Headwater Channels Research Articles

Mapping potential, existing and efficient wetlands using free remote sensing data

Although wetlands remain threatened by human pressures and climate change, monitoring and managing them are challenging due to their high spatial and temporal dynamics within a fine-grained pattern. New satellite time-series at high temporal and spatial resolutions provide a promising opportunity to map and monitor wetlands. The objective of this study was to develop an operational method for managing valley bottom wetlands based on available free remote sensing data. The Potential, Existing, Efficient Wetlands (PEEW) approach was adapted to remote sensing data to delineate three wetland components: (1) potential wetlands, mapped from a digital terrain model derived from LiDAR data; (2) existing wetlands, delineated from land cover maps derived from Sentinel-1/2 time-series; and (3) efficient wetlands, identified from functional indicators (i.e. annual primary production, vegetation phenology, seasonality of carbon flux) derived from MODIS annual time-series. Soil and vegetation samples were collected in the field to calibrate and validate classification of remote sensing data. The method was applied to a 113 000 ha watershed in northwestern France. Results show that potential wetlands were successfully delineated (82% overall accuracy) and covered 21% of the watershed area, while 44% of existing wetlands had been lost. Small wetlands along headwater channels, which are considered as ordinary, cover 56% of wetland area in the watershed. Efficient wetlands were identified as contiguous pixels with a similar temporal functional trajectory. This method, based on free remote sensing data, provides a new perspective for wetland management. The method can identify sites where restoration measures should be prioritized and enables better understanding and monitoring of the influence of management practices and climate on wetland functions.

Assessment of riparian vegetation characteristics in Mediterranean headwaters regulated by check dams using multivariate statistical techniques

In mountainous torrents of the Mediterranean environment the riparian vegetation is strongly influenced by the presence of engineering control works, since these structures bring heavy modifications in channel geometry, hydraulic regime and bed sediment size. Previous investigations have shown high linear correlations between physical (section shape, profile slope, specific discharge, surface and subsurface size of the channel bed) and vegetation (development, structure and biodiversity) indicators in headwater channels with check dams of Calabrian (Southern Italy) torrents. Based on these findings, this study applies multivariate statistical techniques (Principal Component Analysis and Partial Least Square Regression) to identify in the same study headwaters new synthetic explanatory variables, representative of the different transects (upstream, downstream or intermediate, compared to the check dam location) and develop predictive models of riparian vegetation characteristics.The Principal Component Analysis has provided a simple parameter (the first Principal Component, explaining about 60% of the total variance), which is able to discriminate the physical and vegetal characteristics of the different transects close to check dams, thus reducing the large number of factors influencing the fluvial processes. Moreover, cover, height and transversal variability of riparian vegetation have a very high influence (loadings over 0.73) on this component, while its biodiversity is correlated to the second Principal Component (loadings over 0.63). The Partial Least Square Regression has shown that it is possible to estimate with fair accuracy (minimum r2 of 0.70) the development, structure as well as transversal variability of the riparian vegetation, starting from the physical features of the channel. These models may be important in the planning steps of new check dams, since their effects on the development and growth of vegetation upstream and downstream can be forecasted before their installation, at least for the quantification of the order of magnitude of the check dam impacts on torrent ecology.

Characteristics and abundance of large and small instream wood in a Carpathian mixed-forest headwater basin

The effect of instream wood on stream hydraulics and geomorphic processes depends on wood and channel dimensions. We investigated abundance and characteristics (i.e., wood orientation, proportion of spanned channel width, stability and decay classes) of large wood (LW; defined here as having a length ≥ 1 m and a diameter ≥ 0.1 m) and small wood (SW; including two categories with length ≥ 0.5 m and diameter ≥ 0.1 m or length ≥ 1 m and diameter ≥ 0.05 m) in three steep, confined headwater channels of medium-high mountain ranges of the Western Carpathians (Central Europe). Results show that SW is more frequent than LW, however, active-channel LW load varied between 26 and 305 m3∙ha−1, whereas SW showed much lower active-channel load (8–16 m3∙ha−1). Differences between LW and SW active-channel volumes were considerably smaller in streams under dominant deciduous canopy. In these streams, morphological steps – created exclusively by SW – were more frequent than LW steps. This higher frequency of SW in streams surrounded by a deciduous tree canopy can be explained by the continuous supply of branches rather than entire dead trees. On the other hand, the volume and frequency of LW was related to the proportion of conifers in the valley corridor. We observe very high active-channel load in two channel reaches for which values exceeded most of those observed in similarly small streams across the globe. We also observe an unusually large proportion of instream wood (both LW and SW) oriented parallel to the flow direction, which might suggest a higher mobility of bed material in the flysch-dominated headwater channels of our study site.

