Analysis of the triggering conditions and erosion of a runoff-triggered debris flow in Miyun County,Beijing, China

The occurrence of debris flow from channel-bed failure is occasionally noted in small and steeply sloping watersheds where channelized water flow dominates debris flow initiation. On August 12, 2016, a debris flow from channel-bed failures occurred in the Caozhuangzi Watershed of the Longtan Basin, Miyun, Beijing. Rainfall records over 10-min intervals and field investigations including channel morphology measurements were used to study the triggering conditions and erosion process. The results indicated that the occurrence of this event lagged the peak 10-min rainfall interval and that the cumulative rainfall prior to the occurrence time played an important role in its formation. A mean 10-min rainfall intensity–duration expression in the form of I₁₀?=?5.0?×?D^?0.21, where I₁₀ denotes the mean 10-min rainfall intensity and D is the rainfall duration ranging from 10 to 60 h, was proposed. The debris flows have low proportions of grain size fractions <?0.1 mm and higher fractions of grains 0.1–2 mm in size, indicating that the flow had low viscosity and was coarse-grain dominated. Channel morphology analysis revealed that abrupt changes in topography in the study area, including a steep section, a concave stream bank area, and a partial concave stream section were eroded more extensively than other sites. The maximum sediment erosion volume and erosion depth were not proportional to the variation in stream gradient. Consideration of the degree of erosion in the channel at sites with abrupt morphology changes, the maximum sediment erosion volume, and the erosion depth and volume at the initial channel site and downstream region of forest area together showed that the prime factor controlling erosion was entrained sediment volume. This work, thus, provides a case study regarding the triggering conditions of runoff-triggered debris flows and the topographical changes by debris flow erosion.     

Unprecedented rates of landslide and surface erosion along a newly constructed road in Yunnan, China

Field measurements conducted 4 years after the construction of a new portion of the Weixi?CShangri-La road in Yunnan, China, reveal that unprecedented rates of mass wasting occurred along the road with much of this sediment directly impacting the headwaters of the Mekong River. Landslide erosion (including dry ravel) exceeded 33,000 t ha^?1 year^?1 along the most severely eroded sections of the road and averaged more than 9,600 t ha^?1 year^?1 along the surveyed 23.5 km of road; these values are the highest ever reported for road-related landslides. While surface erosion was only about 7% of the total erosion from the road, it is still more than an order of magnitude higher than typical surface erosion rates from disturbed lands in Southeast Asia. Combined landslide and surface erosion from this road delivered an estimated 19 times more sediment to the river than the remaining 99.6% of the contributing catchment. These sediment inputs are aggrading local channels, promoting downstream sediment transport, degrading aquatic habitat, and creating the possibility for a future debris flood or hyperconcentrated flow.     

Field observations of the June 30, 2001 debris flow at Acquabona (Dolomites, Italy)

On June 30, 2001, a debris flow occurred in the Acquabona Creek, a small catchment of the Eastern Dolomites, Italy. This debris flow originated shortly after an intense rainstorm, characterised by a peak intensity of 8.6 mm per 10 min; it transported a total volume of 30,000 m³, consisting of poorly sorted gravely sand with boulders up to 3 m in diameter. The sediment erosion yield rate reached as high as 20 m³/m. In order to verify the accuracy of the field measurements, the total volume of debris deposits have was calculated using three different topographic measurement techniques: 3D laser scanning, terrestrial stereo-photogrammetry survey and total topographic station survey. Data collected so far show that no debris flow has occurred at Acquabona with a rainfall intensity lower than 4.6 mm per 10 min. Channel cross section measurements indicate that debris flow velocity ranges from 2.0 to 7.2 m/s along the lower flow channel and peak discharge ranges between 22 and 300 m³/s. Field estimates of the rheological properties indicate a yield strength ranging from 2,088 to 5,313 Pa and Bingham viscosity between 70 and 337 Pa · s. It is not still possible to identify a rainfall intensity and amount threshold for debris flow triggering, but the data so far collected emphasise that debris flows do not occur with a rainfall intensity lower than 4.6 mm per 10 min.     

Sedimentation in a river dominated estuary

The Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta. Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 10³ m³ s^?1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level. The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents. During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change. If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology.     

