**1. Introduction**

Floods are the most devastating of global natural disasters; they caused billions of dollars in damage and the loss of life of thousands of people in 2017 [1]. Flood hazards can be categorized into different types: e.g., coastal storm surges, river floods, river- and flash floods, and local inundations caused by extreme precipitation. Climate change and sea-level rise further increase the frequency and severity of flood hazards, while population and economic growth further exacerbate flood exposure in low-lying coastal areas [2]. Urgent action is needed to anticipate future losses, but designing and evaluating long-term adaptation strategies is a complex and challenging process for decision makers [3].

In recent decades, a vast array of studies have been conducted to assess and evaluate options for flood adaption so as to reduce current and future flood risk [4–9]. Such studies provide insights into the effects of sea level and climate change on flood hazard (e.g., depth, extent, and duration) [10] and related socio-economic effects on, for example, exposed populations and economic assets [2]. More recently, researchers have started to evaluate the costs and benefits of flood adaptation by using various future projections. In such analyses, benefits are expressed as reduced flood risk (or "expected annual damage," EAD) achieved by implementing proposed adaptation measures [9,11,12]. For example, a recent study show the future benefits of investing in flood protection are much higher than the cost, assuming different future scenarios [9].

Estimating flood adaptation cost is no sinecure, and detailed cost estimates are usually made on the local level—for example, during the engineering design of individual measures [13]. In most cases, however, only aggregate maintenance and investment costs are available from case studies after implementation of the adaptation measure. Furthermore, cost information is often "hidden" in non-peer-reviewed reports, and efforts to collect such information and check its quality are time-consuming. Despite these challenges, enhanced cost estimates of flood adaptation are urgently required to support the economic analysis of flood adaptation research and decision making [14].

With regard to estimating the unit cost of flood adaptation, progress has been made at various scales over the past 10 years regarding flood managemen<sup>t</sup> measures. In terms of flood protection measures, Linham et al. [15] estimate flood adaptation costs for global cities, while Jonkman et al. [14] provide an overview of coastal flood protection for three case studies in The Netherlands, Vietnam, and the United States. Aerts et al. [16,17] and Lasage et al. [18] provide detailed cost estimates for flood protection in New York City, Los Angeles, and Ho Chi Minh City, respectively; they also provide cost estimates for the flood-proofing of individual buildings. Recently, novel literature has emerged regarding the role of nature-based solutions and how these could be developed to reduce flood risk. Such measures include mangrove restoration, rehabilitation of coral reefs, and the development of coastal ecosystems in general. Cost estimates for such activities have been reported in extended reviews by, for example, Bayraktarov et al. [19], Lamond et al. [20], and Narayan et al. [21].

While researchers have made significant progress in providing cost estimates for different categories of flood adaptation, no study has ye<sup>t</sup> combined these estimates into one overview. The main goal of this paper is therefore to increase our empirical database on the cost of flood adaptation by compiling existing peer-reviewed literature and additional reports. The purpose is to provide regional to global flood risk assessment studies with an overview of different flood adaptation measures and their unit costs so they can be included in, for example, cost–benefit analyses or other evaluation studies [12].