Characteristics of large wood in a headwater channel after an extraordinary event: The roles of transport agents and check dams

High-magnitude hydrogeomorphic events in steep streams (e.g., debris flows and debris floods) are capable of transporting considerable quantities of bed material, including large wood, although wood pieces that are longer than the channel width are often perceived as relatively stable elements in narrow headwater channels. We investigated present large wood distribution along 1.7 km of a steep headwater channel (the Klepáčský Stream; Eastern Sudetes; NE Czech Republic) surrounded by managed forests and engineered by twelve check dams (presently at various stages of destruction). The total wood volume was measured at 123.0 m3, including wood deposited on adjacent terraces by a high-magnitude hydrogeomorphic event in 2010 representing a ca. 20–30 year recurrence interval. Approximately a half of the total wood volume (58%) was stored in large jams (≥10 large wood pieces) that originated during that event. We observed reach controls (based on transport agents and valley morphology) and local controls (represented by individual check dams) of LW depositional patterns. The uppermost debris flow-affected reach produced high volumes of mobilised wood, including many pieces that were two-to-three times longer than the local active channel width; significantly shorter and thinner pieces were found within large jams in downstream reaches void of debris-flow processes. We also documented that this high-magnitude event was responsible for the notable out-of-channel storage of large wood, which decreases the volume of wood that might be transported by a future event. In addition, 30.5 m3 of large wood, that is ~one-quarter of the total volume measured along the stream, was deposited in relation to twelve damaged check dams despite their lowered trapping efficiency.

Ecosystem changes following the 2016 Kumamoto earthquakes in Japan: Future perspectives.

Major earthquakes cause widespread environmental and socioeconomic disruptions that persist for decades. Extensive ground disturbances that occurred during the shallow-focus Kumamoto earthquakes will affect future sustainability of ecosystem services west of Aso volcano. Numbers of earthquake-initiated landslides per unit area were higher in grasslands than forests, likely owing to greater root reinforcement of trees, and mostly initiated on ridgelines and/or convex/planar hillslopes. Most landslides traveled short distances and did not initially evolve into debris flows; resultant sediments and wood accumulating in headwater channels can be mobilized into debris flows during future storms. Fissures along ridgelines may promote water ingress and induce future landslides and debris flows that affect residents downstream. Native grasses are at risk because of habitat fragmentation caused by ground disturbances, extensive damage to rural roads, and abandonment of traditional pasture management practices. Sustainable management of affected areas needs to consider future risk of cascading hazards and long-term socioeconomic impacts.

The geomorphic impacts of culverts at paved forest roads: Examples from Carpathian headwater channels, Czech Republic

