Climate Change Adaptation of Elbe River Flood Embankments via Suction-Based Design

Flood embankments are generally designed by assuming steady-state flow conditions and dry soil above the phreatic surface. However, steady-state conditions are rarely achieved and a significant portion of the embankment remains unsaturated upon a flood event. If transient water flow and partial saturation are considered, the flood embankment can be designed with steeper slopes on the landside, which may lead to significant savings in terms of earthfill material (i.e., embodied carbon) and footprint (i.e., habitat suppression and expropriation costs). This paper examines the case of flood embankments in the tidal area of the Elbe River in Germany. These embankments must be retrofitted by raising their crest from 5 to 7 m because of the new projection of extreme river levels due to climate change. In this paper, the conventional “prescriptive” design consisting of raising the embankment by maintaining the 1:3 inclination of the landside slope is compared with the “performance-based” design where the inclination of the slope on the landside could be potentially increased up to 1:1, which is shown to be sustainable if partial saturation and transient water flow are considered. Raising the flood embankment with 1:1 landside slope (rather than 1:3) could lead to expropriation cost savings on the order of €3.9 M/km. For the case of a newly built embankment of 7 m height, the saving would become €4.5 M/km. An approximate estimation of embodied carbon suggests that the carbon saving would be on the order of 3,100–4,200 tCO2e/km.