The 1786 earthquake-triggered landslide dam and subsequent dam-break flood on the Dadu River, southwestern China — Reply

A short reply to discussion by Chanson.     

Discriminant analysis of the geomorphic characteristics and stability of landslide dams

Landslides can cause the formation of dams, but these dams often fail soon after lake formation. Thus, rapidly evaluating the stability of a landslide dam is crucial for effective hazard mitigation. This study utilizes discriminant analysis based on a Japanese dataset consisting of 43 well documented landslide dams to determine the significant variables, including log-transformed peak flow (or catchment area), and log-transformed dam height, width and length in hierarchical order, which affect the stability of a landslide dam. The high overall prediction power (88.4% of the 43 training cases are correctly classified) and the high cross-validation accuracy (86%) demonstrate the robustness of the proposed discriminant models PHWL (with variables including log-transformed peak flow, and log-transformed dam height, width and length) and AHWL (with variables including log-transformed catchment area, and log-transformed dam height, width and length). Compared to a previously proposed “DBI” index-based graphic approach, the discriminant model AHV – which uses the log-transformed catchment area, dam height, and dam volume as relevant variables – shows better ability to evaluate the stability of landslide dams. Although these discriminant models are established using a Japanese dataset only, the present multivariate statistical approach can be applied for an expanded dataset without any difficulty when more completely documented worldwide landslide-dam data are available.     

Prolonged impact of earthquake-induced landslides on sediment yield in a mountain watershed: The Tanzawa region, Japan

To determine for how long a landslide affects sediment discharge, the sediment yields of 15 check-dam basins were compared with the time series of landslide distributions in a mountain basin in the Tanzawa region, central Japan. The distribution of sediment yield was quantitatively estimated from deposition in the sediment pools of check dams. The relationship between the landslide history and sediment discharge in the Nakagawa River basin was examined for an approximately 80-year period. Two major landslide events occurred during this period: the 1923 Kanto Earthquake and the 1972 disaster caused by heavy rainfall. The resulting trend in sediment discharge of the whole basin, estimated using reservoir sedimentation in the Miho Dam at its base, was nearly constant, with high sediment discharge (2897 m³ km^− 2 yr^− 1) in the intervening quarter-century, despite the recovery of vegetation on landslide areas in this period. Comparisons of the landslide distributions resulting from the two disasters, the sediment yields of check-dam basins, and the sediment discharge of the whole basin indicate that recent sediment discharge contains landslide debris that was originated by the Kanto Earthquake that occurred over 80 years ago. Thus, to understand high sediment discharge, it is essential to investigate not only the current basin condition and recent events, but also the landslide history of the basin for at least the previous 100 years.     

A Neogene giant landslide in Tarapacá, northern Chile: A signal of instability of the westernmost Altiplano and palaeoseismicity effects

Giant landslides, which usually have volumes up to several tens of km³, tend to be related to mountainous reliefs such as fault scarps or thrust fronts. The western flank of the Precordillera in southern Peru and northern Chile is characterized by the presence of such mega-landslides. A good example is the Latagualla Landslide (19°15′S), composed of ~ 5.4 km³ of Miocene ignimbritic rock blocks located next to the Moquella Flexure, a structure resulting from the propagation of a west-vergent thrust blind fault that borders the Precordillera of the Central Depression. The landslide mass is very well preserved, allowing reconstitution of its movement and evolution in three main stages. The geomorphology of the landslide indicates that it preceded the incision of the present-day valleys during the late Miocene. Given the local geomorphological conditions 8–9 Ma ago (morphology, slopes and probably a high water table), large-magnitude earthquakes could have provided destabilization forces enough to cause the landslide. On the other hand, present seismic forces would not be sufficient to trigger such landslides; therefore the hazard related to them in the region is low.     

