Delineation of alluvial fans from Digital Elevation

Geomorphology 273 (2016) 134–149

Contents lists available at ScienceDirect

Geomorphology journal homepage: www.elsevier.com/locate/geomorph

Delineation of alluvial fans from Digital Elevation Models with a GIS algorithm for the geomorphological mapping of the Earth and Mars Gianluca Norini a,⁎, Maria Clara Zuluaga b, Iris Jill Ortiz c,d, Dakila T. Aquino c, Alfredo Mahar F. Lagmay c,d a

Istituto per la Dinamica dei Processi Ambientali - Sezione di Milano, Consiglio Nazionale delle Ricerche, Milano, Italy Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Napoli, Italy c Nationwide Operational Assessment of Hazards, Department of Science and Technology, Philippines d National Institute of Geological Sciences, University of the Philippines, Diliman, Quezon City, Philippines b

a r t i c l e

i n f o

Article history: Received 1 October 2015 Received in revised form 4 August 2016 Accepted 5 August 2016 Available online 6 August 2016 Keywords: Geomorphological mapping Geologic hazard assessment Geographic Information Systems Geomorphology of Mars

a b s t r a c t Alluvial fans are prominent depositional geomorphic features present in nearly all global climates on Earth, and also found on Mars. In this study, we present a Geographic Information System (GIS) algorithm designed for the semi-automated detection of alluvial fans that are connected to their contributing upstream drainage network, from the analysis of Digital Elevation Models (DEMs). Through a combination of spatial analysis procedures, the GIS algorithm generates maps of alluvial fans and their upstream source drainage and watersheds. Tests of the algorithm in areas with well-known alluvial fans indicate that this new GIS procedure is capable of high-accuracy mapping of the fan apexes and correct delineation of fan deposits, in both arid and humid climates. Possible future applications of the GIS algorithm presented in this study include the systematic survey of alluvial fans at the local, regional and planetary scales, important for geologic hazard assessment, studies on the evolution of climate, analysis of continental sedimentary environments, understanding of the interplay between the endogenous dynamics and exogenous processes, and the evaluation of natural resources. © 2016 Elsevier B.V. All rights reserved.

1. Introduction The extraction of geologic information from large topographic datasets is an important issue for the study of Earth and other terrestrial planets, especially with the growing availability of Digital Elevation Models (DEMs) at any resolution and the need to provide reliable and homogeneous mapping of geologic features over wide areas (e.g. regional, country, continental or planetary scale) (e.g. Fleming et al., 2010). Tools have been developed for unsupervised or supervised geologic analysis of DEMs for systematic and objective mapping of volcanic edifices, faults, subglacial bedforms, slope instability features and alluvial landforms. With the aim to rapidly and efficiently survey large areas and improve knowledge of their evolution, geological hazard and possible natural resources, DEMs are analyzed based on measured ranges of elevation, slope, curvature, aspect, texture, contour shape and drainage pattern and their association with specific geomorphologic features, like volcanic and alluvial landforms (e.g. Miliaresis and Argialas, 2000; van Asselen and Seijmonsbergen, 2006; Gloaguen et al., 2007; Minár and Evans, 2008; Roberts and Cunningham, 2008; Crippen, 2010; Van Den Eeckhaut et al., 2012; Euillades et al., 2013; Di Traglia et al., 2014; Eisank et al., 2014).

⁎ Corresponding author. E-mail address: [email protected] (G. Norini).

http://dx.doi.org/10.1016/j.geomorph.2016.08.010 0169-555X/© 2016 Elsevier B.V. All rights reserved.

Some of the most powerful geomorphologic processes are erosion, transport and deposition of alluvial sediments along river systems, which are active over wide areas of the Earth's surface. Similar processes have also been documented on the surface of Mars, manifested as relict landforms generated by water flow and alluvial sediment transport and emplacement during past periods of humid climate (e.g. Moore and Howard, 2005; Kraal et al., 2008). On the surface of the Earth, the systematic survey of alluvial landforms over large regions is of particular importance for mapping geologic hazards (e.g. flooding and debris flows), and for studying the complex interplay between endogenous dynamics (e.g. tectonics) and exogenous processes (e.g. erosion and emplacement of alluvial deposits) controlling the geological evolution of the crust (e.g. Roberts and Cunningham, 2008). Among the alluvial landforms, alluvial fans are prominent depositional geomorphic features present in nearly all global climates. They are also found on Mars and on Titan, the largest moon of Saturn, where they are observed in long river systems, deep impact craters and at the base of steep slopes (e.g. Harvey, 1997; Moore and Howard, 2005; Kraal et al., 2008; Blair and McPherson, 2009; Radebaugh et al., 2013). Alluvial fans are located downstream of watersheds, in which sediments transported via tributaries emerge in a radial fan-shaped pattern from a point of topographic change (Drew, 1873; Bull, 1977; Oguchi and Ohmori, 1994; Field, 2001; Viseras et al., 2003; FEMA, 2003; Saito and Oguchi, 2005; Goswami et al., 2009; Lancaster et al., 2012; Bahrami, 2013) (Fig. 1). The mapping of these alluvial landforms is usually straightforward in desert regions,

G. Norini et al. / Geomorphology 273 (2016) 134–149

135

Fig. 1. Schematic representation of a typical active alluvial fan at a mountain front.

but can be very complex and uncertain in forested areas. Moreover, the systematic survey of alluvial fans in large regions require time-consuming photogeologic interpretation and/or extensive fieldwork (e.g. Pandey, 1987; Foster and Beaumont, 1992). This work presents a new Geographic Information System (GIS) algorithm that was motivated by the recent occurrence of debris flows in Barangay Andap, Philippines, spawned by rains from Supertyphoon Pablo (Lagmay et al., 2013). The algorithm is designed for the analysis of large DEMs and the detection of alluvial fans and their upstream source drainage and watersheds. The GIS algorithm identifies alluvial fans that are connected to their contributing upstream drainage network, which generally refer to active alluvial fans in any climate or to abandoned alluvial fans in areas that shifted to dry climate. Through a combination of spatial analysis procedures, the GIS algorithm presented in this study attempts to map out all the alluvial fan apexes and delineate the extension of the corresponding alluvial fan deposits in a DEM. In this paper, the proposed GIS algorithm is first described, then tested over two areas with well-known alluvial fans, and finally applied to case studies from both Earth and Mars. The method is based on the analysis of the drainage network, the reclassification of slope and curvature values and the reconstruction of the conical shape of the alluvial fans over the topographic surface. The main aim of this paper is to contribute to the systematic, rapid, reliable and reproducible mapping of geomorphological and geological features, useful for the analysis of the geological evolution, hazard and resources of wide regions of the Earth and other terrestrial planets. 2. Background: alluvial fans and geomorphological mapping The development of an alluvial fan requires certain conditions, including: (1) a topographic setting wherein the contributing channel from the catchment area drains to a relatively flat lowland (adjoining valley or alluvial plain); (2) a sufficiently large source of sediment from an upland area (mountainous catchment); (3) a triggering mechanism that can induce a discharge of water to transport sediments from the drainage basin to the fan; and (4) a topographic breakpoint with a sudden drop in gradient that causes sediments to fan out (Bull, 1977; Schick and Lekach, 1987; Webb et al., 1987; Blair and McPherson, 1994, 2009; Harvey, 1997, 2003; Mather et al., 2000; Leeder and Mack, 2001; Florsheim, 2004). The topographic break, where the fan apex is located, corresponds with the downstream transition from a