Design and performance of a novel multi-function debris flow mitigation system in Wenjia Gully,Sichuan

The post-earthquake debris flows in the Wenjia Gully led to the exposure of the shortcomings in the design of the original conventional debris flow mitigation system. A predicament for the Wenjia mitigation system is a large amount of loose material (est. 50 × 10⁶ m³) that has been deposited in the gully by the co-seismic landslide, providing abundant source material for debris flows under saturation. A novel design solution for the replacement mitigation system was proposed and constructed, and has exhibited excellent performance and resilience in subsequent debris flows. The design was governed by the three-phase philosophy of controlling water, sediment, and erosion. An Early Warning System (EWS) for debris flow that uses real-time field data was developed; it issues alerts based on the probabilistic and empirical correlations between rainfall and debris flows. This two-fold solution reduces energy of the debris flow by combining different mitigation measures while minimizing the impact through event forecasting and rapid public information sharing. Declines in the number and size of debris flows in the gully, with increased corresponding rainfall thresholds and mean rainfall intensity-duration (I-D) thresholds, indicate the high efficacy of the new mitigation system and a lowered debris flow susceptibility. This paper reports the design of the mitigation system and analyzes the characteristics of rainfall and debris flow events that occurred before and after implementation of the system; it evaluates the effectiveness of one of the most advanced debris flow mitigation systems in China.     

Paraglacial response of steep,sediment‐mantled slopes to post‐‘Little Ice Age’ glacier recession in the central Swiss Alps

This research assesses the morphological consequences of recent (post‐‘Little Ice Age’) paraglacial reworking of valley‐side sediment mantles in the European Alps. It aims to identify the extent and conditioning factors of slope adjustment at sites in the Swiss Alps, model the temporal pattern, and assess the rates of sediment reworking involved. Gully systems have cut into steep, high‐level lateral moraines, and debris cones have accumulated downslope. Debris flow is the dominant agent of sediment transfer. Factors controlling the extent of this activity include moraine slope gradient, relief and moisture availability. Gullies appear to have reached their maximum dimensions within ca. 50 yr of deglaciation, after which gully relief is reduced by removal of inter‐gully slopes and gully infilling (within 80–140 yr). On the most recently deglaciated terrain, minimum erosion rates average ca. 95 mm yr^?1 since gully initiation, greatly exceeding ‘normal’ erosion rates in other environments. Mean annual accumulation of a single debris cone since ice retreat was calculated to be ca. 30 mm yr^?1. Implications of these findings are applied to patterns of paraglacial sediment‐mantled slope adjustment, conceptualising paraglacial landscape response in terms of a sediment release exhaustion model, and paraglacial landform succession. Copyright © 2005 John Wiley & Sons, Ltd.     

山西河曲黄河阶地序列初步研究

黄河干流奇特的"几"字形格局是其复杂发育历史的表现。由于流域内的地质与气候条件复杂多样,分段开展深入研究是全面认识黄河的基础。晋陕峡谷是研究黄河演化的关键地段之一,河流阶地忠实地记录着河流发育的历史。文章选择山西河曲县城附近黄河阶地发育典型的河段进行研究,在实测河流阶地地貌剖面的基础上,系统采集了20个年代样品进行光释光(OSL)测年。依据地貌类型、沉积特征以及定年结果,建立了该河段黄河阶地的演化序列,得出以下结论:1)河曲地区黄河曲流凸岸形成有4级阶地,T₄阶地的形成主要受构造控制,而T₃,T₂和T₁阶地的形成主要与气候变化有关,各阶地的年龄分别是T₄为90ka,T₃为30ka,T₂为20ka,T₁为3.4ka。2)河曲地区约140ka以来河流地貌的演化经历了5个阶段,各阶段以下切侵蚀开始,结束于各阶地堆积面的塑造。约90ka以来,该地区河谷谷底下降速度和曲流可能最大侧蚀速度的平均值分别为0.9mm/a和33.4mm/a。在不同阶段,二者的大小变化及组合状况各异,在构造相对稳定条件下,河流以侧蚀作用为主,其侧蚀速度与气候和岩性条件有关。3)河曲地区的黄河曲流是在河流下切过程中逐渐侧蚀、演化而成的,具有内生曲流的特点。4)T₄阶地的泥流沉积和加积堆积,可能记录了地方性气候变化,其范围和意义有待进一步研究,另外,T₃,T₂和T₁形成过程中气候变化的作用也有待探讨。     