Comparison of landslide susceptibility based on a decision-tree model and actual landslide occurrence: The Akaishi Mountains, Japan

This paper proposes a statistical decision-tree model to analyze landslide susceptibility in a wide area of the Akaishi Mountains, Japan. The objectives of this study were to validate the decision-tree model by comparing landslide susceptibility and actual landslide occurrence, and to reveal the relationships among landslide occurrence, topography, and geology. Landslide susceptibility was examined through ensemble learning with a decision tree. Decision trees are advantageous in that estimation processes and order of important explanatory variables are explicitly represented by the tree structures. Topographic characteristics (elevation, slope angle, profile curvature, plan curvature, and dissection and undissection height) and geological data were used as the explanatory variables. These topographic characteristics were calculated from digital elevation models (DEMs). The objective variables were landslide occurrence and reactivation data between 1992 and 2002 that were depicted by satellite image analysis. Landslide susceptibility was validated by comparing actual data on landslides that occurred and reactivated after the model was constructed (between 2002 and 2004).This study revealed that, from 2002 to 2004, landslides tended to occur and reactivate in catchments with high landslide susceptibility. The landslide susceptibility map thus depicts the actual landslide occurrence and reactivation in the Akaishi Mountains. This result indicates that the decision-tree model has appropriate accuracy for estimating the probabilities of future landslides. The tree structure indicates that landslides occurred and reactivated frequently in the catchments that had an average slope angle exceeding ca. 29° and a mode of slope angle exceeding 33°, which agree well with previous studies. A decision tree also quantitatively expresses important explanatory variables at the higher order of the tree structure.     

Spatial and temporal variations in the grain-size characteristics of historical flood plain deposits, Blue River, Wisconsin, USA

This study examined vertical, lateral, and downstream variations in the grain-size characteristics of historical (post-1830) overbank deposits in a watershed that has experienced high rates of accelerated flood plain sedimentation. More than 800 samples were collected from 53 cores along nine flood plain transects. Overbank deposits exhibit a coarsening-upward sequence attributed to historical changes in the sand content of source materials. The erosion of loess-capped soils increased the exposure, erosion, and transport of sandy parent materials. The average sand content of near-channel cores increases moderately downstream along two of the reaches because sandy source materials are increasingly exposed in larger main valleys in the northern part of the watershed. The two northernmost reaches are coarser overall, but do not display significant downstream trends. The sand content of surface and early historical overbank deposits generally decreases laterally as an exponential function of distance from the channel, suggesting transport by turbulent diffusion. The presence of sand throughout the transects and lateral coarsening at two of the transects, however, suggests that sediment transport by convection is also important.     

The landslide response of alpine basins to post-Little Ice Age glacial thinning and retreat in southwestern British Columbia

The role of post-Little Ice Age (LIA) Neoglacial retreat on landslide activity is investigated in 19 alpine basins along the upper Lillooet River Valley, British Columbia. We examine how Neoglacial scouring and glacial recession have modified hillslope form and slope stability, and construct a decision-making flowchart to identify landslide hazards associated with glacial retreat. This work is based on field mapping, GIS analysis, statistical associations between landslides and terrain attributes, and a comparison between Neoglaciated and non-Neoglaciated terrain within each basin.The bedrock landslide response to glacial retreat varies appreciably according to lithology and the extent of glacial scour below the LIA trimline. Valleys carved in weak Quaternary volcanics show significant erosional oversteepening and contain deep-seated slope movement features, active rock fall, rock slides, and rock avalanches near glacial trimlines. Basins in stronger granitic rock rarely show increased bedrock instability resulting from post-LIA retreat, except for shallow-seated rock slides along some trimlines and failures on previously unstable slopes. In surficial materials, landslides associated with post-LIA retreat originate in till or colluvium, as debris slides or debris avalanches, and are concentrated along lateral moraines or glacial trimlines.Significant spatial association was also observed between recent catastrophic failures, gravitational slope deformation, and slopes that were oversteepened then debuttressed by glacial erosion. Eight out of nine catastrophic rock slope failures occurred just above glacial trimlines and all occurred in areas with a previous history of deep-seated gravitational slope movement, implying that this type of deformation is a precursor to catastrophic detachment.     

