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A topographic fingerprint to distinguish alluvial fan formative processes
Geomorphology 88 (2007) 34 – 45 www.elsevier.com/locate/geomorph
A topographic fingerprint to distinguish alluvial fan formative processes H.X. Volker a,b , T.A. Wasklewicz a,⁎, M.A. Ellis a,c a
c
Department of Earth Sciences, University of Memphis, Memphis, TN 38152, USA b Fugro West, Inc., Ventura, CA 93003-7778, USA Center for Earthquake Research and Information, University of Memphis, Memphis, TN 38152, USA Received 30 May 2006; received in revised form 19 October 2006; accepted 19 October 2006 Available online 29 November 2006
Abstract We demonstrate here a topographic fingerprint, derived from a 1 m high-resolution elevation model (DEM), developed from Airborne Laser Swath Mapping (ALSM) that distinguishes between formative processes from surface form in alluvial fans generated through relatively dry debris flows (DF) and those surfaces formed primarily by fluvial sediment transport (MF). We selected alluvial fans with primarily Holocene age surficial deposits along the eastern side of Death Valley, California. Fans were initially classified in the field into DF and MF types, largely on the basis of sedimentological characteristics and surface morphometry. Trend surface analyses were performed on DEMs of the individual fans in order to remove the long-wavelength trends (both profile and planimetric). From the residual data, we found that high-resolution surface data distinguishes process information in two ways; 1) local-relief increases as a function of length-scale, especially at length scales less than 40–60 m, and 2) local-relief is significantly higher for debris-flow dominated fans than for fluvially dominated fans. This fingerprint, which is directly related to the dominant surface processes, is unavailable from standard 30 m resolution data, providing significant support for the acquisition of high-quality high-resolution data. This fingerprint can be used to help calibrate numerical simulations of alluvial fan development, and has the potential for similar applications to distinguish between processes of similar landforms on other planetary bodies within our solar system. © 2006 Elsevier B.V. All rights reserved. Keywords: Geomorphometry; LiDAR; GIS; Death Valley
1. Introduction A long-standing goal in geomorphology is to use form to reveal process (Horton, 1945; Ashmore et al., 1992; Ferguson et al., 1992; Lane, 1998; Knighton and Nanson, 2000). This is not an easy task, as landscapes ⁎ Corresponding author. Tel.: +1 901 678 4452; fax: +1 901 678 2178. E-mail address: [email protected] (T.A. Wasklewicz). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.10.008
are time-integrated surfaces generated by multiple processes, each coupled to the other or to boundary conditions, and each with response times that likely depend on boundary conditions. The alluvial fan literature is replete of studies employing digital elevation data that portray fan surface topography to distinguish formative processes. Here we use local relief as a function of length-scale, r(l), derived from one-meter planimetric resolution elevation data, to
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distinguish between debris and mixed-flow alluvial fans in Death Valley, Basin and Range Province, California (Fig. 1). The relative contribution of any particular process to the development of alluvial fans, as with every other landscape, is likely to change over time. This is particularly so for modern landscapes, many of which have been exposed to significant changes in climate throughout the Quaternary period. The compound stratigraphy of alluvial fans present clear evidence to support different formative processes dominate at various times (e.g., Ritter et al., 2000; Ortega-Ramírez et al., 2004). We minimize this issue by examining surficial deposits from
Fig. 1. Location of study area within Death Valley National Park (DVNP), California (CA), USA. Shaded-relief of topography along the Black Mountain range-front is shown using both the high-resolution, ALSM-derived DEM and the 30 m USGS NED DEM. BW is the Badwater fan and CC is the Copper Canyon fan. Outline of Fig. 3 is shown by the thicker white box.
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relatively young fans along the eastern flank of Death Valley. The lack of lake sediments and shorelines on these surfaces indicates the fan surface developed after the drawdown of pluvial Lake Manly (Hooke, 1972; Smith, 1976; Li et al., 1996), which likely occurred ∼ 12 ka (Anderson and Wells, 2003). While the relative age of the fan surficial units is constrained to the Holocene, differences in the age of the surficial depositional units are evident in the various degrees of varnish rubification and pavement development. Furthermore, the timing of depositional and erosional processes among the fans in the current population is not synchronized, such that not all fan surfaces have developed at the same time or from the same regional or local storm events during the Holocene. Some of the fans may also possess an internal core of material that dates to the pre-Holocene, but this has not been adequately tested to date. The current study provides an analysis of the recent fan surface and we do not attempt to trace the complex continuum of processes that have likely changed throughout the evolution of the fan as a whole. Alluvial fans in the Basin and Range are widely recognized as either debris-flow dominated (DF), fluvially dominated, or a mixture of debris-flows and ephemeral stream flow (termed mixed-flow fans (MF), largely on the basis of their sedimentary characteristics (Beatty, 1963; Hooke, 1967; Bull, 1968; Whipple and Dunne, 1992; Blair and McPherson, 1999). Debris flows leave characteristic topographies across alluvial fans: long and often sinuous channels bordered by prominent levees and in some instances terminated by a snout of relatively coarse sediment (Whipple and Dunne, 1992) (Fig. 2a). By contrast, fluvially dominated surfaces contain rectangular channels and generally smoother, flatter interfluves (likely a result of secondary processes as explained in Blair and McPherson, 1994a) (Fig. 2b). The fluvially dominated fans display a greater proportion of well-sorted fine sediments. Physical models, in combination with field analyses, corroborate variations in fan surface topography are a function of water content within the sediment-laden flows that both build and incise these surfaces (Hooke, 1968). The MF fans contain a combination of the aforementioned sedimentary and topographic characteristics. The aforementioned differences and our field observations lead us to hypothesize that fan topography contains information indicative of the processes associated with fan development. A variety of DEM values and analyses in geomorphic research have been applied to acquire information on surface topography variability. For example, landslide studies have generated various measures of topographic roughness to automatically delineate landslides within
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Fig. 2. A detailed view of some of the DF (a) and MF (b) fan surface features and stratigraphic units. Debris flows, which typically have coarser sediment loads, leave characteristic topographies across alluvial fans such as the channels bordered by prominent levees. These features are found consistently along the length of the debris flow fans (Staley et al., 2006). By contrast, wetter sediment-laden flow surfaces are intermixed with rectangular channels and generally smoother, flatter interfluves. Stratigraphically, the MF fans display a greater proportion of well-sorted fine sediments, as viewed along incised channels by the authors.
mountainous landscapes and to generate mechanical inferences on the recent behavior of landslides (McKean and Roering, 2004). Surface roughness has also been applied to the identification of landslide topographic elements and to correlate these elements with material
properties and localized landslide motions (Glenn et al., 2006). Another approach to interpreting topographic variability is a comparison of relative- or local-relief. Previous research has shown topographic relief in mountainous terrain is often associated with tectonic
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Fig. 3. A detailed view of the shaded relief derived from the ALSM high-resolution DEM of two of the fan types (DF6 is a debris flow fan and MF6 is a mixed-flow fan). Note the detail that can be readily observed in the fan and range-front morphology.
processes as opposed to climatically induced isostatic uplift (Whipple et al., 1999). At a more localized scale, Oguchi (1997) verified a negative correlation between drainage density and relative relief (the maximum height dispersion normalized by its length or area) and considered it to be a general characteristic of Japanese mountains. The relationship remains relatively constant when zero-order streams are added to the data analyses. Oguchi (1997) attributed the correlation to spatial variations in hillslope processes, whereby mass wasting in the zero-order streams promote the varying relations between drainage density and relative relief. However, none of these analytical approaches have been applied to alluvial fans to extract a process-oriented topographic signature. We show here the rate of change of localrelief derived from a high-resolution DEM enables fan
surfaces to be objectively classified and linked to a predominant formative process at a much shorter lengthscale than in previous studies. 2. Methods 2.1. Derivation of the digital elevation model (DEM) A one-meter DEM was derived from airborne laser swath mapping (ALSM) (Shrestha et al., 1999), generated by the National Science Foundation funded National Center for Airborne Laser Mapping (NCALM) within the University of Florida's Geosensing Systems Engineering Program. The data were acquired over a three day period: 5/29/03, 5/30/03, and 6/02/03 from heights ranging from 300 to 1100 m above ground-level.
