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12 Ice Jamsand their Impact onUrbanCommunities 199 Ice jamsandsubsequent floods According to the 1986definitionby the InternationalAssociationofHydraulicResearch (IAHR) WorkingGrouponRiver IceHydraulics, an ice jam isa ‘stationaryaccumulationof fragmented ice or frazil that restricts flow’ on a river or stream.6 Contributing factors for ice jams include bothweatherconditionsandthecourseof the river itself.On theonehand, longercoolperiods, even in autumn, and severalweeks of temperatures belowzero inwinter are necessary for the river water to cool down sufficiently. During the so-called ‘Little Ice Age’, as well as due to volcanic forcing, the number of cold autumn and winter seasons was higher than today. In addition, inpre-industrial times, this coolingwasalsomore readilypossiblebecause thewater wasnot–or only rarely–warmedby industrial effluents.On theotherhand, an ice-coverwill be produced on shallow water with a low current flow, and therefore if the riverbed is not straightened, but widely spread, it is more likely to appear. If the thick ice cover breaks up during warm late winter or early spring weather, the ice floes start moving and can become stuck in shallow areas or blocked by bridges or other buildings that constitute an obstacle. If we consider thatmany townshadbeenbuilt onplaceswherebridges across the river couldbe erected easily, ice jams aremore likely in an urban context. Furthermore, the vulnerability is normallyhigher indenselypopulatedareas. Ice jam floods are less predictable andpotentiallymoredestructive thanopen-water flood- ing and can producemuch deeper and faster flooding.7 The immediate increase of the water level due to ice jams can causemuchhigher flooding levels than any flood causedbymelting water or heavy rain. In particular, floodswith floating ice floes can be evenmore destructive than ‘normal’ open-water floodswith driftwood in thewater: the damage to bridges, houses, watermills, industrial complexes, harbours, ships and the cultivated environment as awhole is normallymuchgreater. Finally, people andcattle falling into the coldwater rarely survive. Today, ice jam flooding is mostly a problem in regions of the northern hemisphere with long andvery coldwinters, such as in Canada, thenorthern part of theUnited States, Scandi- naviaandRussia.Mostof the literature,bothrelatedtohistoricaleventsandcurrentones,deals with those regions, but notwith central orWestern Europe.8 In the natural sciences, research on river iceand ice jams isquiteadvanced thanks to theconferencesorganisedby theCommit- tee on River Ice Processes and the Environment (CRIPE) of the Canadian Geophysical Union (CGU) andby the InternationalAssociationofHydraulicResearch (IAHR)with a specificwork- ing group on ‘Ice Research and Engineering’.9 Historical studies focusing on pre-modern ice jam flooding are, however, extremely rare. One exception is ThomasWozniak’s case study of theunique ice jam inConstantinople in thewinter of 763/764.10A recent state-of-the-art paper byPrabinRokaya, SujataBudhathoki andKarl-ErichLindenschmidt also supports this impres- sion. Theauthors found, in total, 188paperspublished in journals andconferenceproceedings before October 2017 by creating aword cloud for the keywords in publications.Most of those papers are dedicated towater resources, engineering and geology (each of these three groups constitutingapproximately aquarter of theoverall publications), and the rest is also related to natural and technical sciences.Mostof thestudiesareonCanadaandAlaskaoron theRussian north,while fewer have consideredEurope.11 Even ifwe consider a strongbias, becausemany 6 Beltaos 1995, 71. 7 Cf. Beltaos 1995, 71–75. 8 Cf. Beltaos 1995 onCanada. For an overviewof the numerous publications of this author, see the bibliography in Rokaya et al. 2018, 1452. For a recent case study of the Aura River in Turku, Finland, covering the spring ice- breakup from1749onwards and thenumerous ice jam floods connected to them, seeNorrgård–Hellama2019. 9 <https://www.iahr.org/Portal/About_US/Technical_Division/Ice_Research_and_Engineering_Committee.aspx> (19. 07. 2019, contentnot available any longer). 10Wozniak 2017.Anenlargedanalysis of ice jam flooding in theEarly andHighMiddleAges is inWozniak 2020. 11 Rokaya et al. 2018, 1444f.