Channel morphology and its impact on flood passage, the Tianjiazhen reach of the middle Yangtze River

The Tianjiazhen reach of the middle Yangtze is about 8 km long, and characterized by a narrow river width of 650 m and local water depth of > 90 m in deep inner troughs, of which about 60 m is below the mean sea level. The troughs in the channel of such a large river are associated with regional tectonics and local lithology. The channel configuration plays a critical role in modifying the height and duration of river floods and erosion of the riverbed. The formation of the troughs in the bed of the Yangtze is considered to be controlled by sets of NW–SE-oriented neotectonic fault zones, in which some segments consist of highly folded thick Triassic limestone crossed by the Yangtze River. Several limestone hills, currently located next to the river channel, serve as nodes that create large vortices in the river, thereby accelerating downcutting on the riverbed composed of limestone highly susceptible to physical corrosion and chemical dissolution. Hydrological records indicate that the nodal hills and channel configuration at Tianjiazhen do not impact on normal flow discharges but discharges > 50,000 m³s^− 1 are slowed down for 2–3 days. Catastrophic floods are held up for even longer periods. These inevitably result in elevated flood stages upstream of prolonged duration, affecting large cities such as Wuhan and a very large number of people.     

Geomorphic hazard assessment of landslide dams in South Westland, New Zealand: fundamental problems and approaches

Quantitative assessments of landslide hazard usually employ empirical, heuristic, deterministic, or statistical methods to derive estimates of magnitude–frequency distributions of landsliding. The formation and failure of landslide dams are common geomorphic processes in mountain regions throughout the world, causing a series of consequential off-site hazards such as catastrophic outburst floods, debris flows, backwater ponding, up- and downstream aggradation, and channel instability.Conceptual and methodological problems of quantifying geomorphic hazard from landslide dams result from (a) aspects of defining “landslide-dam magnitude”, (b) scaling effects, i.e. the geomorphic long-range and long-term implications of river blockage, and (c) paucity of empirical data. Geomorphic hazard from a landslide dam-break flood on the basis of conditional probabilities is being analysed for the alpine South Westland region of New Zealand, where formation and failure of landslide dams is frequent. Quantification of the annual probability of landsliding and subsequent dam formation in the area is limited by historical and only partially representative empirical data on slope instability. Since landslide-dam stability is a major control governing the potential of catastrophic outburst flooding, the ensuing hazard is best assessed on a recurring basis. GIS-based modelling of virtual landslide dams is a simple and cost-effective approach to approximate site-specific landslide dam and lake dimensions, reservoir infill times, and scaled magnitude of potential outburst floods. Although crude, these order-of-magnitude results provide information critical to natural hazard planning, mitigation, or emergency management decisions.     

Estimation of water conductivity of the natural flood channels on the Tisza flood-plain, the Great Hungarian Plain

The rapidly subsiding central part of the Pannonian Basin, the flood-plain system of the Tisza River, is analyzed. Natural flood-conducting channels that were functioning prior to the construction of the water control measures of the 19th century have been identified and mapped. By now these channels have mostly disappeared; only small traces of them can be found on modern maps. The identification of these channels was achieved by studying historical maps of the region and by geomorphologic studies. Drawing the outline of the channels and the estimation of their initial meander size was supported by elevation models and satellite imagery. Eight flood breakout points and five independent channel systems were identified. The flood conveying capacity of each channel was also estimated, based on meander wavelengths. The reliability of this estimation is discussed. The channels functioned as anabranches, conducting the floods of the Tisza to its tributary, the Körös River. The efficiency of the flood control system could be increased by regeneration of these natural channels. The summarized water conveying capacity of the abandoned channels is estimated as 1000 m³/s, a value that is slightly under a half of the flood discharge of the Tisza River.     