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The variations in altitude during the data acquisition reflect a change in topography from the fan into the drainage basin as well as variability in surface topography along the length of the mountain front. The laser scanning was completed with a scan angle of 20° (1/2 Field Of View), a scan frequency of 28 Hz and a pulse rate of 33,333 Hz. The data were referenced to the North American Datum of 1983 (NAD83) horizontal and the Geodetic Reference System 1980 (GRS80) ellipsoid heights vertical datums. Laser shots were collected along separate flight strips (swaths) that overlapped at greater than 30%. Post-processing of the ALSM data was conducted at NCALM in late summer and early fall of 2003. Grid nodes were obtained through the application of Kriging to the combined overlapping swaths of laser shots to produce a best estimate elevation on a onemeter grid. The vertical precision is ± 10 cm, while the vertical resolution is b 0.01 m (Shrestha et al., 1999). Data were converted to UTM zone 11 coordinates, and analyses were conducted from one-meter raster grids in ESRI ArcInfo and MATLAB. 2.2. Classification and definition of alluvial fans Twenty-nine alluvial fans exposed contiguously along the range-front for which we had high-resolution topographic data (Fig. 1) were selected for the study. Each fan was classified as either debris flow (DF) or mixed flow (MF) (Figs. 2 and 3) using a field-based criteria described below. The general morphometric characteristics of the fan classes are specified in Table 1. Surficial features and sedimentological characteristics were recorded and photo-documented during field observations of all twenty-nine alluvial fans. The observations and documentation served as a framework for classifying the two fan groups. In particular, deposits along incised channels were examined in accordance with previous studies in Death Valley and adjacent areas in the Mojave Desert (Blair and McPherson, 1992, 1994a). Surface forms used in the classification of debris flow fans included; levees, sieve deposits, lobes, snouts, and debris plugs, as observed by Whipple and Table 1 The average morphometric statistics for the two fan classes (MF — mixed-flow fans; DF — debris flow fans) Fan class
Fan length (m)
Fan width (m)
Fan area (km2)
Fan slope (°)
DBA (km2)
MF DF
816.57 244.36
977.83 198.89
0.90 0.08
5.73 15.05
10.02 0.60
DBA is the drainage basin area of the fans.
Fig. 4. Geomorphological and geological characteristics of fans and source basins. (a) Plot of drainage basin-area vs. fan-area for each of 29 alluvial fans along the Black Mtn. range-front (see Fig. 1). Triangle symbols are DF fans, circles are MF. These data are consistent with Death Valley fans reported by Hooke (1968), shown by the broad gray line, and with data from other sets of alluvial fans: 1–2, Bull, 1964; 3–5, Hooke, 1968. The relationship of the fan area (Af; km2) in the current study with the drainage basin area (Ad; km2) is defined as Af = − 1.94 A0.95 . (b) A ternary compositional plot for the f three most dominant lithologies in the drainage basin, which are metamorphic (predominantly) schists (Xmi), Volcanic (intermediate and felsic) lava flows and tuffs (Tar), and intrusive (primarily) gabbro and diorite (Tws) (Workman et al., 2002), provided as a general indicator of the drainage basin geology. Note that the single DF symbol at 100% Xmi corresponds to 17 occurrences of this fan-class (denoted by the number 17 in the ternary diagram); the two occurrences of MF at 100% Xmi plot within the same range as the occurrences of DF also lie underneath the triangles. DF and MF symbols marked with an asterisk correspond to fans that are identified by a topographic fingerprint that does not match the initial classification (see text for detailed explanation).
Dunne (1992) and Hooke (1987) (Fig. 2a). Forms associated with fluvially derived fan surfaces included; terraces, interfluves, islands, and rectilinear channel banks (Blair and McPherson, 1994a,b) (Fig. 2b). The sedimentological distinctions were based largely on particle imbrication (Iverson, 1997) and matrix supported clasts (Blair and McPherson, 1994b), both indicative of debris flow deposition as well as the upward fining of particles, which is indicative of fluvial deposition (Blair and McPherson, 1992, 1994a).
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Fig. 5. A graphical portrayal of the local relief calculation derived from a series of hypothetical numbers. Each 1-m cell value is replaced with r(l). The r(l) was calculated at each of the following length scales: 1, 2, 3, 5, 10, 20, 30, and 50, 75, and 100-m, using the maximum amount of relief in the window (white box) divided by the total relief of the fan. The grey boxes to the right of the window represent the next series of raster cells to be measured during the moving window process used to generate the local relief values.
We mapped the fan outlines by deriving stream networks and by noting that higher order streams are often found at the margins of coalescing fan surfaces. This is particularly useful in cases where multiple fans comprised a bajada complex. The edge of each fan in the bajada complexes was demarcated by the top bank of the higher order stream incising between two adjacent fans. Additionally, slope, aspect, and curvature grids were used to lend further information to support our decision making process in separating fans from the bajada complex. This was necessary when hydrological networks did not completely define the extent of the entire fan surface. For example, convexity–concavity
variation can help delimit the top bank of the high-order streams between adjacent fans. The contributing drainage areas were delineated (from the 30-m USGS NED DEM) in ArcGIS for each fan surface. The general relationship between drainage area and fan area was assessed from these data. Our findings concur with previous results identified by investigators of similar fans in arid settings (Bull, 1964; Hooke, 1968; Hooke and Rohrer, 1979; Leece, 1991; Whipple and Trayler, 1996) (Fig. 4). This similarity provides a level of confidence that our method of defining fans is accurate and/or at least consistent with earlier cited methodologies.
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Fig. 6. The series of steps required to remove the broad-scale slope trends and subsequently permit us to analyze only the local-relief differences in the fan surfaces. First, second, and third order polynomial surfaces were investigated to assess the output from each. The first and third order surfaces were rejected as they respectively did not remove the slope trend or convoluted the surface. A second order polynomial surface provided the best approximation of the fan surface, while removing the planimetric and profile trends.
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2.3. Derivation of local-relief Local-relief r(l), defined here as the maximum difference in elevation within a square window of length l. Each 1-m grid cell was assigned an r(l) value based on the maximum relief value within the window. Relief values were normalized by the total relief of the fan (Fig. 5). For each of the fans, r(l) was derived at different length-scales (increasing window size) and maintained a 1 m cell size at the varying length-scales. An initial assessment of r(l) as a function of length-scale found it was dominated by the slope of the fan surface, especially at length-scales that approached the size of the fan. We performed a trend-surface analysis (using standard ESRI ArcInfo routines) of each fan in order to define and remove the long-wavelength shape of the fan (Fig. 6). For each fan, this process resulted in the removal of a second-order (quadratic) polynomial surface that best fit the original fan surface (Borrough and McDonnell, 1998) (Fig. 6). The trend surface removal allowed us to examine the topographic variability of the fan surface in a standardized manner without including global trends (e.g., the removal slope trends in both the planimetric and profile directions). The residual surface was used in subsequent analyses.
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these characteristics is quantified by the relatively low rate of increase in MF fans of r(l) values. Interestingly, there are apparent exceptions to the general result described above. Two MF fans exhibit r(l) values that place them unambiguously in the DF group (Fig. 7a), and likewise, one DF fan lies within the MF grouping (Fig. 7a). We re-examined these anomalous fans to see if we had overlooked evidence in their initial classification or to find an alternative interpretation for the inconsistent r(l) values identified in these three fans (one DF and two MF). Active faults have generated relatively high-relief scarps across some of the fan surfaces, but notably not
3. Results Mean r(l) is higher in most cases for fans constructed from debris flows than those formed by a combination of debris flows and fluvial flows (Fig. 7a). The distinction between MF and DF fans is even more evident in a comparison of fan groups (Fig. 7b). Here, the rate of increase of mean r(l) as a function of lengthscale in DF fans is significantly higher than that observed in MF fans at length-scales below 40–60 m. This is a significant result and indicates fans generated from disparate formative processes produce differing surface complexity. Greater differences in elevation at both short and long length-scales on DF fans reflect formative processes that contrast those active on MF fans. Debris flows, for example, produce lobes, levees, and snouts adjacent to channels. The proximity of such features in DF fans relative to MF fans yield higher r(l) values in the DF fans. A combination of fluvial erosion and secondary processes in the MF fans leads to a higher degree of surface dissection and erosion of interfluves. Field observations show a reduction in the elevation of interfluves, a higher density of small incised channels, small erosional terraces cut into the fan surface by fluvial erosion, and an infilling of channels. The sum of
Fig. 7. Plots of normalized local-relief vs. length-scale, r(l) for individual fans (a) and for means of fan classes (b). DF fans are solid lines, MF are dashed. The topographic fingerprint is revealed by the significantly higher rate of increase in r(l) at relatively short lengthscales (40–60 m) for DF fans than for MF fans. Standard 1 sigma uncertainties in mean values of r(l) (not shown here for clarity) are significantly smaller than the ordinate separation distances. The DF and two MF curves marked with asterisks are clearly anomalous, and if the corresponding fan-classes were reclassified to, respectively, MF and DF, the quality of the topographic fingerprint would be improved further still. The anomalous fans are similarly anomalous in their lithological composition (Fig. 4, ternary diagram) and in their drainage-basin areas (Fig. 8). See text for further explanation of this variation from the predominant trend.
