Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 Alluvial fans: geomorphology, sedimentology, dynamics - introduction. A review of alluvial-fan research ADRIAN M. HARVEY 1, ANNE E. MATHER 2 & MARTIN STOKES 3 1Department of Geography, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK (e-mail: amharvey @liverpool, ac. uk. ) 2 School of Geography, University of Plymouth, Drake Circus, Plymouth PIA 8AA, UK (e-mail: [email protected], uk.) 3 School of Earth, Ocean and Environmental Sciences, University of Plymouth, Drake Circus, Plymouth PL4 8AA, UK This volume presents a series of papers on the geo- may range from small debris cones (<50 m in morphology, sedimentology and dynamics of alluvial length: e.g. Wells & Harvey 1987; Brazier et al. fans, selected from those presented at the 'Alluvial 1988; Harvey & Wells 2003) to fluvially dominated Fans' Conference held in Sorbas, SE Spain in June megafans (up to 60 km in length: Gohain & Parkash 2003. The conference was sponsored primarily by the 1990). Alluvial fans, although common in desert British Geomorphological Research Group and the mountain regions (Harvey 1997), may occur in any British Sedimentological Research Group, both climatic environment, in arctic (e.g. Boothroyd & organizations affiliated to the Geological Society of Nummendal 1978), alpine (e.g. Kostaschuk et al. London. 1986), humid temperate (e.g. Kochel 1990) and It is some time since an international conference even humid tropical environments (e.g. Kesel & has been held that was exclusively devoted to the geo- Spicer 1985). morphology and sedimentology of alluvial fans. The In all climatic environments alluvial fans may play previous such conference was that organized by Terry an important buffering role in mountain geomorphic Blair and John McPherson in 1995, and held in Death or sediment systems (Harvey 1996, 1997, 2002b). Valley, a classic setting for alluvial fans (Denny 1965; They trap the bulk of the coarse sediment delivered Blair & McPherson 1994a). Although many of the from the mountain catchment, and therefore affect papers presented there have since been published, no the sediment dynamics downstream, either in rela- dedicated volume on alluvial fans as a whole resulted tion to distal fluvial systems or to sedimentary basin from that meeting, so even longer has elapsed since environments. In doing so, fans preserve a sensitive there has been a specific publication devoted wholly sedimentary record of environmental change within to a series of papers on the geomorphology and sedi- the mountain sediment-source area, rather than a mentology of alluvial fans (Rachocki & Church broad regional record as would, for example, pluvial 1990). lake sediments (Harvey et al. 1999b). South-east Spain was chosen as the venue for this One of the aims of the 2003 conference was to conference, partly for logistic reasons and partly bring together current research on the geomorph- because it is a tectonically active dry region within ology, sedimentology and dynamics of alluvial fans, which there is a wide range of Quaternary alluvial ranging from studies of modern processes through fans. These fans exhibit differing relationships to studies of Quaternary fans and to those of ancient between tectonic, climatic and base-level controls fan sequences within the geological record. We (Harvey 1990, 2002a, 2003; Mather & Stokes 2003; sought papers relating to a range of scales and of Mather et al. 2003), core themes in consideration of climatic and tectonic contexts. Some of the papers the dynamics of alluvial fans. presented at the conference were 'in press' in other An emphasis within the previous alluvial fan liter- publications at the time (e.g. Harvey & Wells ature has been on fans within the deserts of the 2003) or are being published elsewhere (Garcia American South-west (Bull 1977), with a focus & Stokes in press; Saito & Oguchi 2005; Stokes especially on the alluvial fans of Death Valley et al. in press). In this volume we present a selection (Denny 1965; Blair & McPherson 1994a). However, of the papers presented at the Sorbas meeting that alluvial fans are not exclusive to drylands, nor to the spans a wide range of alluvial fan research. We particular combination of processes and morphol- group them into three main themes: those dealing ogy that characterize the Death Valley fans. with processes on fans; those dealing with the Depositional processes may range from debris flows dynamics and morphology of Quaternary alluvial to sheet and channelized fluvial processes. Scales fans; and those dealing with the interpretation of From: HARVEY,A.M., MATHER,A.E. & STOKES,M. (eds) 2005. Alluvial Fans: Geomorphology, Sedimentology, Dynamics. Geological Society, London, Special Publications, 251, 1-7. 