Depositional process and alluvial fan-drainage basin morphometric relationships near banff,Alberta, Canada

Two distinct types of alluvial fans occur in the Bow River Valley, Alberta, Canada: fluvially dominated and debris flow dominated. Large, gently sloping fans dominated by fluvial processes are associated with large and less rugged drainage basins, and small rugged basins have produced small, steep fans dominated by debris flow processes. Quantitative analysis demonstrates that strong fan-basin morphometric relationships occur despite a short fan history. Statistical analysis of fan area-basin area relationships indicate that debris flow fan areas do not increase in size as quickly as contributing basins. The relationship of fluvial fan area to basin area is not statistically significant. However, this relationship is probably affected by fan erosion. Examination of fan slope to basin ruggedness relationships indicates that fan slope increases more rapidly than basin ruggedness for both fan types. This is likely related to non-linear discharge and sediment size effects on fluvial fans, and reworking of larger fan surfaces by fluvial processes on debris flow fans.     

Geomorphological evolution and sediment stratigraphy of numerically simulated alluvial fans

When a sediment laden river reaches a flat basin area the coarse fraction of their sediment load is deposited in a cone shaped structure called an alluvial fan. In this article we used the State Space Soil Production and Assessment Model (SSSPAM) coupled landform–soilscape evolution model to simulate the development of alluvial fans in two- and three-dimensional landforms. In SSSPAM the physical processes of erosion and armouring, soil weathering and sediment deposition were modelled using state-space matrices, in both two and three dimensions. The results of the two-dimensional fan showed that the fan grew vertically and laterally keeping a concave up long profile. It also showed a downstream fining of the sediments along the fan profile. Both of these observations are in agreement with available literature concerning natural and experimental fan formations. Simulations with the three-dimensional landform produced a fan with a semicircular shape with concave up long profiles and concave down cross profiles which is typical for fans found in nature and ones developed in laboratory conditions. During the simulation the main channel which brings sediment to the fan structure changed its position constantly leading to the semicircular shape of the fan. This behaviour is similar to the autogenic process of ‘fanhead trenching’ which is the major mechanism of sediment redistribution while the fan is developing. The three-dimensional fan simulation also exhibited the downstream fining of sediments from the fan apex to the peripheries. Further, the simulated fan also developed complex internal sediment stratification which is modelled by SSSPAM. Currently such complex sediment stratification is thought to be a result of allogenic processes. However, this simulation shows that, such complex internal sediment structures can develop through autogenic processes as well. © 2020 John Wiley & Sons, Ltd.     

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.     

Influence of lithologic erodibility on alluvial fan area,western white mountains,California and Nevada

Differences in lithologic erodibility and sediment storage within a drainage basin affect the relationship between alluvial fan area and drainage basin area along the western White Mountains. Large fans are produced by basins underlain by resistant rocks, which have steep and narrow trunk stream canyons with little sediment in storage. Small fans are produced by basins composed of erodible lithologic units, which have wider valley floors, lower valley-side slopes, and considerably more sediment stored along trunk stream canyons than is the case in basins underlain by resistant rocks.     

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.     

Anthropic aggradation of the Waiho River,Westland, New Zealand: microscale modelling

Long‐term aggradation of the Waiho River, South Westland, New Zealand, has now raised the head of its alluvial fan to unprecedented elevations. In its natural state the river would, like all other major rivers in the area, be somewhat incised into its fanhead. The only relevant factor able to account for the aggradation is the presence of control banks (‘stopbanks’ in local parlance) that restrict the ability of the river to move over the whole of its natural fanhead. A 1 : 3333 scale physical hydraulic model (a ‘microscale’ model) was used to study this situation. An alluvial fan was generated in the model and allowed to develop to equilibrium with steady inputs of water and sediment within boundaries geometrically similar to those of the natural unrestricted Waiho River. The boundaries were then altered to represent the presence of the stopbanks, and the fan allowed to continue evolving under the same water and sediment inputs. The model fanhead aggraded in a spatial pattern similar to that recorded on the Waiho. Taking into consideration the limitations of microscale modelling, these results indicate that the aggradation in the Waiho is a result of the lateral restriction of the river by stopbanks. This poses fundamental questions about the variables that control the behaviour of alluvial fans. The results also suggest that microscale modelling can be used to make reliable quantitative predictions of the effects of engineering works on rivers, in spite of the low level of dynamic similarity with the prototype compared to that in larger‐scale models. Copyright © 2003 John Wiley & Sons, Ltd.     

