Exploring new methods to analyse spatial impact distributions on debris-flow fans using data from south-western British Columbia

Predicting the spatial impact of debris flows on fans is challenging due to complex runout behaviour. Debris flow mobility is highly variable and flows can sporadically avulse the channel. For hazard and risk assessments, practitioners typically base the probability of spatial impact or avulsion on their experience and expert judgement. To support decision-making with empirical observations, we studied spatial impact distributions on 30 active debris-flow fans in south-western British Columbia, Canada. We mapped 146 debris-flow impact areas over an average observation period of 74 years using orthorectified airphotos, satellite imagery, topographic base maps, LiDAR data, orthophotos, and field observations. We devised a graphical method to convert our geospatial mapping into spatial impact heat maps normalized by fan boundaries, enabling comparison of runout distributions across different fans. About 90% of the mapped debris flows reached beyond the mid-points of fans, while less than 10% avulsed more than half-way across the fan relative to the previous flow path. Most avulsions initiated at distances of 20% to 40% of the maximum fan length from the fan apex and upstream of the fan intersection point. Large volume events tend to be more mobile in the down-fan direction, but the relation between volume and cross-fan runout (e.g., avulsions) is more complex. Differences in spatial impact distributions can be explained, in part, by the degree of fan incision and whether a fan is truncated at its toe by a river or lake. There were no significant differences in spatial impact distributions based on the geology of the source area, sediment supply condition, or hydrogeomorphic process classification.     

EXTRACTING FEATURES OF ALLUVIAL FAN AND DISCUSSING LANDFORMS EVOLUTION BASED ON HIGH-RESOLUTION TOPOGRAPHY DATA: TAKING ALLUVIAL FAN OF LAOHUSHAN ALONG HAIYUAN FAULT ZONE AS AN INSTANCE

Range-front alluvial fan deposition in arid and semiarid environments records vast amounts of climatic and tectonic information. Differentiating and characterizing alluvial fan morphology is an important part in Quaternary alluvial fan research. Traditional method such as field observations is a most important part of deciphering and mapping the alluvial fan. Large-scale automatically mapping of alluvial fan stratigraphy before traditional field observations could provide guidance for mapping alluvial fan morphology, thus improving subsequent field work efficiency. In this research, high-resolution topographic data were used to quantify relief and roughness of alluvial fan within the Laohushan. These data suggest that mean surface roughness plotted against the size of the moving window is characterized by an initial increase in surface roughness with increased window size, but it shows no longer increase as a function of windows size. These data also suggest that alluvial fans in this study site smooth out with time until a threshold is crossed where roughness increases at greater wavelength with age as a result of surface runoff and headward tributary incision into the oldest surfaces which suggests the evolution process of alluvial fan. Researchers usually differentiate alluvial morphology by mapping characteristics of fan surface in the field by describing surface clast size, rock varnish accumulation, and desert pavement development and analysis of aerial photographs or satellite imagery. Recently, the emergence of high-resolution topographic data has renewed interest in the quantitative characterization of alluvial and colluvium landforms. Surface morphology that fan surface initially tends to become smoother with increasing age due to the formation of desert pavement and the degradation of bar-and-swale topography and subsequently, landforms become more dissected due to tectonics and climatic change induced increased erosion and channelization of the surface with time is widely used to distinguish alluvial fan types. Those characteristics would reflect various kinds of morphology metrics extracted from high-resolution topographic data. In the arid and semiarid regions of northwestern China, plenty of alluvial fans are preserved completely for lack of artificial reforming, and there exists sparse surface vegetation. In the meantime, range-front alluvial fan displaced by a number of active faults formed a series of dislocated landforms with different offsets which is a major reference mark in fault activity research. In this research, six map units(Qf₆-Qf₁), youngest to oldest, were observed in the study area by mapping performed by identifying geomorphic features in the field that are spatially discernible using hill-shade and digital orthophoto map. Alluvial fan relief and roughness were computed across multiple observation scales(2m×2m to 100m×100m)based on the topographic parameters of altitude difference and standard deviation of slope, curvature and aspect. In this research, mean relief keeps increasing with increased window size while mean surface roughness is characterized by a rapid increase over wavelengths of 6~15m, representing the typical length scale of bar-and-swale topography. At longer wavelengths, surface roughness values increase by only minor amounts, suggesting the topographic saturation length is 6~15m for those fan surfaces in which saturation length of standard deviation of curvature is less than 8m. Box and whisker plot of surface roughness averaged over 8m² for each alluvial fan unit in the study area suggests that the pattern of surfaces smoothing out with age and then starting to become rougher again as age increases further beyond Qf₄ or Qf₃ unit. The younger alluvial fan is characterized by prominent bar-and-swale while the older alluvial fan is characterized by tributaries headward incision. Cumulative frequency distributions of relief and surface roughness in Figure 8 are determined in an 8m by 8m moving window for the comparison of six alluvial fan units in the northeast piedmont of Laohushan. From these distributions we know that Qf₆ and Qf₁ reflect the prominent relief which is related to bar-and-swale and tributaries headward incision respectively, while Qf₄ and Qf₃ reflect the moderate relief which is related to subdued topography. Surface roughness, in addition to facilitating the characterization of individual fan units, lends insight to alluvial landform development. We summarize an alluvial landform evolutionary scheme which evolves four stages depending on characteristics of alluvial fan morphology development and features of relief and roughness. The initial stage in this study site is defined as the active alluvial fan channels with bars of coarse cobbles and boulders and swales consisting of finer-grained pebbles and sand which could be reflected by high mean relief and mean roughness values. As time goes, bar-and-swale topography is still present, but an immature pavement, composed of finer grained clasts, has started to form. In the third stage, the bar-and-swale topography on the fan surface is subdued, yet still observable, with clasts ranging from pebbles to cobbles in size and there exists obvious headward tributary incision. Eventually, tributary channels form from erosion by surface runoff. Headward incision of these tributaries wears down the steep walls of channels that are incised through the stable, planar surface, transforming the oldest alluvial landforms into convex hillslopes, leaving only small remnants of the planar surface intact. Those evolutionary character suggests that alluvial fans in this area smooth out with time, however, relief or roughness would be translated to increase at greater wavelength with age until a threshold is crossed. This research suggests that relief and roughness calculated from high-resolution topographic data of this study site could reflect alluvial fan morphology development and provide constraint data to differentiate alluvial fan unit.     

