Effects of Woody Debris on Channel Morphology and Sediment Storage in Headwater Streams in the Great Smoky Mountains,Tennessee-North Carolina

Coarse woody debris (CWD) is an important component of headwater streams, however, few studies have investigated the geomorphic effects of CWD in the southern Appalachians. In the Great Smoky Mountains, debris slides supply large volumes of CWD and sediment to low-order streams. This study investigates the effect of CWD on bankfull channel dimensions and in-channel sediment storage along second-order streams. Comparisons are made between streams that have experienced recent debris slides and those that have not. CWD channel obstructions are larger but less frequent along debris-slide-affected streams. Dendrochronological evidence indicates that CWD can remain in channels for over 100 yr. Relatively short residence times of CWD along debris-slide-affected streams suggest that logs are frequently flushed through these streams. CWD causes channel widening along all study streams, but the volume of sediment stored in the channel behind CWD obstructions is up to four times greater than the volume of sediment represented by bank erosion associated with CWD. Two large log jams formed by debris slides at tributary junctions stored approximately 4000 m³ of sediment. Sediment stored by CWD was finer than mean bed particle size, and thus represents a significant sediment source when CWD obstructions are breached.     

Debris flows in Glacier National Park, Montana: geomorphology and hazards

Debris flows are one of the many active slope-forming processes within Glacier National Park, Montana. Most debris flow landforms exhibit classic morphology with a distinct failure scarp, incised channel, channel levees, and toe deposits that often develop a lobate form. The Precambrian metasediments that dominate Glacier National Park's geology weather into angular clasts that range in size from platy gravels to boulders. Classic debris flows occur in areas where the topographic expression provides a debris source from cliff faces and an accumulation of regolith, often in the form of talus slopes. Many of these debris flows have long runout zones and can travel many hundreds of meters. Often they cross hiking trails or roads, including the main east–west highway, Going-to-the-Sun Road. Debris flows impacting the road have resulted in several near fatalities, and hikers have been forced to cross active debris flows to reach safe ground. The magnitude of debris flows varies between high magnitude channel incising events and low magnitude channel filling and/or reworking events. The frequency of debris flow events is irregular and appears to be controlled by the hydrology of triggering storms and antecedent moisture conditions, not by the debris supply. As a result, debris flow magnitude is not a function of frequency, but is more closely related to the characteristics of antecedent conditions and individual storms.     

Contrasting rainfall generated debris flows from adjacent watersheds at Forest Falls, southern California, USA

Debris flows are widespread and common in many steeply sloping areas of southern California. The San Bernardino Mountains community of Forest Falls is probably subject to the most frequently documented debris flows in southern California. Debris flows at Forest Falls are generated during short-duration high-intensity rains that mobilize surface material. Except for debris flows on two consecutive days in November 1965, all the documented historic debris flows have occurred during high-intensity summer rainfall, locally referred to as ‘monsoon’ or ‘cloudburst’ rains. Velocities of the moving debris range from about 5 km/h to about 90 km/h. Velocity of a moving flow appears to be essentially a function of the water content of the flow. Low velocity debris flows are characterized by steep snouts that, when stopped, have only small amounts of water draining from the flow. In marked contrast are high-velocity debris flows whose deposits more resemble fluvial deposits. In the Forest Falls area two adjacent drainage basins, Snow Creek and Rattlesnake Creek, have considerably different histories of debris flows. Snow Creek basin, with an area about three times as large as Rattlesnake Creek basin, has a well developed debris flow channel with broad levees. Most of the debris flows in Snow Creek have greater water content and attain higher velocities than those of Rattlesnake Creek. Most debris flows are in relative equilibrium with the geometry of the channel morphology. Exceptionally high-velocity flows, however, overshoot the channel walls at particularly tight channel curves. After overshooting the channel, the flows degrade the adjacent levee surface and remove trees and structures in the immediate path, before spreading out with decreasing velocity. As the velocity decreases the clasts in the debris flows pulverize the up-slope side of the trees and often imbed clasts in them. Debris flows in Rattlesnake Creek are relatively slow moving and commonly stop in the channel. After the channel is blocked, subsequent debris flows cut a new channel upstream from the blockage that results in the deposition of new debris-flow deposits on the lower part of the fan. Shifting the location of debris flows on the Rattlesnake Creek fan tends to prevent trees from becoming mature. Dense growths of conifer seedlings sprout in the spring on the late summer debris flow deposits. This repeated process results in stands of even-aged trees whose age records the age of the debris flows.     