速流结构防治泥石流的理论及应用

作为一种公路沿线发育的地质灾害，泥石流是路基、路面及相关建、构筑物最强烈的毁损动力。为了防治泥石流公路灾害，作者开发了速流结构。该结构由汇流槽和速流槽组成。其中汇流槽的侧墙长度、高度’、厚度等参数通过泥石流沟形态及其冲击力综合确定；而速流槽的宽度、侧墙及纵横断面则由泥石流排泄量、流速、锁固桩及流体性质综合确定。根据公路穿越速流结构的方式，将速流结构分为底越式和顶越式2种。速流结构对于解决具有大冲大淤特性的冲淤变动型沟谷泥石流灾害具有重要应用价值。据此可以确保穿越泥石流沟的公路构、建筑物的安全与稳定．确保公路交通有序进行。此外，文章对速流结构的一些子结构，如速流槽侧墙及锁固桩进行了力学分析。位于川西南西昌-木里干线公路的平川泥石流属于典型的冲淤变动型沟谷泥石流。该沟长度8．2km，沟床平均比降0．181，平均流速9．0m／s，沉积区宽度500m左右。为了确保该段公路交通的有序进行，作者干1999年实施了平川泥石流速流结构。通过2000年以来的研究及现场测试，显示了该防治结构的优越性。但是，作为一种特殊的水工结构，泥石流对速流结构的磨蚀作用是比较强烈的。因此，开展速流结构的抗冲、抗撞研究是进一步研究具有重要价值。     

A prototype experiment of debris flow control with energy dissipation structures

Large-volume debris flow events are defined when the volume of solid materials exceeds 1 million m³. Traditional engineering measures, such as check dams, diversion channels, and flumes, are effective for normal debris flow control but are not sufficient to control large-volume debris flows. Experiments were conducted with an artificial step-pool system on the new Wenjiagou Gully to mitigate large-volume debris flows. The old Wenjiagou Gully was buried by 81.6 million m³ of loose solid material created by a landslide that was triggered by the Wenchuan earthquake on May 12, 2008. The new gully was formed during the scouring process caused by debris flows in 2008. Large-volume debris flows were initiated by rainstorm flood with high kinetic energy. The artificial step-pool system was constructed with huge and big boulders on the new Wenjiagou Gully in 2009. The step-pool system dissipated flow energy in steps and hydraulic jumps. Analysis proved that the step-pool system dissipated two-third of the kinetic energy of flow; thus, the critical discharge for triggering debris flow increased threefold. Due to the step-pool system maximized the flow resistance and protected the bed sediment and banks from erosion, the rainstorm floods in 2009 did not trigger debris flows. In 2010, the step-pool system was replaced with 20 check dams. Huge boulders were broken into small pieces of diameter less than 0.5 m and were used as building materials for the 20 dams. Without the protection of the step-pool system, a rainstorm flood scoured the base of the dams and caused failures for all of the 20 check dams in August 2010. The flow incised the gully bed by 50 m. The loose bank materials slid into the flow mixed with water and formed a large-volume debris flow with a volume of 4.5 million m³. Many houses were buried by the debris flow, and 12 people were killed. Comparison of the two strategies proved that energy dissipation structures are necessary for controlling large-volume debris flows. Check dams, if they are stable, may reduce the potential of bank failures and control debris flows. The step-pool system dissipates flow energy and control gully bed incision and bank failure. A combination of check dams and step-pool systems may be the most effective for mitigating debris flows.     

Morphology and Dynamics of Alluvial Fan and Its Research Prospects

Alluvial fans can preserve historical records of sediment transport to middle and lower river systems or piedmont basins, which are considered to be sensitive recorders of climate change and tectonic activity. In this paper, the morphological characteristics, control factors and future development trend of alluvial fan are summarized and described. The main understanding is as follows: \mathrm{①} According to the gravity flow and traction flow process, fan can be divided into debris flow alluvial fan and fluvial fan. The former is formed under the action of debris gravity flow deposits, which is related to the occasional flood and burst flow in a short time. The latter is braided tributaries depositions which are gradually shallower and spread radially in the direction of fan toe under the traction water transport. \mathrm{②} The erodibility of underlying bedrock can affect the scale of downstream alluvial fan, which depends on the sediment production and store factors in the catchment. The easily eroded bedrock may produce more sediment, making the alluvial fan area larger. In the contrast, the erodibility of rocks in the source area can also affect the slope and hydrological characteristics of the valley so that more sediment is deposited in the upstream basin and the alluvial fan formed in the downstream is smaller. \mathrm{③} Tectonic activity is the pre-condition for the development of alluvial fans, which provides a space for alluvial fans depositions. Faulting in the piedmont can change the position and morphology of the ancient alluvial fan, and also cause deformation or distortion of the thick sedimentary sequence to record the regional tectonic activity. The quaternary alluvial fan sequence corresponds well to the climate change during the glacial-interglacial period. However, the influence of the flood events caused by extreme meteorological events on alluvial fan deposition should be focused on. \mathrm{④} The application of a series of new techniques and methods will help to carry out deep research on alluvial fan in the future, such as high-resolution observation technique, physical simulation experiment, and precise dating.     