大渡河双家坪古平推式滑坡堆积特征及形成机制

大渡河沿岸分布的大量古滑坡堆积体,是流域内水电能源开发及山区城镇建设的重大隐患,双家坪古滑坡堆积体就是其中的典型案例。2010年8月以来,随着瀑布沟电站库区水位升高淹没堆积体前缘,堆积体西侧区域地面及房屋出现了较大规模的变形迹象。调查表明,出现变形的区域仅是一个古老滑坡堆积体的一部分。根据现场调查和钻探手段,从地貌、物质结构特征查明了该古滑坡堆积体的边界条件、各分区的堆积特征,认为该滑坡堆积体可分为4个区,其中A、C区为主要崩落堆积区,以大块石堆积为主,块石直径1~3 m,甚至更大;B区为残留岩体堆积区,岩层保留原岩层序;D区为细颗粒物质堆积区。初步认为古滑坡堆积体为平推式滑动→推出悬空→重力折断→散落堆积→表生改造等过程的形成机制。     

The 1345 slide and flood disaster in the Gauldalen valley,Mid-Norway: A new interpretation

The Gauldal disaster of September, 1345, with 500 assumed killed, is the largest recorded in Norway. Contemporaneous written accounts, oral tradition and geological interpretations express divergent opinions about both its location and development. We show that, contrary to tradition, the river Gaula found its present course at Gaulfossen more than 2000 years before the disaster. Lakes in a small, elevated tributary valley, Langdalen, probably represented the 'lost lake' Hagevatnet of oral tradition, and the disaster apparently started with a flood from these lakes. Erosion of an old ice-front delta in the main valley induced a large slide from the eastern side of the valley, which dammed the river Gaula. Failure of the resulting sand and gravel dam would have developed very rapidly, compared to the clay dam postulated by previous workers, and would explain the dramatic results of the downstream deluge. Our data and interpretion fit oral tradition, accounts in the contemporaneous Icelandic Skålholt Annals, and the geological constraints better than previous models.     

历史时期黄河下游的淤积、决口改道及其与人类活动的关系

黄河是世界闻名的多沙性河流,历史时期就不断淤积、决口改道,且淤积速度不断加快,决口改道的频率也在不断增加。黄河下游地上河的发展和演变是自然因素和人类活动综合作用的结果。历史时期流域内人口的增加、耕作方式的改变及其对植被造成的破坏都对中游地区侵蚀的加剧、来沙的增加,以及下游地区河道的淤积及决口改道的发生带来不同程度的影响。     

无定河流域产沙量变化的淤地坝效应分析

无定河流域1971~1989年的年均流量、悬移质含沙量及输沙率比1954~1970年的明显变小,而月均水沙过程曲线也发生了明显变化。上述水沙过程的变化受到1970年以来人类活动的强烈影响,而淤地坝建设是主因。为探讨淤地坝的减沙效应,提出淤地坝有效减沙面积这一概念,并拟合了动态变化的淤地坝有效减沙面积与年份之间的关系,发现该关系曲线与无定河流域各年代产沙量的变化情况相符。自1990年以来无定河流域淤地坝有效减沙面积呈明显递减趋势,导致了自上世纪90年代以来该流域的产沙量出现增大现象。为了抑制该流域的产沙量,势必需要加大淤地坝建设的力度。如果想使该流域的产沙量逐渐减少,则至少要使流域内年淤地坝有效减沙面积逐年增加。     

Long-term morphodynamic evolution and the equilibrium mechanism of a flood channel in the Yangtze Estuary (China)