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Fig. 8. Plots of drainage-basin size for each of the 29 fans grouped in the two classes, DF and MF. The anomalous fans identified by the topographic fingerprint (Fig. 4) are marked by asterisks.
across the previously mentioned anomalous MF fans. In any case, the relatively small fraction of fan-area occupied by a fault scarp will not contribute significantly to the fan-average value of local relief. There were also no indications of any anomalous surface features that would skew the data toward a higher relief value and therefore, place them within the same range of localized relief as the DF fans during a subsequent investigation of these two fans. A reassessment (during the subsequent field investigation) of the depositional units exposed along these two fans remains consistent with our initial MF classification. The two MF fans do have similar lithology and basin areas to DF fans (Fig. 8), but as previously stated this link is not supported by field analyses of the sedimentary structures and surficial features. Therefore, the two anomalous MF fans are not suitable for the DF or MF fan classification scheme identified during this study. A subsequent, more detailed field study of the DF fan provided evidence of sorted deposits and rectilinear channels consistent with the other MF fans. Initially, our rapid assessment did not cover this section of the fan and hence misidentified this fan as a DF fan. This reassessment and the low r(l) strongly indicate this fan is more in line with our definition of a mixed-flow fan. This is a case that highlights the strength of the topographic signature from the r(l) values. 4. Discussion The topographic fingerprint discovered here provides an objective method of characterizing the dominant processes that have generated the current alluvial fan surfaces. In this discussion of fan types, we do not want to lose sight of the fact that in reality, there exist a spectrum of fan morphologies that in turn reflect a wide variety of transport and aggradation events, both in time and space. We do not want to leave the reader with the impression that fans are either debris-flow types or mixed-flow types.
We anticipate, for example, that had we been able to examine alluvial fans in a significantly wetter environment, or a tropical climate that our results may have similarly reflected the transport and aggradation events occurring within this part of the spectrum of alluvial fans. At this stage, however, it remains remarkable that the DF and MF fans in this arid setting do appear to objectively fall into two classes based upon an analysis of the localized relief of the fans. These two fan classes appear most simply related to the water-content of sedimentladen flows that build up and incise the surface. The role of lithology identified within two alluvial fan classes cannot be completely ruled out. Lithologic controls have been previously identified as a factor leading to differences in fan processes on the west side of Death Valley (Blair, 1999) as well as at other locations (Al-Farraj and Harvey, 2005; Wagreich and Strauss, 2005). We identify differences in the dominant rock types within the two fan classes. However, caution should be advised in interpreting these data as they are taken from a very coarse-resolution data set (Workman et al., 2002) and the ternary diagram generalizes the variability of rock types present in the drainage basins. When all rock types in each of the MF drainage basins are considered the variability of the MF class increases. Several MF basins have in excess of five rock types present in their drainage basins, while other MF fans exhibit only three rock types. A majority of the DF fan drainage basins contain a single rock type, but two contain at least three rock types. The relatively coarse-scale geologic map utilized to extract the geology also further generalizes the patterns identified in the current study. A more detailed geology map (perhaps aided by the ALSM data) could provide a more definitive picture. Furthermore, it must also be stated that other studies indicate lithology has no control (Harvey, 1988; Ritter et al., 2000; Hartley et al., 2005) or at best plays an ambiguous role (Crosta and Frattini, 2004) in fan development and the spatial variability of formative processes. The unresolved role of lithology highlights the degree of the complexity associated with the factors leading to formative processes. The conflicting findings also speak to the need to further investigate the role of lithology, weathering, and sediment transport within the often-steep fan source areas. These types of studies are required to help fine-tune the topographic fingerprint and enhance our understanding of process-form relations in alluvial fans. The importance of the ALSM data cannot be understated in identifying the relationships explained in the results sections. The topographic signature is greatly improved by high-resolution elevation data acquired through the ALSM techniques. It is highly
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unlikely that standard 30 m DEM data would have produced a similar topographic signature. The lengthscale of many of the features (channels, levees, debris lobes, debris dams, etc.) identified on the fan surface is smaller than 30m grid cells (Staley et al., 2006). Furthermore, the fine-scale topographic signature would also be lost in an analysis of the 30 m DEM data. The results from the assessment of local-relief have broader implications with regard to fan evolution. Local-relief can be used as a means to evaluate both physical and numerical models of fan evolution (Hooke, 1967; Coulthard et al., 2002; Clevis et al., 2003). ALSM and other high-resolution mapping techniques have made it possible to produce analyses that provide definitive measures of surface topography. These quantitative measures of form can be used to test if the surface topography produced by evolutionary models is valid. Our assessment of local-relief has provided evidence topography varies between debris-flow and mixed-flow fans in Death Valley. Models of fans generated from varying processes should also reflect corresponding changes in topography, and if they do not, the validity of process-form relations in the model should be questioned. Long term observation of alluvial fan surfaces through our topographic fingerprint would also potentially provide evidence for changes in the dominate mechanism operating on the fan surface, which could be incorporated into numerical models, for more accurate representation of landform evolution. The calibrated models, with more accurate topography, would supply precise data for predicting flood and debris-flow hazards as well as providing better information for planning as cities expand into the surrounding piedmont areas. Detailed topography of alluvial fan data is required as numerical models attempt to simulate the nature of and interactions between fan process and form. These data are critical to understanding channel avulsion (Field, 2001) and the reactivation of flood prone surfaces. The results from studies of channel avulsion can be used to establish the most probable locations for floods on alluvial fans. Flood losses continue to plague many cities throughout the world where urban areas have expanded onto alluvial fans. More knowledge on process-form relations can lead to better management practices and inform decision-makers where to allocate money to protect certain locations at greater risk of flooding and mass movement hazards. Furthermore, the quantification of form presented in this article lends itself to comparisons of geomorphic forms on other planetary bodies. The authors are not suggesting the processes identified in the current study are acting on other planets. To the contrary, at present not
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enough information exists to determine how long-term patterns of processes and forms within Death Valley have varied in comparison with other planets. However, our results indicate differences in processes are visible in the surface topography of the alluvial fans in Death Valley. A similar approach could be applied to other planets (e.g. Mars) where topographic data are available. A topographic signature from analyses on other planets might provide a simple approach to show different topographies are present and signify different processes have been acting on the planet. More specifically, the topographic data could be used to provide quantitative evidence of water-based vs. mass wasting-based processes. 5. Conclusions The topographic signature identified in this study has provided a quantitative method for identification of the dominate mechanisms operating on alluvial fan surfaces in Death Valley, California. The use of high-resolution DEM gathered with Airborne Laser Swath Mapping techniques was instrumental in unraveling this signature. The unprecedented spatial resolution of the data permit us to generate elevation data that revealed the surface complexity of the alluvial fans. The ability to map the various surficial features in detail led to quantifiable measure of surface topography for comparison between the MF and DF fan classes. The results from the assessment of relative relief also have broader implications. Local-relief may be used as a means to evaluate both physical and numerical models of fan evolution. A more accurate understanding of the fan surface complexity as it relates to local-relief should provide hazards specialists with the ability to predict floods and mass movement events on fans. This is critical as sprawl in many of the cities throughout the world are pushing development onto alluvial fans surfaces. Finally, the methodology applied in the present study may lend itself to studying geomorphic forms on other planets. A topographic measure, similar to local-relief, could imply different processes have acted on other fan-like features identified on other planetary bodies. Acknowledgements This material is based upon work supported by the National Science Foundation under Grant No. 0239749. Any opinions, findings, and conclusions or recommendations are those of the authors and do not necessarily reflect the views of the National Science Foundation. Special thanks go to the National Center for Airborne Laser Mapping (NCALM) for all of their assistance in
