate streamflow discharge punctuated by low‐ to intermediate‐magnitude flood events, causing slower but steady topographic reworking. The most ancient parts of the fan surface date back at least to the first half of the 19th century, but most of the fan surface has been restructured after 1911, mainly during the debris‐flow‐dominated events of 1911 and 1987. Phases of rapid fan toe incision and fan degradation have been recognized; since the 1930s or 1940s, the Tartano fan has been subjected to a state of deep entrenchment and narrowing of the main trunk channel and distributary area. Post‐Little Ice Age climate change and present‐day surface uplift rates have been considered as possible explanations for the observed geomorphic evolution, but tectonic or climatic controls cannot account for the order of magnitude of the erosional pace. Anthropogenic controls plausibly override the natural ones: in particular, the building of a dam in the late 1920s, about 2 km upstream of the fan, seems to have triggered fan dissection, having altered the sediment discharge through sediment retention. Copyright © 2011 John Wiley & Sons, Ltd.     

Analysis of debris flow surges using the theory of uniformly progressive flow

Coarse debris flows develop surges with distinct longitudinal sorting. Although highly unsteady, such flow often appears to attain a steady‐state condition, moving over long distances with approximately constant velocity and maximum depth. Typically, a steep, bouldery front is followed by an accumulation of liquid slurry, which in turn decays into a dilute tail. Such sorting has long been recognized by field workers, but its influence on the dynamic behaviour of debris flow surges has not yet been fully clarified by analysis. A simple model is presented, using the theory of uniformly progressive flow and incorporating zoned longitudinal variation in rheology. It is shown that non‐homogeneity can cause very significant magnification of the peak discharge, depending on the slope angle and on the length of the frontal boulder concentration. The shape of the surge flow profiles is determined not only by the rheology of the retained material, but by the longitudinal variation of material characteristics. As a result, excessive reliance on laboratory‐derived rheological constitutive relationships is not advisable. Models of debris flow surges should be non‐homogeneous and able to incorporate zones of contrasting rheology. Copyright © 2000 John Wiley & Sons, Ltd.     

Analysis of the hydrological behaviour of an urbanizing basin

Suburban areas undergo rapid land‐use changes due to urban growth. Consequently, the mitigation of hydrological impacts is a major issue in the field of flood and water pollution management. Nevertheless, suburban catchments have seldom been studied. This paper presents a method for analyzing the hydrological behaviour of suburban catchments; the particular method is tested on the Chézine catchment, located in a suburban area of Nantes (western France). Chézine provides a typical example of a suburban catchment, yet features the unique behaviour of a response time ranging from 1 to 6 h. It is proposed herein to classify rainfall‐runoff events in homogeneous groups according to their flow coefficient. A group of events is characterized by its mean flow coefficient and by its transfer function, which are considered as the signatures of the hydrological behaviour of these similar events. The transfer function is identified from the available series of rainfall and outflow data. The identified transfer functions serve to estimate the localization of contributing zones over the basin by estimating the basin transfer function from flowpaths. The consistency of these assumptions is then verified by comparing the estimated transfer function with the identified one. The application of this method to the Chézine catchment demonstrates that it is possible to distinguish various types of hydrological behaviour regimes associated with significantly different transfer functions. The joint analysis of the flow coefficient and transfer function of each group confirms that the Chézine catchment reacts like an urban basin with just the urban zones contributing to runoff under dry conditions. Otherwise, the wetter the initial state, the greater the tendency of this basin to react like a natural basin, as reflected by the different transfer function shapes. These results confirm the validity of the proposed method to analyse the various behaviour regimes of suburban catchments. In addition, this method helps define the specifications of hydrological models suited to suburban catchments. Copyright © 2013 John Wiley & Sons, Ltd.     

Wood storage in three mountain streams of the Southern Andes and its hydro‐morphological effects

This study analyses large wood (LW) storage and the associated effects on channel morphology and flow hydraulics in three third‐order mountain basins (drainage area 9–12 km²) covered in old‐growth Nothofagus forests, ranging from the temperate warm Chilean Andean Cordillera to the sub‐Antarctic Tierra del Fuego (Argentina). Amount, characteristics and dimensions of large wood (>10 cm diameter, >1 m long) were recorded, as well as their effects on stream morphology, hydraulics and sediment storage. Results show that major differences in LW abundance exist even between adjacent basins, as a result of different disturbance histories and basin dissection. Massive LW volumes (i.e. >1000 m³ ha^?1) can be reached in basins disturbed by fires followed by mass movements and debris flows. Potential energy dissipation resulting from wood dams is about a quarter of the total elevation drop in two streams, with a gross sediment volume stored behind wood dams of around 1000 m³ km^?1, which appears to be of the same order as the annual sediment yield. Finally, the presence of wood dams may increase flow resistance by up to one order of magnitude. Copyright © 2007 John Wiley & Sons, Ltd.     

Effects of forest harvesting on the occurrence of landslides and debris flows in steep terrain of central Japan

Landslides and debris flows associated with forest harvesting can cause much destruction and the influence of the timing of harvesting on these mass wasting processes therefore needs to be assessed in order to protect aquatic ecosystems and develop improved strategies for disaster prevention. We examined the effects of forest harvesting on the frequency of landslides and debris flows in the Sanko catchment (central Japan) using nine aerial photo periods covering 1964 to 2003. These photographs showed a mosaic of different forest ages attributable to the rotational management in this area since 1912. Geology and slope gradient are rather uniformly distributed in the Sanko catchment, facilitating assessment of forest harvesting effects on mass wasting without complication of other factors. Trends of new landslides and debris flows correspond to changes in slope stability explained by root strength decay and recovery; the direct impact of clearcutting on landslide occurrence was greatest in forest stands that were clearcut 1 to 10 yr earlier with progressively lesser impacts continuing up to 25 yr after harvesting. Sediment supply rate from landslides in forests clearcut 1 to 10 yr earlier was about 10‐fold higher than in control sites. Total landslide volume in forest stands clearcut 0 to 25 yr earlier was 5·8 × 10³ m³ km^?2 compared with 1·3 × 10³ m³ km^?2 in clearcuts >25 yr, indicating a fourfold increase compared with control sites during the period when harvesting affected slope stability. Because landslide scars continue to produce sediment after initial failure, sediment supply from landslides continues for 45 yr in the Sanko catchment. To estimate the effect of forest harvesting and subsequent regeneration on the occurrence of mass wasting in other regions, changes in root strength caused by decay and recovery of roots should be investigated for various species and environmental conditions. Copyright © 2007 John Wiley & Sons, Ltd.