laterally confined stream channel to non-confined lowland. This type of topographic configuration requires a feeder channel upstream of the fan apex that is in a valley flanked by steep slopes (Figs. 1 and 2). Thus, the apex of an alluvial fan marks the change of slope profiles as measured perpendicular to the drainage channel, from steep valley flanks upstream (Profile X–X′ in Fig. 2B), to gentle and unconfined lowland slopes downstream from the apex (Profile Y–Y′ in Fig. 2B). In contrast, when measured parallel to the drainage channel, there is usually less pronounced or evident change across the alluvial fan apex (Profile Z–Z′ in Fig. 2B). This morphology suggests that alluvial fan deposition primarily is the result of the expansion of stream paths, allowing stream current and alluvial sediments to laterally spread (Schick and Lekach, 1987; Blair and McPherson, 2009; Lancaster et al., 2012). At the topographic breakpoint, the alluvial fan apex marks a pronounced change in the flow regime, inducing the aggradational emplacement of sediment deposits and the buildup of the alluvial fan, giving rise to a unique sedimentary environment with a prominent geomorphological character. 2.1. Geomorphological features of alluvial fans Many studies have been devoted to analyze the shape and internal structure of alluvial fans, especially for those located in arid and semiarid regions. These studies describe alluvial fans as conspicuous landforms built by the aggradation of alluvial sediments, downslope from the emergence of a channel from a mountainous drainage basin (e.g. Bull, 1977; Harvey et al., 2005). The alluvial fan sedimentary deposits have a semi-conical shape radiating downslope from the alluvial fan apex, located at the change in slope from the mountain catchment to a contiguous valley or alluvial plain. Thus, alluvial fans have a mostly arcuate shape in plan view, spanning outward in a semi-circle centered on the fan apex (e.g. Troeh, 1965; Gomez Villar, 1996; Clevis et al., 2003; Harvey et al., 2005; Saito and Oguchi, 2005; Sánchez-Núñez et al., 2015) (Figs. 1 and 2A). If confined or channeled by adjacent topographic barriers, their plan view can deviate from the 180° semicircular shape of unconfined alluvial fans, assuming more complex morphologies (Fig. 3). At the front of a linear mountain range, a sequence of adjacent alluvial fans laterally coalesces to form a bajada (e.g. Sorriso-Valvo et al., 1998; Sánchez-Núñez et al., 2015) (Fig. 3). An alluvial fan consists of a fan apex, an incised stream channel, inactive fan lobes and the active depositional lobe (Fig. 1) (e.g. Drew,

136


Fig. 2. Morphological configuration of a major alluvial fan in the Lake Alakol region, Kazakhstan. (A) Satellite view of the alluvial fan, the traces of the X–X′, Y–Y′ and Z–Z′ topographic profiles are shown. (B) X–X′ topographic profile across the feeder channel, Y–Y′ topographic profile across the alluvial fan, and Z–Z′ topographic profile along the feeder channel, alluvial fan and alluvial plain.

1873; Eckis, 1928; Hooke, 1967; Bull, 1977; Blair and McPherson, 1994, 2009). The fan apex is the highest and most proximal point of the alluvial fan. The incised channel, which is not always present, is a prolongation of the feeder stream channel downslope from the fan apex into the

alluvial fan. Inactive fan lobes and the active depositional lobe, usually at the mouth of the incised channel, constitute the surface of an alluvial fan (Fig. 1). In addition, an end point can be identified at the intersection between the main stream channel and the distal edge of the alluvial fan


137

Fig. 3. Schematic representation of the possible plan-view shape of alluvial fans, showing the configuration of unconfined fans, confined fans and bajadas.

(Figs. 1 and 3). As viewed in the radial profile, the end point is located at the intersection of the alluvial fan radius with the fan edge (Fig. 2). The alluvial fan is normally bordered by an alluvial plain behind the distal edge and the end point (Figs. 1 and 3). Alluvial fans are built and shaped by different transport and depositional mechanisms of sediments, from the catchment to the fan (primary processes) and within the fan (secondary processes). The transport mechanisms (e.g. rock avalanches, debris flows, and sheetfloods) emplace sediments on a range of slope angles, depending on the dominant depositional processes. These processes are influenced by bedrock lithology, catchment shape, neighboring environments, climate and tectonics (Blair and McPherson, 2009). The average slope of alluvial fans ranges from 2° to 35°, with most between 2° and 20° (e.g. Blissenbach, 1954; Anstey, 1965; Hooke, 1968; Bull, 1977; Pierson and Costa, 1987; Wells and Harvey, 1987; Ritter et al., 1995, 2000; NCR, 1996; Calvache et al., 1997; USGS, 2002; Harvey, 2003; Staley et al., 2006; Blair and McPherson, 2009; Lancaster et al., 2012). The slope of the distal portions of alluvial fans can also vary from N5° in arid regions to b 1° in humid regions (e.g. Boothroyd, 1972; Saito and Oguchi, 2005). For this reason, no general radial slope range exists to definitively classify the alluvial fans formed in different climate environments and convincingly separate them from the adjacent sedimentary environments. Specific morphometric attributes of alluvial fans have been related to the morphology of their catchment areas. In addition to slope, the most important morphometric relationship is the positive correlation between the plan view area of fans and their catchments. Small fans are generated by small catchments and large fans are generated by large catchments, as suggested by Bull (1962), Lustig (1965), Hooke (1968), Oguchi and Ohmori (1994); Harvey (1997) and Mather et al. (2000). These studies also suggest that the stream lengths within the catchment area may be correlated to the stream lengths within the fan area, as well as to the extent of the alluvial fan deposits. An analysis of a large database of alluvial fans in different regions show that these correlations are distorted because of the wide range of transport and depositional processes active within the catchments and the alluvial fans (Blair and McPherson, 2009). In addition, because of steep slopes, catchment areas measured in plan-view can differ significantly from the surface area, with further dispersion of the area–area relationship. Finally, catchments and alluvial fans are dynamic features, which change shape and extent with time (Blair and McPherson, 2009). For this reason, predicting the alluvial fan area and extension based on

measurements of its catchment is not straightforward but depends instead on many variables, like transport and depositional mechanisms, vertical dimension of the catchment, and temporal evolution of the source watershed and alluvial fan. 2.2. Mapping of alluvial fans Alluvial fans consist of prominent deposits, distinguishable from other sedimentary environments on the basis of their morphology. The inclined semi-conical shape of alluvial fans, with distributary drainage patterns, makes it possible to have an effective mapping through the analysis of topographic data and remote sensing imagery, especially in arid and semi-arid regions. Furthermore, the surface texture (e.g. roughness) of the deposits of alluvial fans can differ from the surface textures in adjacent areas (source catchment and alluvial plain), because of the different transport and depositional mechanisms that dominate these sedimentary environments (e.g. Cavalli and Marchi, 2008). Thus, the overall shape, slope, surface texture and drainage pattern are the main factors that can be used to delineate alluvial fans (e.g. Pandey, 1987; Foster and Beaumont, 1992; Blair and McPherson, 1994; Miliaresis and Argialas, 2000; Argialas and Tzotsos, 2004; Roberts and Cunningham, 2008). The most common means of identifying and mapping alluvial fans is through visual interpretation of aerial photographs, topographic maps, DEMs and satellite imagery, which is time-consuming and dependent upon the skills of the individual analyst (e.g. Pandey, 1987; Foster and Beaumont, 1992). Also, the visual technique is more difficult to apply in tropical regions, where dense vegetation obstructs the remotelysensed imagery of the ground. Some studies have proposed the automatic detection and mapping of alluvial fans through the analysis of topographic and optical multispectral datasets, as an objective and faster alternative to visual interpretation. These techniques are based on slope thresholds, topographic roughness or object-oriented feature extraction and multi-resolution segmentation of DEMs and optical multispectral satellite imagery (e.g. Miliaresis and Argialas, 2000; Argialas and Tzotsos, 2004, 2006; Roberts and Cunningham, 2008). The use of slope thresholds is based on the assumption that, for a specific region with certain climate and lithology, mountain ranges are usually steeper and adjacent basins and alluvial plains are commonly gentler than the expected slope values of alluvial fans (Roberts and Cunningham, 2008). The main drawback of using slope thresholds for alluvial fan mapping is that certain slope values, usually associated with alluvial