A precipiton‐based approach to model hydro‐sedimentary hazards induced by large sediment supplies in alluvial fans

Mountain ranges are frequently subjected to mass wasting events triggered by storms or earthquakes and supply large volumes of sediment into river networks. Besides altering river dynamics, large sediment deliveries to alluvial fans are known to cause hydro‐sedimentary hazards such as flooding and river avulsion. Here we explore how the sediment supply history affects hydro‐sedimentary river and fan hazards, and how well can it be predicted given the uncertainties on boundary conditions. We use the 2D morphodynamic model Eros with a new 2D hydrodynamic model driven by a sequence of flood, a sediment entrainment/transport/deposition model and a bank erosion law. We first evaluate the model against a natural case: the 1999 Mount Adams rock avalanche and subsequent avulsion on the Poerua river fan (West Coast, New Zealand). By adjusting for the unknown sediment supply history, Eros predicts the evolution of the alluvial riverbed during the first post‐landslide stages within 30 cm. The model is subsequently used to infer how the sediment supply volume and rate control the fan aggradation patterns and associated hazards. Our results show that the total injected volume controls the overall levels of aggradation, but supply rates have a major control on the location of preferential deposition, avulsion and increased flooding risk. Fan re‐incision following exhaustion of the landslide‐derived sediment supply leads to sediment transfer and deposition downstream and poses similar, but delayed, hydro‐sedimentary hazards. Our results demonstrate that 2D morphodynamics models are able to capture the full range of hazards occurring in alluvial fans including river avulsion aggradation and floods. However, only ensemble simulations accounting for uncertainties in boundary conditions (e.g., discharge history, initial topography, grain size) as well as model realization (e.g., non‐linearities in hydro‐sedimentary processes) can be used to produce probabilistic hazards maps relevant for decision making. Copyright © 2017 John Wiley & Sons, Ltd.     

Topographic predictors of susceptibility to alluvial fan flooding,Southern Apennines

The flooding susceptibility of alluvial fans in the Southern Apennines has long been neglected. To partly address this oversight, we focus on the region of Campania which contains highly urbanized piedmont areas particularly vulnerable to flooding. Our findings are based on stratigraphic analysis of the fans and morphometric analysis of the basin‐fan systems. Using geomorphological analysis we recognized active alluvial fans while stratigraphic analysis together with statistical analysis of the morphometric variables was used to classify the fans in terms of the transport process involved. The results indicate that in the geological context examined, the best discrimination between debris flow (Df) and water flood (Wf) processes is achieved by means of two related variables, one for the basin (feeder channel inclination, Cg) and one for the fan (fan length, Fl). The probability that an unclassified fan belongs to group Wf is computed by applying a logistic function in which a P value exceeding 0.5 indicates that a basin/fan system belongs to group Wf. This important result led to the classification of the entire basin/fan system data. As regards process intensity, debris flow‐dominated fans are susceptible to the occurrence of flows with high viscosity and hence subject to more severe events than water flood‐dominated fans. Bearing this in mind, the data gathered in this study allow us to detect where alluvial fan flooding might occur and give information on the different degrees of susceptibility at a regional scale. Regrettably, urban development in recent decades has failed to take the presence of such alluvial fans into account due to the long recurrence time (50–100 years) between floods. This paper outlines the distribution of such susceptibility scenarios throughout the region, thereby constituting an initial step to implementing alluvial fan flooding control and mitigation. Copyright © 2012 John Wiley & Sons, Ltd.     