Rainfall-related debris flows in Carhuacocha Valley, Cordillera Huayhuash, Peru

Continuous heavy rainfall hit northern Peru in the second half of the 2008/2009 summer season. From the end of January to the beginning of March, the Cordillera Huayhuash experienced abnormally high precipitations that exceeded 270?mm. The antecedent rainfall, aggravated with a severe rainstorm of 20?mm on March 7 triggered a large debris flow in the upper Carhuacocha Valley early in the morning on March 8. The debris flow interrupted drainage from the upper part of the valley damming a lake in the narrow depression between the trough slope and the lateral moraine. As a result of the drainage interruption, water percolated through the moraine dam of Cangrajanca Lake where a secondary mass movement occurred in its inner slope. In September 2009, we mapped the debris flow and related landforms and estimated the total area and volume of both mass movements using geodetic measurements. About 104,000?m³ of sediments was moved from the trough slope towards the moraine from which 534,000?m³ flowed to Cangrajanca Lake subsequently. We analysed the rainfall conditions that triggered the debris flow using rainfall data from the nearby stations. We also compared the precipitation preceding the event with the rainfall thresholds for debris flow initiation.     

The role of geomorphic processes in the transport and fate of mercury in the Carson River basin, west-central Nevada

 The historic processing of precious metal ores mined from the Comstock Lode of west-central Nevada resulted in the release of substantial, but unquantified amounts of mercury-contaminated mill tailings to the Carson River basin. Geomorphic and stratigraphic studies indicate that the introduction of these waste materials led to a period of valley-floor aggradation that was accompanied by lateral channel instability. The combined result of these geomorphic responses was the storage of large volumes of mercury-enriched sediment within a complexly structured alluvial sequence located along the Carson River valley. Much of the contaminated sediment is associated with filled paleochannels produced by the cutoff and abandonment of meander loops, and their subsequent infilling with contaminated particles. Geochemically, these deposits are characterized by variations in mercury levels that exceed three orders of magnitude. Continued lateral instability, coupled with an episode of channel-bed incision, followed the decline of Comstock mining, and has reexposed contaminated debris within the banks of the river. Erosion of bank sediments reintroduces mercury-enriched particles to the modern channel bed. It is suggested on the basis of geochemical and sedimentological data that during the bank erosion process, much of the mercury associated with fine (<63 μ) valley-fill deposits are carried downstream without being incorporated to any appreciable extent within the channel-bed sediments. In contrast, mercury associated with larger and denser particles, particularly mercury-gold-silver amalgam grains, are accumulated in the channel-bed sediments as the river traverses polluted reaches of the Carson River valley. Concentration patterns developed along the modern channel indicate that the valley fill is the primary source of mercury to the river today. Thus, these data imply that efforts to reduce the influx of mercury to the aquatic environment should examine methods for reducing bank erosion rates. Received: 13 December 1996 · Accepted: 15 April 1997     

Mercury distribution and transport in a contaminated river system in Kazakhstan and associated impacts on aquatic biota