天山南缘北轮台断裂库尔楚段晚第四纪的最新活动和滑动速率

使用全站仪实测了天山南麓北轮台逆断裂库尔楚段阿克艾肯－喀腊萨喀腊阿塔木冲洪积扇断层崖地形地貌，采用^10Be测年方法测定了该冲洪积扇的形成年龄，得到该断裂段垂直位移滑动速率为0.48mm/a、0.85-1.0mm/a和1.52mm/a，并对该断裂段的水平缩短速率和天山第四纪水平缩短量进行了讨论。     

The declining but non-rejuvenating base level—The Lisan lake,the Dead Sea area,Israel

Laboratory experiments investigating the effects of a lowering base level do not simulate natural eustatic lowering along concave river profiles. The field data on this issue are also quite limited. In evaluating the control exercised by lowering base level on a drainage network, distinction must be made between its influence and those of other hydromorphological processes operating within the basin. Field data on morphological relations, based on photogrammetric mapping and longitudinal profiling, have been gathered in the Dead Sea area, Israel, where a rapid fall in base level has occurred during the Holocene. The study area is an entrenched fan delta with a sequence of 14 unpaired fan terraces which die out at intersection points within an alluvial fan system. The results suggest that the intersection points did not function as base levels. The receding base level played only a passive role, allowing entrenchment without transmitting a head-cutting feedback basinwards. The arid environment caused a delay in transmission of information through the system. The results support the model of short, episodic, and discontinuous erosional events, inherent in the evolution of drainage basins. It is suggested that base level effects in temperate and humid regions are not transferrable to arid zones.     

Mechanisms for avulsion on alluvial fans: Insights from high-frequency topographic data

Avulsion is a key process in building alluvial fans, but it is also a formidable natural hazard. Based on laboratory experiments monitored with novel high-frequency photogrammetry, we present a new model for avulsion on widely graded gravel fans. Previous experimental studies of alluvial fans have suggested that avulsion occurs in a periodic autogenic cycle, that is thought to be mediated by the gradient of the fan and fan-channel. However, those studies measured gradients at low spatial or temporal resolutions, which capture temporally or spatially averaged topographic evolution. Here, we present high-resolution (1 mm), high-frequency (1-minute) topographic data and orthophotos from an alluvial fan experiment. Avulsions in the experiment were rapid and, in contrast to some previous experimental studies, avulsion occurrence was aperiodic. Moreover, we found little evidence of the back-filling observed at coarser temporal and spatial resolutions. Our observations suggest that avulsion is disproportionately affected by sediment accumulation in the channel, particularly around larger, less mobile grains. Such in-channel deposition can cause channel shifting that interrupts the autogenic avulsion cycle, so that avulsions are aperiodic and their timing is more difficult to predict.     