Initiation conditions for debris flows generated by runoff at Chalk Cliffs, central Colorado

We have monitored initiation conditions for six debris flows between May 2004 and July 2006 in a 0.3 km² drainage basin at Chalk Cliffs; a band of hydrothermally-altered quartz monzonite in central Colorado. Debris flows were initiated by water runoff from colluvium and bedrock that entrained sediment from rills and channels with slopes ranging from about 14° to 45°. The availability of channel material is essentially unlimited because of thick channel fill and refilling following debris flows by rock fall and dry ravel processes. Rainfall exceeding I = 6.61(D)^− 0.77, where I is rainfall intensity (mm/h), and D is duration (h), was required for the initiation of debris flows in the drainage basin. The approximate minimum runoff discharge from the surface of bedrock required to initiate debris flows in the channels was 0.15 m³/s. Colluvium in the basin was unsaturated immediately prior to (antecedent) and during debris flows. Antecedent, volumetric moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 4–9%, and 4–7%, respectively. During debris flows, peak moisture levels in colluvium at depths of 1 cm and 29 cm ranged from 10–20%, and 4–12%, respectively. Channel sediment at a depth of 45 cm was unsaturated before and during debris flows; antecedent moisture ranged from 20–22%, and peak moisture ranged from 24–38%. Although we have no measurements from shallow rill or channel sediment, we infer that it was unsaturated before debris flows, and saturated by surface-water runoff during debris flows.Our results allow us to make the following general statements with regard to debris flows generated by runoff in semi-arid to arid mountainous regions: 1) high antecedent moisture levels in hillslope and channel sediment are not required for the initiation of debris flows by runoff, 2) locations of entrainment of sediment by successive runoff events can vary within a basin as a function of variations in the thickness of existing channel fill and the rate of replenishment of channel fill by rock fall and dry ravel processes following debris flows, and 3) rainfall and simulated surface-water discharge thresholds can be useful in understanding and predicting debris flows generated by runoff and sediment entrainment.     

Sources of debris flow material in burned areas

The vulnerability of recently burned areas to debris flows has been well established. Likewise, it has been shown that many, if not most, post-fire debris flows are initiated by runoff and erosion and grow in size through erosion and scour by the moving debris flow, as opposed to landslide-initiated flows with little growth. To better understand the development and character of these flows, a study has been completed encompassing 46 debris flows in California, Utah, and Colorado, in nine different recently burned areas. For each debris flow, progressive debris production was measured at intervals along the length of the channel, and from these measurements graphs were developed showing cumulative volume of debris as a function of channel length. All 46 debris flows showed significant bulking by scour and erosion, with average yield rates for each channel ranging from 0.3 to 9.9 m³ of debris produced for every meter of channel length, with an overall average value of 2.5 m³/m. Significant increases in yield rate partway down the channel were identified in 87% of the channels, with an average of a three-fold increase in yield rate. Yield rates for short reaches of channels (up to several hundred meters) ranged as high as 22.3 m³/m. Debris was contributed from side channels into the main channels for 54% of the flows, with an average of 23% of the total debris coming from those side channels. Rill erosion was identified for 30% of the flows, with rills contributing between 0.1 and 10.5% of the total debris, with an average of 3%. Debris was deposited as levees in 87% of the flows, with most of the deposition occurring in the lower part of the basin. A median value of 10% of the total debris flow was deposited as levees for these cases, with a range from near zero to nearly 100%. These results show that channel erosion and scour are the dominant sources of debris in burned areas, with yield rates increasing significantly partway down the channel. Side channels are much more important sources of debris than rills. Levees are very common, but the size and effect on the amount of debris that reaches a canyon mouth is highly variable.     