138


fans, also occurs in the adjacent areas. One good example are colluvial deposits at the mountain range fronts and in mountainous catchments and surrounding highlands. Thus, the use of slope thresholds alone generates a “noisy” alluvial fan map, which requires further visual reinterpretation to remove false positive identification of fan areas. Similarly, the discrimination of alluvial fans using surface texture thresholds has the same drawback, which also requires further visual reinterpretation (Riley et al., 1999; Roberts and Cunningham, 2008). Furthermore, the calculation of both roughness and slope may be affected by DEM quality, which depends on the techniques used to interpolate and derive the elevation datasets (i.e. contour interpolation, digital photogrammetry, radar, etc.), oftentimes with negative effects on the accuracy of the alluvial fan maps (e.g. Bolstad and Stowe, 1994; Robinson, 1994; Walker and Willgoose, 1999; Wang et al., 2011). Combined drainage, slope and spectral signature analysis have been proposed by Miliaresis and Argialas (2000) and Miliaresis (2001) to map outflow points along the drainage network, running a slopebased iterative process to draw alluvial fan polygons downslope from the outflow points, and then identifying the alluvial fan-toes by analysis of digital values of a satellite multispectral optical datasets. More recently, Argialas and Tzotsos (2004, 2006) applied object-oriented image analysis techniques, with multi-resolution segmentation of both DEMs and multispectral optical datasets, to delineate alluvial fans. These approaches have been tested in desert areas (e.g. Death Valley, USA), where the spectral signature of alluvial fans is clearly distinguishable from adjacent areas. In vegetated or urban land the use of spectral signatures is probably less effective in detecting alluvial fan materials below the superficial cover, drastically reducing the performance of methods based on optical datasets (Miliaresis and Argialas, 2000; Miliaresis, 2001; Argialas and Tzotsos, 2004, 2006). It is also worth noting that the application of slope/roughness thresholds and object-oriented extraction of alluvial fans are not appropriate to distinguish among the mapped fans or to separate and associate them with their respective contributing feeder channels and watersheds. The ability to link each alluvial fan to its source catchment is a desirable feature in an alluvial fan mapping procedure, because it defines the alluvial fan-catchment relationship, important in assessing geological hazards and evaluating natural resources (e.g. ore exploration in alluvial placers) (e.g. Kartashov, 1971; Harvey, 1997). 3. Methodology for delineating alluvial fans from DEMs: the GIS algorithm In locations where stream channels of an upland drainage basin intersect a mountain front, catastrophic fluid and sediment gravity flows, including rock falls, rock-slides, rock avalanches, debris flows and sheetfloods, constitute major constructional processes of alluvial fans, regardless of climate (Gomez Villar, 1996; Harvey, 2003; Harvey et al., 2005; Saito and Oguchi, 2005; Blair and McPherson, 2009). This fundamental assumption is at the base of our strategy to develop the semi-automated GIS algorithm for the mapping of alluvial fans. Moreover, the mountain-front setting of many fans, and the resulting nonconfined fluid and sediment gravity flows, generates their distinctive semi-conical form. These conical landforms have a relatively high slope which changes to gentler slopes (for example b 1°) at the distal boundary of the alluvial fans (Wells and Harvey, 1987; FEMA, 2003; Harvey, 2003; Saito and Oguchi, 2005; Harvey et al., 2005) (Profile Z–Z’ in Fig. 2B). Consequently, a set of simple geometric rules can be established to define where an alluvial fan forms, in order to delineate its shape and extent depending on the topography (e.g. DEM) of the area. These geometric rules to automatically delineate alluvial fans are:

(2) An alluvial fan has a relatively high-slope conical shape. This conical surface radiates from the fan apex until it intersects with the topography surrounding the alluvial fan; (3) The slope of an alluvial fan's conical surface is defined by the elevation difference and the plan-view distance between the fan apex and the end points. These end points are located at the intersection between topographic profiles radiating from the alluvial fan apex and the slope change at the distal boundary of the fan (Figs. 1, 2 and 3).

3.1. Drainage analysis and identification of alluvial fan apexes The occurrence of alluvial fans is strictly related to the presence of a drainage network (e.g. Gomez Villar, 1996; Mather et al., 2000; Leeder and Mack, 2001; FEMA, 2003; Blair and McPherson, 2009; Lancaster et al., 2012; Sánchez-Núñez et al., 2015). In our GIS algorithm, the drainage network is defined through a hydrological analysis of the DEM (Fig. 4). The first step is to calculate the flow direction matrix, which is a terrain attribute defining the flow direction from every cell of a depressionless hydrologically-corrected DEM with sinks filled resulting in a connected drainage structure. A flow accumulation matrix is then obtained by calculating the number of upslope cells flowing into each specific location. Finally, a threshold defining the number of contributing upslope cells required to have a drainage channel is iteratively defined and applied to the flow accumulation matrix, allowing the delineation of the drainage network (Figs. 4 and 5A). The value of this threshold depends on the DEM cell size and the drainage characteristics of the site, and needs to be tuned for each study area and resolution of the elevation model (e.g. Tarboton et al., 1991). The first geometric rule predicts that a topographic change along a stream channel is needed to identify an alluvial fan apex. As discussed in Section 2, this topographic breakpoint is marked by a change in slope on both sides of the drainage channel, generated by the transition from a confined valley to an unconfined lowland area. Indeed, steep slopes are expected in a confined valley at a certain distance on the sides of the stream channel (Profile X–X′ of Fig. 2). In the GIS algorithm, a convolution filter (mean value over a circular area) applied to a slope map, depicting the directional rate of change of elevation across the DEM, allows detection of a stream channel that is bound by steep mountain slopes (Fig. 4). The application of a threshold on the slope map (slope threshold 1, Fig. 4) results in the study area being divided into two sub-areas, one where relative steep slopes across drainage are indicative of confined valleys, and the other where relative gentle slopes indicate an unconfined drainage (Fig. 5B). An intersection between the steep-slopes sub-area and the drainage network is then performed to generate a map of the potential feeder channels (Fig. 4). The mean surface curvature value along each potential feeder channel, calculated by a curvature analysis of the DEM, allows the selection of the confined streams, discarding the channels that, even if located in the steep-slopes sub-area, are non-confined (Fig. 4). The alluvial fan apexes are identified at the intersection between the border of the steeply-sloping subarea and the selected feeder channels (Figs. 4 and 5B). As for other thresholds used in the GIS algorithm, the values of the convolution filter radius, slope threshold 1, and curvature threshold, for the delimitation of the two sub-areas and the definition of the map of the feeder channels, need to be tuned for each study area and DEM resolution (trial and error or training, Fig. 4). The tuning of the threshold values is performed on a representative alluvial fan located in the study area. Then, the calculated values are applied to delineate the alluvial fans in the whole area. 3.2. Alluvial fans delineation

(1) An alluvial fan apex occurs where a stream channel running in a laterally confined valley traverses a topographic change to a nonconfined lowland area (adjoining valley side, alluvial plain, tectonic depression, etc.);

The radial profile of an alluvial fan exhibits a characteristic shape, with constant slope or concave upward geometry, changing downstream to a nearly flat slope beyond the distal edge of the fan (Profile Z–Z′ in Fig.