The stability of flood channels has attracted considerable attention because of their complicated interactions with the prevailing hydrodynamics and importance in ship navigation. This research examines long-term morphodynamic evolution in the Yangtze Estuary from 1861 to 2002 and the equilibrium mechanism of the Xinqiao Channel in the Yangtze Estuary by digitizing 15 selected maritime charts and calculating the volume of the channel. Although the total period of channel development is much longer than the historical data used in this paper, three stages are identified during the study period: the first embryonic stage (66 years), the second formation stage (33 years) and the third equilibrium stage (45 years). Variations in coastline location, channel volume, and hydrodynamics in the channel during the three stages indicate that the channel equilibrium was reached and maintained when the channel direction was aligned with the direction of offshore tidal wave propagation. Variations in river and sediment discharges affect erosion and deposition in the channel and thus channel geometry. However, future reduction in sediment supply by 10–33% due to the ongoing river engineering projects would increase the volume of the Xinqiao Channel only by 1–3%. It seems unlikely that the above change in sediment discharge will disrupt the equilibrium of the Xinqiao Channel.     

The influence of flood frequency, geomorphic setting and grazing on plant communities and plant biomass on a large dryland floodplain

Plant communities on semi-arid floodplains are ecologically important and support a diverse local and regional fauna and often pastoral economies. Water resource development may affect these communities and economies by decreasing water supply; determining the nature of these relationships is not straightforward because of the complex nature of plant responses to wetting and possible interactions with other drivers. We investigate the effects of reduced wetting on vegetation by examining spatial patterns in plant communities and above-ground herbaceous plant biomass across a flood frequency gradient, geomorphic settings and grazing exclosures. Community and biomass changes were also examined over time in relation to wetting events. The results demonstrate the importance of wetting on plant communities across timescales. At longer timescales, flood frequency influences community composition; at shorter timescales, wetting increase plant biomass and has a secondary influence on community composition. Plant biomass is also influenced in the short-term by grazing, but there is little influence of grazing on community composition. Soil nutrients do not vary systematically across the floodplain and have little influence on species distributions. We conclude that reduced water availability due to water resource development will result in reduced productivity in the short-term and community composition changes in the long-term.     

Field monitoring of the Corvara landslide (Dolomites, Italy) and its relevance for hazard assessment

The Corvara landslide is an active slow moving rotational earth slide - earth flow, located uphill of the village of Corvara in Badia, one of the main tourist centres in the Alta Badia valley in the Dolomites (Province of Bolzano, Italy). Present-day movements of the Corvara landslide cause National Road 244 and other infrastructures to be damaged on a yearly basis. The movements also give rise to more serious risk scenarios for some buildings located in front the toe of the landslide. For these reasons, the landslide has been under observation since 1997 with various field devices that enable slope movements to be monitored for hazard assessment purposes. Differential GPS measurements on a network of 47 benchmarks has shown that horizontal movements at the surface of the landslide have ranged from a few centimetres to more than 1 m between September 2001 and September 2002. Over the same period, vertical movements ranged from a few centimetres to about 10 cm, with the maximum displacement rate being recorded in the track zone and in the uppermost part of the accumulation lobe of the landslide. Borehole systems, such as inclinometers and TDR cables, have recorded similar rates of movement, with the depths of the major active shear surfaces ranging from 48 m to about 10 m. From these data, it is estimated that the active component of the landslide has a volume of about 50 million m³. In this paper the monitoring data collected so far are presented and discussed in detail to prove that the hazard for the Corvara landslide, considered as the product of yearly probability of occurrence and magnitude of the phenomenon, can be regarded has as medium or high if the velocity or alternatively the volume involved is considered. Finally, it is also concluded that the monitoring results obtained provide a sound basis on which to develop and validate numerical models, manage hazard and support the identification of viable passive and active mitigation measures.     