The morphometric and stratigraphic framework for estimates of debris flow incidence in the North Cascades foothills, Washington State, USA

Active debris flow fans in the North Cascade Foothills of Washington State constitute a natural hazard of importance to land managers, private property owners and personal security. In the absence of measurements of the sediment fluxes involved in debris flow events, a morphological-evolutionary systems approach, emphasizing stratigraphy, dating, fan morphology and debris flow basin morphometry, was used. Using the stratigraphic framework and 47 radiocarbon dates, frequency of occurrence and relative magnitudes of debris flow events have been estimated for three spatial scales of debris flow systems: the within-fan site scale (84 observations); the fan meso-scale (six observations) and the lumped fan, regional or macro-scale (one fan average and adjacent lake sediments). In order to characterize the morphometric framework, plots of basin area v. fan area, basin area v. fan gradient and the Melton ruggedness number v. fan gradient for the 12 debris flow basins were compared with those documented for semi-arid and paraglacial fans. Basin area to fan area ratios were generally consistent with the estimated level of debris flow activity during the Holocene as reported below. Terrain analysis of three of the most active debris flow basins revealed the variety of modes of slope failure and sediment production in the region.Micro-scale debris flow event systems indicated a range of recurrence intervals for large debris flows from 106−3645 years. The spatial variation of these rates across the fans was generally consistent with previously mapped hazard zones. At the fan meso-scale, the range of recurrence intervals for large debris flows was 273−1566 years and at the regional scale, the estimated recurrence interval of large debris flows was 874 years (with undetermined error bands) during the past 7290 years. Dated lake sediments from the adjacent Lake Whatcom gave recurrence intervals for large sediment producing events ranging from 481−557 years over the past 3900 years and clearly discernible sedimentation events in the lacustrine sediments had a recurrence interval of 67−78 years over that same period.     

A morphometric analysis of gullies scoured by post-fire progressively bulked debris flows in southwest Montana, USA

In the fall of 2001, an intense thunderstorm in southwest Montana triggered many debris flows in the burned area of Sleeping Child Creek. In most instances, the debris flows cut deep gullies into previously unchannelized colluvial hollows and deposited large volumes of sediment onto the valley floor. The presence of rill networks above the gullies as well as the absence of landslide features indicate that the gullies were scoured by progressively bulked debris flows, a process in which dilute surface runoff becomes increasingly more laden with sediment until it transforms into a debris flow. In this contribution, we present a morphometric analysis of six of the gullies to better understand this relatively understudied process. We find that the locations of the rill heads and gully heads conform to slope-area thresholds that are characteristic of erosion by overland flow. Our data also suggest that the volumes of the debris flows increase exponentially with normalized drainage area, thus lending support to an assumption used in a recently proposed debris flow incision law. Finally, the debris flow fans have been relatively unaltered since deposition, suggesting that the valley may be currently aggrading while the hillslopes are being denuded.     

中国西南山区沟谷暴雨泥石流危险度判定的基本原理和方法

本研究提出了通用于沟谷暴雨泥石流危险度判定的三项基本原理:主次因子原理、因子权重原理和定量赋值原理。首次提出了泥石流危险度的多因子综合定量判定模式和计算公式,为我国西南(西北)山区沟谷暴雨泥石流灾害评估提供了较为先进的实用方法。在云南部分地区的判定检验和应用实践表明,本法具有60%以上的可靠度。可应用于我国西南(西北)山区一切有可能发生暴雨泥石流的自然沟谷和已确认的暴雨泥石流沟谷。     

Empirical models to predict the volumes of debris flows generated by recently burned basins in the western U.S.

Recently burned basins frequently produce debris flows in response to moderate-to-severe rainfall. Post-fire hazard assessments of debris flows are most useful when they predict the volume of material that may flow out of a burned basin. This study develops a set of empirically-based models that predict potential volumes of wildfire-related debris flows in different regions and geologic settings.The models were developed using data from 53 recently burned basins in Colorado, Utah and California. The volumes of debris flows in these basins were determined by either measuring the volume of material eroded from the channels, or by estimating the amount of material removed from debris retention basins. For each basin, independent variables thought to affect the volume of the debris flow were determined. These variables include measures of basin morphology, basin areas burned at different severities, soil material properties, rock type, and rainfall amounts and intensities for storms triggering debris flows. Using these data, multiple regression analyses were used to create separate predictive models for volumes of debris flows generated by burned basins in six separate regions or settings, including the western U.S., southern California, the Rocky Mountain region, and basins underlain by sedimentary, metamorphic and granitic rocks.An evaluation of these models indicated that the best model (the Western U.S. model) explains 83% of the variability in the volumes of the debris flows, and includes variables that describe the basin area with slopes greater than or equal to 30%, the basin area burned at moderate and high severity, and total storm rainfall. This model was independently validated by comparing volumes of debris flows reported in the literature, to volumes estimated using the model. Eighty-seven percent of the reported volumes were within two residual standard errors of the volumes predicted using the model. This model is an improvement over previous models in that it includes a measure of burn severity and an estimate of modeling errors. The application of this model, in conjunction with models for the probability of debris flows, will enable more complete and rapid assessments of debris flow hazards following wildfire.     

