Land for flood risk management: A catchment‐wide and cross‐disciplinary perspective

River floods are currently among the most devastating natural hazards. In the last two decades, much research has been conducted in the field of flood risk management, but the role of land has received little attention explicitly; important questions are: How do different kinds of land use and land management influence flood risk generation? Whose land should be used to retain water or mitigate vulnerability? Which policy instruments allow for accessing and governing the land? This special issue is an endeavour to explore the various meanings of land for flood risk management with the specific lenses of relevant disciplines ranging from natural sciences to policy science and economics. In general, flood hazard generation mainly depends on the retention capacity of the land, where vulnerability and risk result from exposure of receptors as well as from their susceptibility, value or function, and coping capacity (e.g., Blanco-Vogt & Schanze, 2014). Accordingly, there are three primary risk reduction options for river floods: first, to retain runoff in the headwaters of catchments through decentralised water retention before it reaches the river network; second, to slow the flood propagation down and cut the peak discharge in the river network upstream of areas prone to flood impacts through modifying flood conveyance, centralised water retention, and flood defence; and third, to foster resilient settlements through mitigation of their exposure and vulnerability. Each of these options has specific effects on flood risks also depending on the size of a catchment and the nature of the flood. Moreover, it may involve various side-effects and interdependencies. Hence, the relevance of land for these options significantly varies. The three options are not entirely new. However, their appropriate combination and effective implementation on the catchment scale seem still to be hampered by a lack of knowledge on the site-specific multifunctionality of land, catchment-wide interrelations, and coordinated land governance (Hartmann & Juepner, 2014; Klijn, Samuels, & van Os, 2008). Many existing studies on flood risk management focus on selected hydro-meteorological aspects, vulnerability with resilience of constructions, or management strategies and the governance context. A catchment-wide and consistent view on land from the biophysical processes to the institutional arrangements is lacking and thus likely to remain a major hurdle for effective risk reduction. This special issue frames the respective field with key contributions from relevant disciplines ranging from hydrology to planning science and economics. It starts with articles on the three options and then provides contributions on cross-cutting topics. Prevention of surface runoff mainly through deep drainage or surface detention is the overall task of decentralised flood retention. This option has been largely addressed in hydrological research. Both observations and model-based analyses carried out at the catchment scale indicate that the effects of decentralised flood retention are inversely correlated with the size of the catchment and the severity of an event (e.g., Rogger et al., 2017). Recently, so-called natural water retention measures (NWRMs) and nature-based solutions (NBSs; cf. Schanze, 2017) are put in the fore, such as reforestation or adapted agricultural land management (see Collentine & Futter, this issue). These measures can also support other ecosystem services, such as groundwater recharge, biodiversity conservation, or aspects supporting recreation. The potential of those NWRMs and NBSs on flood prevention is still under investigation. At this stage, it is already obvious that they need more land than the traditional grey infrastructure. This land needs to be accessible at sites determined by the biophysical processes and is often not owned by the public, but by private landowners. Once the surface runoff has reached the river network, centralised flood retention is a means of slowing the flood propagation down and cutting the peak of the flood wave. Widely applied and effective measures are river rehabilitation as well as the construction of dams, controlled flood polders, dikes, and walls. Also for this option, site-specific land accessibility is required. The experience from the room for the river projects (see Warner, van Buuren, & Edelenbos, 2012) shows that the availability of land and the resistance of land users and landowners are crucial constraints for implementing such measures (see also Milman et al., McCarthy, et al., this issue). Bornschein and Pohl (this issue) explore hydraulic aspects of flood retention. One of the issues raised in this paper is the influence of the land's roughness. They plead for further exploration of this issue in hydraulic engineering strongly linked to land-use planning. Based on lessons learnt from a previous extreme event, Jüpner (this issue) shows how the reality of flood management can challenge traditional spatial planning decisions. He makes an argument to take extreme events into account in spatial planning also involving the limits of technical flood protection measures. Both contributions underpin the meaning of land as a hydraulic component of flood risk management and the need for its multi-disciplinary consideration. Decentralised and centralised flood retention can only reduce the flood hazard to a certain degree. To address the remaining risk, mitigation of the exposure and vulnerability are complementary tasks. Minimal impacts of inundation and quick recovery are the basic ideas of flood-resilient cities. In practice, approaches to flood-resilient cities often focus on risk mapping, early warning, and flood proofing of the public infrastructure. The main damage, however, tends to occur on private properties. There is a wide range of risk reduction measures for buildings (Golz, Schinke, & Naumann, 2015). They require actions by individual homeowners, who so far often do not implement such measures effectively (Fuchs et al., 2017). Pant et al. (this issue) show how impacts on critical infrastructure propagate to significant effects outside the flood zone. This is where the spatial dimension of urban areas with their infrastructure can play a substantial role for flood losses. Therefore, the analysis of the impact of flood events on critical infrastructure as well as the meaning of land availability for mitigating the exposure and vulnerability of this infrastructure seem to be an ongoing research requirement. Optimisation and implementation of the three options of risk reduction related to land, risk management, and land governance necessitates respecting site-specific interests and property rights at the plot scale while ensuring consistency on the catchment scale. Collentine and Futter (this issue) explore NWRMs as decentralised flood retention. Inventing proper compensation schemes is one of the key items that comes to the fore from their analysis, next to the conclusion that this involves multiple trade-offs. Milman (this issue) describes how landowners understand and respond to flood risk management. The author concludes that the compensation of land-use restrictions resulting from the retention measures requires more attention in research and practice. McCarthy et al. (this issue) refer to a case in the United Kingdom to illustrate how compensation schemes and the perception of landowners of these schemes matter in flood risk management. Tarlock and Albrecht (this issue) explore the constitutional and legal aspects of property rights and flood risk management by comparing the institutional systems in three countries: United States, Canada, and Germany. The paper by Slavíková (this issue) addresses the effects of flood recovery schemes on land use. She shows that the public schemes for flood expenditure and flood recovery provide an important trigger for behaviour of landowners after—and thus before—a flood. Machac et al. (this issue) explore upstream–downstream interrelations referring to the creation of centralised flood storage capacity from a theoretical perspective, using game theory. This helps to identify new policy instruments to deal with flood risk management on the catchment scale. Finally, Seher and Löschner (this issue) address the issue of land governance on the catchment scale. Their contribution concludes that governance approaches such as upstream–downstream negotiations demand for combination with formal instruments of land policy. In this manner, all contributions of the special issue focus on either land as part of the flood risk system or land as subject of land governance from a public and landowners' view. The papers present state of the art and recent findings, but mainly identify gaps in research and practice that need to be addressed in the future. This special issue results from the network “FLOODLAND” that developed out of a project on Cross-border Flood Risk Management (Grant No. LE14010) funded by the Czech Ministry of Education, Youth and Sports. Additional financial support was provided by the “Future Deltas” seed money of Utrecht University. Finally, this special issue also marks the start of the EU COST Action “Natural Flood Retention on Private Land” (CA16209), which will further tackle the topic in the coming years (www.land4flood.eu).

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