Seite - 200 - in The Power of Urban Water - Studies in premodern urbanism

200 ChristianRohr historical publications are not published in journal papers with keywords, and although the bibliography to this article containsat least somestudies related tohistorical floodevents, it is remarkable thatnatural scientific researchhashardly takennoteof studiesbasedondocumen- tary evidence. The above-mentioned study disqualifies them as ‘descriptive’12 and does not value them as important for understanding past and present-day societies and their coping strategies (e.g. settlementplaces, infrastructure, annual socio-economic living conditions). Ice floods in theMiddleAges: anoverview Ourknowledgeabout larger ice jamfloodson rivers and innarrowmaritimestraits in theEarly andHighMiddleAges (before 1250) is limited to short reports inannals andchronicles ofdam- age and casualties. One exception is thewinter of 763/764,which has been recently analysed by ThomasWozniak. His case study is based on 37 entries in chronicles, ofwhich 16 describe theextremecold intheyear763andanother21dealwiththeyear764.Forcontemporarychroni- clers, the unusually long duration of snow cover was noteworthy. The Byzantine chronicler TheophanestheConfessor (760–818)provides thelongestandmostdetailedeyewitnessdescrip- tion. His report states that the Black Seawas frozen from early October 763 toMarch 764 for 150km from the shoreline and to a depth of about 14m. According to Theophanes, the snow layer covering the icewasabout 9.5mdeep.When the iceburst inFebruary, somebig ice floes drifting through theBosporus strait destroyedparts of the townand citywalls of Constantino- ple.Altogether, thehardwinterwas reported in sources from Ireland,different Europeanmon- asteries andvariousByzantine authors.13 Wozniak’s larger research project has documentedmore than 60 years of extremewinter events during theperiodbetween 500and 1100.Most of the recordsdescribe only specific fea- tures:either theseasonsarementionedingeneral terms,suchashiemsmagna (astrongwinter), or the sourcesprovide indirect temperature indications related to the frozenwaterof riversand lakes, but also about the type and results of precipitation, such as a complete snow cover. In some cases, the chroniclers gave daily data for the onset of snow cover and the melting of the snow. The above-mentioned observations deviate very strongly in extreme years from the determined average values of today’s normal periods, with the winters of 859/860, 875/876, 975/976 or 1067/1068having a four- to five-month snow cover or thewinters of 708/709, 839/ 840,993/994or 1076/1077having snowcover lastingalmosthalf a year.14Wemayassume that in those years the larger rivers also had an ice cover, causing problemswith ice jams in early spring. There are only records for somewinters in annals and chronicles that tell us explicitly about completely frozen rivers and subsequent floods, such as the Elbe River in 814/815, the Rhine, Danube, Elbe and Seine rivers in 821/822, the Seine River in 848/849, the Rhine and Main rivers in880/881, theThamesandHavel rivers in927/928,numerousnot clearly specified rivers incentralandwesternEurope in1060and1068/1069, theWerraRiver incentralGermany and others in 1074/1075, as well as several large river catchments in all of Europe and the Bosporus strait in 1076/1077.15 In 1093,many rivers in England, theNetherlands andBelgium, aswell as theentireRhineRiver catchment,were frozen.Even rivers in Irelandwere sometimes frozen,suchas in thewintersof934/945,939/940and941/942.At least someof thoseextremely coldwintersmaybe connectedwith volcanic eruptions that aredocumented in ice cores, such as in821/822 and939/940.16 12 Rokaya et al. 2018, 1449. 13Wozniak 2017. 14Wozniak 2017, 155f.;Wozniak 2020. 15 Cf.Alexandre 1987, 339–341;Wozniak 2020withdetailed references. 16 Cf.McCormick et al. 2007, 878–889.