**4. Potential of Including Ice-Jam Flood Hazards and Risks in the EU Floods Directive** *4.1. Norwegian Perspective*

Although ice jams are mentioned in the new guidelines for flood zone maps currently, after hearing from the Norwegian Water Resources and Energy Directorate (NVE), it appears that the guidelines may not be very clear on a specific procedure for mapping ice-induced flooding. For regulated rivers, there are some restrictions on operations to avoid ice-induced floods, particularly the placement and adjustment of intakes to prevent exposing open water to frigid air and avoid huge frazil generation. Operational restrictions are mainly related to freeze-up flooding due to frazil and anchor ice, but also to the risk of breakup in the case of accidental shutdowns and water being released into bypass reaches. In the proposed guidelines, it is stated that ice jams rarely exceed the 200-year open water flood in Norway, and therefore it is not a central component in the flood zone mapping procedure, since the 200-year level is critical in the Norwegian building code. The guidelines acknowledge that it can be an issue and recommend considering ice where problems with ice have been observed in the past. This can particularly be an issue where water is diverted from the river (but not on the 200-year return period level). At best, ice jams that have threatened hydro-power generation or are caused by hydropower shutdowns are archived using maps or local images, an example of an ice-jam event that occurred on the Svorkmo River in Norway is shown in Figure 3.

#### *4.2. Swedish Perspective*

The Swedish Meteorological and Hydrological Institute (SMHI) also sees the main issue of ice-jam flood hazards to be frazil ice and ice-jam impacts on hydropower (occasionally there are also problems in nonregulated rivers), but it has no official mandate or task to provide ice information. However, the institute does produce ice-breakup forecasts for the Torne River, including forecasts of breakup dates and a "severity degree" factor indicating the risk for ice-jam complications. The forecasts are provided in collaboration and conjunction with forecasts provided by the Finnish Environment Institute (SYKE), which acquires the necessary ice depth data. Both institutes are involved in research projects to further develop river ice forecasting and monitoring capabilities. An example of a flood hazard map indicating some past flood extents due to ice jams is given in Figure 4. code. The guidelines acknowledge that it can be an issue and recommend considering ice where problems with ice have been observed in the past. This can particularly be an issue where water is diverted from the river (but not on the 200-year return period level). At best, ice jams that have threatened hydro-power generation or are caused by hydropower shutdowns are archived using maps or local images, an example of an ice-jam event that occurred on the Svorkmo River in Norway is shown in Figure 3.

the risk of breakup in the case of accidental shutdowns and water being released into bypass reaches. In the proposed guidelines, it is stated that ice jams rarely exceed the 200 year open water flood in Norway, and therefore it is not a central component in the flood zone mapping procedure, since the 200-year level is critical in the Norwegian building

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**Figure 3.** Ice jam on the Svormo River affecting the operation of a downstream hydro-power generation facility (from [12]). **Figure 3.** Ice jam on the Svormo River affecting the operation of a downstream hydro-power generation facility (from [12]). *Water* **2022**, *14*, x FOR PEER REVIEW 7 of 23

garding ice-jam or frazil-ice floods that have occurred in the past and provide an outlook **Figure 4.** Extent of past ice-jam flood events [13]. **Figure 4.** Extent of past ice-jam flood events [13].

#### for potential future floods. Hence, from a Finnish perspective, the Floods Directive does take ice-jam floods sufficiently into account. In Finland, there are areas of potentially significant flood risk, where designation is *4.3. Finnish Perspective*

*4.4. German Perspective* 

based on ice-jam flood risk (risk for significant adverse consequences by ice jams causing floods). Additionally, goals and measures in the flood risk management plans for those areas are targeted specifically for preventing ice jamming or taking action when there are rapidly rising floodwaters due to ice jams. Finnish government authorities have also prepared flood hazard and risk maps for those areas with past ice-jam flood occurrences (roughly based on similar open water flow velocities), for example for Tornio in Figure 5. The EU Floods Directive does consider floods caused by frazil ice and ice jams, even though they are not specifically mentioned in the directive itself. However, according to the directive, the preliminary flood risk assessment should be based on information regarding ice-jam or frazil-ice floods that have occurred in the past and provide an outlook for potential future floods. Hence, from a Finnish perspective, the Floods Directive does take ice-jam floods sufficiently into account.