139

Fig. 4. Flow chart of the proposed GIS algorithm.

2B) (e.g. Boothroyd, 1972; Saito and Oguchi, 2005; Blair and McPherson, 2009; Sánchez-Núñez et al., 2015). Furthermore, the second and third geometric rules predict that the shape of an alluvial fan can be assimilated to a semi-conical surface. In the GIS algorithm, this conical surface is built joining a series of profiles radiating from the fan apex. Thus, a set of radial profiles is first calculated (for example every 10° around the fan apex) and, then, trimmed at the intersection with the alluvial fan end points, whose location is determined by an analysis of the terrain elevation and slope measured along the profiles (Figs. 4 and 5C). The radial profiles analysis is mainly based on a fixed or variable minimum slope threshold and/or the detection of changes in slope along each profile (slope threshold 2, defined by trial and error or training on a representative alluvial fan in the study area, Fig. 4). With the individual analysis of each radial profile, the distal boundary of irregular confined and/or coalescent alluvial fans can also be correctly delineated in any direction around the fan apex. The semi-conical surface of the alluvial fan is then generated by interpolation using the fan apex, the trimmed radial profiles and the fan end points as input data (Figs. 4 and 5D). Finally, the interpolated semiconical surface is intersected with the surrounding topographic surface to delineate the alluvial fan shape in plan-view (Figs. 4 and 5E,F). The iterative analysis of the whole set of identified alluvial fan apexes, described and shown in Section 3.1, allows the identification of the complete alluvial-fan map in an entire area. The procedure also correlates each alluvial fan with the corresponding source drainage and catchment area defined through hydrological analysis of the DEM. 4. Testing the GIS algorithm Some of the best-known examples of alluvial fans are those located in the arid Death Valley, in the southwestern USA (Blair and McPherson,

2009). The GIS algorithm presented in Section 3 was implemented in ArcGIS (ESRI) and run on a 30 m resolution SRTM-DEM of the eastern sector of the valley, to compare the performance of the method with the alluvial fan map published by Blair and McPherson (2009), based on the visual interpretation of a 1:24,000 scale topographic map. The drainage network defined by the hydrological analysis of the DEM has been improved by adding five very small creeks in the southern portion of the study area, so as to be comparable to the drainage network depicted in the map of Blair and McPherson (2009). The threshold values to be applied for the delineation of the alluvial fans in the study area have been tuned by trial and error on a sample alluvial fan (Fig. 6A). The calculated thresholds have then been applied to the whole area. The algorithm was able to detect all 20 of the fan apexes and alluvial fan deposits identified in the map of Blair and McPherson (2009) (Fig. 6B). The location of the fan apexes is very similar in both maps, indicating that the GIS algorithm is capable of high-accuracy mapping of these alluvial features, provided that the drainage network is correctly represented and the thresholds properly set (Figs. 6C,D and 7A). The areal extent of the mapped alluvial fans is more difficult to compare, because of both (1) the different approach between visual interpretation and the GIS algorithm in the delimitation of adjacent/overlapped alluvial fans, and (2) the different input data, with insufficient accuracy in the gentle areas of the SRTM DEM and 1:24,000 scale topographic map: (1) In the map delineated by visual interpretation, there is no overlap among adjacent alluvial fans, even in areas where fans are probably superimposed and their deposits interfingered (Fig. 6A,C). The GIS algorithm calculates each conical surface, corresponding to an alluvial fan, independently of the others, and the superposition among adjacent fans is common (e.g. Fig. 6A,D);

140


Fig. 5. Main steps in the process of alluvial fan mapping with the GIS algorithm applied to a major alluvial fan in the Zagros Mountains region, Iran. (A) Hydrological analysis of the DEM and delineation of the drainage network. (B) Slope analysis of the DEM and identification of the alluvial fan apex (slope threshold is 6°). (C) Slope analysis along the radial profiles and identification of the alluvial fan end points. (D) Interpolation of the semi-conical surface of the alluvial fan. (E–F) Intersection of the interpolated surface with the topography, to delineate the plan-view shape of the alluvial fan.

(2) Alluvial fans, especially in their lateral and distal portions, are generally characterized by gentle slopes (Section 2.1), where accuracy of the SRTM DEM and contour map is

lower, and the transition between adjacent alluvial fans and between fans and alluvial plain is not always clearly identifiable (e.g. Fig. 6A).


141

Fig. 6. Mapping of alluvial fans in the eastern sector of Death Valley, USA. (A) Satellite view of the alluvial fan used to set the algorithm thresholds, with the fan apex and the fan extent calculated by the GIS procedure and shown in the map of Blair and McPherson (2009). Contour lines are derived from the 30-m DEM. (B) Comparison between the number of alluvial fans recognized by the GIS algorithm and by Blair and McPherson (2009) (C) Map of fans by visual interpretation of a 1:24,000 topographic map (redrawn from Blair and McPherson, 2009). (D) Map of fans calculated by the GIS algorithm.

Thus, both the visually interpreted and GIS algorithm maps have a certain degree of inaccuracy, which makes any quantitative comparison of areas uncertain for a correct analysis of the mapping performance relative to the ground truth. The area of non-overlap of the same fan

mapped by both methods usually is located in the distal portions (transition with the alluvial plain) and in the lateral portions (overlap with adjacent fans) of the alluvial fan (Figs. 6A and 7B). The non-overlap area can be greater than the overlap area in some alluvial fans, because

142


Fig. 7. Plots of the alluvial fans apex location and the fans extent calculated by the GIS algorithm versus the fans shown in the maps used for comparison. (A) Plot of the fan apex coordinates calculated by the GIS algorithm versus the fan apex coordinates shown in the map of Blair and McPherson (2009), using a local metric datum. (B) Log-log plot of the overlap fan area versus the non-overlap fan area of the paired alluvial fans mapped by the GIS algorithm and shown in the map of Blair and McPherson (2009). (C) Plot of the fan apex coordinates calculated by the GIS algorithm versus the fan apex coordinates shown in the maps of ISPRA (2012a,b) and Merri et al. (2013), using a local metric datum. (D) Log-log plot of the overlapping fan area versus the non-overlapping fan area of the paired alluvial fans mapped by the GIS algorithm and shown in the maps of ISPRA (2012a,b) and Merri et al. (2013).

of the mapping uncertainty in these distal and lateral portions (Fig. 7B). Despite the differences in the approach and input data, there is a general area–area positive correlation between the areas of overlap of paired alluvial fans of Death Valley mapped by visual interpretation and the GIS algorithm (Figs. 6C,D and 7B). The GIS delineation of fans was also tested in an example from the Alps, in northern Italy, characterized by humid climate. The GIS algorithm was run over a 20 m contour-interpolated DEM of the central Valtellina valley and the results were compared with the georeferenced database of the 1:50,000 National Geological Map of Italy (ISPRA, 2012a,b) and

the fan map of Merri et al. (2013), drawn by means of photogeology and fieldwork (Fig. 8). The threshold values to be applied for the delineation of the alluvial fans in the Valtellina have been tuned by trial and error on a sample alluvial fan located in the study area and, then, applied to the whole topographic dataset (Fig. 8A). The GIS algorithm detected ≈90% of the 50 fan apexes and correlated fan deposits shown in the maps of ISPRA (2012a,b) and Merri et al. (2013) (Fig. 8B). The ≈10% of fans not detected by our algorithm are small fans (b 90,000 m2), which in some cases are located adjacent/overlapped to bigger fans. The GIS algorithm also mapped 12 small fans (≈25% of the total delineated by the algorithm) confirmed