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data collection and post-processing. We would also like to thank Dennis Staley (USFS, Golden, Colorado) for his assistance with the trend-surface analyses. We are very grateful to Roger LeB. Hooke for markedly improving an earlier version of this manuscript. We also thank Philip Giles and Vic Baker for their review comments, which solidified the final product. CERI contribution number 493. References Al-Farraj, A., Harvey, A.M., 2005. Morphometry and depositional style of Late Pleistocene alluvial fans: Wadi Al-Bih, northern UAE and Oman. Geological Society Special Publication, vol. 251, pp. 85–94. Anderson, D.E., Wells, S.G., 2003. Latest Pleistocene lake highstands in Death Valley, California. In: Enzel, Y., Wells, S.G., Lancaster, N. (Eds.), Paleoenvironments and Paleohydrology of the Mojave and Southern Great Basin Deserts, Boulder, Colorado. Geological Society of America Special Paper, vol. 368, pp. 115–128. Ashmore, P.E., Paola, C., Ferguson, R.I., Prestegaard, K.L., Ashworth, P.J., 1992. Secondary flow in anabranch confluences of a braided, gravel-bed stream. Earth Surface Processes and Landforms 17, 299–311. Beatty, C.B., 1963. Origin of alluvial fans, White Mountains, California and Nevada. Annals of the Association of American Geographers 53, 516–535. Blair, T.C., 1999. Cause of dominance by sheetflood vs. debris-flow processes on two adjoining alluvial fans, Death Valley, California. Sedimentology 46, 1015–1028. Blair, T.C., McPherson, J.G., 1992. The Trollheim alluvial fan and facies model revisited. Geological Society of America Bulletin 104, 762–769. Blair, T.C., McPherson, J.G., 1994a. Alluvial fan process and form. In: Abrahams, A.D., Parsons, A.J. (Eds.), Geomorphology of Desert Environments. Chapman and Hall, London, pp. 354–402. Blair, T.C., McPherson, J.G., 1994b. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. Journal of Sedimentary Research. Section A, Sedimentary Petrology and Processes 64, 450–489. Blair, T.C., McPherson, J.G., 1999. Grain-size and textural classification of coarse sedimentary particles. Journal of Sedimentary Research 69, 6–19. Bull, W.B., 1964. Geomorphology of segmented alluvial fans in western Fresno County, California. U.S. Geological Survey Professional Paper 352-E, 89–129. Bull, W.B., 1968. Alluvial fans. Journal of Geological Education 16, 101–106. Borrough, P.A., McDonnell, R., 1998. Principles of Geographical Information Systems (Spatial Information Systems). Oxford University Press, London. Clevis, Q., de Boer, P., Wachter, M., 2003. Numerical modelling of drainage basin evolution and three-dimensional alluvial fan stratigraphy. Sedimentary Geology 163, 85–110. Crosta, G.B., Frattini, P., 2004. Controls on modern alluvial fan processes in the Central Alps, Northern Italy. Earth Surface Processes and Landforms 29, 267–293. Coulthard, T.J., Macklin, M.G., Kirkby, M.J., 2002. A cellular model of Holocene upland river basin and alluvial fan evolution. Earth Surface Processes and Landforms 27, 269–288.
Field, J., 2001. Channel avulsion on alluvial fans in southern Arizona. Geomorphology 37, 93–104. Ferguson, R.I., Prestegaard, K.L., Ashmore, P.E., Ashworth, P.J., Paola, C., 1992. Measurements in a braided river chute and lobe, Flow pattern, sediment transport, and channel change. Water Resources Research 28, 1877–1886. Glenn, N.F., Thackray, G.D., Dorsch, S.J., Streutker, D.R., Chadwick, D.J., 2006. Analysis of LiDAR-derived topographic information for characterizing and differentiating landslide morphology and activity. Geomorphology 73, 131–148. Hartley, A.J., Mather, A.E., Jolley, E., Turner, P., 2005. Climaticcontrols an alluvial-fan activity, Coastal Cordillera, northern Chile. Geological Society Special Publication, vol. 251, pp. 95–116. Harvey, A.M., 1988. Controls of alluvial fan development: the alluvial fans of the Sierra de Carrascoy, Murcia, Spain. Catena Supplement 13, 123137. Hooke, R. LeB., 1967. Processes on arid-region alluvial fans. Journal of Geology 75, 438–460. Hooke, R. LeB., 1968. Steady-state relationships on arid-region alluvial fans in closed basins. American Journal of Science 266, 609–629. Hooke, R. LeB., 1972. Geomorphic evidence for late Wisconsin and Holocene tectonic deformation, Death Valley, California. Geological Society of America Bulletin 83, 2073–2098. Hooke, R. LeB., 1987. Mass movement in semi-arid environments and themorphology of alluvial fans. In: Anderson, M.G., Richards, K.S. (Eds.), Slope Stability. John Wiley & Sons Ltd, New York, pp. 505–529. Hooke, R. LeB., Rohrer, W.L., 1979. Geometry of alluvial fans; effect of discharge and sediment size. Earth Surface Processes 4, 147–166. Horton, R.E., 1945. Erosional development of streams and their drainage basins: hydrophysical approach to quantitative morphology. Geological Society of America Bulletin 56, 275–370. Iverson, R.M., 1997. The physics of debris flows. Reviews of Geophysics 35, 245–296. Knighton, A.D., Nanson, G.C., 2000. Waterhole form and process in the anastomosing channel system of Cooper Creek, Australia. Geomorphology 35, 101–117. Lane, S.N., 1998. The use of terrain modeling in the understanding of dynamic river channel systems. In: Lane, S.N., Richards, K., Chandler, J. (Eds.), Landforms Monitoring, Modeling, and Analysis. John Wiley & Sons, New York, pp. 311–342. Leece, S.A., 1991. Influence of lithologic erodibility on alluvial fan area, western White Mountains, California and Nevada. Earth Surfaces Processes and Landforms 16, 11–18. Li, J., Shangde, L., Lowenstein, T.K., Brown, C.B., Teh-Lung, K., 1996. A 100 ka record of water tables and paleoclimates from salt cores, Death Valley, California. Palaeogeography, Palaeoclimatology, Palaeoecology 123, 179–203. McKean, J., Roering, J.J., 2004. Landslide detection and surface morphology mapping with airborne laser altimetry. Geomorphology 57, 331–351. Oguchi, T., 1997. Drainage density and relative relief in humid steep mountains with frequent slope failure. Earth Surface Processes and Landforms 22, 107–120. Ortega-Ramírez, J., Fucugauchi, J.U., Mortera-Gutiérrez, C.A., Medina-Sánchez, J., Chacón-Cruz, G.J., Maillol, J.M., Bandy, W., Valiente-Banuet, A., 2004. Late Quaternary evolution of alluvial fans in the Playa, El Fresnal region, northern Chihuahua desert, Mexico: Palaeoclimatic implications. Geofisica Internacional 43, 445–466.
H.X. Volker et al. / Geomorphology 88 (2007) 34–45 Ritter, J.B., Miller, J.R., Husek-Wulforst, J., 2000. Environmental controls on the evolution of alluvial fans in Buena Vista Valley,m Nevada, during late Quaternary time. Geomorphology 36, 63–87. Shrestha, R.L., Carter, W.E., Lee, M., Finer, P., Sartori, M., 1999. Airborne laser swath mapping: accuracy assessment for surveying and mapping applications. Journal of American Congress on Surveying and Mapping 59, 83–94. Smith, R.S.U., 1976. Late Quaternary pluvial and tectonic history of Panamint Valley, Inyo and San Bernadino counties, California: Ph. D. dissertation, Calif. Inst. Tech. Staley, D.M., Wasklewicz, T.A., Blaszczynski, 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. Wagreich, M., Strauss, P.E., 2005. Source area and tectonic control on alluvial-fan development in the Miocene Fohnsdorf intramontane
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basin, Austria. Geological Society Special Publication, vol. 251, pp. 207–216. Whipple, K.X., Dunne, T., 1992. The influence of debris-flow rheology on fan morphology, Owens Valley, California. Geological Society of America Bulletin 104, 887–900. Whipple, K.X., Trayler, C.R., 1996. Tectonic control of fan size: the importance of spatially variable subsidence rates. Basin Research 8, 351–366. Whipple, K.X., Kirby, E., Brocklehurst, S.H., 1999. Geomorphic limits to climate-induced increases in topographic relief. Nature 401, 39–43. Workman, J.B., Menges, C.M., Page, W.R., Taylor, E., Ekren, B., Rowley, P.D., Dixon, G.L., Thompson, R.A., Wright, L.A., 2002. Geologic map of the Death Valley ground-water model area, Nevada and California. U.S. Department of the Interior and US, Geologic Survey.