The effects of vegetative ash on infiltration capacity, sediment transport, and the generation of progressively bulked debris flows

Through the alteration of the physical characteristics of a landscape, such as the destruction of vegetation and the formation of a hydrophobic layer, a fire can dramatically amplify erosion rates. On the basis of field observations, it has been proposed that the deposition of a layer of ash on the ground surface can enhance the erosion of mountainous terrain by surface runoff and might even be a necessary condition for the generation of progressively bulked debris flows. In this study, a flume was constructed to investigate the role of ash in increasing both the volume and the transport capacity of runoff. The experiments demonstrated that the presence of ash on the soil surface reduces the ability of flowing water to infiltrate; this effect is even greater when the ash has been pre-wetted. In addition, the ability of ash slurries to infiltrate decreases with increasing ash concentration. The results also indicate that the transport capacity of runoff is enhanced by the incorporation of ash into the flow because of the increased fluid density. However, the addition of ash reduces the boundary Reynolds number such that, at high ash concentrations and with fine-grained sediment, sediment transport declines as the flow becomes hydraulically smooth. The experimental results were also used to evaluate the ability of steep flow fronts, a common characteristic of debris flows and flash floods, to increase sediment transport rates. Finally, it is proposed that ash slurries may evolve into progressively bulked debris flows through a positive feedback between fluid density, transport capacity, and erosivity.     

Sediment budget analysis of slope–channel coupling and in-channel sediment storage in an upland catchment, southeastern Australia

Slope–channel coupling and in-channel sediment storage can be important factors that influence sediment delivery through catchments. Sediment budgets offer an appropriate means to assess the role of these factors by quantifying the various components in the catchment sediment transfer system. In this study a fine (< 63 µm) sediment budget was developed for a 1.64-km² gullied upland catchment in southeastern Australia. A process-based approach was adopted that involved detailed monitoring of hillslope and bank erosion, channel change, and suspended sediment output in conjunction with USLE-based hillslope erosion estimation and sediment source tracing using ¹³⁷Cs and ²¹⁰Pb_ex. The sediment budget developed from these datasets indicated channel banks accounted for an estimated 80% of total sediment inputs. Valley floor and in-channel sediment storage accounted for 53% of inputs, with the remaining 47% being discharged from the catchment outlet. Estimated hillslope sediment input to channels was low (5.7 t) for the study period compared to channel bank input (41.6 t). However an estimated 56% of eroded hillslope sediment reached channels, suggesting a greater level of coupling between the two subsystems than was apparent from comparison of sediment source inputs. Evidently the interpretation of variability in catchment sediment yield is largely dependent on the dynamics of sediment supply and storage in channels in response to patterns of rainfall and discharge. This was reflected in the sediment delivery ratios (SDR) for individual measurement intervals, which ranged from 1 to 153%. Bank sediment supply during low rainfall periods was reduced but ongoing from subaerial processes delivering sediment to channels, resulting in net accumulation on the channel bed with insufficient flow to transport this material to the catchment outlet. Following the higher flow period in spring of the first year of monitoring, the sediment supplied to channels during this interval was removed as well as an estimated 72% of the sediment accumulated on the channel bed since the start of the study period. Given the seasonal and drought-dependent variability in storage and delivery, the period of monitoring may have an important influence on the overall SDR. On the basis of these findings, this study highlights the potential importance of sediment dynamics in channels for determining contemporary sediment yields from small gullied upland catchments in southeastern Australia.     