Road-stream crossings constructed with culverts have the potential to alter geomorphic processes in adjacent stream channels. We examined changes in channel geometry and bed sediments in longitudinal profiles related to culverts at five paved forest road crossings with perennial headwater streams and two gullies initiated by point releases of water through ditches. A dendrogeomorphic approach was used for dating of exposed tree roots and estimation of bank erosion rates in incised cross-sections downstream of road crossings. The studied forest road crossings concentrate surface runoff from the road net, act as stabilizing elements in stream longitudinal profiles and decelerate bedload transport in the case of culvert clogging, which caused notable upstream and downstream changes in fluvial processes. Notable fining of the coarsest fraction in upstream cross-sections was observed only for two culverts with the highest depositional tendencies. Much lower width-depth ratios (<4) and typically higher channel slopes were measured in channel reaches downstream of the culverts, compared with upstream cross-sections. Two road-induced gullies typically had very low width/depth ratios close to 2. At five of a total of seven studied culverts, the highest documented erosion rates occurred immediately downstream of these structures. The mean annual erosion rates reconstructed from cross-sections at the positions of individual tree root samples ranged from 0.2 to 6.5cm·year−1, and the erosion occurred over 2–5year intervals, depending on the studied location. The highest number of observed changes in cross-sectional geometry caused by channel erosion were connected with relatively high-magnitude flow events (with recurrence intervals >10years; specific discharge >1500l·s·km−2), whereas more frequent flows (with recurrence intervals of 2–10years) were not always recorded by exposed tree roots.

The Winter Environmental Continuum of Two Watersheds

This paper examines the winter ecosystemic behavior of two distinct watersheds. In cold-temperate regions, the hydrological signal and environmental parameters can fluctuate dramatically over short periods of time, causing major impacts to aquatic habitats. This paper presents the results of the 2011–2012 winter field campaign in streams and rivers near Quebec City, QC, Canada. The objective was to quantify water quantity and quality parameters and their environmental connectivity from headwater creeks above to the larger rivers below over the entire freeze-up, mid-winter and breakup periods with a view toward exploring the watershed continuum. The paper presents how aquatic pulses (water level, discharge, temperature, conductivity, dissolved oxygen and turbidity, measured at seven sites on an hourly basis along channels of different sizes and orders) evolve through the aquatic environment. Ice conditions and the areal ice coverage were also evaluated (on a daily time step along each instrumented channel). Some findings of the investigation revealed that water temperatures remained well above 0 °C during winter in headwater channels, that dissolved oxygen levels during winter were relatively high, but with severe depletions prior to and during breakup in specific settings, that high conductivity spikes occurred during runoff events, that annual turbidity extremes were measured in the presence of ice and that dynamic ice cover breakup events have the potential to generate direct or indirect mortality among aquatic species and to dislodge the largest rocks in the channel. The authors believe that the environmental impact of a number of winter fluvial processes needs to be further investigated, and the relative significance of the winter period in the annual environmental cycle should be given additional attention.

Slope, grain size, and roughness controls on dry sediment transport and storage on steep hillslopes

AbstractExisting hillslope sediment transport models developed for low‐relief, soil‐mantled landscapes are poorly suited to explain the coupling between steep rocky hillslopes and headwater channels. Here we address this knowledge gap using a series of field and numerical experiments to inform a particle‐based model of sediment transport by dry ravel—a mechanism of granular transport characteristic of steep hillslopes. We find that particle travel distance increases as a function of the ratio of particle diameter to fine‐scale (&lt;1 m) topographic roughness, in agreement with prior laboratory and field experiments. Contrary to models that assume a fixed critical slope, the particle‐based model predicts a broad transition as hillslopes steepen from grain‐scale to hillslope‐scale mean particle travel distances due to the trapping of sediment on slopes more than threefold steeper than the average friction slope. This transition is further broadened by higher macroscale (&gt;1 m) topographic variability associated with rocky landscapes. Applying a 2‐D dry‐ravel‐routing model to lidar‐derived surface topography, we show how spatial patterns of local and nonlocal transport control connectivity between hillslopes and steep headwater channels that generate debris flows through failure of ravel‐filled channels following wildfire. Our results corroborate field observations of a patchy transition from soil‐mantled to bedrock landscapes and suggest that there is a dynamic interplay between sediment storage, roughness, grain sorting, and transport even on hillslopes that well exceed the angle of repose.