**Figure 5.** Flood extent for (**a**) different fluvial flood probabilities and (**b**) a severe ice-jam flood (flow

The Oder River is the river that is most often affected by ice jams in Germany. This area, in the most eastern part of the country, experiences continental temperatures in winter. Ice jams generally occur during freeze-up events and are greatly influenced by backwater from the wind setup in the Baltic Sea entering the river's mouth at Szczecin. Ice blockage, in particular, is problematic for flooding at river structures such as bridge piers and weirs. Efforts to release ice jams are carried out by German and Polish ice breakers, but the flow of released ice is hampered by the very mild slope of the lower reaches of the Oder River. Vulnerabilities exist at dikes which have breached in the past due to ice-jam

= 3100 m3/s and + 2m sea water level) at the Torne River outlet. (source: SYKE).

In Finland, there are areas of potentially significant flood risk, where designation is based on ice-jam flood risk (risk for significant adverse consequences by ice jams causing floods). Additionally, goals and measures in the flood risk management plans for those areas are targeted specifically for preventing ice jamming or taking action when there are rapidly rising floodwaters due to ice jams. Finnish government authorities have also prepared flood hazard and risk maps for those areas with past ice-jam flood occurrences (roughly based on similar open water flow velocities), for example for Tornio in Figure 5. In Finland, there are areas of potentially significant flood risk, where designation is based on ice-jam flood risk (risk for significant adverse consequences by ice jams causing floods). Additionally, goals and measures in the flood risk management plans for those areas are targeted specifically for preventing ice jamming or taking action when there are rapidly rising floodwaters due to ice jams. Finnish government authorities have also prepared flood hazard and risk maps for those areas with past ice-jam flood occurrences (roughly based on similar open water flow velocities), for example for Tornio in Figure 5.

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**Figure 4.** Extent of past ice-jam flood events [13].

**Figure 5.** Flood extent for (**a**) different fluvial flood probabilities and (**b**) a severe ice-jam flood (flow = 3100 m3/s and + 2m sea water level) at the Torne River outlet. (source: SYKE). **Figure 5.** Flood extent for (**a**) different fluvial flood probabilities and (**b**) a severe ice-jam flood (flow = 3100 m3/s and +2 m sea water level) at the Torne River outlet. (source: SYKE).

#### *4.4. German Perspective 4.4. German Perspective*

The Oder River is the river that is most often affected by ice jams in Germany. This area, in the most eastern part of the country, experiences continental temperatures in winter. Ice jams generally occur during freeze-up events and are greatly influenced by backwater from the wind setup in the Baltic Sea entering the river's mouth at Szczecin. Ice blockage, in particular, is problematic for flooding at river structures such as bridge piers and weirs. Efforts to release ice jams are carried out by German and Polish ice breakers, but the flow of released ice is hampered by the very mild slope of the lower reaches of the Oder River. Vulnerabilities exist at dikes which have breached in the past due to ice-jam The Oder River is the river that is most often affected by ice jams in Germany. This area, in the most eastern part of the country, experiences continental temperatures in winter. Ice jams generally occur during freeze-up events and are greatly influenced by backwater from the wind setup in the Baltic Sea entering the river's mouth at Szczecin. Ice blockage, in particular, is problematic for flooding at river structures such as bridge piers and weirs. Efforts to release ice jams are carried out by German and Polish ice breakers, but the flow of released ice is hampered by the very mild slope of the lower reaches of the Oder River. Vulnerabilities exist at dikes which have breached in the past due to ice-jam flood events. "The stretch along the Oder Bruch, formally an inland delta drained for agricultural use, is particularly vulnerable due to its containment through dikes and the sediment accretion of the riverbed to elevations higher than the surrounding land. A catastrophic event of extended flooding throughout the adjacent low-lying area of the Oder Bruch occurred in March 1947, in which ice jams caused backwaters to overtop and breach dikes along the Oder Bruch at two locations, with breach widths of over 100 m. Flooding was extensive, leading to the evacuation of 20,000 people" [14]. The fact that ice jamming has become less frequent along the Oder River in recent decades, plus the advances in flood protection and ice defense measures, warning systems, and corresponding disaster control measures, have led to a lack of perception by the people of the dangers and risks of ice-jam floods along the German Oder riverbanks.