The River Nura in Central Kazakhstan has been heavily polluted by Hg originating from an acetaldehyde plant. A number of studies were undertaken to investigate the transport, fate and bioavailability of Hg in this river system. The sediments within a 20 km section of the river downstream of the effluent outfall canal are highly polluted and are acting as a strong source of surface water contamination. Mercury transport in the river is dominated by the remobilization of contaminated bed sediments and river bank erosion during the annual spring flood. Peak Hg concentrations in unfiltered surface water samples during a larger than usual flood event in 2004 were in the order of 1600–4300 ng L⁻¹. The majority of the particulate-bound Hg appears to be sedimented in the shallow Intumak reservoir 75 km downstream of the source of the pollution, leading to a drop in aqueous Hg concentrations by an order of magnitude. Nevertheless, background concentrations of Hg in surface water are not reached until at least 200 km downstream, and during the flood period Hg is also detected in the terminal wetlands of the river.Mercury concentrations in sediment cores taken from the river bed in the most contaminated section of the Nura ranged from 9.95 to 306 mg kg⁻¹. Methylmercury (MeHg) levels in shallow sediment cores were highest in surface sediments and ranged between 4.9 and 39 μg kg⁻¹, but were generally less than 0.1% of total Hg (THg). A significant inverse relationship was found between THg concentrations and the percentage of MeHg formed in the sediments, irrespective of the sampling depth. The observed relationship was confirmed by comparison with results from a different river system, indicating that it may be true also for other highly contaminated aquatic systems. It is hypothesized that at high THg levels in severely contaminated sediments, the accumulation of MeHg may be limited by increasingly efficient demethylation processes, and that this underlying trend in sediments is the reason why MeHg levels in surface water are often found to be higher at less contaminated sites compared to upstream sites.Mercury concentrations in biota in the most contaminated section of the river were 15–20 times higher than background levels. Fish were found to be impacted for more than 125 km downstream from the source, indicating significant transport of dissolved MeHg to downstream areas and/or in-situ MeHg production in less contaminated downstream reaches. There were also indications that impoundments may increase the bioavailability of Hg.     

A slope failure caused by drainage cutoff through the advancement of seasonal frost, Hudson Bay Lowland

A failure occurred between December 14 and 17, 2008 in the upper part of a 45-m high, northwest facing bank of the Nelson River in northern Manitoba (56.687°N, 93.777°W). The slope failure occurred at a spring site in a bay associated with a buried valley. The sediment input to the river from this event is roughly 20,000 to 25,000 m³. The source zone is made up of 25  m of water-bearing sand and gravel confined between ice-rich silty clay at the top of the bank and laminated to rhythmically bedded silt and clay at the base of the section. The collapse was confined to the material above the basal silts and clays and was associated with a perched groundwater flow system. A strong argument for drainage cutoff by the advancement of seasonal frost has been demonstrated through the correlation of the bank collapse with the timing of a significant cold snap recorded at two nearby weather stations. The failure illustrates the importance of stratigraphy in controlling bank erosion in this area. Previously, fluvial erosion was seen as an important control on mass wasting in Horseshoe Bay. However, surface information suggests that no toe erosion except to remove the slide deposit has occurred at this site since 2004.     

汶川震区映秀镇"8·14"特大泥石流灾害调查

2010年8月14日强降雨过程导致汶川震区映秀镇红椿沟泥石流暴发,泥石流堰塞堆积体堵断岷江主河道,导致河水改道冲入映秀新镇,引发洪水泛滥;造成映秀镇13人死亡、59人失踪,受灾群众8 000余人被迫避险转移.由于这场泥石流灾害发生在汶川地震震中区,是地震与降雨共同作用下的结果,研究其形成与成灾过程对于进一步认识强震区泥...     

贵阳息烽县阳郎排洪工程水量效应分析——以鱼简河水库至底赛寨为例

在没有河谷水文监测资料的情况下,利用洪丰期水量及引洪流量和河谷断面水文参数,计算出引洪前后鱼简河断面水位、流量的变化,并与河岸农田、村舍等地物分布高程对比,进而评估引洪产生的受灾情况。结果表明在阳朗坝排洪412 m3/s的条件下,鱼简河A-A、C-C地段河谷断面承载流量分别为491.091及476.219 m3/s,相对安全,而B-B、D-D地段,河谷断面承载流量分别为365.002及160.675 m3/s,将有可能被洪水淹没成灾。      

Late Quaternary fluvial aggradation and incision in the monsoon‐dominated Alaknanda valley,Central Himalaya,Uttrakhand, India

The present study aims to explain the spatial and temporal variability in phases of aggradation/incision in response to changes in climate and seismicity during the late Quaternary in the Alaknanda River valley (a major tributary of the river Ganges or Ganga). Geomorphology, stratigraphy and optical dating of the fluvial sediment reveal that the oldest fluvial landforms preserved in the south of the Main Central Thrust are debris flow terraces developed during the early part of pluvial Marine Isotopic Stage 3. Following this, a period of accelerated incision/erosion owing to an increase in uplift rate and more intense rainfall occurred. In the Lesser Himalaya, three phases of valley fill aggradation around 26 ± 3 ka, 18 ± 2 ka and 15 ± 1 ka and 8 ± 1 ka occurred in response to changes in monsoon intensity and sediment flux. The last phase was regionally extensive and corresponds to a strengthening of the early Holocene Indian Summer Monsoon. A gradual decline in the monsoon strength after 8 ± 1 ka resulted in reduced fluvial discharge and lower sediment transport capacity of the Alaknanda River, leading to valley fill incision and the development of terraces. The study suggests that fluvial dynamics in the Alaknanda valley were modulated by monsoon variability and the role of tectonics was subordinate, limited to providing accommodation space and post‐deposition modification of the fluvial landforms. Copyright © 2010 John Wiley & Sons, Ltd.     

