Morphological dynamics of an englacial channel

Geir Vatne,Tristram D. L. Irvine-Fynn

doi:10.5194/hess-20-2947-2016

Abstract

Abstract. Despite an interest in the hydraulic functioning of supraglacial and englacial channels over the last 4 decades, the processes and forms of such ice-bounded streams have remained poorly documented. Recent glaciological research has demonstrated the potential significance of so-called "cut-and-closure" streams, where englacial or subglacial flow paths are created from the long-term incision of supraglacial channels. These flow paths are reported to exhibit step-pool morphology, comprising knickpoints and/or knickzones, exaggerated in dimensions in comparison to supraglacial channels. However, little is known of the development of such channels' morphology. Here, we examine the spatial organisation of step pools and the upstream migration of steps, many of which form knickzones, with repeated surveys over a 10-year period in an englacial conduit in cold-based Austre Brøggerbreen, Svalbard. The observations show upstream step recession to be the dominant process for channel evolution. This is paralleled by an increase in average step height and conduit gradient over time. Characteristic channel-reach types and step-riser forms are consistently observed in each of the morphological surveys reported. We suggest that the formation of steps has a hydrodynamic origin, where step-pool geometry is more efficient for energy dissipation than meanders. The englacial channel system is one in rapid transition towards a quasi-equilibrium form within a decadal timescale. The evolution and recession of knickzones reported here result in the formation of a 37 m deep moulin shaft, suggesting that over time an incising supraglacial channel may evolve towards an englacial channel form exhibiting a stable end-point characterised by a singular vertical descent, which potentially can reach the glacier bed. This challenges the prevailing notions that crevasses or hydrofractures are needed to form deep moulins. Our observations highlight the need to further examine the adjustment processes in cut-and-closure channels to better understand their coupling to supraglacial meltwater sources and potential significance in cold-based glacier hydrology and ice dynamics.

Highlights

Fluvial geomorphologists have long been intrigued by the similarities between supraglacial and alluvial streams when it comes to meandering, especially as ice walled streams typically lack an entrained sediment load (Dozier, 1976; Knighton, 1972; Leopold et al, 1960; Zeller, 1967), and are typically characterised by a rapidly adjusting morphology (Dozier, 1974)
Supraglacial observations have suggested that intrinsic processes lead to the formation of staircase-like longitudinal profiles or step-pool sequences (Carver et al, 1994; Knighton, 1985, 1981); such features appear similar to the forms commonly found both in bedrock rivers (Hayakawa and Oguchi, 2006; Howard, 1998; Whipple and Tucker, 2002; Wohl and Grodek, 1994) and highgradient alluvial channels (Abrahams et al, 1995; Chin, 1998; Church and Zimmermann, 2007; Comiti et al, 2009; Grant et al, 1990; Molnar et al, 2010; Turowski et al, 2009)
Observed channel knickpoints exhibit large morphological variations that are difficult to place into discrete categories, we suggest that knickpoint morphology can be divided into three major types based on the morphology at the knickpoint lip, the step riser, and the presence, size and shape of appurtenant downstream pools and conduit void spaces (Fig. 1)

Summary

Introduction

Fluvial geomorphologists have long been intrigued by the similarities between supraglacial and alluvial streams when it comes to meandering, especially as ice walled streams typically lack an entrained sediment load (Dozier, 1976; Knighton, 1972; Leopold et al, 1960; Zeller, 1967), and are typically characterised by a rapidly adjusting morphology (Dozier, 1974). Due to the rapidity of morphological adjustment in ice, supraglacial and englacial streams represent natural, experimental opportunities to investigate hydrodynamic processes of channel adjustment and meandering in both the horizontal and vertical dimensions Despite this potential, the hydraulics and morphology of supraglacial streams have received surprisingly little attention over the last 25 years, despite a recent resurgence (Isenko and Mavluydov, 2002; Isenko et al, 2005; Jarosch and Gudmundsson, 2012; Karlstrom et al, 2013; Kostrzewski and Zwoliñski, 1995; Mantelli et al, 2015; Raymond and Nolan, 2000; Smith et al, 2015; Stock and Pinchak, 1995)

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Conclusion