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

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

Geomorphic response to an active transpressive regime: a case study along the Chaman strike‐slip fault,western Pakistan

The Chaman left‐lateral strike‐slip fault bounds the rigid Indian plate boundary at the western end of the Himalayan‐Tibetan orogen and is marked by contrasting topographic relief. Deformed landforms along the fault provide an excellent record for understanding this actively evolving intra‐continental strike‐slip fault. The geomorphic response of an active transpessional stretch of the Chaman fault was studied using digital elevation model (DEM) data integrated with Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) Visible and Near Infrared/Short Wave Infrared (VNIR/SWIR) and images from GeoEye‐1. Geologic and geomorphic mapping helped in reconstructing the Late Quaternary landscape history of this transpessional strand of the Chaman strike‐slip fault and the associated Spinatizha thrust fault in western Pakistan. Topographic analysis of a part of the transpression (the thrust bounded Roghani ridge) revealed northward growth of the Spinatizha fault with the presence of three water gaps and two corresponding wind gaps. Geomorphic indices including stream length‐gradient index, mountain front sinuosity, valley floor width to valley height ratios, and entrenchment of recent alluvial fan deposits were used to define the lateral growth and direction of propagation of the Spinatizha fault. Left‐lateral displacement along Chaman fault and uplift along the Spinatizha fault was defined using topographic analysis of the Roghani ridge and geomorphic mapping of an impressive alluvial fan, the Bostankaul fan. The landforms and structures record slip partitioning along the Indian plate boundary, and account for the convergence resulting from the difference in the Chaman fault azimuth and orientation of the velocity vector of the Indian plate. Copyright © 2012 John Wiley & Sons, Ltd.     

Emergent geomorphic–vegetation interactions on a subalpine alluvial fan

Following perturbation, an ecosystem (flora, fauna, soil) should evolve as a function of time at a rate conditioned by external variables (relief, climate, geology). More recently, biogeomorphologists have focused upon the notion of co‐development of geomorphic processes with ecosystems over very short through to very long (evolutionary) timescales. Alpine environments have been a particular focus of this co‐development. However, work in this field has tended to adopt a simplified view of the relationship between perturbation and succession, including: how the landform and ecosystem itself conditions the impact of a perturbation to create a complex spatial response impact; and how perturbations are not simply ecosystem destroyers but can be a significant source of ecosystem resources. What this means is that at the within landform scale, there may well be a complex and dynamic topographic and sedimentological template that co‐develops with soil, flora and fauna. Here, we present and test a conceptual model of this template for a subalpine alluvial fan. We combine detailed floristic inventory with soil inventory, determination of edaphic variables and analysis of historical aerial imagery. Spatial variation in the probability of perturbation of sites on the fan surface was associated with down fan variability in the across‐fan distribution of fan ages, fan surface channel characteristics and fan surface sedimentology. Floristic survey confirmed that these edaphic factors distinguished site floristic richness and plant communities up until the point that the soil–vegetation system was sufficiently developed to sustain plant communities regardless of edaphic conditions. Thus, the primary explanatory variable was the estimated age of each site, which could be tied back into perturbation history and its spatial expression due to the geometry of the fan: distinct plant communities were emergent both across fan and down fan, a distribution maintained by the way in which the fan dissipates potentially perturbing events. Copyright © 2015 John Wiley & Sons, Ltd.     

USING UAV PHOTOGRAMMETRY TECHNOLOGY TO EXTRACT INFORMATION OF TECTONIC ACTIVITY OF COMPLEX ALLUVIAL FAN——A CASE STUDY OF AN ALLUVIAL FAN IN THE SOUTHERN MARGIN OF BARKOL BASIN