Effective mitigation of debris flows at Lemon Dam, La Plata County, Colorado

To reduce the hazards from debris flows in drainage basins burned by wildfire, erosion control measures such as construction of check dams, installation of log erosion barriers (LEBs), and spreading of straw mulch and seed are common practice. After the 2002 Missionary Ridge Fire in southwest Colorado, these measures were implemented at Knight Canyon above Lemon Dam to protect the intake structures of the dam from being filled with sediment. Hillslope erosion protection measures included LEBs at concentrations of 220–620/ha (200–600% of typical densities), straw mulch was hand spread at concentrations up to 5.6 metric tons/hectare (125% of typical densities), and seeds were hand spread at 67–84 kg/ha (150% of typical values). The mulch was carefully crimped into the soil to keep it in place. In addition, 13 check dams and 3 debris racks were installed in the main drainage channel of the basin.The technical literature shows that each mitigation method working alone, or improperly constructed or applied, was inconsistent in its ability to reduce erosion and sedimentation. At Lemon Dam, however, these methods were effective in virtually eliminating sedimentation into the reservoir, which can be attributed to a number of factors: the density of application of each mitigation method, the enhancement of methods working in concert, the quality of installation, and rehabilitation of mitigation features to extend their useful life. The check dams effectively trapped the sediment mobilized during rainstorms, and only a few cubic meters of debris traveled downchannel, where it was intercepted by debris racks.Using a debris volume-prediction model developed for use in burned basins in the Western U.S., recorded rainfall events following the Missionary Ridge Fire should have produced a debris flow of approximately 10,000 m³ at Knight Canyon. The mitigation measures, therefore, reduced the debris volume by several orders of magnitude. For comparison, rainstorm-induced debris flows occurred in two adjacent canyons at volumes within the range predicted by the model.     

VARIABILITY OF IN-CHANNEL SEDIMENT STORAGE WITHIN A SMALL SEMIARID DRAINAGE BASIN

This study determines the spatial and temporal variability of in-channel storage within a small semiarid drainage basin in equatorial East Africa, and establishes a tentative sediment budget for coarse (>200 μm) in-channel sediments. Detailed measurements of in-channel sediment storage (mass) within third and fourth-order ephemeral channels were obtained using channel-pit excavations and probing with metal rods. Eighty-seven monumented cross-sections were established in February 1986 and resurveyed in December 1986, following the last runoff event of the year. These provided data on change in sediment storage on a 30-m channel reach basis. In addition, measurements of bankfull channel width, mean depth, cross-sectional area, wetted perimeter, hydraulic radius, channel slope and distance from the basin outlet were measured at each cross section. Total in-channel sediment storage was approximately 8640 t with 83% of this total stored within the Main (fourth-order) Channel. Stepwise multiple regression of In-transformed data indicated that bankfull channel width and distance from the outlet (which is strongly related with slope) were significantly related to in-channel storage. The variation in the ratio of stream power:critical power along the Main Channel may explain the distribution of in-channel sediments. Net aggradation of 50 to 60 t during 1986 was minor in relation to the total in-channel storage reservoir, but indicates that a static equilibrium condition cannot be assumed. Bedload output during 1986 was approximately 125 t, and the computed input of coarse sediments to the major channels within the basin was approximately 185 t. The sediment delivery ratio for the coarse material was approximately 68%, which indicates a relatively efficient transport system. [Key words: geomorphology, sediment budget, in-channel sediment storage, semiarid, drainage basin.]     

Sediment transportation and deposition by rain-triggered lahars at Merapi Volcano,Central Java,Indonesia