Debris flow magnitude,frequency, and precipitation threshold in the eastern North Cascades,Washington, USA

We examine the magnitude, frequency, and precipitation threshold of the extreme flood hazard on 37 low-order streams in the lower Stehekin River Valley on the arid eastern slope of the North Cascades. Key morphometric variables identify the magnitude of the hazard by differentiating debris flood from debris flow systems. Thirty-two debris flow systems are fed by basins?<?6 km² and deposited debris cones with slopes?>?10°. Five debris flood systems have larger drainage areas and debris fans with slopes 7–10°. The debris flood systems have Melton ruggedness ratios from 0.42–0.64 compared to 0.78–3.80 for debris flow basins. We record stratigraphy at seven sites where soil surfaces buried by successive debris flows limit the age of events spanning 6000 years. Eighteen radiocarbon ages from the soils are the basis for estimates of a 200 to1500-year range in recurrence interval for larger debris flows and a 450?±?50-year average. Smaller events occur approximately every 100 years. Fifteen debris flows occurred in nine drainage systems in the last 15 years, including multiple flows on three streams. Summer storms in 2010 and 2013 with peak rainfall intensities of 7–9 mm/h sustained for 8–11 h triggered all but one flow; the fall 2015 event on Canyon Creek occurred after 170 mm of rain in 78 h. A direct link between fires and debris flows is unclear because several recent debris flows occurred in basins that did not burn or burned at low intensity, and basins that burned at high intensity did not carry debris flows. All but one of the recent flows and fires occurred on the valley’s southwest-facing wall. We conclude that fires and debris flows are linked by aspect at the landscape scale, where the sunny valley wall has flashy runoff due to sparse vegetation from frequent fires.

     

Variation of debris flow/flood formation conditions at the watershed scale in the Wenchuan Earthquake area

A channelized debris flow/flood generally originates from initial gully erosion by superficial runoff that evolves rapidly into massive erosion of the channel bed. Knowledge of the formation conditions of such events is crucial for accurate forecasting, and determination of rainfall and runoff thresholds for such hazards is a primary concern following a strong earthquake. This work proposed a framework for debris flow/flood formation at the watershed scale in two watersheds (area: 2.4 and 32.4 km²) in the Wenchuan Earthquake area (China). The critical runoff and rainfall conditions required for debris flow/flood formation were simulated and their annual variations investigated. Ultimately, the runoff conditions required for debris flow/flood formation in the two studied watersheds were calculated on an annual basis and found to increase in time. Similarly, following consideration of three different rainfall types, critical rainfall conditions were proposed that also showed an increasing tendency. The increase of rainfall and runoff conditions for debris flow/flood formation is attributable to both the recovery of vegetation and the reduction of source materials. In comparison with actual monitored flow behaviors and previously proposed rainfall thresholds, the results showed strong consistency and high forecasting efficiency.

     

Effect of spatial scale on suspended sediment concentration in flood season in hilly loess region on the Loess Plateau in China

Suspended sediment concentration is a major variable influencing soil erosion and loss, study on which at different spatial scales is of great meaning to understand soil erosion mechanism and sediment transport process. Based on data from 4 sloping surfaces and 7 basins ranging from 0.0003 to 187 km² in area, the suspended sediment concentration in flood season (SSC) with drainage area is studied. With increasing drainage area on the slope surfaces, the mean suspended sediment concentration in flood season (MSSC) enhances continuously until a peak value of 685 kg m⁻³ occurs at the whole slope surface No. 7 runoff plot resulting from harder and harder erosion forms downslope. Entering basin systems, the diluted action of subsurface water on the toeslope on MSSC and small water flow power Ω make a minimum MSSC value of 568 kg m⁻³ occur in the first-order basin system Tuanshangou basin at an area of 0.18 km², and then from Tuanshangou basin to larger basins, the positive feedback function among drainage density, water flow energy, and hyperconcentrated flow as well as its reduction of settling velocity of coarser particles generates continuously increasing MSSC with drainage area.