Alluvial fans that are in the process of development always show complex geomorphic features due to natural modification. Accordingly, analyzing these fans whether to be influenced by tectonic deformation is one of the technique difficulties in active tectonic studies. Complex alluvial fans are the focus of the study of active tectonics such as fracture mapping and activity behavior analysis, for they have often retained important structural information. Traditional measurement methods, such as satellite remote sensing, RTK GPS and Lidar, are difficult to meet the demand for the study of micro tectonic deformation because of the reason of accuracy or cost performance. The recent UAV photogrammetry technology, due to its many advantages such as low cost, high resolution, and efficiency of exporting DEM and DOM data, has been widely used in three-dimensional modeling, ground mapping and other fields. In the quantitative study of active tectonics, this technology fills up the deficiency in the research of the micro structure of the traditional measurement. Through detailed field investigations and paleoseismic trenching, we further used this technology to obtain the topographic data of a complex alluvial fan located at the southern marginal fault of Barkol Basin, Xinjiang. Pointing at the alluvial fans that are in the process of development, and on the basis of topographic analysis and image processing for DEM, we take the research method of secondary partitions of the geomorphic surface and cut the alluvial fans longitudinally according to the difference of its age. Through the establishment of profile cluster within each partition, separate analysis and data contrast with the adjacent partitions, we acquired the tectonic activity information during the development of alluvial fan. The tectonic vertical deformation of this alluvial fan is about 2.5m.     

The influence of tectonic subsidence on volcaniclastic sedimentation: The Cretaceous upper Daeri Member,Wido Island,Korea

The Daeri Member, a Cretaceous volcanic–sedimentary succession, can be divided into lower, middle, and upper parts based on vertical changes in its lithologic characters. The lower Daeri Member is composed of siliciclastic deposits formed in a semi‐arid floodplain environment, which is overlain by the middle Daeri Member consisting mainly of andesite lava flow. After the emplacement of the andesite, activities of intrabasinal normal faults created accommodation on hanging wall blocks together with the development of intrabasinal topographic relief. The upper Daeri Member occurs only in hanging wall blocks and is composed of rhyolitic volcaniclastic sediments formed during an explosive volcanic eruption. Following the eruption, owing to semi‐arid climatic conditions and the destruction of vegetation, the eruptive materials were easily remobilized and deposited by episodic sediment gravity flows, resulting in deposition of the resedimented volcaniclastic deposits with sheet‐like geometry. Away from the intrabasinal normal faults, the resedimented volcaniclastic deposits show a decrease in grain size together with changes in inferred depositional processes from debris flows to hyperconcentrated flows and supercritical sheetfloods. This suggests that the resedimented volcaniclastic deposits were stacked on alluvial fan environments induced by intrabasinal topographic relief associated with normal fault activities. In addition, episodic movement of the faults gave rise to periodic fluctuation of the accommodation and an increase in gradient of the alluvial fan surface, resulting in the development of coarsening‐upward trends in the resedimented volcaniclastic deposits. The development of the alluvial fan and the coarsening‐upward trends indicate that dynamic tectonic subsidence and concomitant changes in the intrabasinal physiographic relief influenced the depositional processes and sizes of the transported volcaniclastic sediments of the upper Daeri Member. Thus, it is necessary to carefully observe tectonic signatures in volcaniclastic successions, particularly the syneruptive lithofacies, in order to reconstruct the tectonic and volcanic histories of receiving basins.     

Catchment lithology as a major control on alluvial megafan development,Kohrud Mountain range,central Iran

The relative importance of tectonics, climate, base level and source lithology as primary factors on alluvial‐fan evolution, fan morphology and sedimentary style remain in question. This study examines the role of catchment lithology on development and evolution of alluvial megafans (>30 km in length), along the flanks of the Kohrud Mountain range, NE Esfahan, central Iran. These fans toe out at axial basin river and playa‐fringe sediments towards the centre of basin and tectonics, climatic change and base‐level fluctuations, were consistent for their development. They formed in a tectonically active basin, under arid to semiarid climate and a long term (Plio‐Pleistocene to Recent) change from wetter to drier conditions. The key differences between two of these fans, Soh and Zefreh fans, along the west and south flanks of this mountain range, is that their catchments are underlain by dissimilar bedrock types. The source‐area lithologies of the Soh and Zefreh fans are in sedimentary and igneous terrains, respectively, and these fans developed their geometry mainly in response to different weathering intensities of their catchment bedrock lithologies. Fan surface mapping (based on 1/50000 topographic maps, satellite images, and fieldwork), reveals that the geomorphic evolution of these fans differs in that the relatively large‐scale incision and through trenching of the Soh fan is absent in the Zefreh fan. Whereas the limited sediment supply of the Soh fan has resulted in a deep incised channel, the Zefreh fan has remained aggradational with little or no trenching into proximal to medial fan surface due to its catchment bedrock geology, composed mainly by physically weathered volcaniclastic lithology and characterized by high sediment supply for delivery during episodic flash floods. Sediment supply, which is mainly a function of climate and source lithology, is a dominant driver behind the development of fan sequences in alluvial megafans. Copyright © 2012 John Wiley & Sons, Ltd.     

