Theretreatofatidewaterglacier:observationsandmodel calculationsonHansbreen,Spitsbergen Andreas Vieli, 1,2* Jacek Jania, 3 Lezek Kolondra 3 1 InstituteforAtmosphericandClimateScience,Eidgeno«ssischeTechnischeHochschule,Winterthurerstrasse190,CH-8057Zu« rich,Switzerland E-mail:a.vieli@bristol.ac.uk 2 SectionofGlaciology,Versuchsanstaltfu« rWasserbau,HydrologieundGlaziologie,Eidgeno«ssischeTechnischeHochschule,ETH-Zentrum, CH-8092Zu« rich,Switzerland 3 Department of Geomorphology, Facultyof Earth Sciences, Universityof Silesia, ul. Be°dzin¨ ska 60, PL-41-200 Sosnowiec, Poland ABSTRACT . Based on observations and model calculations, the retreat over the last two decadesof Hansbreen, atidewaterglacier in southern Spitsbergen, Svalbard, isinvesti- gated.The observations of the calving-front position between 1982 and 1998 show an abrupt retreat in 1990, which is suggested to be related to a depression in the glacier bed. The observed seasonal variations of the front position are mainly due to variations of the calving rate.The observations of Hansbreen further indicate that during periods of slow front-position changes, melting at the water-line may play an important role in triggering theprocessofcalving.TheevolutionofHansbreenbetween1982and1998issimulatedwith a numerical model for the dynamics of tidewater glaciers. Using a flotation criterion for calvinginwhichforeachtime-stepthepartoftheglacierterminuswhichisbelowacritical height above buoyancy is removed, we are able to reproduce the observed rapid retreat of Hansbreen through the depression in the glacier bed. From the observations and model calculations, we conclude that the rapid retreat is mainly an effect of basaltopography in the terminus region and not a direct response to a change in massbalance. INTRODUCTION The dynamical behaviour of tidewater glaciers, which are considered as grounded calving glaciers from which icebergs are discharged intothe sea, is important with respect totheir reaction to changes in climate. Length changes of tidewater glaciers are not only a result of a change in surface mass balance, they are additionally affected by calving at the ter- minus. Rapid retreats relatedto increasedcalving rates have been observed for severalgroundedcalvingglaciers (Van der Veen,1996;Warren and Aniya,1999).These rapidchanges in front positions have been interpretedas less a direct response to changes in climate than a strong effect of the local basal topographyneartheterminus(MeierandPost,1987;Warren, 1993; Naruse and Skvarca,2000;Vieliandothers,2001). In this paper, we focus on the dynamical behaviour of Hansbreen, a tidewater glacier, in southern Spitsbergen, Svalbard.With a length of 16km and frontal flow velocities up to 150 m a ^1 , Hansbreen is a relatively small, thin and slowly flowing tidewater glacier. Detailed front-position records between 1982 and 1998 exist, and the basal topog- raphy of the terminus region is known. Such an extensive datasetonly exists for a few tidewater glaciers and provides the opportunity toinvestigatetheeffectof basaltopography on the change of terminus position and to test a numerical model of the dynamics of tidewater glaciers. A special focus is provided on the observed rapid retreat of Hansbreen in 1990 through a depression inthe glacier bed. Calving Calving is avery efficient ablation mechanism, and therefore isanimportantprocessforthedynamicsoftidewaterglaciers. Basedonobservations,empiricalrelationsforthecalving rate u c , which is defined as the difference between ice velocity at theterminusandtherateofchangeoftheglacierlength,have been determined. A linear relationbetween the calving rate and the water depth d at the terminus was suggested fortide- water glaciers by Brown and others (1982). A linear relation has also been suggested for glaciers calving into fresh water, but for a given water depth the calving rate in fresh water is expectedtobe one orderof magnitudebelow the value found in seawater (Funkand Ro« thlisberger,1989). Depressions in the basal topography are observed to be typical in the frontal region of tidewater glaciers. According tothese empirical relations, the calving rate increases with a retreat of the terminus into deeper water, and a rapid unstable retreat through a depression in the glacier bed is therefore expected. This has been observed for several tide- waterandfresh-watercalvingglaciers and suggeststhatthese glaciers undergo cycles of slow advance and rapid retreat through depressions in the glacier bed (Meier and Post, 1987). For the case of rapidlychangingglaciersthe validityof these linear empirical relations has been questioned (Meier and Post,1987;VanderVeen,1996;Vieliandothers,2001). Journalof Glaciology , Vol. 48, No .163, 2002 * Present address: Bristol Glaciology Centre, School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, England. 592 Downloaded from https://www.cambridge.org/core. 09 Nov 2021 at 13:40:35, subject to the Cambridge Core terms of use.