Coarse bedload routing and dispersion through tributary confluences

Abstract. Sediment routing fundamentally influences channel morphology and the propagation of disturbances such as debris flows. The transport and storage of bedload particles across headwater channel confluences, which may be significant nodes of the channel network in terms of sediment routing, morphology, and habitat, are poorly understood, however. We investigated patterns and processes of sediment routing through headwater confluences by comparing them to published results from lower-gradient confluences and by comparing the dispersive behavior of coarse bedload particles between headwater confluence and non-confluence reaches. We addressed these questions with a field tracer experiment using passive-integrated transponder and radio-frequency identification technology in the East Fork Bitterroot River basin, Montana, USA. Within the confluence zone, tracers tended to be deposited towards scour-hole and channel margins, suggesting narrow, efficient transport corridors that mirror those observed in prior studies, many of which are from finer-grained systems. Coarse particles in some confluence reaches experienced reduced depositional probabilities within the confluence relative to upstream and downstream of the confluence. Analysis of particle transport data suggests that variation in the spatial distribution of coarse-sediment particles may be enhanced by passing through confluences, though further study is needed to evaluate confluence effects on dispersive regimes and sediment routing on broader spatial and temporal scales.

Headwater sediment dynamics in a debris flow catchment constrained by high-resolution topographic surveys

Abstract. Debris flows have been recognized to be linked to the amounts of material temporarily stored in torrent channels. Hence, sediment supply and storage changes from low-order channels of the Manival catchment, a small tributary valley with an active torrent system located exclusively in sedimentary rocks of the Chartreuse Massif (French Alps), were surveyed periodically for 16 months using terrestrial laser scanning (TLS) to study the coupling between sediment dynamics and torrent responses in terms of debris flow events, which occurred twice during the monitoring period. Sediment transfer in the main torrent was monitored with cross-section surveys. Sediment budgets were generated seasonally using sequential TLS data differencing and morphological extrapolations. Debris production depends strongly on rockfall occurring during the winter–early spring season, following a power law distribution for volumes of rockfall events above 0.1 m3, while hillslope sediment reworking dominates debris recharge in spring and autumn, which shows effective hillslope–channel coupling. The occurrence of both debris flow events that occurred during the monitoring was linked to recharge from previous debris pulses coming from the hillside and from bedload transfer. Headwater debris sources display an ambiguous behaviour in sediment transfer: low geomorphic activity occurred in the production zone, despite rainstorms inducing debris flows in the torrent; still, a general reactivation of sediment transport in headwater channels was observed in autumn without new debris supply, suggesting that the stored debris was not exhausted. The seasonal cycle of sediment yield seems to depend not only on debris supply and runoff (flow capacity) but also on geomorphic conditions that destabilize remnant debris stocks. This study shows that monitoring the changes within a torrent's in-channel storage and its debris supply can improve knowledge on recharge thresholds leading to debris flow.

Landslide-driven erosion and slope–channel coupling in steep, forested terrain, Ruahine Ranges, New Zealand, 1946–2011

Landslides are an important means of conveying sediment from slopes to channels in steepland environments, but landsliding is a discontinuous process. This means that any single assessment in time of their contribution to the sediment cascade is of limited value. To better understand landscape dynamics and the contribution of landslides to slope–channel coupling over time, this paper quantifies connectivity over a time span of 65years in small, steep headwater catchments in the southern Ruahine Range, New Zealand. Temporal variability in landsliding and slope–channel coupling was assessed using six sets of aerial photography flown between 1946 and 2011, from which over 6900 landslides were mapped in ArcGIS, of which up to 78% connected with the stream network. Estimates of the volume of material delivered by landslide erosion to headwater channels were based on ground survey measurements of selected landslide scars and suggest that between 1946 and 2011 over 5millionm3 of sediment was delivered from slopes to channels in the 221km2 study area. Forest cover is not sufficient to prevent this erosion. These catchment systems are particularly vulnerable to high magnitude storm events, which significantly elevate landslide intensity and enhance sediment delivery, as occurred in the mid-1970s. The legacy of these events remains in these headwater channels, with ongoing consequences for stream and hazard management in and adjacent to the ranges.