A topographic fingerprint to distinguish alluvial fan formative processes H.X. Volker a,b , T.A. Wasklewicz a,⁎, M.A. Ellis a,c a
c
Department of Earth Sciences, University of Memphis, Memphis, TN 38152, USA b Fugro West, Inc., Ventura, CA 93003-7778, USA Center for Earthquake Research and Information, University of Memphis, Memphis, TN 38152, USA Received 30 May 2006; received in revised form 19 October 2006; accepted 19 October 2006 Available online 29 November 2006
Abstract We demonstrate here a topographic fingerprint, derived from a 1 m high-resolution elevation model (DEM), developed from Airborne Laser Swath Mapping (ALSM) that distinguishes between formative processes from surface form in alluvial fans generated through relatively dry debris flows (DF) and those surfaces formed primarily by fluvial sediment transport (MF). We selected alluvial fans with primarily Holocene age surficial deposits along the eastern side of Death Valley, California. Fans were initially classified in the field into DF and MF types, largely on the basis of sedimentological characteristics and surface morphometry. Trend surface analyses were performed on DEMs of the individual fans in order to remove the long-wavelength trends (both profile and planimetric). From the residual data, we found that high-resolution surface data distinguishes process information in two ways; 1) local-relief increases as a function of length-scale, especially at length scales less than 40–60 m, and 2) local-relief is significantly higher for debris-flow dominated fans than for fluvially dominated fans. This fingerprint, which is directly related to the dominant surface processes, is unavailable from standard 30 m resolution data, providing significant support for the acquisition of high-quality high-resolution data. This fingerprint can be used to help calibrate numerical simulations of alluvial fan development, and has the potential for similar applications to distinguish between processes of similar landforms on other planetary bodies within our solar system. © 2006 Elsevier B.V. All rights reserved. Keywords: Geomorphometry; LiDAR; GIS; Death Valley
1. Introduction A long-standing goal in geomorphology is to use form to reveal process (Horton, 1945; Ashmore et al., 1992; Ferguson et al., 1992; Lane, 1998; Knighton and Nanson, 2000). This is not an easy task, as landscapes ⁎ Corresponding author. Tel.: +1 901 678 4452; fax: +1 901 678 2178. E-mail address: [email protected] (T.A. Wasklewicz). 0169-555X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2006.10.008
are time-integrated surfaces generated by multiple processes, each coupled to the other or to boundary conditions, and each with response times that likely depend on boundary conditions. The alluvial fan literature is replete of studies employing digital elevation data that portray fan surface topography to distinguish formative processes. Here we use local relief as a function of length-scale, r(l), derived from one-meter planimetric resolution elevation data, to
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distinguish between debris and mixed-flow alluvial fans in Death Valley, Basin and Range Province, California (Fig. 1). The relative contribution of any particular process to the development of alluvial fans, as with every other landscape, is likely to change over time. This is particularly so for modern landscapes, many of which have been exposed to significant changes in climate throughout the Quaternary period. The compound stratigraphy of alluvial fans present clear evidence to support different formative processes dominate at various times (e.g., Ritter et al., 2000; Ortega-Ramírez et al., 2004). We minimize this issue by examining surficial deposits from
Fig. 1. Location of study area within Death Valley National Park (DVNP), California (CA), USA. Shaded-relief of topography along the Black Mountain range-front is shown using both the high-resolution, ALSM-derived DEM and the 30 m USGS NED DEM. BW is the Badwater fan and CC is the Copper Canyon fan. Outline of Fig. 3 is shown by the thicker white box.
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relatively young fans along the eastern flank of Death Valley. The lack of lake sediments and shorelines on these surfaces indicates the fan surface developed after the drawdown of pluvial Lake Manly (Hooke, 1972; Smith, 1976; Li et al., 1996), which likely occurred ∼ 12 ka (Anderson and Wells, 2003). While the relative age of the fan surficial units is constrained to the Holocene, differences in the age of the surficial depositional units are evident in the various degrees of varnish rubification and pavement development. Furthermore, the timing of depositional and erosional processes among the fans in the current population is not synchronized, such that not all fan surfaces have developed at the same time or from the same regional or local storm events during the Holocene. Some of the fans may also possess an internal core of material that dates to the pre-Holocene, but this has not been adequately tested to date. The current study provides an analysis of the recent fan surface and we do not attempt to trace the complex continuum of processes that have likely changed throughout the evolution of the fan as a whole. Alluvial fans in the Basin and Range are widely recognized as either debris-flow dominated (DF), fluvially dominated, or a mixture of debris-flows and ephemeral stream flow (termed mixed-flow fans (MF), largely on the basis of their sedimentary characteristics (Beatty, 1963; Hooke, 1967; Bull, 1968; Whipple and Dunne, 1992; Blair and McPherson, 1999). Debris flows leave characteristic topographies across alluvial fans: long and often sinuous channels bordered by prominent levees and in some instances terminated by a snout of relatively coarse sediment (Whipple and Dunne, 1992) (Fig. 2a). By contrast, fluvially dominated surfaces contain rectangular channels and generally smoother, flatter interfluves (likely a result of secondary processes as explained in Blair and McPherson, 1994a) (Fig. 2b). The fluvially dominated fans display a greater proportion of well-sorted fine sediments. Physical models, in combination with field analyses, corroborate variations in fan surface topography are a function of water content within the sediment-laden flows that both build and incise these surfaces (Hooke, 1968). The MF fans contain a combination of the aforementioned sedimentary and topographic characteristics. The aforementioned differences and our field observations lead us to hypothesize that fan topography contains information indicative of the processes associated with fan development. A variety of DEM values and analyses in geomorphic research have been applied to acquire information on surface topography variability. For example, landslide studies have generated various measures of topographic roughness to automatically delineate landslides within
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Fig. 2. A detailed view of some of the DF (a) and MF (b) fan surface features and stratigraphic units. Debris flows, which typically have coarser sediment loads, leave characteristic topographies across alluvial fans such as the channels bordered by prominent levees. These features are found consistently along the length of the debris flow fans (Staley et al., 2006). By contrast, wetter sediment-laden flow surfaces are intermixed with rectangular channels and generally smoother, flatter interfluves. Stratigraphically, the MF fans display a greater proportion of well-sorted fine sediments, as viewed along incised channels by the authors.
mountainous landscapes and to generate mechanical inferences on the recent behavior of landslides (McKean and Roering, 2004). Surface roughness has also been applied to the identification of landslide topographic elements and to correlate these elements with material
properties and localized landslide motions (Glenn et al., 2006). Another approach to interpreting topographic variability is a comparison of relative- or local-relief. Previous research has shown topographic relief in mountainous terrain is often associated with tectonic
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Fig. 3. A detailed view of the shaded relief derived from the ALSM high-resolution DEM of two of the fan types (DF6 is a debris flow fan and MF6 is a mixed-flow fan). Note the detail that can be readily observed in the fan and range-front morphology.
processes as opposed to climatically induced isostatic uplift (Whipple et al., 1999). At a more localized scale, Oguchi (1997) verified a negative correlation between drainage density and relative relief (the maximum height dispersion normalized by its length or area) and considered it to be a general characteristic of Japanese mountains. The relationship remains relatively constant when zero-order streams are added to the data analyses. Oguchi (1997) attributed the correlation to spatial variations in hillslope processes, whereby mass wasting in the zero-order streams promote the varying relations between drainage density and relative relief. However, none of these analytical approaches have been applied to alluvial fans to extract a process-oriented topographic signature. We show here the rate of change of localrelief derived from a high-resolution DEM enables fan
surfaces to be objectively classified and linked to a predominant formative process at a much shorter lengthscale than in previous studies. 2. Methods 2.1. Derivation of the digital elevation model (DEM) A one-meter DEM was derived from airborne laser swath mapping (ALSM) (Shrestha et al., 1999), generated by the National Science Foundation funded National Center for Airborne Laser Mapping (NCALM) within the University of Florida's Geosensing Systems Engineering Program. The data were acquired over a three day period: 5/29/03, 5/30/03, and 6/02/03 from heights ranging from 300 to 1100 m above ground-level.