Riparian plant species richness along lateral and longitudinal gradients of water stress and flood disturbance, San Pedro River, Arizona, USA

Diversity theory predicts that species numbers should be highest at intermediate levels of both disturbance and environmental stress. We examined woody and herbaceous plant species richness and cover in the San Pedro River flood plain, along lateral gradients of water availability (ground-water depth), flood disturbance (inundation frequency), and distance from and elevation above the channel, and along longitudinal gradients of water availability (ground-water depth, surface flow permanence, and rainfall) and flood disturbance (total stream power). Herbaceous species were recorded during four sampling periods, and spatial patterns for this group were time-dependent, reflecting temporal variation in limiting factors. During the summer dry season of a dry year, when overall richness was low, richness and cover of herbaceous species declined laterally from the stream channel with increasing ground-water depth, consistent with the idea that low resource levels can limit species richness. Following the summer monsoon rains and floods, when water was less limiting and annuals were seasonally abundant, lateral patterns shifted such that herbaceous species richness and cover increased with increasing plot location above or from the channel. The relationship of herbaceous species richness with tree canopy cover also varied seasonally, shifting from positive (greater richness under canopy) in dry seasons to negative (lesser richness under canopy) in wet seasons. Longitudinally, herbaceous species richness and cover were limited primarily by stream flow and/or ground-water availability during the summer dry season of a dry year. Following the summer monsoon rains and floods, patterns were weighted by the seasonally abundant annuals, and richness increased among sites primarily with distance upstream (and related rainfall gradients). Richness and cover patterns also varied between years with different flood conditions. During the two sampling seasons in the year following a large flood, herbaceous species richness increased with flood disturbance intensity but declined at the few most intensely disturbed sites, consistent with intermediate disturbance theory.For woody species, richness within plant functional groups varied in opposing ways along the lateral gradients: hydromesic pioneer species decreased and hydromesic and xeric competitors increased with distance from or above the channel, with no overall change in species richness. Among sites, woody species richness patterns were related to water availability, but not to flood disturbance. However, richness of woody hydromesic pioneer species increased with both increasing site moisture and flood disturbance. Woody and herbaceous species richness both increased among sites as a function of increasing flood-plain width, likely due to species–area effects. Overall, results indicate that flood disturbance and water availability both influence species richness of riparian plants in the flood plain of this semi-arid region river, with the relative influence of each factor varying among plant groups and over time.     

Effects of large floods on sediment transport and reach morphology in the cobble-bed Sainte Marguerite River

Sediment transport rates were estimated for two flood events on the cobble-bed Sainte Marguerite River in the Saguenay region, Canada. Morphologic methods were used to derive one set of estimates, and a combination of the Meyer-Peter and Muller equation with a dimensionless sediment transport ratio (after Dietrich et al. [Nature 340 (1989) 215]) was used to derive another set of estimates. Both sets of estimates give consistent results for the first event (which had a decade-scale return period), and for the second event (which was the largest flood on record and had a century-scale return period). The transport occurring during the second event was an order of magnitude greater than that occurring during the first event: despite this disparity in the transport intensity of the two events, the channel morphology remained qualitatively similar. The observed degree of channel stability is attributed to a change of channel pattern and the initiation of bed degradation following channel rectification in the 1960s.     

永胜金沙江寨子村古滑坡和古堰塞湖的发现

在滇西北地区永胜南部金沙江的右岸,发现了总方量为2.5×108 m3的超大型古滑坡.滑坡后缘及两侧断壁为近直立的石灰岩峭壁,高5 ～30 m.滑床发育有规模不一的冲沟,覆盖有发育植被的坡积物、倒石堆和巨石.该滑坡曾造成过严重的堵江事件,在金沙江左岸仍然可以看到垄岗状残留滑坡坝,坝体方量约180×104 m3,滑坡坝和坝体后侧基岩界限清晰,两者之间岩性区别较大.滑坡堵江事件在上游形成了沿江连续分布长度约46 km的古堰塞湖沉积,沉积物主要由粉土、粉质粘土及粉细砂构成,偶夹卵石层,以水平层理为主,粉细砂层中见有小型交错层理,湖相沉积特征显著,而沉积物的树枝状平面分布进一步证实其形成于堰塞湖环境.