143

Fig. 8. Mapping of fans in the central Valtellina, Italy. (A) Satellite view of the alluvial fan used to set the algorithm thresholds, with the fan apex and the fan extent calculated by the GIS procedure and shown in the maps of ISPRA (2012a,b) and Merri et al. (2013). Contour lines are derived from the 20-m DEM. (B) Comparison between the number of alluvial fans recognized by the GIS algorithm and by ISPRA (2012a,b) and Merri et al. (2013). (C) Map of fans by photogeologic analysis and fieldwork (redrawn from ISPRA, 2012a,b, and Merri et al., 2013). (D) Map of fans calculated by the GIS algorithm.

by a visual inspection of the 20 m DEM, even if not detected by ISPRA (2012a,b) and Merri et al. (2013). The location of the fan apexes detected by both methods is very similar (Figs. 7C and 8C,D). As discussed in the Death Valley case study, the accuracy of the alluvial fan extent in Valtellina is difficult to compare quantitatively between the two maps, because of the different approaches and input data, even if there is a

positive area–area correlation result (Figs. 7D and 8C,D). Also, some differences in the fans outline may be related to built-up areas and earthworks that modified the topography. The analysis of the two test case studies (Death Valley and Valtellina) shows that the GIS algorithm is capable of high-accuracy mapping of the fan apexes and correct delineation of fan deposits with

144


different plan-view extents, in both arid and humid climates (e.g. Figs. 6A and 8A). 5. Discussion 5.1. Systematic geomorphological mapping of alluvial fans on Earth The systematic mapping of alluvial fans is useful for gaining insight into sedimentary processes, tectonic activity, climatic evolution, base level changes and drainage basin characteristics of an area. Indeed, the geomorphological mapping procedure presented has important implications for geologic hazard assessment, studies on tectonic–climate interaction, the search for water resources, and the analysis of fan-

related reservoirs and alluvial placers by comparison of present-day continental environments with ancient alluvial sedimentary successions (e.g. Kartashov, 1971; Keaton et al., 1988; Moscariello and Deganutti, 2000; Larsen et al., 2001; Viseras et al., 2003; Goswami et al., 2009; Shepherd, 2009; Rohais et al., 2012; Bahrami, 2013). Among the potential applications of a GIS algorithm for the systematic delineation of active alluvial fans, the analysis of geologic hazard is perhaps of paramount importance. Many alluvial fans, owing to their fertile soil, elevation above the alluvial plain, gentle slopes and accessible aquifers, are places where cities and villages are located. However, active alluvial fans also are very dynamic features that can dramatically change and are potentially catastrophic to human presence and activities (e.g. Larsen et al., 2001). Hence, demarcating the boundaries of

Fig. 9. Mapping of selected alluvial fans in a sector of the southern foot of the Sierra Madre Mountains, Luzon Island, Philippines. (A) Topographic map with the apex and extent of the San Vicente alluvial fan mapped by Saito and Oguchi (2005). (B,C,D) Maps of the San Vicente and two other alluvial fans calculated by the GIS algorithm. Location of the alluvial fans is shown in Fig. 10A. Contour lines in B, C and D are derived from the 10-m DEM.


alluvial fans is crucial to disaster preparedness and mitigation, especially in densely populated communities lying within these zones, and in heavily forested and vegetated regions, where thick cover may conceal the geomorphologic features related to the potentially catastrophic alluvial-fan sedimentary environment. Important case studies for identifying geological hazards related to human settlements on alluvial fans can be found in the Philippines, where many rural communities are situated in the upper portion of

145

fans and, thus, in the path of devastating floods and debris flows. Historical records show frequent occurrences of intense rainfall events in the Philippines, which led to a rapid increase in channels' volumetric flow rates, causing floods and debris flows within alluvial fans situated at the bases of mountain slopes (e.g. Saito and Oguchi, 2005; Lagmay et al., 2013; Ferrer et al., 2014). The GIS algorithm presented in this study was applied to identify alluvial fans in a ≈ 12,000 km2 sector of the southern foot of the Sierra Madre Mountains, on the island of

Fig. 10. Mapping of alluvial fans in a sector of the southern foot of the Sierra Madre Mountains, Luzon Island, Philippines. (A) Map of alluvial fans calculated by the GIS algorithm. The location of the detailed maps of Figs. 9 and 10B,C is shown. (B) Plan-view shape of a major alluvial fan, with its catchment area and source drainage, over a satellite view. (C) Plan view shapes of two small alluvial fans hosting rural settlement and cultivations, over a satellite view.

146


Luzon, where debris flows on 14 November 2004 left 135 people dead and hundreds homeless (Ortiz et al., 2014). Using a 10 m synthetic aperture radar (SAR)-derived DEM, the input thresholds for the GIS algorithm have been set on the San Vicente alluvial fan, located in the study area, whose extent has been previously mapped by Saito and Oguchi (2005) (Fig. 9A,B). With these thresholds, the algorithm has been applied to the whole topographic dataset, resulting in a good match between the alluvial fans mapped by the GIS algorithm and their boundaries as suggested by the shape of the contour lines generated from the SAR-derived DEM (e.g. Fig. 9B,C,D). The resulting map includes 148 alluvial fans with plan-view extents from ≈ 100,000 to ≈40,000,000 m2, where many villages are located (Fig. 10A,B,C). Inhabitants of these places often do not have any knowledge or consideration of the highly dynamic and potentially catastrophic nature of the alluvial fan sedimentary environment. The use of the GIS algorithm is particularly important in densely populated areas of humid tropical regions since it is able to detect alluvial fans even with thick vegetation cover, a problem encountered in correct and comprehensive visual interpretation of alluvial fans from aerial photographs, satellite images and DEMs (partially solved with the use of LiDAR data). The successful results in the application of the newly developed GIS algorithm to identify alluvial fans in the Philippines suggests that such hazardous environments can be mapped out rapidly and efficiently, an important and fundamental aspect for geohazards mapping of flash floods and debris flows on a nationwide scale. 5.2. Implications for geomorphological mapping of Mars The systematic mapping of alluvial fans in large regions could also have significant implications for regional studies and global surveys of Mars. Such kind of studies, focused on gullies, paleolakes and alluvial fans, already played an important role in understanding the climatic evolution and the presence and behavior of surface water on the planet. The identification of geomorphologic features in these regional and global studies were mainly based on visual interpretation of panchromatic and multispectral images obtained from orbit (e.g. Cabrol and

Grin, 1999; Heldmann and Mellon, 2004; Moore and Howard, 2005; Kraal et al., 2008; Williams et al., 2011). The presented GIS algorithm opens the possibility of performing regional and global mapping of alluvial fans on Mars based on topographic data. In the last decade, exploration missions to the planet produced DEMs of wide regions with increasing resolution and accuracy (e.g. Neukum et al., 2004; Jaumann et al., 2007). Indeed, available DEMs in the 50–200 m resolution range (e.g. Dumke et al., 2008) or better may allow fast and reliable mapping of alluvial fans from the local to the planetary scale, sustaining studies on the climatic evolution of Mars and the interplay with other geologic processes, like tectonics and volcanism (e.g. Hauber et al., 2006; Pacifici et al., 2007; Jaumann and the Mars Express HRSC Team, 2014). The GIS algorithm has been run on 125-m DEMs, obtained from the High Resolution Stereo Camera (HRSC) onboard the European Space Agency (ESA) Mars Express spacecraft, covering the northwestern sector of the Holden crater in the southern Margaritifer Terra region (0511_0000DA4) and the southern wall of an unnamed crater in the southwestern Terra Sabaea region (6520_0000DA4), where Moore and Howard (2005) identified the fan apex location and the extent of large alluvial fans (Fig. 11A,B). The input thresholds for the algorithm have been set on the smallest, southernmost, alluvial fan in the Holden crater, and then applied to the H0511_0000DA4 and H6520_0000DA4 DEMs, resulting in a good match between the fan apexes location and alluvial fans extent mapped by the GIS algorithm and the locations and sizes reported by Moore and Howard (2005) (Fig. 11A,B). Also, a 100-m DEM (H2459_0009DA4), derived from HRSC data, has been used to run the GIS algorithm in a sector of the Harris Crater in the far western Terra Tyrrhena region using the same input thresholds calculated for the Holden crater case study area (Neukum et al., 2004; Jaumann et al., 2007; Dumke et al., 2008). An alluvial fan complex has previously been recognized by Moore and Howard (2005) and Williams et al. (2011) in this crater (Fig. 12A). A visual comparison between the total extent of the alluvial fan complex and catchments as mapped by Williams et al. (2011) and the output of the algorithm shows that the GIS procedure is able to correctly delineate these geomorphological features on Mars (e.g. Fig. 12). These results indicate