0305-8719/05/$15 9 The Geological Societyof London 2005. Downloaded from http://sp.lyellcollection.org/ by guest on September 28, 2021 2 A.M. HARVEY ETAL. alluvial fan sedimentary sequences from the ancient (by Arzani in Iran, and by Gabris & Nagy in rock record. Hungary). Processes on alluvial fans Dynamics of Quaternary alluvial fans In recent years there has been growing recogni- Research on Quaternary alluvial fans tends to focus tion of the range of sedimentary signals of fan on the relationships between fan sediments and mor- depositional processes. The importance of sheet- phology, but has moved beyond earlier descriptive flood processes as opposed to channelized fluvial morphological studies (e.g. Denny 1965; Hooke processes has been recognized on desert alluvial 1967, 1968; Bull 1977) and basic morphometric fans (Blair & McPherson 1994a), and there is studies (Bull 1977; Harvey 1984, 1997). More atten- growing recognition of the distinctions within mass- tion has been paid to the morphological style of allu- flow processes between true debris-flow processes vial fans insofar as this reflects fan setting, including and those associated with hyperconcentrated flows the conventional tectonically active mountain-front (Wells & Harvey 1987; Blair & McPherson 1994a, setting and backfilled fans on tectonically stable 1998), aided by advances in the understanding of the mountain fronts (Bull 1978), or tributary-junction rheology of mass flows in the lahar literature settings (Harvey 1997). Fan setting, in turn, affects (e.g. Cronin et al. 1997; Lavigne & Suwa 2004) and the degree of confinement (Sorriso-Valvo et al. in hydraulic engineering (e.g. Engelund & Wan 1998; Harvey et al. 1999a) and accommodation 1984; Julian & Lan 1991; Rickenmann 1991; Wang space (Viseras et al. 2003). In addition, fan style et al. 1994). In this volume we include two papers on reflects the geomorphic regime of the fan, from fan sedimentation processes, in --lllal --"- l~CUl -" ...........y ~Ull LL ,'1 ~ I.K~ld "~ ag~xaum~, u.uu~;n tu p~ug~au~n~; tc~cat, uplC fans and environments, by Mather & Itartley and by to fans undergoing dissection (Harvey 2002a). This Wilford et al. in hyper-arid and humid environ- is controlled fundamentally by the relationships ments, respectively. between sediment supply to the fan and by flood Building on previous work (e.g. Bull 1977; power, in other words by the threshold of critical Kostaschuk et al. 1986), it is realized that distinct fan stream power (Bull 1979). surface gradients result from different depositional Perhaps the main emphasis over recent years in processes, but the concept of a specific 'slope gap' research on Quaternary alluvial fans has been to (Blair & McPherson 1994b) between alluvial fan and consider how the gross factors controlling alluvial- river gradients has since been demonstrated to have fan environments (catchment controls, tectonics and been flawed (Kim 1995; McCarthy & Candle 1995; long-term geomorphic evolution, climatic controls, Harvey 2002c; Saito & Oguchi 2005). To some base level) interact to create fan morphological and extent Blair & McPherson's (1994a) classification of sedimentary styles, and how fans respond to changes alluvial-fan styles, based on process combinations, in the controlling factors. Studies of fan morphome- reinforces the traditional concept of 'wet fans and try have developed to focus on these interactions dry fans' (Schumm 1977); however, the application (Silva et al. 1992; Calvache et al. 1997; Harvey et al. of that concept to a climatic association with humid 1999a; Harvey 2002c; Viseras et al. 2003). and arid climates, respectively, is clearly oversim- It has long been realized that of the controlling plistic. On the contrary, arid environments tend to be factors, catchment characteristics (including drainage more fluvially dominant than do humid regions basin area, relief and geology) control the supply of (Baker 1977), and many studies have demonstrated water and sediment to the fan, and therefore the an increase in fluvial activity on fans, resulting from process regime on the fan and the resulting fan mor- climatic aridification (e.g. Harvey & Wells 1994), phology. Although emphasized in the geological or an increase in hillslope sediment supply from literature (see below), tectonic controls may influence debris-flow activity following a climatic change sediment production in the source area, and, together towards greater humidity (e.g. Gerson 1982; Bull with gross topography, appear primarily to control 1991; A1-Farraj 1996). fan location, fan setting and gross fan geometry There is increasing recognition that alluvial fans (Harvey 2002a) rather than fan-sediment sequences. represent a continuum of depositional processes Tectonics appear to influence fan morphology and (Saito & Oguchi 2005) from small debris cones, sedimentary sequences primarily through an influ- characteristic of many mountain environments (e.g ence on accommodation space (Silva et al. 1992; Brazier et al. 1988), especially in paraglacial zones Viseras et al. 2003). (Ryder 1971), to large fluvially dominant fans, such For Quaternary alluvial fans in a wide variety of as the Kosi megafan in India (Gohain & Parkash environments, climatic factors appear to have an 1990). In this volume we include two papers on large overwhelming control on fan morphology and sedi- fluvially dominated fans in contrasted environments mentary styles.