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The variations in altitude during the data acquisition reflect a change in topography from the fan into the drainage basin as well as variability in surface topography along the length of the mountain front. The laser scanning was completed with a scan angle of 20° (1/2 Field Of View), a scan frequency of 28 Hz and a pulse rate of 33,333 Hz. The data were referenced to the North American Datum of 1983 (NAD83) horizontal and the Geodetic Reference System 1980 (GRS80) ellipsoid heights vertical datums. Laser shots were collected along separate flight strips (swaths) that overlapped at greater than 30%. Post-processing of the ALSM data was conducted at NCALM in late summer and early fall of 2003. Grid nodes were obtained through the application of Kriging to the combined overlapping swaths of laser shots to produce a best estimate elevation on a onemeter grid. The vertical precision is ± 10 cm, while the vertical resolution is b 0.01 m (Shrestha et al., 1999). Data were converted to UTM zone 11 coordinates, and analyses were conducted from one-meter raster grids in ESRI ArcInfo and MATLAB. 2.2. Classification and definition of alluvial fans Twenty-nine alluvial fans exposed contiguously along the range-front for which we had high-resolution topographic data (Fig. 1) were selected for the study. Each fan was classified as either debris flow (DF) or mixed flow (MF) (Figs. 2 and 3) using a field-based criteria described below. The general morphometric characteristics of the fan classes are specified in Table 1. Surficial features and sedimentological characteristics were recorded and photo-documented during field observations of all twenty-nine alluvial fans. The observations and documentation served as a framework for classifying the two fan groups. In particular, deposits along incised channels were examined in accordance with previous studies in Death Valley and adjacent areas in the Mojave Desert (Blair and McPherson, 1992, 1994a). Surface forms used in the classification of debris flow fans included; levees, sieve deposits, lobes, snouts, and debris plugs, as observed by Whipple and Table 1 The average morphometric statistics for the two fan classes (MF — mixed-flow fans; DF — debris flow fans) Fan class
Fan length (m)
Fan width (m)
Fan area (km2)
Fan slope (°)
DBA (km2)
MF DF
816.57 244.36
977.83 198.89
0.90 0.08
5.73 15.05
10.02 0.60
DBA is the drainage basin area of the fans.
Fig. 4. Geomorphological and geological characteristics of fans and source basins. (a) Plot of drainage basin-area vs. fan-area for each of 29 alluvial fans along the Black Mtn. range-front (see Fig. 1). Triangle symbols are DF fans, circles are MF. These data are consistent with Death Valley fans reported by Hooke (1968), shown by the broad gray line, and with data from other sets of alluvial fans: 1–2, Bull, 1964; 3–5, Hooke, 1968. The relationship of the fan area (Af; km2) in the current study with the drainage basin area (Ad; km2) is defined as Af = − 1.94 A0.95 . (b) A ternary compositional plot for the f three most dominant lithologies in the drainage basin, which are metamorphic (predominantly) schists (Xmi), Volcanic (intermediate and felsic) lava flows and tuffs (Tar), and intrusive (primarily) gabbro and diorite (Tws) (Workman et al., 2002), provided as a general indicator of the drainage basin geology. Note that the single DF symbol at 100% Xmi corresponds to 17 occurrences of this fan-class (denoted by the number 17 in the ternary diagram); the two occurrences of MF at 100% Xmi plot within the same range as the occurrences of DF also lie underneath the triangles. DF and MF symbols marked with an asterisk correspond to fans that are identified by a topographic fingerprint that does not match the initial classification (see text for detailed explanation).
Dunne (1992) and Hooke (1987) (Fig. 2a). Forms associated with fluvially derived fan surfaces included; terraces, interfluves, islands, and rectilinear channel banks (Blair and McPherson, 1994a,b) (Fig. 2b). The sedimentological distinctions were based largely on particle imbrication (Iverson, 1997) and matrix supported clasts (Blair and McPherson, 1994b), both indicative of debris flow deposition as well as the upward fining of particles, which is indicative of fluvial deposition (Blair and McPherson, 1992, 1994a).
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Fig. 5. A graphical portrayal of the local relief calculation derived from a series of hypothetical numbers. Each 1-m cell value is replaced with r(l). The r(l) was calculated at each of the following length scales: 1, 2, 3, 5, 10, 20, 30, and 50, 75, and 100-m, using the maximum amount of relief in the window (white box) divided by the total relief of the fan. The grey boxes to the right of the window represent the next series of raster cells to be measured during the moving window process used to generate the local relief values.
We mapped the fan outlines by deriving stream networks and by noting that higher order streams are often found at the margins of coalescing fan surfaces. This is particularly useful in cases where multiple fans comprised a bajada complex. The edge of each fan in the bajada complexes was demarcated by the top bank of the higher order stream incising between two adjacent fans. Additionally, slope, aspect, and curvature grids were used to lend further information to support our decision making process in separating fans from the bajada complex. This was necessary when hydrological networks did not completely define the extent of the entire fan surface. For example, convexity–concavity
variation can help delimit the top bank of the high-order streams between adjacent fans. The contributing drainage areas were delineated (from the 30-m USGS NED DEM) in ArcGIS for each fan surface. The general relationship between drainage area and fan area was assessed from these data. Our findings concur with previous results identified by investigators of similar fans in arid settings (Bull, 1964; Hooke, 1968; Hooke and Rohrer, 1979; Leece, 1991; Whipple and Trayler, 1996) (Fig. 4). This similarity provides a level of confidence that our method of defining fans is accurate and/or at least consistent with earlier cited methodologies.
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Fig. 6. The series of steps required to remove the broad-scale slope trends and subsequently permit us to analyze only the local-relief differences in the fan surfaces. First, second, and third order polynomial surfaces were investigated to assess the output from each. The first and third order surfaces were rejected as they respectively did not remove the slope trend or convoluted the surface. A second order polynomial surface provided the best approximation of the fan surface, while removing the planimetric and profile trends.
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2.3. Derivation of local-relief Local-relief r(l), defined here as the maximum difference in elevation within a square window of length l. Each 1-m grid cell was assigned an r(l) value based on the maximum relief value within the window. Relief values were normalized by the total relief of the fan (Fig. 5). For each of the fans, r(l) was derived at different length-scales (increasing window size) and maintained a 1 m cell size at the varying length-scales. An initial assessment of r(l) as a function of length-scale found it was dominated by the slope of the fan surface, especially at length-scales that approached the size of the fan. We performed a trend-surface analysis (using standard ESRI ArcInfo routines) of each fan in order to define and remove the long-wavelength shape of the fan (Fig. 6). For each fan, this process resulted in the removal of a second-order (quadratic) polynomial surface that best fit the original fan surface (Borrough and McDonnell, 1998) (Fig. 6). The trend surface removal allowed us to examine the topographic variability of the fan surface in a standardized manner without including global trends (e.g., the removal slope trends in both the planimetric and profile directions). The residual surface was used in subsequent analyses.
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these characteristics is quantified by the relatively low rate of increase in MF fans of r(l) values. Interestingly, there are apparent exceptions to the general result described above. Two MF fans exhibit r(l) values that place them unambiguously in the DF group (Fig. 7a), and likewise, one DF fan lies within the MF grouping (Fig. 7a). We re-examined these anomalous fans to see if we had overlooked evidence in their initial classification or to find an alternative interpretation for the inconsistent r(l) values identified in these three fans (one DF and two MF). Active faults have generated relatively high-relief scarps across some of the fan surfaces, but notably not
3. Results Mean r(l) is higher in most cases for fans constructed from debris flows than those formed by a combination of debris flows and fluvial flows (Fig. 7a). The distinction between MF and DF fans is even more evident in a comparison of fan groups (Fig. 7b). Here, the rate of increase of mean r(l) as a function of lengthscale in DF fans is significantly higher than that observed in MF fans at length-scales below 40–60 m. This is a significant result and indicates fans generated from disparate formative processes produce differing surface complexity. Greater differences in elevation at both short and long length-scales on DF fans reflect formative processes that contrast those active on MF fans. Debris flows, for example, produce lobes, levees, and snouts adjacent to channels. The proximity of such features in DF fans relative to MF fans yield higher r(l) values in the DF fans. A combination of fluvial erosion and secondary processes in the MF fans leads to a higher degree of surface dissection and erosion of interfluves. Field observations show a reduction in the elevation of interfluves, a higher density of small incised channels, small erosional terraces cut into the fan surface by fluvial erosion, and an infilling of channels. The sum of
Fig. 7. Plots of normalized local-relief vs. length-scale, r(l) for individual fans (a) and for means of fan classes (b). DF fans are solid lines, MF are dashed. The topographic fingerprint is revealed by the significantly higher rate of increase in r(l) at relatively short lengthscales (40–60 m) for DF fans than for MF fans. Standard 1 sigma uncertainties in mean values of r(l) (not shown here for clarity) are significantly smaller than the ordinate separation distances. The DF and two MF curves marked with asterisks are clearly anomalous, and if the corresponding fan-classes were reclassified to, respectively, MF and DF, the quality of the topographic fingerprint would be improved further still. The anomalous fans are similarly anomalous in their lithological composition (Fig. 4, ternary diagram) and in their drainage-basin areas (Fig. 8). See text for further explanation of this variation from the predominant trend.