Fig. 11. Mapping of alluvial fans on Mars, with a comparison of the fan apexes and extents calculated by the GIS algorithm and reported by Moore and Howard (2005). (A) Map of alluvial fans in the northwestern sector of the Holden crater in the southern Margaritifer Terra region. Background image extracted from the ESA Mars Express HRSC data H0511_0000(4). (B) Map of an alluvial fan in the southern wall of an unnamed crater in the southwestern Terra Sabaea region. Background image extracted from the ESA Mars Express HRSC data H6520_0000(4).


147

Fig. 12. Mapping of an alluvial fan complex within the Harris Crater, far western Terra Tyrrhena, Mars. (A) Total extent of the alluvial fan complex derived from the map of Williams et al. (2011) drawn by photogeologic analysis. (B) Map of the alluvial fan complex calculated by the GIS algorithm. Background image extracted from the ESA Mars Express HRSC data H2459_0009(4).

that the systematic regional/global surveys of alluvial fans on Mars would be feasible, provided that accurate DEMs of the planet surface are available, with important applications to the climatic and geologic studies of the planet, e.g. analysis of the spatial variability of the alluvial fans, assessment of the sediment volumes in the fans using the mapping results and DEMs, and comparative studies with terrestrial alluvial fans. 6. Concluding remarks The principal achievements of our study can be summarized by the following points. 1) A new semi-automated GIS algorithm for the mapping of alluvial fans has been developed. This algorithm is based on a combination of spatial analysis procedures for the processing of DEMs, to delineate the catchments and source drainage, the fan apexes and the extent of the alluvial fan deposits. Tests of the GIS algorithm performance show that this tool can produce reliable maps of alluvial fans in both arid and humid climates. 2) This GIS algorithm may be a fundamental step to develop objective and efficient methodologies for defining features relevant to geologic hazard assessment, especially in densely populated areas and vegetated regions, where thick cover may conceal the geomorphologic features related to the potentially catastrophic alluvial fan sedimentary environment. 3) Application of the new GIS algorithm provides the possibility of performing objective and reliable systematic surveys of alluvial fans on planetary surfaces (Earth and Mars), to support studies on comparative planetology, climatic evolution, tectonics–climate interaction, water resources, and fan-related reservoirs and alluvial placers. Acknowledgments This work was supported by the NOAH grant–Nationwide Operational Assessment of Hazards Project of the Department of Science and Technology of the Philippines. We acknowledge S. Salvosa, F. Llanes, J.A.M. Galang and R.N. Eco for useful discussions that improved the GIS algorithm and the manuscript. We also thank Takashi Oguchi, Ian S. Evans and an anonymous reviewer for their valuable and constructive comments.

References Anstey, R.L., 1965. Physical characteristics of alluvial fans, In: Technical Report ES-20. Army Natick Laboratory, USA. Argialas, D.P., Tzotsos, A., 2004. Automatic extraction of alluvial fans from ASTER L1 satellite data and a digital elevation model using object-oriented image. 20th Congress of the International Society for Photogrammetry and Remote Sensing, Istanbul, Turkey, pp. 12–23. Argialas, D., Tzotsos, A., 2006. Automatic extraction of physiographic features and alluvial fans in Nevada, USA, from digital elevation models and satellite imagery through multiresolution segmentation and object-oriented classification. American Society for Photogrammetry and Remote Sensing Annual Conference, pp. 1–6 (Reno, Nevada). Bahrami, S., 2013. Tectonic controls on the morphometry of alluvial fans around Danehkhoshk anticline, Zagros, Iran. Geomorphology 180–181, 217–230. Blair, T., McPherson, J., 1994. Alluvial fan processes and forms. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments. Chapman & Hall, London, pp. 354–402. Blair, T.C., McPherson, J.G., 2009. Processes and forms of alluvial fans. In: Parsons, A.J., Abrahams, A.D. (Eds.), Geomorphology of Desert Environments, second ed. Springer, Berlin, pp. 413–467. Blissenbach, E., 1954. Geology of alluvial fans in semiarid regions. Geol. Soc. Am. Bull. 65, 175–189. Bolstad, P.V., Stowe, T., 1994. An evaluation of DEM accuracy: elevation, slope, and aspect. Photogramm. Eng. Remote Sens. 60, 1327–1332. Boothroyd, J.C., 1972. Coarse-grained sedimentation on a braided outwash fan, northeast Gulf of Alaska. Technical Report 6. University of South Carolina Coastal Research Division, USA. Bull, W.B., 1962. Relations of alluvial fan size and slope to drainage basin size and lithology in western Fresno Country, CA. US Geol. Surv. Prof. Pap. 450-B, 51–53. Bull, W.B., 1977. The alluvial fan environments. Geography 1, 222–270. Cabrol, N.A., Grin, E.A., 1999. Distribution, classification, and ages of Martian impact crater lakes. Icarus 142, 160–172. Calvache, M.L., Viseras, C., Fernández, J., 1997. Controls on fan development – evidence from fan morphometry and sedimentology, Sierra Nevada, SE Spain. Geomorphology 21, 69–84. Cavalli, M., Marchi, L., 2008. Characterization of the surface morphology of an alpine alluvial fan using airborne LiDAR. Nat. Hazards Earth Syst. Sci. 8, 323–333. Clevis, Q., De Boer, P., Wachter, M., 2003. Numerical modelling of drainage basin evolution and three-dimensional alluvial fan stratigraphy. Sediment. Geol. 163, 85–110. Crippen, R.E., 2010. Global topographical exploration and analysis with SRTM and ASTER elevation models. In: Fleming, C., Marsh, S.H., Giles, J.R.A. (Eds.), Elevation Models for Geoscience. Geological Society, London, Special Publication 345, pp. 5–15. Di Traglia, F., Morelli, S., Casagli, N., Garduño Monroy, V.H., 2014. Semi-automatic delimitation of volcanic edifice boundaries: Validation and application to the cinder cones of the Tancitaro–Nueva Italia region (Michoacán–Guanajuato Volcanic Field, Mexico). Geomorphology 219, 152–160. Drew, F., 1873. Alluvial and Lacustrine Deposits and Glacial Records of the Upper-Indus Basin. Geol. Soc. Lond. Q. J. 29, 441–471. Dumke, A., Spiegel, M., Schmidt, R., Michael, G., Neukum, G., 2008. Mars: high-resolution digital terrain model and ortho-image mosaic on the basis of mex/hrsc data. Int. Arch. Photogramm. Remote. Sens. Spat. Inf. Sci. XXXVII (Part B4), 1037–1042 (Beijing, China).