Connectivity of the coarsest fraction in headwater channels: imprints of fluvial processes and debris‐flow activity

Some steep headwater streams of the mid‐mountains landscape of the flysch belt of the estern arpathians are affected by debris flows. Connectivity of the largest boulder and cobble fractions has been evaluated in steep streams found in the transitional zones between the hillslopes and alluvial cones of similar lithology and watershed morphometry. Two longitudinal profiles affected by past debris‐flow activity and two longitudinal profiles void of such processes were selected. In the first case, active channels were characterised by the presence of the coarsest fraction, while a downstream trend of sediment coarsening or fining lacked any influence of contemporary fluvial processes and sediment supply. In addition, debris‐flow‐affected streams showed greater differences between the middle axis of the coarsest particle and the particle‐size index including the lengths of all three axes, which is most likely the result of limited active fluvial transport. All the studied streams demonstrated general downstream fining of the largest particles, which resulted from local flysch lithology containing more resistant sandstones in the upper parts of the watershed. The trend of sediment fining was more rapid in the longitudinal profiles unaffected by past debris flows. These types of streams also showed adjustment of the coarsest bed fraction to contemporary processes in channels by sediment coarsening in incised reaches and sediment fining in depositional reaches as well as by downstream response to some lateral sediment inputs. The index of the unit stream power showed no correlation with the considered coarsest fraction in both fluvial‐dominated channels and debris‐flow‐affected channels.

Do the coarsest bed fractions and stream power record contemporary trends in steep headwater channels?

Three stream channels that were devoid of evidence of past debris flows and one headwater channel that contained debris flow deposits in the flysch western Carpathians, Czech Republic were selected to test relationships between in-channel processes, bed sediments, and unit stream power calculated for bankfull and Q20 flows. Contemporary depositional or erosional trends in the examined headwaters were linked with bed sediments that were represented by the coarsest cobble and boulder fraction with a mean calculated from the five largest particles. The downstream trends of the unit stream power were derived for a bankfull discharge and a well-documented 20-year flood event. In addition, the flow competences during the discharges were calculated using indirect bedload transport measurements. Downstream fining of the cobble and boulder fraction was observed in all of the studied headwaters, and unique downstream variations of the unit stream power were calculated for the longitudinal profiles. The single-thread streams that were devoid of evidence of debris flow events exhibited direct relations between the coarsest sediment size and the unit stream power, especially as calculated for the 20-year flood event and for erosional/depositional trends of the channel. The downstream coarsening of the bed material that was accompanied by an increase in the unit stream power was usually observed in the case of deeply incised (>0.5m above the assumed bankfull depth) channel reaches. The calculated competence of the 20-year flow was up to twofold higher than that required to entrain the largest bed particle diameters in those channel reaches, and even the bankfull flow was potentially capable of transporting the coarsest bed particles in certain of the reaches. On the other hand, some depositional channel reaches evidently led to the disconnectivity of transport of the coarsest bed material even in the case of the 20-year flood event. The longitudinal profile of the channel that had evidence of past debris flows did not clearly show a direct relationship between boulder size and calculated unit stream power and contemporary channel trends. This finding reflected the presence of significantly coarser bed material, which points to a higher stability of the debris flow-influenced channel during ordinary floods when compared to fluvially dominated headwaters that are devoid of debris flows.

Reach-scale geomorphic differences between headwater streams draining mountaintop mined and unmined catchments

Mountaintop surface mining (MTM) is a controversial coal extraction method commonly practiced in the central and southern Appalachian Mountains, USA, that drastically reengineers previously steep, forested landscapes and alters sediment and water delivery processes to and along headwater channels draining mined areas. Although sediment delivery and hydrologic response from MTM operations remain highly variable and poorly resolved, the inherent close coupling between hillslopes and headwater channels is expected to result in geomorphic differences in stream channels draining MTM landscapes relative to unmined landscapes. Dedicated geomorphic studies are severely lacking in comparison to extensive research on water quality impacts of MTM. This study reports moderate geomorphic differences between headwater (catchment area <~6km2) stream channels draining MTM and unmined catchments in tributaries of the Mud River in southern West Virginia. Univariate and multivariate analyses indicate that MTM streams are characterized by deeper maximum channel depths, smaller width-to-depth ratios, increased bedrock exposure along the streambed, and increased frequency of very fine silt and sand deposition relative to channels draining unmined catchments. Geomorphic differences are most pronounced for streams draining the smallest catchment areas (<3.5km2). Collectively, geomorphic differences provide evidence for relatively rapid channel adjustment of accelerated bedrock incision attributed to potential increased hydraulic driving forces and altered sediment regimes in MTM channels, notably sustained delivery of very fine sediment and potentially reduced coarse sediment delivery. More rapid delivery and transfer of water in addition to excess delivery of very fine sediments to and through headwater channels will have consequences to flooding and water quality in the short term and landscape evolution processes over longer time scales. Given the extent of MTM operations in this region, additional studies are urgently needed to more rigorously evaluate geomorphic response to mining at the reach and at the network scales.

