Erosion and deposition in the proglacial zone: the 1996 jôkulhlaup on Skeiôarârsandur, southeast Iceland

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1 The Extremes ofthe Extremes: Eylraordinarv Floods (Proceedings of a symposium held al Reykjavik. Iceland. July 2000). IAHS Publ. no Erosion and deposition in the proglacial zone: the 1996 jôkulhlaup on Skeiôarârsandur, southeast Iceland BASIL GOMEZ Geomorphology Laboratory, Indiana State University. Terre Haute, Indiana 47S09, USA bgomez@indslate.edu ANDREW J. RUSSELL School of Earth Sciences and Geography, Keele University, Keele, Staffordshire ST5 5BG, UK LAURENCE C. SMITH Department of Geography, University of California, Los Angeles, California 90095, USA OSKAR KNUDSEN Klettur Consulting Engineers, Bildshofda 12, Reykjavik IS-112, Iceland Abstract The November 1996 jôkulhlaup from Skeiôarârjôkull Glacier, Iceland, had little impact on the proximal surface of Skeiôarârsandur, though most channel change occurred in the proximal zone. Patterns of erosion and deposition were revealed by aerial photography, repeat-pass interferometry and field survey. The jôkulhlaup bypassed the proximal zone because meltwater ponded in an icemarginal depression, which regulated the flow of water and calibre of sediment supplied to Skeiôarârsandur, and most drainage was through a single primary outlet (the Gigjukvisl River). The geomorphic impact of jôkulhlaups may vary between periods of glacier advance when a glacier and sandur are coupled and active aggradation occurs in the proximal zone, and periods of glacier retreat when the glacier is decoupled from the sandur. The style of sedimentation in rivers which route water and sediment directly on to the sandar will also differ from that in rivers buffered by ponding in the proglacial zone. Key words sandur; Iceland; interferometry; jôkulhlaup; proglacial zone INTRODUCTION We discuss the influence that topographic controls had on patterns of discharge and sedimentation within the proximal zone of Skeiôarârsandur, during the November 1996 jôkulhlaup from Skeiôarârjôkull Glacier, south Iceland (Fig. 1(a)). Aerial photographs obtained on 6 November, 12 h after the flood peak (Fig. 1(b)), and repeat-pass interferometry using synthetic aperture radar (SAR) images acquired on 24 October 1996 and on 2 January 1997 (Fig. 1(c)) provide a synoptic perspective on patterns of erosion and deposition associated with the event. These independently derived measures of topographic change afford the first detailed, macroscale perspective on the erosional and depositional characteristics of a single jôkulhlaup on a sandur. This, in turn, allows us to discern how routing jôkulhlaups through different proglacial settings may affect sandur morphology.

2 218 Basil Gomez et al. SUHi-wirâriôKull 0 Fig. 1 (a) Location map (area covered by radar imagery is delimited by solid rectangle), and physiography of the proglacial zone and meltwater drainage routes during the 1996 jôkulhlaup. (b) Photomosaic: the proglacial zone showing ponding and meltwater drainage routes during the 1996 jôkulhlaup. Compiled from photographs obtained at 11:45 local time, on 6 November 1996 National Land Survey of Iceland (area shown is delimited by rectangle in (a)), (c) Interferometric phase correlation on Skeiôarârsandur between 24 October 1996 and 2 January 1997, overlain on 2 January radar backscatter image. High phase correlation (black) indicates surfaces that were undisturbed by the 1996 jôkulhlaup (after Smith et al, 2000).

Erosion and deposition in the proglacial zone: the 1996jôkulhlaup on Skeiôarârsandur 219 STUDY AREA Skeiôarârsandur has an area of 1350 km 2, and is the largest active glacial outwash plain on Earth