FAULTED LANDFORM AND SLIP RATE OF THE JINGHE SECTION OF THE BOLOKENU-AQIKEKUDUKE FAULT SINCE THE LATE PLEISTOCENE

The Bolokonu-Aqikekuduke fault zone(Bo-A Fault)is the plate convergence boundary between the middle and the northern Tianshan. Bo-A Fault is an inherited right-lateral strike-slip active fault and obliquely cuts the Tianshan Mountains to the northwest. Accurately constrained fault activity and slip rate is crucial for understanding the tectonic deformation mechanism, strain rate distribution and regional seismic hazard. Based on the interpretation of satellite remote sensing images and topographic surveys, this paper divides the alluvial fans in the southeast of Jinghe River into four phases, Fan1, Fan2, Fan3 and Fan4 by geomorphological elevation, water density, depth of cut, etc. This paper interprets gullies and terrace scarps by high-resolution LiDAR topographic data. Right-laterally offset gullies, fault scarps and terrace scarps are distributed in Fan1, Fan2b and Fan3. We have identified a total of 30 right-laterally offset gullies and terrace scarps. Minimum right-lateral displacement is about 6m and the maximum right-lateral displacements are(414±10)m, (91±5)m and(39±1)m on Fan2b, Fan3a and Fan3b. The landform scarp dividing Fan2b and Fan3a is offset right-laterally by (212±11)m. Combining the work done by the predecessors in the northern foothills of the Tianshan Mountains with Guliya ice core climate curve, this paper concludes that the undercut age of alluvial fan are 56~64ka, 35~41ka, 10~14ka in the Tianshan Mountains. The slip rate of Bo-A Fault since the formation of the Fan2b, Fan3a and Fan3b of the alluvial-proluvial fan is 3.3~3.7mm/a, 2.2~2.6mm/a and 2.7~3.9mm/a. The right-lateral strike-slip rate since the late Pleistocene is obtained to be 3.1±0.3mm/a based on high-resolution LiDAR topographic data and Monte Carlo analysis.     

Building damage concentrated in Longtoushan town during the 2014 Ms. 6.5 Ludian earthquake,Yunnan, China: examination of cause and implications based on ground motion and vulnerability analyses

Though the 2014 Ludian Earthquake had only a moderate magnitude (Ms 6.5), high-level ground motions of almost 1g occurred at Longtoushan Town (seismic station 53LLT), which located near the intersection of a conjugate-shaped seismogenic fault. The building damages on the pluvial fan and the river terrace at Longtoushan was clearly different. In order to examine the generation of the large acceleration at 53LLT, the focal mechanisms and the rupture processes of the conjugate-shaped seismogenic fault were determined. We found that there were two continuous impulsive waves in the records of 53LLT that were generated from two different faults, the Baogunao fault and the Xiaohe fault, respectively. Site effects on the pluvial fan and the river terrace at Longtoushan Town and their relations to different building damages were examined. We found that the predominant period at the pluvial fan was about 0.25 s, close to the fundamental natural period of multi-story confined masonry buildings. Ground motions on the pluvial fan and the river terrace were simulated through convolving synthesized bedrock motions with the transfer functions, which were analyzed using the one-dimensional underground velocity structures identified from H/V spectral ratios of ambient noise. Building collapse ratios (CRs) are estimated based on the vulnerability function of the 2008 Wenchuan Earthquake and are compared with the observed values. We found that the observed building CRs on the pluvial fan are much higher than the estimated values. High-level ground shaking that is far beyond the design level was a reason for serious building damage.     

Erosion of fixed dunes in the Sahel,Central Niger

During the 1974 rainy season gullies of the order of 150–300 m long were active on the flanks of fixed Pleistocene dunes in the vicinity of Janjari, central Niger. The gullies terminate on small (0.5–1 × 10⁴m²) alluvial fans, where sand deposition had occurred below the intersection point. Gully activity was probably the result of a particularly severe storm. Comparison between fixed dunes subject to differing climatic conditions suggests that gullying and alluvial fan formation may be characteristic of a restricted morphoclimatic zone.     