148


Eckis, R., 1928. Alluvial fans in the Cucamonga district, Southern California. J. Geol. 36, 111–141. Eisank, C., Smith, M., Hillier, J., 2014. Assessment of multiresolution segmentation for delimiting drumlins in digital elevation models. Geomorphology 214, 452–464. Euillades, L., Grosse, P., Euillades, P., 2013. NETVOLC: An algorithm for automatic delimitation of volcano edifice boundaries using DEMs. Comput. Geosci. 56, 151–160. FEMA, 2003. Guidelines and specifications for flood hazard mapping partners, Appendix G. Federal Emergency Management Agency, Guidance for alluvial fan flooding analyses and mapping, Washington, D.C., USA. Ferrer, P.K., Llanes, F., dela Resma, M., Realino II, V., Obrique, J., Gacusan, R.C., Ortiz, I.J., Quina, C., Aquino, D.T., Eco, R.N., Lagmay, A.M.F., 2014. Debris Flow Hazard Map Simulation using FLO-2D for Selected Areas in the Philippines. Project NOAH Open-File Report, Manila, Philippines. Field, J., 2001. Channel avulsion on alluvial fans in southern Arizona. Geomorphology 37, 93–104. Fleming, C., Marsh, S.H., Giles, J.R.A., 2010. Introducing elevation models for geoscience. In: Fleming, C., Marsh, S.H., Giles, J.R.A. (Eds.), Elevation Models for Geoscience. Geological Society, London, Special Publication 345, pp. 1–4. Florsheim, J.L., 2004. Side-valley tributary fans in high-energy river floodplain environments: sediment sources and depositional processes, Navarro River basin, California. Geol. Soc. Am. Bull. 116, 923–937. Foster, N.H., Beaumont, E.A., 1992. Photogeology and photogeomorphology. American Association of Petroleum Geologists, Tulsa, OK, USA. Gloaguen, R., Marpu, P.R., Niemeyer, I., 2007. Automatic extraction of faults and fractal analysis from remote sensing data. Nonlinear Process. Geophys. 14, 131–138. Gomez Villar, A., 1996. Abanicos aluviales: Aportación teórica a sus aspectos más significativos. Cuatern. Geomorfol. 10, 77–124. Goswami, P.K., Pant, C.C., Pandey, S., 2009. Tectonic controls on the geomorphic evolution of alluvial fans in the Piedmont Zone of Ganga Plain, Uttarakhand, India. J. Earth Syst. Sci. 118, 245–259. Harvey, A.M., 1997. The role of alluvial fans in arid zone fluvial systems. In: Thomas, D.S.G. (Ed.), Arid Zone Geomorphology: Process, Form and Change in Dry Lands. John Wiley and Sons Limited, Chichester, pp. 231–259. Harvey, A.M., 2003. The response of dry-region alluvial fans to the late Quaternary climatic change. In: Alsharhan, A.S., Wood, W.W., Goudie, A.S., Fowler, A., Abdellatif, E.M. (Eds.), Desertification in the Third Millennium. Balkema, Rotterdam, pp. 83–98. Harvey, A., Mather, A.E., Stokes, M., 2005. Alluvial fans: geomorphology, sedimentology, dynamics-introduction, a review of alluvial fan research. In: Harvey, A., Mather, A.E., Stokes, M. (Eds.), Alluvial fans: geomorphology, sedimentology, dynamics. Geological Society Special Publication 251, London, pp. 1–8. Hauber, E., Gwinner, K., Scholten, F., Neukum, G., Jaumann, R., Hoffmann, H., van Gasselt, S., 2006. Geomorphologic mapping of Mars with the High Resolution Stereo Camera (HRSC). European Planetary Science Congress. Berlin, Germany, p. 353. Heldmann, J.L., Mellon, M.T., 2004. Observations of Martian gullies and constraints on potential formation mechanisms. Icarus 168, 285–304. Hooke, R.L., 1967. Processes on arid-region alluvial fans. J. Geol. 75, 438–460. Hooke, R.L., 1968. Discussion: model geology: prototype and laboratory streams. Geol. Soc. Am. Bull. 79, 391–394. ISPRA, 2012a. Carta Geologica d'Italia, Foglio 056 - Sondrio. Servizio Geologico d'Italia, Roma. ISPRA, 2012b. Carta Geologica d'Italia, Foglio 057 - Malonno. Servizio Geologico d'Italia, Roma. Jaumann, R., the HRSC Co-Investigator Team, et al., 2007. The high resolution stereo camera (HRSC) experiment on Mars Express: instrument aspects and experiment conduct from interplanetary cruise through nominal mission. Planet. Space Sci. 55, 928–952. Jaumann, R., the Mars Express HRSC Team, 2014. The Martian geomorphology as mapped by the Mars express high resolution stereo camera (HRSC): implications for geological processes and climate conditions. EGU General Assembly, Vienna, EGU2014-3777. Geophysical Research Abstracts vol. 16. Kartashov, I.P., 1971. Geological features of alluvial placers. Econ. Geol. 66, 879–885. Keaton, J.R., Anderson, L.R., Mathewson, C.C., 1988. Assessing debris flow hazards on alluvial fans in Davis County, Utah. Twenty-fourth Annual Symposium on Engineering Geology and Soils Engineering. Washington State University, Pullman, Washington, pp. 98–108 (Publications and Printing). Kraal, E.R., Asphaug, E., Moore, J.M., Howard, A., Bredt, A., 2008. Catalogue of large alluvial fans in Martian impact craters. Icarus 194, 101–110. Lagmay, A.M.F., Eco, R.N., Alconis, J., Salvio, B., 2013. What hit Barangay Andap, New Bataan, Compostela Valley (initial assessment). Project NOAH Open-File Report, Manila, Philippines. Lancaster, J., Spittler, T., Short, W., 2012. Using digital geologic maps to assess alluvial-fan flood hazards. In: Soller, D.R. (Ed.), Digital Mapping Techniques Workshop Proceedings. U.S. Geological Survey Open-File Report 1171, Washington, USA, pp. 53–64. Larsen, M.C., Wieczorek, G.F., Eaton, L.S., Torres-Sierra, H., 2001. Natural hazards on alluvial fans: the debris flow and flash flood disaster of December 1999, Vargas State, Venezuela. Proceedings of the 6th Caribbean Islands Water Resources Congress, Mayaguez, Puerto Rico, pp. 1–7. Leeder, M.R., Mack, G.H., 2001. Lateral erosion (“toe-cutting”) of alluvial fans by axial rivers: implications for basin analysis and architecture. J. Geol. Soc. 158, 885–893. Lustig, L.K., 1965. Clastic sedimentation in Deep Springs Valley, California, U.S. Geological Survey Professional Paper 352-F, pp. 131–190 (Washington, USA). Mather, A.E., Harvey, A.M., Stokes, M., 2000. Quantifying long-term catchment changes of alluvial fan systems. Geol. Surv. Am. Bull. 112, 1825–1833. Merri, A., Bonomi, T., Fumagalli, L., Dubricich, S., 2013. Modello idrogeologico concettuale dell'acquifero di fondovalle valtellinese. GIT – Geology and Information Technology,