3 Erosion and deposition in the proglacial zone: the 1996jôkulhlaup on Skeiôarârsandur 219 STUDY AREA Skeiôarârsandur has an area of 1350 km 2, and is the largest active glacial outwash plain on Earth (Fig. 1(c)). It is being constructed by braided rivers emanating from Skeiôarârjôkull Glacier; a lobate outlet glacier on the southern margin of the Vatnajôkull ice cap. The ice-cored terminal moraine and its associated suite of icedisintegration and ice-contact landforms at the head of Skeiôarârsandur date to These features lie conformably on an older sandur surface. Between 1920 and 1960 the glacier retreated 1-3 km from the moraine complex, though the rate of recession subsequently slowed. The amount of recession was greatest along the western margin of the ice front and the proglacial zone behind the moraine complex is asymmetric. It is widest in the west, where the proglacial zone incorporates the moraine complex and a 1-km-wide, 25-m-deep ice-marginal depression that developed between 1931 and 1960 (Fig. 1(b)). No terminal moraine is visible beyond the eastern margin of Skeiôarârjôkull Glacier, where the ice-marginal depression is 250 m wide and 14 m deep. Jôkulhlaups occur periodically on Skeiôarârsandur, though flood peaks have declined since the late 1930s as event frequency and geothermal activity have increased (Bjôrnsson, 1988). Meltwater emerges through numerous subglacial tunnels at the ice front, but three large, braided stream systems (the Gigjukvisl River, the Skeiôarâ River and the Nûpsvôtn River) route water and sediment onto Skeiôarârsandur (Fig. 1(c)). Overflow channels, such as the Hâôldukvisl Channel and Saeluhûsskvisl Channel, are also activated during jôkulhlaups when water ponds in the proglacial zone (Fig. 1(b)). The 5-6 November 1996 jôkulhlaup was the largest recorded on Skeiôarârsandur. Flood water entered the Skeiôarâ River on the morning of 5 November, and in the succeeding two days 3.8 km 3 of water drained across the sandur. The Gigjukvisl River and the Skeiôarâ River conveyed 60% and 34% of the total volume of flood water, respectively (Snorrason et al, 1997), and the peak discharge in the Gigjukvisl Rivetwas estimated to be 1.9 x 10 4 m 3 s" 1 (Russell et al, 1999). TOPOGRAPHIC CONTROL ON EROSION AND DEPOSITION The ice-marginal depression that separates the proximal zone of Skeiôarârsandur from the glacier terminus played a fundamental role in regulating the flow of water and sediment on to the sandur during the November 1996 jôkulhlaup (Fig. 1(b)). Meltwater which issued from a plethora of sub- and supra-glacial sources around the west-central and central margin of Skeiôarârjôkull Glacier including the spectacular the Gigjukvisl River ice-walled channel (Russell et al, 1999), ponded in the depression. Ponding attenuated the jôkulhlaup hydrograph in the Gigjukvisl River (Russell et al, 1999), and activated the Hâôldukvisl Channel and the Sasluhusskvisl Cannel (Fig. 1(b)); though the Gigjukvisl River functioned as the primary outlet for flood water routed on to Skeiôarârsandur. Because meltwater discharge was focused on the Gigjukvisl River the jôkulhlaup bypassed Skeiôarârsandur's proximal zone (Fig. 1(c)), less than 25% of which was inundated (Gomez et al, 2000). The topographic surveys and repeat-pass SAR interferometry also revealed that the ice-marginal depression acted as a sediment trap and was a zone of net deposition, with major proximal deposition occurring in front of the primary meltwater outlets (Smith et al., 2000).

4 220 Basil Gomez et al. There was some reworking of bed and bank materials in the Hâôldukvisl Channel and the Sseluhusskvisl Channel (which appear to have primarily transported relatively fine, suspended sediments), but most erosion occurred in the Gigjukvisl Channel. Erosion doubled the width of the Gigjukvisl River at the point where it flows through the ice-cored terminal moraine, and x 10 6 m 3 of sediment were eroded from the reach immediately downstream, creating a wider, shallower, less sinuous channel (Russell & Knudsen, 1999; Smith et al, 2000). The nature and evolution of the contemporary integrated, ice-marginal drainage network that routes water and sediment on to Skeiôarârsandur through a small number of primary outlets are discussed elsewhere (Gomez et al, 2000). This point-source distributary system is in marked contrast to the more diffuse meltwater distributary system that was in operation early last century (Churski, 1973). At that time, the Gigjukvisl River and numerous other topographic lows in the moraine complex constituted a fully functional multipoint dispersal system that routed water and sediment directly on to the active sandur surface, which began directly in front of Skeiôarârjôkull Glacier. This surface consisted of an apron of small, coalescing, coarse-grained outwash fans that merged with the older sandur surface at a point some 4 km beyond the gaps in the moraine complex from which the feeder channels emerged. It also contrasts with conditions in the Skeiôarâ River and the Nûpsvôtn River which route meltwater from the flanks of Skeiôarârjôkull River directly onto the sandur (Fig. 1(c)). The geometry of the distributary channels and presence of inactive surfaces in the proximal zone during the 1996 jôkulhlaup contrasts with the picture of a complex network of braided channels, and coalescing outwash fans that are coupled to the ice margin, that the historical and stratigraphie evidence suggests (Churski, 1973; Maizels, 1991). The event had little impact on the proximal surface of Skeiôarârsandur beyond the confines of the entrenched channels that traverse it because Skeiôarârjôkull Glacier Fig. 2 Coupled and decoupled meltwater dispersal systems. Inset graph summarizes the effect ponding in the proglacial zone has on the jôkulhlaup hydrograph (after Russell & Knudsen, 1999).