Estimation of bedload transport in headwater streams using a numerical model (Moravskoslezké Beskydy Mts, Czech Republic)

The TOMSED numerical model, originally developed for Alpine torrents, was successfully applied on the May, 2010 flood event in headwater streams of the Czech part of the Western Flysch Carpathians. The Manning equation was used together with application of a reduced energy gradient and potential erosion limits. Contributions of form resistance and grain resistance to Manning’s roughness parameter were considered separately. The absence of bedload discharge measurement made it difficult to validate the model for local headwater channels, although channel reaches with recent erosional and depositional trends were recognized fairly well by the simulations. Bedload transport was calculated in the range from 480–1240 m3 for the richly-supplied headwater stream and 380–860 m3 for the stream with limited sediment-supply conditions. It turns out that the channel reaches with higher sediment supply have less developed bedforms (e.g. steps) and thus lower flow resistance resulting in a higher intensity of bedload transport.

Coarse Bed Sediments in a Headwater Channel as Indicators of Fluvial Processes and Slope-Channel Coupling: A Case Study from the Carpathian Mountains (Czech Republic)

Abstract The character of riverbed sediments usually reflects fluvial processes and the dynamics of sediment transport in fluvial systems. The approach in this study was based on the measurement of the largest boulders located within a bankfull channel, and on the observation of changes in their size in the longitudinal profile of a headwater stream in the Moravian-Silesian Beskids Mountains. The resulting trends in a particle-size index reflect the character of sediment delivery into channel segments and the recent channel-forming processes. The largest boulders were observed in channel sections with a strong interaction of slope and fluvial processes, and a slight coarsening of sediments was recorded in the incised downstream sections of the longitudinal stream profile. In contrast, the refining of bed sediments was typical of the transitional zone between slope-channel coupled reaches and an alluvial cone characterised by a tendency to material aggradation.

Cryologic continuum of a steep watershed

Ice processes taking place in steep channels are sensitive to the thermal and hydrological regimes of upstream reaches and tributaries as well as to the local channel morphology. This work presents freezeup, mid-winter, and breakup data from four channels of increasing order located in a cold temperate watershed during the winter 2010–2011. From headwater channels to the main drainage system, water temperature, ice coverage, and ice processes are reported and related to weather conditions and to channel characteristics. Headwater channels only formed ephemeral ice features, and their water temperature reached as much as 4 °C in mid-winter. On the other hand, larger channels formed impressively large ice dams, some of them reaching 2 m in height. The development of a suspended ice cover partially insulated the channels; as a result, water temperatures remained above 0 °C even for air temperatures well below freezing. This work presents steep channels ice processes that have not been described in previous publications. The concept of a watershed cryologic continuum (WCC) is developed from the data collected at each channel order. This concept emphasizes the feedback loops that exist between morphology, hydrology, heat, and ice processes in a given watershed and can lead to a better understanding of ice processes taking place at any channel location within that watershed. The WCC can also contribute in improving our understanding of the impacts of climate change on the cryologic and thermal regimes of steep channels. Copyright © 2012 John Wiley & Sons, Ltd.

