Modelling of Ice-Jam Floods in the Lower Part of the Danube Basin: Case Studies During the Winters of 2011/2012 and 2016/2017

Abstract

The design flood (DF) for flood defence systems for large rivers is typically determined by taking into account the maximum water levels of the year, which typically occur in Central Europe between May and June (rivers like the Danube, Tisza, and Sava). The DF is typically calculated using a 1% probability of a flood peak and an additional 0,8-1,2 meters of freeboard height. Hydraulic models have been also employed to analyze scenarios of probable maximum floods (PMF). Nevertheless, despite their substantial influence on flood processes, ice-jam floods (IJFs) are rarely taken into account. Compared to open-channel flooding, the processes controlling the formation of peak flood waves under ice-jam conditions are very different. Consequently, these processes must be incorporated into calculations for probable maximum floods due to ice jamming (PMFice). Despite recent advancements in instrumentation, modelling techniques, and laboratory research, accurately defining hydraulic parameters under ice-covered conditions still requires direct field measurements. This study utilizes published hydrometric data from gauging stations along the Serbian part of the Danube River during ice-jam events in the winters of 2011/2012 and 2016/2017. Case studies were selected on a river section identified as critical during these periods. The analyses presented in this paper indicate that the Manning roughness coefficient for the entire river channel, including the ice cover, is approximately nc ≈ 0.0186 ms-1/3, while the coefficient for the ice sheet alone is estimated at ni ≈ 0.0145 ms-1/3. Hydraulic modelling of the studied river section yielded a Manning coefficient for the main channel of n ≈ 0.0225 ms-1/3, which may pose a challenge for designers. Since air-water friction is negligible in open-channel flow conditions, the hydraulic gradient under ice-covered flow conditions can be explained by the fact that it is roughly half of that in open-channel flow (R = H/2).