Hyperconcentrated flows on a forested alluvial fan of eastern Canada: geomorphic characteristics,return period,and triggering scenarios

The assessment of the dominant flow type on alluvial fans usually refers to two categories: debris‐flow fans (i.e. sediment gravity flows) and fluvial fans (i.e. fluid gravity flows). Here we report the results of combined morphometric, stratigraphic and sedimentological approaches which suggest that hyperconcentrated flows, a transitional process rheologically distinct from debris flows and floods and sometimes referred to as debris floods, mud floods, or transitional debris flows, are the dominant fan building process in eastern Canada. These flows produce transitional facies between those of debris flows which consist of a cohesive matrix‐supported diamicton, and those of river flows which display more distinct stratification. The size of the blocks in the channels and the abrasion scars at the base of several trees attest to the high transport capacity of these flows. The fan channels are routed according to various obstacles comprised primarily of woody debris that impede sediment transit. However, these conditions of sediment storage are combined with readily available sediment due to the friable nature of the local lithology. Tree‐ring analysis allowed the reconstruction of eight hydrogeomorphic events which are characterized by a return period of 9.25 years for the period 1934–2008, although most of the analyzed events occurred after 1970. Historical weather data analysis indicates that they were related to rare hydrometeorological events at regional and local scales. This evidence led to the elaboration of weather scenarios likely responsible for triggering flows on the fan. According to these scenarios, two distinct hydrologic regimes emerge: the torrential rainfall regime and the nival regime related to snowmelt processes. Hydrogeomorphic processes occurring in a cold‐temperate climate, and particularly on small forested alluvial fans of north‐eastern North America, should receive more attention from land managers given the hazard they represent, as well as because of their sensitivity to various meteorological parameters. Copyright © 2014 John Wiley & Sons, Ltd.     

A cellular model of Holocene upland river basin and alluvial fan evolution

The CAESAR (Cellular Automaton Evolutionary Slope And River) model is used to simulate the Holocene development of a small upland catchment (4·2 km²) and the alluvial fan at its base. The model operates at a 3 m grid scale and simulates every flood over the last 9200 years, using a rainfall record reconstructed from peat bog wetness indices and land cover history derived from palynological sources. Model results show that the simulated catchment sediment discharge above the alluvial fan closely follows the climate signal, but with an increase in the amplitude of response after deforestation. The important effects of sediment storage and remobilization are shown, and findings suggest that soil creep rates may be an important control on long term (>1000 years) temperate catchment sediment yield. The simulated alluvial fan shows a complex and episodic behaviour, with frequent avulsions across the fan surface. However, there appears to be no clear link between fan response and climate or land use changes suggesting that Holocene alluvial fan dynamics may be the result of phases of sediment storage and remobilization, or instabilities and thresholds within the fan itself. Copyright © 2002 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.     

Geometry of alluvial fans: Effect of discharge and sediment size

The slope of an alluvial fan increases with increasing debris size and sediment concentration in the flow, and decreases with increasing discharge. Laboratory studies suggest that the discharge which controls this slope, or dominant discharge, is that which is equalled or exceeded one quarter to one third of the time that flow occurs on the fan. In contrast, the dominant discharge in perennial alluvial rivers is equalled or exceeded only about 5 per cent of the time that flow occurs in the river. The dominant discharge on fans increases with increasing debris size, reflecting the importance of threshold stress. The slope of some natural and most laboratory alluvial fans is steepest on the flanks and gentlest along the axis. Consideration of the momentum of water debouching onto a fan at its apex suggests that the difference in slope between axis and flank should be greatest on steep fans composed of relatively non-cohesive materials because on such fans higher discharges tend to flow down the axis, whereas lower discharges can be turned to course down the flanks. On fans with gentle slopes or composed of more cohesive material the higher discharges can also be turned toward the flanks, so on such fans the difference in slope between the axis and flank is less pronounced. Field and laboratory observations support this interpretation. Because deposition at any one time on an alluvial fan is localized, some areas aggrade while others remain at a fixed elevation. This process is treated as a Markov process with the probability of diversion from an area of active deposition into an adjacent lower area increasing as the height of the active area above the mean or ‘ideal’ surface increases. Analysis of data from laboratory and natural fans suggests that the amplitude of such surface irregularities is greater on fans composed of coarser material. The data on natural fans also suggest an increase in amplitude of the irregularities with increasing fan area.     