5 Erosion and deposition in the proglacial zone: the 1996jôkulhlaup on Skeiôarârsandur 221 is decoupled from the sandur. Erosion and deposition occurred throughout the proglacial zone, but most geomorphic change occurred within the ice-marginal depression, not on Skeiôarârsandur. The presence or absence of an ice-marginal depression may act as a major control on patterns of erosion and deposition during high-magnitude jôkulhlaups. We therefore expect the geomorphic impact of jôkulhlaups to vary between periods of glacier advance associated with active sandur aggradation in the proximal zone and periods of glacier retreat characterized by depression formation (Fig. 2). In the former case, when the glacier is coupled to the sandur, drainage is down the topographic slope. Moreover, the source areas from which sediment and water originate are free to shift in both space and time. In the latter case, the proximal surface of the sandur is decoupled both from the ice front and the rivers that traverse it (Fig. 1(c)). The existence of a reverse slope causes water to pond in the proglacial zone and runnels water through a small number of primary outlets that have fixed locations. This has the effect of attenuating the flood hydrograph and delaying the flood peak (Fig. 2). The ice-marginal depression also acts as a sediment trap, but it does not affect the transport of sand-sized material through channels in the proximal zone. However, the coarse sediment is filtered out, and the development of a point-source dispersal system may thus give the appearance of increasingly distal conditions in the channels that traverse the proximal zone. For this reason, we expect the style of sedimentation in the Skeiôarâ River and the Nûpsvôtn River, which route water and sediment directly on to Skeiôarârsandur, to differ from that in the Gigjukvisl River at comparable points downsandur. This variation is the subject of further investigation. Acknowledgements Our programme of fieldwork and image interpretation would not have been possible without the assistance of numerous friends and colleagues, or the support provided by NSF grant SBR , NERC grant GR3/10960, NASA grant NAG5-7555, the Icelandic Public Roads Administration, and the Icelandic Research Council. REFERENCES Bjôrnsson, H. (1988) Hydrology of Ice Caps in Volcanic Regions. Societas Scienlarium Islandica, Reykjavik. Churski, Z. (1973) Hydrographie features ofthe proglacial area of Skeiôarârjôkull. Geogr. Polonica 26, Gomez, B., Smith, L. C., Magilligan, F. J., Merles, L. A. K. & Smith, N. D. (2000) Glacier outburst floods and outwash plain development: Skeiôarârsandur, Iceland. Terra Nova 12, Maizels,.1. K. (1991) The origin and evolution of Holocene sandur deposits in areas of jôkulhlaup drainage, Iceland. In: Environmental Change in Iceland (ed. by.1. K. Maizels & C. Caseldine), Kluwer, Dordrecht, The Netherlands. Russell, A..1. & Knudsen, Ô. (1999) Controls on the Sedimentology of November 1996 Jôkulhlaup Deposits. Skeiôarârsandur, Iceland, Int. Assoc. Sediment. Spec. Publ. no. 28, Blackwell Scientific, Oxford. Russell, A..1., Knudsen, Ô., Maizels, J. K. & Marren, P. M. (1999) Channel cross-sectional area changes and peak discharge calculations in the Gigjukvisl River during the November 1996 jôkulhlaup, Skeiôarârsandur, Iceland. Jôkull 47, Smith, L. C, Alsdorf, D. E., Magilligan, F..1., Gomez, B., Mertes, L. A. K., Smith, N. D. & Garvin,.1. B. (2000) Estimation of erosion, deposition, and net volumetric change caused by the 1996 Skeiôarârsandur jôkulhlaup, Iceland, from SAR interferometry. Wat. Resour. Res. 36, Snorrason, Â., Jônsson, P., Pâlsson, S., Ârnason, O., Sigurôsson, S., Vikingsson, S., Sigurôsson, Â. & Zôphaniasson, S. (1997) Hlaupiô â Skeiôarârsandi haustiô 1996: Ûtbreiôsla, rennsli og aurburôur. In: Vatnajôkull: Gos oghlaup (ed. by H. Haraldsson), Vegagerôin, Reykjavik.

Controls on the development of supraglacial flood water outlets during jôkulhlaups

The Extremes of the Extremes: Extraordinary Floods (Proceedings of a symposium lieid at Reykjavik, Iceland. July 2000). IAHS Publ. no. 271. 2002. 71 Controls on the development of supraglacial flood water