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Fig. 8. Plots of drainage-basin size for each of the 29 fans grouped in the two classes, DF and MF. The anomalous fans identified by the topographic fingerprint (Fig. 4) are marked by asterisks.
across the previously mentioned anomalous MF fans. In any case, the relatively small fraction of fan-area occupied by a fault scarp will not contribute significantly to the fan-average value of local relief. There were also no indications of any anomalous surface features that would skew the data toward a higher relief value and therefore, place them within the same range of localized relief as the DF fans during a subsequent investigation of these two fans. A reassessment (during the subsequent field investigation) of the depositional units exposed along these two fans remains consistent with our initial MF classification. The two MF fans do have similar lithology and basin areas to DF fans (Fig. 8), but as previously stated this link is not supported by field analyses of the sedimentary structures and surficial features. Therefore, the two anomalous MF fans are not suitable for the DF or MF fan classification scheme identified during this study. A subsequent, more detailed field study of the DF fan provided evidence of sorted deposits and rectilinear channels consistent with the other MF fans. Initially, our rapid assessment did not cover this section of the fan and hence misidentified this fan as a DF fan. This reassessment and the low r(l) strongly indicate this fan is more in line with our definition of a mixed-flow fan. This is a case that highlights the strength of the topographic signature from the r(l) values. 4. Discussion The topographic fingerprint discovered here provides an objective method of characterizing the dominant processes that have generated the current alluvial fan surfaces. In this discussion of fan types, we do not want to lose sight of the fact that in reality, there exist a spectrum of fan morphologies that in turn reflect a wide variety of transport and aggradation events, both in time and space. We do not want to leave the reader with the impression that fans are either debris-flow types or mixed-flow types.
We anticipate, for example, that had we been able to examine alluvial fans in a significantly wetter environment, or a tropical climate that our results may have similarly reflected the transport and aggradation events occurring within this part of the spectrum of alluvial fans. At this stage, however, it remains remarkable that the DF and MF fans in this arid setting do appear to objectively fall into two classes based upon an analysis of the localized relief of the fans. These two fan classes appear most simply related to the water-content of sedimentladen flows that build up and incise the surface. The role of lithology identified within two alluvial fan classes cannot be completely ruled out. Lithologic controls have been previously identified as a factor leading to differences in fan processes on the west side of Death Valley (Blair, 1999) as well as at other locations (Al-Farraj and Harvey, 2005; Wagreich and Strauss, 2005). We identify differences in the dominant rock types within the two fan classes. However, caution should be advised in interpreting these data as they are taken from a very coarse-resolution data set (Workman et al., 2002) and the ternary diagram generalizes the variability of rock types present in the drainage basins. When all rock types in each of the MF drainage basins are considered the variability of the MF class increases. Several MF basins have in excess of five rock types present in their drainage basins, while other MF fans exhibit only three rock types. A majority of the DF fan drainage basins contain a single rock type, but two contain at least three rock types. The relatively coarse-scale geologic map utilized to extract the geology also further generalizes the patterns identified in the current study. A more detailed geology map (perhaps aided by the ALSM data) could provide a more definitive picture. Furthermore, it must also be stated that other studies indicate lithology has no control (Harvey, 1988; Ritter et al., 2000; Hartley et al., 2005) or at best plays an ambiguous role (Crosta and Frattini, 2004) in fan development and the spatial variability of formative processes. The unresolved role of lithology highlights the degree of the complexity associated with the factors leading to formative processes. The conflicting findings also speak to the need to further investigate the role of lithology, weathering, and sediment transport within the often-steep fan source areas. These types of studies are required to help fine-tune the topographic fingerprint and enhance our understanding of process-form relations in alluvial fans. The importance of the ALSM data cannot be understated in identifying the relationships explained in the results sections. The topographic signature is greatly improved by high-resolution elevation data acquired through the ALSM techniques. It is highly
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unlikely that standard 30 m DEM data would have produced a similar topographic signature. The lengthscale of many of the features (channels, levees, debris lobes, debris dams, etc.) identified on the fan surface is smaller than 30m grid cells (Staley et al., 2006). Furthermore, the fine-scale topographic signature would also be lost in an analysis of the 30 m DEM data. The results from the assessment of local-relief have broader implications with regard to fan evolution. Local-relief can be used as a means to evaluate both physical and numerical models of fan evolution (Hooke, 1967; Coulthard et al., 2002; Clevis et al., 2003). ALSM and other high-resolution mapping techniques have made it possible to produce analyses that provide definitive measures of surface topography. These quantitative measures of form can be used to test if the surface topography produced by evolutionary models is valid. Our assessment of local-relief has provided evidence topography varies between debris-flow and mixed-flow fans in Death Valley. Models of fans generated from varying processes should also reflect corresponding changes in topography, and if they do not, the validity of process-form relations in the model should be questioned. Long term observation of alluvial fan surfaces through our topographic fingerprint would also potentially provide evidence for changes in the dominate mechanism operating on the fan surface, which could be incorporated into numerical models, for more accurate representation of landform evolution. The calibrated models, with more accurate topography, would supply precise data for predicting flood and debris-flow hazards as well as providing better information for planning as cities expand into the surrounding piedmont areas. Detailed topography of alluvial fan data is required as numerical models attempt to simulate the nature of and interactions between fan process and form. These data are critical to understanding channel avulsion (Field, 2001) and the reactivation of flood prone surfaces. The results from studies of channel avulsion can be used to establish the most probable locations for floods on alluvial fans. Flood losses continue to plague many cities throughout the world where urban areas have expanded onto alluvial fans. More knowledge on process-form relations can lead to better management practices and inform decision-makers where to allocate money to protect certain locations at greater risk of flooding and mass movement hazards. Furthermore, the quantification of form presented in this article lends itself to comparisons of geomorphic forms on other planetary bodies. The authors are not suggesting the processes identified in the current study are acting on other planets. To the contrary, at present not
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enough information exists to determine how long-term patterns of processes and forms within Death Valley have varied in comparison with other planets. However, our results indicate differences in processes are visible in the surface topography of the alluvial fans in Death Valley. A similar approach could be applied to other planets (e.g. Mars) where topographic data are available. A topographic signature from analyses on other planets might provide a simple approach to show different topographies are present and signify different processes have been acting on the planet. More specifically, the topographic data could be used to provide quantitative evidence of water-based vs. mass wasting-based processes. 5. Conclusions The topographic signature identified in this study has provided a quantitative method for identification of the dominate mechanisms operating on alluvial fan surfaces in Death Valley, California. The use of high-resolution DEM gathered with Airborne Laser Swath Mapping techniques was instrumental in unraveling this signature. The unprecedented spatial resolution of the data permit us to generate elevation data that revealed the surface complexity of the alluvial fans. The ability to map the various surficial features in detail led to quantifiable measure of surface topography for comparison between the MF and DF fan classes. The results from the assessment of relative relief also have broader implications. Local-relief may be used as a means to evaluate both physical and numerical models of fan evolution. A more accurate understanding of the fan surface complexity as it relates to local-relief should provide hazards specialists with the ability to predict floods and mass movement events on fans. This is critical as sprawl in many of the cities throughout the world are pushing development onto alluvial fans surfaces. Finally, the methodology applied in the present study may lend itself to studying geomorphic forms on other planets. A topographic measure, similar to local-relief, could imply different processes have acted on other fan-like features identified on other planetary bodies. Acknowledgements This material is based upon work supported by the National Science Foundation under Grant No. 0239749. Any opinions, findings, and conclusions or recommendations are those of the authors and do not necessarily reflect the views of the National Science Foundation. Special thanks go to the National Center for Airborne Laser Mapping (NCALM) for all of their assistance in