Unravelling the patterns of alluvial fan development using mineral magnetic analysis: examples from the Sparta Basin,Lakonia, southern Greece

Mineral magnetic analysis of B‐horizons of soils developing upon the surfaces of alluvial fans was undertaken in order to: (i) differentiate and rank discrete fan surfaces by order of formation; (ii) establish whether fan surfaces formed simultaneously in adjacent fan systems; and (iii) deduce probable patterns of fan development. The results of the analysis indicate that the greatest concentration of ferrimagnetic and anti‐ferromagnetic minerals occurs within soils which have developed upon the proximal fan surfaces with a progressive reduction in magnetic minerals in soils associated with medial and distal surfaces. The build‐up of magnetic minerals in the proximal fan soils suggests that these surfaces formed first followed by the medial and distal surfaces. With the exception of the Kalivia Sokas fan, the majority of depositional events responsible for fan surface formation occurred simultaneously, suggesting that adjacent fan systems share broadly similar depositional histories. Although the precise timing of depositional events is uncertain, it is probable that by the end of the late Pleistocene, small, largely undissected fans comprising two to three surfaces had formed. At the start of the Holocene, fan systems experienced significant fanhead incision. A net distal extension of the fan trench coupled with a progressive basinward shift of the locus of deposition during the middle and late Holocene resulted in formation of medial and distal fan surfaces. Changes in climate are deemed to be the major control of fanhead incision, fan trenching and fan surface formation. However, the likely effects of long‐term tectonic activity and approximately five thousand years of human occupation upon fan development in the Sparta Basin remain unclear. Copyright © 2000 John Wiley & Sons, Ltd.     

THE ROLE OF ALLUVIAL FANS IN THE MOUNTAIN FLUVIAL SYSTEMS OF SOUTHEAST SPAIN: IMPLICATIONS OF CLIMATIC CHANGE

The mountain fluvial systems of southeast Spain involve sediment supply from steep mountain slopes into headwater channels. Alluvial fans often occur where these headwater channels emerge from the mountain areas, and may influence the connectivity of the sediment transport system from the mountain source areas to the main lowland drainages. Critical in this role is whether the alluvial fans are aggrading or dissecting, and whether there is a break or continuity in the channel through the fan environment. Previous work has identified some of the factors influencing the behaviour of the alluvial fans in southeast Spain. This paper deals with the mountain front alluvial fans in the semi-arid areas of Murcia and Almeria provinces. It attempts, by mapping the location of alluvial fans, then their classification into aggrading or dissecting fans, to identify the extent to which the mountain fluvial systems are buffered by aggrading alluvial fans or exhibit channel continuity through the mountain front environment. It further considers the implications of climatically induced changes between aggradational and dissectional behaviour on alluvial fans.     

Assessing morphological changes in a human-impacted alluvial system using hydro-sediment modeling and remote sensing

Construction of managed aquifer recharge structures(MARS)to store floodwater is a common strategy for storing depleted groundwater resources in arid and semi-arid regions,as part of integrated water resources management(IWRM).MARS divert surface water to groundwater,but this can affect downstream fluvial processes.The impact of MARS on fluvial processes was investigated in this study by combining remote sensing techniques with hydro-sediment modeling for the case of the Kaboutar-Ali-Chay aquifer,northwestern Iran.The impact of MARS on groundwater dynamics was assessed,sedimentation across the MARS was modeled using a 2D hydrodynamic model,and morphological changes were quantified in the human-impacted alluvial fan using Landsat time series data and statistical methods.Changes were detected by comparing data for the periods before(1985e1996)and after(1997 e2018)MARS construction.The results showed that the rate of groundwater depletion decreased from 2.14 m/yr before to 0.86 m/yr after MARS construction.Hydro-sediment modeling revealed that MARS ponds slowed water outflow,resulting in a severe decrease in sediment load which lead to a change from sediment deposition to sediment erosion in the alluvial fan.Morphometric analyses revealed decreasing alluvial fan area and indicated significant differences(p<0.01)between pre-and post-impact periods for different morphometric parameters analyzed.The rate of change in area of the Kaboutar-Ali-Chay alluvial fan changed from
0.228 to
0.115 km2/year between pre-and post-impact periods.     