基于微型无人机摄影技术的微构造信息提取研究——以博-阿断裂乌苏通沟东岸为例

利用微型无人机摄影测量技术,获取了博-阿断裂在乌苏通沟东岸的高精度地形、地貌数据,解译DEM数据,并结合野外调查工作,明确了断裂在乌苏通沟东岸冲洪积扇上19.3～31.1m的水平位错。分析获取的陡坎剖面,且对比陡坎两侧地貌的剥蚀程度,认为陡坎形成后受到后期水流的侵蚀,部分陡坎的高度在一定程度上被放大,断裂的实际垂直位错在0.7m左右。通过实例展示了无人机摄影技术在活动构造研究中的巨大潜力以及在微构造信息提取中的独特优势。     

A fluid dynamics approach to modelling the 18th March 2007 lahar at Mt. Ruapehu,New Zealand

Lahars are water-sediment mass flows from a volcanic source. They can be triggered by a variety of mechanisms and span a continuum of flow rheology and hydraulic properties, even within the same event. Lahars are extremely powerful landscaping agents and represent a considerable hazard potential. However, this highly dynamic character and a lack of direct measurements has made modelling lahars difficult. This study therefore applies a fluid dynamics model; Delft3D, to analyse the 18th March 2007 dam break lahar at Mount Ruapehu, New Zealand. The modelled lahar routed through the Whangaehu gorge in ~30 min, crossed the Whangaehu fan in ~60 min, and then over a further 3 h travelled an additional ~22 km distance along the Whangaehu River to the Tangiwai bridge. The modelled mean frontal velocity was 6.5 m s⁻¹ along the gorge although peak velocity reached up to 19.6 m s⁻¹. The modelled lahar flow front progressively slowed across the fan but along the River it accelerated from 2.1–3.3 m s⁻¹. Calculated peak velocity along the River was <4.5 m s⁻¹. These results generally compare well with gauged records, with historical records, and with other modelling approaches. However, discrepancies in frontal velocity and time to peak stage arise due to (1) specifying roughness, which arises from slope variations between adjacent computational nodes, and which is stage-dependant, and (2) due to rapid topographic changes that produce frequent hydraulic jumps, which are inadequately accommodated in the numerical scheme. The overall pattern of discharge attenuation, and of relationships between topographic and hydraulic variables, is similar to that calculated for lahars on other volcanoes. This modelling method could be applied at other similar sites where a likely source hydrograph and high-resolution topographic data are available. These results have important implications for hazard management at Ruapehu and for examining geomorphic and sedimentary impacts of this lahar.     

The role of biota in the initiation and growth of islands on the floodplain of the Okavango alluvial fan,Botswana

A group of islands of varying size on the floodplain of the Okavango alluvial fan, were studied to establish the processes which lead to the initiation and growth of islands. It was found that islands are initiated by the mound-building activities of the termite Macrotermes michaelseni. These termites import fine grained materials to use as a mortar for the construction of epigeal mounds. Their activities create a topographic feature, raised above the level of seasonal flooding, and also change the physical properties and nutrient status of the mound soil. Shrubs and trees are able to colonize these mounds, which results in increased transpiration. As a result, precipitation of calcite and silica from the shallow ground water occurs preferentially beneath the mounds, resulting in vertical and especially lateral growth, causing island expansion. © 1998 John Wiley & Sons, Ltd.     

Geological setting of hydrothermal activity at 12°50′N on the East Pacific Rise: A submersible study

A detailed submersible investigation of a 20-km segment of the East Pacific Rise near 12°50′N between the Orozco and Clipperton fracture zones has resulted in the localization of 24 active hydrothermal vent fields and over 80 sites of sulfide accumulations. The active vents range from low-temperature vents characterized by exotic benthic communities to high-temperature “black smokers” and the deposition of polymetallic sulfides. The study is based upon a combination of fine scale topography obtained using the SEABEAM sonar system on N/O “Jean Charcot”, camera lowerings along the axis using the RAIE vehicle, and 32 dives by the submersible “Cyana” operating from N/O “Le Suroit”. The observations made between the Orozco and Clipperton fracture zones show topographic highs situated along the strike of the accreting plate segment separated by a small ridge offset at 11°49′N. This offset divides this portion of the ridge into two separate spreading segments each of which has a primary topographic high along strike. Secondary highs are associated with each segment of the ridge separated by either small offsets (or relay zones) or in some cases, zones where spreading centers overlap. Dives made on the tops of both primary highs (12°50′N and 11°30′N) confirm the presence inferred from previous surface work of high-temperature vent fields while one reconnaissance dive (14°20′N) near the Orozco fracture zone/ridge axis intersection reveals the absence of any hydrothermal activity in the present or recent past. The vast majority of vent fields investigated were found at the topographic high near 12°50′N, are associated with the most recent period of volcanism, and are confined to lava ponds situated within the axial graben.     