A lahar is a general term for a rapidly flowing mixture of rock debris and water (other than normal streamflow) from a volcano and refers to the moving flow. Located in the populated area of Central Java, the stratovolcano Merapi (2965 m) is prone to lahar generation, due to three main factors: (1) millions of cubic meters of pyroclastic deposits are the product of frequent pyroclastic flows, which have occurred on 2- to 4-year intervals; (2) rainfall intensity is high (often 40 mm in 2 h on average) during the rainy season from November to April; and (3) drainage pattern is very dense. Following the 22 November 1994 eruption of Merapi, 31 rain-triggered lahar events were recorded in the Boyong River between December 1994 and May 1996.On Merapi's slopes, instantaneous sediment concentration at any given time of the lahars varies widely over time and space. Lahars are transient sediment-water flows whose properties are unsteady, so that the sediment load fluctuates during the flow. The boundary between the flow types (debris flow, with sediment concentration >60% volume, or hyperconcentrated flow, with sediment concentration ranges from 20% to 60% volume) may fluctuate within the flow itself. Grain-size distribution, physical composition of sediments, shear stress, yield stress, and water temperature play each a role on this boundary. Natural self-damming and rapid breakout are partly responsible for the sediment variations of the flows.Debris-flow phases at Merapi typically last a few minutes to 10 min, and are often restricted to the lahar front. Debris-flow surges are sometimes preceded and always followed by longer hyperconcentrated flow phases. As a result, mean sediment concentration of the lahars is low, commonly from 20% to 50% volume. Besides, transient normal streamflow phases (sediment concentration <20% volume) can occur between two debris-flow surges.Low sediment load and frequent transient flows in the Merapi channels may result from at least three factors: (1) several breaks-in-slope along the channel increase the deposition rate of sediment, and hinder the bulking capacity of the lahars; (2) source material is mainly coarse debris of “Merapi-type” block-and-ash flows. Consequently, the remobilization of coarse debris is more difficult and the clast deposition is accelerated; (3) variations of rainfall intensity over time and space, common in tropical monsoon rainfall, also influence the sediment load variations of the lahars.Sedimentologic analyses of the lahar deposits in the Boyong River at Merapi encompass clast-supported and matrix-supported debris-flow deposits, hyperconcentrated flow deposits, and streamflow deposits. The stratigraphic succession of massive and stratified beds observed immediately after any given lahar event in the Boyong River indicates that the sediment concentration varies widely over time and space during a single lahar event. Sedimentation rate varies from 3 to 4.5 cm/min during relatively long-lived surges to as much as 20 cm/min during short-lived surges. These results indicate that the sediment load fluctuates during lahar flow, further demonstrating that lahars are transient sediment-water flows with unsteady flow properties.     

Reconceptualising coarse sediment delivery problems in rivers as catchment-scale and diffuse

This paper assesses river channel management activities in the context of the interaction between coarse sediment delivery, climate change, river channel response and flood risk. It uses two main sources of evidence: (1) an intensive instrumentation of an upland river catchment using both traditional hydrometric and novel sediment sensing methods; and (2) a sediment delivery model that combines a treatment of sediment generation from mass failure with a treatment of the connectivity of this failed material to the drainage network. The field instrumentation suggests that the precipitation events that deliver sediment from hillslopes to the drainage network are different to those that transfer sediment within the network itself. Extreme events, that could occur at any time in the year (i.e. they are not dependent on wet antecedent conditions), were crucial for sediment delivery. However, sustained high river flows were responsible for the majority of transfer within the river itself. Application of three downscaling methods to climate model predictions for the 2050s and 2080s suggested a significant increase in the number and potential volume of delivery events by the 2050s, regardless of the climate downscaling scenario used. First approximations suggested that this would translate into annual bed level aggradation rates of between 0.10 and 0.20 m per year in the downstream main channel reaches. Second, the importance of this delivery for flood risk studies was confirmed by simulating the effects of 16 months of measured in-channel simulation with river flows scaled for climate change to the 2050s and 2080s. Short-term sedimentation could result in similar magnitude increases in inundated area for 1 in 0.5 and 1 in 2 year floods to those predicted for the 2050s in relation to increases in flow magnitude. Finally, we were able to develop an alternative approach to river management in relation to coarse sediment delivery, based upon reducing the rates of coarse sediment delivery through highly localised woodland planting, under the assumption that reducing delivery rates should reduce the rate of channel migration and hence the magnitude of the bank erosion problem. Thus, the paper demonstrates the need to conceptualise local river management problems in upland river environments as point scale manifestations of a diffuse sediment delivery process, with a much more explicit focus on the catchment scale, if our river systems are to become more insulated from the impacts of future climate changes.     

泥石流的结构两相流模型:Ⅱ.应用

本文应用泥石流的结构两相流模型对各类泥石流的运动机理进行探讨,并成功地解释了泥石流运动中的一系列特殊现象,如可能出现的颗粒浓度“上大下小”型分布、泥石流垂线速度的“反Ｓ”型分布、泥石流中颗粒脉动速度分布变化的特殊性、泥石流的输移特性以及在一定条件下出现的“流核”现象,等等。模型计算结果得到了实验资料的验证。     