8a Riunione del Gruppo di Geologia Informatica, Sezione della Società Geologica Italiana, Chiavenna, Italy, pp. 1–27. Miliaresis, G., 2001. Extraction of bajadas from digital elevation models and satellite imagery. Comput. Geosci. 27, 1157–1167. Miliaresis, G., Argialas, D., 2000. Extraction and delineation of alluvial fans from digital elevation models and landsat thematic mapper images. Photogramm. Eng. Remote Sens. 66, 1093–1101. Minár, J., Evans, I.S., 2008. Elementary forms for land surface segmentation: the theoretical basis of terrain analysis and geomorphological mapping. Geomorphology 95, 236–259. Moore, J.M., Howard, A.D., 2005. Large alluvial fans on Mars. J. Geophys. Res. 110, E04005 (1–24). Moscariello, A., Deganutti, A.M., 2000. Sedimentary and hydrologic processes of a debrisflow dominated alluvial fan-Moscardo Fan, Italy. Proceedings of the Second International Conference, Debris-Flow Hazards Mitigation: Mechanics, Prediction, and Assessment, Taipei, Taiwan, pp. 301–309. NCR, 1996. Alluvial fan flood. Technical Report, National Research Council, Committee on Alluvial Fan Flooding, Water Sciences and Technology Board, Commision on Geosciences, Environment, and Resoures Washington, D.C., USA. Neukum, G., Jaumann, R., HRSC Co-Investigator and Experiment Team, 2004. HRSC: the high resolution stereo camera of Mars Express. In: Wilson, A. (Ed.), Mars Express: The Scientific Payload. ESA, Noordwijk, The Netherlands, pp. 17–35. Oguchi, T., Ohmori, H., 1994. Analysis of the relationships among alluvial fan area, source basin area, basin slope, and sediment yield. Z. Geomorphol. Neue Folge 38, 405–420. Ortiz, I.J., Aquino, D.T., Norini, G., Salvosa, S., LLanes, F., Galang, J.A.M., Eco, N., ZuluagaVelez, M.C., Lagmay, A.M.F., 2014. Developing automatic delineation of alluvial fans for rapid hazard assessment in Aurora Province, Philippines. Japan Geosciences Union Meeting, Yokohama, Japan, pp. 1–2. Pacifici, A., Ori, G.G., Komatsu, G., Pondrelli, M., 2007. Geomorphological analysis of Ares Vallis (Mars) by using HRSC (MEX) data: catastrophic foods and glacial morphologies. Mem. S.A.It. Suppl. 11, 119. Pandey, S.N., 1987. Principles and applications of photogeology. John Wiley & Sons, Chichester. Pierson, T.C., Costa, J.E., 1987. A rheologic classification of subaerial sediment-water flows. Geol. Soc. of Am. Rev. Eng. Geol. 7 (1987), 1–12. Radebaugh, J., Lorenz, R.D., Farr, T.G., Kirk, R.L., Lunine, J.I., Ventra, D., Le Gall, A., Lopes, R.M.C., Barnes, J.W., Hayes, A., Stofan, E.R., Wall, S.D., Wood, C., 2013. Alluvial fans on Titan reveal materials, processes and regional conditions. 44th Lunar and Planetary Science Conference, The Woodlands, Texas, USA, p. 2641. Riley, S.J., Degloria, S.D., Elliot, R., 1999. A terrain ruggedness index that quantifies topographic heterogeneity. Intermountain J. Sci. 5, 1–4. Ritter, J.B., Miller, J.R., Enzel, Y., Wells, S.G., 1995. Reconciling the roles of tectonism and climate in Quaternary alluvial fan evolution. Geology 23, 245–248. Ritter, J.B., Miller, J.R., Husek-Wulforst, J., 2000. Environmental controls on the evolution of alluvial fans in Buena Vista Valley, north central Nevada, during the late Quaternary time. Geomorphology 36, 63–87. Roberts, N., Cunningham, D., 2008. Automated alluvial fan discrimination, quaternary fault identification, and the distribution of tectonically reactivated crust in the Gobi Altai region, southern Mongolia. Int. J. Remote Sens. 29, 6957–6969. Robinson, G.J., 1994. The accuracy of digital elevation models derived from digitised contour data. Photogramm. Rec. 14, 805–814. Rohais, S., Bonnet, S., Eschard, R., 2012. Sedimentary record of tectonic and climatic erosional perturbations in an experimental coupled catchment-fan system. Basin Res. 24, 198–212. Saito, K., Oguchi, T., 2005. Slope of alluvial fans in humid regions of Japan, Taiwan and the Philippines. Geomorphology 70, 147–167. Sánchez-Núñez, J.M., Macías, J.L., Saucedo, R., Zamorano, J.J., Novelo, D., Mendoza, M.E., Torres-Hernández, J.R., 2015. Geomorphology, internal structure and evolution of alluvial fans at Motozintla, Chiapas, Mexico. Geomorphology 230, 1–12. Schick, A.P., Lekach, J., 1987. A high magnitude flood in the Sinai Desert. In: Myer, L., Nasch, O. (Eds.), Catastrophic Flooding. Allen and Unwin, Boston, pp. 381–409. Shepherd, M., 2009. Less common reservoir types. In: Shepherd, M. (Ed.), Oil Field Production GeologyAAPG Memoir 91. American Association of Petroleum Geologists, Tulsa, OK, USA, pp. 311–312. Sorriso-Valvo, M., Antronico, L., Le Pera, E., 1998. Controls on modern fan morphology in Calabria, southern Italy. Geomorphology 24, 169–187. Staley, D.M., Wasklewicz, T.A., Blaszczynki, J.S., 2006. Surficial patterns of debris flow deposition on alluvial fans in Death Valley, CA using airborne laser swath mapping data. Geomorphology 74, 152–163. Tarboton, D.G., Bras, R.L., Rodriguez-Iturbe, I., 1991. On the extraction of channel networks from digital elevation data. Hydrol. Process. 5, 81–100. Troeh, F.R., 1965. Landform equations fitted to contour maps. Am. J. Sci. 263, 616–627. USGS, 2002. Natural hazards on alluvial fans: the Venezuela debris flow and flash flood disaster. U.S. Geological Survey, Science for the Changing World Fact Sheet FS-10301s (Washington, USA). van Asselen, S., Seijmonsbergen, A.C., 2006. Expert-driven semi-automated geomorphological mapping for a mountainous area using a laser DTM. Geomorphology 78, 309–320. Van Den Eeckhaut, M., Kerle, N., Poesen, J., Hervás, J., 2012. Object-oriented identification of forested landslides with derivatives of single pulse LiDAR data. Geomorphology 173, 30–42. Viseras, C., Calvache, M.L., Soria, J.M., Fernandez, J., 2003. Differential features of alluvial fans controlled by tectonic or eustatic accommodation space. Examples from the Betic Cordillera, Spain. Geomorphology 50, 181–202. Walker, J.P., Willgoose, G.R., 1999. On the effect of digital elevation model accuracy on hydrology and geomorphology. Water Resour. Res. 35, 2259–2268.

G. Norini et al. / Geomorphology 273 (2016) 134–149 Wang, L., Chen, J., Zhang, H., Chen, L., 2011. Difference analysis of SRTM C-band DEM and ASTER GDEM for global land cover mapping. International Symposium on Image and Data Fusion ISIDF 2011, Yunnan, China, pp. 1–4. Webb, R.H., Pringle, P.T., Rink, G.R., 1987. Debris flows from tributaries of the Colorado River, Grand Canyon National Park, Arizona. U.S. Geological Survey Open-File Report 1492, Washington, USA.

149

Wells, S.G., Harvey, A.M., 1987. Sedimentologic and geomorphic variations in storm generated alluvial fans, Howgill Fells, northwest England. Geol. Soc. Am. Bull. 98, 182–198. Williams, R.M.E., Rogers, A.D., Chojnacki, M., Boyce, J., Seelos, K.D., Hardgrove, C., Chuang, F., 2011. Evidence for episodic alluvial fan formation in far western Terra Tyrrhena, Mars. Icarus 211, 222–237.