Persistent alluvial fanhead trenching resulting from large,infrequent sediment inputs

We present laboratory and field evidence that in mountainous catchment‐fan systems persistent alluvial fanhead aggradation and trenching may result from infrequent, large sediment inputs. We suggest that the river‐fan systems along the fault‐bounded range front of the western Southern Alps, New Zealand, are likely to be in a dynamic equilibrium on ≥10³‐yr timescales, superimposed on which their fanheads undergo long‐term cumulative episodic aggradation. These fanheads are active only in rare events, do not take part in the usual behaviour of the catchment‐fan system and require much longer to exhibit dynamic equilibrium than the rest of the fan. These findings (1) increase our knowledge of the effects of extreme events on alluvial fan morphodynamics in humid climates, (2) question the general applicability of inferring past climatic or tectonic regimes from alluvial‐fan morphology and stratigraphy and (3) provide a conceptual basis for hazard zonation on alluvial fans. Copyright © 2006 John Wiley & Sons, Ltd.     

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.     

A numerical modelling and experimental study of flow width dynamics on alluvial fans

Alluvial fans are dynamic landforms, the evolution of which is controlled by both external environmental forcing (climate, tectonics and base level change) and internal process‐form feedbacks. The latter include changes in flow configuration (between sheetflow and channelized flow states), driven by aggradation and degradation, which may in turn promote changes in sediment transport capacity. Recent numerical modelling indicates that such feedbacks may lead to dramatic and persistent fan entrenchment in the absence of external forcing. However, the parameterization of flow width within such models is untested to date and is subject to considerable uncertainty. This paper presents results from an experimental study of flow width dynamics on an aggrading fan in which spatial and temporal patterns of fan inundation are monitored continuously using analysis of digital vertical photography. Observed flow widths are compared with results from a simple theoretical model developed for non‐equilibrium (aggradational) conditions. Results demonstrate that the theoretical model is capable of capturing the first‐order characteristics of width adjustment over the course of the experiment, and indicate that flow width is a function of fan aggradation rate. This illustrates that models of alluvial flow width derived for equilibrium conditions may have limited utility in non‐equilibrium situations, despite their widespread use to date. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

Controls on sediment evacuation from glacially modified and unmodified catchments in the eastern Sierra Nevada,California

The degree of glacial modification in small catchments along the eastern Sierra Nevada, California, controls the timing and pattern of sediment flux to the adjacent fans. There is a close relationship between the depth of fan‐head incision and the pattern and degree of Late Pleistocene catchment erosion by valley glaciers; catchments with significant glacial activity are associated with deeply incised fan heads, whereas fans emerging from glacially unmodified catchments are unincised. We suggest that the depth of fan‐head incision is controlled by the potential for sediment storage during relatively dry ice‐free periods, which in turn is related to the downstream length of the glacially modified valley and creation of accommodation through valley floor slope lowering and glacial valley overdeepening and widening. Significant storage in glacially modified basins during ice‐free periods leads to sediment supply‐limited conditions at the fan head and causes deep incision. In contrast, a lack of sediment trapping allows quasi‐continuous sediment supply to the fan and prevents incision of the fan head. Sediment evacuation rates should thus show large variations in glacially modified basins, with major peaks during glacial and lows during interglacial or ice‐free periods, respectively. In contrast, sediment removal from glacially unmodified catchments in this type of setting should be free of this effect, and will be dominated instead by short‐term variations, modulated for example by changes in vegetation cover or storm frequency. This distinction may help improve our understanding of long‐term sediment yields as a measure of erosional efficiency. Copyright © 2008 John Wiley & Sons, Ltd.