Detailed debris flow hazard assessment in Andorra: A multidisciplinary approach

In many mountainous areas, the rapid development of urbanisation and the limited space in the valley floors have created a need to construct buildings in zones potentially exposed to debris flow hazard. In these zones, a detailed and coherent hazard assessment is necessary to provide an adequate urban planning. This article presents a multidisciplinary procedure to evaluate the debris flow hazard at a local scale. Our four-step approach was successfully applied to five torrent catchments in the Principality of Andorra, located in the Pyrenees. The first step consisted of a comprehensive geomorphologic and geologic analysis providing an inventory map of the past debris flows, a magnitude–frequency relationship, and a geomorphologic–geologic map. These data were necessary to determine the potential initiation zones and volumes of future debris flows for each catchment. A susceptibility map and different scenarios were the principal outcome of the first step, as well as essential input data for the second step, the runout analysis. A one-dimensional numerical code was applied to analyse the scenarios previously defined. First, the critical channel sections in the fan area were evaluated, then the maximum runout of the debris flows on the fan was studied, and finally simplified intensity maps for each defined scenario were established. The third step of our hazard assessment was the hazard zonation and the compilation of all the results from the two previous steps in a final hazard map. The base of this hazard map was the hazard matrix, which combined the intensity of the debris flow with its probability of occurrence and determined a certain hazard degree. The fourth step referred to the hazard mitigation and included some recommendations for hazard reduction. In Andorra, this four-step approach is actually being applied to assess the debris flow hazard. The final hazard maps, at 1 : 2000 scale, provide an obligatory tool for local land use planning. Experience achieved during the study showed that the collaboration between geologists, geomorphologists, engineers, and decision makers is essential and that only a multidisciplinary approach allows for solving all the problems of such a complex process as debris flows. Finally, we propose that our approach may be applied to other mountainous areas, adapting the hazard matrix to new local conditions.     

Does hydrological connectivity improve modelling of coarse sediment delivery in upland environments?

Modelling the delivery of landslide-generated sediment to channel networks is challenging due to uncertainty in the magnitude–frequency distribution of failures connected to the channel network. Here, we investigate a simplified treatment of hydrological connectivity as a means for improving identification of coarse sediment delivery to upland rivers. Sediment generation from hillslopes and channel banks and its delivery to the channel network are modelled based on a modified form of SHALSTAB coupled to a network index version of TOPMODEL. The network index treatment has two important hydrological effects: (a) it only allows saturated areas to connect to the hydrological network when there is full saturation along the associated flow path; and (2) overland flow associated with unconnected but saturated zones is assumed to remain within the catchment and to contribute to a reduction in the catchment-averaged saturation deficit. We use this hydrological treatment to restrict sediment delivery to situations where there is surface hydrological connection (i.e. saturation) along the complete flow path that connects failure areas to the drainage network. This represents an extreme restriction on the possibility of connected failure as it does not allow for failed material to connect if failures are associated with partial saturation or where delivery involves runout across areas where hydrological connection is not maintained. The impact of this restriction is assessed by comparing model predictions with field mapping of connected failures and data from continuously recording coarse sediment sensors, for two storm events. The hydrological connection requirement restricted connected failures to zones closer to the drainage network and resulted in a better level of agreement with the field mapped failures. Simulations suggested that in the study catchment the majority of sediment inputs occur from hydrologically-connected areas close to the channel network during moderate sized rainstorms that occur relatively frequently.     

Can the narrowing of the Lower Yellow River by regulation result in non-siltation and even channel scouring?

This study considered whether the narrowing of the upper (broad and wandering) reaches of the Lower Yellow River could result in a reduction in sedimentation and even an increase in channel erosion in both the upper and the lower (narrow and meandering) reaches. Analysis of field data and numerical modeling results both justify the proposal to narrow the channel. A positive correlation was found between channel eroded-area and the channel width. Therefore narrowing under conditions of low flow will reduce the amount of erosion in the reach, which, in turn, will reduce the amount of sediment transported into the lower channel. This will reduce the amount of siltation in the lower reaches of the river. However, narrowing under conditions of high flow with a low concentration of sediment will reduce both the extent of flood attenuation along the narrowed channel and the amount of lateral channel bank collapse, which results in increased flows and less sedimentation in the lower channel, leading to increased erosion. When flows with a high concentration of sediment are released from the Xiaolangdi Reservoir, both the lower narrow channel and the upper channel can transport a large amount of the sediment load. It is concluded that the narrowing of the upper broad channel will result in a reduction in sedimentation, or even in channel erosion, in both the upper and the lower channels if the reservoir is operated such that the volume of sediment added during low flows is balanced by the volume eroded during high flows with a low concentration of sediment.