A Process-Based Hierarchical Framework for Monitoring Glaciated Alpine Headwaters

Recent studies have demonstrated the geomorphic complexity and wide range of hydrologic regimes found in alpine headwater channels that provide complex habitats for aquatic taxa. These geohydrologic elements are fundamental to better understand patterns in species assemblages and indicator taxa and are necessary to aquatic monitoring protocols that aim to track changes in physical conditions. Complex physical variables shape many biological and ecological traits, including life history strategies, but these mechanisms can only be understood if critical physical variables are adequately represented within the sampling framework. To better align sampling design protocols with current geohydrologic knowledge, we present a conceptual framework that incorporates regional-scale conditions, basin-scale longitudinal profiles, valley-scale glacial macroform structure, valley segment-scale (i.e., colluvial, alluvial, and bedrock), and reach-scale channel types. At the valley segment- and reach-scales, these hierarchical levels are associated with differences in streamflow and sediment regime, water source contribution and water temperature. Examples of linked physical-ecological hypotheses placed in a landscape context and a case study using the proposed framework are presented to demonstrate the usefulness of this approach for monitoring complex temporal and spatial patterns and processes in glaciated basins. This approach is meant to aid in comparisons between mountain regions on a global scale and to improve management of potentially endangered alpine species affected by climate change and other stressors.

Effect of forest harvesting on hydrogeomorphic processes in steep terrain of central Japan

Forest harvesting activities affect various hydrogeomorphic processes in forest terrain, including increases in occurrence of mass movements (i.e., landslides and debris flows), and changes in sediment transport rate in channels. Thus, the influence of harvesting on these processes needs to be assessed to protect aquatic ecosystems. In steep terrain with frequent mass movement, relationships between rainfall magnitude and frequency of mass movements associated with forest harvesting are needed to develop improved strategies for disaster prevention. We examined the effects of forest harvesting on the frequency of mass movements in the steep Sanko catchment (central Japan) using aerial photographs for six different years covering 1976 to 2003. We also investigated the spatial distribution of sediment stored in the system as well as sediment transport rate in selected subcatchments by field surveys. Landslides occur during moderate rainfall events (return period of daily rainfall<10years) in younger forests (<25years after harvesting), whereas the threshold magnitude of rainfall for landslides is higher in older forests (>25years after harvesting). The occurrence of mass movements substantially altered the volume of sediment storage in the channel that links sediment supply from hillslopes with sediment yield downstream. However, the volume and spatial distribution of sediment deposits in headwater channels were not clearly related to forest age because of the redistribution and evacuation of sediment by debris flows. Bedload transport rate was highly affected by sediment storage volume, whereas suspended sediment concentration was affected by the area of exposed sediment along channels that was carved by debris flows. Thus, the effect of harvesting on hydrogeomorphic processes cannot be estimated simply by the elapsed time after forest harvesting; rainfall magnitude and the history of mass movements need to be considered to better understand these linkages.

Sediment budget monitoring of debris-flow and bedload transport in the Manival Torrent, SE France

Abstract. Steep mountain catchments typically experience large sediment pulses from hillslopes which are stored in headwater channels and remobilized by debris-flows or bedload transport. Event-based sediment budget monitoring in the active Manival debris-flow torrent in the French Alps during a two-year period gave insights into the catchment-scale sediment routing during moderate rainfall intensities which occur several times each year. The monitoring was based on intensive topographic resurveys of low- and high-order channels using different techniques (cross-section surveys with total station and high-resolution channel surveys with terrestrial and airborne laser scanning). Data on sediment output volumes from the main channel were obtained by a sediment trap. Two debris-flows were observed, as well as several bedload transport flow events. Sediment budget analysis of the two debris-flows revealed that most of the debris-flow volumes were supplied by channel scouring (more than 92%). Bedload transport during autumn contributed to the sediment recharge of high-order channels by the deposition of large gravel wedges. This process is recognized as being fundamental for debris-flow occurrence during the subsequent spring and summer. A time shift of scour-and-fill sequences was observed between low- and high-order channels, revealing the discontinuous sediment transfer in the catchment during common flow events. A conceptual model of sediment routing for different event magnitude is proposed.