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data collection and post-processing. We would also like to thank Dennis Staley (USFS, Golden, Colorado) for his assistance with the trend-surface analyses. We are very grateful to Roger LeB. Hooke for markedly improving an earlier version of this manuscript. We also thank Philip Giles and Vic Baker for their review comments, which solidified the final product. CERI contribution number 493. References Al-Farraj, A., Harvey, A.M., 2005. Morphometry and depositional style of Late Pleistocene alluvial fans: Wadi Al-Bih, northern UAE and Oman. Geological Society Special Publication, vol. 251, pp. 85–94. Anderson, D.E., Wells, S.G., 2003. Latest Pleistocene lake highstands in Death Valley, California. In: Enzel, Y., Wells, S.G., Lancaster, N. (Eds.), Paleoenvironments and Paleohydrology of the Mojave and Southern Great Basin Deserts, Boulder, Colorado. Geological Society of America Special Paper, vol. 368, pp. 115–128. Ashmore, P.E., Paola, C., Ferguson, R.I., Prestegaard, K.L., Ashworth, P.J., 1992. Secondary flow in anabranch confluences of a braided, gravel-bed stream. Earth Surface Processes and Landforms 17, 299–311. Beatty, C.B., 1963. Origin of alluvial fans, White Mountains, California and Nevada. Annals of the Association of American Geographers 53, 516–535. Blair, T.C., 1999. Cause of dominance by sheetflood vs. debris-flow processes on two adjoining alluvial fans, Death Valley, California. Sedimentology 46, 1015–1028. Blair, T.C., McPherson, J.G., 1992. The Trollheim alluvial fan and facies model revisited. Geological Society of America Bulletin 104, 762–769. Blair, T.C., McPherson, J.G., 1994a. Alluvial fan process and form. In: Abrahams, A.D., Parsons, A.J. (Eds.), Geomorphology of Desert Environments. Chapman and Hall, London, pp. 354–402. Blair, T.C., McPherson, J.G., 1994b. Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages. Journal of Sedimentary Research. Section A, Sedimentary Petrology and Processes 64, 450–489. Blair, T.C., McPherson, J.G., 1999. Grain-size and textural classification of coarse sedimentary particles. Journal of Sedimentary Research 69, 6–19. Bull, W.B., 1964. Geomorphology of segmented alluvial fans in western Fresno County, California. U.S. Geological Survey Professional Paper 352-E, 89–129. Bull, W.B., 1968. Alluvial fans. Journal of Geological Education 16, 101–106. Borrough, P.A., McDonnell, R., 1998. Principles of Geographical Information Systems (Spatial Information Systems). Oxford University Press, London. Clevis, Q., de Boer, P., Wachter, M., 2003. Numerical modelling of drainage basin evolution and three-dimensional alluvial fan stratigraphy. Sedimentary Geology 163, 85–110. Crosta, G.B., Frattini, P., 2004. Controls on modern alluvial fan processes in the Central Alps, Northern Italy. Earth Surface Processes and Landforms 29, 267–293. Coulthard, T.J., Macklin, M.G., Kirkby, M.J., 2002. A cellular model of Holocene upland river basin and alluvial fan evolution. Earth Surface Processes and Landforms 27, 269–288.
Field, J., 2001. Channel avulsion on alluvial fans in southern Arizona. Geomorphology 37, 93–104. Ferguson, R.I., Prestegaard, K.L., Ashmore, P.E., Ashworth, P.J., Paola, C., 1992. Measurements in a braided river chute and lobe, Flow pattern, sediment transport, and channel change. Water Resources Research 28, 1877–1886. Glenn, N.F., Thackray, G.D., Dorsch, S.J., Streutker, D.R., Chadwick, D.J., 2006. Analysis of LiDAR-derived topographic information for characterizing and differentiating landslide morphology and activity. Geomorphology 73, 131–148. Hartley, A.J., Mather, A.E., Jolley, E., Turner, P., 2005. Climaticcontrols an alluvial-fan activity, Coastal Cordillera, northern Chile. Geological Society Special Publication, vol. 251, pp. 95–116. Harvey, A.M., 1988. Controls of alluvial fan development: the alluvial fans of the Sierra de Carrascoy, Murcia, Spain. Catena Supplement 13, 123137. Hooke, R. LeB., 1967. Processes on arid-region alluvial fans. Journal of Geology 75, 438–460. Hooke, R. LeB., 1968. Steady-state relationships on arid-region alluvial fans in closed basins. American Journal of Science 266, 609–629. Hooke, R. LeB., 1972. Geomorphic evidence for late Wisconsin and Holocene tectonic deformation, Death Valley, California. Geological Society of America Bulletin 83, 2073–2098. Hooke, R. LeB., 1987. Mass movement in semi-arid environments and themorphology of alluvial fans. In: Anderson, M.G., Richards, K.S. (Eds.), Slope Stability. John Wiley & Sons Ltd, New York, pp. 505–529. Hooke, R. LeB., Rohrer, W.L., 1979. Geometry of alluvial fans; effect of discharge and sediment size. Earth Surface Processes 4, 147–166. Horton, R.E., 1945. Erosional development of streams and their drainage basins: hydrophysical approach to quantitative morphology. Geological Society of America Bulletin 56, 275–370. Iverson, R.M., 1997. The physics of debris flows. Reviews of Geophysics 35, 245–296. Knighton, A.D., Nanson, G.C., 2000. Waterhole form and process in the anastomosing channel system of Cooper Creek, Australia. Geomorphology 35, 101–117. Lane, S.N., 1998. The use of terrain modeling in the understanding of dynamic river channel systems. In: Lane, S.N., Richards, K., Chandler, J. (Eds.), Landforms Monitoring, Modeling, and Analysis. John Wiley & Sons, New York, pp. 311–342. Leece, S.A., 1991. Influence of lithologic erodibility on alluvial fan area, western White Mountains, California and Nevada. Earth Surfaces Processes and Landforms 16, 11–18. Li, J., Shangde, L., Lowenstein, T.K., Brown, C.B., Teh-Lung, K., 1996. A 100 ka record of water tables and paleoclimates from salt cores, Death Valley, California. Palaeogeography, Palaeoclimatology, Palaeoecology 123, 179–203. McKean, J., Roering, J.J., 2004. Landslide detection and surface morphology mapping with airborne laser altimetry. Geomorphology 57, 331–351. Oguchi, T., 1997. Drainage density and relative relief in humid steep mountains with frequent slope failure. Earth Surface Processes and Landforms 22, 107–120. Ortega-Ramírez, J., Fucugauchi, J.U., Mortera-Gutiérrez, C.A., Medina-Sánchez, J., Chacón-Cruz, G.J., Maillol, J.M., Bandy, W., Valiente-Banuet, A., 2004. Late Quaternary evolution of alluvial fans in the Playa, El Fresnal region, northern Chihuahua desert, Mexico: Palaeoclimatic implications. Geofisica Internacional 43, 445–466.
H.X. Volker et al. / Geomorphology 88 (2007) 34–45 Ritter, J.B., Miller, J.R., Husek-Wulforst, J., 2000. Environmental controls on the evolution of alluvial fans in Buena Vista Valley,m Nevada, during late Quaternary time. Geomorphology 36, 63–87. Shrestha, R.L., Carter, W.E., Lee, M., Finer, P., Sartori, M., 1999. Airborne laser swath mapping: accuracy assessment for surveying and mapping applications. Journal of American Congress on Surveying and Mapping 59, 83–94. Smith, R.S.U., 1976. Late Quaternary pluvial and tectonic history of Panamint Valley, Inyo and San Bernadino counties, California: Ph. D. dissertation, Calif. Inst. Tech. Staley, D.M., Wasklewicz, T.A., Blaszczynski, 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. Wagreich, M., Strauss, P.E., 2005. Source area and tectonic control on alluvial-fan development in the Miocene Fohnsdorf intramontane
45
basin, Austria. Geological Society Special Publication, vol. 251, pp. 207–216. Whipple, K.X., Dunne, T., 1992. The influence of debris-flow rheology on fan morphology, Owens Valley, California. Geological Society of America Bulletin 104, 887–900. Whipple, K.X., Trayler, C.R., 1996. Tectonic control of fan size: the importance of spatially variable subsidence rates. Basin Research 8, 351–366. Whipple, K.X., Kirby, E., Brocklehurst, S.H., 1999. Geomorphic limits to climate-induced increases in topographic relief. Nature 401, 39–43. Workman, J.B., Menges, C.M., Page, W.R., Taylor, E., Ekren, B., Rowley, P.D., Dixon, G.L., Thompson, R.A., Wright, L.A., 2002. Geologic map of the Death Valley ground-water model area, Nevada and California. U.S. Department of the Interior and US, Geologic Survey.