Application of kinematic wave theory for predicting flash flood hazards on coupled alluvial fan–piedmont plain landforms

A rainstorm that caused a severe flash flood on the piedmont plain at the toe positions of two alluvial fans located to the west of the Organ Mountains in Dona Ana County, New Mexico, USA, is analysed. The space–time distributions of rainfall are evaluated from the Next Generation Weather Radar (NEXRAD) and overland flow is modelled as kinematic wave. The spatial distribution of rainfall shows a topographic control. The greatest rainfall depth, duration, and intensity occurred at the higher elevation mountain slopes and decreased with decreasing elevation from the alluvial fans to the piedmont plain. The alluvial fan–piedmont plain system is modelled by coupling divergent and rectangular overland flow planes. Explicit finite difference approximations, hybridized with the analytical method of characteristics, are made to the kinematic wave equations to account for the spatial and temporal distribution of the rainfall and variable boundary conditions. Simulation results indicate that sheet‐flow floodwater elevations rise (1) in a nonlinear fashion from the apex to toe positions of the alluvial fans, and (2) near linearly from the toe positions of the alluvial fans onto the piedmont plains with the formation of kinematic shocks near the middle to the upstream end of the plane at times between the initiation of the rainstorm and the time of concentration of the plane. Thus, the maximum flooding occurs at the middle or upstream sections of the piedmont plains regardless of the pattern of space–time variability of rainfall. These results are in agreement with observed geomorphologic features suggesting that piedmont plains are naturally flood‐prone areas. This case study demonstrates that flood hazards on piedmont plains can exceed those on alluvial fans. The models presented in this study suggest that the flood hazard zones on coupled alluvial fan–piedmont plain landforms should be delineated transverse to the flow directions, as opposed to the flood hazard zones with boundaries in the longitudinal direction of the axis of an alluvial fan. Copyright © 2003 John Wiley & Sons, Ltd.     

Possible Coseismic Large-scale Landslide off the Northern Coast of Papua New Guinea in July 1998: Geophysical and Geological Results from SOS Cruises

— The epicentral area of the 1998 Papua New Guinea Earthquake and Tsunami off Sissano Lagoon, northern coast of Papua New Guinea (M=7.1; 17 July, 1998) was surveyed by the Research Vessel Kairei in January 1999. Precise bathymetric survey by use of SEABEAM-2112 and other geophysical surveys (bottom reflectivity, sub-bottom profiling, surface ship gravity and geomagnetic surveys) were carried out and four piston core samples were collected during the nine days' survey. The area was also surveyed by use of the Japanese deep sea research ROV Dolphin-3K and research submersible Shinkai-2000 after the regional geophysical survey, in order to locate the possible seismic faults and/or underwater landslides as the source of tsunami and to study the process which took place off Sissano Lagoon and the driving force of the event. The study area is characterised by substantial fan sediment supply from Sissano Lagoon. Straight small-scale submarine canyons and valleys are eroding the shelf slope constructed by the fan sediment. Topographic features of arcuate slumps caused by landslides are recognised at numerous sites of the study area. Most of them are old and the most recent is located 25 km northeast off the Sissano Lagoon. Two major topographic depressions on the shelf were located off Sissano Lagoon. The western is a depression of about 10-km width and the eastern is a meandering deep-sea canyon. Amphitheatre topographic features with slumps caused by landslides that were discovered by Kairei's cruise were extensively surveyed in the study area. Six dives by Dolphin-3K and seven dives by Shinkai-2000 revealed that a fresh crack of about 15 km in total length is located along the slope of the amphitheatre. The crack is apparently the upper slope of a large-scale slumping on the amphitheatre, characterised by tensional stress on it. The easternmost part of the fresh crack is accompanied by living chemosynthetic organisms such as mussels and tube worms. The chemosynthetic community is apparently associated with cold seepage along the crack suggesting that the crack was constructed very recently. The areas other than the crack were rather old with bioturbation.